String Theory

;; 2004-12-28 T22:29:43-0500 (Tuesday) D. Goel

My Quick Notes on the "Elegant Universe" Documentary by Greene.

Copyright (C) 2004 and onwards D. Goel.

"Elegant Universe" documentary by Greene, based on the book of same name by the same author --- (for self: See also MovieHistory, -- this documentary was shown on PBS channel (channel 8) this evening at 08.00 PM and is recorded in my video cassette number 1.) Note that these and other documentaries can be seen at: http://www.pbs.org/wgbh/nova/elegant/program.html

It is a beautiful documentary, but since I am no physics professor, I sometimes lost track of the points being made, so I reran the recorded version, and produced these notes for self.

1. The QM world is full of chaos (not the literal physics meaning chaos, but the chaos in the English sense), while the macroscopic world is not -- for example, in Einstein's regime, topology is invariant -- a donut will never become a ball. But at the Quantum level, topology-changing rips occur all the time. There needs to be some mechanism that calms the QM level disturbances before they show up in the macroscopic Einstein's regime -- string theory does just that -- it calms the quantum world -- it's as if the spacetime path of string creates a tube which encloses the quantum rips, thus preventing them from getting too big.
2. The idea of Large Extra Dimensions is no longer a "crackpot" idea. These extra dimensions are not "small" or "too curled up" to be seen. Rather, they are large -- same scale as everyday dimensions. Then, why don't we see them? We will explain -- Our 3D (space) +1 (time) universe is like a single slice in a loaf of bread representing the 11D "bulk". More formally, it may be a brane in the 11D universe. Why don't we see the other universes? There's no interaction -- all particles in our universe result from vibrations on strings. Almost all of these strings are open-ended, and the open ends are tied to the surface that our universe is. Thus, these strings never interact with the other universes (hm, what if 2 such universes get too "close" -- will the strings "bump" into the other one?). Thus, we never see the other universe in any fashion. The only exception is the graviton -- the string for graviton is a closed loop, with no open ends tied into any brane. This particle is thus free to float around, and not tied to our universe.
3. IIUC, in this picture, Quarks (and other equivalent "base" particles) --> vibrations on strings.
4. There were 5 string theories in the 10D universe, which were later seen to be nothing but different representations of a single M-theory in an 11D universe. In M-theory, each string became a membrane, or for short, brane.
5. 11 dimensions = 1 time + 3 space + 7 extra dimensions -- of these 7 dimensions, 6 were originally expected in the string-theory representation, and 7 were expected in the M-brane theory. A string theory is nothing but one of 5 possible equivalent 10-D representations of M-theory.
6. People came up with more possible structures (rather than just the usual branes) possible in the bulk, once M-theory came forth.
7. A brane can be 3-dimensional or higher, and can grow very large (size of our universe), depending on the energy supplied to it.
8. Our universe, in fact, may be nothing but a 3-D brane in the n-dimensional space. Adjacent 3-D branes may exist -- we might be like a slice of bread in a loaf of bread.
9. String theory may offer one possible explanation for the weakness of gravity -- gravity is 10^(-39) times weaker than the EM force, for example. This explanation: Maybe gravity isn't weaker at all, it is merely perceived to be -- since a graviton easily slides in and out of our universes, maybe its perceived effect is diluted and we feel "rather little" of it. (Me: Isn't this interesting? This would mean that on a small scale, you wouldn't find a ball's grav. pull as strong as a magnet's, but on a large scale, you would find that even though masses in your universe get "diluted", you find yourself affected by masses from neighboring universe -- could this have answers for the dark matter problem? If so, why didn't Green talk on that?)
10. (Me: Why 3+1Ds? Why not 4+1? or 5+1? And why 11D? Why not 12?)
11. Generating gravity waves may, thus, be our only way of communicating with intelligent beings in adjacent universes.
12. BTW, these parallel universes may in fact, play a role in our very existence, as explained in the Big Bang point below.
13. One of the puzzles of physics is what caused the big bang, why a big bang (and why should we need the inflation model with its weirdities to go with the big bang)? String theory provides one possibility as an answer to the why of big bang, but note that the technical assumptions that go into that solution leave us back with the same questions as before, so nothing groundbreaking here at all. Here's that solution: Imagine 2 large branes in the bulk. These 2 large branes move, and collide with each other. Imagine a picture a bump in one of the 2 branes where they collide. At that bump, a lot of energy is released, resulting in a "big bang". In this picture, such collisions may take place again and again..
14. String theory has no testable predictions yet. The hope is to get some such predictions -- two possibilities for validation are vanishing gravitons and observation of supersymmetry. Of course, even if supersymmetry is experimentally verified, that is not a direct confirmation of string theory, but just a confirmation of one of the predictions of string theory. Presumably, supersymmetry is a valid hypothesis with or without string theory, thus somewhat diluting the ramifications of observing supersymmetry, as far as testability of string theory are concerned.
15. Vanishing Gravitons -- smash particles into each other, to produce a particle-circus. In this circus, hope that you observe the signature of a graviton escaping to other universes -- to other branes.
16. Supersymmetry -- here's what I (not Green) understand about supersymmetry: Though supersymmetry is predicted by string theory, it is a valid and nice hypothesis in itself, which seeks to explain away the imbalance between strengths of different forces of nature and makes the basic laws more symmetric. The laws appear asymmetric merely because we locally find them asymmetric due to the fact that in a space representing law-strengths, we have come to reside at a particular point at the bottom of a pit which is shaped like a circular shell (due to symmetry breaking), and hence are not located at the center of the symmetry. BTW, simpler example of symmetry breaking: you stand on a symmetric, circular hill, surrounded by a circular trench, and you find the world nice and circularly symmetric around you. You let a ball drop, which lands somewhere in the trench. In the trench, the ball doesn't find the world circularly symmetric at all -- on one side is mountain, but on the other side lie fields.
17. Observation Of Supersymmetry One of the predictions of string theory is supersymmetry. Supersymmetry means that for every particle we usually observe, there must be a counterpart -- a much heavier sparticle. If any of the particle-colliders observe a sparticle, that will not only be a validation of supersymmetry, but will also support string theory.
18. Could string theory ultimately turn out just be a futile exercise? SURE! Especially since no testable predictions yet. Weinberg (an outsider to string theory, (right?)) comments that string theory has such elegance and mathematical beauty that it would be hard to believe that it is merely "fun math".
19. String theory is now about membranes or branes, and should be more properly called M-theory (but who cares? :-))