Wednesday, October 04, 2006

Open Letter to the Theoretical Physics Community

As a retired physicist, I have been spending some time trying to learn about the new physics of strings and quantum gravity, ie the post-Standard Model theories of fundamental physics. The popular literature is chock full of books like The Elegant Universe (Brian Greene), Warped Passages (Lisa Randall), The Trouble with Physics (Lee Smolin), and Not Even Wrong (Peter Woit). Wonderful reading, but if you want to understand it like a physicist you have to be able to follow the calculations; and these books are non-mathematical.

So I decided to go to the source papers themselves, looking for some that were basic or reviews of the field. The physics research literature is rich in excellently written papers that the average graduate physicist could learn a lot from. I refer to the following well known example.

“It is known that Maxwell's electrodynamics--as usually understood at the present time--when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relative to the “light medium,” suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.”


This passage is from the introduction to Albert Einstein’s 1905 paper “On the Electrodynamics of Moving Bodies.” Within two pages Einstein was defining simultaneity, showing how coordinates transform, and calculating physical consequences. This was the defining paper of Special Relativity.

Here is another excellent example.

“For most practical calculations in quantum electrodynamics the solution is ordinarily expressed in terms of a matrix element. The matrix is worked out as an expansion in powers of the fine structure constant, the successive terms corresponding to the inclusion of an increasing number of virtual quanta.”

This is from Dick Feynman’s seminal 1949 paper “Space-Time Approach to Quantum Electrodynamics.” Three pages later Feynman is busy calculating the interaction between charges using his new Feynman diagrams. A few pages further on he calculates the self-energy of the electron.

Now fast forward to present days and take a look at the following passage from a typical paper in theoretical physics.

“If one wants to have a well posed initial value formulation then the metric fields g that live on M are such that (M, g) is globally hyperbolic which implies that M is diffeomorphic to the direct product R x S where S is an n-dimensional smooth manifold. Since the action is invariant under Diff(M), the diffeomorphisms Y: R x S --> M; (t, x) --> Y are a symmetry of the action. For each Y we obtain a foliation of M into a one parameter family of spacelike hypersurfaces.”

This snippet is from the introduction to a 2004 paper by T. Thiemann called “The LQG-String: Loop Quantum Gravity Quantization of String Theory I. Flat Target Space.” After 39 pages of similar gobbledygook, Thiemann admits that “We have so far hardly mentioned matter.”

These recent papers are unintelligible except to a select few adherents of String Theory (stringers) or Loop Quantum Gravity (q-graviters). And Thiemann described his paper as “a (relatively) non-technical summary of the status of the quantum dynamics in Loop Quantum Gravity (LQG).” Come on now!

My blog buddy Mahndisa responded to my complaint: “Sure, Mr. Thiemann's work is a bit mired in formalism and the physical significance gets lost.” I must agree with her there. Then Mahndisa goes on to discuss another paper “String Quantization: Fock vs. LQG Representations” by Robert C. Helling and Giuseppe Policastro “who use Thiemann's Pohlmeyer state to quantize the harmonic oscillator. But the spectrum doesn't look anything like what we are used to seeing since the quantization yields only one bound state and the rest are scattering states!” All states except for the ground state have diverging energy and the energy spectrum is not bounded from below. This is clearly in conflict with the energy spectra of harmonic oscillators found in nature.

Well that doesn’t sound too good. But I looked at the paper anyway. After many pages of abstract theory, I found on page 13 that “The Hilbert spaces we have constructed so far are by themselves not physical: The diffeomorphisms of the string do not act trivially on them and they would describe the quantum theory of parametrised strings.” This sounds a lot like Thiemann.

Frustrated, I complained to Lubos Motl that "as an old time physicist (PhD in 1971) I find it impossible to understand the papers on strings and LQG, but the popular books are too superficial. I wonder if you would consider writing a few posts for physicists like me who learned how to do calculations with Feynman diagrams and sort of understand the Standard Model, but don't have the math for the new theories. It would be so much appreciated."

Motl answered: “Arguably, physics is still about Feynman diagrams, the Dirac equation, quantum mechanics, etc. Yes, there are additions: chiral symmetry breaking, confinement, supersymmetry, holographic duality and the dual theories of quantum gravity with all of their solutions, especially new phenomena with additional dimensions, etc.

Still, in general, physics more or less is about the elementary particles, their spin, and their interactions with coupling constants. A theory is meant to say something about them and allow us to calculate the results of the experiments that include the spectrum of matter - arguably different kinds of matter that can include new particles or black hole microstates if needed - plus the probability of different interactions and possible transitions.

AMEN to that, and thank you, Lubos.

Now how about you stringers and q-graviters start writing your papers with a slightly wider audience in mind? Give us a chance to follow your theory and calculations. Put all the preliminaries in an appendix. Get to the physical quantities sooner; that is, if there are any physical quantities.

Or, perhaps, the situation in modern theoretical physics is as Burton Richter described in the Oct. 2006 issue of Physics Today. In his article entitled “Theory in particle physics: Theological speculation versus practical knowledge,” Richter states:

“Today, there seems to be nearly an infinity of solutions, each with different values of fundamental parameters, and no relations among them. What we have is a large number of very good people trying to make something more than philosophy out of string theory. Some, perhaps most, of the attempts do not contribute even if they are formally correct.”

Philosophy -- Hmmm, maybe that’s why the papers are so hard to read.

5 Comments:

Blogger Mahndisa S. Rigmaiden said...

10 05 06

Hey Bill:
I couldn't have said it better myself. As much as I love reading about the hypothetical physical theories that can emerge when one ignores this rule or that, I really do wish to know about the PHYSICS! Sometimes, we can get lost in the forest looking for one tree and I think that has happened with a lot of high level theoretical physics. The only thing that separates some of those guys from pure mathematicians is that the pure math folks still think physicists aren't rigorous enough! heheheheheh Did they have that rivalry going on when you were in school?

Anyway, I must admit that there is often a disconnect between the physical world and the math used to describe it or some other abstract world.

For now, I read what I can and reread over and over and until I get it. But your point is well taken, it would be nice if some of those guys did two things:

1.Produce a paper that is LESS than thirty pages for a change!
2.Discussed these complex ideas in a way that everyone can understand.

My parents are both elementary school teachers, and have been for quite a while. One thing they emphasize is learning and touching and feeling knowledge. Sometimes the PhD's don't get courses in teaching methods or pedagogy and it is difficult for them to translate their knowledge into something tangible for kids to digest...Great post Bill.

3:00 PM  
Blogger Mahndisa S. Rigmaiden said...

10 05 06

Well kids or anyone else...

3:01 PM  
Anonymous Anonymous said...

hahaha @ gobbledygook

The research papers are incomprehendable, unnecessarily so in my opinion.

You have to laugh, or you will surely cry.

3:10 PM  
Anonymous Anonymous said...

Bill

Good luck with this mission, though I won't be holding my breath. I'd rather read about this than the Foley/Hastert implosion story!

Tex

10:37 PM  
Blogger Faust said...

Hi Dr. Lama,

I might suggest you have a look through:

ArXiv Structure (INR, Russia)
http://xstructure.inr.ac.ru

There are many helpful topical reviews on all aspects of string / M-theory.

I am trying to build a group of interested individuals for discussing physics (and mathematics), primarily at the undergraduate level. Why not drop by and share some ideas on Quantum Optics? Would appreciate it greatly.

8:50 AM  

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