Register to reply

Tidal forces of a black hole

by Soule
Tags: black hole
Share this thread:
Matterwave
#55
Jun22-14, 12:41 PM
Sci Advisor
Matterwave's Avatar
P: 2,801
Quote Quote by PeterDonis View Post
Actually, if "black hole" means "event horizon", then the horizon *does* start as a point and grows in radius. This happens inside the collapsing matter; the event horizon reaches the Schwarzschild radius corresponding to the total mass of the collapsing matter at the same instant as the outer surface of the matter reaches that radius as it collapses. After that instant, the horizon stays at the same radius forever (unless more matter falls in).

The tidal forces at the horizon as it grows, however, are never larger than the tidal forces at the horizon after the matter has collapsed through it. So if the black hole's tidal gravity at the horizon isn't enough to separate quarks, the tidal gravity while it's forming won't be either.
From a purely GR standpoint (with all the assumptions of spherical symmetry, and isotropy, etc.) you might be correct. But from an astrophysical standpoint, I don't think you can really say for sure any one point where an event horizon starts to grow. The collapse of a stellar core is a catastrophic process, occurring in time scales of micro-seconds to milliseconds. Additionally, there is no guarantee that all of the assumptions of isotropy and homogeneity, for example, are met.

I felt it prudent, to inform the OP that the stellar collapse process is a catastrophic one, in order to distance him from the idea that stellar collapse happens starting from one tiny point and slowly growing outwards. But if there is a problem with this idea, then be sure to correct me. :)
PeterDonis
#56
Jun22-14, 02:07 PM
Physics
Sci Advisor
PF Gold
P: 6,135
Quote Quote by Matterwave View Post
From a purely GR standpoint (with all the assumptions of spherical symmetry, and isotropy, etc.) you might be correct. But from an astrophysical standpoint, I don't think you can really say for sure any one point where an event horizon starts to grow.
Dropping the assumption of spherical symmetry certainly makes things more complicated; AFAIK there are no analytical solutions for the general case, only numerical simulations. But AFAIK that does not change the qualitative features that I described. Bear in mind that the event horizon is the boundary of the region of spacetime that can't send light signals to future null infinity; qualitatively, such a region *has* to start with a single point (more precisely, there has to be some earliest spacelike hypersurface that the EH intersects, and it must intersect that hypersurface at a single point), even if the details of the process are not spherically symmetric.

Quote Quote by Matterwave View Post
I felt it prudent, to inform the OP that the stellar collapse process is a catastrophic one, in order to distance him from the idea that stellar collapse happens starting from one tiny point and slowly growing outwards. But if there is a problem with this idea, then be sure to correct me. :)
Bear in mind that I was only talking about the event horizon, not about the entire process of collapse. I agree that the collapse process does not start at a single point and grow outwards. Only the EH does.
Matterwave
#57
Jun22-14, 04:32 PM
Sci Advisor
Matterwave's Avatar
P: 2,801
Quote Quote by PeterDonis View Post
Dropping the assumption of spherical symmetry certainly makes things more complicated; AFAIK there are no analytical solutions for the general case, only numerical simulations. But AFAIK that does not change the qualitative features that I described. Bear in mind that the event horizon is the boundary of the region of spacetime that can't send light signals to future null infinity; qualitatively, such a region *has* to start with a single point (more precisely, there has to be some earliest spacelike hypersurface that the EH intersects, and it must intersect that hypersurface at a single point), even if the details of the process are not spherically symmetric.
But if the process is not necessarily spherically symmetric, which point would the universe know to choose to begin creating an EH? Wouldn't it be plausible that at many different places in the core, you have densities high enough to create multiple EHs and then they merge to form a large black hole?

But anyways, my point was only that the supernova process is a very chaotic one. And one should not so quickly jump to conclusions based on very nice initial conditions.
Soule
#58
Jun22-14, 05:04 PM
P: 27
Quote Quote by Matterwave View Post
But if the process is not necessarily spherically symmetric, which point would the universe know to choose to begin creating an EH? Wouldn't it be plausible that at many different places in the core, you have densities high enough to create multiple EHs and then they merge to form a large black hole?
I was thinking that too >.> lol.
PeterDonis
#59
Jun26-14, 08:36 PM
Physics
Sci Advisor
PF Gold
P: 6,135
Quote Quote by Matterwave View Post
But if the process is not necessarily spherically symmetric, which point would the universe know to choose to begin creating an EH?
It's not a question of "choosing to create an EH". The EH is globally defined; it's the boundary of the region that can't send light signals to future null infinity. That boundary must be a null surface, i.e., a surface generated by light rays. And for a single collapsing object (this is to distinguish from the case of multiple black holes merging--see below), the null surface forming the boundary must intersect any spacelike hypersurface in either a point, or a 2-surface; and given a slicing of spacetime into spacelike hypersurfaces, the hypersurface which the EH boundary (of the single collapsing object) intersects in a point must be to the past of any hypersurfaces in the same slicing that the EH intersects in a 2-surface.

Quote Quote by Matterwave View Post
Wouldn't it be plausible that at many different places in the core, you have densities high enough to create multiple EHs and then they merge to form a large black hole?
Yes, that's true, and I should have clarified that my statement was talking about each black hole individually, not about the set of all black holes in the universe. In other words, each black hole individually starts out with its own "section" of event horizon that works as I described above; later on, the "sections" of EH associated with different black holes can merge, but I wasn't intending to describe the merge process (it must be to the future of each of the individual sections anyway, so it doesn't invalidate what I said above).

Quote Quote by Matterwave View Post
my point was only that the supernova process is a very chaotic one. And one should not so quickly jump to conclusions based on very nice initial conditions.
Nothing that I stated depends on any symmetry in the initial conditions. It's purely a consequence of global geometric facts about *any* null surface and *any* set of spacelike hypersurfaces. It's easier to visualize for a highly symmetric collapse, but that's all.


Register to reply

Related Discussions
Black holes and tidal forces! Astronomy & Astrophysics 4
Radial tidal forces and tidal forces on moving objects redux Special & General Relativity 0
Tidal forces near a black hole Special & General Relativity 1
Black Hole Tidal Forces Astronomy & Astrophysics 1
Tidal Forces Near a Black Hole Advanced Physics Homework 12