Why Don't Singularities Instantly Suck Everything In?

[uknova]

I have an interest in physics, I have not studied it at any high levels (as u will soon find out ), i just like to read and think about it, but I have a question, please don't blind me with maths!

If a singularity at a black hole has infinity density, I'm guessing this means it has infinite gravity. If someone has infinite gravity, won't everything in the universe get sucked into it instantaneously? Why does this not happen?

Sorry if my question sounded a bit dumb, which i guess it probably did!

 

[JesseM]

If a singularity at a black hole has infinity density, I'm guessing this means it has infinite gravity. If someone has infinite gravity, won't everything in the universe get sucked into it instantaneously? Why does this not happen?

The answer is that it doesn't have infinite gravity, gravity is based on total mass rather than density, and the mass of the singularity is not infinite. For example, if the sun were replaced by a black hole of equal mass, the gravitational pull felt by the Earth wouldn't change, so it would remain in the same orbit.

 

[uknova]

how does something that has no size have a finite mass ?

 

[JesseM]

how does something that has no size have a finite mass ?

mass doesn't really have anything to do with size, except in the sense that normally mass = density*volume...but obviously this formula breaks down when the density is infinite and the volume is zero, infinity times zero doesn't have a well-defined answer. But obviously if you keep cutting the volume in half while doubling the density, the mass will stay constant, and that's basically what's going on when the matter in a star is being compressed into a singularity.