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I'm doing my science fair project on black holes. I learned that inside a black hole there is a very strong gravitational pull. My hypothesis is that around the black hole there might be a spinning force (similar to tornadoes)as the galaxy rotates around it. This spinning force should draw objects toward the black hole. Am I on the right track? Do you have any suggestions about how to test my hypothesis?
Question Date: 2013-01-03
Answer 1:

Actually, it sounds like you're talking about a rotating black hole, also called a Kerr black hole. Kerr black holes are described by two numbers: the mass of the black hole, and something we call its angular momentum (basically, how fast the black hole is "spinning"). Now, there are two effects that describe how an object falls into a Kerr black hole: there's the usual gravitational pull from the black hole's mass, but there's also an effect from the black hole's rotation. It turns out that if an object is falling into the black hole while rotating in the same direction as the black hole, the object is "repelled," so that it falls in more slowly. If instead an object falls into the black hole while rotating in the opposite direction as the black hole, the object is "attracted," so it falls in more quickly. So, what you call the "spinning force" (which is more correctly called a "centrifugal force") can either make objects fall in more quickly or more slowly, depending on how they fall into the black hole.

Unfortunately, black holes are incredibly massive things, so we can't create them in a lab to perform experiments on them. Everything we know about them comes from either studying Einstein's theory of gravity, which is what predicts their existence, or observing black holes that are far away (usually, the types of black holes we can observe are large black holes that are feeding at the center of far-away galaxies, in which case they're called "quasars"). So, it's very difficult to test your hypothesis with an experiment. The best we can do is to look at Einstein's equations of gravity and see what they tell us about the way a black hole behaves, and that's how we know everything I said above about something either falling in faster or slower, depending on which way it falls in.

Answer 2:

This is not a good science fair project because it's impossible for you to test. You would need a black hole to test it, and there are no black holes available on or near the Earth, even in a laboratory. The only black holes we know about are all many light years away, and the only way we know about them is we observe the effects of their gravity on stars and other objects which tells us that there is an extremely small, extremely massive object that really cannot be anything except for a black hole.

According to Einstein's theory of general relativity, which is currently the best theory we have that describes black holes (as well as the rest of gravity), anything that has mass exerts a force that causes space to fall inward into them, and this inward flow of space creates the force that we call gravity: objects are pulled in with the space that they sit in. At the event horizon of a black hole, the speed of space falling in equals the speed of light, which means that nothing can escape, since nothing can move in space any faster than the speed of light. However, the speed of the infall of space continues to increase toward the center of a black hole, reaching infinity at a central point inside of the black hole called a singularity. Mathematically, the equations of general relativity require you to divide by zero at the singularity, which means that this is where the theory breaks down and we don't know what happens.

Black holes don't need to spin, but because spinning matter falling into a black hole can't stop spinning (another law of physics), virtually all black holes in nature do spin. A spinning black hole, or for that matter any object with mass, causes the flow of space to not only fall inward, but to spiral inward, like a whirlpool. This means that an object falling into a spinning black hole, even if it is diving straight for the singularity, will actually spiral in, because it will get caught in the swirl of space as it falls in. The same thing happens as I said with any massive body: if you drop a pencil from your hand, it will start a very steep spiral toward the center of the Earth as it falls before it hits the ground, because the Earth is spinning. The Earth just isn't massive enough or spinning fast enough for you to notice the difference from a straight fall downward.

Spinning black holes also have some other odd characteristics - the central singularity of a spinning black hole is ring-shaped, for example, not a point. I am not enough of a physicist to explain to you how this works.

Galaxies, meanwhile, do not spin because of the spinning black hole in the center. Galaxies spin for the same reason that the Earth spins: you can't stop spinning unless you have something to stop you. This is true even if you are spinning in a chair, say - you slow down and stop because of friction with the joint in the base of the chair. If it weren't for friction and you were spinning, you would continue spinning forever.

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