First look
The idea, in plain words
A black hole is the one thing nothing escapes from. Whatever crosses its edge is gone for good, light included. So a black hole can only ever grow, and should last for ever. That was the settled view until 1974. It is probably wrong.
In that year Stephen Hawking, a 32-year-old physicist at Cambridge, did a sum that joined two parts of physics which do not usually meet: gravity, and the rules that govern atoms and smaller things. To his own surprise, the answer said that a black hole is not perfectly black. It should glow. The glow is very faint, and it comes from just outside the edge, so nothing has to break out from inside. But the energy for it has to come from somewhere, and it comes out of the black hole’s mass.
How faint? A black hole with the mass of the Sun would glow like something at 60 billionths of a degree above the coldest possible temperature. Space itself is warmer than that: the faded light of the Big Bang fills it at 2.7 degrees. So every black hole known today takes in more than it gives out. They are still growing. And the rule runs backwards from what you would expect: the bigger the black hole, the colder it is.
But the universe is cooling. One day, a very long time from now, space will be colder than the black holes, and they will begin to shrink. Then the backwards rule turns into a runaway. As a black hole loses mass it gets hotter. Hotter, it glows more brightly and loses mass faster. It ends in a flash. In its last second it gives out the energy of about five million of the biggest hydrogen bombs.
The times are beyond imagining. A black hole with the mass of the Sun would take about 2 followed by 67 noughts of years to disappear. The universe is 14 billion years old, which is 1 followed by only 10. The giant black holes in the middles of galaxies would take far longer still. If the theory is right, they will be the last things left, and then they will go too.
Is it right? Nobody has ever seen this glow. From any real black hole it is far too faint to measure. There is one hope. If very small black holes were made in the first moments of the universe, some would be reaching their final flash about now, and telescopes in space are watching for flashes of that kind. None has been found. Almost every physicist believes Hawking’s result all the same, because it fits so well with everything else.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- Event horizon
- The edge of a black hole: the boundary from inside which nothing can get out.
- Kelvin
- The temperature scale that starts at absolute zero, the coldest anything can be.
- Cosmic microwave background
- The oldest light there is: a faint glow of microwaves coming from every direction in the sky.
- Gamma rays
- Light with the shortest wavelengths and the most energy of all, made by the most violent events in the universe.
- Mass
- How much matter a thing holds. It is the same wherever you take it.
- Photon
- The smallest packet light comes in.
- Supermassive black hole
- A black hole of hundreds of thousands to billions of times the mass of the Sun, found at the centre of a galaxy.
Where this comes from
- Wikipedia, Hawking radiation
- S. W. Hawking, Black hole explosions?, Nature 248 (1974)
- NASA image library, Stephen Hawking at NASA's 50th anniversary lecture
- NASA image library, Fermi sees the gamma-ray sky
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.