This visualization shows gravitational waves emitted by two black holes of nearly equal mass as they spiral together and merge. Orange ripples represent distortions of space-time caused by the rapidly orbiting masses. These distortions spread out and weaken, ultimately becoming gravitational waves (purple).
Hawking @ Harvard
At packed Sanders Theatre, theoretical physicist and cosmologist tackles the contradictory qualities of black holes
It's said that fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamed up by science fiction writers, but they are clearly matters of science fact.
Apart from mass, angular momentum, and electrical charge the black hole preserves no other details of the object that collapsed. For example, the final black hole state is independent of whether the body that collapsed was composed of matter or antimatter, or whether it was spherical or highly irregular.
This created a paradox about the nature of black holes. One theory suggested that black holes with identical qualities could be formed from an infinite number of different types of stars; another suggested that the number could be finite. This is a problem of information.
If the information about the bodies that form black holes is not lost, then black holes contain a lot of information that is hidden from the outside world. If the amount of hidden information inside a black hole depends on the size of the hole, one would expect, on general principles, that a black hole would have a temperature and would glow like a piece of hot metal. But that was impossible because, as everyone knew, nothing could get out of a black hole - or so it was thought.
In early 1974, Hawking began to challenge that axiom when he discovered particles emitting from a black hole at a steady rate.
That outflow, later dubbed Hawking radiation, was among the key ideas that revolutionized science's understanding of black holes by suggesting that at least some energy could be emitted by the mysterious phenomena.
What happens to all the particles that fell into the black hole? They can't just emerge when the black hole disappears. The particles that come out of a black hole seem to be completely random and bear no relation to what fell in. It appears that the information about what fell in is lost, apart from the total amount of mass and the amount of rotation.
If that information is truly lost, Hawking said, that strikes at the heart of our understanding of science.
For more than 200 years, we have believed in the science of determinism. If information were lost in black holes, we wouldn't be able to predict the future because the black hole could emit any collection of particles.
It might seem that it wouldn't matter very much if we couldn't predict what comes out of black holes — there aren't any black holes near us. But it's a matter of principle. If determinism — the predictability of the universe — breaks down in black holes, it could break down in other situations. Even worse, if determinism breaks down, we can't be sure of our past history either. The history books and our memories could just be illusions. It is the past that tells us who we are. Without it, we lose our identity.
To understand whether that information is in fact lost, or whether it can be recovered, Hawking and colleagues are currently working to understand “supertranslations” to explain the mechanism by which information is returned from a black hole and encoded on the 'event horizon'.
Following the lecture, Hawking answered three questions from audience members, including one about his recently announced Breakthrough Starshot project, which aims to send probes to the nearest star system, Alpha Centauri.
“The Solar System contains nowhere that is as favorable to human life as the Earth,” Hawking said. “The moon is small and has no atmosphere. Mars is also smaller than the Earth. It has a thin atmosphere, but it is not enough to breathe or protect us from cosmic radiation, so astronauts will have to live underground. To find somewhere like the Earth, we have to boldly go to the stars.”
