Knocking on Heaven's Door - Lisa Randall [87]
Some did, however, still worry that Hawking’s derivation, although consistent with all known laws of physics, could be wrong and that black holes might be completely stable. After all, Hawking radiation has never been tested by observations since the radiation from known black holes is too weak to see. Physicists are rightfully skeptical of these objections since they would then have to throw away not only Hawking radiation, but also many other independent and well-tested aspects of our physical theories. Furthermore, the logic underlying Hawking radiation directly predicts other phenomena that have been observed, giving us further confidence in its validity.
Nonetheless, Hawking radiation has never been seen. So to be super-safe, physicists asked the question: If Hawking radiation was somehow not correct and the black holes the LHC might create were stable and never decayed, would they be dangerous then?
Fortunately, even stronger proof exists that black holes pose no danger. The argument makes no assumptions about black hole decay and is not theoretical but is based instead solely on observations of the cosmos. In June 2008, two physicists, Steve Giddings and Michelangelo Mangano,39 and soon afterward, the LHC Safety Assessment Group,40 wrote explicit empirically based papers that convincingly ruled out any black hole disaster scenario. Giddings and Mangano calculated the rate at which black holes could form and what their impact would already have been in the universe if they were indeed stable and didn’t decay. They observed that even though we haven’t yet produced the energies required to create black holes—even higher-dimensional black holes—at accelerators here on Earth, the requisite energies are reached quite frequently in the cosmos. Cosmic rays—highly energetic particles—travel through space all the time, and they often collide with other objects. Although we have no way to study their consequences in detail as we can with experiments on Earth, these collisions frequently have energies at least as high as that which the LHC will achieve.
So if extra-dimensional theories are correct, black holes might then form in astrophysical objects—even the Earth or the Sun. Giddings and Mangano calculated that for some models (the rate depends on the number of additional dimensions), black holes simply grow too slowly to be dangerous: even over the course of billions of years, most black holes would remain extremely small. In other cases, black holes could indeed accrete enough matter to grow big—but they often carried charge. If these had indeed been dangerous, they would have been trapped in the Earth and in the Sun, and both of the objects would have disappeared long ago. Since the Earth and Sun seem to have remained intact, the charged black holes—even those that rapidly accrete matter—can’t have dangerous consequences.
So the only possibly dangerous scenario that remains is that black holes don’t carry charge but could grow big sufficiently quickly to be a threat. In that case, the Earth’s gravitational pull—the only force that could slow them down—wouldn’t be sufficiently strong to stop them. Such black holes would pass right through the Earth so we couldn’t use the Earth’s existence to draw any conclusions about their potential danger.
However, Giddings and