BernardWhiting

Gravity explains how heavenly bodies, such as stars and planets, hold together, while quantum mechanics explains what are the properties of ordinary matter. We are trying to understand how these two theories can work together when stars become very compact and collapse to form a black hole. So far, our understanding suggests that black holes should not remain black, but should eventually evaporate. This leaves us with a puzzle about how information of what formed the black hole can still be preserved after the end of the evaporation. That is the puzzling problem we have been discussing all week.

As Francesca Vidotto explained in answering another question, a theory in which information is not preserved is usually considered to be problematical from a physics perspective. Normally, quantum mechanics does preserve information very well, but there is a potential problem when black holes are introduced. In 1976, Stephen Hawking described this problem in a paper entitled "Breakdown of predictability in gravitational collapse" published in Physical Review D. Ever since then, there has been a quest to discover a theory in which this breakdown would be absent. That topic was the focus of our meeting here at Nordic this week.

We think the detection of gravitational waves would be cool. They are predicted by Einstein's theory of gravity but, so far, their effects have only been seen indirectly, such as in the slow spiral of two neutron stars towards each other as energy is carried away by the gravitational waves. Next month, the upgraded LIGO gravitational wave detectors will begin taking science data again. We look forward to their success.

At our conference, we are looking forward to another breakthrough. For forty years we have understood that black hole horizons can have an entropy, and we have thought of this in terms of them hiding the information about how the black hole was formed. At out meeting, Stephen Hawking and Malcolm Perry proposed a new way for how the black hole horizon might encode that information. We will consider it a great achievement if we can explain how that information may be carried away from the black hole (and hence not lost) as the black hole evaporates. That's the really cool result all of us at the conference trying to understand.

It is surprisingly simple to name those theories, namely Einstein's theory of gravity and the theory of quantum mechanics, which explains the properties of ordinary matter, such as tables and chairs. At their core, these theories are both rather complicated. Nevertheless, the reason you fall down toward the ground (and not just float as if in free fall) is due to the influence of gravity, the lasers in your CD player were developed through our understanding of quantum mechanics. And there is one more thing. Stephen Hawking's discovery that black holes can evaporate meant that they could also be described as having a temperature. Temperature and entropy (which measures information loss) are both notions in thermodynamics, which is believed to apply to all physical systems. So that too is always part of our discussions. Your AC or heat exchanger work by using thermodynamical principles and, of course, energy.

Yes, you cannot just make information disappear. It takes some some energy to do a calculation and create a piece of information. In her talk at this conference, Fay Dowker showed how to replace some of our usual energy arguments by arguments about entropy instead. Nevertheless, energy and entropy are not the same thing.

So here is one of the problems we used to help us understand all this. Suppose you write a message on a piece of paper, and then burn the paper with the message on it. We would argue that all the information in the message must be contained in the motion of the molecules and heat radiated from the burning paper. But you need never fear that someone would be able to gather the molecules and heat photons and reconstruct your message, so the information in your message would be effectively lost, and even safely lost if that is what you intended.