Caleb Scharf is Director of Columbia University’s Astrobiology Center, and his recent book is Gravity’s Engines: the Other Side of Black Holes; the American edition has the subtitle How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos which is more descriptive as well as more alliterative. Last night he gave a very entertaining talk at the Royal Institution based on his book, illustrated with some astonishing images and animations.
I have read quite widely about black holes, and although I am no physicist and have no understanding whatsoever of the mathematics underpinning the theories, I do consider I know a fair bit about them and understand the basic principles. Even so, Caleb Scharf’s inspiring talk caused me to buy a copy of the book at the end of the lecture.
The history of the study of black holes really began in 1767, with the Reverend John Michell postulating ‘dark stars’; his science was wrong, but his concept of a body from which light could not escape wasn’t. It wasn’t until the early twentieth century that physics reached the point where it could not only show that black holes could exist, but explain how – the work of Einstein forming the basis from which Karl Schwarzchild fleshed out the mathematics.
The statistics of black holes are mind-boggling. The Earth would produce a black hole with an event horizon 9mm in diameter; a black hole of ten solar masses would roughly fit inside the M25; the black hole at the centre of our galaxy affects the orbits of proximate stars so that its mass can be estimated at 4.3 million solar masses (this is quite modest compared with some elsewhere in the universe which can be measured in billions of solar masses).
Nearly all galaxies have a supermassive black hole at the centre, which can be detected as the energy of motion of matter falling into them is converted into electromagnetic radiation as the accretion disc orbiting the black hole loses matter across the event horizon, and also because space around black holes is extremely, well, extreme. The enormous mass which is rotating drags spacetime round with it, and being electrically charged produces enormous voltages. This conversion of matter to energy is six times more efficient than nuclear fusion.
The conclusion of the lecture was to ask ‘what does this energy do to the rest of the galaxy’. Recent research has shown that black holes have a fundamental effect on the rest of the universe, and on star and galaxy formation in particular, with black holes belching out bubbles into the atmosphere of their galactic cluster and sending out ripples, sound waves at a frequency a million billion times below anything human hearing can detect, with energies of 1037 Watts. This slows down the growth of galaxies, so the cosmic environment is a product of the co-evolution of black holes and galaxies.
Perhaps most exciting is the discovery that the black hole at the centre of the Milky Way is probably not as inactive as previously thought; an object has been detected falling towards the event horizon, and it will probably reach it in about six months’ time. The black hole at the centre of our galaxy might be due to burp a big bubble quite soon.