# 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?

• (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.] Jul 12, 2012 at 12:27
• Lenny Susskind has once shown a simplified mathematical argument that one needs 26 dimensions to allow for the tachionic ground state of bosonic string theory. The number of dimensions can then reduced to 9 + 1 dimensions turning to superstrings. Jul 12, 2012 at 13:53
• What dimensions are in general, Prof Strassler nicely explains in a series of articles starting with this one. As the later articles in the series explain, the (large) extra dimensions could in principle have been detected by the discovery of Kaluza-Klein particles at the LHC for example. Jul 12, 2012 at 13:53
• One would need to analyze a whole spectum of such particles to experimentaly determin their shape as is explained here. From a theoretical point of view, the shape of the extra dimensions is discribed by moduli fields. Darn, now this has become too long for one comment :-P Jul 12, 2012 at 13:54
• @Jiminion As in you have to arbitrarily label them; there isn't a universal "up". Apr 18, 2015 at 9:41

Let me take parts 2. and 3. of the question first:

The 10 dimensions of string theory are, a priori, not "coiled up" or anything else. They are derived for a string theory where the classical version of the string propagates in d-1 spatial dimensions and 1 temporal dimension, i.e. Minkowski space $\mathbb{R}^{1,d-1}$. "Dimension" here is dimension of a manifold in the usual sense of differential geometry - number of coordinates needed to uniquely distinguish a point on the manifold from all points close to it.

Now, as for why (super)string theory in flat space requires $d=10$:

One way to see string theory is by certain two-dimensional conformal field theories living on the world sheet the string traces out in the target space. I give a quick explanation of the structure of such theories here. The total conformal charge of the full combined CFT on the worldsheet can be seen as the quantum anomaly of the classical Weyl symmetry of the string - for a general discussion of the relation between anomalies and central charges see this answer by DavidBarMoshe, for a general discussion of the relation between central charges and quantization see this Q&A of mine.

The quantization of the bosonic (or "naive") string has d coordinate fields that each correspond to a free bosonic CFT with central charge $c=1$ plus a "ghost system" incurred from BRST quantization that has a central charge $c=-26$. Ghost systems are allowed to have negative central charge because they decouple from all physical processes.

Now, the procedure used to quantize this string in the first place makes use of the Weyl symmetry being non-anamalous, i.e. $c=0$ for the full theory - which only happens at $d\cdot 1 - 26 = 0$, i.e. $d=26$. Therefore, the bosonic string exists consistently as a quantum theory only in 26 dimensions.

The superstring is now what you get when you additionally have fermions living on the worldsheet. It's called the "super"string because the new action is supersymmetric, but it might as well be called the "spinning string", since trying to write down a worldline action for a particle with spin also introduces such fermions.

In any case, the ghost system for the larger symmetry of the superstring has $c=-15$, and the fermions each contribute $c=1/2$. This gives the requirements $\frac{3}{2}d - 15 = 0$, which is solved by $d=10$.

I'm afraid the full derivation is rather technical and it would serve little use to reproduce it here. Lastly, one should remark that there are many equivalent ways to arrive at this constraint on dimensions, this is by far not the only one, but the one that's easiest to tell for me. Others might find a presentation discussing ordering constants related to the vacuum energy more physically intuitive, for example.

• Does this prove that M-theory doesn't exist? Nov 25, 2022 at 3:41
• @IsaacNewton No, it just shows that M-theory is not just "string theory". To say anything more definitive we would actually need a complete definition of what M-theory even is :P Nov 25, 2022 at 9:15

(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.

• 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. Jul 13, 2012 at 1:42

For bosonic string theory, see this. I'll be using the same standard notation in this answer.

# Superstrings (in the RNS formalism)

## Ramond sector

$$\begin{array}{l}0 = {{\hat G}_0}\left| \psi \right\rangle \\{\rm{ }} = \sum\limits_{n = - \infty }^\infty {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n}} \left| \psi \right\rangle {\rm{ }}\\{\rm{ }} = \left( {{{\hat \alpha }_0}\cdot\,{{\hat d}_0} + \sum\limits_{n = 1}^\infty {\left( {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n} + {{\hat d}_{ - n}}\cdot\;\;{{\hat \alpha }_n}} \right)} } \right)\left| \psi \right\rangle {\rm{ }}{\kern 1pt} \,\\{\rm{ }} = \left( {\left( {\frac{1}{2}{\ell _P}{p^\mu }} \right)\,\cdot\,\left( {\frac{1}{{\sqrt 2 }}{\gamma ^\mu }} \right) + \sum\limits_{n = 1}^\infty {\left( {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n} + {{\hat d}_{ - n}}\cdot\;\;{{\hat \alpha }_n}} \right)} } \right)\left| \psi \right\rangle \\{\rm{ }} = \left( {\frac{1}{{2\sqrt 2 }}{\ell _P}{\gamma ^\mu }{p_\mu } + \sum\limits_{n = 1}^\infty {\left( {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n} + {{\hat d}_{ - n}}\cdot\;\;{{\hat \alpha }_n}} \right)} } \right)\left| \psi \right\rangle \left( {} \right)\\{\rm{ }} = \left( {\frac{1}{{2\sqrt 2 }}{\ell _P}{\gamma ^\mu }{p_\mu } + \sum\limits_{n = 1}^\infty {\left( {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n} + {{\hat d}_{ - n}}\cdot\;\;{{\hat \alpha }_n}} \right)} } \right)\left| \psi \right\rangle \\\left( {\frac{1}{{2\sqrt 2 }}{\ell _P}{\gamma ^\mu }{p_\mu } + \sum\limits_{n = 1}^\infty {\left( {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n} + {{\hat d}_{ - n}}\cdot\;\;{{\hat \alpha }_n}} \right)} } \right)\left| \psi \right\rangle = 0\\\left( {{\gamma ^\mu }{p_\mu } + \frac{{2\sqrt 2 }}{{{\ell _P}}}\sum\limits_{n = 1}^\infty {\left( {{{\hat \alpha }_{ - n}}\cdot\;\;{{\hat d}_n} + {{\hat d}_{ - n}}\cdot\;\;{{\hat \alpha }_n}} \right)} } \right)\left| \psi \right\rangle = 0\end{array}$$

This is the Dirac-Ramond Equation.

Still in the Ramond sector,

$${\hat L_0}\left| \psi \right\rangle = \hat G_0^2\left| \psi \right\rangle$$=

$${\hat L_0}\left| \psi \right\rangle = \hat G_0^2\left| \psi \right\rangle$$

$$a = 0$$

Now, consider some Level 1 Neveu-Schwarz Spurious State Vector $$\left| \varphi \right\rangle = {\hat G_{ - 1/2}}\left| \chi \right\rangle$$

$$0 = {\hat G_{1/2}}\left| \chi \right\rangle = {\hat G_{3/2}}\left| \chi \right\rangle = \left( {{{\hat L}_0} - a + \frac{1}{2}} \right)\left| \chi \right\rangle$$

So, $$a = \frac{1}{2}$$ in the Neveu - Schwarz sector.

Now, we consider a Ramond Spurious State Vector $$\left| \varphi \right\rangle = {\hat G_0}{\hat G_{ - 1}}\left| \chi \right\rangle$$ ; where $${\hat F_1}\left| \chi \right\rangle = \left( {{{\hat L}_0} + 1} \right)\left| \chi \right\rangle = 0$$

$$0 = {\hat L_1}\left| \psi \right\rangle = \left( {\frac{{{{\hat G}_1}}}{2} + {{\hat G}_0}{{\hat L}_1}} \right){\hat G_{ - 1}}\left| \chi \right\rangle = \frac{{D - 10}}{4}\left| \chi \right\rangle$$

Thus, $$D=10$$.

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 $90^{\circ}$ 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, the $90^{\circ}$ does not hold there. It would hold on the surface of a cylinder , $z$ from $-\infty$ to $\infty$, $x$ from $0$ to $2\pi r$.

• 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. Jul 13, 2012 at 1:45
• @RonMaimon Sure, they will not make physics sense but the mathematics will be there,no? As for spherical coordinates on the sphere surface, the angles are not 90degrees. think of the poles. en.wikipedia.org/wiki/Spherical_trigonometry . I will make it clear I mean the surface coordinates. Thanks. Jul 13, 2012 at 4:35
• 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. Jul 13, 2012 at 4:52
• @RonMaimon Actually there's a whole arc of discontinuity in spherical co-ordinates. If $\phi$ is the angle from the $z$-axis, $\theta$ the planar, then fixing a value $\phi_0$, there's a discontinuity in $\theta$ at the point $0,2\pi$. The poles are bad in the sense that you have a redundant degree of freedom there, the arc defined by $\theta=0$ or $\theta=2\pi$ is bad because you lose continuity in the chart there. Jul 13, 2016 at 16:23

To make the math work. Ever since Einstein determined that time is actually another dimension, Physicists have used that notion to expand the conception of the Universe to include added (by not sensible) dimensions to get their math and theories to work. Of particular note is Witten's unification of string theories which "only" required the addition of yet another dimension.

What string theorists fail to realize is that each dimension represents an added degree of freedom and thus may very well be under constraining the system.

In part because of this, that is why some critics of string theory (Woit, Smolin) have called string theory "Not Even Wrong" and "The Theory not of 'Everything' but 'Anything' ".

• Yes, ST seems to be pure math with "dimensions" added as necessary to fit a given problem. This reminds me of John Von Neumanns skeptical reply to a fellow theorist, who claimed to have solved a problem by including an extra free parameter (dimension/degree of freedom): "With 3 free parameters I can draw an elephant. Give me 4 and I can make it wiggle its trunk!"
– Jens
Jan 18, 2017 at 12:14