If you’ve ever wondered about the chances of accidentally stumbling on a black hole during a leisurely outing without your telescope, I’ve got bad news for you: black holes cannot be seen with a telescope. Or anything else for that matter: by definition, they are invisible. Or are they?
Ever since they were first theorized, black holes have captured both scientific interest and public imagination. Nowadays most scientists agree that they certainly exist, given the abundant observational evidence which has been uncovered.
But wait a minute, you might be thinking. Haven’t we already determined that black holes exist?
Well, not exactly. Though the possibility of black holes always lurked in the field equations of Albert Einstein’s General Theory of Relativity, it took another physicist by the name of Karl Schwartzchild to find a solution to these equations which predicted the existence of black holes.
But a mathematical solution isn’t the same as a real, observed phenomenon, and Einstein himself had his doubts about this prediction of his own theory. In fact, it took several decades before black holes began to be taken seriously as a physical phenomenon and not just a mathematical curiosity.
What is a Black Hole?
We tend to think of them as objects, but they can be more accurately described as places: regions of space which are causally disconnected from the rest of our universe. In General Relativity, mass and energy cause spacetime to become curved or warped to some degree. For a massive object like the Earth, this curvature is sufficient to hold smaller objects like the moon in orbit around it, and to keep us from floating off its surface into space.
If an object like a giant star is compressed beyond a certain limit, a sort of runaway collapse occurs until the density of the object goes completely off the scales, and the curvature of space around it soars to infinity.
It has become a gravitational singularity, essentially a single point containing all the mass of the original object. More importantly, there is now a boundary surrounding this point called the event horizon from which no information can escape. When we talk about black holes, we are usually referring to the space enclosed by the event horizon and not the singularity itself.
Going back to the original question, the impossibility of seeing such an object with a telescope becomes clear. To see something means to obtain information from it, whether by visible light or other types of emissions like x-rays, radio waves or infrared radiation.
Some information-carrying signal must either be emitted by or reflected from the object. Within the event horizon, all paths through space are bent toward the singularity and no information can escape, rendering black holes invisible by nature.
How to Detect a Black Hole
But is it possible to detect a black hole without actually seeing it? As it turns out, yes, at least in principle. Black holes can leave certain visible signatures outside of their event horizons, though for the most part it would take a professional telescope to detect them:
Accretion disk
Material close to the event horizon doesn’t all get sucked into the black hole. Some gases and dust may fall into a close orbit instead, accelerated by the extreme gravitational field until friction within the disk causes the temperature to rise enough for the disk to glow. Though accretion disks occur around regular stars, around black holes they are hot enough to emit x-rays. These x-ray emissions from accretion disks are a prime indicator for black holes.
Relativistic jets
In certain, mysterious circumstances, blazing beams of charged particles are ejected from the black hole, extending thousands of light years into space. Though their origin remains unclear, we know that the jets are emitted not from the black hole itself but rather by the spinning material of the accretion disk, releasing an incomprehensible amount of energy in the process. Such jets have been photographed by the Hubble and other space telescopes.
Gravitational effects
A black hole exerts the same gravitational influence on its surroundings as any object of the same mass. The main difference is that because the mass is compressed into such a small area, such effects become much more pronounced close to the event horizon (as in the case of the accretion disk).
Evidence for the presence of a supermassive black hole at the centre of our Milky Way galaxy, for example, was discovered by studying the orbital motions of stars close to the galactic core. Our current understanding of physics offers no other explanation for the motions we observe.
M87 Black Hole
But what about the much-celebrated 2019 photograph of the M87 black hole from the Event Horizon Telescope (EHT)? Hasn’t this image proved that it is indeed possible to see and photograph a black hole directly?
Again, not exactly. Though it was touted as the first direct image of a black hole, there are some important distinctions to be made here. As we’ve already discussed, black holes neither emit nor reflect any signal (if we ignore that troublesome Hawking radiation).
Though the black hole has certain properties which can be detected from the outside—mass, spin, and electric charge—the event horizon itself is a featureless boundary, indistinguishable from empty space.
What we are seeing in the M87 image is the accretion disk, warped by relativistic effects, surrounding a dark circular shadow which presumably contains the black hole. It is important to note that this circular shadow is not the event horizon, but rather an effect of light-bending around the event horizon predicted by General Relativity.
It is equally important, for those hoping to see this phenomenon for themselves, that this image was taken using radio telescopes which capture invisible radio waves, not visible light, allowing the EHT to see through the dense dust clouds surrounding the core of the galaxy.
Thousands of terabytes of radio data were collected from telescopes around the world, providing the unprecedented, mind-boggling resolution required to produce the image. But first all that data had to be calibrated and processed to render a coherent, visible image from the invisible wavelengths collected by the telescopes.
In Summary
Until we have a working theory of quantum gravity, we may never fully understand the inner workings of these celestial monstrosities. But though we can’t view black holes with our telescopes, we shouldn’t let that diminish our sense of awe and wonder as we scan the darkness above through the eyepiece. It is enough to know that they are out there, resisting our attempts to unlock their secrets.
