Leonard Susskind discusses duel with Stephen Hawking

For two decades, Stanford University physicist Leonard Susskind battled cosmologist Stephen Hawking over the behavior of black holes. Hawking said that when black holes eat their fill, they disappear, taking with them everything they consumed over their billions of years of existence. Susskind found this idea so disturbing that he publicly declared war – a conflict he describes in his new book, aptly titled “The Black Hole War.” In a conversation before a recent appearance at the Los Angeles Public Library, Susskind recounted his long struggle to “make the world safe for quantum mechanics.”

How did this war with Stephen Hawking come about?

I was a particle physicist when I was invited to an event at Werner Erhard’s house in 1981. Erhard (founder of the est self-awareness movement) admired scientists and liked to listen to them debate. At one of his events, I met Stephen Hawking. Stephen discovered an amazing fact, which is that black holes evaporate. It’s like a puddle of water out in the sun.

OK.

So the question is, what happens to the information trapped in the black hole? Stephen said it was lost forever. Stephen didn’t just say it, he proved it. At least he convinced himself and everybody else mathematically that it was true.

And you felt that was wrong.

It violates one of the fundamental principles of physics, which says nothing is ever lost completely. You may say, “How can you say information isn’t lost. I can erase information on my computer.” But every time a bit of information is erased we know it doesn’t disappear. It goes out into the environment. It may be horribly scrambled and confused, but it never really gets lost. It’s just converted into a different form.

In your book, you compare Stephen Hawking to the White Whale and yourself to Ahab.

I obsessed over this. This was never a matter of personal animosity. But he couldn’t see how damaging this would be to the rest of physics. And he didn’t see what a great resolution might come out of it if thought about in the right way. I love the man but I wanted to grab him by the neck and shake him a little bit. Stephen would just smile and say, “I’m right and you’re wrong.”

That’s a pretty heady debate for someone who started out as a plumber.

I was from a poor Jewish family in the South Bronx. My father was a plumber, but when I was 16 he got sick and I had to take over. Being a plumber in the South Bronx wasn’t fun.

When did physics come along?

I was going to engineering school but fell in love with physics. When I told my father I wanted to be a physicist, he said, ‘Hell no, you ain’t going to work in a drug store.’ I said no, not a pharmacist. I said, ‘Like Einstein.’ He poked me in the chest with a piece of plumbing pipe. ‘You ain’t going to be no engineer,’ he said. ‘You’re going to be Einstein.’

What is the great resolution you referred to?

One result is something called Black Hole Complementarity. Let’s say Alice falls into a black hole while Bob stays on the outside and watches. Nothing drastic happens to her when she crosses the event horizon (the point of no return around a black hole). Of course she’s eventually going to get it. On the other hand, there is another picture of the black hole, where every bit of information that you throw onto the horizon of a black hole gets sort of stuck on the horizon and builds up a soup of information bits. And this soup is hot, about a 100 billion, billion, billion degrees.

So Alice would get burned up?

We have a dilemma. One theory, based on general relativity, simply says Alice just floats past the horizon. That would be Alice’s view of things. But Bob’s view of things, if he believes in quantum mechanics, is that Alice falls into this soup of hot bits and her molecules are ripped apart. So, which one is correct? Alice can’t both be killed at the horizon and not killed at the horizon. The answer is they are both correct.

How can that be?

These two ideas are not in conflict because to be in conflict, there has to be a contradiction. Well, nobody can see a contradiction for the simple reason that nobody can send a message from the inside of a black hole. Alice can’t send a message saying, “Bob, I’m OK, don’t worry about me,” because the message can’t get out of the black hole. Yet everything Bob sees is consistent with saying that Alice was thermalized.

It’s difficult to see how both can be true.

We’ve had these things before in Einstein’s thought experiments. Einstein, in the special theory of relativity, proved that different observers, in different states of motion, see different realities.

There’s another strange theory that’s come out of this battle isn’t there?

Yes, the holographic principle. A hologram is a two-dimensional sheet, such as film, which codes three-dimensional information. A simple way to say it is that the black hole horizon is like a hologram. The horizon of the black hole is like the film, and the image is the stuff that falls into the black hole. It’s extremely unintuitive. But the bottom line is the exact description of a region of space, no matter how big, is in terms of a theory which you can imagine has a film on the boundary in which complicated and extremely scrambled versions of that space are going on. So in that sense, the universe is like a hologram.

Stephen now agrees that the information is not lost when a black hole evaporates.

Yes, he’s seen the light (pointing to a page in the book where Hawking’s concession letter appears). When he sees the light, he’s very magnanimous.

Are there are any evaporating black holes in our region of the universe?

No. They are all accreting [still eating]. Black holes are much, much colder than their surroundings in space. That means heat flows from the surrounding space into the black hole. If we wait for a long, long time the universe will expand, it’ll cool, and eventually empty space will become colder than the black holes. When that happens, they will start to evaporate. But don’t hold your breath.

 john.johnson@latimes.com