5 ms·

What if two blackholes with 50 solar masses collide into each other?

lostmsu 7y ago
AFAIK, nothing prevents that. You'd want blackholes with 100 solar masses though, as the collision will apparently shed significant part of the mass in gravitational waves.
- TCR19 7y ago
  So does that mean during a collision between blackholes mass actually does/can escape a blackhole? I thought mass couldn't escape, at least not in any observable way that we can detect. [I don't know a ton about astrophysics, so apologies if this question doesn't make sense].
  - saagarjha 7y ago
    Mass (or rather, energy) escaping a black hole through gravitational waves has been detected experimentally. Similarly, Hawking radiation allows for black holes to emit energy as well (though this is due to quantum effects near the event horizon).
  - ncmncm 7y ago
    No, it cannot. The only energy that can escape in the collision comes from the kinetic energy of the two bodies, and from their colliding accretion discs, and energy bound up in magnetic fields outside the holes. It is interesting that the kinetic energy of a black hole is technically outside the hole. Energy radiated as a consequence of pair production on the event horizon would fluctuate as the surface changed shape and area, but not much.
    - saagarjha 7y ago
      > It is interesting that the kinetic energy of a black hole is technically outside the hole. How so?
      - raattgift 7y ago
        It's not. Even in Newtonian mechanics kinetic energy is an observer-dependent quantity; in General Relativity we care about the stress-energy-momentum tensor, and you can slice that up into kinetic and potential energies as you wish by applying coordinates. The vast majority of the stress-energy in the immediate neighbourhood of an accreting black hole formed by stellar collapse is found inside the horizon, even if there is a substantial accretion structure. General Relativity doesn't make any predictions about the forms stress-energy-momentum can take; we get that from matter theories like classical (but relativistic) Maxwell's equations, quantum electrodynamics, or the full Standard Model. (One also runs into "toy" or "test" forms of matter -- various idealized space-filling fluids or dusts, mainly, that approximate matter in the large: huge numbers of stars or huge numbers of galaxies, for example). So General Relativity also has nothing much to say about kinetic energy vs potential energy: just that they each must contribute to the Einstein Field Equations and that typically means being encoded in the stress-energy tensor. The perfectly elastic bouncing of microscopic particles of a gas each bouncing in one dimension between opposite sides of the inside of a gas container produces a beautiful relationship between the kinetic energy of a gas and its pressure. https://en.wikipedia.org/wiki/Kinetic_theory_of_gases#Pressure_and_kinetic_energy https://en.wikipedia.org/wiki/Kinetic_theory_of_gases#Pressu... (PV = 2/3 K) Assuming isotropic pressure at time t, we encode an identical contribution into the pressure components of the stress-energy tensor (the green diagonals here https://en.wikipedia.org/wiki/Stress–energy_tensor#/media/File:StressEnergyTensor_contravariant.svg https://en.wikipedia.org/wiki/Stress–energy_tensor#/media/Fi... ) for every point within the gas cylinder. The inner regions of massive stars have a lot of pressure; collapsars like neutron stars have even more pressure deep within them. In a runaway collapse that leads to the formation of a black hole, pressure typically dominates the stress-energy tensor, driving the formation of the event horizon. By contrast, a Schwarzschild black hole is a vacuum solution of the Einstein Field Equation, meaning that the stress-energy tensor is everywhere zero. Thus there is no pressure. The source of the "central" mass inside the horizon of a Schwarzschild is best thought of as gravity self-gravitating, or if you like, "it's just defined as curvature alone until you throw a test object in". If we perturb the Schwarzschild black hole by throwing neutral test objects through the horizon, the stress-energy tensor inside the horizon must be somewhere nonzero, but once through the horizon (ignoring quantum effects) the nonzero stress-energy stays in there, and everywhere outside the stress-energy tensor returns to zero. An astrophysical black hole by stellar collapse locks up stress-energy in the same way: once it's inside the event horizon, it stays there (ignoring quantum effects, principally Hawking radiation). Stress-energy outside the horizon might cross the horizon in various ways, or it might form some long lived arrangement sufficiently far from the horizon. You wouldn't say that a white dwarf partner in a white-dwarf/black-hole binary forms "the kinetic [or other] energy of a black hole", would you? If not, then neither does any matter near -- but outside -- the horizon. The no-hair theorem(s) mean(s) that in general you cannot distinguish between an uncharged, zero-angular-momentum black hole formed by stellar collapse (or black hole mergers) and a Schwarzschild (vacuum) black hole (say, formed primordially from nothing but gravitational radiation), even in binaries. A binary of uncharged, no-angular-momentum black holes may be two Schwarzschild BHs or two astrophysical BHs or one of each. The momentum-energy that leaves the binary cannot come from the stress-energy tensor of a Schwarzschild black hole, because it's zero everywhere in the horizon. If no-hair is true, it can't come from the stress-energy tensor in the interior of a black hole formed by stellar collapse, either. A very large primordial BH will have had a bunch of things fall into it (if nothing else, lots of CMB photons), but can still have essentially no stress-energy inside. Primordial black holes are not especially crazy: that's one possible way to explain supermassive black holes ( https://en.wikipedia.org/wiki/Supermassive_black_hole#Formation https://en.wikipedia.org/wiki/Supermassive_black_hole#Format... and note it's possible that primordial black holes can start with and retain essentially zero angular momentum). A pair of primordial SMBHs, each near the centre of mass of merging galaxy clusters, may have eaten a bunch of stellar masses worth of dust and gas in their lifetimes, but not nearly enough to account for the gravitational radiation that will be emitted late in their inspiral, let alone during the merger and ringdown. Instead, it is the angular momentum of the binary system (as a whole, since in this sketch neither BH rotates) that must power the gravitational radiation during the inspiral.
sandworm101 7y ago
The chances of two random black holes ever colliding with each other (not a binary-pair situation) is astronomically rare. On cosmic scales, black holes are microscopic. They are far smaller than stars of similar masses, but have we ever seen two random stars just collide? Two black holes colliding would be like two bullets colliding if fired randomly from guns on different continents. The likelihood is so low that any detection within our sphere of observation would be very suspect.
- phkahler 7y ago
  We have not seen stars collide, but black hole mergers seem to be a regular thing now that we have a detector. I would posit the difference is that you have to be looking at a the stars at the time of collision, but for black holes we have an omnidirectional detector with high SNR.
  - sandworm101 7y ago
    We have seen binary pairs collide, objects that were orbiting each other and who in all probability started their lives as a pair of objects and share a common formation story. We have not seen two non-associated stars born in different places randomly cross paths, which is the sort of merger necessary to form some sizes of black holes.
    - Retric 7y ago
      Binary’s don’t always start from the same nebula. Over millions of years stars move around quite a bit and can form or break apart binary’s.
      - sandworm101 7y ago
        To form a binary, for one star to capture another, requires some careful planning. (A binary pair grabbing a passing star and exchanging momentum.) They don't just get close and start orbiting each other. If a star was captured by another, it wouldn't be a tight pair that may one day merge. They would be very far apart, so far that merger would take far longer than the age of the universe.

AFAIK, nothing prevents that. You'd want blackholes with 100 solar masses though, as the collision will apparently shed significant part of the mass in gravitational waves.

TCR19 7y ago
So does that mean during a collision between blackholes mass actually does/can escape a blackhole? I thought mass couldn't escape, at least not in any observable way that we can detect. [I don't know a ton about astrophysics, so apologies if this question doesn't make sense].
- saagarjha 7y ago
  Mass (or rather, energy) escaping a black hole through gravitational waves has been detected experimentally. Similarly, Hawking radiation allows for black holes to emit energy as well (though this is due to quantum effects near the event horizon).
- ncmncm 7y ago
  No, it cannot. The only energy that can escape in the collision comes from the kinetic energy of the two bodies, and from their colliding accretion discs, and energy bound up in magnetic fields outside the holes. It is interesting that the kinetic energy of a black hole is technically outside the hole. Energy radiated as a consequence of pair production on the event horizon would fluctuate as the surface changed shape and area, but not much.
  - saagarjha 7y ago
    > It is interesting that the kinetic energy of a black hole is technically outside the hole. How so?
    - raattgift 7y ago
      It's not. Even in Newtonian mechanics kinetic energy is an observer-dependent quantity; in General Relativity we care about the stress-energy-momentum tensor, and you can slice that up into kinetic and potential energies as you wish by applying coordinates. The vast majority of the stress-energy in the immediate neighbourhood of an accreting black hole formed by stellar collapse is found inside the horizon, even if there is a substantial accretion structure. General Relativity doesn't make any predictions about the forms stress-energy-momentum can take; we get that from matter theories like classical (but relativistic) Maxwell's equations, quantum electrodynamics, or the full Standard Model. (One also runs into "toy" or "test" forms of matter -- various idealized space-filling fluids or dusts, mainly, that approximate matter in the large: huge numbers of stars or huge numbers of galaxies, for example). So General Relativity also has nothing much to say about kinetic energy vs potential energy: just that they each must contribute to the Einstein Field Equations and that typically means being encoded in the stress-energy tensor. The perfectly elastic bouncing of microscopic particles of a gas each bouncing in one dimension between opposite sides of the inside of a gas container produces a beautiful relationship between the kinetic energy of a gas and its pressure. https://en.wikipedia.org/wiki/Kinetic_theory_of_gases#Pressure_and_kinetic_energy https://en.wikipedia.org/wiki/Kinetic_theory_of_gases#Pressu... (PV = 2/3 K) Assuming isotropic pressure at time t, we encode an identical contribution into the pressure components of the stress-energy tensor (the green diagonals here https://en.wikipedia.org/wiki/Stress–energy_tensor#/media/File:StressEnergyTensor_contravariant.svg https://en.wikipedia.org/wiki/Stress–energy_tensor#/media/Fi... ) for every point within the gas cylinder. The inner regions of massive stars have a lot of pressure; collapsars like neutron stars have even more pressure deep within them. In a runaway collapse that leads to the formation of a black hole, pressure typically dominates the stress-energy tensor, driving the formation of the event horizon. By contrast, a Schwarzschild black hole is a vacuum solution of the Einstein Field Equation, meaning that the stress-energy tensor is everywhere zero. Thus there is no pressure. The source of the "central" mass inside the horizon of a Schwarzschild is best thought of as gravity self-gravitating, or if you like, "it's just defined as curvature alone until you throw a test object in". If we perturb the Schwarzschild black hole by throwing neutral test objects through the horizon, the stress-energy tensor inside the horizon must be somewhere nonzero, but once through the horizon (ignoring quantum effects) the nonzero stress-energy stays in there, and everywhere outside the stress-energy tensor returns to zero. An astrophysical black hole by stellar collapse locks up stress-energy in the same way: once it's inside the event horizon, it stays there (ignoring quantum effects, principally Hawking radiation). Stress-energy outside the horizon might cross the horizon in various ways, or it might form some long lived arrangement sufficiently far from the horizon. You wouldn't say that a white dwarf partner in a white-dwarf/black-hole binary forms "the kinetic [or other] energy of a black hole", would you? If not, then neither does any matter near -- but outside -- the horizon. The no-hair theorem(s) mean(s) that in general you cannot distinguish between an uncharged, zero-angular-momentum black hole formed by stellar collapse (or black hole mergers) and a Schwarzschild (vacuum) black hole (say, formed primordially from nothing but gravitational radiation), even in binaries. A binary of uncharged, no-angular-momentum black holes may be two Schwarzschild BHs or two astrophysical BHs or one of each. The momentum-energy that leaves the binary cannot come from the stress-energy tensor of a Schwarzschild black hole, because it's zero everywhere in the horizon. If no-hair is true, it can't come from the stress-energy tensor in the interior of a black hole formed by stellar collapse, either. A very large primordial BH will have had a bunch of things fall into it (if nothing else, lots of CMB photons), but can still have essentially no stress-energy inside. Primordial black holes are not especially crazy: that's one possible way to explain supermassive black holes ( https://en.wikipedia.org/wiki/Supermassive_black_hole#Formation https://en.wikipedia.org/wiki/Supermassive_black_hole#Format... and note it's possible that primordial black holes can start with and retain essentially zero angular momentum). A pair of primordial SMBHs, each near the centre of mass of merging galaxy clusters, may have eaten a bunch of stellar masses worth of dust and gas in their lifetimes, but not nearly enough to account for the gravitational radiation that will be emitted late in their inspiral, let alone during the merger and ringdown. Instead, it is the angular momentum of the binary system (as a whole, since in this sketch neither BH rotates) that must power the gravitational radiation during the inspiral.

sandworm101 7y ago

The chances of two random black holes ever colliding with each other (not a binary-pair situation) is astronomically rare. On cosmic scales, black holes are microscopic. They are far smaller than stars of similar masses, but have we ever seen two random stars just collide? Two black holes colliding would be like two bullets colliding if fired randomly from guns on different continents. The likelihood is so low that any detection within our sphere of observation would be very suspect.

phkahler 7y ago
We have not seen stars collide, but black hole mergers seem to be a regular thing now that we have a detector. I would posit the difference is that you have to be looking at a the stars at the time of collision, but for black holes we have an omnidirectional detector with high SNR.
- sandworm101 7y ago
  We have seen binary pairs collide, objects that were orbiting each other and who in all probability started their lives as a pair of objects and share a common formation story. We have not seen two non-associated stars born in different places randomly cross paths, which is the sort of merger necessary to form some sizes of black holes.
  - Retric 7y ago
    Binary’s don’t always start from the same nebula. Over millions of years stars move around quite a bit and can form or break apart binary’s.
    - sandworm101 7y ago
      To form a binary, for one star to capture another, requires some careful planning. (A binary pair grabbing a passing star and exchanging momentum.) They don't just get close and start orbiting each other. If a star was captured by another, it wouldn't be a tight pair that may one day merge. They would be very far apart, so far that merger would take far longer than the age of the universe.