It may well have been the liveliest hour and a half I’ve ever spent in the company of theoretical physicists. In April, during a workshop I was attending on black holes, Bill Unruh gave a talk that challenged his colleagues on a point almost all of them thought had been settled in the mid-1980s. His colleagues challenged him back. The room throbbed with debate. At most conferences I’ve been to, one speaker presents his or her ideas, the next speaker presents his or her ideas, which might be exactly the opposite, nobody responds to what any else says, and nothing gets resolved. Everyone shuffles off to lunch, leaving onlookers not knowing what to think. Well, I still don’t know what to think of Unruh’s arguments, but it was invigorating to see great minds engage with one another.
Unruh bit off a piece of the central question in the search for a unified theory: What happens to stuff that falls into a black hole? No place else in the universe brings modern theories into such direct conflict. Einstein’s general theory of relativity says black holes are one-way streets: their gravity is so intense that nothing going down the drain can ever get back out again. Quantum theory says black holes are two-way streets: all processes are reversible in time, so whatever falls in has to be able to get back out in some form or other.