If I ask you what you think the most fascinating thing in the universe is, what would you say? Of course, the obvious answer is The Nexus blog. However, there’s also something else out there that is spectacular on a similar astronomic scale. I am, indeed, referring to the mysterious matter-inhaling bodies known as… black holes. The name itself is enough to induce a feeling of helplessness and insignificance in oneself. Today we’re going to see what we can learn about a black hole’s true profile, and perhaps by the end they won’t appear to be so scary after all.
When you hear the words “black hole” being uttered, thoughts are generally conjured of a large purely black sphere floating in space that consumes all that approaches, bending space-time and breaking the laws of physics as we know them. This definition largely holds, but let’s try to refine it slightly from layman’s terms. A black hole is essentially an area in space that has such a strong gravity that not even electromagnetic radiation such as light can escape it. In order for it to achieve this, the black hole must be of infinite density. But make no mistake – this does not mean the black hole has infinite mass, it only means that it has zero volume. Since density is equal to mass per unit volume, as volume decreases to zero, density increases to infinity. The reason that the volume is zero rather than the mass being infinite can be intuitively seen from the creation of the black hole. Ignoring the specifics of the black hole’s formation which we shall go onto explore in greater depth later, consider some mass being compressed due to gravity within a volume of space. Normal matter is no longer compressible at a certain point due to electromagnetic repulsion between atoms, but if the gravity is strong enough, you might be able to get past that. You can continue compressing it infinitely, until is has zero volume, but still contains mass. The mass can’t just disappear or become infinite through this process. What results is a point in space that does not occupy any three-dimensional space yet still contains mass, known as a singularity.
A black hole has a certain area of influence, and anything that enters this area will never escape (well that’s what we think, anyway). The boundary surrounding this zone is known as the event horizon, and the space within that is what we refer to as the black hole, since anything within it is truly invisible. The reason it is named as such is because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if the event happened or not. For all black holes, the radius of the event horizon is equivalent to the Schwarzschild radius, which is the radius of a sphere such that if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface of the sphere would equal the speed of light. Karl Schwarzschild derived a formula from the theory of general relativity which could calculate this radius for any object.
R = 2GM/c2
We aren’t actually able to ‘see’ a black hole since ‘sight’ is made possible through the reflection of light from an object into our eyes. No light escapes a black hole, so we don’t see anything. Rather, we see a complete lack of anything, also known as nothing at all. We are only able to observe this absence of light when there’s material around the event horizon which has not been sucked in, and this reflects or projects light which is warped by the black hole’s gravity, an effect known as gravitational lensing. But again, this is somewhat of a lie. Currently, astronomical instruments don’t have the resolution to really see the disk of light surrounding a black hole, since they are so distant from us. Every ‘image’ that you see of a black hole is actually an artist’s rendering, rather than an actual picture. Don’t let their vivid creative skills fool you.
However, this may be about to change. The Event Horizon Telescope is a project where radio telescopes from around the world will have their data combined through interferometry in order to produce images of the Milky Way’s black hole, Sagittarius A*. In effect, the telescope network acts as a single Earth-sized telescope, giving it a very high resolution. The images of the black hole and its surrounding materials will allow astronomers to study in greater depth the structure of the disk around the black hole and how the black hole feeds off it, as well as allowing them to test Einstein’s general relativity at the extreme. The telescope came online on the 5th of April, and will observe for about a week and a half, until this Friday. The information and data collected, however, will be so immense that it’ll take a lot of time to process, perhaps a few months. The wait will be worth it, as we will finally have our first picture of the immediate region around a supermassive black hole.
I will keep you updated on the progress of this project, as well as provide more information as to the formation of a black hole and how it functions in upcoming posts, so stay tuned. I hope you can agree that black holes are truly fascinating… they just seem to suck you in.