The great physicist Stephen Hawking died yesterday, as I’m sure you know by now. Hawking, and in particular his 1988 book, A Brief History of Time, got me interested in cosmology as a teenager, and while I’m a landlubber in any hard science, the interest has never worn off. I know that Hawking was important to many others for similar reasons.
Hawking’s most important scientific contribution was the idea of Hawking radiation, or radiation that comes from black holes. To landlubbers that sounds crazy—the whole point of the black hole is that it’s black, and nothing comes out of it. But if “things fall apart, the center cannot hold,” or if, as a scientist would put it, the entropy in the world always increases, then the blackness of the black hole is troublesome, because matter, along with the entropy it contains, can be lost to the world when it falls in. Hawking imagined that the entropy wasn’t really lost: the black hole had it. And since anything with entropy has a temperature, and anything with a temperature can radiate its heat, a black hole can radiate! But what about the fearsome gravity that has destroyed so many starships in science fiction novels, and from which not even light is supposed to be able to escape? Hawking reasoned that at the border separating the black hole from the rest of the world, the rules of quantum mechanics tell us that pairs of particles were constantly flickering into and out of existence, and that in some cases one of the pair would be drawn in toward the black hole while the other would escape, the escaping particles being visible to us as radiation. This predicted radiation will keep astronomers and physicists busy for the foreseeable future. Can we observe it? What about its philosophical implications for the conservation of energy in the world? Does it help us understand how gravity, the calling card of the black hole, is to be harmonized with quantum phenomena? Most ideas, like the doomed particles at the threshold of the black hole, live for only a very short time and have no lasting consequences in our world, but some, like Hawking’s, radiate outward.