I was re-watching an episode of Stargate: SG-1, “A Matter of Time”, and tried to explain to a friend how they got the physics wrong. Hence this article. Also, the photo isn’t mine–it’s from Astronomy.com.
A popular misconception of black holes is that they are like evil cosmic vacuum cleaners on a continuing mission to devour strange new worlds and obliterate everything that comes remotely close to them. For instance, if some trickster god like Loki or Q snapped his fingers and replaced the Sun with a black hole of similar mass, most folk assume the black hole would instantly start gobbling up the inner planets and eventually turn its hungry gaze on the gas giants. Part of this misconception comes from, I think, artist’s depiction of black holes in binary systems, with the hydrogen tail of a black hole’s companion star slurping down it’s companion’s mass through a giant gravity straw.
Not quite how it works. Interstellar actually does a decent job of depicting what a black hole would act like (and look like). When you see Gargantua in that movie, you see that planets have retained a stable orbit around their parent star that had lost its battle to gravity long ago and already become a black hole.
The truth is, if the Sun were magically replaced by a black hole of similar mass in an instant (and it would have to be magic, because the Sun isn’t massive enough to ever collapse into a black hole), the orbits of the planets would not be substantially affected. The “vacuum effect” of a black hole is just gravitational attraction. And mass and distance are the only determiners of gravitational attraction that we know about. So, if you replace one body of a given mass with another body of the same mass, you are gonna get the same results–which, in our case–is relatively stable orbits in the solar system. The “object so dense that not even light escapes” has only do with objects within the black hole’s event horizon–which is the term we use to describe the distance away from the black hole’s “center” in which any matter, any “stuff” at all, can escape the black hole’s gravity field.
Okay, but what makes a black hole collapse in on itself after all? We all know that “gravity” does it. But maybe a better question is, Well, why doesn’t everything else collapse? And what happens to a black hole that isn’t happening to everything else?
To understand this best, consider a single atom. Let’s go with the simplest atom, hydrogen. One proton in the nucleus, and one electron “out there” in the electron cloud surrounding the nucleus of the atom. The positive and negative charges of the proton and neutron keep them “together”. Consider, though, a larger atom like oxygen. Oxygen has 8 protons, 8 neutrons, and 8 electrons typically. The protons and neutrons hang out in the nucleus, but electrons are dispersed around the center of the atom in a sort of order, and they are repelled away from each other by means of their shared negative “charge”. Electrons arrange themselves in the electron cloud depending on how much energy they have–thank you, photonic energy packs.
Speaking of photons, you may have noticed that photons are “stackable”, that is, you can either spread photons out in a wide area, or you can concentrate them into a super intensely refined area. Think flashlights verses laser pointers. You can’t do that with electrons, or with protons, or neutrons, or other matter particles scientists have fancy names for. When you lump a bunch of these kinds of particles together, you get orderly arrangements like atoms and molecules that have volume. A hydrogen atom only has one electron, oxygen can have a max of 8, and that’s really it. You can’t really smash much more into an atom. (Okay, you can do a couple of things, but not much.) There are “forces” at work inside atomic nuclei that keep matter from stacking, and hence you get the world as we know it.
So, what happens in the creation of a black hole is that all of those forces that keep matter particles from “stacking” like photons give way to the immense gravity of the star. Usually, scientists consider gravity a kinda wimpy force in comparison with the other fundamental forces that govern electromagnetism and what happens on the inside of atomic nuclei. Usually, those forces beat out gravity, but under the extreme condition of extreme mass, gravity is the most relevant force, and gravitational attraction wins over the other attraction/repulsion forces happening between matter particles. Gravity, in fact, breaks everything down so you lose volume itself. That is why black holes are also called singularities. Basically, a black hole is a hugely massive object that looks and behaves like a subatomic particle. Imagine: an object many times more massive than the Sun, vanishing into a space so small it can’t be observed by the naked eye. Also, do charge, mass, and spin sound familiar to you? Remind you of a proton or a photon, maybe? If so, that’s pretty much all there is to a singularity.
There’s one semantic thing I’ll mention before concluding. When we talk about black holes in popular science and science fiction, I get the feeling most people mean singularity+event horizon+accretion disk. Meaning, we are both talking about some observable phenomenon surrounding the “black hole proper”, that is, the singularity, and the singularity itself. It is much easier to think about the visible swirl of gas surrounding the event horizon that is the accretion disk, or the event horizon, than it is to think about a dimensionless object of infinite density and a crapton of stellar masses.
You wanna read about this for real, for real? I would recommend the following:
Brian Green’s The Elegant Universe. (He will totally make you work for this, he doesn’t do black holes until three-quarters of the way through the book.)
Stephen Hawking’s A Brief History of Time. (I know it is in there somewhere.