Posted by : varun in (Science)

Is a black hole a giant cosmic vacuum cleaner?

“Not really” would be the appropriate answer to this question. Now let us try to understand why the pull of the gravity is stronger towards the black hole. A black hole itself has a very minute. It cannot pull matter at large distances differently than any other object. At a long distance from the black hole the force of gravity falls off  as the distance grows, just as it does for normal objects.

Technically, the gravity of any spherical object behaves as if all the mass were intensified at one central point. Since most objects have surfaces, you will feel the strongest gravity of an object when you are on its surface. This is as close to its total mass as you can get. If you penetrated a spherical object with a unchanging mass density, getting nearer to its core, you would seem the force of gravity get weaker, not stronger. The force of gravity you feel depends on the mass that is interior to you, because the gravity from the mass behind you is precisely negated by the mass in the converse direction. Therefore, you will feel the strongest force of gravity from an object, for example a planet, when you are standing on the planet’s surface, because it is on the surface that you are nearest to its gross mass. Penetrating the surface of the planet does not expose you to more of the planet’s total mass, but actually exposes you to less of its mass. Now remember the size of a black hole is very small. Gravity near a black hole is very strong because objects can get extremely close to it and still be subject towards to its gross mass.

There is not anything exceptional about the mass of a black hole. A black hole is different from our experience not because of its mass, but because its radius has vanished. Far away from the black hole, you would feel the same strength of gravity as if the black hole were a normal star. But the force of gravity close to a black hole is tremendously powerful because you can get so close to its gross mass!

For demonstration, the surface of the Earth where we are currently right now is 6400 km away from the core of the Earth. The surface is as close as you can get and still be exposed to the total mass of the Earth. Thus, it is where you will feel the strongest gravity. If suddenly the Earth became a black hole (impossible!) and you remained at 6400 km from the new Earth-black hole, you would feel the same pull of gravity as you do today. For example, if you normally weigh 65 kgs, you would still weigh 65 kgs. The mass of the Earth remained constant, your distance from it has not changed, and hence you would experience the equal gravitational force as you feel on the surface of normal Earth. But with the Earth-black hole, it would be possible for you to get closer to the total mass of the Earth. Let’s say that you weigh 65 kgs standing on the surface of normal Earth. As you venture closer toward the Earth-black hole you would feel a stronger and stronger force. If you went to within 3200 km (half the radius of normal Earth) of the Earth-black hole you would weigh 260 kgs! Now let us consider that you dug your way to core of the Earth, you would weigh less than what weight at the surface, could be around 30 kgs or so, because there would be less Earth mass interior relative to you!

As another example, consider the Sun. If the Sun suddenly became a black hole (equally impossible!), the Earth would continue on its normal orbit and would feel the same force of gravity from the Sun as usual!

Hence, to be “sucked up” by a black hole, somebody has to get closer to it, otherwise, one will experience the equal force of gravity as compare to any normal star.