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By Root 677 0

you are worried, rest assured that these small, very short-lived black holes won’t pose a danger to us or to our planet: they’ll be gone well before they could do any damage. Black holes don’t last for ever: they evaporate by emitting radiation through the phenomenon known as Hawking radiation. But just as a small drop of coffee evaporates more quickly than a full cup, a small black hole evaporates more quickly than a big one, so the small black holes that could conceivably be produced at colliders will evaporate almost immediately. Nevertheless, if they are produced, these higher-dimensional black holes would last long enough to leave visible signs of their existence at a detector. They would have a very distinctive appearance since they would produce many more particles than you would find in ordinary particle decays, and these particles would go off in all directions.

Furthermore, if the ADD model is correct, black holes and KK partners of the graviton might not be the only exotic new discoveries. If ADD and string theory are both right, colliders could produce strings at very low energies, almost as low as a TeV. Once again, this is because the fundamental gravity scale is so low in the ADD models. Higher-dimensional gravity would become strong at about a TeV, and quantum gravity could contribute measurable effects.

The strings of the ADD theory would not be nearly as massive as the inaccessible Planck scale mass. If you think of strings as notes, the strings of the ADD proposal are far less high-pitched. The low-pitched strings of the ADD models would have mass not much bigger than a TeV. If we’re lucky, they’ll be light enough for the LHC to produce. Collisions with high enough energy would then produce the light strings of this model in abundance, along with new objects called string balls, containing many long strings.

However, despite the appeal of such potential discoveries, you should bear in mind that in all likelihood the energy at the LHC will be close to, but not as high as, the energy needed to make strings and black holes. Whether or not the ADD strings and black holes will be visible depends on the precise energy of higher-dimensional gravity (and, of course, on whether the proposals are correct).

The Fallout

The ADD proposal was fascinating. Who would have thought that extra dimensions could be so large, or that they could have so much bearing on problems of immediate interest (to particle physicists at least), such as the hierarchy problem? However, this proposal did not actually solve the hierarchy problem. It turned the hierarchy problem into another question: can additional dimensions be this large? This remains an outstanding question for the ADD scenario. Without some new and as yet undetermined physical principles, dimensions are not expected to be so extraordinarily large. At the very least, according to known theories, you would still need supersymmetry to maintain the large flat space that is needed for the ADD proposal. In essence, supersymmetry would stabilize and reinforce large dimensions that would otherwise collapse. Since one nice feature of ADD seemed to be that it could eliminate the need for supersymmetry, this is a bit disappointing.

The other weakness of the theory is its cosmological implications. For the theory to agree with known facts about the evolution of the universe, some of its numbers have to be very carefully chosen. And the bulk has to contain very little energy, or else cosmological evolution won’t agree with observations. Again, this might be possible, but the whole point of a solution to the hierarchy problem is to eliminate the necessity for large fudges.

Nonetheless, many physicists were open to the idea of taking extra-dimensional theories seriously and trying to devise ways to search for them. Experimenters, especially, were excited. As Joe Lykken, a particle physicist working at Fermilab, said to me when describing experimenters’ reaction to large extra dimensions, “To them, all this ‘beyond the Standard Model’ research is kooky. Supersymmetry or extra