What are black holes? The facts, the theories, and the confusion about how black holes are formed and behave

Hear the words ‘black hole’ and you may think about a spinning vortex, sucking everything into its maw like your uncle at a wedding buffet. You may picture a star being pulled towards it like a piece of spaghetti, flapping around the void until it is slurped out of existence. The reality isn’t as simple, or as delicious. Here’s our primer.

What is a black hole?

Black holes have confounded physicists for centuries but a simple description would be an area in spacetime that has such strong gravity, even light can’t escape its pull.

One of the first things to understand about black holes is that they’re not empty space, but rather matter compacted into a very small area. A black hole may have the mass of 20 stars, for example, but only be the size of central London. That density does strange things to the laws of time and space, one of these being the appearance of a theoretical barrier called the event horizon.

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In Albert Einstein’s theory of general relativity, the event horizon is a point of no return. As an object gets closer to the event horizon, more and more paths taken by its particles bend towards the black hole. Once the event horizon has been breached, the deformation of spacetime becomes so great that there is no way for the particles to move outwards.

This is the point where the hole becomes “black”, when light is unable to escape from being pulled forward, onwards, towards a gravitational singularity; a point where spacetime has become so distorted, its curvature is infinite. Here, all the laws of physics as we understand them are thrown out of the window. No one really knows what happens at the singularity.

How big are black holes?

There are four classes of black holes, although two of these are hypothetical. At the top end of the spectrum are supermassive black holes, believed to reside at the centre of most galaxies. The one in our own Milky Way, known as Sagittarius A*, is thought to have the mass of 4 million suns with an event horizon that stretches 44 million kilometres.

At the bottom end of the spectrum is the micro black hole, which could be as small as a single atom, although we’ve yet to actually measure one this size. In between these two are hypothetical intermediate-mass black holes and stellar black holes – created by the collapse of stars with about three times or more the mass of our sun.

(Artists impression of the stellar black hole Cygnus X-1. Credit: NASA/CXC/M.Weiss)

Another intriguing possible category is the primordial black hole, first proposed by Stephen Hawking in 1971. These hypothetical black holes could have been formed during the birth of the universe, before stars came into existence, and may hold crucial explanations for the existence of dark matter.

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Efforts to find them have so-far come up dry, although this could change thanks to the Ligo-Virgo partnership and its detection of gravitational waves. Being able to detect ripples in the fabric of spacetime could pave the way for further investigations, looking back in time for black hole mergers that pre-date stars.

What would happen to me if I fell into a black hole?

If you were unlucky enough to find yourself being sucked into a black hole (my commiserations), some weird things are going to happen to you, and some even weirder things are going to happen to someone watching you from a safe distance.

What happens when an object breaches the event horizon is not easily comprehensible, and that’s because this is the point when two different models of physics – quantum mechanics and general relativity – contradict each other.

There’s an excellent write up of this quadrum – called the Black hole information paradox – by Amanda Gefter for the BBC. The gist of it is this: to someone watching you breach the event horizon, you would be slowly obliterated by the distortion of space, the slowing of time and the heat of Hawkings radiation – something that will also, by the by, eventually dissipate the black hole.

So you’re dead, right? Not exactly. According to general relativity, you would actually pass through the event horizon, not noticing the effects of gravity because you would be in freefall (something Einstein called his “happiest thought”), tumbling towards the singularity.

But how can you be travelling through space and be burnt to a crisp at the same time? This quandary happens because quantum physics says information cannot be lost, and therefore your body has to stay outside the horizon. If you don’t cross the event horizon, however, you’ll violate the laws of general relativity. Scientists have postulated a number of solutions to reconcile these conflicting laws of nature, but the paradox is an ongoing question mark at the heart of physics.

When it comes to black holes, then, it’s not only light that can’t escape, but also the brains of generations of thinkers.

Lead image credit: Wikimedia Commons