Thursday, November 28, 2013

Black Holes

How about a bit of science?

Everyone's heard about Black Holes, but what the heck are they?

First, a little background.  If you throw a ball up from the Earth, it will slow down and eventually fall back to the Earth.  No matter how fast you throw it up, it will always come back down.  Suppose you used a nice, fancy machine to throw it up faster than you can.  Will it always come back down?

Happily, the answer is no.  If you have a very powerful machine, (and we don't have one yet that can do this), that can throw the ball so that when it leaves the machine it is traveling at around 7 miles per second, the ball will go up and never come back down.  It will indeed slow down continuously, but not enough to head back toward Earth.  This speed that an object needs to leave a planet or any other mass, is called its escape velocity.  For instance the Moon has an escape velocity of about 1.5 miles per second.

You can probably guess that the more massive and dense the object, the higher its escape velocity is.

But can an object be so massive and dense so that nothing can escape from it, not even light?  Once again, we are happy, because the answer is yes, and such an object is called, not surprisingly, a Black Hole.

And that's why they are black. Not even light can escape from one.  Do we know for sure that such objects exist?  Not quite.  There is some pretty good evidence for Black Holes, and most astronomers believe they exist.

Black Holes were first predicted as a result of Einstein's Theory of General Relativity, which is actually mostly a theory of gravity. And they are a bit more complicated that they seem.

The outer 'surface' of a Black Hole is called the event horizon.  From the outside, anyone looking toward a Black Hole would see only black within the event horizon.

What would happen if Jill unwisely decided to take a trip in a rocket ship into a Black Hole.  Well, it somewhat depends on the size of the Black Hole.  Right now, our best evidence in that most galaxies have huge Black Holes at their centers.  So she travels toward the center of our galaxy (which would take millions or years at the current speeds of our rockets, so Jill will get very bored and hungry). She could actually travel right through the event horizon and wouldn't even know it.  Sadly, if she tried to get back out, it ain't going to happen.  Just sit back and enjoy the ride, Jill.  Because she will inexorably be pulled toward the center of the Black Hole, a mysterious place called the Singularity. This Singularity contains all the mass of the Black Hole.  Einstein's equations predict that this is a point of infinite density, and no size.  Try not to think about that.

Once she gets close enough to this Singularity, she and her rocket will begin to be stretched toward the Singularity. This is because gravity will be stronger at the tip of the spacecraft than it is at its end. This stretching is actually called spaghettification.  It would not be a pleasant experience, even though it sounds delicious. Once she reaches the Singularity, she will be completely crushed into it, and have no volume.  Do not be concerned about this, as she will be dead long before this happens.

Here's a very interesting phenomena associated with Black Holes.

Let's switch the frame of reference to an observer, Jack, quite a distance from the Black Hole. Jack would see Jill fly toward the large black hole in her spaceship. But Jack would see her ship slow down as it got closer to its event horizon – which is the point of no return.  As time went by for Jack, he would see her ship going slower and slower, and the light he would see coming from her ship would get shifted to the red part of the spectrum, and eventually to shorter and shorter radio waves, so that Jack will never see her ship cross the event horizon- ever!

Who is right, Jack or Jill? That's the thing about relativity - they are both right, from their own frames of reference.  How can that be?  Either Jill flies through the event horizon into the Singularity or she never makes it to the Singularity.

Take about 5 university-level physics courses, and it will be almost make sense.



Tim Farage is a Senior Lecturer in the Computer Science Department at The University of Texas at Dallas. You are welcome to comment upon this blog entry and/or to contact him at



Anonymous said...

So you've explained black holes. What are worm holes?

Tim Farage said...

Wormholes were thought to be at the center of Black Holes and were sort of a tunnel from the center of a Black Hole to somewhere else in space and time. Physicists no longer consider them likely to exist. But movie makers absolutely know they exist because people in space ships travel through wormholes all the time. For instance in the 2009 Star Trek movie, Mr Spock and a bunch of Romulans travel through a wormhole back in time. But don't worry, aliens traveling back in time to get us should not be in your top 10 list of concerns.

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