OK, physics brains, I was under the impression that there had to date been no experimental evidence to back up the mathematics of string theory indicating there are numerous higher dimensions. This guy I’m quoting here seems to think differently, but although experiments for the LHC are being developed, I don’t see anything to back up his claims. Does what he is saying make sense?
Additional dimensions go well beyond mere speculation. Certain theories that work would not work if there weren’t extra dimensions. How many and their sizes is yet to be determined but competing theories actually converge on the fact that there are more than 3 dimensions to space (4, if you count time, which you should). The truth is, the dimensions may be too small for direct detection, in which case, we may only end up inferring them with a high degree of confidence. I’m relatively confident that there are more than 3-4 dimension and lean toward the 10 dimensions (11 with time). Calabi-Yau space assumes 6 but that may be an artifact. Since these competing theories are actually related or overlapping in non-trivial ways, we have fairly decent evidence that would allow us to assume extra dimensions.
There has been considerable controversy about the lack of testable predictions from string theory, Peter Woit has been a leading critic - see his book Not Even Wrong.
However, I think most string theorists are quite candid about the situation, and maintain a sense of humor. There’s a nice book by Joseph Conlon called Why String Theory. Chapter 7 is entitled Direct Experimental Evidence for String Theory. The entire contents of the chapter is one sentence:
There are some papers that claim certain observations are evidence for extra dimensions (either directly or indirectly), but the claims of these papers are not widely accepted. I.e. you (GinoC) are correct.
The basic argument of the quote that certain theories lean towards more dimensions is flawed because the evidence for such theories over theories that do not lean that way is scant to non-existent.
My understanding is that String Theory could be a lot like the Geocentric model of the solar system. Geocentric model - Wikipedia
Geocentric model can be used to predict when planets and stars will appear and other celestial events, but it is also completely wrong. In that sense, String Theory answers a lot of questions, but no one knows if it is anymore accurate than the Geocentric model.
Not quite. The main problem with the String Model (note: It is not a theory) is that it’s not just one model. There are a combinatorically-large number of different string models. Let’s say that you perform a bunch of experiments and make a bunch of measurements, and eliminate (say) 99.9999% of all string models. Now you ask a question about something that you haven’t measured yet, and try to get a prediction on it. There will still be so many different models left that you’ll get plenty that predict each different answer, and you still won’t know which are correct until you perform the new experiment. In effect, the string model can’t tell you anything that you don’t already know, which makes it useless as a theory.
I do think the string theory criticism can be a little unfair sometimes. To some extent, there’s a general problem that physics has reached a point where we know so much amazing stuff - down to unimaginably small scales / high energies, and back almost to the inception of a 13-billion-year-old universe - that the practical constraints make experimentation incredibly difficult.
Beyond that general problem, I think the issues peculiar to string theory are:
(1) The unconstrained nature of the theories themselves, described by Chronos above.
(2) A much more mundane practical issue: despite its problems, string theory still dominates theoretical physics. There is no better theory out there, but perhaps that’s somewhat self-fulfilling if it’s consuming the bulk of research funding. Whether this is a valid criticism is extremely hard to judge for the outsider, because by definition you have to be at the leading edge to have any inkling of the potential of various avenues of research.
In my perfect world (2) would not be a concern. Theoretical physicists consume such a trivial amount of money in the pursuit of such profound truths that we should fund anyone with the talent and motivation to do the work. These guys cost nothing. The big dollar questions are in experimental funding.
Well, that’s a bit harsh. There are certain predictions that are consistent across all of the various string theories. The number of extra dimensions is one, for example: it has to be 6 in vanilla string theory. The reason is simply that you can’t write down a consistent supersymmetric (classical) string theory in dimensions other than 3, 4, 6 and 10. The first three cases are then ruled out by quantization conditions (anomaly cancellation). So that’s a fairly generic one—albeit one unlikely to be tested in the near future (provided certain large extra dimension scenarios don’t pan out—which seems unlikely by now—and provided the dimensions are actually compactified, and not just hidden in some other way, as e.g. in Randall-Sundrum models).
Another fairly generic prediction is, of course, supersymmetry. Indeed, should superpartners be observed, that would seem to be a pretty strong hint towards some flavor of string theory—because if we want to couple things to gravity, SUSY must be a local symmetry, which implies supergravity, the only consistent quantization of which that we know of is string theory.
There may be some other generic predictions, e.g. regarding early-universe cosmology and its signatures in the cosmic microwave background, but I’m not sure about that.
