Black Holes for Beginners

Source: NASA

Black holes: there is no better name for these elusive behemoths of the universe. Not only does the term aptly describe their enigmatic nature, but it also alludes to the vast gaps in our knowledge about them. In both science and philosophy, their perplexing properties point out flaws in our comprehension; with all the beauty and perceived order of the universe, black holes throw a wrench into all our preconceived notions and make us question reality, even on a day-to-day level. It is this, combined with their mystique and inaccessibility, that fuels our fascination with them.

Paradoxically, their simplicity makes them all the more complex. A black hole is really only a region in space of intense gravity. According to the theory of relativity (read my overview here), its high but extremely dense mass warps spacetime to a drastic degree, resulting in time dilation and such strong gravity that not even massless light particles (photons) can escape.

A black hole’s constituents are relatively simple. As explained by NASA, its outer sphere-like ‘limit’ is called the event horizon, and it’s pretty much the boundary after which even light is stuck inside; this is why a black hole appears… black. Once something passes this frontier, it will fall towards the center of the black hole: the singularity. A point of infinite density, spacetime is infinitely curved here (according to relativity), and the laws of physics break down. Scientists are still scratching their heads at what exactly that entails; there is just no way to calculate what would happen, and that has some interesting implications.

Back to the structure: black holes also have an ergosphere, a region surrounding it in which spacetime is significantly warped by the black hole, but it is still possible to escape from it with enough speed. It also has some funky consequences that come with considering a black hole’s spin, but we’ll get to that later. Finally, black holes may have an accretion disk: a collection of dust and gas, among other matter, that orbits the black hole at intense speeds as its gravity pulls it in.

You may have noticed that it was only in 2019 that we actually got the first picture of a black hole (or rather, its shadow and accretion disk). So how is it that we’ve known about them for so long? It all started with Einstein’s theory of relativity (general relativity, to be exact), and with it, Einstein’s field equations, which quantify the theory. This fiendishly complicated bout of math is summed up by American physicist John Wheeler: ‘Spacetime tells matter how to move; matter tells spacetime how to curve’.

But it was the German astronomer Karl Schwarzschild who, in 1916, solved these equations for the first time, impressing even Einstein. I won’t torture you with the math, but essentially, the consequences of Schwarzschild’s solutions showed that very dense objects can warp spacetime in such an extreme way that not even light can escape: a black hole. Work around this time also pointed out the possibility of a singularity, as the equations suggested its infinite values.

Schwarzschild theorized a body needs to have a certain radius – compared to its mass – to be a black hole, called the Schwarzschild radius. For example, the Sun has a radius of 696,340 km, and a mass of 1.9885×1030 kg (about 332,950 Earths). Were it to become a black hole (which is practically impossible as it is not massive enough), all this matter must be squished into a body with a radius of only 3 kilometers, making it incredibly dense. In 1958, David Finkelstein described the Schwarzschild radius as proportional to the event horizon we know today, and theorized that matter can cross it in only one direction: in.

Despite these discoveries and many more, the first actual black hole was only discovered in 1971. The science world was already aware of a massive source of X-rays in the sky known as Cygnus X-1, but it was a group of scientists at the Royal Greenwich Observatory, and another working independently from the University of Toronto, who connected it with the possibility of a black hole’s involvement. X-rays are emitted by incredibly fast stellar dust, which can occur when a star’s material is being sucked into a black hole. A star known as HDE 226868 was detected near the X-ray source, and by analyzing the motion and orbit of that star, they were able to tell it was orbiting an invisible neighbor that was much more massive than anything else could be: a black hole.

There are several different ways black holes can form; one is through the death of a star. But not just any star. The Sun, for example, is way too small to turn into a black hole. After expanding into a red giant in around 5 billion years and frying our planet to a crisp in the process, it will become a white dwarf: a small, dense leftover of its former self. Slightly larger stars than the Sun will become neutron stars: extremely dense remnants of a dead star – though not as dense as a black hole – and a complex topic on their own. But stars that are over 5 times the mass of our Sun will eventually use up their ‘fuel’, and become unable to perform the nuclear fusion that creates the energy that holds its structure up. These massive stars collapse into themselves as a result of their own gravity, and form a stellar black hole. Those are sprinkled throughout space: one for every big, dead star. But they are not the only kind.

Primordial black holes are (as of now, unobserved) remnants of the infant universe. They would have formed within the first second after the Big Bang; in the madness, regions of dense mass may have collapsed into themselves, forming these black holes. Their mass falls into two categories: under the mass of the moon, or over ten times the mass on the Sun. This means that while large ones may exist, tiny ones – with the mass of a feather – might too; those cannot be explained by a dying star. Though this is still technically a theory, their scattered presence in the Universe may help us understand unexplained gravitational mysteries, such as how galaxies are held together; this is often thought to be due to the infuriatingly undetectable dark matter, but the gravitational influence of primordial black holes may provide at least part of the answer.

The king of the monsters is undoubtedly the supermassive black hole. These typically lurk in the centers of most galaxies (the Milky Way’s is Sagittarius A*), and their mass can range from millions to billions of times that of the Sun. Despite what it seems, a galaxy is not actually held together by them; the galaxy’s mass is much, much larger than one black hole. Their formation is a bit of a mystery, since their mass is way too big to be caused by just one stellar implosion. Theories on this range from a black hole devouring so much material that it grows to this size, or that its evolution involved collisions with others.

The fact that these gravitational beasts are so elusive has everything to do with their (perceived) simplicity. The no-hair theorem states that a black hole’s only detectable physical properties (external, that is, because we frustratingly cannot see inside) are its mass, electric charge, and angular momentum. No other information (or ‘hair’, as John Wheeler put it) can be measured; black holes are bald (well, they might have a few hairs, but we’ll get to that later).

These properties are dependent on the black hole’s formation and what it absorbs. A stellar black hole’s mass is a result of the mass of its former star’s core, but it may also absorb matter and grow throughout its lifetime (though it can theoretically decay due to Hawking radiation, which we’ll also get to later…). As with the example of Cygnus X-1, a black hole’s mass can be deduced by monitoring nearby objects that we can see and get information from, like stars. By determining, for example, their orbits, we can use their mass and motion to calculate how massive the nearby black hole must be.

Electric charge, though a theoretical rabbit hole, is relatively minimal in nature, as black holes are rarely charged; the universe itself is neutral, and the black hole would balance itself out by attracting particles of the opposite charge.

The final property to distinguish black holes from each other is angular momentum, or spin. Measuring this is a little crazier than mass. It involves analyzing the spectrum of X-rays emitted by the black hole’s accretion disk, and determining the disk’s distance from the event horizon. Black holes whose accretion disks spin in the opposite direction to its own spin have the furthest distance between the event horizon and disk; non rotating black holes have a little less distance. The least distance is between black holes and disks that spin in the same direction. Essentially, the faster the net spin, the closer the accretion disk; as a result, when the disk is very close to the black hole, the X-rays are scattered by the black hole’s heavy gravity, so less of it reaches us on Earth. Therefore, the dimmer the X-rays are, the closer the accretion disk must be to the black hole, and the faster the black hole must spin.

In nature, most black holes do spin. Many black holes come from stars: these themselves are formed by whirling dust clouds, resulting in spin. If a star like that collapses into a black hole at death, its spin can be ‘taken over’ by the new black hole. But here’s the catch: because it has to stuff all that mass into such a small area, the spin of the comparatively tiny black hole is much, much faster than that of the star. This is due to the so-called conservation of angular momentum, which states that ‘if you halve the radius that an object has as it rotates, its rotational speed increases by a factor of four’ (Forbes). You can think of this as an ice skater bringing her arms in as she spins, which speeds her up. In doing this, she ‘compresses’ herself, or brings her mass closer to her center of gravity. When a star dies, its core material is also compressed, resulting in faster spin.

Now apply this concept to the scale of a black hole: a massive star squashed into a tiny area. This means that even if the star spins relatively slowly, the angular momentum translated to the resulting black hole will be insane. In fact, the faster a black hole spins, the smaller its event horizon gets. They also absorb the angular momentum of anything they eat, so even if a non-spinning black hole exists and it swallows a little space rock, if that rock has even the tiniest bit of spin, it will be translated into the black hole’s. However, it cannot speed up infinitely; the singularity would eventually be completely exposed as the event horizon gets smaller with increasing spin. This idea of a ‘naked singularity’ is so intimidating that nobody really dares to know what exactly that would look like – what with the laws of physics breaking down in open space without the isolation of an event horizon – but there are theories that they might somehow exist.

Another wacky effect of the spin of black holes is that it drags spacetime with it, known as frame-dragging. This actually happens with all objects in space that spin, even Earth (though Earth’s gravity is so low that we wouldn’t notice). But a massive object like a black hole can wrap itself in its surrounding spacetime, with the closest spacetime being dragged along the most. For (a very rough) example: when tie-dying a t-shirt (spacetime), before you apply the dye, you wrap it in a spiral form. To do this, you stick the handle of a wooden spoon (black hole) in the middle of it and twist (spin). The fabric closest to the spoon is twisted the tightest. This is what happens with a black hole; if you were standing on its edge in the ergosphere, even if you did not have any velocity at all, the black hole’s dragging of spacetime would make it appear like you were moving through space, when it is the actual space (with you in it) that is moving.

So, now to the question that I’m sure you’re pondering at this point: what would happen if you fell into a black hole? Let’s forego all the deadly gases and rays that hang out near black holes for this example, and also forget about charge and spin. In the case of a stellar black hole, which is comparatively lower mass, the tidal forces exerted on your body before you even reach the event horizon would be off the wall; so much so that if you fell in feet first, the lower half of your body would feel a stronger gravitational pull than the upper. Your sides would also be pushed together, and this results in your body being completely stretched out and eventually torn apart, delightfully described as spaghettification. You would no longer be in one piece before you reached the fun part, so it’s best to avoid stellar black holes, if possible.

A supermassive black hole, on the other hand, is much, much bigger than a stellar one – millions to billions the mass of our Sun. This unimaginable difference between the black hole and our puny human bodies means that the gravitational differences between your feet and head are insignificant, so you can actually reach the event horizon this time. The black hole looks bigger than expected due to its warping of spacetime, and you’ll see stars, their light warped into mere lines, around the edge of it, until you cross the event horizon. But don’t get too excited; chances are, you may still get spaghettified as you approach the singularity.

You’d have a nice view, though; in a stellar black hole, your ripped-apart body would hit the singularity immediately, but in the more spacious supermassive one, you’d have a few moments to enjoy the scenery. You wouldn’t be completely in the dark, because some light will come in with you, but most impressively, you can look outside. Because of the extreme warping of spacetime, time will run much more slowly for you than for someone waiting outside. This allows you to watch the entire progression of the universe in extreme fast-forward mode, until the black hole’s own demise, which will happen after the last stars have died. But, according to Ahron Maline, you’d probably get zapped with some high frequency light before that, since the time warp squishes the light waves as they enter the black hole. Even if you survive that, you’d quickly get broken down into your smallest subatomic particles and mashed into the singularity.

Safe to say, it would be a thrilling but short ride into a black hole. But for someone watching you from the outside, it’s a different story. To them, you would be stuck at the event horizon for the foreseeable future, due to time dilation; for your friend, time runs much faster. Also, you appear to grow dimmer and redder as time goes on, because light waves become redshifted; in other words, the black hole’s gravity causes the light waves to stretch, from visible to infrared. Eventually, only radio waves give any clue to your existence on the edge of the event horizon. They can send you signals to ask if you’re alright, but you can’t send any out… or can you?

It was originally thought that nothing could escape a black hole. However, in 1974, Stephen Hawking theorized that black holes emit thermal radiation, called Hawking radiation. He argued that this eventually leads to the (very slow) decay, or evaporation, of a black hole as long as it does not maintain itself by absorbing enough matter. But the most groundbreaking part of this discovery was that logically, this means that something does come out of black holes, which was not thought possible before.

The problem, however, is that this something seems completely random. And after a while, the black hole would disappear, leaving you with nothing; once dropped into a black hole, an object, or information, is gone forever. This led to the puzzle known as the Information Paradox. On the one hand, you have what seems like a complete loss of information in a black hole; on the other, you have quantum mechanics stating that information simply cannot be lost, ever. And both statements are assumed to be correct. There have been a few baffling theories on this, but in his 2018 book Brief Answers to the Big Questions, Hawking stated he believed the paradox was not yet solved. Still, there is much research happening in this area, with some interesting recent developments.

Remember the no-hair theorem? Well, as it turns out, there is evidence that black holes may not be completely bald. Until that came to light, it was thought that there was no way to retrieve information that fell into a black hole; all one could say about a given one was its mass, charge, and spin, but nothing else. Recently, it was shown that some of that information may have been left on the event horizon in the form of gravitational differences (among other discoveries). However, this theory holds only for black holes approaching their limits in terms of spin and charge, known as extremal. But theoretically, they do have hair. Researching distinguishable differences between black holes may be the key for the information paradox.

But why do we care? Anyone with half a brain knows it’s not a great idea to jump into a black hole, and there are none even remotely close to Earth, so we really shouldn’t worry. Though this all seems like highly theoretical work, Hawking makes a great point in his book mentioned above: if we accept that information can indeed be lost completely, it puts our whole understanding of the world and our history in limbo. How can we be sure what happened if the loss of information is real, and if we can’t even understand black holes, how do we know where else it could happen? Also, how can we try to predict anything at all if we can’t understand how exactly the laws of physics and information might break down?

The mysterious giants might just be lurking in space, minding their own business, but even with no threat to humans, their nature makes us question ours. Even without knowing every last detail of their being, their existence sheds light (no pun intended) on how far away we are from fully comprehending ourselves and our universe; opening the door to possibilities such as wormholes and even so-called white holes, there is no end to what they may teach us. Their deconstruction of what we know, combined with their befuddling simplicity, makes them a phenomenon with no equal.