29 January 2017

A supermassive black hole is really, really big

In the schematic image above, there is a little dot in the center for size compairson.

That's not the earth.   That's our entire solar system.
"Space is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space."
Discussed at the Space subreddit.


  1. More stars than grains of sand in all of earth's beaches...

  2. Thanks for the Douglas Adams reference. I need to remember that there are good and funny things in the world!

  3. Interestingly we don't know how big the material that makes up a black hole is. The diagram accurately describes how big the "event horizon" is. Beyond there nothing can escape.

    The thing is all these various astronomical bodies are basically a game of "what's stopping gravity from crushing this matter into a point mass?" Small rocky bodies (planets, moons, asteroids) can be held up by the crystal structures in rocks. More massive objects like our sun are held up because the outward force generated by all that nuclear fusion neatly balances the inward force of gravity. A strange one is neutron stars which are even more compressed but they appear to avoid complete gravitational collapse by a strange kind of outward pressure derived from the Pauli exclusion principle. (oversimplification alert) The neutrons refuse to occupy the same space at the same time because that would make them the same particle and they "hate" that.

    Beyond that there is a black hole. Where our current understanding says: there is nothing that would prevent gravity from smashing this finite amount of matter into a perfect geometric point.

    Now a perfect geometric point is a problem because it has a size (volume) of zero. So for density : a finite amount of matter divided by zero area gives... uh oh... infinite (or undefined) density. This is why they are sometimes called "singularities".

    I don't know any physicists who think the actual size is zero (a mathematical point). I think most everyone believes that the collapse is halted somehow at a finite size by some outward force generated by some crazy quantum gravity theory we don't have yet making them extremely extremely dense, but not infinitely so.

    Thus the true "size" of the matter making up a black hole hiding deep in that event horizon remains unknown.


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