When it comes to the universe, gravity runs the show.
Even light can be pulled by gravity.
It’s not a strong effect, quite subtle in fact. Usually we need very, very strong gravity from really, really massive objects like stars, to observe the effect. But it’s there.
Gravity traps us on the surface of the Earth. If we want to escape and break out into space, we need sufficient kinetic energy to overcome it. In simple kinematic terms, this can boil down to obtaining a single velocity… the escape velocity. Exceed the escape velocity… you’re free. Move at less than the escape velocity… you fall back to the Earth.
But light has a finite speed: 299,792,458 meters per second (or about 186,000 miles per second).
Gravity can keep getting stronger.
So if you keep adding mass to a body, eventually its gravity can get so strong that the required escape velocity is faster than the speed of light. Since nothing can go faster than light, nothing can escape!
That’s a black hole.
Here’s a Very, Very Brief History of Stars
Stars start out as blobs of hydrogen. The more hydrogen that collects, the more gravitational pull the object has. The more gravitational pull it has, the more hydrogen it collects. Eventually that hydrogen gets squeezed together with enough pressure to overcome the electromagnetic repulsion of the positively charged hydrogen nuclei, and you get a nuclear fusion reaction, releasing light energy and generating helium nuclei… a star is born. Over billions of years, the hydrogen is used up. If the mass is larger enough and the gravity is strong enough, the helium nuclei can fuse into heavier elements like carbon, nitrogen and oxygen and this can keep going until fusing the nuclei no longer releases any energy (that happens at about iron).
The light energy produced by the fusion reactions produces and outward force that balances gravity. This allows the star to maintains a stable size. But when all the light elements (the fuel) is used up, that outward pressure is no longer there. The star collapses in on itself.
There are barriers to the complete collapse. When matter is compressed, electrons slowly fill their available quantum states. Once that happens, the matter exerts an outward pressure, called the electron degeneracy pressure that can balance against the inward compression due to gravity. But if the star has more than about 1.4 times the mass of our sun, even this can be overcome and the matter gets crushed down into one giant nucleus supported by neutron degeneracy pressure. The outer material rebounds off the degenerate core in a very violent explosion–a supernova.
If the star is massive enough, the gravitational force can overcome even this. And the star keep shrinking, concentrating more and more mass in a smaller and smaller volume. Eventually, you reach a state where you have enough mass concentrated into a small enough volume, that the gravity around it is too strong for even light to escape–the black hole.
Anatomy of a Black Hole
By its definition, you can’t actually “see” a black hole. But you can certainly see the light from the space and matter around it, and its effects on that light, as in the image above. This particular black hole and the center of a galaxy about 55 million light years away from us, is estimated to be about 6.5 billion times the mass our sun. All around it, matter is being pulled in and smashed together generating light. In the center, the gravitational force is so strong that light can’t escape, which is why there’s a black spot. We’re looking at a void. The light around it is reddish because the light that is in fact far enough away from the hole to escape still has to climb out of a gravitational energy well. In doing so, it loses energy and shifts toward the red side of the spectrum.
Black holes themselves are pretty simple things. They have three properties: mass, charge, and spin, and otherwise don’t seem to have and distinguishing features, (aside perhaps from differences in the matter accreting around them). They have an event horizon. This can be thought of as it’s surface, the boundary beyond which light can’t escape.
That said, in a very recent paper, a group of physicists looking at quantum gravity have suggested that matter inside a black hole may leave a subtle quantum signature on the gravitational field around it. This is exciting because in quantum mechanics, there is a fundamental idea that information cannot be lost. But if everything inside a black hole simply disappeared, that rule would be violated. This new ideas, sometimes referred to has the black holes having “hair” reconciles these ideas.
Gravitational tides are also worth mentioning to. A tidal force arises due to a difference in force at two separate points. Here on Earth the difference in gravity between your head and feet is extremely small, arising from difference between your head and feel relative to their distance from the center of the Earth. But near a black hole it can be substantial, enough to stretch you out quite violently. Physicists occasionally refer to this phenomenon informally as “spaghettification.”
At the black hole’s core is a singularity–a place where matter is compressed into an infinitely tiny space. Here, our concepts of space and time break down. Some physicists argue that there’s a Plank limit to this, that there’s a fundamental granularity to the universe and that nothing gets smaller than about 10-35 meters, so it’s not technically infinitely small. Interestingly, you can find solutions to general relativity for a rotating singularity that enable backward time travel. Others have argued that you can produce wormholes connecting different places in spacetime, and you might even generate white holes elsewhere in the universe that spew forth all that swallowed energy at the speed of light. The truth of the matter is that no one really knows. And no one is likely to know.