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By Root 746 0

scales we have experimentally studied. And even if we don’t know the most basic elements of spacetime, we do understand its properties for distances far away from the Planck scale length. In those regimes, we can apply physical principles we understand and deduce the sorts of consequences I’ve described. We’ve encountered many unexpected features of extra dimensions and branes, and those features might play a critical role in solving some of the puzzles of our universe. Extra dimensions have opened our eyes and our imaginations to amazing new possibilities. We now know that extra-dimensional setups can come in any number of shapes and sizes. They could have warped extra dimensions, or they could have large extra dimensions; they might contain one brane or two branes; they might contain particles in the bulk and other particles confined to branes. The cosmos could be larger, richer, and more varied than anything we imagined.

Which, if any, of these ideas describes the real world? We’ll have to wait for the real world to tell us. The fantastic thing is that it probably will. One of the most exciting properties of some of the extra-dimensional models I’ve described is that they have experimental consequences. I can’t overemphasize the significance of this remarkable fact. Extra-dimensional models—with new features that we might have thought were either impossible or invisible—could have consequences that we might see. And from these consequences, we might be able to deduce the existence of extra dimensions. If we do, our vision of the universe will be irrevocably altered.

There might be tests of extra-dimensional spacetime in astrophysics or cosmology. Physicists are now developing detailed theories of black holes in extra-dimensional worlds, and have found that although they are similar to their properties in four dimensions, there are subtle differences. The properties of extra-dimensional black holes could turn out to be sufficiently distinctive that we will be able to discern recognizable differences.

Cosmological observations might also ultimately tell us more about the structure of spacetime. Observations today probe what the universe looked like billions of years ago. Many agree with predictions, but several important questions remain. If we live in a higher-dimensional universe, it must have been very different earlier on. And some of those differences might help to explain perplexing features of observations. Physicists are now studying the implications of extra dimensions for cosmology. We might learn about dark matter hidden on other branes, or cosmological energy stored by hidden higher-dimensional objects.

But one thing is certain. Within the next five years, the Large Hadron Collider particle accelerator at CERN will turn on and probe physical regions no one has ever observed before. My colleagues and I are eagerly awaiting that time. The LHC is a great bet—for scientists it doesn’t get much better. Experiments at the LHC will almost certainly discover particles whose properties will give us new insights into physics beyond the Standard Model. The exciting thing is that no one yet knows what those new particles will be.

During the time I’ve been doing physics, the only new particle discoveries have been particles that theoretical considerations told us we were pretty sure to find. Not to undermine those discoveries—they were impressive accomplishments—but finding something genuinely new and unknown will be far more thrilling. Until the LHC starts running, no one can be really certain where to best concentrate their efforts. Results from the LHC are likely to change the way we view the world.

The LHC will have enough energy to produce the new types of particle that promise to be so revealing. These particles could turn out to be superpartners or other particles that four-dimensional models predict. But they might also be Kaluza-Klein particles—particles that traverse extra dimensions. If and when we see those KK particles will depend entirely on the size and shape of the cosmos in which we live. Do we live