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In my imagination, I always thought we could put a black box around a black hole, and it would be indistinguishable from any other mass - that is, any other mas

In my imagination, I always thought we could put a black box around a black hole, and it would be indistinguishable from any other mass - that is, any other mass that can be treated as a point mass. I.e. put a black hole with solar mass 1 in a black box. Put a star with solar mass 1 in another black box. From a gravitational point of view, you couldn't tell the difference, yes? But this result implies that the black box with the black hole will gain mass over time, even without adding any mass into the black box? So you could distinguish it from another mass? Or do I have that wrong? My understanding is as someone who is interested but has no real education on these topics.

bmitc 4y ago
It is my understanding that, from a gravity-only standpoint, you are right. But I actually thought that black holes slowly evaporate, i.e., lose mass, from emitting Hawking radiation. It isn't clear from the article whether the vacuum energy black holes still have that property. The article confuses me on something else. It mentions a link between black hole mass and the expansion of the universe, but then it seems to imply that the expansion causes the black holes to gain mass which in turn causes the expansion to accelerate. It doesn't seem to address why the universe is expanding in the first place. But I guess dark energy was proposed as the thing that was doing the expansion acceleration, and not the expansion cause.
- pdonis 4y ago
  > I actually thought that black holes slowly evaporate This is believed to be true, but the time scale is something like 60 or more orders of magnitude longer than the age of the universe, so (a) no evidence for this effect exists or is likely to be found any time soon, and (b) it's irrelevant for the dynamics of our current universe anyway.
  - bmitc 4y ago
    That makes sense. I forgot about the timescales for the evaporation. Thanks!
  - justinpombrio 4y ago
    I thought we had evidence that black holes evaporate in that Earth hasn't been swallowed up yet. A while back there were concerns (notably not from physicists) about the LHC forming black holes. I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions, but black holes emit more radiation the smaller they are(!), so these tiny black holes evaporate nearly instantly. (Thus the same would happen if the LHC made any.) A tiny black hole that didn't evaporate would be scary because it could grow larger but not smaller.
    - andrewflnr 4y ago
      To be more precise, if LHC was capable of forming black holes, then they would also be regularly formed in the upper atmosphere by cosmic rays... but more likely neither of those is the case. I don't think many serious physicists actually think particle collisions create black holes.
      - ISL 4y ago
        Collisions of sufficient energy could potentially create tiny black holes. GP has the right idea though -- evaporation goes power-law faster the smaller the black hole. No matter what the mechanism for protection from cosmogenic-collision black holes, if they were problematic, the Sun would have been destroyed long ago through a black-hole creation, black-hole capture, solar-collapse process with cosmic rays much higher in energy than anything humans will ever generate. So, as long as you can look outside and see the sun, you need not ever sweat the particle-collision destroys the world hypothesis, no matter whence the particles are generated.
    - pdonis 4y ago
      > A while back there were concerns (notably not from physicists) about the LHC forming black holes. There were concerns, but they were not well founded in actual physics. > I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions I'm not aware of any such response. The response I'm aware of was that events with higher energy than the LHC is capable of creating happen routinely in cosmic ray collisions, and no black hole formation has ever been observed in such collisions, so black hole formation is not going to happen at the LHC either. That is consistent with our best current theoretical prediction, which is that you would need an accelerator capable of reaching the Planck scale, many orders of magnitude higher energy than the LHC, for black hole production to be possible.
- ikrenji 4y ago
  evaporation could be relevant for small blackholes, eg the tiniest ones quickly disappear
- mrtweetyhack 4y ago
  [dead]
pdonis 4y ago
> I always thought we could put a black box around a black hole, and it would be indistinguishable from any other mass - that is, any other mass that can be treated as a point mass. Yes, that's what the standard theory of black holes says. > this result implies that the black box with the black hole will gain mass over time, even without adding any mass into the black box? Sort of. First, it's important to note that the paper is talking about a special type of "black hole", an object that has "vacuum energy" inside it (which means something that acts like a cosmological constant in the Einstein Field Equation)--which isn't a standard black hole (those have zero stress-energy inside). The claim is basically that the total vacuum energy inside such an object can increase as the universe expands. However, this does not mean that the ordinary "mass" of the black hole would increase. Vacuum energy doesn't work like ordinary mass. The effect that this model is claimed to account for is the accelerated expansion of the universe due to dark energy; basically this model is supposed to provide a mechanism for how dark energy could come into existence as a result of black hole formation (but, again, it's a special kind of "black hole", not the ordinary kind).
- pmontra 4y ago
  If I understood the paper [1] correctly, the idea is that all black holes don't contain a singularity. They have vacuum energy instead and that leads to the increase of mass and dark energy. [1] https://iopscience.iop.org/article/10.3847/2041-8213/acb704/pdf https://iopscience.iop.org/article/10.3847/2041-8213/acb704/...
  - pdonis 4y ago
    > the idea is that all black holes don't contain a singularity More precisely, theoretically, we can construct models of compact objects that look like standard black holes, but don't have a singularity (and also don't have an event horizon, they only have apparent horizons). Any such compact object must contain "vacuum energy" or something equivalent, i.e., something that looks similar to a cosmological constant in the Einstein Field Equation--that is the only way to evade the conclusions of the various singularity theorems that apply to standard black holes. That type of compact object is what is being hypothesized in the paper under discussion.
smath 4y ago
You mention point mass. Yes, the volume also matters. If your second black box contains the same mass but over a bigger volume, then the spacetime curvature it will cause will be less extreme than the black hole in the first box. The book I most like on this topic is Kip Thorne's Black Holes and Time Warps. IMO Thorne is a better explainer than Hawking.
- ianred 4y ago
  Are we talking about the volume of the event horizon? If I understood it correctly, the total of the mass of a black hole is in its singularity. The volume of the event horizon will depend on the total mass of the black hole.
  - smath 4y ago
    Oh I just mean when comparing (A) block hole in a black box, vs (B) a non-black-hole start of the same mass, B will likely be over a large volume, and hence will produce different spacetime curvature.
- andrewflnr 4y ago
  I'm pretty sure by the time you're outside the box, assuming it's the same size for both, you can't tell anymore. I'm quite confident this is the case for classical gravity and a spherically symmetric "box", and I don't think tides or relativistic corrections are noticeably different far away from the horizon. (Yeah, you'll feel the black hole's tides, but stars have tides too.)
  - aqfamnzc 4y ago
    I'm not a relativity expert, but couldn't you tell the difference between a point mass (or just significantly smaller volume) and a star in this black box scenario? At a great distance, they would appear the same gravitationally, but as you get closer, the star would appear less massive. Since more mass would be pulling on you at an angle, rather than directly toward the center.
fsakura 4y ago
Why do you think the black box with the black hole will gain mass over time? AFAIK: On the contrary it will lose mass over time due to Hawking Radiation and evaporate eventually (though that might take literally forever). Also spacetime curvature will be slightly different for point mass vs distributed mass.
- andrewflnr 4y ago
  Because of the ideas in the article?
DiogenesKynikos 4y ago
The "no-hair theorem" says that black holes only have three properties: mass, angular momentum and electric charge. If a black hole is perturbed (for example, by merging with another black hole or swallowing a star), it will temporarily be more complicated, but then it quickly goes back to having only above three properties. The extra properties (such as the gravitational quadrupole moment) asymptomatically decay, over a relatively short timespan.
- btilly 4y ago
  The "no hair theorem" is a theorem of classical general relativity. Attempts to try to model it with some quantum mechanics thrown in show a tremendous amount of additional state that scales with the surface area of the black hole. This work suggests even more complications to that picture. That it looks very different from the classical theory. All of this should come with disclaimers and fudge factors because of our lack of a real theory reconciling GR with QM.
  - DiogenesKynikos 4y ago
    Black holes probably have additional state in a full quantum theory, such as lepton number. However, to an extremely good approximation, they'll still look like objects with just three properties.
    - pmayrgundter 4y ago
      In this paper they're talking about spinning black holes, the "Kerr" solutions to GR. Iiuc, there's a lot more structure with these type. See the table on this page for the categories: https://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric https://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric
    - btilly 4y ago
      Current thinking is that black holes have a LOT of state, not just a little. Anything else would result in loss of entropy. See https://physics.stackexchange.com/a/163046/6796 https://physics.stackexchange.com/a/163046/6796.
AmericanOP 4y ago
The expansion of space-time is an observed property of space. It has always been expanding, but at different rates. My interpretation of this theory is that spacetime beyond the event horizon is also expanding. This expansion increases vacuum space, which contains vacuum energy. This either correlates or is coupled with vacuum energy in our observable universe.
deleted 4y ago
[deleted]

bmitc 4y ago

It is my understanding that, from a gravity-only standpoint, you are right. But I actually thought that black holes slowly evaporate, i.e., lose mass, from emitting Hawking radiation. It isn't clear from the article whether the vacuum energy black holes still have that property. The article confuses me on something else. It mentions a link between black hole mass and the expansion of the universe, but then it seems to imply that the expansion causes the black holes to gain mass which in turn causes the expansion to accelerate. It doesn't seem to address why the universe is expanding in the first place. But I guess dark energy was proposed as the thing that was doing the expansion acceleration, and not the expansion cause.

pdonis 4y ago
> I actually thought that black holes slowly evaporate This is believed to be true, but the time scale is something like 60 or more orders of magnitude longer than the age of the universe, so (a) no evidence for this effect exists or is likely to be found any time soon, and (b) it's irrelevant for the dynamics of our current universe anyway.
- bmitc 4y ago
  That makes sense. I forgot about the timescales for the evaporation. Thanks!
- justinpombrio 4y ago
  I thought we had evidence that black holes evaporate in that Earth hasn't been swallowed up yet. A while back there were concerns (notably not from physicists) about the LHC forming black holes. I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions, but black holes emit more radiation the smaller they are(!), so these tiny black holes evaporate nearly instantly. (Thus the same would happen if the LHC made any.) A tiny black hole that didn't evaporate would be scary because it could grow larger but not smaller.
  - andrewflnr 4y ago
    To be more precise, if LHC was capable of forming black holes, then they would also be regularly formed in the upper atmosphere by cosmic rays... but more likely neither of those is the case. I don't think many serious physicists actually think particle collisions create black holes.
    - ISL 4y ago
      Collisions of sufficient energy could potentially create tiny black holes. GP has the right idea though -- evaporation goes power-law faster the smaller the black hole. No matter what the mechanism for protection from cosmogenic-collision black holes, if they were problematic, the Sun would have been destroyed long ago through a black-hole creation, black-hole capture, solar-collapse process with cosmic rays much higher in energy than anything humans will ever generate. So, as long as you can look outside and see the sun, you need not ever sweat the particle-collision destroys the world hypothesis, no matter whence the particles are generated.
  - pdonis 4y ago
    > A while back there were concerns (notably not from physicists) about the LHC forming black holes. There were concerns, but they were not well founded in actual physics. > I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions I'm not aware of any such response. The response I'm aware of was that events with higher energy than the LHC is capable of creating happen routinely in cosmic ray collisions, and no black hole formation has ever been observed in such collisions, so black hole formation is not going to happen at the LHC either. That is consistent with our best current theoretical prediction, which is that you would need an accelerator capable of reaching the Planck scale, many orders of magnitude higher energy than the LHC, for black hole production to be possible.
ikrenji 4y ago
evaporation could be relevant for small blackholes, eg the tiniest ones quickly disappear
mrtweetyhack 4y ago
[dead]

pdonis 4y ago

> I always thought we could put a black box around a black hole, and it would be indistinguishable from any other mass - that is, any other mass that can be treated as a point mass. Yes, that's what the standard theory of black holes says. > this result implies that the black box with the black hole will gain mass over time, even without adding any mass into the black box? Sort of. First, it's important to note that the paper is talking about a special type of "black hole", an object that has "vacuum energy" inside it (which means something that acts like a cosmological constant in the Einstein Field Equation)--which isn't a standard black hole (those have zero stress-energy inside). The claim is basically that the total vacuum energy inside such an object can increase as the universe expands. However, this does not mean that the ordinary "mass" of the black hole would increase. Vacuum energy doesn't work like ordinary mass. The effect that this model is claimed to account for is the accelerated expansion of the universe due to dark energy; basically this model is supposed to provide a mechanism for how dark energy could come into existence as a result of black hole formation (but, again, it's a special kind of "black hole", not the ordinary kind).

pmontra 4y ago
If I understood the paper [1] correctly, the idea is that all black holes don't contain a singularity. They have vacuum energy instead and that leads to the increase of mass and dark energy. [1] https://iopscience.iop.org/article/10.3847/2041-8213/acb704/pdf https://iopscience.iop.org/article/10.3847/2041-8213/acb704/...
- pdonis 4y ago
  > the idea is that all black holes don't contain a singularity More precisely, theoretically, we can construct models of compact objects that look like standard black holes, but don't have a singularity (and also don't have an event horizon, they only have apparent horizons). Any such compact object must contain "vacuum energy" or something equivalent, i.e., something that looks similar to a cosmological constant in the Einstein Field Equation--that is the only way to evade the conclusions of the various singularity theorems that apply to standard black holes. That type of compact object is what is being hypothesized in the paper under discussion.

smath 4y ago

You mention point mass. Yes, the volume also matters. If your second black box contains the same mass but over a bigger volume, then the spacetime curvature it will cause will be less extreme than the black hole in the first box. The book I most like on this topic is Kip Thorne's Black Holes and Time Warps. IMO Thorne is a better explainer than Hawking.

ianred 4y ago
Are we talking about the volume of the event horizon? If I understood it correctly, the total of the mass of a black hole is in its singularity. The volume of the event horizon will depend on the total mass of the black hole.
- smath 4y ago
  Oh I just mean when comparing (A) block hole in a black box, vs (B) a non-black-hole start of the same mass, B will likely be over a large volume, and hence will produce different spacetime curvature.
andrewflnr 4y ago
I'm pretty sure by the time you're outside the box, assuming it's the same size for both, you can't tell anymore. I'm quite confident this is the case for classical gravity and a spherically symmetric "box", and I don't think tides or relativistic corrections are noticeably different far away from the horizon. (Yeah, you'll feel the black hole's tides, but stars have tides too.)
- aqfamnzc 4y ago
  I'm not a relativity expert, but couldn't you tell the difference between a point mass (or just significantly smaller volume) and a star in this black box scenario? At a great distance, they would appear the same gravitationally, but as you get closer, the star would appear less massive. Since more mass would be pulling on you at an angle, rather than directly toward the center.

fsakura 4y ago

Why do you think the black box with the black hole will gain mass over time? AFAIK: On the contrary it will lose mass over time due to Hawking Radiation and evaporate eventually (though that might take literally forever). Also spacetime curvature will be slightly different for point mass vs distributed mass.

andrewflnr 4y ago
Because of the ideas in the article?

DiogenesKynikos 4y ago

The "no-hair theorem" says that black holes only have three properties: mass, angular momentum and electric charge. If a black hole is perturbed (for example, by merging with another black hole or swallowing a star), it will temporarily be more complicated, but then it quickly goes back to having only above three properties. The extra properties (such as the gravitational quadrupole moment) asymptomatically decay, over a relatively short timespan.

btilly 4y ago
The "no hair theorem" is a theorem of classical general relativity. Attempts to try to model it with some quantum mechanics thrown in show a tremendous amount of additional state that scales with the surface area of the black hole. This work suggests even more complications to that picture. That it looks very different from the classical theory. All of this should come with disclaimers and fudge factors because of our lack of a real theory reconciling GR with QM.
- DiogenesKynikos 4y ago
  Black holes probably have additional state in a full quantum theory, such as lepton number. However, to an extremely good approximation, they'll still look like objects with just three properties.
  - pmayrgundter 4y ago
    In this paper they're talking about spinning black holes, the "Kerr" solutions to GR. Iiuc, there's a lot more structure with these type. See the table on this page for the categories: https://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric https://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric
  - btilly 4y ago
    Current thinking is that black holes have a LOT of state, not just a little. Anything else would result in loss of entropy. See https://physics.stackexchange.com/a/163046/6796 https://physics.stackexchange.com/a/163046/6796.

AmericanOP 4y ago

The expansion of space-time is an observed property of space. It has always been expanding, but at different rates. My interpretation of this theory is that spacetime beyond the event horizon is also expanding. This expansion increases vacuum space, which contains vacuum energy. This either correlates or is coupled with vacuum energy in our observable universe.

deleted 4y ago

[deleted]