The Search For Extra Dimensions

What extra dimensions, you probably think, having just read the title. We know very well that the world around us is three-dimensional. We know East from West, North from South, up from down - what extra dimensions could there possibly be if we never see them?

A revolution in thinking about extra dimensions
If modern physics has taught us anything, it is that our normal intuition of how the universe works cannot be trusted. A perfect example is Einstein's Special Theory of Relativity, which states that the speed of light is the only constant, length and time can vary, and our notion of the simultaneity of two events is incorrect. Another example is quantum mechanics, which says that properties of an object such as momentum and position cannot be determined precisely, only probabilistically. A concept regarding the structure of the universe that again opposes our instincts is that of extra dimensions.

However, a new study demonstrates that the shapes of extra dimensions can be "seen" by deciphering their influence on cosmic energy released by the violent birth of the universe 13 billion years ago. The method provides evidence that physicists can use experimental data to discern the nature of these elusive dimensions - the existence of which is a critical but as yet unproven element of string theory. According to string theory, the extra dimensions could adopt any of tens of thousands of possible shapes, each shape theoretically corresponding to its own universe with its own set of physical laws.

What might a higher-dimensional form look like?
Our three-dimensional world includes the coordinates X, Y, and Z, extending infinitely throughout the universe. But, some researchers have proposed that extra dimensions may be finite, and compacted around a sphere, pole, or other geometrical shape. Others have said that quarks, the standard-model particles, may have "technicolor" cousins in another realm. Or, quarks and neutrinos may exist in a mirror-world, as "squarks" and "sneutrinos."

Why would it matter to us if the universe has more than 3 dimensions, if we can not feel them?
Well, in fact we could "feel" these extra dimensions through their effect on gravity. While the forces that hold our world together are constrained to the 3+1-"flat" dimensions, the gravitational interaction always occupies the entire universe, thus allowing it to feel the effects of extra dimensions. Unfortunately, since gravity is a very weak force and since the radius of extra dimensions is tiny, it could be very hard to see any effects, unless there is some kind of mechanism that amplifies the gravitational interaction. Such a mechanism was recently proposed by Arkani-Hamed , Dimopoulos, and Dvali, who realized that the extra dimensions can be as large as one millimeter, and still we could have missed them in our quest for the understanding of how the universe works!

Our search for extra dimensions is not over yet. In fact, it has only just started. We are looking for events where gravitons are produced in the collisions and then leave our three-dimensional world, traveling off into one of the other dimensions. This would cause an apparent non-conservation of energy from the point of view of our three dimensional world. Therefore we will have a significantly extended sensitivity to large extra dimensions. We very well might see them!

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