What's inside a Black Hole?

  • By Greg Kestin
  • Posted 01.10.18
  • NOVA

Black holes devour stars, light, and anything that comes too close. But from our spot far outside the horizon of any black hole, all we can see is the glow of swirling radiation from objects falling over the horizon.

What could be inside? Explore three stunning theories in this episode of What the Physics?!

Running Time: 03:19


What's Inside a Black Hole?

Published January 10, 2018

What's inside a black hole?

You may have heard there's a smooth surface called the horizon, out of which nothing can escape. You may have also heard there's something called a singularity at the center of a black hole where everything is crushed into an infinitesimally small infinitely dense point. Even these two simple facts are likely wrong, or at least more complicated.

While nobody knows for sure what's in a black hole, there are some pretty awesome ideas. Inside there could be a ball of string, there could be an incredibly compact star, or there could be visions of the past.

Black holes challenge the two pillars of modern physics. First the space bending theory of gravity, that's Einstein's theory of general relativity, and second the theory of tiny stuff, or in other words quantum theory. Combining these would give us what's called a theory of quantum gravity. So what would the most popular theory of quantum gravity, called string theory, predict is in a black hole?

In string theory everything at its fundamental level even you is made of tiny vibrating strings. So maybe black holes are no different. One theory is that black holes are a giant fuzzball of strings. There's no infinitely dense singularity, and no smooth horizon. There's just this ball of strings. So if something like a particle, or you, fall into a black hole, well you're made of strings, so your strings will combine with the black hole strings, and they'll get longer. And the black hole fuzzball grows.

String theory isn't the only theory of quantum gravity. There's also a theory called loop

quantum gravity, which also contends that there's no singularity at the center of

a black hole. When a star collapses into a black hole, all its mass is gravitationally attracted toward the center, but according to loop quantum gravity there's a force preventing the matter from being completely crushed to a point. It can only be crushed until this outward pressure-like force counters the crushing force of gravity.

It's kind of like when you sit on an exercise ball. It squishes down, but only so much because of the air pressure inside the ball. So the matter inside the black hole would

be crushed into a very very tiny star called a Plank Star that sits at the center of the black hole. For a black hole the mass of our Sun, its Plank Star would be the size of an atom.

Even the most conventional theories of black holes predict some crazy stuff inside. According to Einstein's Theory of General Relativity, if you fall into a black hole, you'll see bright lights all around you. That's right, to us out here it looks like it's black, but inside it's incredibly bright.

Black holes Bend space so much that even light can't escape, but the light that falls in doesn't just disappear, some of it is just inside the horizon, and you may see it as you fall more quickly inward toward the center. Some of the light you see may have fallen into the black hole centuries, even millennia, before you existed. So maybe inside a black hole you could see visions of the past.

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Host, Writer, Producer: Greg Kestin

Scientific Consultant: Samir Mathur

Editorial Input from: Julia Cort

Animation: Edgeworx

Animation and Editing: Greg Kestin

Special thanks: Entire NOVA team

From the producers of PBS NOVA © WGBH Educational Foundation

Funding provided by FQXi

Music provided by APM

Sound effects: Freesound.org

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