Weyl and Higgs Theory -- Posted on Friday, December 9 2011
Our hope of finding a gauge theory of weak interactions with massive gauge bosons looks forlorn. It appears that we shall also have unwanted (unobserved) massless scalar particles to worry about. Nevertheless, let us proceed from a global to a local gauge theory. A miracle is about to happen. — Halzen and Martin
Today's
Los Angeles Times has an article on the elusive Higgs boson, the "God particle" that the Large Hadron Collider (LHC) is (hopefully) zeroing in on. The Higgs particle represents the final missing piece of the Standard Model of physics, and its existence (or rather the associated Higgs field) would explain why elementary particles such as electrons and muons have the masses they have.
The LHC has been running for nearly two years now, and its scientists have accumulated enough data to make a fairly accurate assessment of whether they have found the Higgs among the trillions of collision events stored in the collider's massive computer system. A public announcement is expected to be made early next week, possibly on 13 December.
It should be remembered that the Higgs theory is critically dependent on Hermann Weyl's gauge invariance principle which, prior to Peter Higgs' theoretical work of 1964, was believed to be strictly applicable only to theories involving particles of zero mass. However, unlike the zero-mass carriers of the strong, electromagnetic and gravitational forces (gluons, photons and the as-yet undiscovered graviton), the weak force is mediated by the \(W^{\pm}\) and \(Z^{0}\) vector bosons, which are anything but massless. The work of Higgs (and a few others) showed how gauge invariance could be preserved in the Lagrangian describing spin-zero particles. I've given an
elementary mathematical treatment of how this is done, although there are much better explanations available: Griffiths'
Introduction to Elementary Particles is one, while Halzen and Martin give a somewhat older treatment in their
Quarks and Leptons: An Introductory Course in Modern Particle Physics. At any rate, the Higgs mechanism is profoundly ingenious if subtle, and if the LHC finds the Higgs (which Leon Ledermann has also called the "goddamned particle") Higgs should surely be considered for the Nobel prize.
By the way, the journal
New Scientist has an article that posits the question "What if there is no Higgs particle?" It answers the question by saying that the Standard Model will surely survive through other theoretical avenues. But I liken confirmation of a non-existent Higgs boson with faster-than-light neutrinos: the world will go on spinning, but it will be a much more c
onfusing place.
Lastly, and by way of apology, those of you who have waded through my dreary write-ups will have probably noticed that I base my belief in the existence of
God on the fact that mathematical symmetry governs the laws of nature, yet that symmetry cannot be directly observed. Whatever your own personal preference is regarding this matter, I wish you all a joyous and safe holiday season!
UPDATE, 12-13-2011:
Gotta wait some more. "Tantalizing" data, but no definitive proof that the Higgs particle exists.
Peter Higgs in 2008.
Weyl at the IAS -- Posted on Sunday, September 18 2011
One more.
The
Institute for Advanced Study, where Hermann Weyl went after fleeing Germany in 1933, has three pages on Weyl's life and legacy. I don't know how long these pages have been up, but they provide a very nice summary of the man and his contributions to mathematics and physics. One page has a photo of a bust of Weyl that his widow, Ellen Bär, sculpted in 1957, two years after his death.
The IAS site includes this beautiful (but undated) photo of Weyl, which I'd like to believe was taken early in his tenure at the school. If true, then he's either working on a math problem or trying to understand the Internal Revenue Service's tax tables of which, as a newly naturalized American citizen, he later wrote about!
Co-founded by the noted American educator Abraham Flexner in 1930, the IAS has been home to many of the world's greatest scientists and mathematicians. The Institute imposes no teaching or administrative requirements on its faculty, and they are generally free to conduct whatever research interests them.
Photo from the Shelby White and Leon Levy Archives Center, Institute for Advanced Study, Princeton, NJ, USA, with special thanks to Ms. Erica Mosner, Assistant Archivist.
Weyl's Princeton Home -- Posted on Saturday, September 17 2011
Here's a very recent photo of Hermann Weyl's home in Princeton, New Jersey, close to the Institute for Advanced Study (where Weyl taught from 1933 to 1951). Located at 284 Mercer Street, it's a few blocks from Einstein's home at 112 Mercer Street. Fellow mathematician Kurt Gödel also lived nearby, and he and Einstein often walked to and from the IAS together.
Much thanks to Dr. Gualtiero Badin of Princeton University for sending me the picture and Prof. C.N. Yang for sending me the address. Yang bought Weyl's house in 1957 (the year Yang won the Nobel prize in physics) and lived there until 1966.
Weyl Gravity Again -- Posted on Thursday, August 18 2011
I don't post to this site anymore, but I wanted to toss in this last item.
There are a zillion papers on
arXiv.org dealing with Weyl conformal gravity theory, and I really can't contribute anything to the literature. But I did derive a much simpler alternative to Cornelius Lanczos' treatment of the mathematics, and
here it is.
Two of the papers I reference can be found
here and
here. A snapshot of the DeWitt reference can be found
here (specifically equations 16.34 and 16.35). Enjoy.
Whether or not Weyl gravity is a valid theory with regard to the dark matter/dark energy problem is anyone's guess. I suppose it's as good as any right now.
Cornelius Lanczos was a Hungarian-American mathematical physicist (1893-1974). After receiving his PhD in physics in 1921, he discovered an exact solution to Einstein's field equations representing a cylindrically symmetric distribution of matter. Lanczos was Einstein's assistant in Berlin in 1928-29 before settling at Purdue University, where he spent the bulk of his academic career.
Classes of Infinities -- Posted on Monday, August 1 2011
Somewhere in the infinite sequence of digits comprising the transcendental number pi (π) is the string
9142085257914481477154
This apparently random sequence of numbers is guaranteed to be there, as π has an infinite number of digits.
But perhaps the string "In the beginning God" is too easy. It doesn't take much imagination to realize that the entire Old Testament is transcripted in π as well, along with the New Testament (in koine Greek), and all of Shakespeare's plays and sonnets (in Kurdish, yet). Somewhere, many, many trillions of decimal places into π, those books are there. So is My Pet Goat.
It was the German mathematician Georg Cantor (1845-1918), the inventor of set theory, who was the first to rigorously study infinite sets of numbers. He apparently was also the first to recognize that there are infinities that are fundamentally different from other infinities, such as the difference between the infinite set of cardinal numbers (0, 1, 2, 3) and real numbers (pretty much everything else). There are infinitely more real numbers (13.002123 and π, for example) than integers and infinitely more integers than prime numbers. But they're all infinite, so big deal, right?
Today's
New Scientist has an article that examines this question in light of recent and current mathematical research. You can read the article for yourself, but to me it implies that infinity might have a kind of mathematical structure all its own, a structure in which infinite sets display individual, distinct properties that are only now being examined.
One aspect that the structure idea touches on has to do with the continuum hypothesis, which basically posits that there are two kinds of infinity—discrete infinities (e.g., integers) and continuous infinities (real numbers), with nothing in between. They're both without limit, but each member of the discrete set is entire to itself (the number 13, for example), while \(&pi\) has an infinite number of digits and cannot be written down save by the purely human invention of designating a symbol to represent the number.
So why is this a big deal? Infinity is infinity, right?
Here's my take. In quantum physics we have a number Z that represents Feynman's path-integral transi
tion amplitude for some initial and final state. The integral must be integrated over an infinite number of space-time points and so cannot be done in closed form. But the integration can be done in piecewise fashion, i.e., by perturbation. Each step in the integration gets exponentially more difficult, though, so the calculations must be taken only as far as absolutely necessary. With each step, more and more state/particle paths (or virtual particles) enter into the calculation, but 6-digit accuracy is usually achievable after only a few steps.
Assume now that some genius comes along who figures out a way to do the integration exactly, without any approximation at all. That is, when given the path integral for some specific problem, she goes to the blackboard and simply writes down the complete answer, presumably a complex number that contains combinations of Planck's constant \(h\), the speed of light \(c\), the electronic charge \(q\), \(\pi\) and maybe even the gravitational constant \(G\). Question: if we could dispense with the perturbation approach, what form would the answer have? What would it look like? How would particles be represented? And what would it mean?
It seems to me that here we would be crossing over from an infinite number of discrete quantities to a continuous set that could be written down at once. Although the two approaches might yield the same result (to some specified number of digits), the mathematical, physical and philosophical consequences would be enormous.
I think we would, in a very real sense, finally behold infinity itself.
Causality Holds -- Posted on Wednesday, July 27 2011
A group of physicists at the Hong Kong University of Science and Technology has
reported that the velocity of a photon in a vacuum is indeed limited to the usual `3×10^8` meters/second speed limit.
However, this finding, which appears in
Physical Review Letters, is being picked up by the
news media as proof that time travel is impossible. Their reasoning is that, according to relativity, time slows down as velocity is increased. At the speed of light, time freezes; ergo, if something goes faster than light, time must go backward. This is a logic I don't understand at all. Republican "science"?
Nothing v Something -- Posted on Tuesday, July 26 2011
New Scientist's Amanda Gefter's article
Existence: Why is There a Universe? touches on the problem of entropy and nothingness: although a zillion moles of a randomly-dispersed gas has a very high entropy (the gas has almost no order), a state of nothingness must also have a very high entropy because "you can shuffle it around all you want and it still looks like nothing."
By the same logic, according to physics nobelist Frank Wilczek of MIT, nothingness is a state of perfect symmetry. But in physics, broken symmetries are the rule, so nothingness would have to be an unstable state.
Arizona State astrophysicist Lawrence Krauss has a new book out (I can't recall the title just now, as I haven't read it) that deals with this kind of phenomena. A flat universe (one in which there is zero overall curvature) would have a total energy content of exactly zero: the positive energy of matter and fields would be canceled precisely by the negative binding energy of gravitation. Thus, asserts Krauss, a state of nothingness (zero energy) could give rise to a state of somethingness, which still has zero energy. This is surely the most extreme example of mass-energy conservation one could think of.
One version of the Heisenberg uncertainty principle states that the energy content of a vacuum can be very high over very small intervals of time:
\( \Delta E \Delta t > \frac{1}{2}\hbar \)
When the universe was created time did not exist, so the energy uncertainty must have been very great, indeed.
But, as Gefter rightly points out, Heisenberg's principle is a law of physics, and physical laws must have existed prior to the creation of the universe. How did this come about? This question takes us back to Gefter's original inquiry, which is: Why is there something rather than nothing?
Weyl Gravity Theories -- Posted on Saturday, July 16 2011
Weyl gravity appears to be a very peculiar theory. — Sophie Pireaux
Some rambling thoughts tonight. Research involving Hermann Weyl's conformal gravity theory continues unabated—many papers are appearing on
arXiv that deal with it in one form or another. I can't keep up with the ones I can actually understand, while the ones I can't understand I ignore. Here's a small glimpse of what I see going on.
Weyl's 1918 theory was an attempt to explain electrodynamics as a purely geometrical phenomenon. In spite of its elegance the theory failed, although the manifold that Weyl had invented (Weyl geometry) has remained a topic of great interest to this day. Why? In my opinion it is because (apart from the purely conformal aspect of the theory, which is itself aesthetically beautiful) it provides a tantalizing new approach to current problems, including the vacuum energy and dark matter problems (galactic rotation rates, etc.) and quantum gravity. This view is supported by many recent papers, of which the following are just a few:
These papers are all very readable and interesting, but I have a few beefs regarding these applications of Weyl geometry.
The primary problem involves the Weyl vector \(\phi_\mu\) (which Weyl tried so valiantly to identify with the electromagnetic potential in his theory). In almost all current theories this vector has been scrubbed out (though I doubt that anyone seriously thinks that Weyl geometry has anything to do with electrodynamics nowadays). But it is this vector that makes the Weyl geometry work; in particular, it makes the Weyl connection conformally invariant. When \(\phi_\mu \) is zero this is no longer the case; Weyl's geometry reverts back to ordinary Riemannian geometry, so Weyl's gravity theory no longer makes any sense.
In addition, the Weyl Lagrangian provides a theory of gravitation at the expense of making the associated Lagrangian of fourth order wi
th respect to the metric tensor \(g_{\mu\nu}\) and its derivatives. Such higher-order theories involve "ghost fields" that are non-physical (of negative norm).
Furthermore, none of these approaches deal with the "divergence problem." That is, mass-energy conservation still relies on covariant derivatives rather than ordinary derivatives. Also, no one knows what the mass-energy tensor should look like for 4th-order theories.
Lastly, with respect to quantum gravity theories, we see unappealing (at least to me) combinations of classical geometrical quantities (like the Ricci scalar \(R\) and similar scalars) mixed up with scalar and spinor quantum fields (like \(\Psi\) ).
But what the hell. Let's move on.
Weyl proposed using the square of the Ricci scalar ( \(R^2\) ) as the basis for his gravity theory solely because the density of the Einstein-Hilbert term \(R\) alone is not conformally invariant; that is, it's not invariant with respect to some arbitrary change of scale
\(g_{\mu\nu} \rightarrow \lambda(x) g_{\mu\nu}\). Weyl's \(R^2 \) is conformally invariant, and is the simplest one available (Weyl was aware that some combination of \(R_{\mu\nu} R^{\mu\nu}\) and
\(R_{\alpha\beta\mu\nu} R^{\alpha\beta\mu\nu} \) would also work, but I suspect he considered these scalars to be less "pretty.")
The gravitational equations of motion for empty space are easy to derive for the Weyl Lagrangian (I've worked it out in at least one of the write-ups available on this site, so I won't do it here). The answer is
The trivial solution is \(R = R_{\mu\nu} = 0\), which is Einstein's equation. But for a constant, non-zero \(R\) we also have the second-order expression
\(R_{\mu\nu} - \frac{1}{4} g_{\mu\nu} R = 0\)
For a Schwarzschild-like line element of the type
\(ds^2 = A c^2 dt^2 - B dr^2 - r^2 d\theta^2 - r^2 \sin^2 \theta d\phi^2 \)
where \(A\) and \(B\) are functions only of the radius parameter \(r\), the equations of motion give the identifications
\(A = B^{-1} = 1 - \beta/r + k r^2\)
with \(\beta\) and \( k\) non-zero constants. This is just the Schwarzschild line element with an additional term with one very interesting property—it can be used to explain the observed cosmological acceleration effect, in which the universe seems to be expanding at an ever-increasing rate (the constant parameter \(k\) is in fact proportional to the cosmological constant \(\Lambda\)).
But there's a better candidate for Weyl gravity, one that even Weyl himself seems to have overlooked. As I mentioned, the quantities \(\sqrt{-g}R^2 \), \(\sqrt{-g}R_{\mu\nu} R^{\mu\nu}\) and \(\sqrt{-g}R_{\alpha\beta\mu\nu} R^{\alpha\beta\mu\nu} \) are all available, and it would appear that some linear combination of these scalar densities might serve as a good Lagrangian candidate for gravity. As luck would have it, there exists a scalar density in which all these terms appear together automatically. It is, appropriately, the Weyl conformal density \(\sqrt{-g}C_{\alpha\beta\mu\nu} C^{\alpha\beta\mu\nu}\) itself. Best of all, this density is conformally invariant in ordinary Riemannian geometry; you don't need Weyl's \(\phi\)-field.
I've talked about the Weyl conformal \(C\) tensor several times here before; it is responsible for the tidal, deforming (but volume-preserving) effects of gravitational fields in the absence of matter, and in some respects is more fundamental than the Riemann-Christoffel tensor \(R_{\alpha\beta\mu\nu}\), which vanishes in the absence of matter. But it never dawned on me to actually compute the \(C^2 \)
scalar itself, which turns out to be
and so has everything built into it. Two notable researchers who worked out the equations of motion associated with this quantity are Mannheim and his associate D. Kazanas (the pdf file is
here). By varying the Weyl Lagrangian with respect to the metric \(g^{\mu\nu}\) they got the Bach equation \(B_{\mu\nu} = 0\), where
Note that this tensor is traceless (\(B = B_{\mu\nu} g^{\mu\nu} = 0\)) and admits the identification
\(R_{\mu\nu} = \Lambda g_{\mu\nu}\)
as a solution (\(\Lambda\) is usually identified with the cosmological constant). This (along with \(R = 0\)) is called the trivial solution. ArXiv has many papers that detail searches for other solutions.
Mannheim and Kazanas solve the Bach equation (not a fun job!) and get the surprisingly
simple result
\(A = B^-1 = 1 - \beta/r + k r^2 + \gamma r\)
along with a constant term. This is the Schwarzschild/Weyl result with an additional parameter \(\gamma\) that might also have something to do with dark matter (note that the \(k\) and \(\gamma\) coefficients would have to be very small to have gone unnoticed, particularly on galactic scales).
Researchers now seem to have plenty of parameters to play with to explain their cosmological and quantum gravity theories. None of them, to me, is very attractive, particularly in light of Dirac's old admonition that "mathematical equations must be beautiful" if they are also to describe the truth.
Please give these papers a look. They're at least interesting, if not mathematically beautiful.
Bored Tonight -- Posted on Wednesday, July 13 2011
Several people have asked where the imaginary "`i`" comes from in Penrose's treatment of the Elitzur-Vaidman bomb problem that I talked about earlier. I didn't know myself (hell, I just pass this stuff along) but suspected it could be derived by considering a `90^o` momentum change in the photon wave function. Assuming an overall arbitrary phase factor of \( \exp(i\theta) = \cos \theta + i \sin \theta\) would seem to do it (let \(\theta\) go from zero to \(\pi/2\) ), but that's not very satisfactory (and probably wrong). But then I asked what the wave function of a photon might actually look like, and realized that I'd never given it any thought. Neither, it seems, has anyone else (just try looking for a photon ket state vector on the Internet).
The fundamental quantity that describes a photon is the four vector \(A_\mu\), which is described by the source-free Lorentz gauge condition as
`◻^2 A_mu = 0`
The solution to this partial differential equation is
`A_\mu = `exp` (-ip⋅x/ℏ) \epsilon_\mu (p)`
where `\epsilon_\mu` is the polarization vector (which actually has only two independent terms, not four). Numerous writers call `A_\mu` the photon wave function, but it would seem that you can't actually do anything with it, much less pull down a factor of `i` from it under a reflection.
Penrose's treatment references the book Optics by Miles V. Klein and Thomas E. Furtak. I borrowed the text from Caltech, but it still doesn't explain where the damned `i` comes from in the Mach-Zehnder beam splitter experiment. But more searching l
ed me to a very interesting paper that presents the photon wave function in a form I've never seen before.
It's a 2006 paper by M.G. Raymer and Brian J. Smith of the University of Oregon at Eugene called
The Maxwell wave function of the photon. This little five-page paper (suitable for undergraduates) includes a nifty flow chart that takes Einstein's `E^2 = m^2 c^4 + c^2 p^2` mass-energy relation in two directions, one for massive particles (Dirac's equation) and one for massless photons (Maxwell's equations). Assuming a three-component vector wave function `\Psi(x, t)` for the photon having the simple form
`\Psi(x, t) = E(x, t) + iB(x, t)`
(where `E` and `B` are the electric and magnetic field vectors), we easily see that
`i {\partial\Psi}/{\partial t} = c∇ × \Psi`
reproduces
all four Maxwell equations for free space. In addition, it is easy to see that the integral
`∫ |\Psi|^2 d^3 x`
normalizes `\Psi` with respect to the total energy of the electromagnetic field.
By the way, this version of the photon wave function `\Psi` isn't new; Iwo Bialynicki-Birula of the Polish Academy of Sciences has published extensively on it and its role in quantum field theory (but to me this is no wave function; it's just an interesting combination of vectors with some interesting properties).
Still, I haven't been able to use any of this to get the `i` factor from the Mach-Zehnder experiment. Let me know if you find the solution.
Entropy and Environment -- Posted on Saturday, July 9 2011
Roger Penrose's Road to Reality is a comprehensive overview of just about everything in mathematical physics, and it includes detailed discussions of the role of entropy, particularly as it relates to the birth, evolution and fate of the universe.
Penrose views the Sun as a nearly perfect black body that radiates low-entropy light in the form of short-wavelength radiation to the Earth, which then re-radiates much of this light back into space. The difference? Earth's radiation goes out as long-wavelength light with added entropy:
In the interim, Earth's various life forms (predominantly plants) utilize the Sun's radiation as a low-entropy energy source; the stripped energy goes back out as high-entropy infrared radiation. So why is this important?
In his July 9 blog, UC Riverside mathematical physicist
John Baez (now on a long sabbatical in Indonesia) writes about this same issue but with a more important message— Earth's long-wavelength re-radiated light is preferentially absorbed by atmospheric carbon dioxide, which acts to trap infrared radiation that would ordinarily escape into space. End result: Earth is heating up because of anthropomorphic carbon loads.
To a great extent, Earth operates off of the Sun's low-entropy energy supply
It is primarily elevated levels of CO2 (along with atmospheric water vapor and methane) that are contributing to global warming. The extent and import of this warming is highly controversial, but what is not disputed is the annual observed increase in atmospheric CO2 levels, which have risen continuously since regular measurement began in the 1800s. It is now approaching 390 parts per million, and atmospheric scientists are predicting associated global temperature increases on the order of several degrees Celcius by 2050. That doesn't seem like much, but if true it could be catastrophic for our planet.
Baez, whose career emphasis shifted radically in 2010 from quantum gravity and n-dimensional gauge theory to environmental science, appears to believe that our quest for a Theory of Everything will not materialize unless we start taking care of this planet. His website is followed by millions of scientists, and I encourage you to bookmark his site and read it on a regular basis.
Welcome to Dollyworld -- Posted on Thursday, July 7 2011
All gods are homemade, and it is we who pull their strings, and so, give them the power to pull ours. — Aldous Huxley
Genetics is not my strong suit, so I welcome any useful reference that even I can understand. A good one is Spencer Wells' latest
bookPandora's Seed: The Unforeseen Costs of Civilization. Wells, Professor of Genetics and Anthropology at Cornell University, is the director of the Genographic Project, an effort funded by IBM and the National Geographic Society (NGS) that deals with historical worldwide human migration patterns.
The book doesn't really talk a lot about genetics per se, but uses it instead to explore the inevitable rise of civilization as a consequence of man's shift from hunter-gatherer to farmer around 10,000 years ago. Along the way, Wells asserts, mankind unwittingly sowed the seeds of overpopulation, disease, mental illness, climate change and even religious extremism. But one of Well's primary concerns deals with the problem of obesity as a consequence of modern civilization, at least in the developed countries.
As a correspondent with the NGS Wells seems to have traveled everywhere, but his description of a summertime visit to Pigeon Forge, Tennessee with his family is particularly interesting. Wells went to
Dollywood, Dolly Parton's music- and food-themed amusement park, where he witnessed obesity on a truly astonishing scale. The narration is amusing, given Well's descriptions of endless funnel cakes and fried Snickers bars, waddling guests and amusement rides that cannot safely accommodate a large percentage of the patrons, but it's also tragic in view of the fact that so much of this fatness epidemic is avoidable.
Wells attributes part of the reason for the obesity epidemic to thrifty genotyping. As he explains, thousands of years ago humans were often required to do strenuous activity in order to acquire enough food to survive. This gave rise to ectomorphic Africans, whose bodies could shed extreme solar heat efficiently while running many miles to hunt down game. Similarly, Polynesians had to paddle for days in dugout canoes to secure fish, giving rise to powerfully-built, energy-efficient bodies. Although much of this strenuous activity persisted during the advent of farming, the primary cause of human mortality shifted from trauma (getting gored by an irate mastodon, for example) to infectious diseases such malaria, which spread easily in closely-knit farming communities (and exponentially more so with the rise of cities). But modern man has found himself screwed by thrifty genotyping: sedentary lifestyles are the rule now, and the easy availability of non-nutritious, calorie-rich processed food has given rise to a third wave of mortality statistic, chronic non-infectious diseases such as heart disease, diabetes and cancer, the high incidence of which is largely attributable to behavior. American Samoa and some American Indian groups are now experiencing Type I and Type II diabetes rates that are approaching 25% of their total populations.
Wells' three waves of killers, from the Neolithic Revolution (10,000 BCE) to the present
While Mississippi remains by far America's fattest state, the overwhelming national trend is toward even greater per capita weight gain and its attendant adverse health impacts. In their July 2011 report on national obesity statistics, the Trust for America's Health and the Robert Wood Johnson Foundation
report that in 1990 not a single state had an obesity rate of over 15% but, by 2010, 38 states had rates exceeding 25% and every state was above 15%. It's no wonder that diabetes is exploding across the country.
Obesity dominates in the Bible Belt. God loves you guys, and you love it deep fried!
Wells does not admit it, but it is abundantly obvious that American obesity predominates in the nation's Bible Belt. And while Wells discusses the rise of fundamentalist Christianity in his book as an indirect consequence of our disastrous food consumption behavior, he wisely avoids the associated issue of how and why the American right—undereducated, overfed, armed and hyper-proselytized—is being exploited by the likes of Sarah Palin and Michele Bachmann, who extol the presumed God-given right of Americans to feed themselves and their children whatever garbage they want free of government intervention.
This is, as Wells does imply, mythos versus logos—unscientific, wishful-thinking mythology fighting against science and rationality. And in 2012, the myth-believers might just win.
Bombs Away -- Posted on Tuesday, June 21 2011
While re-reading Shadows of the Mind by Roger Penrose (1994) I came across his description of a fascinating quantum problem that I had completely forgotten about. It's called the Elitzur-Vaidman bomb problem, and it neatly demonstrates how a real problem that cannot be solved classically can be solved using quantum mechanics. Here we'll walk through it using Pen
rose's diagrams but with a few changes to shorten the solution (Penrose's treatment is many pages long).
You're given a quantity of bombs, many of which are duds. The live ones are equipped with a highly sensitive trigger device—the trigger is so sensitive that it will respond to the action of a single photon and set off the bomb. The duds are also equipped with triggers, but they are defective—the triggers are jammed and an impinging photon has no effect. Your task is to sort out the duds and to secure a quantity of live bombs.
Obviously, there is no classical way you can test any of the weapons without destroying the live ones; the best you can do is find the duds. However, the principle of superposition in quantum mechanics can be used to find the live ones. Here's how.
First a preliminary situation to acquaint you with the math and experimental setup (which is known as a Mach-Zehnder interferometer):
A single photon emerges from a light source. It will have some state vector associated with it; call this vector |A〉. The photon encounters a half-silvered mirror, so that it has an equal chance of being reflected or transmitted. If it is transmitted, it will enter the state designated as |B〉; if reflected, it will be in the state |C〉. The photon thus finds itself in a superposition of the two states,
|A〉 = |B〉/√2 + i|C〉/√2
(The square root is there just to make things look better at the end, and the imaginary number i is added to the C state to account for the fact that a reflection changes the photon's phase by π. Also, I'm using equal signs, but they should be read here as "goes like.") If transmitted, the photon encounters a fully-silvered mirror and is reflected upward. We thus have a new state given by
|B〉 = - i|D〉/√2
(The minus sign is purely arbitrary.) If it is reflected by the original half-silvered mirror, the photon proceeds to another fully-silvered mirror, and its state is described by
|C〉 = - i|E〉/√2
The total superposition thus far is then described by
The photon thus collapses back to its original state (|F〉 = |A〉), which was inevitable—the photon's initial energy and momentum must be conserved. The photometer at F always registers the photon, while the photometer at G always remains silent.
Now consider the revised set-up above, where we have replaced the fully-silvered mirror on the bottom right with a similar mirror attached to the trigger of a bomb. Again, we send one photon out from the source.
If the bomb is a dud, the jammed trigger will not budge. The mirror remains in place; we have the same situation as before, and the photometer at G will not register. All dud bombs will activate the photometer at F and leave the photometer at G alone.
However, if the bomb is live the situation changes drastically. Consider the scenario where the photon emerges from the source and is actually transmitted through the original half-silvered mirror. The photon encounters the bomb mirror and triggers the bomb, which explodes. In this case, the photon's state has collapsed in accordance with
|A〉 = |B〉
The bomb thus acts as a detector (and in this case a rather noisy one). However, if the photon is reflected the photon's state still collapses, this time in accordance with
|A〉
= i|C〉
The photon is now reflected by the fully-silvered mirror at the upper left, giving it the new state
|C〉 = i|E〉
so that
|A〉 = i(i|E〉) = - |E〉
Now, when the photon (still collapsed) encounters the half-silvered mirror at the photometers, it goes back into a superposition of states in accordance with
|E〉 = |F〉/√2 + i|G〉/√2
so that overall
|A〉 = -|E〉 = -|F〉/√2 - i|G〉/√2
The photon's state will now collapse either at the photometer at F or at G, with a 50% probability for each.
Thus, overall a live bomb has only a 25% chance of triggering the photometer at G (the photon has a 50% chance of being reflected at the first half-silvered mirror, then another 50% chance of being reflected to G). But the only ones that can possibly set off the photometer at G are definitely live bombs (or bad, depending on how they will be used).
Summary: if you test a bomb, and the photometer at G registers a photon, then the bomb is definitely live.
Convince yourself that the live-bomb case makes the state |D〉 unavailable to the photon, and this makes all the difference.
Israeli physicist Avshalom Elitzur earned a PhD in physics without any formal education, but his accomplishments are notable and he has wor
ked with some great physicists, including Yakir Aharonov. You can find the 1993 Elitzur-Vaidman paper
here. In 1994, the bomb-testing problem was verified experimentally by Anton Zeilinger and his colleagues.
[Of course, things are more complicated than what's described here. Real beam splitters aren't made from infinitely thin reflecting surfaces; they consist of slabs of glass or other dielectric material of finite thickness. For a simple explanation of why this is important, see
How does a Mach-Zehnder interferometer work?]
Many Worlds -- Posted on Monday, June 20 2011
PBS reran Parallel Worlds, Parallel Lives on NOVA last night, which chronicles the re-acquaintance of Mark Everett, leader of the alt-rock band EELS, with his father, Hugh Everett III. Everett III was the brilliant quantum physicist who came up with the
"many worlds" interpretation of quantum mechanics as an alternative to the conventional view,
wave function collapse.
In what is surely one of the most hopeful acts of any cable science show, NOVA's
website includes a
link to Everett's 1957 PhD dissertation (pdf warning), which details the many-worlds theory. Being a geek, I downloaded the 137-page paper and read it last night. It's interesting, but the terminology is ancient by today's standards and so is harder to follow than it should be.
Mark Everett is a bit of a geek himself, although he readily confesses that he has no understanding of physics nor has any inkling of what his father accomplished. In the NOVA episode he travels to Princeton to meet several quantum gurus and others who knew his father as a grad student, including Charles Misner (who wrote the classic Gravitation with John Wheeler and Kip Thorne) and Max Tegmark, who is a keen and noted proponent of many-worlds. The younger Everett gets to see the loft where his father wrote his dissertation and other neat stuff (his father seems to have been very fond of cheap sherry), but the show mainly serves to demonstrate the huge gulf that existed between father and son.
Mark hardly knew his father, a brooding, distracted, hard-drinking and hard-smoking type who would certainly have never won a Father of the Year award. Mental illness also seems to have be the birthright of Mark and his sister, having received it from their mother and brilliant but equally depressed father. When the elder Everett died at 51 from a heart attack in 1982, Mark's sister committed suicide (the suicide note indicated she would be with her Dad in some "parallel world"). Later Mark's mother died of cancer, rounding out a trio of deaths that hit him hard. Mark himself experienced suicidal thoughts, but fortunately was able to find a more productive outlet through his music.
The show's background music includes some of Mark Everett's work, although it's too Elvis Costello-y for me. I read his 2008 autobiography a few years ago, mainly to find out what more I could learn about his physicist father, but that too was disappointing. What I did learn is that Hugh Everett was an atheist who nevertheless believed in quantum immortality, and that the younger Everett is himself an ordained minister!
After completing his dissertation, Hugh Everett's advisor, John Wheeler, was so impressed with the idea of many-worlds that he took Everett to Denmark to meet and confer with the famous Niels Bohr. Bohr, the founder of the conventional Copenhagen interpretation of quantum mechanics (wave function collapse), received Everett politely but dismissed his ideas, which he saw as radical and wrong-headed. Everett, sorely disappointed, shortly thereafter left academia and worked for the government the rest of his short life. Sadly, he did not live to see the resurgence of his many-worlds idea, which today has more proponents than critics.
Every grad student's dream: Punk kid (Everett, second from right) meets Niels Bohr (center)
Many worlds and quantum consciousness and immortality—what wonderful ideas! As a Christian myself, I think that there might just be something behind all that (John 14:2).
Lots of Black Holes -- Posted on Saturday, June 18 2011
Scientists, using a combination of data obtained from the Hubble telescope and the Chandra X-ray orbital observatory, have discovered a large group of supermassive
black holes at the edge of the universe, hiding within a larger group of galaxies. The finding adds to growing evidence that black holes and galaxies are somehow intimately related, and that this relationship stretches back to the time of the Big Bang.
Artist's conception of an early-universe black hole
The presence of the black holes was deduced using "image stacking." The Chandra observatory, which was trained continuously on the study region for 46 days, detected glimmers of X-ray radiation but discerned no individual candidates for black holes. But when combined with ultra-deep field data from the Hubble, the black holes were revealed. Of the 250 or so galaxies studied, a large percentage (perhaps 100%) appeared to contain black holes. (The Chandra observatory was specially designed to detect and measure X-rays given off by cosmologically-distant celestial objects.)
X-ray radiation is a high-energy form of light that can be emitted by stars and matter as they accrete closer and closer to a black hole. As matter piles up in the accretion disk (or is compressed as it falls into the hole), friction heats up the matter to the point of iridescence. For this reason some black holes, while actually invisible to the naked eye, are considered "bright" objects.
X-rays, unlike visible or infrared light, can escape the powerful gravitational pull of a black hole.
No, light is light, and X-rays are no better at escaping the pull of a black hole than any other light.)
The red shift of light from the galaxies studied had an average Z of about 6, meaning that the galaxies are receding from us at 96% of the speed of light. The Hubble equation can be used to show that the light from these galaxies was emitted some 12.8 billion years ago, or less than a billion years after the Big Bang. The existence of black holes so early in the universe's history seems to imply that they are common and fundamental celestial objects. Numerous scientists familiar with the study have expressed their belief that intelligent life might not have arisen without them.
The Weyl Curvature Tensor and the End of Everything -- Posted on Thursday, June 9 2011
Roger Penrose's fanciful view of God setting up the universe.
In the phase space representing the entire universe, the Creator would have had to set the initial conditions to within one part
in 1010123 (depicted here with a very fine placement needle!) to produce the universe that we see today (this is not, however, proof of the
existence of God). From Road to Reality.
I've touched on the Weyl curvature hypothesis once or twice (an odd term,
considering the curvature tensor is Weyl's but the hypothesis is due to Penrose), but here I'll mention two very readable papers that discuss the hypothesis
in greater detail. First, some background.
Roger Penrose published his idea in 1977 in an attempt to explain a second law of thermodynamics that is
compliant with gravitation. By way of example, Penrose began by noting that the second law (which basically states that entropy can only increase with time)
requires that a high-pressure volume of gas will invariably expand into a larger volume of lower pressure, the idea being that the entropy S of the gas molecules
will increase via Boltzmann's equation S = k log V, where k is Boltzmann's
constant (a variation of this equation is inscribed on Ludwig Boltzmann's head
stone; see below). However, a large volume of diffuse gas molecules will also tend
to collapse under the influence of gravity, resulting in just the opposite effect. The question is: how can this contradiction be resolved, given the
assumption that entropy always increases?
Penrose decided that there must be a form of entropy contained in gravitational fields, such that a
large volume of gas would have a smaller gravitational entropy than, say, a coalesced object like a star or a black hole. This line of thought took Penrose
into deep considerations of the second law at the time of the Big Bang singularity and its continuing role in the evolution of the universe (including its
possible collapse back to the singularity under the Big Crunch, if that should be the case).
When the universe was born, Penrose reasoned, the
expanding ball of mass-energy would likely have had a minimal Weyl curvature, while the Ricci curvature would have been large (for a brief description of the two, see my write-up on the
Weyl conformal tensor). As the universe evolved, gravitational coalescence
occurred with a corresponding increase in Weyl curvature. Penrose therefore reasoned that Weyl curvature might have something to do
with entropy, given the fact that they both increase as the universe gets older. Penrose maintained that
as the universe ages, more and more black holes will accumulate, so much so that at some point nearly all the mass of the universe will reside
in black holes, with a corresponding large Weyl curvature. This will not quite mark the end, however, as the black holes themselves will all slowly
evaporate via Hawking radiation. In the end, the universe will consist only of stray photons, and entropy will have reached its maximum extent
(perhaps it will be infinite). In this sense, Penrose believes, Weyl curvature and entropy are related. Indeed, the Bekenstein-Hawking black hole entropy-area formula
\( S = \frac{1}{4}\frac{kc^3 A}{G \hbar} \)
(which can be derived a number of different ways) demonstrates the intimate relationship between entropy and the surface area A of
the ultimate gravitational object, the black hole. Most interestingly, the entropy and all the information it contains exists on the surface of
the event horizon!
In 2006, O. Rudjord and O. Gron of the University of Oslo investigated the Weyl curvature hypothesis and its possible
relationship to gravitational entropy in a beautiful paper,
The Weyl curvature conjecture and black hole entropy. The authors
assumed that, if Penrose is right, the Weyl conformal tensor Cαβγδ would have to be proportional to gravitational
entropy. However, since the contracted trace of this tensor is zero, the only scalar they could form
is CαβγδCαβγδ, which they call P2. Then begins an amusing search for
a radial vector field whose divergence is proportional to the entropy. After laboriously trying out three alternatives for the proportionality term,
they finally settle on the Kretschmann scalar, RαβγδRαβγδ. They then explore this
choice in both Schwarzschild and de Sitter spacetimes. Conclusion: at the cosmological horizon (end of time), entropy must be of non-geometrical origin.
Oh well. Maybe something else will work.
Penrose has written numerous papers himself on the subject, of course, but the
most readable I've seen to date is the one he presented to a
conference in Edinburgh in 2006. It also introduces the idea that at the universe's end, only photons will exist, so that the geometry of the universe
must be conformal (again, see my write-up for a explanation of this point). Weyl himself explored a strictly conformal space-time in his 1918 theory,
but it failed, mostly because the line element ds of matter cannot be made scale-invariant.
Penrose's latest book is
Cycles of Time.
I have not read it yet but the reviews, which have been very
positive, indicate that Penrose now believes that at the end of the universe all information will have been destroyed via black hole evaporation.
The universe, consisting of nothing but radiation, will then have no concept of time or history, and so will "reset" itself by forgetting about its large
entropy content. At that point there will be nothing to distinguish the universe from its pre-Big Bang state. Thus, a new Big Bang will occur, possibly
with a different set of fundamental physical constants; then another, and another after that, for all eternity.
This is pretty heavy stuff, and
I have not nearly done it justice in this overly-long post. Penrose's earlier books,
The Emperor's New Mind and
The Road to Reality, along with the papers cited above, will be of much greater use to those of you want more information. Personally, the
subject matter just blows me away.
In the book Shermer, who holds advanced degrees in experimental psychology and science history (and lives about a mile from me), focuses on why we believe what we believe, his primary thesis being that our beliefs c
ome first (childhood exposure, environment, etc.) with the rational, thinking aspect following in support of
those beliefs (rationalization, confirmation bias, etc.). He explains the two basic primordial errors we human make as being based in patternicity (the tendency to discern patterns in meaningful as well as meaningless data) and agenticity (the tendency to ascribe unexplained or coincidental events as the actions of an external agent like God or fate).
Anyway, Shermer's book includes a detailed discussion on the nature of God and the possibilities of the afterlife, both from the point of view of brain function, neurochemistry, and learned and even inherited predisposition. That's all very interesting, and worthy of being read for its own sake, but Shermer (an avowed atheist) also provides four lines of "evidence" for afterlife, which goes something like this:
1. Information fields and the universal life force. Something about Nature preserving information in a nebulous, universal field that can be "sensed" by certain enchanted people.
2. ESP and evidence of mind. This includes psychic abilities, ESP and telepathy, again involving very special people with "The Power."
3. Quantum consciousness.
This is the idea that the universe is a giant quantum field in which all things, including information, are interconnected in some fundamental sense.
4. Near-death experiences. "Then I saw a bright halo, and a voice called to me saying 'Go into the light.' " Enough said.
As a skeptic, Shermer pretty much destroys all these ideas, but he gives particularly short shrift to the quantum consciousness idea, which Roger Penrose (a hero of mine) and numerous other notable physicists see as the only logical explanation of afterlife, assuming there is one. Penrose has addressed the QC idea in several books, notably The Emperor's New Mind and The Road to Reality, both of which I have talked about at length on this site. I consider his Road book to be a masterpiece, an opinion that many seem to share: although crammed with fairly difficult mathematics, it became a bestseller when it came out in 2004.
The conventional ("Copenhagen") interpretation of quantum mechanics. A quantum system evolves in time unitarily and continuously (U), only to collapse discontinuously under human observation or measurement (R). Either the human mind collapses wave functions (how?), or some other picture ("many-worlds") is involved in which the wave function does not collapse but maintains its set of superposed eigenfunctions, with the mind simply going into the universe associated with the observed outcome. Either way, the human mind seems to play an indispensabl
e role. (From Road to Reality)
What I do find neat about Shermer and Penrose is that they seriously attempt to address the issues of mind, God and reality from a purely scientific (as opposed to metaphysical) point of view, something that most scientists do not want to get involved with. I find this approach most refreshing, and much preferred to the usual dogmatic adherence to holy books and writings of questionable provenance that would force people into believing all kinds of nonsensical myths and miracles (as a Christian, I find following Jesus' teachings difficult enough without having to believe in a bunch of impossible events).
While Shermer is not nearly in the same intellectual league as Penrose, his book is stimulating and well worth reading its 400 pages. (Downside: Shermer's unnecessary and disconnected asides on the blessings of libertarianism and the philosophy of objectivist writer Ayn Rand, whom I detest.)
Weyl and the Continuum -- Posted on Thursday, May 19 2011
I think it's safe to say that very few of you who studied Fourier analysis in school were much concerned that a continuous, differentiable function ƒ(t) could be represented by an infinite number of very discrete sine and cosine terms. If your instructor said anything about it at all, she probably just remarked that it is assumed from the outset that the terms form a complete set, which in the n → ∞ limit allows the sines and cosines to represent the objective function exactly. Typically, however, there was always one wise guy in the class who would ask that if the term cos(1243194063 π t/p) was removed, would the objective function "miss" it in any critical way, or if the set could still be considered complete. And that is the danger one faces when dealing with complete sets, particularly those that contain an infinite number of elements. Somehow, life goes on.
And you did move on, happy in the knowledge that you could forget about completeness and focus instead on getting a decent grade in the class. However, about that same time the instructor had you consider the fact that when the objective function itself becomes a single, discrete, unrepeated pulse, that complete set of sines and cosines becomes the very continuous exponential function exp. Confusion reigned, and you congratulated yourself that you were not majoring in mathematics. You probably also heaved a sign of relief in the knowledge that mathematicians had to study "analysis," but you only had to deal with a watered-down version called "calculus." At any rate, you most likely forgot all about that jabber concerning continuity and complete sets.
In 1918 Hermann Weyl wrote a book on the continuum of numbers, which he conveniently titled
The Continuum. In the book he asked very difficult questions concerning the seemingly infinite divisibility of real numbers. There are only two chapters in the book; the first is pretty simple, mostly basic mathematical concepts and such, while the second takes up the bulk of the book and deals with much tougher material. Toward the end, Weyl moves on from the continuity of numbers to that of time, which he considered to be the most fundamental continuum.
But unlike the real number system, in which concepts such as "smaller," larger" and "equal" can be defined (or at least reasoned to exist), Weyl considered the related concepts of "earlier," later" and "now" to be much more difficult. In particular, Weyl considered consciousness to be an aspect of our concept of time, because time forces us to wonder why "This now is, but
now no longer is", and that question necessarily involves conscious thought. It is human consciousness that must wrestle with the puzzling fact that an experiential "now" simultaneously becomes "memory of."
Although Weyl likened the experiential "now" to a mathematical point in the real number system, in 1918 it is doubtful that anyone believed that space might be atomically and indivisibly "granular," or that "granules" of time exist. The nearest thing we have to granular time would be Planck time, which is (Gℏ/c5)½ ≈ 5.4×10-44 second. But this is preposterously small, given that human reaction times (
not to mention biochemical reaction rates) are on the order of milliseconds at best. But Weyl did admit to the possibility that mathematical points and what he called "exact-time points" might not be the ultimate underlying elements of human experiential reality, and he noted that human reason alone could differentiate between physical and perceptual points of time and space.
Today, string and loop quantum gravity theories indicate that space-time might very well be granular or indivisible in some way, so that there is no true continuum except that which exists conceptually in the human mind. I think Weyl would have felt very much at home with these theories.
The Limits of Knowledge -- Posted on Monday, May 9 2011
I'm struggling to get through
Scot McKnight's authoritative 2005 book Jesus and His Death: Historiography, the Historical Jesus, and Atonement Theory. It's a tough read, certainly not meant for beginners like me, I guess, but fascinating all the same.
Anyway, in the book McKnight reflects on the writings of noted British historian Keith Jenkins on what can and cannot be known about historical events:
No, what Jenkins is accusing us of is
far more profound, and it closes the books on nearly every historical Jesus study ever done. He is saying that we are not finding the ?real? Jesus behind the texts, the rediscovery of whom sheds light both on the real Jesus and a more genuine and authentic and historical faith. He is arguing that we are simply fooling ourselves: what we think we are doing is not what we are doing. We are not finding Jesus back there, hidden for all these years by th
e church and others. What we are "finding" is nothing; we are "imposing" pleasing narratives about our own ideologies in order to assert our own power. We impose our power in the form of rhetoric about Jesus. Historical Jesus scholars don?t have a goose by the neck, after all; instead, they have a mirror by the top and they are looking at themselves. History, he is saying, is not the past. History is a narrative using discrete facts about the past. This sort of history is more imagined than it is found. The past remains there, discoverable in its historiographical representations (like the Gospels), but meaningless until it is spun into a narrative. History makes discoverable and discrete and existential facts meaningful; but the meaning one finds is not what happened, not the past itself, but a narrative spun in the mind of the historian.
In short, Jenkins asserts that objective historical truth about Jesus Christ may simply be unattainable, and we have to learn to live with our ignorance. More importantly, we have to admit our ignorance as well, and that requires the abandonment of unbased certainty.
The other day I talked about a New Scientist article on Heisenberg's uncertainty principle, which states quite clearly that the limits to physical truth are set by Nature. The journal now has another
article about the limits to knowledge th
at
includes overviews of the "cosmic horizon," Gödel's 1931 incompleteness theorem ("This sentence is false"), black holes and the origin of life on Earth
(you need a subscription to read the article, but you can also see it
here).
Uncertainty Principle Not So Uncertain? -- Posted on Tuesday, May 3 2011
Werner Heisenberg's (1901-1976) famous Uncertainty Principle states that there is a fundamental limit to how accurately we can measure certain quantities simultaneously. The best known example is position and momentum; in summary, the principle says that if you measure a particle's position and its momentum at the same time, there will be a certain finite amount of "fuzziness" in the values you get. This fuzziness is a limit imposed by Nature; it cannot be eliminated or even reduced.
Or can it? The journal
New Scientist is reporting the recent work of several researchers who believe that quantum entanglement might provide a way to improve upon Heisenberg's famous principle. If it's true, one of the foundational tenets of quantum mechanics is going to be seriously shaken up.
You can find the two most recent papers
here and
here. For a very elementary overview of the quantum entanglement problem, see my write-up on Bell's inequality
here.
While a rigorous derivation of the precise uncertainty principle is usually given in beginning graduate school, I can give you a very simplified derivation here that any undergraduate can follow.
Start with any two arbitrary hermitian operators A and B in a space spanned by some arbitrary state vector |ψ〉. Now define the two associated operators
ΔA = A −〈A〉 ΔB = B − 〈B〉
where 〈A〉 = 〈ψ|A|ψ〉 is the expectation value of the operator A, with a similar definition for B. The new operators thus define the "distance" of A and B from their "average" values.
If we now take the expectation values of the squares of ΔA and ΔB themselves, we get
〈(ΔA)2〉 = 〈A2〉 − 〈A〉2 〈(ΔB)2〉 = 〈B2〉 − 〈B〉2
Let us call these quantities σ2A and σ2B, respectively. The significance of this notation will be apparent shortly.
Now let's construct a new arbitrary state vector |β〉 from ΔA and ΔB and some real scalar λ via
|β〉 = (ΔA + i λΔB)|ψ〉
Its associated bra vector is |β〉*, or
〈β| = 〈ψ|(ΔA − i λΔB)
The inner product is then
〈β|β〉 = 〈σ2A +
λ2 σ2B + i λ [A, B]〉 (1)
where [A, B] is the anticommutor AB - BA and we have used the fact
that [ΔA, ΔB] = [A,B].
Now, 〈β|β〉 ≥ 0 but is otherwise just a number. Its extremal value is found by taking the derivative of (1) with respect to λ and setting the resulting expression to zero. This gives
λ = − ½ i [A, B]/σ2B
Plugging this into (1), we have the condition
σ2A
σ2B ≥ − ¼ [A, B]2
or
σA σB ≥ − ½ i [A, B] (we take the negative root for a good reason)
Convince yourself that whether λ maximizes or minimizes 〈β|β〉, the inequality still holds.
For the case where A is the position operator x and B is the momentum operator p we then have, from the basic quantum identity [x, p] = iℏ, the precise Heisenberg uncertainty principle,
σA σB ≥ ½ ℏ
For classical systems, the anticommutator [A, B] is usually zero, so there is no uncertainty at all in the measurements of classical parameters, such as the position and velocity of an automobile. For quantum systems, the situation can be drastically different!
So what are σA and σB? In elementary statistics, these quantities are nothing more than the standard deviations of the position and momentum measurements, while their squares σ2A and σ2B are called variances. (Why the statistical nature of these identities is never made clear in most textbooks escapes me.)
Weyl and Ellen -- Posted by on Wednesday, April 27 2011
I often get emails from people who want to know more about Hermann Weyl's personal life. Here's a clue: I actually know very little about Weyl's life outside of his physics and mathematics. I've been to Switzerland, but I haven't even been able to locate the guy's grave. (Photo: Weyl in Jena, Germany, sometime in 1930.)
But here's something I was able to share recently. Weyl's first wife Hella died in 1948, and two years later he married Ellen Lohnstein Bär (1902-1988), the daughter of noted physicist Richard Bär. She was the member of a very prominent and wealthy Swiss family with ties to banking and finance. Ellen was herself a noted sculptor (Bildhauerin), artist and violinist, and her works can still be found on online auction sites.
Here is her son's online family album (warning: 4.7 MB). Ellen and her family moved in high circles: Wolfgang Pauli, Isaac Stern, Georg Solti and Chaim Weizmann were all family friends.
It is said (see Wheeler's tribute to Weyl elsewhere on my website) that Weyl's last uttered word on this Earth was "Ellen," when he died of a heart attack while posting some letters on 8 December 1955.
Update: I couldn't find any copyright information on the Bär album, so here is a photo taken of Weyl and wife Ellen, presumably around the time they were married in 1950. He was around 65 at the time, while Ellen (still beautiful) was 48. They had five years together before Weyl passed on in 1955. As Ellen's previous husband had been a physicist (she became a widow in 1940), I suspect this had something to do with her attraction to the rather frumpy-looking Weyl.
Weyl at 35 -- Posted on Tuesday, April 26 2011
Yesterday I found a couple of new photos of Hermann Weyl on the
ETH-Zürich website. This is a portion of a picture taken during a visit Weyl and his wife made in 1921 to the home of Fritz Medicus, a Swiss-German philosopher (go to the website to see the copyrighted photo).
Weyl married his first wife Fredericke Berta Hellene Joseph ("Hella," 1893-1948) in 1913. She came to the University of Göttingen in 1911 to study under the great German phenomenological philosopher Edmund Husserl, who Weyl had known there since his student days. A translator of Spanish novels to German and English, Hella died in 1948, with Weyl remarrying two years later. Weyl and Hella had two sons who both became noted mathematicians.
Speaking of philosophy (not my best subject), some years ago I recommended Thomas Ryckman's excellent book
The Reign of Relativity: Philosophy in Physics, 1915-1925. It's an invaluable source of information on Weyl's physics and philosophy at a time when Einstein's general theory of relativity had changed the scientific and philosophical world. I still recommend it.
(Copyrighted photo courtesy of the Eidgenössische Technische Hochschule.)
Krauss on Nothingness -- Posted on Sunday, April 24 2011
Noted Arizona State University astrophysicist Lawrence Krauss recently gave an interesting talk called A Universe from Nothing, which you can watch on
YouTube. In his talk Krauss addresses the cosmological constant, which Einstein fudged into his 1915 gravitational field equations in order to "freeze" the universe, which he believed was static, neither expanding nor contracting.
In a few years, observational evidence appeared that seemed to show that the universe was not static but expanding. H
ermann Weyl was one of the first physicists to suggest that the universe was indeed expanding, and even proposed an early form of Hubble's Law (which states that cosmological objects are receding from one another at a rate proportional to their distance). The famed astronomer Edwin Hubble observationally proved the expanding universe concept in 1929 (right here in Pasadena!)
Einstein initially thought the idea "
abominable," but by 1923 he admitted the possibility that it was true. His 1923 postcard to Weyl (reproduced above) includes the fascinating remark "Wenn schon keine quasi-statische Welt, dann fort mit dem kosmologischen Glied" (If there is no quasi-static world, then away with the cosmological term).
The noted Swiss physicist (and fellow Weyl enthusiast)
Norbert Straumann writes about Einstein's struggles with the cosmological issue in this short
paper from 2002 (which includes the translation of Einstein's postcard remark to Weyl).
By the way, Krauss is also a noted proponent of evolution (you can watch many of his talks on this subject online), and is a dedicated (nay, virulent) atheist. In the linked video he states that all of creation could simply be the result of a vacuum fluctuation some 13.7 billion years ago. Interesting, but this does not account for the universe's apparent need to produce complex structures, such as human beings (although such "islands" of extremely low entropy
would be permissible). The "driving force" of the universe (which doesn't necessarily have to be God) remains a mystery.
Where is Everything? -- Posted on Friday, April 15 2011
For decades, astronomers have realized that there is simply not enough observed matter in most galaxies to keep them from flying apart. The measured orbital velocity of galactic stars, particularly those on the rims of their galaxies, is too great for gravity to maintain them in orbit.
This puzzle has led to the dark matter conjecture, which posits that much of the matter in galaxies in unseen or "dark." The most likely candidates for dark matter, which doesn't interact with ordinary matter except through gravity, are weakly interacting massive particles, or WIMPS. Neutrinos (which hardly interact with matter at all) are now considered to have mass, but the upper bound of neutrino mass is to
o small for them to be considered WIMPS.
Two days ago, scientists in Italy studying WIMPS
released a paper summarizing the results of the most ambitious WIMP detection project to date. The heart of the detector (buried
deep in a mine to minimize cosmic ray interference) held 161 kilograms of liquid xenon, which the scientists hoped would interact with any passing WIMPS. The results:
The graph depicts all the detection events that occurred over the project's 101-day observation period. All but three events (red dots) were the result of background noise or other extraneous events considered statistically meaningless. However, the scientists also expected to see two or three hits as a result of stray cosmic rays penetrating the detector.
Their conclusion: the red dots were cosmic rays, and so no WIMPS were detected.
All matter (or mass-energy) in the universe is now considered to consist of dark energy (73%), dark matter (23%), and baryonic/fermionic matter, the ordinary stuff we all know and love (4%).
Incredibly, 96% of the universe is missing.
Neutrinos Again -- Posted on Thursday, April 14 2011
Yesterday I posted a blog about neutrinos, then quickly took it down because (knowing little about the subject) I had no right to say what I did. But I did want to reaffirm my recommendation for the second edition (2008) of David Griffiths'
Introduction to Elementary Particles, as the revised edition now includes a good overview chapter on neutrinos. (Most of the revised edition is available on
Google Books.)
In 1929, Hermann Weyl took Dirac's relativistic electron equation (which actually holds for any fermion) and found that the four coupled, simultaneous differential equations decouple into just two equations when the particle mass is set to zero. This gave Weyl two equations
which have since been traditionally viewed as the mathematical foundation for neutrinos
, particles of zero mass and spin ½ that travel at the speed of light. Alas, recent observational evidence now pretty clearly demonstrates that the three neutrino flavors can all oscillate into one another, meaning that an electron neutrino νe can be spontaneously converted to a muon neutrino νμ or a tau neutrino ντ, etc.
Neutrino oscillation solves the so-called neutrino problem, which describes the discrepancy between the observed flux of solar electron neutrinos and that predicted by the Standard Model. You can read about the problem
here.
Anyway, Griffiths presents the derivation for a simple 2-neutrino oscillation situation which can be followed by any undergraduate (last night I repeated the calculation for the three neutrino situation, which is far messier but yields the same results). For a more detailed discussion, Griffiths refers us to the work of Fermilab's Boris Kayser, who seems to be a leading researcher on the topic; a good, accessible reference (and with one of the wildest graphics I've ever seen) can be found here at
arXiv.org.
Back in 1933 Weyl got sucker-punched
by Wolfgang Pauli for his (Weyl's) neutrino equations because they did not preserve parity symmetry. The experimental verification of broken parity was not made until 1956; regretfully, Weyl's victory over Pauli was unknown to him, as he passed away the year before. Pauli died in 1958.
For an excruciatingly detailed treatment of neutrino oscillation at a relatively accessible level, see Giuti and Kim's 2007 book
Fundamentals of Neutrino Physics and Astrophysics (chapter six has no fewer than 438 numbered equations!)
Eddington, Just Once More -- Posted on Tuesday, April 5 2011
The British astronomer and mathematical physicist Arthur Stanley Eddington died in November 1944. His last book, Fundamental Theory, was published posthumously. I pieced it together from portions available on the Internet. It's a large pdf file (18.8 MB), but
here it is if you want it.
The book includes a concise summary of Weyl's 1918 gauge theory, a fascination for which I guess Eddington took with him to his grave. His understanding of the replacement φμ → i φμ is most interesting.
The book's appendix also supplies the basis for Eddington's calculation of the number of protons and electrons in the universe, which
I posted a few days ago in full. For brevity, I used Mathematica to calculate it to only 20 decimal places:
(Notice the 136 figure in the input. It seems Eddington never could get over that integer, which he earlier surmised was the exact inverse of the fine-structure constant.)
Oddly, the last line of Fundamental Theory reports the number to be 3/2 times the above figure. It looks as if Eddington inserted it in light of the thermodynamic formula for the molar kinetic energy of an ideal gas, E = 3/2 nRT. (In fact, the last chapter of the book is called "The Molar Electromagnetic Field".) Huh? I really don't get it. Maybe you can figure it out.
Nonsymmetric -- Posted on Tuesday, April 5 2011
You probably already know that the affine connection of differential geometry is usually taken to be symmetric in its lower indices (Γλμν = Γλνμ). But scores of researchers over the decades have sought to generalize Einstein's gravity theory by abandoning this condition. Notably, Einstein himself tried it (this page of handwritten connection calculations was found near his death bed), and Schrödinger also gave it a shot in a series of papers he wrote in 1947 (google "The Final Affine Field Laws," the full text of which I have been unable to find in pdf format). You can take my word that this generalization led nowhere, but it's interesting.
I haven't seen any evidence that Hermann Weyl ever investigated a nonsymmetric connection. He probably saw it as a waste of time.
A very readable review of what a nonsymmetric affine connection entails can be found in the 2007
paper "On the Nonsymmetric Purely Affine Gravity" by Indiana physics professor Nikodem Poplawski, whose math can be followed by any undergraduate.
"Purely affine" simply means that the metric tensor gμν is not taken into account.
By the way, I scanned the above page of Einstein's calculations from the book
Albert Einstein: Creator and Rebel by Banesh Hoffman (Einstein's friend and colleague) and Helen Dukas, who was Einstein's personal secretary for many years. It's a great book that any Einstein admirer should have in her library.
Eddington, Once More -- Posted on Monday, April 4 2011
A last note here on Eddington (shown here at the age of 38), and how he proposed to rescue Hermann Weyl's 1918 theory from Einstein's criticisms, which fatally injured the theory.
Reasoning that space-time should be metrically "flexible," in 1918 Hermann Weyl proposed that the rescaling of the fundamental metric tensor gμν → λ(x) gμν should have no effect on physics. He worked out the mathematical details of this
invariance and discovered that Maxwell's electrodynamics seemed to automatically pop out of the theory. Einstein initially loved the idea, but then noted that the line element ds2 = gμν dxμ dxν would also be rescaled according to ds2 → λ ds2. Since ds can be made to serve as a measuring rod or clock, Einstein reasoned that certain absolute quantities, such as the spacing of atomic spectral lines and the Compton wavelength of an electron, would change arbitrarily and thus have a prehistory whose properties would depend on their past. Einstein concluded that Weyl's theory must therefore be nonphysical.
Weyl countered with the confusing idea that physical observables can be determined either by persistence (Beharrung) or adjustment (Einstellung). He argued that intrinsic quantities such as the Compton wavelength are unobservable and "persistent" (they just exist), while rods and clocks measure "adjustable" quantities that involve the properties of the rods and clocks themselves, as they are part of the measurement process. While this view eliminated Einstein's objection, they seemed artificial at best, and most physicists at the time agreed with Einstein.
But not Eddington. He took Weyl's theory and made it even stranger.
Eddington reasoned that if the non-invariance of ds was due to the metric tensor gμν, then the metric tensor had to go. The only other covariant tensor
of rank two available was the Ricci tensor Rμν, so he wrote
ds2 = Rμν dxμ dxν
Now, the Ricci tensor is composed solely of a quantity known as the coefficient of affine connection Γαμν, which Eddington used to base his entire theory. He conveniently ignored the fact that in Riemannian geometry the connection is itself composed of the metric tensor and its first derivatives. Instead, he just assumed the connections were themselves fundamental quantities.
Eddington still needed an action for his theory, and he used
I = ∫ |R|1/2 L d4x
where |R| is the determinant of the Ricci tensor "matrix." Because the Lagrangian L has to be both coor
dinate- and scale-invariant, Eddington had to use the Ricci tensor and its inverse to raise and lower the indices of other tensors (such as the Maxwell tensor F
μν) that he incorporated into the Lagrangian.
The resulting theory was a total mess. Weyl, who had already moved on from his theory, was probably embarrassed to see his idea butchered in this way, while Wolfgang Pauli wrote to Eddington
"In contrast to you and Einstein, I consider the invention of the mathematicians that one can found a geometry on an affine connection without a primary line element as for the
present of no significance for physics."
Nevertheless, Eddington's idea motivated Einstein to conduct a similar search for a generalized affine connection, a quest that sadly led the great scientist away from the quantum revolution and down a fruitless, thirty-year road (1925-1955) that ended only with his death in 1955.
"I believe there are 15 747 724 136 275 002 577 605 653 961 181 555 468 044 717 914 527 116 709 366 231 425 076 185 631 031 296 protons in the universe, and the same number of electrons." — Sir Arthur Stanley Eddington, 1938
I've written
about Eddington (1882-1944) before, and whatever you want to say about him, imprecise would
definitely not describe the man.
The above number is known as the Eddington Number (quite a coincidence, yes?), usually given as 1.57×1079. Clearer minds have since truncated that figure to simply 1080, which is the basis of Hermann Weyl's large number hypothesis (and subsequently extended by Paul Dirac).
It was Eddington who, back in the days when the fine structure constant α was thought to be very close to 1/136, stepped up with a detailed proof that the inverse was exactly 136. Later, when experiment showed it was closer to 137, Eddington provided a revised proof showing the inverse was exactly 137 (he blamed an earlier algebraic error on the discrepancy).
Like Weyl, Eddington was also by Einstein's general theory of relativity, and again like Weyl he attempted a generalization of Einstein's tensor mathematics in the development of a unified field theory. Weyl gave up his own 1918 theory within a few years, but Eddington was more dogged. He in fact loved Weyl's theory, which
he described as "unquestionably the greatest advance in the relativity theory after Einstein's work." When Einstein showed that Weyl's theory was invalid, Eddington resolutely set about generalizing Weyl's work. Alas, Weyl subsequently referred to Eddington's revision as "not fit for discussion" (undiskutierbar).
Eddington with friend.
Eddington wrote two books in the period 1921-22 on Einstein's gravity theory, both of which included a detailed analysis of Weyl's theory. I have a raggedy copy of Eddington's The Mathematical Theory of Relativity (1922), which is interesting if something of a quaint mathematical antique, along with a reprint of his Space, Time and Gravitation (1921), which covers much the same material. Eddington also wrote a curious book called Relativity Theory of Protons and Electrons (1936), which appears to be more philosophical (though I haven't read it). If you have nothing better to do, you can download it legally
here.
I recently discovered a book on Eddington by King's College professor of mathematics C.W. Kilmister called
Eddington's Search for a Fundamental Theory: A Key to the Universe (1994). I've only just started reading it, and it looks to be very interesting. Also interesting is the author's unabashed fascination with Eddington's theories, which he reveals in statements such as
"It is half a century since I succumbed to the Eddington magic — I paraphrase Thomas Mann's phrase to try to do justice to my youthful if uncritical absorption in Relativity Theory of Protons and Electrons, which Eddington had published five years or so earlier, in 1936."
Imagine that—a grown man fascinated by the work of an obscure scientist! :-)
But Eddington deserves praise if only for the fact that he led the 1919 solar eclipse expedition whose results famously proved Einstein's theory predicting the bending of light by gravity (it also made Einstein a scientific superstar).
Newton's Belated Triumph -- Posted on Monday, March 28 2011
While I'm on the subject of newspapers, here's an amusing story about an editorial that ran in The New York Times on January 13, 1920, ridiculing American rocket pioneer Robert H. Goddard about his proposal to shoot rockets into the vacuum of outer space. It read, in part:
That Professor Goddard, with his 'chair' in Clark College and the countenancing of the Smithsonian
Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react—to say that would be absurd. Of course he only seems to lack
the knowledge ladled out daily in high schools.
It wasn't until July 17, 1969, with the Apollo moon landing, that the newspaper printed an apology:
A Correction. On Jan. 13, 1920, "Topics of the Times," an editorial-page feature of the The New York Times, dismissed the notion that a rocket could function in vacuum and commented on the ideas of Robert H. Goddard, the rocket pioneer. Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error.
The Times' apology took nearly 50 years.
"Further investigation and experimentation," indeed.
Scientific Honesty -- Posted on Monday, March 28 2011
Just a note to say that the April 2011 issue of
Scientific American has a nice article by Princeton physics professor Paul Steinhardt on the cosmological theory known as inflation, the idea that the very early universe experienced an extremely
brief (∼ 10-30 second) period of hyper-expansion. Inflation was predicted by Alan Guth some 30 years ago and has become a cornerstone of conventional modern cosmology. It not only accounts for many observations, it is also highly predictive. Scientifically speaking, it has passed all the tests.
But Steinhardt, a 30-year adherent of inflation theory, isn't so sure, and he brings up some good arguments as to whether inflation is correct or even necessary. In short, Steinhardt puts inflation on trial in his article, and finds that there are numerous arguments that could easily overturn it or make it redundant.
But what's particularly interesting is that Steinhardt's article is a good example of how scientists—even when they have a perfectly valid, testable theory on their hands—continue to objectively test and re-evaluate their ideas to the point where, even when their theories are shown to be right but unnecessary, are willing to throw them out and start over. How many politicians, theologians and neoconservative ideologues are willing to do the same?
The article cites a recent paper by G. W. Gibbons and Neil Turok
(The Measure Problem in Cosmology), which you may find enlightening. Another paper that I have found interesting is
Inflation with a Weyl Term, or Ghosts at Work by Nathalie Deruelle et al., in which Hermann Weyl's conformal tensor is tossed into a model Lagrangian for the universe. (The authors refer to it as constantly popping up "on the market of gravity theories." Ha.)
Just the stuff for the beginning of the week!
It Begins -- Posted on Monday, March 28 2011
The New York Times has announced it is starting its long-awaited
digital subscription process. Although unpaid access to The Times will still be possible, it will be very limited.
In his prophetic 2009 book
Empire of Illusion: the End of
Literacy and the Triumph of Spectacle, author Chris Hedges correctly predicted that print-based information would give way to image-based information. So what's wrong with that? It has to do with the fact that
print requires reading and intellectual digestion, while images are essentially emotion-based and require no thought. As more and more newspapers go the subscription route (and you can't blame them—they need to make money to stay in business), fewer and fewer Americans will be reading, while Internet images will remain free and ubiquitous.
One can only wonder what the intellectual level of America is going to look like in ten years. In the meantime, speaking of images, and while you're still reading, here's an
article reporting on Abercrombie & Fitch's new line of push-up bikini tops for eight-year-old girls. I predict that in ten years pedophiles will be able to get off on the images without the distraction of having to read that there might be something sick about it.
Please be patient, the girls themselves will soon be shown posing provocatively in the ads. But if you can't wait, you can always fall back on last year's controversial YouTube video of gyrating 7-year-old girls emulating Beyoncé:
Nice going, America. To quote Hedges in a
recent interview, "It's over."
Weyl and Dark Matter -- Posted on Saturday, March 26 2011
A little looking around on the online physics journal
arXiv.org shows that there continues to be much interest in identifying Weyl's vector field φμ with dark matter. A particularly simple example was published recently by MIT's
H. Cheng, who also reserves a few choice comments regarding the basis for Einstein's rejection of Weyl's original 1918 conformal theory. Cheng believes that fermions would not interact with an all-permeating bosonic Weyl field, negating Einstein's assertion that the presence of the field would alter the history of electrons passing through it (making distinct atomic spectral lines impossible).
Cheng does not explain why he says that "Weyl made the mistake of identifying [the φ-field] with the photon," but he is actually referring to the fact that the line element ds2 in Weyl's conformal theory is invariant only for photons (i.e., ds = 0). In reality, Weyl struggled valiantly to find a general invariant form for
ds2. He never did, and it was primarily for this reason that he abandoned the theory.
I would prefer to think that dark energy, not dark matter, would have more in common with Weyl's conformal geometry, but I have little to justify that point of view other than a layperson's sense of aesthetics. In my naïve
write-up on Weyl's theory, I used Weyl's lagrangian along with φμ = 0 to indicate that just about any alteration of the Einstein-Hilbert action will result in something
that looks interesting, but isn't.
Still, the holy grail
of the Large Hadron Collider is the Higgs field, and the Higgs is predicted to be a boson. The Higgs field is presumed to permeate all space, and it also presumes to explain the basis of particle mass, as particles of varying kinds slog their way through the field. Could Weyl's field have anything to do with the Higgs? Probably not, but it's an interesting thought, and one that seems to keep popping up in the literature.
I think Weyl would be amused were he around today.
The New Romans -- Posted on Saturday, March 26 2011
We all have friends and relatives who choke up in prideful tears when they see billion-dollar Stealth bombers flying overhead (like they do here in Pasadena every Rose Parade). Now they can choke up watching the latest US Navy commercial, which highlights multi-billion-dollar cruise missile carriers along with the unreal assertion that
Until the world can live in peace … until natural disasters no longer strike … until Freedom can protect itself — until that day, th
e men and women of America's Navy stand ready to answer the call.
It should have added that "Until that day when all these imaginary situations come about (and we all know they they never have and never will), the American taxpayer will gladly fork over a trillion dollars a year in defense spending."
Come on, have we become so stupid and arrogant that we can listen to this kind of jingoistic crap without laughing our heads off or shedding torrents of remorseful tears?
After every successful military campaign, Julius Caesar Augustus' victory parades would feature the emperor in a special chariot carrying a huge phallus symbolizing Rome's strength and virility. This was the image that immediately entered my head when George W. Bush landed on the US Navy carrier Abraham Lincoln in a crotch-padded flight suit to give his "Mission Accomplished" speech on May 1, 2003.
"President Bush can land on my deck any time he wants!" — one Red State woman during a CNN interview
"Navy Seals rock!" — Katie Couric, CBS anchorwoman (still pert, spunky and cute as a button)
In 1953 Eisenhower noted that
Every gun that is made, every warship launched, every rocket fired signifies, in the final sense, a theft from those who hunger and are not fed, those who are cold and not clothed. This world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children. This is not a way of life at all in any true sense. Under the cloud of threatening war, it is humanity hanging from a cross of iron.
That came from a man who had personally witnessed the horrors and atrocities of World War II. We now watch ads directed at gun- and videogame-crazed Red State morons who have few employment prospects other than legalized mass murder.
What's a Dord? -- Posted on Saturday, March 26 2011
I've finally finished the Text of the New Testament: Its Transmission, Corruption and Restoration by the late, esteemed Princeton theological seminarian and Bible expert
Bruce Metzger. This is the book's 4th edition (2005), and in its preparation Metzger was joined as co-author by
his star pupil
Bart D. Ehrman of the University of North Carolina at Chapel Hill. Unlike Metzger, Ehrman
is almost universally despised by fundamental Christians for having published numerous books (e.g., Misquoting Jesus) that cast doubt on the supposed inerrancy of the Old and New Testaments.
Ehrman himself has published extensively in works ranging from professorial research papers and textbooks
to his more
recent, controversial, popularized "exposés" of Bible inaccuracies, ambiguities and outright contradictions. Unlike his mentor Metzger, who went to his grave in 2007 a confirmed and faithful Christian, Ehrman underwent a rather sudden transformation to agnosticism, the result not of biblical consistency problems but of his inability to reconcile God's allowance for evil in the world.
[It's amusing to look at the schizophrenic, bimodal reviews of Ehrman's books on Amazon.com. They're either highly praised by liberals who have actually read the books, or panned by conservatives who either haven't read them or can't.]
Regardless of how you stand on all this, everyone needs to know about all the well-known Bible "problems" that
every pastor learns about (and then promptly forgets later when preaching) in seminary school. To Ehrman (and probably Metzger as well), many of
these problems are both serious and irreconcilable. Bible apologists (going back even to Calvin's day) have long known about these issues and attempted numerous (and sometimes
ingenious) fixes. Ehrman has re-examined many of the problems, and his arguments for biblical errancy are convincing. At least I am convinced. I do not believe that any rational human being can dismiss the conclusion that the Old Te
stament is largely pseudo-historical religious mythology, while the New Testament is primarily a construction of the human mind. If God did indeed inspire any of these writings (and I still believe He did inspire some of them), His messages have been deeply corrupted by a combination of accidental and intentional human tampering.
Here's a simple, amusing and secular example pulled from Metzger's Text of the New Testament book:
As Metzger would be the first to admit, many such innocent errors found their way into the Old and New Testaments as they were being laboriously hand-copied over the hundreds of years preceding the invention of the printing press. Other errors were intentional. If, as Ehrman has asserted, God had intended His words to be scrupulously preserved, He
was more than undone by all-too-human scribes who were too tired, stupid, lazy or agenda-driven to respect those intentions.
So what are we
to believe? For me, Matthew 22:34-40 just about says it all. And I think Ehrman would agree with that: rather than allow Bible errors to stymie one's faith, he would prefer that we adjust our faith to allow for a God whose real messages come to us from a variety of sources—the Bible, mathematics, science, nature, and the unparalleled reasoning ability of the unfettered human mind.
Enough of my
silly philosophy. In addition to your Bible, may I suggest that you read what is arguably Ehrman's best book, Jesus, Interrupted, which can be comfortably read and digested in about 4 hours. Metzger's book is a scholarly text and will take you much, much longer to get through (skim through it on a first reading). Then go for Israel Finkelstein's best-selling The Bible Unearthed: Archaeology's New Vision of Ancient Israel and the Origin of Its Sacred Texts (2002), which you can easily read in maybe 8 hours. Then get Donald B. Redford's scholarly and comprehensive Egypt, Canaan and Israel in Ancient Times (1992). The latter two books will shatter any myths you've entertained over the years concerning the biblical story of Exodus and other nonsensical tales.
Emmy Noether -- Posted on Thursday, March 17 2011
Physicist and writer
Ransom Stephens gave a lecture last year on Emmy Noether, the
female German mathematician (1882-1935) and friend/colleague of Hermann Weyl that I occasionally
talk about on this site. Below is Stephens' lecture on YouTube.
Stephens' talk is not perfect, and it's an hour long, and there's a pitch thrown in at the end for his book, but I recommend that you watch it because he provides a pretty decent overview of the relationship between mathematical symmetries and conservation laws (which Noether formalized
in her famous 1918 theorem). The humanity of the woman is also much in evidence in the lecture. Plus, Hermann Weyl's 1935
eulogy
of Noether is described.
I was heartened to hear Stephens admit that, as a physicist, he doesn't understand Noether's pure mathematics at all (ring theory, non-commutative algebra, that sort of thing). Hell, I don't, either.
"It's got to be there, damn it!" -- Posted on Wednesday, March 16 2011
After straining innumerable brain cells trying to understand supersymmetry (SUSY, see my post of 25 January), I now learn that it may be
all wrong, after all.
In her article "What if Supersymmetry is Wrong?", science writer Amanda (and New Scientist book editor) posits the very real possibility that decades of forlorn searching for the tell-tale signs of SUSY (including preliminary results from the Large Hadron Collider) might be telling us that one of the leading contenders for a "theory of everything" might simply be a wild goose chase. String theorists, rejoice.
For me, this leaves loop quantum gravity, as I have given up trying to figure out string theory (it's just too damned difficult). Hopefully, by the time
Gefter writes "What if LQG is Wrong?" I'll be comfortably ensconced in a nursing home.
Why You Want to Believe -- Posted on Wednesday, March 16 2011
In our hunter-gatherer days, a rustling sound in the bushes could mean either the wind or a predator, so the fledgling human mind had to make a decision—stay or run. Running could mean saving your life (predator) or just wasting your time (the wind). Not running meant being eaten (predator), or being relieved that it was only the wind. Even a dim-witted Australopithecus could figure that one out: she ran, because it was the best survival option.
←Here's a young Australopithecus I snapped at the Museum für Naturkunde in Berlin. She evidently did not run.
New Scientist has an interesting article called "Why You Want to Believe in the Paranormal" by British psychologist
Richard Wiseman (you have
to log in to read the New Scientist article, but reader membership is free).
We all have eccentric friends and relatives who believe in psychic abilities, paranormal phenomena, dowsing and magnet therapy. I think it's all nonsense, and science has shown it to be nonsense, but people will believe what they want to believe, and they will never be convinced they are wrong. Wiseman's article tells us why, and it seems to go back to our hominid days. Here's a snippet:
Every moment of our waking lives we are bombarded with huge amounts of sensory information which we struggle to make sense of. Our visual system tries to detect objects and faces, and our auditory system works hard to identify sounds and understand conversation. These pattern-detecting processes are so important to our survival that we have evolved to err towards false positives, preferring to "see" nonexistent patterns than miss a genuine one. As a result, our brains have a tendency to perceive meaning in random input. This is why some people believe astrological predictions, see faces on Mars, mistake lenticular clouds for UFOs and hear the voices of the dead in static noise.
George W. Bush, who in my humble opinion should have been hanged, used Americans' fear of "rustling sounds" to scare us into invading Iraq. He lied, and thousands of our military people died for nothing. But
better safe than sorry*, right?
America still hears those "rustling sounds," which is why we spend a trillion dollars a year on weapons systems, covert surveillance and secret prisons. That we do this and still call ourselves Christians is another neat psychological trick, but I'll talk about that some other time.
* A chemistry student of mine brought up this excuse after class. I replied that it's easy to say when you're sending other people off to die.
Weyl and Gravity Waves -- Posted on Monday, March 14 2011
I was flipping through an old book this afternoon when
this clipping fell out. I'd forgotten all about it — it's an obituary notice (dated 10 October 2000) I had saved for physicist
Joseph Weber, who was a UC Irvine laser expert. He was also known as the father of gravitational radiation detection.
Shortly after publishing his theory of general relativity in November 1915, Einstein discovered that
gravity waves, traveling at the speed of light, should exist. Yet despite ninety years of dedicated effort, these waves have not been conclusively detected. Weber (who used 1.4-ton cylinders of aluminum outfitted with highly sensitive strain gauges to detect the anticipated slight warping effect of passing gravitational waves from outer space), obtained inconclusive (and most likely null) results.
Einstein's gravitational field equations are highly non-linear and difficult to solve for all but the most simple and symmetric applications (Schwarzschild solved them for the static, spherically symmetric case in 1916, and it took almost another 50 years to do it for a simple rotating mass). Researchers today use high-speed computers to solve the equations with a variety of numerical approximation techniques, but in Einstein's day there was only one known approximate approach — linearizing the field equations. This entails assuming that the metric tensor gμν can be expressed as the constant Lorentz metric coupled with a new metric that represents a slight departure from flat space-time:
gμν = ημν + εγμν
where ε is some small constant
such that all Einstein field term
s proportional to order ε2 and higher can be ignored.
It is interesting that Hermann Weyl, in his investigations of the linearized field equations in 1918, discovered a metric γμν that satisfied the wave equation, thus showing that gravitational effects must propagate at the speed of light. It was typical of Weyl to employ a clever mathematical trick* to do this: he decomposed the metric according to
γμν = ∂μφν + ∂νφμ
where φμ(x) is an arbitrary, twice-differentiable quantity (but not the vector 4-potential of his unified theory of gravitation and electromagnetism). This
allowed Weyl to write the linearized field equations as
◻2γμν = (∂0∂0 - ∇2) γμν = 0
which is the wave equation (I've avoided a subtlety here for simplicity). Thus, gravity
propagates through space at the speed of light. For the static case (time independence) the μ, ν = 0 equation reduces to
∇2 γ00 = 0
which is Newton's law of gravitation. Surprisingly, Weyl's metric itself is not a gravitationally significant quantity!
It is amazing how
much information can be extracted from the linearized field equations, despite the fact that they are only approximations to reality. By further decomposing the γμν metric it can be shown that gravity,
like electromagnetism, propagates at the speed of light but, unlike electromagnetism, has far different polarization properties. These properties can be utilized in the design of gravity wave detectors, the most modern of which is the
LIGO detector.
Although gravitational radiation has not
been directly detected, its existence has been confirmed by observing the rotational characteristics of binary neutron star systems. Neutron stars rotate at precise rates, making them excellent clocks. However, binary systems can radiate gravity waves as they rotate about one another, with the resulting loss of energy; this in turn results in the slow orbital collapse of the system. Detailed observations by Russell Hulse and Joseph Taylor of Princeton University precisely (±0.2%) confirmed the theoretical energy loss of the
PSR B1913+16 binary system (which they discovered in 1974), an effort which garnered them the Nobel prize in physics in 1993.
* Adler, Bazin & Schiffer, Introduction to general relativity, 2nd ed., 1975.
Weyl's Postulate -- Posted on Thursday, March 10 2011
First, a definition and a quote:
The world lines of galaxies on average form a family of non-interacting geodesics converging toward the past in accordance with the existence of a global Gaussian time coordinate. — Weyl's Postulate, 1923
"It appears that the velocities between distant celestial objects on average increase with their mutual separations." — Hermann Weyl,
1923
In 1923, Hermann Weyl proposed what is now called Weyl's postulate, which has to do with the large-scale behavior of the universe.
I've experienced some difficulty following the reasoning behind Weyl's postulate, but there are those who do understand it, and I wanted to pass along the thoughts behind one short and very readable article that explains Weyl's reasoning about as well as anyone. It's interesting because Weyl's postulate, according
to the authors, predates Edwin Hubble's 1929 discovery that our universe is expanding
by six years.
The
paper is entitled Weyl's Principle, Cosmic Time and Quantum Fundamentalism by S.E. Rugh and H. Zinkernagel, dated 30 June 2010. The following is not a review of the paper, but rather my rough take on Weyl's postulate according to the authors.
The cosmological principle basically states that the universe on average is both isotropic (meaning that it looks the same in every direction) and homogeneous (meaning that the universe looks the same to any two observers, no matter how far apart they might be). Although stars, galaxies and and their interactions are very complicated, at some large enough scale galaxies and other megastructures can be considered as point objects uniformly distributed throughout the universe. These points undoubtedly interact to some extent (galactic collisions, for example, are known to occur), but on a truly grand scale all interaction can be ignored.
If the universe is isotropic and homogeneous, it might resemble a uniform ball of matter points, with each particle of matter following a geodesic world-line (assuming no interaction). Spherical symmetry would require that each particle have a counterpart on the other side of the ball, and the movement of each "shell" of particles would define a smooth 3-hyperspace in which the particles are "co-moving" or fixed with respect to one another at every moment in time. Weyl reasoned that with such a model, a "cosmic time" could be imagined in which the particle shells converge at earlier times and diverge as cosmic time increases. At every moment, particle movement is orthogonal to the
shell.
Two views of the universe: On the left,
galaxies move chaotically, their world lines are allowed to cross, and a consistent mathematical description of the universe is impossible. On the right, world lines follow geodesics that are orthogonal to smooth hyperplanes of constant cosmic time. From J. Narlikar, Introduction to Relativity, Cambridge Univ. Press, 2010.
(Weyl did not really assume any spherical symmetry, but I've used it to help me visualize the idea.) More conventionally, in the above graph we imagine hyperplanes of 3-space at every point in a universal, cosmic time. If these hyperplanes are all orthogonal to the world lines, then (with the unphysical exception of "parallel" matter flow) convergence and divergence of the world lines becomes unavoidable. Cosmic time thus has a "direction" consistent with the concept of universal expansion or contraction.
Weyl's cosmic time thus becomes a global, standard clock time that applies to every observer in the universe, making possible simultaneity of events. Unfortunately, this kind of cosmic time flies in the face of relativity, where time is always relative, depending on things like particle velocity and gravitational effects. Consequently, Weyl's postulate appears to prevent a completely covariant treatment of the simple cosmological models that utilize his postulate
(however, realistic cosmological models have been developed that are remarkably similar to the universe we
actually observe).
One way of seeing this is to recall the Schwarzschild metric of general relativity, which exhibits radial-dependent terms in the time and space coefficients. With the global time marker assumed in Weyl's postulate we can set the time coefficient equal to unity, so that the line element becomes `ds^2 = c^2 dt^2 - g_{ij} dx^i dx^j` (i, j = 1,2,3), where the `g_{ij} ` are functions of space and the global time `t`. This line element resembles that of ordinary flat space, but it's actually much more complicated than the Schwarzschild case. The most important contemporary model of the inverse is the Friedmann-Lemaître-Robertson-Walker (FLRW) model, which utilizes a line element of this type.
The big pay-off of these global-time models is the fact that they provide a solution to Einstein's fields equations that includes an unambiguous formula for the Hubble equation, which relates the velocity of a galaxy or galactic cluster to its distance from the observer. They also also yield the usual formula for the red shift of the object as a function of its velocity.
Weyl's postulate would appear to be essential to global-time cosmological models like the FLRW model, but Rugh and Zinkernagel note that Weyl has gotten scant attention or credit for his idea. They
point out that many recent texts on general relativity
don't even mention Weyl in the derivation of the models, preferring to focus instead on the cosmological principle. For example, Adler, Bazin and Schiffer (Introduction to General Relativity, 1975) ignore Weyl's cosmic time idea altogether and state only that a global time coordinate "is the price one has to pay to simplify the cosmological models and to describe physical reality in convenient mathematical terms."
Rugh and Zinkernagel's paper is only 14 pages, but provides much more information on the history of Weyl's postulate and its role in cosmology. The final section of the paper considers the fate of the postulate as global time is reversed back to the poin
t of the Big Bang, when quantum effects dominated.
A good undergraduate-level derivation of the FLRW cosmological model, Hubble's law and the redshift formula can be found
here (courtesy Prof. Michael Kachelrieß of the Norway Institute of Physics).
CMB and Graviton Handedness -- Posted on Saturday, March 5 2011
In his excellent 2004 book The Road to Reality, Sir Roger Penrose (a hero of mine) discusses the similarities and differences between the two most famous of Nature's massless particles of integer spin—the photon, a particle of spin 1 that is the carrier of the electromagnetic force, and the graviton, a hypothetical spin 2 particle that is the carrier of the gravitational force. Penrose notes that, like photons, gravitons may come in chiral (right-handed and left-handed) forms, adding that
This may seem strange, from the physical point of view, because there is no evidence of any left/right asymmetry in the gravitational field, and there is certainly none in the standard Einstein theory of general relativity.
Still, he reasons, there is definitely something asymmetrical about Nature ("left" being preferred by neutrinos in the weak interaction), so gravity might have a chiral asymmetry at the quantum level.
This is indeed the point of view of loop quantum gravity, and it is also the subject of some new research as reported by an article in the latest
New Scientist journal.
In an upcoming paper by João Magueijo and Dionigi Benincasa of Imperial College London, it is suggested that any neutrino-like behavior of the graviton might be discovered by a more detailed study of the cosmic microwave background (CMB), the relic radiation from the Big Bang that is responsible for the observed residual 2.7 Kelvin temperature of our universe. The researchers posit that powerful gravitational waves at the time of the Big Bang might have polarized the CMB's photons in some measurable way.
For those of you who have fast Internet connections,
here is the most highly-detailed map available of the latest Wilkinson Microwave Anisotropy Probe
(WMAP) data
. The discreteness is due to micro-level temperature gradients in today's universe, which resulted from the extremely fine anisotropy of the Big Bang (if the Big Bang had been perfectly uniform, you would not seen any temperature differences across the universe).
Astrophysicists who have at least some concept of a Creator have
called the WMAP the closest thing yet to seeing the face of God. If this is true, then God has a distinct preference for imperfection, slight though it may be. Indeed, if not for the observed
spontaneous symmetry breaking of quantum field theory, there would arguably be no quantum theory at all.
Loopy -- Posted on Sunday, February 27 2011
The following is presented here off the top of my head and mostly out of boredom. If it sounds like I don't know what I'm talking about, I probably don't—it wouldn't be the first time. But perhaps it will be of help if you're interested in the same things I am.
I've finished reading Rovelli's book on quantum gravity. I won't pretend that I understood everything, or even a fraction of it, but the book did lead into a direction that I have some familiarity with.
What God hath put asunder, let no man join. — Wolfgang Pauli I stand aloof; and will no reconcilement — Hamlet, Act 5 Scene 2
Quantum gravity remains the great open question of physics. Unlike the electric and magnetic fields of Maxwell''s day, quantum theory and Einstein's general theory of relativity (gravitation) have resisted unification for almost one hundred years, and it has not been for lack of trying.
In the early days, Einstein and Weyl both struggled to unify the two forces known at the time
(electromagnetism and gravity) without success. With the subsequent advent of quantum mechanics and the discovery of the weak and strong interactions and a dizzying plethora of very short-lived elementary and composite particles, the situation has only gotten worse. The relative simplicity of Einstein's and Weyl's efforts have given way to truly bizarre, mathematically abstruse unified theories that defy comprehension by all but the most sophisticated and clever researchers. String theory, perhaps the leading contender, requires 11 space-time dimensions in its fullest description, while supersymmetry (SUSY) remains 4-dimensional but requires the existence of a zoo of as-yet unseen "partner" particles such as gluinos, photinos and selectrons. The Large Hadron Collider, while not yet fully operational, has seen no evidence that either theory is even remotely correct.
This leaves a third alternative, loop quantum gravity (LQG), in which space is 3-dimensional with no weird particles flying about. Best of all, being background independent in a very real sense it doesn't even require space or time for its description. Plus, unlike the other contending theories, it
is inherently finitely non-perturbative, offering the promise of theory free of perturbative infinities. And, since it
ultimately views "space" as a discrete, Planck-scale weaving of finite closed loops, areas and volumes, it satisfies a growing conceptual belief that, at the smallest scales, space-time is "foamy" or "grainy" and not continuous.
Some background: Einstein's gravitational field equations can be derived by varying the Einstein-Hilbert action, which is simply the integral of the Lagrangian L = √-g R, where R is the Ricci scalar. But to get a quantum version of the equations one is led to replace the Lagrangian with its Hamiltonian cousin, H = pk dqk/ds - L, which involves only terms containing the canonical position and momenta vectors q and p. The Hamiltonian variant is more convenient for quantizing gravity because the commutation relation [qi, pj] = iℏδij is arguably the starting point for all quantum theory. This canonical approach to a quantum theory of gravity can be traced back to Dirac (I think he attempted it in 1932).
But the Hamiltonian approach necessarily involves various mathematical constraints (at least in classical field theory), and these constraints make using the p and q problematic. A breakthrough occurred in 1986, when Abhay Ashtekar discovered a
change of variables that appeared to alleviate the problem. Ashtekar's new variables involve the Weyl conformal tensor Cμναβ (along with its dual, which I won't go into here). One of the surprising aspects of Ashtekar's approach is that it involves the chirality (right- and left-handedness) of the graviton, the as-yet undiscovered
spin-2 particle that carries the gravitational force. Thus, the Ashtekar quantum gravity theory necessarily involves Weyl spinors.
Anyway, as I read it (and please remember that I'm an idiot), Ashtekar's new variables consist of a "position" coordinate Γ, which seems to be essentially Weyl's spin connection ω, and its conjugate "momentum," which is logically expressed as the functional partial derivative operator
iℏ δ/δΓ. The connection is used to define parallel-transported "round trips" around hypothetical "loops" of space. These loops are like "line Dirac deltas" in the sense that they have no meaning inside or outside the loops, but at each point on a loop they have a definite meaning and value. (There's only one "x-coordinate," which is along the loop!) Loops can kink, join and "weave" with other loops, and the points of intersection are called "nodes," which represent Planck-scale volumes of the space-time defined by the loops in three dimensions.
Although LQG is an attempt to unify gravity with
quantum mechanics, it is limited in the sense that it does not attempt to describe matter or particles. While string theory purports to describes particles as closed or open-ended pieces of "string" whose vibrational characteristics define a particular particle or field, LQG is
only trying to make gravity understandable within the usual non-commuting mathematics of quantum physics. If it ever gains traction over string theory and SUSY, the particle aspects of LQG will undoubtedly have to be addressed.
As stated, it's a Sunday and I brought all this up out of boredom. But I also see evidence of Weyl's work in LQG, primarily because Ashtekar's Hamiltonian coordinates consist of Weyl's spin connection, and the background metric gμν (such as it is) seems to be conformally invariant; that is, the local metric transformation gμν → λ(x) gμν has no effect on the theory. Weyl's conformal tensor Cμναβ is thus automatically built into the theory; in many ways, it is more fundamental than the Riemann-Christoffel tensor Rμναβ. But perhaps I am seeing
Weyl where he does not belong.
For additional information, you can read my write-up on Weyl's conformal tensor
here. Some elementary stuff on Weyl's spin connection can be found
here and
here.
The Lord has woven the universe in a way we cannot see. — Anon.
Rovelli Again -- Posted on Thursday, February 17 2011
I obtained a copy of Carlo Rovelli's Quantum Gravity from Cal Tech and have been poring over it for the past week. Its primary focus is loop quantum gravity, a subject I am now desperate to learn. However, Rovelli's book is devilishly difficult, at least for me, and I hope I come across something more at my level (Loop Quantum Gravity DeMystified, anyone?)
At the same time, it's neat to find elements of the theory that relate to Hermann Weyl's gauge ideas, at least those he applied to quantum theory in 1929. Rovelli's book includes an appendix that summarizes efforts to quantize gravitation going back as far as 1927, but it was not until Yang-Mills theory came along in 1955 that conclusively tied Weyl's gauge invariance concepts to modern particle physics.
Rovelli notes that the late Nobelist Julian Schwinger subsequently (1963) pointed out that the gravitational field could be characterized as a kind of gauge field. Indeed, gauge invariance can be viewed simply as any local symmetry that leaves the laws of physics invariant which, in the case of gravity, can be characterized as an arbitrary change in local Lorentz frames.
Rovelli also notes that researchers in the late 1970s proposed a variation of Weyl's action that resulted in a renormalizable theory (to what order the book doesn't say):
where the coefficients are suitably chosen constants. Note that this action is of fourth order in the metric tensor, a presumed defect that Einstein criticized when Weyl first proposed it over 90 years ago. Similarly, Stephen Hawking in 1978 proposed a kind of Feynman "sum over metrics" quantum gravity theory in which the action is only linear in the Ricci scalar R (the quantity Z is a probability amplitude):
(Here, Dg means integration over every metric gμν(x). Ouch—how would one even begin to carry out such an integration?!)
Loop quantum gravity, according to expert Lee Smolin, represents one leg of a triad of cutting-edge quantum gravity theories, the other two being
string theory and supersymmetry. To me, LQG is the more logical and beautiful; for why, see Smolin's excellent (though now outdated) book
Three Roads to Quantum Gravity.
Loop Quantum Gravity -- Posted on Monday, February 7 2011
I managed to scrounge a draft copy of Italian physicist
Carlo Rovelli's excellent book
Quantum Gravity off the Internet (I don't know if it's legal, so I won't cite my source here). Anyway, in the book's acknowledgments Rovelli writes
... I have had the joy of talking about a physics which is far from [just] problem-solving, from outsmarting each other, or from making weapons which
make us stronger than them. I think that physics is about escaping the prison of the received [dogmatic] thoughts and searching [instead] for novel ways about thinking [about] the world, [and] about trying to clear [away] a bit [of] the misty lake of our insubstantial dreams,
which reflect the reality like the lake that reflects the mountains.
(I think I may be forgiven if I have altered the draft
text somewhat here, as I have heard Rovelli talk and his Italian doesn't always translate perfectly. I assume that the final text is more or less perfect.)
But Rovelli's thoughts here are
beautiful, and confirm the idealism that all scientists certainly share. Part II of Rovelli's book deals with Loop Quantum Gravity (LQG), which is arguably the most popular alternative to string theory (or maybe supersymmetry).
LQG has the distinction of being technically and mathematically much simpler than string theory or supersymmetry (at least on the surface) because it does away with the presumed existence of an independent background space-time, replacing it with a kind of graph-theoretic spin network composed of links and nodes (I used to do a lot of computerized hydraulic modeling, and the similarity is remarkable). LQG has had some notable successes (it correctly predicts the entropy of a black hole, for example) but, like the other theories, it has its share of divergence problems and other issues.
You may recall that Hermann Weyl's gravity theory of 1918 suffered for the most part because of Weyl's formulation of space-time and how he allowed it to stretch arbitrarily from one space-time point to another. In LQG, there is no space-time to stretch. This idea has enormous appeal to me, because concepts such as existence, consciousness and an all-pervasive Creator would, IMHO, be more understandable without a background metric or space-time "stage" upon which everything acts. The "timelessness" of the theory is also appealing.
Rovelli's book is technical but quite readable. Its primary detraction comes from the fact that the author writes mostly in mathematicalese, which physicists tend to struggle with.
Update 1: I just discovered that a good portion of the finalized book is available for preview at
Google Books.
Update 2: My draft copy of the book is illegal. I erased the pdf file from my computer and have ordered the book through my library's inter-library loan program.
Update 3: This
brief introduction to loop quantum gravity (also written by Rovelli) should be useful to the beginner.
And Another -- Posted on Sunday, February 6 2011
Here's
another recent paper by Erhard Scholz, which includes much of the material alluded to in my earlier post. It has a nice, succinct introduction to Weyl geometry (Eq. 2 is Weyl's phase factor, which I still believe is important in the scheme of things). The paper also presents a mathematically unbeautiful theory involving a scalar field approach to the Weyl lagrangian.
Scholz on Weyl -- Posted on Saturday, February 5 2011
Here is a fairly recent (2009) overview of Hermann Weyl's geometry as applied to late 20th century physics, cosmology in particular. The author is
Erhard Scholz of Germany's Wuppertal University, who, in all matters
concerning Weyl historically, scientifically, mathematically, morally, intellectually and philosophically, is by leaps and bounds my superior.
Plot of observed supernovae type Ia magnitude/redshift data (191 points) with that predicted by Scholz' variant of Weyl geometry (line). A pretty good fit.
I'm still reading the article, so I won't comment on it here. Instead, I offer it as an example of how Weyl's geometry retains its relevance today.
The Last Word -- Posted on Wednesday, February 2 2011
Herb Silverman is Distinguished Professor Emeritus of Mathematics at the College of Charleston, South Carolina (I have one of his books on complex variables). He also writes on religious and secular topics, although he is an avowed atheist.
His latest thoughts can be read
here in the Washington Post, to which I responded:
Herb, I appreciate all your comments and your shared wisdom regarding the role and extent of religion, not only in politics but in how this country views itself.
As a life-long Southern Baptist, I have become disgusted with the hypocrisy of American Christians—their love of war, of the military, of lies, of torture, of moral exceptionalism, of money, of greed and of corporate criminality. It makes no sense whatsoever to call this a "Christian nation" when we do not follow the teachings of Jesus Christ. The gospel of Matthew mentions hypocrisy no fewer than 15 times, and exactly how America's Christians can read that and not see their faces in the mirror truly escapes my understanding.
The Christian churches of America no longer have anything to teach me. In 2000, our pastor told the congregation that "a vote for Al Gore is a vote for Satan." The very last time I attended
church in 2008, the pastor talked about the threats from North Korea and Iran and the need to protect ourselves by "striking first." This is precisely what happens when religion and politics mix.
I will continue to seek Christ's guidance from the New Testament, but my ears and heart are now closed to the churches. I now see America in a new light—very possibly, it's the Great Satan of the world after all.
Weyl Spinors in SUSY -- Posted on Tuesday, January 25 2011
I'm a little over halfway through Patrick Labelle's
Supersymmetry Demystified. It has been of enormous help to me in finally understanding the basics of this theory, and I wholeheartedly recommend the book for the beginner.
Still, like string theory there is no experimental support for supersymmetry, so tackling the subject (which is quite difficult) is basically a leap of faith.
Supersymmetry (SUSY) fascinates me because it spells everything out in terms of the spinors that Hermann Weyl discovered in 1929. The Dirac spinor mixes the two Weyl spinors, but
SUSY separates them so that an action lagrangian can be constructed solely out of the right-chiral or left-chiral Weyl factors. In combination with scalar terms, the basic supersymmetric lagrangian represents a first step toward a nearly divergenceless theory that successfully couples fermonic and bosonic fields (at least at low energies).
Those of you who have noted the many remarkable properties of the 2×2 Pauli and 4×4 Dirac matrices will be even
more impressed with their power in SUSY. In particular, the only purely imaginary Pauli matrix, σ2, takes on a starring role in the construction of lagrangian terms that are invariant with regard to Lorentz rotations and boosts; it also allows the use of a single Weyl spinor (usually the left-chiral variety) in the development of the theory. Indeed, the real antisymmetric matrix iσ2 represents the theory's "metric tensor," which raises and lowers spinor indices (compare this with the completely symmetric metric tensor of general relativity).
However, the notation of SUSY presents a significant learning difficulty. It is highly convoluted
because of the many indices and types of matrix-spinor combinations that are used in the theory. I still do not like the dotted and undotted spinor approach, but I suppose once you get used to them they're useful (Dirac once remarked that he wished he had invented the notation).
Keep in mind that SUSY is a quantum field theory, so it can be explored using a rather complicated form of the Feynman calculus; it can also be expressed with both
global and local gauge symmetries (but I haven't gotten that down yet). While all of this is quite complicated, the fact that all spinor components are anticommuting Grassmann quantities simplifies the algebra somewhat (see my write-up on
Gaussian integrals for an elementary overview of Grassmannian integration).
I haven't reached the part of the book that deals with superspace and supergravity (where space-time components are added to Grassmann spinor fields), but so far so good.
Hopefully, a preview of Supersymmetry Demystified will be put up on
Google Books so you can gauge the text's usefulness before you buy it (but then it's only about fifteen dollars on Amazon). Lots of luck.
Weyl on God -- Posted on Friday, January 14 2011
Hermann Weyl spent much of his life examining the interconnections between mathematics, science, religion and philosophy, and many of his thoughts are summarized in three lectures he prepared in 1931-32. These lectures were subsequently incorporated into his The Open World (reprinted in
Mind and Nature: Selected Writings on Philosophy, Mathematics and Physics). He begins with
A mathematician steps before you, speaks about metaphysics, and does not hesitate to use the name of God. That is an unusual practice nowadays. The mathematician, according to the ideas of the modern public, is occupied with very dry and special problems, he carries out increasingly complicated calculations and more and more intricate geometrical constructions, but he has nothing to do with those decisions in spiritual matters which are really essential for man. In other times this was different.
Weyl goes on to talk about how natural philosophy (what we today call science) in the times of Aristotle, Plato, Bruno and Kepler was shaped if not controlled by religious thought and beliefs; science, then, was not to
tally rational, but heavily (though not exclusively) mythological.
Weyl then speaks about the Copernican revolution, which removed the Earth from the center of the universe and set the stage for the Age of Enlightenment some centuries later, though
[through] the act of redemption by the Son of God, crucifixion and resurrection are no longer the unique cardinal point in the history of the world, but a hasty performance in a little corner of the universe repeating itself from star to star: this blasphemy displays perhaps in the most pregnant manner the precarious aspect which a theory removing the Earth from the center of the world bears for religion.
Battles between science and religion were fought constantly, with many scientists (notably Giordano Bruno) coming up on the short end. But inevitably, rational science began its ascent, though initially it was still couched in religion (Kepler: ?The science of space is unique and eternal and is reflected out of the spirit of God. The fact that man may partake of it is one of the reasons why man is called the image of God?).
Weyl ultimately brings us to the era of Einstein's relativity theory, which Weyl sees akin to the "finger of God in Nature" to "those of us who are Christians and not heathens," writing that The world is not a chaos, but a cosmos harmoniously ordered by inviolable mathematical laws. Weyl goes on to say
Thus the mere postulation of the external world does not really explain what it was supposed to explain, namely, the fact that I, as a perceiving and acting being, find myself placed in such a world; the question of its reality is inseparably connected with the question of the reason for its lawful mathematical harmony. But this ultimate foundation for the ratio governing the world, we can find only in God; it is one side of the Divine Being. Thus the ultimate answer lies beyond all knowledge, in God alone; flowing down from him, consciousness, ignorant of its own origin, seizes upon itself in analytic self-penetration, suspended between subject and object, between meaning and being.
Many
scientists today unabashedly describe themselves as atheists. In The Open World, Weyl makes it clear that he was not one of them. To me personally, it provides hope that one day science and religion will be spoken of as one.
Finally, Weyl finishes his book with this:
Many people think that modern science is far removed from God. I find,
on the contrary, that it is much more difficult today for the knowing person to approach God from history, from the spiritual side of the world, and from morals;
for there we encounter the suffering and evil in the world which it is difficult to bring into harmony with an all-merciful and all-mighty God. In this domain we have evidently not yet succeeded in raising the veil with which our human nature covers the essence of things. But in our knowledge of physical
nature we have penetrated so far that we can obtain a vision of the flawless harmony which is in conformity with sublime reason. Here is neither suffering nor evil nor deficiency, but perfection only.
In this week of horror in Tucson and countless other heartless places on this planet, Weyl's words provide hope that God-given reason may still guide us witless humans.
Supersymmetry Demystified -- Posted on Wednesday, January 5 2011
I know I've previously mentioned this quote from Lewis Ryder's Quantum Field Theory in his chapter on supersymmetry, but I can't help bringing it up once again:
This morning I visited the place where the street-cleaners dump the rubbish. My God, it was beautiful. — Van Gogh
I think I've also mentioned once or twice that I haven't been able to make heads or tails out of supersymmetry, so you'll forgive me if I think even less of Van Gogh upon reconsideration of the above quote (although his Wheatfield with Crows is nice). However, Ryder goes on to note that
Despite its exasperating complexity [supersymmetry] has an undoubted mathematical fascination and some physicists incline to the view that God must surely have made use of such an interesting theory.
This remark calls to mind one made by Einstein, who famously noted that Raffiniert ist der Herrgott, aber boshaft ist er nicht (the Lord is subtle, but not malicious). Well, supersymmetry is damned malicious, in my opinion, and, if God had anything to do with it, I'd be more inclined to agree with Hermann Weyl's retort to Einstein that "Maybe God is malicious after all!"
But supersymmetry is a theory that mixes bosons with fermions and, since Nature likes diversity, I suppose such a symmetry has merit. It's undoubtedly a neat idea. Not only that, but the theory's mathematics relies heavily on Weyl spinors, which are of some interest to me. And so, despite the fact that I have attended
introductory lectures on supersymmetry and come away with nothing but headaches, and have also tried valiantly but unsuccessfully to learn the subject from introductory
texts like Ryder's, I have decided to do something I'm ashamed to admit — today I ordered Patrick Labelle's
Supersymmetry Demystified (about
fifteen dollars).
This is not to knock the excellent "Demystified" series of books, but they're really designed for high school kids and older but stupid people like me (from the Amazon.com description: "It's a no-brainer!") At the same time, I feel desperate to find a comprehensible text, and maybe this is it.
In many ways, supersymmetry is tougher than string theory (especially the notation), and to me there's the additional issue of meaning and relevance—to date, neither theory has any experimental support, and both may turn out to be physically worthless, though mathematically beautiful.
Weyl Neutrinos -- Posted on Monday, January 3 2011
In 1929 Hermann Weyl showed that for massless spin-½ particles the Dirac equation decouples into two equations, each describing a 2-component spinor (now called the Weyl spinor) with left- or right-handed helicity. These spinors were subsequently assumed to describe neutrinos. The situation is summarized in L.H. Ryder's Quantum Field Theory (2nd edition):
Since Weyl's spinors violate parity symmetry, Wolfgang Pauli denounced them as unphysical. However, in 1956 parity was shown to be violated by the weak interaction, and Weyl's spinors were vindicated.
It is now believed that neutrinos have very small but non-negligible masses. These masses have not been measured precisely, but they have definite upper mass limits as compared with their associated leptons:
Electron mass: 0.511 Mev; electron neutrino mass: < 5.1×10-6 MeV
I occasionally get emails asking me that if neutrinos have mass, then the Dirac equation cannot be decoupled and Weyl's prescription must be in error. My response is always this: I'm not an expert on neutrinos, so your assessment is as good if not better than mine.
It is always possible to say that the presence of sma
ll masses in the Dirac equation makes the Weyl spinors at least approximately correct, but then one could always counter by saying that the tau neutrino's mass is hardly small, being 60 times that of the electron (which is the particle the Dirac equation was originally intended to describe).
But to me an even more important issue is the concept of neutrino oscillation, a relatively recent idea that describes the interconversion of neutrinos from one type to another. This idea perfectly explains the observed disparity between the number of solar neutrinos predicted to be generated by the Sun and the number seen here on Earth. The terrestrial neutrino flux is very close to one-third that predicted by certain neutrino models for the solar neutrino flux. If solar neutrinos are, say, 100% of the electron variety, they would convert roughly into 33% electron neutrinos, 33% muon neutrinos and 33% tau neutrinos by the time they arrived at Earth.
But this oscillation can only occur if the neutrinos have mass, and this throws the physical interpretation of the Weyl spinors into question.
She did not forget Jesus!
"Long live freedom!"
UPDATE, 12-13-2011:
One more.
I don't post to this site anymore, but I wanted to toss in this last item.
Somewhere in the infinite sequence of digits comprising the transcendental number pi (π) is the string 
Here's my take. In quantum physics we have a number Z that represents Feynman's path-integral transi
tion amplitude for some initial and final state. The integral must be integrated over an infinite number of space-time points and so cannot be done in closed form. But the integration can be done in piecewise fashion, i.e., by perturbation. Each step in the integration gets exponentially more difficult, though, so the calculations must be taken only as far as absolutely necessary. With each step, more and more state/particle paths (or virtual particles) enter into the calculation, but 6-digit accuracy is usually achievable after only a few steps.
I think it's safe to say that very few of you who studied Fourier analysis in school were much concerned that a continuous, differentiable function ƒ(t) could be represented by an infinite number of very discrete sine and cosine terms. If your instructor said anything about it at all, she probably just remarked that it is assumed from the outset that the terms form a complete set, which in the n → ∞ limit allows the sines and cosines to represent the objective function exactly. Typically, however, there was always one wise guy in the class who would ask that if the term cos(1243194063 π t/p) was removed, would the objective function "miss" it in any critical way, or if the set could still be considered complete. And that is the danger one faces when dealing with complete sets, particularly those that contain an infinite number of elements. Somehow, life goes on.
Werner Heisenberg's (1901-1976) famous Uncertainty Principle states that there is a fundamental limit to how accurately we can measure certain quantities simultaneously. The best known example is position and momentum; in summary, the principle says that if you measure a particle's position and its momentum at the same time, there will be a certain finite amount of "fuzziness" in the values you get. This fuzziness is a limit imposed by Nature; it cannot be eliminated or even reduced. 
I often get emails from people who want to know more about Hermann Weyl's personal life. Here's a clue: I actually know very little about Weyl's life outside of his physics and mathematics. I've been to Switzerland, but I haven't even been able to locate the guy's grave. (Photo: Weyl in Jena, Germany, sometime in 1930.)
Yesterday I found a couple of new photos of Hermann Weyl on the
Yesterday I posted a blog about neutrinos, then quickly took it down because (knowing little about the subject) I had no right to say what I did. But I did want to reaffirm my recommendation for the second edition (2008) of David Griffiths'
You probably already know that the affine connection of differential geometry is usually taken to be symmetric in its lower indices (Γλμν = Γλνμ). But scores of researchers over the decades have sought to generalize Einstein's gravity theory by abandoning this condition. Notably, Einstein himself tried it (this page of handwritten connection calculations was found near his death bed), and Schrödinger also gave it a shot in a series of papers he wrote in 1947 (google "The Final Affine Field Laws," the full text of which I have been unable to find in pdf format). You can take my word that this generalization led nowhere, but it's interesting. 
A last note here on Eddington (shown here at the age of 38), and how he proposed to rescue Hermann Weyl's 1918 theory from Einstein's criticisms, which fatally injured the theory. 
For a brief but more detailed discussion of Eddington's idea, see Abraham Pais' foundational scientific biography
Physicist and writer
In our hunter-gatherer days, a rustling sound in the bushes could mean either the wind or a predator, so the fledgling human mind had to make a decision—stay or run. Running could mean saving your life (predator) or just wasting your time (the wind). Not running meant being eaten (predator), or being relieved that it was only the wind. Even a dim-witted Australopithecus could figure that one out: she ran, because it was the best survival option.
I was flipping through an old book this afternoon when
Although LQG is an attempt to unify gravity with
quantum mechanics, it is limited in the sense that it does not attempt to describe matter or particles. While string theory purports to describes particles as closed or open-ended pieces of "string" whose vibrational characteristics define a particular particle or field, LQG is
only trying to make gravity understandable within the usual non-commuting mathematics of quantum physics. If it ever gains traction over string theory and SUSY, the particle aspects of LQG will undoubtedly have to be addressed.
I managed to scrounge a draft copy of Italian physicist
Herb Silverman is Distinguished Professor Emeritus of Mathematics at the College of Charleston, South Carolina (I have one of his books on complex variables). He also writes on religious and secular topics, although he is an avowed atheist. 
I'm a little over halfway through Patrick Labelle's
Hermann Weyl spent much of his life examining the interconnections between mathematics, science, religion and philosophy, and many of his thoughts are summarized in three lectures he prepared in 1931-32. These lectures were subsequently incorporated into his The Open World (reprinted in
