Random physics questions regarding black holes and electrons

Post Reply New Topic

I'm pretty sure black holes form because the star collapses and the density becomes so high that the compact ball of atoms basically rips a hole in spacetime. But I read that density does not in fact increase gravitational attraction. At least not in classical mechanics. Maybe it does in general relativity.

So the first question is, does gravity increase with density according to general relativity, and if not, how exactly does a black hole manage to be so catastrophically spacetime-ripping?

Now, assuming black holes do rip holes in spacetime because of high density:

A couple of months ago, I was thinking about electrons, and it occurred to me that they, according to a book I was reading, were "point particles", with no spacial extent. Now, if an electron has no spatial extent, than it must have no volume. If it has a volume of zero, and a finite, non-zero mass, then m/0=infinity, which means an infinite density, and thus, the electron becomes a black hole that sucks everything into its bottomless stomach. Or something. Which obviously doesn't happen, cause then there wouldn't be any life at all.

Is that a valid example of general relativity clashing with quantum mechanics? Or is it messed up somehow?

The only possible solution that I can think of is that the electron does have a non-zero volume, because Heisenberg's uncertainty principle smears it out over some region of space, so it can have a volume. Not sure that works, though.

I don't know much about relativity, or black holes, but I do know that gravitational pull doesn't increase with density. Take a giant star and shrink it to a point, and it's gravitational pull will be no greater than if it were still a star. However, if you get closer than the star's original radius, then you'll get a stronger gravitational pull.

All electrons are virtually the same. What if they are the same. What if electrons were more of a single dimensional field than individual units. That would make the universe seem a lot smaller than it appears if that were true.

What you say makes sense, unloser. It also removes the whole electrons-turning-into-black-holes thing. I think.

And aharon, i like the theory about the single dimensional field. Another theory is that there was one electron at the beginning of the universe. This electron lives out its lonely existence as the sole thing in the universe (we might as well have one representative from every other fundamental particle, but in the meantime, for simplicity's sake, we'll assume it's just an electron) until it reaches the end of the universe. Then it decides to go back in time to the beginning of the universe. Now since an electron traveling back in time is an anti-electron, and vice-versa, the electron is now an antielectron. But, just like in Back to the Future when Doc meets himself, the original electron going forward in time is still there, from the moment it was created to the moment it turned back in time.
The antielectron, having reached the beginning of the universe, turns around, starts traveling forward in time again, and becomes an electron. Because of the time loops, there are now three electrons in the universe. If it keeps going like this, then the universe will eventually be full of electrons, perhaps an infinite number of them.
And that, folks, is a guess (made by some famous scientist who I can't remember the name of) as to why all electrons look the same. Because, really, they are the same electron.

The black-hole problem was discovered by Einstein and Rosen as part of their arguments against quantum mechanics. Bohr and Heisenberg countered with the notion of point particles and probabilistic reality. Some things with infinite density just don't form event horizons. It's part of the weirdness of QM.

Bending of time is an interesting concept to me. Hawking described it in one of his PBS programs utilizing, as an example, a ship at sea caught in an eddy. As the ship got pulled towards the center, the area around the ship (time) was also affected. As Spock would say, "Fascinating."

Just curious. I haven't studied it, but where are quarks in relation to the makeup of atoms or electrons?

Electrons are fundamental particles as far as we know (i.e. not made up of any others). Protons and neutrons are made up of 3 quarks (a combination of the up and down quarks that give their overall charge of 1 or 0, respectively) and are called baryons. There are also particles made of quark anti-quark pairs and those are called mesons.

Even in general relativity, gravity does not increase with density, only with total mass. Let's compare the the situation with electric forces. I'm sure you know Gauss's law which states that if you have an object with continuous charge with an imaginary sphere, then the the electric force field flowing through the surface of that sphere is the same as it would be if the total charge enclosed by that sphere were a single point charge in the centre. So, if you had a ball of continuous electric charge with total charge Q and radius R, say, and measure the electric field strength at a distance r from its centre, where r > R, then the electric field strength is just the one given by Coulomb's law: E = k Q/r^2, where k is Coulomb's constant. However, if r < R, then according to Gauss's law, we would then have to treat the total charge Q as the total charge enclosed by a sphere of radius r, in which case, it is the charge density, rho, times it's volume: Q = (4/3) * pi * rho * r^3, substituting this into the first equation gives, E = (4/3) * pi * rho * r. Gauss's law also works for gravity except that instead of using Coulomb's law as in the case electrostatics, you would use Newton's law of gravitational attraction in the case of Newtonian gravity or use post-Newtonian corrections in the case of general relativity.

Black holes don't suck. If you were to replace the Sun with a black hole of the same mass, the Earth would still have the same orbit as it always did. The gravity of the black hole would only get stronger as you get closer to it.

Thanks, jono. I'm glad gravitation doesn't increase with density. Cause if it did, that would be pretty weird.

It must be all those physics books for laymen that I read. That's where I learned it all. I don't think they explain everything right.