On hadron colliders, dark matter and black holes

[töff]

I have some good popular-level books on modern physics and quantum-mechanic theories, including string theory ...

... I just haven't read any of them ...

[torstan]

That's a very common issue with physics books.

For those that are interested in this stuff (and if you've got to p26 of this thread I'm going to assume you are!) then I can recommend J.S.Bell's Speakable and Unspeakable in Quantum Mechanics as a really good book to flick through.

[töff]

I can't take credit for the analogy. But yeh, it's a good one, huh ... as analogies go, which isn't far.

It's like the rubber sheet thing to explain spacetime curvature. Same concept.

[Karro]

I can't take credit for the analogy. But yeh, it's a good one, huh ... as analogies go, which isn't far.

It's like the rubber sheet thing to explain spacetime curvature. Same concept.

Yeah, which I always thought was a pretty good analogy. It had the added benefit of being demonstrable in how it worked.

[torstan]

My favourite analogy for explaining compact dimensions has always been asteroids, the game. It is entirely possible for extra dimensions to be wrapped up in a way that if you travel in a straight line you return to where you started. If the distance you need to travel to return is very small then we might not even notice that the dimension is there. Alternatively the dimension could be very large, but we might not be able to travel in it at all. In that case it would be more like our own existence on the earth's surface before we designed planes. We cannot travel up in the air, but we can see things that do. We are restricted to moving in the 2D world of the earth's surface. In the same way string theory predicts that the universe has many more dimensions than we are aware of, but that we are confined to 3D objects within that space called branes (Actually 3+1 dimensions as we always count time as well). So even though the universe has these other dimensions, the universe we deal with is 3D. Stretching the analogy further, the prehistoric man could see that birds travel in the air. In our case, it may be that gravity can travel in the extra dimensions. This has been proposed as a reason for gravity being weaker than the other forces as it spreads out across many more dimensions. Just as if you drop ink on paper and drop ink in a glass of water. If you compare the darkness of the ink on the paper at a given distance from the point of impact to the darkness of the ink in the glass at the same distance, the ink on paper will be darker.

So we expect that there are more dimensions - 11 in total - from string theory. This is required for the theory to be in any way consistent and is one of the clear predictions that it makes. There are then a number of explanations for why we don't see those extra dimensions. In some scenarios we would see hints of those dimensions turn up in the data from the LHC. An example would be if those extra dimensions were hidden by being wrapped up, like the ant on the string. From a distance the string looks 1 dimensional - you can only go one direction on it, along its length. However if you have a good magnifying glass, you can see that the surface of the string is 2 dimensional. If you were very small you could walk around the string as well as along it. Indeed if you were very small indeed the string would seem almost flat. So it's all about the degree of magnification. There will be a degree of magnification at which it becomes clear that the surface of the string has width as well as length.

Now the LHC is a microscope in a very real sense. As you increase the energy of a collider you probe smaller length scales. Therefore if the extra dimensions are large enough, then when we increase the energy of our colliders we may cross the magnification threshold for the extra dimension and start to see effects from its existence.

[Karro]

So... what are the implications, practically if there are any, of (a) the existence of additional dimensions or (b) the concept of tiny, pulsating, multi-dimensional strings as the most fundamental building blocks of all matter?

[torstan]

The implications will really depend upon the precise form the string theory takes. There are many (10^500 at least) ways to create a consistent universe with string theory. Each of those has different implications. No most of those wouldn't create the kind of universe we see around us, so we can discard the vast majority as reasonable explanations. That still leaves an awful lot of options, and far too many than we can reasonably study to discover possible implications. We'll need a major theoretical or experimental breakthrough to get clear predictions from it currently.

On the other hand, there are some things that all of those cases agree upon. They would result in gravity being transmitted by a particle with clear properties. That would be the graviton. It would have a partner, the gravitino that could be the dark matter in the universe. Because we would have a scenario with gravity being mediated by a particle that is fundamentally governed by quantum mechanics we would have a consistent model of quantum gravity, solving one of the longest running mysteries in physics. That sort of revolution in thinking would rebuild the foundations of modern physics. When those kind of revolutions happen it is impossible to tell what will come out of it, but it will be big.

On the more hypothetical side of things, possible implications would be that there are a number of universe separated in the extradimensions, allowing for the possibility of colliding branes. This could be a cause for the big bang in some theories. I'll mention some other possibilities later today, but right now I'm going to get lunch.

[töff]

We are restricted to moving in the 2D world of the earth's surface.

A good example of the closed-in-on-itself dimension. The surface of a globe is finite but boundless! That's mathematically awesome.

[torstan]

It certainly is

[töff]

... but somewhat of a pain in the ass for cartographers.

Maybe we should all upgrade to cosmography.