Why black holes?
I thought there was a fascinating, and largely untold, story about how black holes, once considered so ridiculous as to not even be the preserve of science fiction, have moved relentlessly into the centre of science over the past century. They evidently play some key but mysterious role in the universe, creating all we see around us and even explaining why we are here at all. But what that is nobody knows.
What I am talking about is 'supermassive' black holes. There is one in the heart of every galaxy, and some have huge masses of tens of billions of times that of the sun. What they are doing there? How did they get so big so soon after the big bang? Are they the seeds around which galaxies of stars coalesced? Or did galaxies of stars form first and later spawn giant black holes in their hearts? Nobody knows.
Actually, it is likely you are reading these words because our galaxy, the Milky Way, has a tiddler of a supermassive black hole, thousands of times smaller than the biggest. The reason is the big black holes tend to have titanic jets, which lance outwards from their poles across millions of light years, driving away the gas, which is the raw material of stars, and snuffing out star formation. Our supermassive black hole, Sagittarius A* - incidentally, discovered 50 years ago this year – has never been powerful enough to do this. And so star formation has continued, and a star like the Sun and a planet like the Earth have been possible.
Oh, and I forgot to say, in 1972, my dad took me to a meeting of the Junior Astronomical Society in London. An astronomer called Paul Murdin described his discovery, with Louise Webster, of the first stellar black hole – Cygnus X-1. It blew my 12-year-old mind!
Why have you focused more on history and biography than usual?
All my books are different. Some are about people such as The Magic Furnace – one of my first books - and The Magicians. And some are pure science like Quantum Theory Cannot Hurt You, We Need to Talk about Kelvin and Infinity in the Palm of Your Hand. I really like telling the stories of the people who actually made discoveries because it makes subjects come alive and because it is possible to interweave often hard science in a hopefully painless way.
You mention in a footnote that Kerr has suggested black holes don’t have to be singularities - what would be the implication if this were true?
The implication is that Einstein’s theory of gravity might not break down at the heart of a black hole, as everyone suspects. In other words, it would be a reliable guide – if anyone could figure out how to use it – to what happens inside a black hole. The flip-side is that the “singularity” was thought to be where quantum theory takes over. If it isn’t, then where in the universe is a place that can give us a clue about how quantum theory – our description of the small-scale universe – meshes with Einstein’s theory of gravity – our description of the large-scale universe? These two theories – wonderfully successful in their own domains – must surely unify into a seamless picture of reality: quantum gravity. But how?
You’ve highlighted key points in our discovery of black holes - what do you think the next might be?
Did supermassive black holes originate before galaxies? Already, NASA’s James Webb Space Telescope is hinting that galaxies at the very dawn of the universe have unexpectedly large complements of stars and unexpectedly big supermassive black holes. It is hard to imagine how those black holes grew so big in the short time available since the big bang. Were supermassive black holes spawned by some as-yet-unimagined exotic processes in the fireball of the big bang? Maybe that is what we will discover next. But, frankly, there are so many things we don’t know about supermassive black holes, the next discovery could be anything!
What’s exciting you at the moment?
Space experiments like EUCLID and telescopes like the upcoming Vera C. Rubin Observatory in Chile will tell us how 'dark energy' has varied over the history of the universe. Dark energy is the major mass component of the universe, accounting for 70% of all there is. It is invisible, fills all of space and has repulsive gravity, which is speeding up the expansion of the universe. But, when we use our best theory of physics – quantum theory – to predict the energy density of the vacuum – that is, dark energy – we get a number which is 1 followed by 120 zeroes bigger than what astronomers observe. This is the biggest discrepancy between a prediction and an observation in the history of science. It’s fair to say that something in our science is badly wrong!
Previously, the biggest discrepancy between a prediction and an observation concerned the lifetime of atoms: theory predicted a lifetime that was 1 followed by 40 zeroes smaller than observed. That discrepancy, in the 1920s, was resolved by quantum theory, the biggest upheaval in science since the scientific revolution of the 17th century. Who knows – the dark energy discrepancy could spawn another revolution. But first we need to figure out how dark energy has evolved through time. That will at least be a clue to what dark energy is and what in physics needs to change.
Interview by Brian Clegg - See all of Brian's online articles or subscribe to a digest free here
Comments
Post a Comment