Why only 11 dimensions?

If string theory posits 11 dimensions, why can't there be more? Seems a rather arbitrary number.

 

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For unification of forces and supersymmetry, it appears that is how the math works out.

M-theory is the most promising unifiying theory out there, but there is still the issue of an assumed space/time background (not to mention the lack of falsifiability, experimental evidence, etc.)....

 

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Is there any experiment we can devise to verify the existence of higher dimensions?

 

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Maybe at extremely high energy/small scales. Since the dimensions are tiny and curled up, you could possibly experimentally verify their effects through particle collision or observation of forces at the CERN LHC or future accelerators.

I don't really know I am not a physicist but an interested observer.

 

The number of dimensions is required for consistency. If there aren't 11 dimensions, then we get something called anomalies, which we know don't exist. Because in string theory the anomalies only cancel in 11 dimensions, we know that Nature must be 11 dimensional if string theory is right.

This is in contrast to everything that we've done before---in string theory, one actually FINDS the dimension of space-time, as opposed to just guessing four. For example, general relativity is perfectly happy in ANY number of dimensions.

The easiest experiment would be to build a particle accelerator the size of the galaxy. If we were to do that, then we could probe the energy levels predicted by string theory quite easily.

If some of the extra dimensions are larger than others, some evidence may pop up at CERN when it turns on. But most theorists aren't putting money on this outcome.

 

And the hardest experiment?

 

What is the latest thinking as to how the most energetic primary cosmic rays get their energy. Any way to use them to test "string theory"

 

The problem with high energy cosmic rays is that we don't know where they will come from. If we could isolate a source of very high energy cosmic rays, it is concievable that we could build a detector to see what happens.

 

There is a very big advance coming about now on the origin of Cosmic rays. Huge new auger telescope for them being built in the extremely dark pampus region of argintina. I made a post about it approximately a year ago when 1000 of the chrinkoff water tanks were operational. I for get the details but this detector used also the faint sky glow and very precise timeing of the shower in these tanks, which are scatered of many square miles. Do you know about it?

 

Yeah I know there is a professor (or two?) here at OSU who's on the experiment. The problem is luminosity. They only expect a few events a year, or less. This is not enough data to do statistics on, so I don't know if one could do any particle physics experiments.

The other problem is that we don't know any sources of high energy cosmic rays. If we knew (or found) a source, then we could set up a detector (in space perhaps) that always faced that source. Then, if we got, say, ten events a year, we may have enough data after a decade or so to do some statistics on.

I am kind of talking out of my ass, of course. This is mostly speculation

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While looking for something else, I noticed by old post to the detector in Argintina:

http://www.sciforums.com/showpost.php?p=911416&postcount=1

 

If you haven't already seen this video it's worth a look.

 

Thanks for link. I think the discussion of the straight line on the mobeus strip as seen by a "flat lander" traveling along this straight line back to his starting point may help some understand how we 3D creatures can travel along a straight line and return to our starting point. - I.e. a closed universe you can not escape from by always going straight away from where you started.

 

In other dimensions GR would imply the gravitational force will drop with distance with exponent other than -2, so given the exponent -2, in a sense can't you say GR predicts the dimension of space?

 

You are certainly correct in asserting this. The point that I wanted to make is that GR has no constraints on number of dimensions, and is perfectly consistent in any number of dimensions. So, for example, you rightly pointed out that in 4 large dimensions, GR gives an inverse square law. But if there are an additional 50 small dimensions that were small enough so that we couldn't test gravity (i.e. about a micrometer), GR wouldn't tell us anything.

String theory, on the other hand, REQUIRES 11 dimensions to be consistent.

 

I heard about 26 before, so it is now 11? Why is it keep changing?

 

Different flavors of string theory. The same quantum consistency conditions give a different dimension, depending on the matter that is present.

For example, if your theory only has bosons, then consistency required 26 dimensions. This, of course, is only a toy model because we know that matter in our universe is fermionic.

So if you have fermions, 10 dimensions are required.

So why 11? It turns out that we actually missed some conditions when string theory was first written down. As we have begun understanding the finer details of string theoy (now called M theory), we have to admit one more dimension beyond the 10.

 

What I've never understood is how these dimensions are manifested physically. Whenever I read about them they're described as being "curled up" into incredibly tiny spaces at every point in the universe, and I have to be honest; that doesn't make sense to me.

I guess what I'm asking is how do these extra dimensions interact with the matter or energy in the universe?

 

I saw some video online that completely explained it to me. I'll try and dig it up, because I sure as hell cant remember it anymore. When it got up to 8 or 9d, it seemed like they were just making stuff up. And stopped when they couldn't make it feasable anymore, at 11d.