Knocking on Heaven's Door - Lisa Randall [85]
BLACK HOLES AT THE LHC
Black holes are objects with such strong gravitational attraction that they trap anything that approaches too closely. Whatever comes within a radius known as the event horizon of the black hole gets engulfed and becomes imprisoned inside. Even light, which seems rather inconspicuous, succumbs to a black hole’s enormous gravitational field. Nothing can escape a black hole. A Trekkie friend jokes that they are the “perfect Borgs.” Any object that encounters a black hole gets assimilated, since the laws of gravity dictate that “resistance is futile.”
Black holes form when enough matter gets concentrated inside a small enough region that the force of gravity becomes indomitable. The size of the region required to make a black hole depends on the amount of mass. Smaller mass must congregate in a proportionately smaller region, while larger mass can be distributed over a larger region. Either way, when the density is enormous and a critical mass is within the required volume, the gravitational force becomes irresistible and a black hole is formed. Classically (which means according to calculations that ignore quantum mechanics), these black holes grow as they accrete nearby matter. Also according to such classical calculations, these black holes wouldn’t decay.
Before the 1990s, no one thought about creating black holes in a laboratory since the minimum mass required to make a black hole is enormous compared to a typical particle mass or the energies of current colliders. After all, black holes embody very strong gravity, whereas the gravitational force of any individual particle that we know of is negligible—far less than other forces such as electromagnetism. If gravity jibes with our expectations, then in a universe composed of three dimensions of space, particle collisions at accessible energies fall far short of the requisite energy. Black holes do, however, exist throughout the universe—in fact they seem to sit at the center of most large galaxies. But the energy required to create a black hole is at least fifteen orders of magnitude—a one followed by fifteen zeroes—bigger than anything a lab will create.
So why did anyone even mention the possibility of black hole creation at the LHC? The reason is that physicists realized that space and gravity could be very different from what we have observed so far. Gravity might spread not just in the three spatial dimensions we know, but also in as-yet-invisible additional dimensions that have so far eluded detection. Those dimensions have had no identifiable effect on any measurement made so far. But it could be that when we reach the energies of the LHC, extra-dimensional gravity—if it exists—could manifest itself in a detectable manner.
As we will explore further in Chapter 17, the extra dimensions that were briefly introduced in Chapter 7 are an exotic idea—but have reasonable theoretical underpinnings and might even explain the extraordinary feebleness of the gravitational force we know. Gravity can be strong in the higher-dimensional world but diluted and extremely weak in the three-dimensional world that we observe, or—according to the idea Raman Sundrum and I worked out—it could vary in an extra dimension so that it is strong elsewhere but weak in our location in higher-dimensional space. We don’t know yet whether such ideas are correct. They are far from certain, but as Chapter