October 17, 2021

I, Science

The science magazine of Imperial College

It’s been a very challenging year for Einstein: first the neutrino saga threatened his postulated upper limit to the speed of light, and now astronomers hope to use the Earth-sized Event Horizon Telescope to peer into the very heart of our galaxy and make the first ever observations of a supermassive black hole, again testing the theory of relativity.

Today astronomers from around the world will gather in Arizona for a conference on the technological means needed to make such observations, which only a few years ago would have been considered an impossible task. As it is, operating such an enormous telescope will require the co-operation of scientists from all corners of the globe.

If you’ve been keeping up with BBC’s Stargazing Live, you’ll have seen Professor Brian Cox explaining the “enigmatic inhabitants of the cosmos” that are black holes. Countless observations of galaxies and other astronomical objects appear to confirm their existence, but their very nature means that they are very shady characters (pun intended). Their intensely strong gravitational field means that not even light can escape their pull, resulting in an area of complete blackness. Considering the inky darkness of the surrounding space, it is perhaps not surprising that they are so elusive.

So, how can we image such an object? Supermassive black holes can be up to billions of times the mass of stars like our sun, and create such an effect that they twist, knot and warp the environment around them. Matter such as dust and interstellar gas swirls around such a hole, getting progressively faster and faster as it nears the centre. At such high speeds the matter slams together and compresses, and the incredible friction it experiences turns it into a scorching plasma – at temperatures around a billion degrees. These incredible temperatures cause the material to release energy and ‘glow’ in a detectable way.

However, this is a bit of a cheat – this still won’t result in images of the black hole itself. As the glowing material circles the black hole it will reach a point known as the Event Horizon – a point beyond which even light cannot escape the gravitational pull of the hole. Beyond this horizon, all we see is blackness – therefore, by imaging this radiating material astronomers will be able to capture an outline or shadow of the hole itself.

How does this affect Einstein? Well, general relativity predicts that this outline must be a perfect circle. If it is found to be any other shape, this will imply that elements of the theory must be incorrect – as was the case with the apparently superluminal neutrinos.

It is thought that there is a supermassive black hole at the centre of most, if not all, galaxies. This includes our own Milky Way, which is known to be home to around 25 small black holes that range from 5 to 10 times the mass of our sun. The black hole at the centre of our Milky Way is the chosen one to observe as it is the right size and the right distance from us – closer holes are too small, and bigger ones are unfortunately too far away.

Even if Einstein is found to be correct, the amount of information we stand to gain from exploring this black hole will be invaluable, and will help us to understand black hole theory to an unprecedented level.

For some basics on black holes and black hole theory (with the added bonus of the lovely Dara O’Briain to guide you), catch up with the aforementioned BBC Stargazing Live (originally airing 16/17/18 January on BBC2).

Image: NASA/Dana Berry