wowt explains: what is a black hole?

We know that black holes are bizarre. We know they exist. We even have a picture of one. But what exactly are they?

gargantua interstellar black hole

Over the century or so that we’ve been interested in black holes, we’ve gotten so comfortable with the term that it barely phases us when it’s mentioned. But much of what we typically think we know about them is either a paradox or a misnomer. They’re not actually black, they’re not holes, and they aren’t cosmic vacuum cleaners as they’re often pictured in movies and on TV. If you were to pick an accurate term to describe them, it would be gravitational zombies. They’re dead stars which do only one thing: feed on anything that strays too close.

Because they’re so distant, mysterious, and bizarre, it’s easy to project pretty much anything we want on them and get confused about what they actually are and how the physics around them works. So, in the interest of boosting general black hole awareness and helping to bury a few persistent myths about these objects, I decided to tackle some frequently asked questions about them.

how are black holes formed?

Black holes are created when a massive star undergoes a violent explosion at the end of its life. Underneath the layers being blown off into space, vast quantities of plasma collapse into the star’s core, creating a massive implosion that annihilates matter as we understand it. Neutron stars are created by a similar mechanism, but because they come from lighter suns, the energy of the implosion isn’t enough to completely destroy all the matter, although some neutron stars should have become black holes but didn’t due to a quirk of angular momentum. While neutron stars get their gravitational heft from the hyper-dense states of matter of which they’re made, black holes’ power comes from the raw, self-gravitating energy retained from the implosion which created them.

how can we detect black holes?

As soon as a new black hole is formed, it starts devouring matter around itself and belching out radiation because it has a limit on how much matter it can consume. These stellar objects are extremely compact — more on that later because it’s really important — and while they will eat anything in their gravitational grasp, they can only cram so much gas, dust, and other matter down their gullets at a time. But as they draw in what they’re about to consume, their tidal forces warp, accelerate, and heat up matter until it glows, forming an accretion disk we can detect with radio observations.

Another way to find a black hole is to look for its gravitational pull. For example, we know that our own Milky Way galaxy has a supermassive black hole in its center because we can see stars flying around it in eccentric orbits much faster than they should considering the size of the massive object they should be circling. The only reasonable explanation for seeing stars swing wildly around a comparatively small sphere emitting nothing at all is the tidal force of a black hole because we know of no other physical phenomenon capable of doing something like this.

do black holes spin?

Yes, and not only do they spin around these axes, they’re probably the fastest objects in the universe when they do thanks to the conservation of angular momentum. Because stars spin, when they implode into black holes all that energy generated by their rotation is now moving a much more compact object. Because that energy can’t dissipate into the cosmos around it, the only direction it can go is forward, pushing the newly born black hole faster and faster. On top of that, collisions and gravitational interactions with other objects can accelerate it further. In theory, they can be as fast as light itself.

Consider that the monster at the heart of galaxy NGC 1365 is estimated to be some 44 million kilometers across but does a complete revolution in less ten minutes, meaning that it’s spinning at 84% the speed of light. Another black hole called 4U 1630-47 was clocked at 90% the speed of light. As you can imagine, this creates a lot of very interesting and energetic phenomena around these objects, including massive magnetic storms and powerful distortions of time and space called frame dragging.

what’s inside a black hole?

Because we can’t look inside black holes by definition, we don’t know exactly what lays under their event horizons, we can only make educated guesses. From our understanding of basic physics, the environment would have to be chaotic and energetic. Forget traveling back in time or seeing other universes. You’d most likely be pelted with enough radiation to scramble the very atoms you’re made of within moments while to outside observers, the black hole itself would seem to be the coldest object in the universe, registering at just a fraction of a fraction of a fraction of absolute zero, even as its core could be burning at trillions of degrees.

what happens if you fall into a black hole?

Absolutely nothing good. You’d be swept up by tidal forces so extreme, if you were falling in feet first, your feet would be pulled in faster than your head, stretching your body until it rips apart, then warping the limbs through a process known as spaghettification. What happens after that gets murky, invoking very complex quantum mechanics and paradoxes required to explain how matter can be effectively erased from the universe. Nuances aside, you would absolutely die in the process, the question is just the exact degree of how horribly dead you would be.

how heavy are black holes?

Black hole mass varies greatly. We’ve seen one as light as 3.8 times the mass of our Sun and one tipping the scales at 17 billion solar masses. (Those titles belong to XTE J1650-500, a binary star system in our galaxy, and the black hole at the center of galaxy NGC 1277, respectively.) Their physical size could range from 15 miles to 10 billion miles across, and seemingly just about anything in between. However, there is a hypothesis that any quantity of matter squeezed into what physicists call the Schwarzschild radius can become a black hole and there may be objects with the mass of Jupiter or Neptune crushed to the size of a tennis ball or marble floating in deep space. However, because they would be so tiny and faint, we don’t have the tools to detect them.

can black holes die?

According to existing theories, yes. Various quantum instabilities around their event horizons would cause them to slowly evaporate over trillions and trillions of years until there would be nothing left. This would still make them the longest living things in the universe and perfect havens for any advanced alien species at the heat death of the cosmos, but ultimately just as finite as anything else. While we lack the tools to observe these processes in the wild, we have been able to recreate similar phenomena in scale experiments with lasers and supercooled gas which behaves like the event horizon of a black hole.

can anything form in orbit around black holes?

It’s unlikely that frame dragging, magnetic storms, and radiation would let planets form around stellar black holes. However, supermassive ones can trigger star formation on a galactic scale, allowing countless new planets, some with environments where life could survive and thrive to be born. In fact, without collisions between galaxies and those titanic gravitational zombies, we wouldn’t exist because there would be very little to create extremely heavy elements necessary for living things and few stellar nurseries actively creating stars and planets.

could we ever harvest the energy of black holes?

Actually, yes. It’s possible to use them as dynamos to generate vast amounts of energy, and if you can contain them, direct their output to navigate through interstellar, if not intergalactic space, or run massive civilizations. Of course, artificial black holes would require vast amounts of energy and would be inherently unstable, so this technology is out of our reach today. Even if we were able to generate microscopic black holes with massive particle colliders, they wouldn’t be of any use to us. Even the smallest practical black hole would require something like Mount Everest crunched into the diameter of an atom. We’d need to be able to compress the mass-energy equivalent of Ceres into an object barely visible with the naked eye to get real benefit out of this approach.

could you link two black holes to form a wormhole?

It would require more than enough power to warp space and time yourself, and if you could link two black holes, the tunnel they’d form would be extremely energetic and unstable. Trying to travel through it would be like trying to surf the shockwave of a nuclear blast at point blank range. You’d be vaporized, and the tunnel would collapse so quickly that you probably wouldn’t even reach the other side. It really is difficult to overstate just how much energy you will be trying to control and how violently it will end for you. So, in short, yes but you really shouldn’t.

# space // astronomy / astrophysics / black holes


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