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Q & A: Why does matter spiral into a black hole?

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Most recent answer: 08/24/2014
Why in artists' renditions of black holes consuming a star we always see the infalling stream of matter being torn apart from the star by the side and then spiraling towards the black hole instead of seeing the stream of matter being ripped apart in a straight line towards the black hole? I mean, if I'd jump off from the 50th floor, I wouldn't move foward instead of down and I wouldn't spiral down.
- Anonymous

This is a great question to think about for understanding gravity and motion. Your example of jumping off a building is a good place to start:

I mean, if I'd jump off from the 50th floor, I wouldn't move foward instead of down and I wouldn't spiral down.

If you really jump straight down (maybe you hang onto a ledge with your hands and then let go) this will be true. However, if you give yourself any initial velocity in the forward direction (for example, by pushing off from a ledge with your feet) you will continue to move forward as you fall down. The force of gravity from the mass of the earth accelerates you (increases your velocity) in one direction only--towards the center of the earth, or "down." It has no effect on your velocity in the forward direction, so as you fall down, you also keep moving forward at the same speed you had at the moment of jumping. Air resistance, unlike gravity, will act to decelerate you in any direction, so that will slow you down a bit.

Of course, you're not going to make it very far in the forward direction, because your downward velocity is going to make you hit the earth. But if the building were really tall and you gave yourself a really huge initial velocity in the forward direction, you could move forward so fast that you'd never hit the ground. As fast as you fell towards the ground, the earth would curve away from you, and you could make it all the way back around to where you started. You'd be in orbit!

If the building were very tall, but not quite tall enough to be in space, there would still be some air resistance up there. That would slowly decrease your velocity in the forward direction, and you would start to get closer to the earth on each orbit, moving in a spiral. This is what happens to satellites that get too low in their orbits and start to touch the atmosphere.

This is also what's happening to the stream of matter often shown spiraling into a black hole. The matter has some initial velocity in the forward direction (either because it came from a star orbiting the black hole, or just because it didn't happen to be heading straight for the black hole when it passed nearby), so it follows a curved path around the black hole. On the way, it collides with other stuff orbiting the black hole, which slows it down, similar to the effect of air resistance. This causes it to move in a spiral, getting closer and closer to the black hole until it disappears inside the event horizon. (There are other, more complicated things going on here, like tidal forces and some consequences of general relativity, but this is the basic idea.)

If you dropped an object straight "down" into a black hole without pushing it in the forward direction, it would just fall straight in without spiraling. You can try this in one of those spiral "wishing well" coin funnels, which are a little bit like black holes because they model the curved spacetime that causes the force of gravity. Dropping the coin in on the little ramp that you're supposed to use gives it some forward velocity and causes it to spiral, but if you just drop the coin straight towards the center, it will fall right in.

The straight path isn't nearly as fun to watch, or for artists to draw.

Rebecca Holmes

(published on 08/24/2014)

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