Exploring the Physics of Making Black Holes

[docnet]

TL;DR Summary

How much physics do you need to understand how black holes are created?

Hello,

This is a question I've been pondering on my own for some time. I have no formal education in advanced physics, only at the introductory undergrad level.

I read that immense pressure can create black holes by compressing matter. I've read Laurence Krauss explain you'd need to compress Earth to the size of an apple. After approaching a certain density, the gravitational pull of the object on itself becomes great enough to overcome the electromagnetic repulsion between atoms. This makes sense because gravity has an inverse square relationship with distance. If an object is imploding, it will continue imploding until it has zero volume.

My question is do physicists explain how gravity becomes greater than the electromagnetic repulsion between atoms? Electromagnetic repulsion has the same relationship with distance, and the electromagnetic forces between atoms are much stronger than gravity at large distances. It seems like the repulsive forces should just get stronger with increasing density. Why does after a certain density does the EM force give?

Another wacky question: If we assume that atoms of different elements have varying strengths of electromagnetic repulsion, would a black hole created from titanium have a different density than a black hole created from an equal mass of helium?

 

[PeterDonis]

I read that immense pressure can create black holes by compressing matter.

Thinking of it this way is probably not really useful. Compression of matter is not required for gravitational collapse, and is not really a significant factor in any of the astronomical models of gravitational collapse of things like massive stars to black holes. In fact, the key ingredient is usually loss of pressure; see further comments below.

I've read Laurence Krauss explain you'd need to compress Earth to the size of an apple.

A specific reference would be helpful. I suspect it's going to be a pop science reference, which will then lead to the usual response that pop science sources, even when they are by scientists, aren't good sources if you want to actually understand the science.

After approaching a certain density, the gravitational pull of the object on itself becomes great enough to overcome the electromagnetic repulsion between atoms.

Note, first, that "gravitational pull" is not the same as "compression".

Second, the process being described here is not really a description of collapse to a black hole. It's a description of collapse to a white dwarf or neutron star. See further comments below.

This makes sense because gravity has an inverse square relationship with distance.

This Newtonian view of gravity works OK for white dwarfs, but not for neutron stars or black holes. For those, you need relativity, and in relativity gravity is not a force. It's spacetime geometry.

do physicists explain how gravity becomes greater than the electromagnetic repulsion between atoms?

Yes, but, as noted above, the explanation is not an explanation of how black holes form; it's an explanation of how white dwarfs and neutron stars form.

Basically, as stars reach the end of their main sequence lifetime and fusion reactions in their cores stop, they can no longer produce thermal pressure to balance their gravity, so they shrink. Eventually they reach a small enough size and a large enough density that degeneracy pressure--pressure due to the Pauli exclusion principle as fermions get squeezed into a smaller and smaller volume--can hold them up. In the case of white dwarfs, it is electron degeneracy pressure--at white dwarf densities and pressures, the ordinary structure of atoms cannot be maintained, and the electrons get squeezed out of the atoms and behave like an "electron gas". In neutron stars, densities and pressures are so high that the electrons get squeezed into the protons of the atomic nuclei, forming neutrons that behave like a "neutron gas".

In other words, what you are calling the "electromagnetic repulsion" between atoms is really a particular kind of quantum state that cannot be maintained once pressures and densities get high enough. It's not really a matter of the repulsion getting "overcome", it's more a matter of the possible quantum states changing as pressures and densities get higher.

It turns out that there is a maximum possible mass for both white dwarfs and neutron stars, so if a gravitational collapse occurs with an object that is over both of those maximum masses, then the collapse will not stop at either the white dwarf stage or the neutron star stage, but will continue on into a black hole. But whether or not that happens is not driven by whether or not the electromagnetic repulsion between atoms is overcome (as described above). That can happen without a black hole ever forming.

If we assume that atoms of different elements have varying strengths of electromagnetic repulsion, would a black hole created from titanium have a different density than a black hole created from an equal mass of helium?

A black hole doesn't have a well-defined density at all. But it is true that there is no way of telling what the matter that collapsed to form a black hole was made of; all black holes of the same mass are identical (we are here ignoring spin and charge), no matter what they were formed from.

 

[Janus]

Long answer short: The molecules/ atoms are electrically neutral ( as many positive charges as negative). Under the immense pressure, the electrons are squeezed down towards the positively charged nucleus, where they merge with the protons and form neutrons(which are neutral, and don't repel each other.)

 

[docnet]

A black hole doesn't have a well-defined density at all. But it is true that there is no way of telling what the matter that collapsed to form a black hole was made of; all black holes of the same mass are identical (we are here ignoring spin and charge), no matter what they were formed from.

I'm always amazed at the breadth of knowledge of physicists. Thank you for typing out an informative reply. I didn't know there were so many complicated factors that lead to black holes. I'm studying differential geometry in uni next semester. I'm an aspiring biologist and I will have time to learn some general relativity on my own :)

 

[PeterDonis]

Thank you for typing out an informative reply.

You're welcome!