# Gravity on a torus

by Gerard Westendorp
Tags: gravity, torus
 P: n/a I was playing with a 2D model with point particles and gravitational attraction. To avoid particles going off to infinity, I thought I might turn the 2D space into a topological torus: connect x=L = x=0 and y=L to y=0. But now a problem occurs: What is the distance between 2 points? Because of the periodic boundary, you get for the distance (s): s^2 = (x2-x1+i*L)^2 + (y2-y1+j*L)^2 You could set i = j = 0, but that would destroy translation symmetry, it would make the coordinates at 0 and L physically different from other points. So there are a multitude of distances, a grid of them described by the integers (i,j). But that does seem a bit weird. Any comments on this? Gerard
 P: n/a In article <46cfe90b$0$240\$e4fe514c@news.xs4all.nl>, Gerard Westendorp wrote: >I was playing with a 2D model with point particles and gravitational >attraction. To avoid particles going off to infinity, I thought I might >turn the 2D space into a topological torus: connect x=L = x=0 and y=L to >y=0. > >But now a problem occurs: What is the distance between 2 points? > >Because of the periodic boundary, you get for the distance (s): > > s^2 = (x2-x1+i*L)^2 + (y2-y1+j*L)^2 > >You could set i = j = 0, but that would destroy translation symmetry, it >would make the coordinates at 0 and L physically different from other >points. > >So there are a multitude of distances, a grid of them described by the >integers (i,j). > >But that does seem a bit weird. Any comments on this? This is kind of a funny situation. The most natural-seeming thing to do is to take all of the different values of (i,j) as "legitimate" distances. That is, assume that any given particle is attracted to any other particle at all of the other particle's locations in this hall of mirrors. But if you do this, you find that the force on any given particle is a divergent sum. I suppose one way to get around this would be to give the graviton a mass -- that is, to replace the usual gravitational potential G m1 m2 / r with a Yukawa potential (G m1 m2 / r) exp(- lambda r) for some parameter lambda. That would tame the divergence, but it'd mean you were talking about a physically different theory. Whether that's what you should do or not depends largely on what you're planning to use this model for, I suppose. -Ted -- [E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]
 P: n/a Gerard Westendorp wrote: > I was playing with a 2D model with point particles and gravitational > attraction. To avoid particles going off to infinity, I thought I might > turn the 2D space into a topological torus: connect x=L = x=0 and y=L to > y=0. > > But now a problem occurs: What is the distance between 2 points? I think you must try to solve Poisson equation on the torus.
P: n/a

## Gravity on a torus

Gerard Westendorp wrote:
> I was playing with a 2D model with point particles and gravitational
> attraction. To avoid particles going off to infinity, I thought I might
> turn the 2D space into a topological torus: connect x=L = x=0 and y=L to
> y=0.
>
> But now a problem occurs: What is the distance between 2 points?
>
> Because of the periodic boundary, you get for the distance (s):
>
> s^2 = (x2-x1+i*L)^2 + (y2-y1+j*L)^2
>
> You could set i = j = 0, but that would destroy translation symmetry, it
> would make the coordinates at 0 and L physically different from other
> points.
>
> So there are a multitude of distances, a grid of them described by the
> integers (i,j).
>
> But that does seem a bit weird. Any comments on this?

Think I get it now.

The particles simply see each other particle, plus all the infinite
number of images of of them. In 2D (ie gravity force varies as 1/r) this
would be a problem, because the series 1/x + 1/(x+L) + 1/(x+2L) + ...
doesn't converge. But with the inverse square law it is OK.

Gerard

 P: n/a This discussion is funny to me because apparently all the string theory guys have solved this already---since they speak of the description of gravity in an alternative world say 2 space dimensions, where one dimension is compactified and they claim to know what the gravitational potential would be in such a case. I wonder what their criterion is for what the potential of gravity should be in such a case, since they need such a criterion to make the claims they do. I wonder if one of the guys "in the know" would read this and tell us how their argument works.
 P: n/a > This discussion is funny to me because apparently all the > string theory guys have solved this already---since they > speak of the description of gravity in an alternative world > say 2 space dimensions, > where one dimension is compactified and they claim to know > what the gravitational potential would be in such a case. > I don't think string theory is meant to work as a gravity theory in 2-d. One is instead building a gravity theory, say, in ten dimensions using (quantum) fields in 2-d. > I think you must try to solve Poisson equation on the torus. This would seem to be exactly what is needed. A complication is that a necessary condition for the Poisson equation (with a well-behaved, non-negative mass density) to have a solution on the torus is that the mass density vanishes everywhere! To see this, just integrate the equation over the torus and use the divergence theorem. This implies the total mass vanishes. Since the mass density is non-negative, it must vanish everywhere. charlie torre
 P: n/a On 2007-10-28, pirillo wrote: > This discussion is funny to me because apparently all the > string theory guys have solved this already---since they > speak of the description of gravity in an alternative world > say 2 space dimensions, > where one dimension is compactified and they claim to know > what the gravitational potential would be in such a case. Sure, string theorists sometimes include gravity on the 2D worldsheets of strings, but most of the time it doesn't have much effect. The way that gravity is defined in other than 4 dimensions, nowadays, is using the Einstein-Hilbert action of general relativity (which is the integral of the Ricci curvature scalar over space-time). In 2 and 3 dimensions, this version of gravity behaves very differently from what you get by reducing Newtonian gravity to 2 or 3 dimensions (which is described by the 1 or 2 dimensional Poisson equation for the gravitational potential). In 2 and 3 dimensions, general relativity is in a sense trivial. Einstein's field equations are R = T, where R is the Ricci curvature tensor and T is the energy-momentum tensor. In 2 and 3 dimension, the full Riemann curvature tensor can be altebraically related to the Ricci tensor R. So, in the absence of matter, R = 0 implies the Riemann curvature is also zero, which in turn implies that the metric is flat. A flat metric implies no acceleration due to gravity. This is very different from lower dimensional Newtonian gravity. Solving the Poisson equation in in 1 or 2 dimensions still produces a non-trivial gravitational potential. For higher dimensional extensions of gravity, Newtonian equations still work in the weak field limit. However, if some of these higher dimensions are compact, then the shape behavior of the gravitational potential as a function of the compactified coordinates strongly depends on what kind of sources are used for matter. I'm not sure if point sources or other kinds of sources are used in those cases. It is possible that some matter distributions are not allowed, because there will exist no solution to Einstein's equations compatible with them. One would have to look at this issue in more detail. Hope this helps. Igor
 P: n/a To Igor and the others, I have not read your thread fully yet, but from the first sentence I see a misinterpretation of what I meant. I never said that the 2dimensions (Torus) I was talking about, are the dimensions of the string world sheet. I'm saying the torus here is a toy model for --space--of spacetime itself E.g., a simpler version of R^{10} with one dimension made into a circle. Regardless, due to many of the same concerns many of the posters before me expressed. I do not know how the write the potential for a Gauge (EM--like) theory in such a compactified space. But since people like Lisa Randall have in their book that "at long distances, gravitation on a cylinder behaves a certain way and differently when the distance is small compared to the zize of the compact dimension-- as evidenced by the potential" eg somethis that at long distances behaves like r^2 will behave as r^4 at shoter distances etc. How do they get this potential? Is it some local interaction index(to extract potential As in QCD) or is it a classical potential that one puts on the cylinder (say, emanating from a source).
 P: n/a On 2007-11-04, pirillo wrote: > Regardless, due to many of the same concerns many of the posters > before me expressed. I do not know how the write the potential for a > Gauge (EM--like) theory in such a compactified space. But since > people like Lisa Randall have in their book that "at long distances, > gravitation on a cylinder behaves a certain way and differently when > the > distance is small compared to the zize of the compact dimension-- > as evidenced by the potential" eg somethis that at long distances > behaves > like r^2 will behave as r^4 at shoter distances etc. How do they get > this potential? Is it some local interaction index(to extract > potential As in QCD) > or is it a classical potential that one puts on the cylinder (say, > emanating > from a source). The behavior of the potential that Lisa Randall and other string theorists allude to in this context is just the classical Newtonian one (there are arguments for why the more general general relativistic situation reduces to this one). To get the Newtonian potential for a stationary point mass, you discard the time axis and solve the Poisson equation in the spatial directions for the point mass distribution. In R^n, for n > 2, the solution is proportional to 1/r^(n-2) (n=3 gives the usual 1/r potential). If you solve for the Newtonian potential on a space where all dimensions except three have been compactified, that is on R^3 x C^n, then the full expression for the potential will be different. Here, C^n is some compact manifold, it could be a circle (n=1), an n-torus (a product of n circles), an n-sphere, or something more exotic. However, its behavior very close and very far from the source is easy to obtain. If you are very close to the source, much closer than the diameter of C^n, then the local behavior of the potential is exactly the same as in the R^(3+n) case: the potential diverges as 1/r^(1+n). On the other hand, if you are very far from the source, the spatial dependence of the potential is very weak in the compactified directions. Assuming that the potential is constant in these directions, the potential becomes the same as for R^3: the long distance decay of the potential will go as 1/r. If the compactified dimensions are there and are very small, it makes sense that we would only see the 1/r behavior on ordinary length scales. However, when two particles would come very close to each other, they would feel the stronger 1/r^(1+n) short range gravitational potential. This hypothesis has been used to test for the existence of large (centimeter or millimeter scale) extra dimensions, so far with negative results (http://www.npl.washington.edu/eotwash/). Hope this helps. Igor
 P: n/a On 2007-11-04, pirillo wrote: > Regardless, due to many of the same concerns many of the posters > before me expressed. I do not know how the write the potential for a > Gauge (EM--like) theory in such a compactified space. But since > people like Lisa Randall have in their book that "at long distances, > gravitation on a cylinder behaves a certain way and differently when > the > distance is small compared to the zize of the compact dimension-- > as evidenced by the potential" eg somethis that at long distances > behaves > like r^2 will behave as r^4 at shoter distances etc. How do they get > this potential? Is it some local interaction index(to extract > potential As in QCD) > or is it a classical potential that one puts on the cylinder (say, > emanating > from a source). The behavior of the potential that Lisa Randall and other string theorists allude to in this context is just the classical Newtonian one (there are arguments for why the more general general relativistic situation reduces to this one). To get the Newtonian potential for a stationary point mass, you discard the time axis and solve the Poisson equation in the spatial directions for the point mass distribution. In R^n, for n > 2, the solution is proportional to 1/r^(n-2) (n=3 gives the usual 1/r potential). If you solve for the Newtonian potential on a space where all dimensions except three have been compactified, that is on R^3 x C^n, then the full expression for the potential will be different. Here, C^n is some compact manifold, it could be a circle (n=1), an n-torus (a product of n circles), an n-sphere, or something more exotic. However, its behavior very close and very far from the source is easy to obtain. If you are very close to the source, much closer than the diameter of C^n, then the local behavior of the potential is exactly the same as in the R^(3+n) case: the potential diverges as 1/r^(1+n). On the other hand, if you are very far from the source, the spatial dependence of the potential is very weak in the compactified directions. Assuming that the potential is constant in these directions, the potential becomes the same as for R^3: the long distance decay of the potential will go as 1/r. If the compactified dimensions are there and are very small, it makes sense that we would only see the 1/r behavior on ordinary length scales. However, when two particles would come very close to each other, they would feel the stronger 1/r^(1+n) short range gravitational potential. This hypothesis has been used to test for the existence of large (centimeter or millimeter scale) extra dimensions, so far with negative results (http://www.npl.washington.edu/eotwash/). Hope this helps. Igor
 P: n/a Igor said: " However, its behavior very close and very far from the source is easy to obtain. If you are very close to the source, much closer than the diameter of C^n, then the local behavior of the potential is exactly the same as in the R^(3+n) case: the potential diverges as 1/r^(1+n). On the other hand, if you are very far from the source, the spatial dependence of the potential is very weak in the compactified directions. Assuming that the potential is constant in these directions, the potential becomes the same as for R^3: the long distance decay of the potential will go as 1/r. " Igor , how could one justify this claim? In other words, roughly, whats your argument that very close to the source the potential behaves like that? Do you know The greens function for the torus explicitly? I've heard people talk of field lines but I dont know how to argue using field lines or if such arguments are very precise.