5 ms·

Wouldn’t these black holes have evaporated by now?

nealabq 9y ago
According to https://phys.org/news/2016-09-cold-black-holes.html https://phys.org/news/2016-09-cold-black-holes.html a black-hole greater than the mass of the moon would have an apparent temperature less than that of the cosmic background radiation, so it wouldn't have started shrinking yet. And it'd be very hard to see. Temperature is inversely proportional to mass ( https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation ) so heavier black holes are even colder. According to https://what-if.xkcd.com/129/ https://what-if.xkcd.com/129/ a moon-mass black hole would be as big as a grain of sand. I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.
comnetxr 9y ago
Reasonable sized black holes evaporate extremely slowly. A black hole the mass of the moon would take 10^45 years to evaporate, so none would have evaporated yet in the lifetime of this universe which is about 1.4*10^10 years old. See https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation. Only black holes around 10^11 kilos or less will have evaporated by now. Couldn't think of any objects around that size to compare to, but there are some here: https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass) https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)
- comnetxr 9y ago
  The radius of the biggest black hole that would have evaporated by now is about a femtometer, or the size of a proton.
  - Semirhage 9y ago
    Of course a black hole’s temperature is inversely proportional to its mass, so a tiny black hole would be terrifically hot. Ironically a much more massive black hole is undetectable unless it’s lensing something, or “feeding” on something. A small black hole would have a very high temperature just from Hawking radiation.
    - m_mueller 9y ago
      Aren‘t those the most energy dense objects known to us? Not sure how large the black holes in the ‚Kugelblitz‘ idea are, but supposedly if we could harvest the energy output of a significant fraction of the sun, we could manufacture them and then use it to travel to other stars rather easily... or destroy entire planets, as those two things unfortunately go hand in hand (in that regard Star Wars may be more realistic than other SciFi funnily enough).
      - stephengillie 9y ago
        Vehicles are too easily weaponized. The amount of energy it takes to move a human and a cabin is more than enough energy to destroy a small building. People have driven their cars into houses and businesses for as long as cars have existed. Not to mention aircraft. And driving cars into cars is a too common tragedy. The odd part about Star Wars is the relative weakness of their weapons. Star Trek have "warp core breaches in a bottle" AKA the photon torpedo, which is much better at damaging starships than ramming.
- dogma1138 9y ago
  So next question how does this account for the required spin? won't black holes that small would be spinning beyond what is possible through GR? can these black holes be just normal ones or would they be a "naked black hole/singularity"?

According to https://phys.org/news/2016-09-cold-black-holes.html https://phys.org/news/2016-09-cold-black-holes.html a black-hole greater than the mass of the moon would have an apparent temperature less than that of the cosmic background radiation, so it wouldn't have started shrinking yet. And it'd be very hard to see. Temperature is inversely proportional to mass ( https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation ) so heavier black holes are even colder. According to https://what-if.xkcd.com/129/ https://what-if.xkcd.com/129/ a moon-mass black hole would be as big as a grain of sand. I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.

comnetxr 9y ago

Reasonable sized black holes evaporate extremely slowly. A black hole the mass of the moon would take 10^45 years to evaporate, so none would have evaporated yet in the lifetime of this universe which is about 1.4*10^10 years old. See https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation. Only black holes around 10^11 kilos or less will have evaporated by now. Couldn't think of any objects around that size to compare to, but there are some here: https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass) https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)

comnetxr 9y ago
The radius of the biggest black hole that would have evaporated by now is about a femtometer, or the size of a proton.
- Semirhage 9y ago
  Of course a black hole’s temperature is inversely proportional to its mass, so a tiny black hole would be terrifically hot. Ironically a much more massive black hole is undetectable unless it’s lensing something, or “feeding” on something. A small black hole would have a very high temperature just from Hawking radiation.
  - m_mueller 9y ago
    Aren‘t those the most energy dense objects known to us? Not sure how large the black holes in the ‚Kugelblitz‘ idea are, but supposedly if we could harvest the energy output of a significant fraction of the sun, we could manufacture them and then use it to travel to other stars rather easily... or destroy entire planets, as those two things unfortunately go hand in hand (in that regard Star Wars may be more realistic than other SciFi funnily enough).
    - stephengillie 9y ago
      Vehicles are too easily weaponized. The amount of energy it takes to move a human and a cabin is more than enough energy to destroy a small building. People have driven their cars into houses and businesses for as long as cars have existed. Not to mention aircraft. And driving cars into cars is a too common tragedy. The odd part about Star Wars is the relative weakness of their weapons. Star Trek have "warp core breaches in a bottle" AKA the photon torpedo, which is much better at damaging starships than ramming.
dogma1138 9y ago
So next question how does this account for the required spin? won't black holes that small would be spinning beyond what is possible through GR? can these black holes be just normal ones or would they be a "naked black hole/singularity"?