Black Holes: The Reith Lectures

by Stephen Hawking

American scientist John Wheeler, who in many ways is the hero of the black hole story.

A singularity is what you end up with when a giant star is compressed to an unimaginably small point. This concept has been a defining theme in Stephen Hawking’s career. It refers not only to the end of a star but also to a far more fundamental idea about the starting-point for the formation of the entire universe. It was Hawking’s mathematical work on this that earned him global recognition.

Quasars are the brightest objects in the universe, and possibly the most distant detected so far. The name is short for ‘quasi-stellar radio sources’ and they are believed to be discs of matter swirling around black holes.

‘naked’ singularity is a theoretical scenario in which a star collapses but an event horizon does not form around it – so the singularity would be visible.

When John Wheeler introduced the term ‘black hole’ in 1967, it replaced the earlier name ‘frozen star’.

if you want to explore the inside of a black hole, make sure you choose a big one.

Although you wouldn’t notice anything particular as you fell into a black hole, someone watching you from a distance would never see you cross the event horizon. Instead, you would appear to slow down, and hover just outside. Your image would get dimmer and dimmer, and redder and redder, until you were effectively lost from sight. As far as the outside world is concerned, you would be lost for ever.

With no light escaping from the black hole, there is no way that anyone watching from a distance could actually witness your descent. In space, no one can hear you scream; and in a black hole, no one can see you disappear.

Entropy means the tendency for anything that has order to become more disordered as time passes –so, for example, bricks neatly stacked to form a wall (low entropy) will eventually end up in an untidy heap of dust (high entropy). And this process is described by the Second Law of Thermodynamics.

When a black hole is created by gravitational collapse, it rapidly settles down to a stationary state, which is characterized by only three parameters: the mass, the angular momentum (state of rotation) and the electric charge. Apart from these three properties, the black hole preserves no other details of the object that has collapsed.

The more disordered something is – the higher its entropy – the more information is needed to describe it. As the physicist and broadcaster Jim Al-Khalili puts it, a well-shuffled pack of cards has higher entropy than an unshuffled one and therefore its description requires far more explanation, or information.

Using the science of the very small to study the very large was one of Stephen Hawking’s pioneering achievements.

a vacuum is never totally empty. According to the uncertainty principle of quantum mechanics, there is always the chance that particles may come into existence, however briefly. And this would always involve pairs of particles, with opposite characteristics, appearing and disappearing.

there could be much smaller ‘mini’ black holes with the mass of say, a mountain. A mountain-sized black hole would give off X-rays and gamma rays at a rate of about ten million megawatts, enough to power the entire world’s electricity supply. It wouldn’t be easy, however, to harness a mini black hole. You couldn’t keep it in a power station, because it would drop through the floor and end up at the centre of the earth. If we had such a black hole, about the only way to keep hold of it would be to have it in orbit around the earth.

we might be able to create micro black holes in the extra dimensions of space-time.

According to some theories, the universe we experience is just a four-dimensional surface in a ten- or eleven-dimensional space. The movie Interstellar gives some idea of what this would be like. We wouldn’t see these extra dimensions because light wouldn’t propagate through them, but only through the four dimensions of our universe. Gravity, however, would affect the extra dimensions and would be much stronger there than in our universe. This would make it much easier for a little black hole to form in the extra dimensions.

The Nobel Prize in Physics is awarded when a theory is ‘tested by time’ which in practice means confirmed by hard evidence. For example, Peter Higgs was one of the scientists who, back in the 1960s, suggested the existence of a particle that would give other particles their mass. Nearly fifty years later, two different detectors at the Large Hadron Collider spotted signs of what had become known as the Higgs Boson. It was a triumph of science and engineering, of clever theory and hard-won evidence; and as a result Peter Higgs and François Englert, a Belgian scientist, were jointly awarded the prize. No physical proof has yet been found of Hawking Radiation and some scientists suggest that it will be too difficult to detect. Still, with black holes being studied in ever closer detail, some day confirmation may come.

A scientific law is not a scientific law if it only holds when some supernatural being decides to let things run and not intervene.

Since the Reith Lectures were recorded, Professor Hawking and his colleagues have published a paper which makes a mathematical case that information can be stored in the event horizon. The theory hinges on information being transformed into a two-dimensional hologram in a process known as supertranslation. The paper, entitled ‘Soft Hair on Black Holes’, offers a highly revealing glimpse into the esoteric language of this field – as the abstract reproduced at the end of this lecture shows – and the challenge that scientists face in trying to explain it.

What does this tell us about whether it is possible to fall into a black hole and come out in another universe? The existence of alternative histories with and without black holes suggests this might be possible. The hole would need to be large, and if it were rotating, it might have a passage to another universe. But you couldn’t come back to our universe.