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Do Black Holes Compress Forever?

11 Jun 2015, 18:59 UTC
Do Black Holes Compress Forever?
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Once a star collapses into a black hole, does it keep on compressing forever?
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© Fraser for Universe Today, 2015. |
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Once a star collapses into a black hole, does it keep on compressing forever?

Just like Thailand, what happens in a black hole stays in a black hole. The event horizon keeps everything a complete mystery. So, once a star collapses into a black hole, does it keep on compressing forever?

Here we are, back at black holes, like a moth to the flame. I just can’t resist talking about them, reading about them, or listening to the hidden messages inside their darkest secrets.

No matter how many times I check Wikipedia, clicking refresh-refresh-refresh, can I get an answer to the question: do black holes ever stop compressing? No, and by that I mean that I don’t use Wikipedia or have any kind of research assistance, I just know all this stuff.

Imagine you’ve got a star like our Sun. You go away for a couple weeks, forget to water it and then the poor thing dies. It puffs up as a red giant for a bit, blows off its outer layers and then compresses down.

Without nuclear fusion at its core, there’s nothing to stop the star from collapsing inward. This compression halts when everything left in the star, mostly a carbon core left over from the days when it was a star, crushes together as tightly as possible.

Congratulations! It’s a white dwarf! Essentially one big chunk of high density carbon - a diamond wishes it had carbon atoms crushed together that closely. It still has the mass of a whole star, but it’s only about the size of the Earth. If you ever land your shuttle on one of these, you’ll need to be going about 6000 kilometers per second to escape its gravity.

The only thing stopping the white dwarf from collapsing any further is the Pauli Exclusion Principle. This is the physical law that prevents two electrons from occupying the same quantum state. If the star has enough mass, somewhere around 1.4 to 3 times the Sun, the structure of the matter itself can’t hold out against the gravity pulling it inward. Electrons are squished into protons and make neutrons.

Then it’s neutrons all the way down, and congratulations! You’ve got yourself a neutron star, and it’s only a freaky teeny tiny 25 km across. Neutrons are tough and they hold up under the pressure.

Want to escape with your spacecraft now? You need to hit 250,000 kilometers per second to escape. Light can do that no problem, it just whizzes, by flipping the bird. Neutron stars are very bright, hot, and pour out gamma radiation. If the star gets any more massive, even the neutrons give way and the star collapses into something even more exotic… a black hole.

The light that could just barely escape from a neutron star, making rude gestures and leaving you in its dust? It can’t even get off the surface. Well, it sort of can, it just gets red shifted into oblivion. Who’s laughing now, light?

But is there another limit? A place where the compression stops? Astrophysicists have no idea.

Maybe there’s another fundamental particle, deep down inside there, and it’s made of stronger stuff than neutrons. Perhaps, it can hold on against the black hole’s continued compression.

Or maybe, the compression continues forever. The black hole continues to compress faster and faster until it’s compressing at nearly the speed of light.

Is there a fundamental limit to the size of things in the Universe? Is there something so small that things can’t get any smaller? There’s the idea of the Planck Length, first derived by the physicist Max Planck.

It’s an insanely small length: 1.616 x 10^-35 meters. For the sake of scale, it’s so small, a hydrogen atom is 10 trillion trillion Planck lengths across. The Planck length is just a mathematically derived value, it’s not a fundamental limit in the Universe.

It’s not the resolution of existence. Maybe things can’t get as small as a Planck Length, and maybe they can get way smaller than a Planck Length. And maybe they can get infinitely small.

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