5 ms·

In principle, yes. The Einstein field equations are time symmetric, so you could reverse the situation by pumping in gravitational waves from a great distance

In principle, yes. The Einstein field equations are time symmetric, so you could reverse the situation by pumping in gravitational waves from a great distance in a spherically symmetric way, and have them converge in some central region in such a way as to increase the mass of a black hole at the center, or have it split up into two more massive black holes.

iaw 10y ago
I think the practical limitation here is generating and concentrating/focusing the gravitational waves. EDIT: And there would probably have to be some control of the relative phase of each component wave as well.
- deleted 10y ago
  [deleted]
dandare 10y ago
Isn't it in principle equally difficult as generating concentrial waves on the surface of a swimming pool in such way as to eject a swimmer back on the podium where he jumped in from?
- antognini 10y ago
  Yes, it's almost an exactly analogous situation. The difficulty is that you're going against the second law of thermodynamics in both cases.
  - Aelinsaar 10y ago
    ...Which is at least one going notion as to why the arrow of time exists at all.
  - deleted 10y ago
    [deleted]
- JonnieCache 10y ago
  Like this? ;) https://youtu.be/WffR6HrEqTA?t=39 https://youtu.be/WffR6HrEqTA?t=39
  - ar0b 10y ago
    So you could theoretically build a black whole generator by building a sphere that could emit gravitational waves with precision?
    - Aelinsaar 10y ago
      You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible. Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard; your black hole generators inevitably keep collapsing into black holes or at least neutron stars.
      - b34r 10y ago
        Some A+ structural integrity fields I suppose.
        Aelinsaar 10y ago
        Oh yeah, with daily baryon sweeps to keep the tribbles out!
      - fizx 10y ago
        > You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible. I don't think that's technically true. You could build a bunch of catapults on the edge of the sphere, and when they all launch rocks at the center of the sphere, they would eventually form a black hole. The catapults could be arbitrarily far from each other as the radius of the sphere increases, such that they would not really do much to each other gravitationally. You'd just have to wait a real long time for the rocks to hit the middle. > Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard; Well yes, galaxy-sized intelligently designed structures don't really happen.
- therein 10y ago
  That analogy was great. It does an awesome job at explaining the disproportionate amount of energy that needs to be focused at a point in space.
- im3w1l 10y ago
  No. It's in principle as easy as using array of antennas designed to reinforce each others signals. It's used in normal FM radio.
keyme 10y ago
So can "specially crafted" gravity waves converge into a point of negative mass?
drauh 10y ago
It won't be difficult once we understand how quantum gravity works. :)
lisper 10y ago
Hm, that's interesting. That means that a black hole could spontaneously disappear (or split into two) depending on its internal state. Observing that a black hole does not do this would then give you information about its internal state. This would be a counter-example to the no-hair conjecture, which would be a major breakthrough. So something must be wrong here. It can't be that simple.
- gjm11 10y ago
  No-hair is about stable black holes in a steady state. When (e.g.) two black holes merge to form a third, its transient state may be more complicated; I guess the "hair" decays exponentially. (Disclaimer: Not an actual physicist.)
  - antognini 10y ago
    Yes, that's right. When two black holes first merge, the resulting black hole has a sort of peanut shape which contains lots of information (i.e., hair). Such a black hole is not stable, however, and emits lots of gravitational waves that carry all this information away (a process known as balding).
    - lisper 10y ago
      Thanks! But why then is that not a general solution to the black hole information paradox?
      - raattgift 10y ago
        The paradox is an almost wholly thoretical problem. Our civilization will never observe even a stellar black hole evaporating, and UMBHs might never evaporate even in the enormously far future limit where their Hubble volumes are effectively de Sitter vacuum. If a black hole is always colder than the relic and cosmological horizon radiation even as we approach de Sitter vacuum, then it won't evaporate. No evaporation, no missing information -- it's just somewhere else in spacetime. If a black hole only mostly evaporates, the remnant holds information, and so no information loss paradox. Just a problem about how to assess the entropy. If a black hole generically isn't no hair (i.e., if no hair doesn't hold up at all, or only holds up in cases like exactly spherical, stationary black holes in true vacuum) then the hair holds information, and so no information loss paradox. Just a problem about the nature of the hair. Hawking, Perry and Strominger are leaning towards "soft hair" as extremely long wavelength particles (wavelengths on the order of the Hubble diameter), and while there are lots and lots of as-yet-unanswered questions in that approach, it is not ridiculous. There are other possibilities too. This is an issue which has had forty years of study. The problem is that when you take a _model_ black hole deliberately arranged so that it has the greatest chance of being wholly determined by the eleven free parameters of no hair, and put it in dS electrovac and run that to the far future, so it also has the greatest chance of evaporating, there are a couple of problems. Firstly, fully classically, we cannot know (because of no hair) if we grew the black hole by throwing in one shell of matter or two shells of matter of half the mass each, or ten shells of matter whose differing masses sum up to the same as the previous two cases. If we don't evaporate, we don't care; if we have a remnant, we don't care; if we fully evaporate, we have lost the information about the number of shells and their individual masses. Worse, when we throw in the shells we have the Hawking radiation temperature rise from T_start to T_end. If we arrange it so that an observer can measure the Hawking radiation as we throw in each shell, that observer will see T_start, T_start+1, ... T_end-1, T_end, so that observer has information that is otherwise lost. So, hair? Secondly, we can make this worse by turning classical matter shells into individual quantum particles. I need to simplify here. If we throw in two alphas and a handful of electrons, and have no hair, an outside observer will not be able to know if you threw those in, or threw in two 4-He atoms, or 8 2-H atoms, and so forth. The no hair black hole evaporates into photons, and thus you have a version of the Knapsack Problem, with the unfortunate side effect that by the AMPS argument, you must have violated at least one of unitarity for at least some observers, the equivalence principle (in particular the "no drama" condition at the horizon of a sufficiently massive black hole that spacetime curvature is effectively flat at the horizon), the holographic principle (in particular, AdS/CFT's view of it), or semiclassical gravity as an effective (in the Wilson sense) theory of gravitation outside the horizon. People argue about which the worst thing to give up would be, and many qc-gr papers have been written in the past couple of years where one or more is gleefully abandoned, with the consequences followed to logical extremes. The Hawking-Perry-Strominger approach only deals with the classical problem, hoping to extend that into the quantum realm. There are several other ideas theirs competes with, many of which start with some quantum view hoping to extend that to realistically massive black holes. In any event, resolution has proven to be non-trivial. However, the tl;dr version is that yes, adding in some hair MAY solve the information loss paradox, but there are other approaches, and no good way to choose among them. Real black holes might indeed be very hairy at all times. They certainly don't live in de Sitter vacuum (or even dS electrovac) today or any time soon, and they even less sit in AdS (and our universe's quantum fields are not conformally invariant). However, these still seem like acceptable conditions for _models_ of black holes where one is probing the nature of the mathematical theories that describe them in detail, and in particular General Relativity.

iaw 10y ago

I think the practical limitation here is generating and concentrating/focusing the gravitational waves. EDIT: And there would probably have to be some control of the relative phase of each component wave as well.

deleted 10y ago
[deleted]

dandare 10y ago

Isn't it in principle equally difficult as generating concentrial waves on the surface of a swimming pool in such way as to eject a swimmer back on the podium where he jumped in from?

antognini 10y ago
Yes, it's almost an exactly analogous situation. The difficulty is that you're going against the second law of thermodynamics in both cases.
- Aelinsaar 10y ago
  ...Which is at least one going notion as to why the arrow of time exists at all.
- deleted 10y ago
  [deleted]
JonnieCache 10y ago
Like this? ;) https://youtu.be/WffR6HrEqTA?t=39 https://youtu.be/WffR6HrEqTA?t=39
- ar0b 10y ago
  So you could theoretically build a black whole generator by building a sphere that could emit gravitational waves with precision?
  - Aelinsaar 10y ago
    You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible. Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard; your black hole generators inevitably keep collapsing into black holes or at least neutron stars.
    - b34r 10y ago
      Some A+ structural integrity fields I suppose.
      - Aelinsaar 10y ago
        Oh yeah, with daily baryon sweeps to keep the tribbles out!
    - fizx 10y ago
      > You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible. I don't think that's technically true. You could build a bunch of catapults on the edge of the sphere, and when they all launch rocks at the center of the sphere, they would eventually form a black hole. The catapults could be arbitrarily far from each other as the radius of the sphere increases, such that they would not really do much to each other gravitationally. You'd just have to wait a real long time for the rocks to hit the middle. > Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard; Well yes, galaxy-sized intelligently designed structures don't really happen.
therein 10y ago
That analogy was great. It does an awesome job at explaining the disproportionate amount of energy that needs to be focused at a point in space.
im3w1l 10y ago
No. It's in principle as easy as using array of antennas designed to reinforce each others signals. It's used in normal FM radio.

keyme 10y ago

So can "specially crafted" gravity waves converge into a point of negative mass?

drauh 10y ago

It won't be difficult once we understand how quantum gravity works. :)

lisper 10y ago

Hm, that's interesting. That means that a black hole could spontaneously disappear (or split into two) depending on its internal state. Observing that a black hole does not do this would then give you information about its internal state. This would be a counter-example to the no-hair conjecture, which would be a major breakthrough. So something must be wrong here. It can't be that simple.

gjm11 10y ago
No-hair is about stable black holes in a steady state. When (e.g.) two black holes merge to form a third, its transient state may be more complicated; I guess the "hair" decays exponentially. (Disclaimer: Not an actual physicist.)
- antognini 10y ago
  Yes, that's right. When two black holes first merge, the resulting black hole has a sort of peanut shape which contains lots of information (i.e., hair). Such a black hole is not stable, however, and emits lots of gravitational waves that carry all this information away (a process known as balding).
  - lisper 10y ago
    Thanks! But why then is that not a general solution to the black hole information paradox?
    - raattgift 10y ago
      The paradox is an almost wholly thoretical problem. Our civilization will never observe even a stellar black hole evaporating, and UMBHs might never evaporate even in the enormously far future limit where their Hubble volumes are effectively de Sitter vacuum. If a black hole is always colder than the relic and cosmological horizon radiation even as we approach de Sitter vacuum, then it won't evaporate. No evaporation, no missing information -- it's just somewhere else in spacetime. If a black hole only mostly evaporates, the remnant holds information, and so no information loss paradox. Just a problem about how to assess the entropy. If a black hole generically isn't no hair (i.e., if no hair doesn't hold up at all, or only holds up in cases like exactly spherical, stationary black holes in true vacuum) then the hair holds information, and so no information loss paradox. Just a problem about the nature of the hair. Hawking, Perry and Strominger are leaning towards "soft hair" as extremely long wavelength particles (wavelengths on the order of the Hubble diameter), and while there are lots and lots of as-yet-unanswered questions in that approach, it is not ridiculous. There are other possibilities too. This is an issue which has had forty years of study. The problem is that when you take a _model_ black hole deliberately arranged so that it has the greatest chance of being wholly determined by the eleven free parameters of no hair, and put it in dS electrovac and run that to the far future, so it also has the greatest chance of evaporating, there are a couple of problems. Firstly, fully classically, we cannot know (because of no hair) if we grew the black hole by throwing in one shell of matter or two shells of matter of half the mass each, or ten shells of matter whose differing masses sum up to the same as the previous two cases. If we don't evaporate, we don't care; if we have a remnant, we don't care; if we fully evaporate, we have lost the information about the number of shells and their individual masses. Worse, when we throw in the shells we have the Hawking radiation temperature rise from T_start to T_end. If we arrange it so that an observer can measure the Hawking radiation as we throw in each shell, that observer will see T_start, T_start+1, ... T_end-1, T_end, so that observer has information that is otherwise lost. So, hair? Secondly, we can make this worse by turning classical matter shells into individual quantum particles. I need to simplify here. If we throw in two alphas and a handful of electrons, and have no hair, an outside observer will not be able to know if you threw those in, or threw in two 4-He atoms, or 8 2-H atoms, and so forth. The no hair black hole evaporates into photons, and thus you have a version of the Knapsack Problem, with the unfortunate side effect that by the AMPS argument, you must have violated at least one of unitarity for at least some observers, the equivalence principle (in particular the "no drama" condition at the horizon of a sufficiently massive black hole that spacetime curvature is effectively flat at the horizon), the holographic principle (in particular, AdS/CFT's view of it), or semiclassical gravity as an effective (in the Wilson sense) theory of gravitation outside the horizon. People argue about which the worst thing to give up would be, and many qc-gr papers have been written in the past couple of years where one or more is gleefully abandoned, with the consequences followed to logical extremes. The Hawking-Perry-Strominger approach only deals with the classical problem, hoping to extend that into the quantum realm. There are several other ideas theirs competes with, many of which start with some quantum view hoping to extend that to realistically massive black holes. In any event, resolution has proven to be non-trivial. However, the tl;dr version is that yes, adding in some hair MAY solve the information loss paradox, but there are other approaches, and no good way to choose among them. Real black holes might indeed be very hairy at all times. They certainly don't live in de Sitter vacuum (or even dS electrovac) today or any time soon, and they even less sit in AdS (and our universe's quantum fields are not conformally invariant). However, these still seem like acceptable conditions for _models_ of black holes where one is probing the nature of the mathematical theories that describe them in detail, and in particular General Relativity.