## Black Hole or Bust

Posted in: Gravity Simulator, Lesson Time! | June 14, 2014 | No Comments

Last week I reported on General Relativity in the Gravity Simulator.  And now that relativity is in place — this means the sim can have BLACK HOLES!  YEAAAAA!

Created in the Gravity Simulator

Black Holes

How hard could it be to add in black holes, right?

We all know that black holes are extremely massive, extremely dense objects.  (Mostly.)  Get close enough to them, and gravity pulls so strong, that not even light can escape.  Whoa!  So we just need to make a big star in the Gravity Simulator — and we get a black hole!

Well, no.  You’ll never get a black hole if you’re dealing with boring, old Newtonian gravity, though.  In the old simulator, say, you could make a star bigger and bigger and bigger, and all you’d get is a bigger star.  Any astroid or planet or star can escape its pull, so long as it’s moving fast enough.

Event Horizon

Around black holes, there’s a line of no return called the Event Horizon.  If you’re outside of this boundary — you could escape the black hole.  But the moment you cross it, you’re sunk.  You’ll get swept ever further into the black hole.

This happens because General Relativity contains our old friend Special Relativity.  And, if you’ll recall, a key part of Special Relativity is that nothing can travel faster than light.  The speed of light is the speed limit for everything.  The Event Horizon represents the line near enough to the black hole where, if you wanted to escape, you’d have to travel at light speed.  Fall in a bit closer, and gravity gets a bit stronger, and you’d need to go even faster than light to escape.  No can do.  You’re stuck.

Masses: 1000, 2000, 4000, 8000…

In the pictures above, I draw where the Event Horizon would be on each the star.  The smaller the star is, the weaker the gravity it, and the closer you’d have to get to reach the Horizon.  In fact, most of the time, you’d actually have to go deep inside the star to find this line. Which means, it isn’t really an Event Horizon.  The calculations I used to draw these assumed that all the mass of the star is inside the Horizon.  As you can see above, that’s not the case.  The stars aren’t dense enough, which means: no Event Horizon and no black hole.

Masses: 8000, 10000, 14000, 16000…!

But if we get enough mass in place, the Horizon grows big enough that it swallows up the whole star — and we finally get our black holes!  Now let’s have some fun with them!

An orbit that precesses, much like Mercury. (Created in the Gravity Simulator)

Black holes merging! (Created in the Gravity Simulator)

Black Holes have tons of neat properties, which you’ll all be able to check out in the next update to the Gravity Simulator.  Stay tuned for more General Relativistic fun!

-Andy

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