Q&A: Hawking and black holes

Hawking says he has changed his mind about black holes

Stephen Hawking has put forward a new theory that changes the way scientists view black holes, saying he was wrong about them in the past.

The physicist told a conference on gravitation in Dublin that he has revised his belief that black holes destroy everything that falls on them.

Our science editor David Whitehouse explains black holes.

What is a black hole?

It is an object from which nothing can escape because its escape velocity is greater than the speed of light. Since nothing can travel faster than light, nothing can get out.

Inside a black hole strange things are done to space and time, and at its centre could be a so-called singularity where space and time are squeezed to an infinitely small point.

But some scientists are unhappy with this idea.

Have they been detected?

Probably. There are many objects that could be black holes seen orbiting stars. Often these objects pull material from their stellar companions and drag it on to themselves.

It becomes hot and gives off high-energy radiation. If astronomers are lucky they can use this radiation to determine the size and mass of the compact object.

It is also believed by many astronomers that there are supermassive black holes at the centres of many galaxies.

There are many lines of observational evidence, such as the motion of stars in the vicinity of these mysterious objects, which lend support to this idea.

What was Hawking's black hole theory?

In 1975, Hawking calculated that once a black hole forms, it radiates energy and starts losing mass by giving off so-called "Hawking radiation".

Scientists were astounded because this work established a connection between gravity and entropy, which is a measure of how energy changes from one form to another.

Entropy has a lot to do with the information in a system.

For example, a pile of bricks has more entropy than when they have been made into a house. It takes bricks and information to turn them into a house.

Can anything make a black hole?

Yes. It was said that black holes had no air, meaning that it did not matter what came together to make them.

All that mattered was that a sufficiently large mass be squashed into a sufficiently small space.

Before Hawking's latest thinking it was thought that, once formed, it would be impossible to tell what went in; once something had fallen in, it was lost forever and the only information that remained was its mass and spin.

What exactly has he changed his mind about?

Hawking now believes that black holes may allow information to leak out.

For several years many scientists had been unhappy with the idea that a black hole could just disappear, because it represented a loss of information from the Universe.

This ran contrary to the laws of quantum physics, which are the rules to describe the behaviour of the Universe at the smallest scales.

These laws say that information can never be totally lost.

Whether information is or is not lost has important practical and philosophical consequences.

Is it complicated?

Certainly is. Here is the summary of Professor Hawking's presentation.

Professor Stephen Hawking (Cambridge)

The information paradox for black holes

The Euclidean path integral over all topologically trivial metrics can be done by time slicing and so is unitary when analytically continued to the Lorentzian. On the other hand, the path integral over all topologically non-trivial metrics is asymptotically independent of the initial state. Thus the total path integral is unitary and information is not lost in the formation and evaporation of black holes. The way the information gets out seems to be that a true event horizon never forms, just an apparent horizon.