Memories of Hawking

I first saw Hawking when I was an undergrad at Princeton, sometime around 1981. I heard he was going to speak at the Institute for Advanced Studies - a bunch of buildings in the woods past the golf course. Back then it was easy to get in, even in the middle of the night: sometime my friends and I would walk over and admire the oriental rugs and the bust of Einstein. Now it's not so easy. But anyway, I went over and sat myself among the eager crowd of physicists waiting to hear what Hawking would say.

He gave a lecture on virtual black holes. He argued that microscopic virtual black hole / anti-black hole pairs were constantly forming and annihilating in the vacuum... just like other particles are always doing... but with a difference: they would gradually increase the entropy of the universe. This process, he claimed, made the future fundamentally different than the past.

This was wildly heretical - and later Hawking himself ceased to believe it. But it was a reasonable extrapolation of his earlier work on the decay of black holes. He had showed black holes emit thermal radiation and slowly shrink away. This process seems to increase the entropy of the universe. So, it made some sense that the same thing could happen with virtual black holes.

I barely understood his lecture, since while I'd been studying quantum field theory on my own, and taking a course on general relativity, I was far from an expert on either, and he was combining them in a brand-new way. Still, it was exciting. By the end of my undergraduate years I decided to work on quantum gravity. I eventually did, years later.

I saw him a few other times, including in the math department cafeteria in Cambridge. I was always too intimidated to talk to him - which was, anyway, a complicated and slow process, involving first a human translator who could understand his barely audible speech, and then later, as his disease progressed and technology improved, a computer.

One time that really sticks in my mind is the general relativity conference in Dublin in 2004. This is when he changed his mind about black holes increasing the entropy of the universe as they decay. There was a huge crowd in attendance, and reporters, who were kept at the back. People were trying to sneak in, so guards were checking IDs. There was a kind of circus-like atmosphere that's rare in physics conferences.

The photo shows Hawking in front of a copy of the famous bet he and Kip Thorne made against John Preskill. They bet that information was lost in black holes. Preskill bet they did not. The prize was an encyclopedia. Preskill won that bet, and chose an encyclopedia of baseball facts. Here you see him waving that encyclopedia over his head, which he won after Stephen Hawking gave his talk arguing that, contrary to his earlier claims, the entropy of the universe does not go up when black holes emit Hawking radiation.

This photo appeared in Time magazine. For more on this Dublin meeting, including some photos of my own, see this:

http://math.ucr.edu/home/baez/dublin/

Back in the 1970s, Hawking revolutionized quantum gravity and inspired everyone in that subject. His discovery of Hawking radiation is one of the few solidly accepted predictions about how quantum mechanics and gravity combine to yield strange new phenomena. It's been checked in many ways - calculations, not experiments, though experiments have been done on analogous physical systems.

But the can of worms opened up by this discovery has not yet been sorted out. In recent years, the so-called "firewall paradox" showed we're still far from understanding it. This paradox comes, in part, from accepting the claim that black hole decay does not increase the entropy of the universe. So, maybe Hawking was right in the first place.

There's a lot of thinking left to be done. He will no longer be with us to help out. Individuals come and go, but the grand tale continues.
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