Can black holes merge to create larger event horizons?

[PeterDonis]

If you have a block of wood among smaller blocks of wood, that block look bigger in comparison. If you take that same block and put it in a pile of bigger blocks then that block looks smaller in comparison. But the block is the same size in both instances.

I understand all that. What I'm saying is that you need to think carefully about what specific concept of "energy" you are talking about, that "looks smaller in comparison" in some places and "looks bigger in comparison" in others, and what that is in comparison to. Just using the same word "energy" does not work, because the word "energy" does not denote a single concept the way "size" does in your example above; it can mean different concepts. When you do all that, I think you will find that energy does not work quite the way size does in your simple example above.

 

[Nugatory]

Isn't potential energy the direct result of gravitational energy?

I'm not even sure what you mean by "gravitational energy". Gravitational potential energy is created when you add energy in some other form to a system in such a way that work is done against the force of gravity, and is released when the force of gravity is allowed to do work on a mass... and it's a long ways and a lot of lost precision between there and "the direct result".

I understand your point about physics being a hobby for you, but what you're doing here isn't physics. Even a hobby requires some investment in the basic skills of that hobby before you can engage in it.

 

[Soule]

I edited to say:

You could also react the word "energy" with "temperature" in my previous post and it conveys the same concept.

 

[Soule]

I'm not even sure what you mean by "gravitational energy". Gravitational potential energy is created when you add energy in some other form to a system in such a way that work is done against the force of gravity, and is released when the force of gravity is allowed to do work on a mass... and it's a long ways and a lot of lost precision between there and "the direct result".

I understand your point about physics being a hobby for you, but what you're doing here isn't physics. Even a hobby requires some investment in the basic skills of that hobby before you can engage in it.

I have taken classes. In none of them was the concept described like this. This makes more sense. Though to be fair, my professor for the class that covered it was a consultant for cern and had very little interest in his students. This class was happening when cern thought it had measured neutrinos moving faster than light. So again, that's understandable

So a particle outside a black hole already has that energy intrinsically?

 

[PeterDonis]

You could also react the word "energy" with "temperature" in my previous post and it conveys the same concept.

That doesn't help, because "temperature" has the same ambiguity in general relativity that "energy" does.

 

[Soule]

That doesn't help, because "temperature" has the same ambiguity in general relativity that "energy" does.

:/ how would you describe the concept?

 

[Soule]

"Fry, for the love of god , I can't teach! I'm a professor!"
-Prof. Hubert J. Farnsworth

 

[Dale]

I dint think this analogy works because a new particle forms with its own potential energy leaving the original hadron (am i using that right?) With it's original energy. Forgive me. I'll have to rethink this when i have slept. I have a night schedule and it's past bedtime and i can hardly think.

The analogy works fine. Essentially, what you have is two particles in a static background gravitational potential interacting through some other attractive potential. It doesn't matter if that other potential is electromagnetic, strong, or weak. All that does is determine the exact mathematical form of the potential, but you are not looking at that level of detail.

Regardless of the details of the other potential, the only way for the tidal forces to separate the particles is for the loss in gravitational potential energy to be greater than the increase in the other potential energy.

 

[Dale]

Isn't potential energy the direct result of gravitational energy?

I am not sure what you are asking here. There is the gravitational potential and the other potential. There is an energy associated with each, as well as kinetic energy. You can exchange anyone of them for the other.

 

[pervect]

Cus I don't think potential energy would come into play here. New mass is created. Unless its a photon I guess XD.

I don't know why you think that. Ignoring the technical issues related to the concepts of mass and energy in GR, you might want to review the following simple question:

In the reaction of deuterium and tritum to form helium, $^2_1D + ^3_1T -> ^4_2He$ is the "mass" on the left hand side of the equation the same as the mass on the right? Assuming that you agree that they are not, then where does the discrepancy come from if not potential energy?

 

[PeterDonis]

:/ how would you describe the concept?

As I already have: by carefully distinguishing "energy at infinity", which is a constant of the photon's motion but is not directly measured by any observer at finite altitude, from "energy as measured by observers at finite altitude", which changes as a photon climbs out of a gravity well. The latter change is usually called "gravitational redshift".

Whether you attribute gravitational redshift to "the photon's energy changing" or "the photon's energy staying the same but its relationship to the observers changing" is a matter of interpretation, not physics; it depends on which concept, energy at infinity or energy as measured by observers at finite altitude, you want to label with the word "energy". The physics works the same and makes all the same predictions either way; it doesn't care which words you use to label which concepts.

 

[Soule]

As I already have: by carefully distinguishing "energy at infinity", which is a constant of the photon's motion but is not directly measured by any observer at finite altitude, from "energy as measured by observers at finite altitude", which changes as a photon climbs out of a gravity well. The latter change is usually called "gravitational redshift".

Whether you attribute gravitational redshift to "the photon's energy changing" or "the photon's energy staying the same but its relationship to the observers changing" is a matter of interpretation, not physics; it depends on which concept, energy at infinity or energy as measured by observers at finite altitude, you want to label with the word "energy". The physics works the same and makes all the same predictions either way; it doesn't care which words you use to label which concepts.

Ah. Sorry. I had not been exposed to the language yet. It just was what made sense to me.

 

[Soule]

To everyone else:

Yeah. That clarification of potential energy cleared things right up. Thanks for the help! I learned a lot! By far the neatest thing I learned was that gravity was far more literally a "negative energy" than I had originally pictured. This is useful :3.

 

[Soule]

OH! This reminds me.

There may not be small enough and stable enough black holes to produce those tidal forces, however what about when a black hole begins forming inside of a star? I'm not sure how it forms exactly (outside of the mass collapsing under its own gravity of course), but i assume it grows starting from a microscopic point. Would the tidal forces of a forming black hole be strong enough for quark separation? How does that work?

 

[Matterwave]

OH! This reminds me.

There may not be small enough and stable enough black holes to produce those tidal forces, however what about when a black hole begins forming inside of a star? I'm not sure how it forms exactly (outside of the mass collapsing under its own gravity of course), but i assume it grows starting from a microscopic point. Would the tidal forces of a forming black hole be strong enough for quark separation? How does that work?

A black hole is not really formed starting from a microscopic point, it's being formed as the entirety of the stellar core's mass is being compressed catastrophically (when the neutron degeneracy pressure is overwhelmed) into the Schwarzschild radius.

 

[mfb]

I'm not sure if I understand that. Inside the event horizon, tidal forces grow without bound for any black hole, don't they? So are you talking about the tidal forces for something hovering outside the event horizon?

I don't consider "inside" a region where current physics works. Sure, if you just ignore quantum effects at all, you can use GR to describe that region. But is that how real black holes look like? Where "look" is a bit ironic because we can never "see" it...

 

[Soule]

A black hole is not really formed starting from a microscopic point, it's being formed as the entirety of the stellar core's mass is being compressed catastrophically (when the neutron degeneracy pressure is overwhelmed) into the Schwarzschild radius.

Oh ok. That happens the instant the core reaches its Scwarzschild radius. I should have figured that out as that is the one thing about black holes I can calculate XD.

 

[PeterDonis]

A black hole is not really formed starting from a microscopic point, it's being formed as the entirety of the stellar core's mass is being compressed catastrophically (when the neutron degeneracy pressure is overwhelmed) into the Schwarzschild radius.

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.

 

[PeterDonis]

I don't consider "inside" a region where current physics works. Sure, if you just ignore quantum effects at all, you can use GR to describe that region. But is that how real black holes look like? Where "look" is a bit ironic because we can never "see" it...

I don't think the fact that we can't directly "see" inside the hole's horizon means that "current physics doesn't work" inside the horizon. For a black hole of stellar mass or larger, the curvature at the horizon is much smaller than any sort of curvature threshold where quantum effects should become important, at least according to our best current understanding. So Occam's razor says that physics inside the horizon, at least down to the point where the curvature *does* get large enough for quantum effects to be important, should work the same as physics outside the horizon--meaning that we should be able to use GR to describe it.

Yes, various speculations about how to resolve the black hole information paradox involve assuming that the above is not the case, that there are quantum effects that *are* important even at the horizon of a black hole of this size. But those are speculations. Unless and until we get some actual experimental evidence to back up those speculations, I don't think it's fair to just say that "current physics doesn't work" inside the horizon. Using GR to describe physics inside the horizon, for cases where the curvature there is small enough, is just extrapolating a well-confirmed theory into a regime in which it should still be valid, given our best current knowledge, even though we can't experimentally explore that regime directly. We do that in physics all the time; in fact, if we couldn't do that, physics would be useless, because it could only tell us about experiments that we've already done and regimes that we've already explored.

 

[Matterwave]

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 anyone 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]

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 anyone 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.

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]

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]

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]

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.

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).

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.