Expert: James Gort - 12/2/2006

Question
Dear James,
I am a student from Israel. I'm doing a school project about "Black Holes", and i have a question I would like to ask you.
I have read an article about black holes, which said black hole have an extremely strong gravitational field.
I would like to ask you, what causes the strong gravitational field around the black hole?

I would really appreciate you taking the time to answer my question.

Yours,
Alex.  

Answer
Hello Alex,

In the center of a black hole, there is mass.  According to Einstein, all mass distorts "space-time", a phenonemon which we can measure, and we call it "gravity".  No one knows why mass causes all other masses to be attracted to it (a property of gravity), but it does.

When a mass is very diffuse, like a large one kg sponge ball, all the little particles of matter making it up (atoms or molecules) are widely separated.  So another piece of mass (say, a billiard ball) can't get very close to a clump of the sponge ball, so the attraction (gravity) would be small.  Newton said that the force of gravity varies inversely with the distance apart.  If we can't very close (because the sponge won't let us), then the gravity would be weak at the surface of the sponge.

But say we have a small lead ball, also one kg in mass.  Because it's small, our billiard ball can get pretty close to all those lead atoms, so the gravity at the surface of the lead ball is greater.

Now, what if we could somehow shrink the entire one kg lead ball to a size smaller than an atom?  If we could, then the gravity at the surface of our very tiny lead ball (it wouldn't actually be lead anymore, because all the lead atoms would have broken done into a glob very small and compact) would have a very large gravity at its surface.  In fact, we could create a black hole like this.  All it takes is to squeeze matter into a very small space, so its gravity would be so high than nothing could escape from its surface - not even light.

So black holes all have to have mass (that's what makes the gravity), but they don't all need a lot of mass.  A small mass just needs to be smaller than a large mass (to have the same high gravity at its surface).

So to answer your question, what causes the high gravity of a black hole is its mass, together with its small size.

Hope that helps.

Prof. James Gort