(Alpha) String Theory Questions

I will be a bit delayed in replying to the comments made. But for now, a comic that I thought was quite apropros, from Cliff Johnson at asymptotia.com:

strings_cafe1.jpg
 
Can you give or link to a description of string theory that a layman can understand?

Try www.superstringtheory.com

This is John Schwartz's website and has a good split discussion---one a bit more advanced and one a bit more elementary.

The basic idea is that one assumes that fundamental particles are not point-like, but are made of tiny vibrating strings. Everything else follows.:)

What are strings "made" out of?

This is a bit like asking, what is an electron made out of? Well, electrons. To my knowledge, string theory itself doesn't answer this question (at least to my knowledge), however, if I were to venture a guess, the $$E=mc^2$$ equation that some people are so fond of misusing implies that a string (at rest, p = 0) is made of energy (inasmuch as this equation implies that anything is made of energy).

If it's going to be a struggle to verify String Theory, can it really be considered to be science?

Good question. You should check out a recent book by Lee Smolin called "The Trouble with Physics" which is highly critical of string theory. Some people would definitely not consider string theory a science. String theorists will admit that if they cannot come up with something at the end of the day, then it has all been for naught.

Perhaps "all for naught" is a strong statement. If string theory is wrong, then it has produced some new mathematics, and has given us a wonderful dictionary called the AdS/CFT duality. (If you want, I can explain the AdS/CFT more carefully---otherwise, just take it on faith that it has produced some very beautiful insight into the way quarks interact.)

So why should string theory be considered a science? Well, we can make predictions, they just aren't unique. For example, I could tell you the precise value for the higgs mass in a particular string model. The problem is, there is something like $$10^{500}$$ similar models, and there is no reason to suspect that the model I have built is any more favorable than the $$10^{500} - 1$$ other models, which don't give the same anwer!

The debate in string theory right now is over something called the Anthropic Principle, that says, loosely, things are the way they are because otherwise we wouldn't be around to observe them. Think of it this way: let all fundamental constants (speed of light, strength of the forces, cosmological constant, higgs mass, etc.) take on all possible values. Only for a very limited set of values is life able to exist---the reason that we have intelligent life is because we live in the remote corner of parameter space that makes this possible. Basically, we impose the requirement of intelligent observers, which is a very stringent one, and a requirement I feel that we don't adequately understand. The idea is that string theory predicts a very large number of universes, with all manner of values for the fundamental constants, and by imposing the condition that there exist conscience observers, we can get the number of reasonable solutions down to a more managable size.

If this sounds like it is not science, then many agree with you. A more palatable version of the Anthropic Principle uses the value of the cosmological constant as the delimeter, as opposed to the whole "intelligent observers" thing. (The cosmological constant is the thing that tells us our universe is currently expanding and accelerating.) But, the smallness of the cosmological constant is required for intelligent observers, so the two (at least to me) seem interchangable.

Perhaps this has given you an overly pessimistic view of string theory. The other answer (that is becomming less and less popular) is that we just don't know enough about string theory to make predictions yet. The only type of answers that we can get from string theory right now are perturbative answers---that is, approximate answers. So, there may be something in the exact answer (or, the non-perturbative limit) which tells us uniquely what things like the cosmological constant should be.

Sorry if that was long, but it is a very important question.

Is there a danger that people "believe" in it so much that they are unreceptive to other ideas offering valuable insights that can be verified?

It depends. So many people are so interested in string theory because it has a lot of promise, and has passed every test that we have thrown at it. String theory offers some terrific insight into the universe, as well (see earlier posts). And what would the danger be? That we lose 100 years chasing a rabbit that turns out to be wrong? The history of physics has shown that robust scientific proposals survive despite prevailing opinion (like Galileo).

This may not be the answer you were looking for.

As an aside, I thought this was interesting. It's a rating of a Briane Greene article on string theory using the Baez "crackpot" index:

Yes I have seen this index, and I think John Baez (who has worked on Loop Quantum Gravity, String Theory's chief competitor) specifically listed a few of the points to prove a point with string theory.
 
IMHO, this is not the "correct" answer to MT question, because MT may not accept "string theory" as correct.
Better answer:
"I would ignore you, until you provide some evidence or equally elegant theory with 28 dimensions."

Yes you are probably right.

Perhaps my point is better made only within QM: In some analyis of observation light must be treated as packets of energy (Photovoltatic effect in dim light) and in others as waves, even in the same experiment if the detector is an array of photo detectors behind a double slit. I am not bothered by the need to use one "theory" for one partof the observation and another theory for another part, if these separate theories are more simple (easily applied) than an integrated theory few can understand.

Yes, this is a very valid view point. But there are others who would wonder why we need two separate theories:)

I just wish the prediction were more observable, as Maxwell's prediction of radio waves were - Hertz went off and found them. Etc.

Yes I do, too. There are some proposals of stringy effects that we might see at the LHC in CERN in the coming years---microscopic black holes is the example that leaps to mind. The normal argument goes like this: if we have extra dimensions, it is possible that some are bigger than others. A paper by three guys in 1998 (Arkani-Hamed, Dvali, Dimopoulos I think) showed that two of the extra dimensions could be much larger, and possibly observable at LHC. The implication of this is the appearance of black holes in the LHC data---the black holes will be easy to see because they decay into everything (by Hawking Radiation). So it is possible that we could see things like this at the LHC.

From the cosmological end, string theory could elegantly explain the observation of something called cosmic strings, of which I am not intimately familiar.

The problem with string theory predictions is that they are never firm in the sense that they can disprove the theory. So, for example, if motion through a magnetic field didn't produce an electric current, Maxwell's equations would be wrong. String theory can always evade limitations because the energy scales are so huge.

Perhaps string theory's "one step exactness" is also the reason why few can understand it?

Well, I should be clear. If there is a single formulation of string theory, it should give exact answers to simple questions. But the way we currently know how to get any real results out of the theory is to make approximations, like ignoring the recoild of the nucleus. That is, we can get approximate answers to simple questions.

String theory is a complex subject, and there are few who can really appreciate the theory as it currently stands, myself included. There are branches of the theory that are more accessible than others, and to contribute in some areas is no more difficult than making contributions in any other are of physics.

SUMMARY: Perhaps "divide and conquer" is better for most people and should not be dropped for beauty, if that is all there is to an untestable, but unified theory.

This is a very valid point, and one that most physicists would agree too. The chief job is to try to predict how nature behves. If we cannot do that with string theory, then we should try something else.
 
Thanks for the lengthy, considered, and impartial answers Ben.

As it happens, I think I know what electrons are made out of! I tend to view strings as being made out of the same thing as "the crease in my pants". I don't mean cotton, I mean discontinuity - I take a rather "pure marble" topological view.
 
I'm confused by "pure marble"---what do you mean? Like a cusp or kink in space-time?
 
No, Einstein's "pure marble" geometrical idea as opposed to the "wood" of matter and radiation.

http://209.85.165.104/search?q=cach..."+"pure+marble"+wood&hl=en&gl=uk&ct=clnk&cd=7

" Ironically, the source of Einstein’s frustration was the structure of his own equation. For 30 years, he was disturbed by a fundamental flaw in this formulation. On one side of the equation was the curvature of space-time, which he likened to “marble” because of its beautiful geometric structure. However, he hated the other side of this equation describing matter-energy which he considered to be ugly and he compared to “wood”. While the “marble” of space-time was clean and elegant, the “wood” of matter-energy was a horrible jumble of confused, seemingly random forces from subatomic particles, atoms, polymers, and crystals to rocks, trees, planets, and stars. Einstein’s grand strategy was to "turn wood into marble" – that is, to give a completely geometric origin to matter. In retrospect, we can probably spot Einstein’s error..."
 
Back
Top