Apologies for the lengthiness between posts all! We had a combination of immense amounts of work and a slightly confusing topic (though, mainly the former). But wait, that’s a perfect progenitor to create the title object!
Black holes—one of the great mysteries of the universe. They made Stephen Hawking’s head hurt, helped spawn relativistic theory…they are, indeed, wonderful objects that may or may not confuse the heck out of you.
In general terms, black holes are warped regions in spacetime where singularity points (singularities) with infinite density have enough gravity to pull in anything—including light, which means that we can’t see them. It is infinitely dense because density = mass/volume, and the volume of a singularity is a point or zero. There are a lot of technicalities in black holes—we’ll cover them one by one.
Firstly, let’s look at how black holes are formed. They are giant—maybe even supergiant—stars that explode into a supernova and then collapse in on themselves. Normally, they would turn into a neutron star and stay there, but if the neutron star exceeds the Tolman-Oppenheimer-Volkoff limit, then it collapses and turns into—drumroooooll…a black hole.
Now we’ll go over general black hole anatomy. The fabric of spacetime is usually smooth and even, with paths going out equally in all directions that can be traveled on. A black hole warps that—it makes a hole, essentially, that pulls everything in. The paths don’t go equally in all directions anymore, but instead all lead towards the center of the black hole. The centre of a black is a singularity of no mass, which warps spacetime to an infinite degree. Around it, there is this thing called an event horizon, which is the “point of no return”. Once an object goes past the event horizon, it cannot escape the black hole’s gravitational pull. A black hole may grow in size by accreting other objects of mass (contrary to popular belief, black holes do not “suck”), slowly growing larger and larger.
A black hole has three independent physical properties (which means properties that can be observed from outside the black hole): mass, charge, and angular momentum. Any two black holes found to have the same values for all of these are said to be indistinguishable and therefore black hole “twins”, if you will. This is what we call the no-hair theorem. There are two main kinds of black holes—the kind that rotate, and the kind that don’t.
The ones that don’t rotate are called Schwarzschild black holes. These black holes have no electric charge or angular momentum, but only mass. They are as simple as you get—unmoving, gravitational vortexes in the spacetime continuum. These are also just about the only objects in the universe that are perfect—and I mean perfect—spheres. They’re also special in another way—once you get past the event horizon, there is absolutely no way you will be able to avoid getting sucked into the singularity. None. You don’t stand a chance against the forces of a black hole.
Kerr black holes, on the other hand, do rotate. There is a theory—just a theory, mind—that states that it is possible, under perfect conditions, for matter to travel through its rotating ring of a singularity. The theory says that when you exit the black hole, you will most likely end up in another universe, with the black hole acting as an Einstein-Rosen bridge, or wormhole. This is because the spinning produces what’s called frame dragging, producing an ergosphere. This actually spins the fabric of space time, making objects able to travel around the black hole, and sometimes around the singularity as we said.
Theoretically, charged black holes exist as well, but in reality, the charge tends to attract particles of the opposite charge, which quickly cancel it out. Charged black holes that don’t rotate are called Reissner-Nordström black holes, and the ones that do rotate are called Kerr-Newman black holes.
There is also something called the Schwarzschild radius (we know, we know, you’re tired of hearing about Schwarzschild, but bear with us) that tells you a lot about this kind of black hole. The Schwarzschild radius is the radius at which the Schwarzschild metric, or gravitational field of a black hole, becomes singular—it becomes one, massless point. Therefore, the Schwarzschild radius is often referred to as the radius of a black hole and gives its size. The event horizon is actually situated right at the edge of the Schwarzschild radius, so the radius can even be thought of as the distance from the singularity to the event horizon. The weirdest part to this is that for the whole black hole itself this gives it an actual density. Okay, maybe what would be weirder is that some black holes can have a low density.
The Schwarzschild radius (derived from escape velocity for those who are curious) is given by the equation , where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light.
Another way to change up black holes is by mass, which is shown with that there Schwarzschild radius. Black holes are normally created by stars with masses between lower and upper mass limits of about 3 to 100 solar masses. In real life…weird results have come up with impossibly massive stars or stars very close to the lower limit). But even then, there are all sorts, theoretical or not. The types include:
- Micro black holes—Hawking theorized that low-mass black holes could be formed by high pressures in the early universe, if these exist, they might emit bursts of Hawking radiation in the (relatively) near future as they evaporate. They also may, for an instant, be created here on Earth in the LHC! Alternatively, we hear of work being done with lasers to make them (oh, how science is awesome).
- Stellar-mass black holes—3 to 15 solar masses, the major example used is usually Cygnus X-1, and they can form high energy X-ray binaries.
- Intermediate mass black holes—100 to 1000 solar masses, theoretically the center of globular clusters, may form from mergers of stellar-mass black holes, and may eventually merge into a supermassive black hole.
- Supermassive black holes—10^5 to 10^9 solar masses, the theoretical centers of spiral galaxies, thought to explain AGNs and quasars.
- Primordial black holes—very large and small ones that formed in the early universe, 10^-8 to 10^5 solar masses. They aren’t fully understood.
Black holes also have another effect called gravitational lensing, which is when gravity causes light to bend so much that we can see multiple images of the same object. We follow the light rays in a straight path (which is normally what’s done with lenses), so we can see their apparent locations. Cool, huh? And if the black hole is perfectly symmetrical with respect to the line from Earth to whatever object we’re viewing, well, we can see an entire ring of the object. It looks something like this:
Unfortunately, however, the black hole is rarely ever perfectly in line with Earth and the object we’re trying to view. So, instead, we see two images, like this:
So what happens if you fall into a black hole, you ask? Well, it’s really not pretty. Once you get past the event horizon, you’re basically dead meat. There are ways to slow your descent into the singularity, but really…it’s hopeless. Once you get to a certain point, you just get sucked into the singularity, where your body is crushed into infinite density and added to the mass of the black hole. Right before that, however, you get torn apart by tidal forces of gravity and turn into a long, noodle-ish looking thing. That’s called spaghettification. It is really not pleasant. I would advise you to never try that.
But wait! There’s more! How do we know black holes exist if we can’t see them? The answer is this: Even though we can’t see them, they have effects on the surrounding regions that lets us know that they are indeed there. Their gravity is still great enough that they have effects on surrounding objects, though really, it’s not obvious enough that we’d be able to find it. But the most obvious thing they do? Because they accrete so much stuff, they radiate X-rays and energy like you wouldn’t believe. Just tons and tons of X-rays and energy—and best of all? Usually, they’re accompanied by these giant relativistic jets of gas, spouting from either end of the black hole. We don’t know why, or how, but there you go. Huge jets of glowing gas blowing out of black holes. And that is how we know they’re there.
There’s also something else called a naked singularity, which technically isn’t a black hole because it lacks an event horizon, but for all other purposes and intents, is the same thing. It is a massless singularity of infinite density that sucks in just about everything around it—it just doesn’t have an event horizon. However, no naked singularities have been discovered yet, so it seems that black holes like to stay clothed (as stated by the Cosmic Censorship Theory).
We mentioned Hawking radiation earlier in the post, but what exactly is it? Well, the real explanation is beyond the comprehension of any of your resident astro geeks, but we’ll just say that sometimes particle-antiparticle pairs are created at the event horizon, and one escapes and one does not. These particles result in evaporation, which takes away energy and therefore mass from the black hole. Hawking radiation increases as the black hole loses mass, so at the very end of a black hole’s life, it releases a powerful burst of high-energy radiation. In fact, some primordial black holes are thought to be going through this right about now.
Another point to make is what happens when “stuff” falls into a black hole. This “stuff” has various properties that are dubbed information. From there, there is a debate. Perhaps not the Great Debate, but a fairly important one that has resulted in the black hole information loss paradox. This pretty much debates whether information is lost by a black hole. Some say it is, some say it isn’t (cue the Thorne-Hawking-Preskill debate). It is a paradox because two scientists, Hawking and Bekenstein, found that with Hawking radiation, a black hole evaporating information thrown into it would make that information permanently disappear. Forever. But then this violates some more quantum mechanics that are somewhat difficult to comprehend. Some say that the information should also exist just in a slightly…changed form. Again, we don’t even pretend to really understand these theories, but it just goes to show how complicated these strange astronomical objects are.
TL;DR: Black holes are the condensed remnants of extremely massive stars. Their density is so high that all sorts of pressures that would normally oppose gravity can’t fight anymore, and the battle for equilibrium is over. So the mass basically condenses down to a point with many properties that aren’t fully understood. First of all, black holes technically can’t directly be seen, as light would warp around it. But we can still manage to infer properties with this major disadvantage! For example, properties of a black hole can be focused to mass, charge, and spin—these factors result in different types of black holes, but that’s beside the point (see what we did there?). Black holes also have tons of anatomy to them, even if this isn’t biology. Most importantly, their “size” is defined by a Schwarzschild radius, which defines the event horizon, the outer limit where the black hole’s effects exists. Hawking radiation is where black holes lose mass through some fancy quantum physics that we don’t pretend to begin to fully comprehend, but it does show that black holes don’t actually last forever. Interestingly, they also end with a bang.
Sources and links for further reading:
Carroll and Ostlie, An Introduction to Modern Astrophysics 2nd edition, p. 633 to 646