Over 100 years ago, in 1915, Albert Einstein developed his theory of general relativity. This was a mathematical framework which described the curvature of spacetime; how matter causes this curvature, and how this curvature affects the movement of matter and light. Just like bowling balls on a trampoline, massive objects curve the spacetime around them, causing “wells” which draw other objects towards them.
As if curved spacetime wasn’t already strange enough, only a few months later, another physicist, Karl Schwarzschild, found that Einstein’s theory seemed to predict some very strange behaviour near extremely massive, compact objects: black holes. The maths showed that an extremely dense, heavy object would have a surface around it where the equations “break”, with some of the terms becoming infinite. This is what is called the Event Horizon, and it is predicted to be a boundary which can only ever be crossed in one direction. Once this boundary has been crossed, nothing, not even light, can return. This is the point where the spacetime has been curved and warped so much that nothing can move fast enough to escape.
Over the last century, Einstein’s theory has evolved from abstract mathematics to being the most precisely tested theory in human history. The miniscule effects of the earth’s mass on the spacetime around it must be accounted for to extreme precision for such mundane things as allowing our GPS systems to work. The theory predicted gravitational waves; the rippling of spacetime caused by the collision of massive objects such as neutron stars or black holes, which were finally directly observed for the first time (by the LIGO experiment) in 2015. However, while the existence of black holes have been an accepted part of astrophysics for decades, being invoked to describe many different phenomena, they have never been directly observed.
The effects of black holes on the spacetime and material surrounding them are well studied. At the centre of our own galaxy, and at the centre of almost every large galaxy, there is known to be an extremely compact, massive, invisible object, which is taken to be a supermassive black hole.Their existence is fundamental to our current theories of galaxy evolution. However, we have not known what a black hole looks like, or if the event horizon is a real physical thing instead of just a quirk of mathematics. Until now.
On the 10th of April, the Event Horizon Telescope (EHT) collaboration revealed the first ever direct image of a black hole. This image, showing the supermassive black hole at the centre of the distant M87 galaxy, was 10 years in the making. The intricacies involved in this feat have pushed the observational abilities of the human race to the very limit.
Using general relativity, and state-of-the-art computer simulations, it was predicted that a black hole would appear as a shadow. The hot material it is swallowing up shines bright, however the region within the event horizon can emit no light, so when looking here there would be a circular region which would be emitting no light compared to the area around it. In real terms, this shadow would be roughly the same size as our whole solar system, however on a cosmic scale, this is tiny. M87 is so far away from us (55 million light years, or just over 30 billion trillion miles, that’s 30 with 19 zeros after it), that this is the same as trying to see a grape on the surface of the moon. Doing this would require a telescope the size of the planet.
This didn’t stop us. By using a network of radio observatories across the globe (from Greenland to Antarctica, and Hawaii to France), a special technique called long base interferometry allows the combined observations to reach the same resolution as a planet-sized telescope. This required extremely careful work. Each observatory had to take data at the same time remotely, using precisely synchronised clocks. This data was so large that it had to be transported physically by plane from each location to a central supercomputer where it was combined. This caused a delay in waiting for the Antarctic winter to pass and allow flights to reach the observatory. From the radio data, a lot of work had to be done to process the signals, removing any noise and combining them into an image.
The resulting image is incredible. It agrees perfectly with Einstein’s predictions from 100 years ago, and is almost identical to the simulations. The shadow is of a size which corresponds to a black hole with a mass 6.5 billion times that of the Sun. The brighter part on one side indicates that the material is spinning around the black hole; a relativistic effect makes the side which is moving towards us brighter than the side moving away. The predictions were so perfect that this almost feels disappointing, with no big surprises. However, this is just the beginning. By proving that imaging a black hole is possible, this opens a whole new field of astronomy for observational black hole research. The group has indicated that with further observation, there can be more information found on how jets can form from a black hole, the magnetic fields generated by the black hole, and more can be done to understand these enigmatic objects. For now though, it is truly breathtaking to watch the human race work together and now stand at the edge of a whole new era of astronomy.
One researcher from the EHT group stated that he would now look at his work in astronomy as being split into the time before this image was taken, and the time after. It is hard to view this as an overstatement of the importance of this discovery. This will be the image shown in every school science class, every science documentary, and every physics lecture, inspiring future astronomers for many years to come.
This post was written by Stephanie Campbell and edited by Ella Mercer.