# Hawking's recent work: The Information Paradox

## Interview with

Stephen Hawking’s earliest work considered Black Holes and ultimately led to his discovery of radiation from black holes that is known as Hawking Radiation in his honour. But his exploration of our puzzling cosmos didn’t stop there: he continued exploring the workings of the Universe throughout his life. Speaking to Chris Smith, Claudia de Rham is a theoretical physicist at Imperial College London...

Chris - Claudia, Andrew Pontzen earlier rather craftily set you up in the sense that he made a reference to the “information paradox” and we said we would cover this later. Over to you, take up the baton and tell us what this is, and why it’s relevant to Stephen Hawking?

Claudia - As Andrew mentioned, one of Hawking’s biggest discoveries is the fact that a black hole can radiate, so they can evaporate; they can lose energy over time. That means if a black hole evaporates, after a finite time, they just disappear. There’s no more black hole there. And that’s really lead to one of biggest paradoxes in physics, which is the information loss paradox, because the information that leaks out through this Hawking radiation is minimal.

Chris - Can you just clarify what you mean by “information”?

Claudia - When you have a black hole, stuff may fall into the black hole. They may be stars, they may be other planets, there may be a whole civilisation and so they encode information. Just like a book would encode information, we have information about what’s going on in the star. Just think of it as knowledge. There’s some knowledge there.

Chris - It’s like order? So, in a disordered universe, you’re feeding material into a black hole that’s ordered and organised in a certain way, whatever that way is, so it is ingesting order?

Claudia - It is ingesting order, but really think of it as some type of knowledge that you send into the black hole. Now, in your everyday life, when you have information it gets moved around, it may change state, but the information remains conserved. So if you take a book - this is one of Hawking’s famous analogies - if you took a book and burnt it, the information of all the words that were written in the book is still there. You’re going to need to work extremely hard to retrieve it so you need to analyse all the fumes, and the temperature, and the ashes but ultimately it’s still there, the information is conserved. It may be harder to get access to but it’s still there.

Chris - So what’s the problem with the black hole then?

Claudia - While the black hole is still there, you may think well, the information is stored inside the black hole. Maybe you don’t have access to it but, physicists, it makes us happy because we know here’s where the information is stored. But now, if the black hole is able to evaporate and it entirely disappears, then where has the information gone? That’s one of the biggest paradoxes. It’s not only philosophical, it’s not just that we like to conserve information its that, from a quantum mechanical point of view, the information should be there, so where is it?

Chris - So something’s missing? Is there something missing from our understanding of how a black hole works? Is that information leaking out of a black hole in some other way, or is something else happening to the information to balance this equation and we’re missing a term somewhere? What do people like you speculate may account for this?

Claudia - Exactly. You should be a theoretical physicist! This is exactly the questions people are asking themselves and the answer is we don’t know. This is still an open question but Hawking himself, throughout his career, tried to come up with different possibilities.

When I was finishing my PhD in 2005, he had a revolutionary paper; he thought he possibly had an answer to that and that he had to concede a new configuration. He was using some of the framework he’s been working on earlier in his career on quantum gravity where you have the sum of all possible geometries, if you want, according to the laws of quantum mechanics. While the dominant ones may seem to be losing information, maybe you need to take into account all the ones, the sum of all of them, all of the information would be restored.

This was one possibility, but the community as a whole wasn’t necessarily entirely convinced. And then later on himself may not have been convinced.

Chris - So, in a nutshell, what can we learn from what Stephen Hawking has laid as the foundation for people like you are now working on?

Claudia - Throughout his career he several times tried to find different ways to tackle this problem and find different resolutions and he was never entirely satisfied and I think this a tremendous legacy for how research can be done. He really had physics at heart, well above his ego. He really wanted to get to the bottom of the questions and even if he may have thought he had a solution originally, he kept going on to try to understand what the real resolution could be.

Chris - He was certainly motivated to have perseverance wasn’t he?

Claudia - Exactly.

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