Brief Answers to the Big Questions by Stephen Hawking

Author:Stephen Hawking
Language: eng
Format: epub, mobi, azw3
Publisher: Random House Publishing Group
Published: 2018-10-15T16:00:00+00:00

Since that time, the mathematical evidence that black holes emit thermal radiation has been confirmed by a number of other people with various different approaches. One way to understand the emission is as follows. Quantum mechanics implies that the whole of space is filled with pairs of virtual particles and antiparticles that are constantly materialising in pairs, separating and then coming together again, and annihilating each other. These particles are called virtual, because, unlike real particles, they cannot be observed directly with a particle detector. Their indirect effects can nonetheless be measured, and their existence has been confirmed by a small shift, called the Lamb shift, which they produce in the spectrum energy of light from excited hydrogen atoms. Now, in the presence of a black hole, one member of a pair of virtual particles may fall into the hole, leaving the other member without a partner with which to engage in mutual annihilation. The forsaken particle or antiparticle may fall into the black hole after its partner, but it may also escape to infinity, where it appears to be radiation emitted by the black hole.

Another way of looking at the process is to regard the member of the pair of particles that falls into the black hole, the antiparticle say, as being really a particle that is travelling backwards in time. Thus the antiparticle falling into the black hole can be regarded as a particle coming out of the black hole but travelling backwards in time. When the particle reaches the point at which the particle–antiparticle pair originally materialised, it is scattered by the gravitational field, so that it travels forward in time. A black hole of the mass of the Sun would leak particles at such a slow rate that it would be impossible to detect. However, there could be much smaller mini black holes with the mass of, say, a mountain. These might have formed in the very early universe if it had been chaotic and irregular. 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 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.

People have searched for mini black holes of this mass, but have so far not found any. This is a pity because, if they had, I would have got a Nobel Prize. Another possibility, however, is that 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 is like. We wouldn’t

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