The Many Dimensions of Dimensions III continued

Continued from Previous Post...

PHYSICS

Now we move on to the physical effects of extra dimensions and the reasons why they are interesting:

- Kaluza-Klein particles. One of the most universal effects of extra dimensions is the existence of massive particles known as Kaluza-Klein states. If a particle lives in five-dimensions, it will have a component of momentum in the fifth dimensions as well. Since quantum mechanics equates momentum with wavelength, and since the fifth dimension is assumed to be curled up into a ring, it becomes clear that this fifth component of momentum is restricted to have certain values - those values which give a wavelength that is a multiple of the length of the dimension.
And that leads to one of the interesting aspects of Kaluza-Klein states. When viewed in four-dimensions, these discrete values of the fifth component of momentum have the same effect as making the particle heavier. A massless particle in 5D appears in 4D as being an infinite number of particles, with each one having a different mass. One of the main methods of searching for extra dimensions is to look for heavier copies of known particles.
Another effect is the creation of particles. In a higher-dimensional Universe that contain no particles at all, the properties of the higher dimensions can mimic particles. For example the length of a curled up dimension behaves as a massive particle with no spin (called the radion) while other properties of the same curled up dimension can reproduce spin-1 particles such as the photon (which is responsible for electromagnetism) and possibly the particles known as fermions which are responsible for all known forms of matter. This leads to the possibility that there are no such things are particles, only 4D effects of higher dimensional spaces.

- Weakened Gravity : Another interesting effect of higher dimensions is as a solution to the hierarchy problem. It has been known for many years that electromagnetism and the two types of nuclear forces are actually identical at high energies - they are in fact different forms of a single force. But gravity (the only other know force) doesn't fit into this picture. It is far too weak to even be related to the other forces. It is this extreme difference in strengths of forces that is known as the hierarchy problem.
One solution is the presence of extra dimensions. If gravity is travelling in dimensions that the other forces cannot penetrate, it could easily be diluted in strength. Imagine transporting a small amount of water through a hose, and through a wide channel. In the hose the water is kept together and is forced (by water pressure) to travel quickly to its destination. In the channel, the water spreads out and can travel slowly to its destination. The same amount of water is moved, but in the second case it is so spread out that the movement is a lot weaker.
This also leads to another method of detecting extra dimensions. Since they affect only gravity, it is possible to build experiments which measure gravity on tiny distances. Because the extra dimensions dilute it, we expect to see gravity get stronger than expected as we get to smaller distances, where there is less room to dilute it.

- Black Holes: We will probably never have experiments on Earth capable of making black holes in 4D. They require too much energy. But higher dimension black holes can be made with very little energy! So little that the next big experiment - the Large Hadron Collider - could easily produce such things. (And contrary to what some irresponsible journalists have reported, there is no danger involved. These black holes decay too quickly to leave the experiment - if they didn't we would be hit by several each year coming from space. )
The possibility of producing black holes on Earth opens up the possibility of studying gravity in far more detail than we could have ever expected in a four-dimensional Universe.

These are just three of the interesting effects of extra dimensions. The professional journals are filled with articles on all the interesting properties of higher dimensions!