Tuesday, July 06, 2010

How could I have been so dense?

Admittedly, Wikipedia is not the world’s most reliable source, but this one from its list of common misconceptions is really incredibly obvious:

Black holes, unlike the common image, do not act as cosmic vacuum cleaners any more than other stars. When a star evolves into a black hole, the gravitational attraction at a given distance from the body is no greater than it was for the star. That is to say, were the Sun to be replaced by a black hole of the same mass, the Earth would continue in the same orbit (assuming spherical symmetry of the sun). Due to a black hole's formation being explosive in nature, the object would lose a certain amount of its energy in the process, which, according to the mass–energy equivalence, means that a black hole would be of lower mass than the parent object, and actually have a weaker gravitational pull.

A moment’s thought confirms that this makes perfect sense, and yet it had never occurred to me.

Yes, black holes have a huge gravitation pull for their size, but for their mass their pull is normal. So the reason nothing can escape the pull from inside a hole’s event horizon would simply be that of close proximity to so much densely-packed mass; by contrast, if you’re on the surface of (or even inside) a large, heavy star, you’re still quite a distance from most of the star’s mass. I think that’s right. It sounds right…

(Via Matt.)

1 comment:

Eunoia said...

Surely the Black Hole formation is IMplosive in nature???????