The Black Hole
Of all of the absolutely fascinating concepts of astronomy and physics which we can encounter, and bump into throughout our lives — one stands out still as the most mysterious, the most captivating, and in many respects, the most scary: The Black Hole.
In this article I will attempt to write everything I know about black holes, followed by my questions about them. These questions I have thus far not found the answers for. I know that perhaps I would find the answer to some of these questions if I looked into it more deeply, however, one does not have infinite time to research everything one is interested in! And often once you’ve dug into a certain area of astrophysics deeply enough you find that scientists themselves are, of course, still discovering things.
What is a black hole? An explanation from scratch.
In order to understand black holes, certain fundamental building block concepts need to be at least mentioned beforehand:
One of the best definitions I’ve ever heard for black holes was this; “gravity gone mad”. So I guess any explanation of a black hole starts from what gravity is. Gravity is the law that any object, no matter how light or heavy, attracts other objects to it. For example, when a guy walks past a cute girl, some magical property of physics will propel that guy towards the girl. It’s undeniable, we’ve all experienced the “attraction” of gravity.
But in all seriousness, the gravity we exert on each other is actually extremely infinitesimal. The aforementioned “gravitational attraction” is likely more like “hormonal attraction”. The gravity that something like our earth or the moon exert are a lot more tangible. For example, it is the gravity of the moon that causes tides, palpable and real to us.
Now, over to light. We all know what light is (to a basic level) — interestingly, gravity also attracts light. As invincible as light may sound, and as fast as it may move, it is still finite. What is light? It can be represented by several models, light is an electromagnetic wave, light can also be represented by photons, which in essence represent a quantised amount of light of a very specific frequency. Either way, it is important to understand something quite significant: light moves at the fastest possible speed. The speed of light is 299 792 458 m / s which has also been called “the ultimate speed”. By calling it that, that is a reference to the fact that no matter can be accelerated to that speed, (or it could, but it would theoretically take infinite energy).
The “space-time continuum” is not merely some science fiction concept. Space-time is the fabric of space, it determines, by Einstein’s “special relativity”, the relative progression of time as viewed from other inertial reference frames. (An inertial reference frame is a point of view based on where you are and at what speed you and your surroundings are moving).
I guess of these concepts the hardest to understand would be space-time. An easy way to imagine it is as a kind of three-dimensional matrix (space) and then a fourth dimension (time) that covers the whole universe.
The escape velocity is how fast an object has to travel away from another object in order for it to completely escape that objects gravitational field. This concept is critical to understanding black holes. For example, if you wanted to completely escape the earth’s gravitational field you’d have to travel straight upwards at an initial speed of 11.2 km/s. That’s quite the high jump.
Definition: The black hole
With these concepts put into place as building blocks, defining the black hole is really quite simple.
Definition: A black hole is an object that has sufficient mass within in compacted in a small enough area such that the escape velocity at the surface of that object is greater than the speed of light. What does that mean? It means unlike our earth where we can escape its gravity if we move fast enough, with a black hole, there is literally no escape. Even light, which moves faster than anything else in the universe cannot escape it.
More Advanced Topics
The Event Horizon
The gravitational effect an object exerts decreases with distance. This means the further you are away from an object, the less you’ll feel its gravitational effect. With Black Holes, the same principle applies. Since at the surface of a black hole, the escape velocity is greater than the speed of light, as you move further away from the surface, the escape velocity decreases. At some point you will reach an imaginary “sphere” around the black hole wherein the escape velocity at the surface of that sphere is exactly the speed of light, that imaginary sphere is called the event horizon.
The reason it is called the event horizon is interesting. It is because nothing inside that sphere escapes (even light itself) — which means we have no information about what goes on inside that horizon. The coolest movie in the universe could be playing in there, and we’d have no idea. Hence any event that happens inside that horizon has no influence whatsoever on the outside universe. This is because light in astronomy and astrophysics is information, and that’s why we have those big fat radio telescopes out in the outback somewhere, to absorb light in the radio frequencies.
The Schwarzschild radius
Look let’s face it, you know you’ve hit it big as a scientist when something gets named after you. Karl Schwarzschild was a clever German cookie who sat down one day and realised something; if we could take any object, any object at all, and compress the matter in that object into a small enough area, we could make a black hole, albeit a small one.
The Schwarzschild radius is the theoretical event horizon radius of that object, if it were compressed small enough. For example, if we could take all of the matter of the earth and compress it small enough, we’d make a black hole with an event horizon of 9.0 mm. A nine millimetre radius black hole. But never fear, just like all black holes, as it sucks in other matter it would grow larger. Please note when we say “larger” we actually mean “more massive”, since as black holes clearly show, size is not everything! (they’re actually quite small)
If the sun were compressed small enough, it’s Schwarzschild radius would be about 3.0 KM.
The Accretion Disc
It’s a bit hard to imagine how “things” would generally enter a black hole. Should they form a queue? Is it first come first serve? Actually what generally happens is an accretion disc forms around the event horizon. The accretion disk is the disk of debris which orbits around stellar objects, in this case, the black hole, as matter approaches black holes, due to the spiralling motion of the black hole, matter goes into a disc, ever spiralling inward. This disc goes on until the event horizon, at which point no information relating to the matter there is receivable.
Supermassive Black Hole
Have you ever wondered why the spiral arms of the milky way keep rotating around the centre? Me neither. It’d probably have something to do with “dark matter” which is just a place-holder word because we actually have no idea why they rotate so fast. But in all seriousness, one of the reasons why our galaxy has a centre at all and has such a complex, beautiful structure around its centre is because there is a supermassive black hole in the middle that attracts all of the matter of the galaxy around it. The milky way’s supermassive black hole is believed to have around 4 million solar masses — that’s a lot of stuff in there keeping things in order.