Manyfold Universe Theory

Geek-from-parallel-Universe writes "In the HEP preprints database a preprint ">appeared in which the authors propose that a world is a brane folded many times in extra sub-millimiter spatial dimensions. We see other folds only through gravity as a dark matter because light must go around the folds. If this is true then I am waiting for Star Trek-like devices: 'portable submillimeter wormhole generator' and 'personal parallel universe transmitter' to appear on the market. :-)"

Comments

Some context for this...

[zunger]

There's actually been a lot of fuss about what's called "large extra dimensions" recently. The original problem was that the energy scale associated with gravity is about 10^19 GeV (1GeV = the energy an electron would get going through a potential gap of 10^9 V = approximately the mass of a proton) while the energy scale associated with all the other forces of nature is only 10^3 GeV. This is really bad because it means that (for instance) particles would get gravitational fields surrounding them that give them masses on the order of 10^19 GeV, which would turn everything in sight into a black hole.

This problem can be solved in a number of ways - notably supersymmetry, which causes those giant gravitational fields to cancel out. But there's one other odd problem to deal with, which are "extra dimensions." Basically string theory requires that the universe is actually 10-dimensional, and the other 6 dimensions are simply wrapped up very tightly. (Mental picture: If you wrap up a sheet of paper (which is 2-dimensional) into a very tight tube and look at it from far away, it looks 1-dimensional. Unless you're scanning it on distance scales comparable to the radius of the tube.) The problem is that you have to somehow wrap up these 6 dimensions on a really small distance scale (the length scale of gravity, about 10^-42 cm) and keep the other 4 really big. (the size of the universe) This again happens because the energy scale of gravity is big.

So about a year ago, Nima Arkani-Hamed, Savas Dimopoulos, Gia Dvali and John March-Russell had an interesting thought: We don't *know* that gravity really behaves like anything in particular at length scales below about a millimeter. (The current limit of experiment is about 0.8mm) So they noticed that the following setup gives the right answers too:

* We live in a universe with however many "extra" (small, rolled-up) dimensions, but these are rolled up with radii on the order of somewhere between 1fm (10^-15m, the size of a nucleus) to 0.1mm. (The range of sizes is because there are several different models)

* In this loosely rolled-up world, there are these 4-dimensional objects called "branes" floating around.

Then several amazing things happen. First of all, all matter particles (electrons, quarks, people) are bound to the surface of the brane and can't leave it. So are all the non-gravity force particles. (Photons, gluons, etc.) This just follows from the physical properties of branes in string theory, and it means that as far as anything but gravity is concerned, the universe is 4-dimensional and we won't see the extra dimensions.

Second, gravity completely ignores the brane (except insofar as there's matter, and therefore sources of gravity, there) and flies around freely in all of the dimensions. But because some of them are rolled up, what happens is that at long distances (bigger than the radius) all the gravity gets "squeezed" along the extra dimensions and gravity behaves like ordinary 4-dimensional gravity. At short distances, this changes -- for instance, the 1/r^2 force of gravity becomes something like 1/r^4.

But the real magic is, if the fundamental energy scale of gravity was 10^3 GeV, (the same as the scale for everything else) the distortion of gravity by the rolling up of space would make it seem like the scale was 10^19 GeV to any observer looking at distance scales bigger than the radius!

So the bonus of the Large Extra Dimensions (LED) scenario is, everything has the same energy scale, and it only seems that gravity has this high energy scale because we're looking at too long a distance. And all of the problems of a high energy scale indeed go away.


Of course, you can ask what the hell any of this has to do with reality. The thing is that all of this is consistent with all experiments to date and explains several tricky points. More importantly, it is experimentally testable; part of the testing happens in tabletop experiments (groups at Stanford and at NIST in Boulder are working on measuring gravity at distances down to about 10^-6 m) and part of it in accelerators. The final tests (thumbs up or thumbs down) will come from experiments at the LHC accelerator in Geneva, which should (knock on wood) be up to spin around 2004/5. Final results should take a few more years after the machine comes on-line.

But disclaimer: At this point this entire scenario is conjecture. People are already working out "observational experiments" to check these models -- for instance, whether these are consistent with the known spectrum of cosmic rays -- which are strong experimental constraints. But until the final experiments happen we can't be certain, one way or the other.

Also, since the original paper came out there have been several modified versions of the conjecture, which differ essentially in technical (but very important) points. The Randall-Sundrum model is especially important, and today's model looks to join the list of candidates.

So what does this mean for us? First of all, if it's right then the underlying scale of gravity is only 10^3 GeV, which is definitely accessible with the next generation (LHC) of accelerators. This means we can start to directly monkey around with the processes associated with black hole formation and the origins of the universe. Apart from completely changing physics (by making quantum gravity experiments practical) this is one of those things that creates more applications than we know what to do with. Making small black holes (and no, they wouldn't eat up the planet. :) is one thing. In some of the models effective FTL travel may be possible.

But possibly the most interesting thing is that there's no reason at all for our brane -- the one that our universe lives on -- is the only one. In fact, the most reasonable model suggests that there is some unbelievable number of branes floating out there, maybe 10^24 of them. It's not clear that the laws of physics would be the same on all of them -- e.g. the speed of light may be different, or the charge of the electron, or whatever -- but if the scenario turns out to be true, it is possible (though difficult) to communicate between two different worlds.

And for my money, that's the neatest thing of all.