EARLY one Saturday when I was 10, my mother dragged me across Los Angeles to see, of all things, a documentary. It was Errol Morris’s A Brief History of Time, about Stephen Hawking’s life and work. As Hawking discussed the “singularities” at the centre of black holes, I was stunned to learn that there was something Albert Einstein had been unable to resolve. That black hole captured me for life.
Black holes gain form in Einstein’s general theory of relativity, which proposes that space and time are unified into a space-time curved by the presence of massive objects. General relativity encourages us to move away from thinking about a mysterious force called gravity. Instead, it says that a body such as the sun is so massive that it bends space-time. Planets orbit the sun because the straightest line they can take in space-time near it is an elliptical path around it.
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Take Einstein’s picture to its logical conclusion and you end up asking yourself what happens when an object is so massive that it effectively folds space-time in on itself. This is a black hole.
Black holes are often described as being akin to a deep well: if you fall in, you can’t get out. But what happens inside its boundary, or “event horizon”, is even more fascinating. The normal properties of space and time seem to switch places. In everyday life, we can only move forward in time, but inside a black hole, things can only move forward in space – like being on an irreversible conveyor belt.
What is that conveyor belt moving towards? Mathematically, space-time curvature becomes infinite at a black hole’s central singularity, whatever that means. But we don’t know whether that reflects physical reality or just the theory breaking down.
As a university student, I wanted to get involved in the latest research on black holes. I learned the hard way that this isn’t straightforward. There is the way black holes are discussed in popular literature and the type of research portrayed in Morris’s documentary, focusing on the quantum properties and space-time properties of black holes. Then, there are astrophysical black holes: real objects that seem to act like black holes and form, we think, when massive objects such as stars collapse.
It is hard to see black holes, so black hole astrophysics focuses on observations of what happens near them. As exotic as black hole theory seems, we apparently have one supermassive black hole relatively close to us. Sagittarius A* is a bright radio source at the Milky Way’s centre, and we are fairly certain that it is a black hole with the mass of a few million suns.
“There is the way black holes are discussed in popular literature and then there are black holes that really exist”
Some of the most intriguing questions about black holes come from studying objects known as active galactic nuclei (AGN): extremely bright, compact galactic centres believed to have black holes of even greater mass at their cores. These come in a variety of classes. Quasars give out a lot of radio waves – they are “radio-loud”. But maybe my favourite AGNs are blazars, which are not only very bright and radio-loud, but also spew out jets made of particles travelling close to light speed. Now there’s a thing: if black holes suck in and hide everything, why is it that some of them have particles flying away at high speeds? And why do only some do this, and not others?
, but we can only see some of them. This would hardly lessen the core mystery, however, and that is before you get to the fact that blazar jets and their host galaxies have a variety of different colours that change over decades.
The exciting thing is that we are making progress on a lot of fronts. from a team led by at Dartmouth College in New Hampshire showed that we might be able to find a unifying model of blazars that explains how their colour changes in time.
Meanwhile, the big development earlier this year was when the Event Horizon Telescope imaged the region immediately at a black hole boundary. Over the past two years, gravitational wave experiments have also found multiple small, star-mass black holes – most recently seeing one eating a neutron star.
After an undergraduate thesis focused on astrophysical black holes and doctoral work that included considering how black holes would work if gravity were slightly different from how Einstein proposed, I have moved on to thinking about neutron stars, which are thought to form when objects not massive enough to make black holes collapse. But for me, as for many others, black holes will remain my first gateway to the wonders of the universe.
- This column appears monthly. Up next week: Graham Lawton
