# Why does string theory require 9 dimensions of space and one dimension of time?

String theorists say that there are many more dimensions out there, but they are too small to be detected.

1. However, I do not understand why there are ten dimensions and not just any other number?

2. Also, if all the other dimensions are so coiled up in such a tiny space, how do we distinguish one dimension from the other?

3. If so, how do we define dimension?

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(1) It's all in the mathematics. (2) Can you distinguish your everyday 3 dimensions? Nope. So then there's no problem if the curled up ones are indistinguishable (not saying they are, though). [Atleast, I think it's this.] – Manishearth♦ Jul 12 '12 at 12:27
– Dilaton Jul 12 '12 at 13:53
– Dilaton Jul 12 '12 at 13:53

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– Dilaton Jul 12 '12 at 13:54
– Rody Oldenhuis Jul 12 '12 at 18:58

## 2 Answers

One can posit mathematical string theories in any dimensions of any kind.

However, I do not understand why there are ten dimensions and not just any other number?

The specific dimensions arise from the requirements of the known physics encapsulated in the Standard Model and other data coming from particle physics, plus the requirement of General Relativity and its quantization. The Special Unitary groups whose representations accommodate the SM need at least these dimensions. There are models with more dimensions than this.

Also, if all the other dimensions are so coiled up in such a tiny space, how do we distinguish one dimension from the other?

We cannot move into the coiled ones, only in x.y,z. We do not need to distinguish them, as we do not distinguish the molecules in the air. The predictions from this type of theory on the behavior of particles is the only way of checking for their existence: consistency of theory with data.

If so, how do we define dimension?

A space variable ( centimeters) or time one ( seconds) that is continuous and maps the real numbers, each dimension at 90degrees to the rest, an extension of how we define normal x,y,z.That some are curled should not bother one. The coordinates over the earth are curled over the sphere's surface, for example, though the 90degree does not hold there. It would hold on the surface of a cylinder , z from -infinity to infinity, x from 0 to 2*r*pi.

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This answer is not accurate, you cannot formulate any sort of string theory in 60 dimensions, if you have too many spatial dimensions, there are too many degrees of freedom on the horizon. Regarding spherical coordinates, these are orthogonal. – Ron Maimon Jul 13 '12 at 1:45
– anna v Jul 13 '12 at 4:35

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I see what you mean. But if you have ghosts in the theory, and divergent loop integrals, what does the mathematics mean? I agree that in all dimensions less than 10 and in all dimensions less than 26 you can formulate a fermionic/bosonic string theory (if you use a Polyakov noncritical string or linear dilaton), but in general I don't like to say this for dimensions higher than 26, because ghosts are not the same kind of problem qualitatively. BTW, only the poles are bad in spherical coordinates. – Ron Maimon Jul 13 '12 at 4:52

(1) String Theory is a very mathematical theory based on some natural assumptions, and this ends up relating Quantum Mechanics and General Relativity, as we want. Some of the equations in String Theory, however, have a proportionality constant $c$ in it, called the central charge. And when we manipulate these equations and set them equal to each other, we see that they ONLY make sense if $c=26$. This $c$ is the dimension of space that String Theory is a priori defined over, so now we see that we need 26 dimensions to not have absurdities... BUT that only made use of the bosonic particles in the world -- we forgot about fermions!! This is where Supersymmetry comes into play, and it throws in the fermions, and the equations are perturbed and leads to a new dimension of 10 for everything to make sense.

(2) Just because we can't see it, doesn't mean it's not there... we can't see atoms with the eye, but we can use tools to see them... same thing happens here, our current technology can't see them, but we hope to change this in the future. EVEN BETTER though, is that the formula for gravitational force should actually be different because of these extra small dimensions -- thus we plan to figure these extra dimensions out by testing the gravitational force at small distances and seeing a perturbation to the standard inverse-square law of Newton. These extra dimensions are what is supposed to make gravity so weak compared to the other forces of nature.

(3) a dimension is just a coordinate axis... so time is a dimension too. And just like your clock, this axis can repeat itself and not stretch to infinity.

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I don't know how to make sense of compactified time, especially in string theory. Even if you compactify euclidean time, thermal string theory is hard to make sense of too, because gravity doesn't allow thermal ensembles of infinite extent. Regarding the central charge argument, it's fine, but it doesn't require 10 dimensions per-se, just an equivalent central charge, so you can have a non-geometric compactification. – Ron Maimon Jul 13 '12 at 1:42
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