4 ms·

So black holes can't evaporate? How does Hawking radiation works if the back hole are has to stay the same?

SiempreViernes 5y ago
I think it's more a case of the press release people wanting to call it a unqualified "law", as far as we know black holes do evaporate slowly. But there remains is a statement about how the final area relates to the area of the two merging black holes.
edem 5y ago
They _do_ evaporate, but they also absorb CMB, and right now CMB > evaporation. Later, when CMB fully dissipated they can evaporate in practice.
- kstrauser 5y ago
  > Later, when CMB fully dissipated That's one of the most understated uses of "later" I've heard.
  - edem 5y ago
    Hehe, true.
    - kstrauser 5y ago
      According to https://en.wikipedia.org/wiki/Timeline_of_the_far_future https://en.wikipedia.org/wiki/Timeline_of_the_far_future, that may be around 150 billion years from now. Also, I have a hard time reading that page without a sense of existential dread.
jfengel 5y ago
Black holes can't evaporate now because the cosmic background radiation is too hot. The black holes are colder the CMBR, so they absorb heat and grow (albeit very, very slightly). Eventually the CMBR will cool down and the holes will be able to evaporate, but not for an insanely long time.
- __MatrixMan__ 5y ago
  I imagine this holds only for black holes that are massive enough to be stable? All those itsy bitsy ones created by the LHC, they've evaporated, yes?
  - wruza 5y ago
    I believe that LHC has no chance ever to harvest enough energy to create a sensible-sized black hole. It’s still mc-squared (give or take an order of magnitude and my layman mistakes), so 1g BH takes about 1e14 joules or 5 minutes of average EU electricity output. Also, there is no sea nearby to cool it off afterwards. Also, a planck-sized BH weighs 1e-5 g. A mass similar to Mount Everest[13][note 1] has a Schwarzschild radius much smaller than a nanometre.[note 2] Its average density at that size would be so high that no known mechanism could form such extremely compact objects It seems that at energies available to us they are basically either virtual or non-existent. This contradicts the common notion that cosmic rays create microbhs occasionally, but I guess we have to wait for a physicist to clarify this.
    - jfengel 5y ago
      In theory, it might have made a black hole. It would have lasted a ridiculously small period of time, but be quite recognizable by the energy it gave off. Instead of the usual decay patterns, it would give off a spray just of photons, like a black body at a recognizable (and very high) temperature. We didn't see that, and in fact theory predicted that it was insanely unlikely that we would. But there's nothing wrong with the possibility of a black hole much, much, much smaller than a gram, with a radius smaller than the Planck length. If we had seen it, it would have been insanely informative. But it wasn't ever gonna happen.
      - morei 5y ago
        Do you know what theory that might be? The difficulty in producing a black hole is getting the energy density high enough. We have no known mechanism to get an energy density that's even close the right order of magnitude. Maybe you meant that in theory quantum fluctuations might do it? Unfortunately, this is really a non-answer. The probability is so ridiculously low that it's not practicably distinguishable from zero. (It's _vastly_ more likely that every measurement ever taken and that _will_ be ever taken is wrong, than that the event actually happened).

I think it's more a case of the press release people wanting to call it a unqualified "law", as far as we know black holes do evaporate slowly. But there remains is a statement about how the final area relates to the area of the two merging black holes.

edem 5y ago

They _do_ evaporate, but they also absorb CMB, and right now CMB > evaporation. Later, when CMB fully dissipated they can evaporate in practice.

kstrauser 5y ago
> Later, when CMB fully dissipated That's one of the most understated uses of "later" I've heard.
- edem 5y ago
  Hehe, true.
  - kstrauser 5y ago
    According to https://en.wikipedia.org/wiki/Timeline_of_the_far_future https://en.wikipedia.org/wiki/Timeline_of_the_far_future, that may be around 150 billion years from now. Also, I have a hard time reading that page without a sense of existential dread.

jfengel 5y ago

Black holes can't evaporate now because the cosmic background radiation is too hot. The black holes are colder the CMBR, so they absorb heat and grow (albeit very, very slightly). Eventually the CMBR will cool down and the holes will be able to evaporate, but not for an insanely long time.

__MatrixMan__ 5y ago
I imagine this holds only for black holes that are massive enough to be stable? All those itsy bitsy ones created by the LHC, they've evaporated, yes?
- wruza 5y ago
  I believe that LHC has no chance ever to harvest enough energy to create a sensible-sized black hole. It’s still mc-squared (give or take an order of magnitude and my layman mistakes), so 1g BH takes about 1e14 joules or 5 minutes of average EU electricity output. Also, there is no sea nearby to cool it off afterwards. Also, a planck-sized BH weighs 1e-5 g. A mass similar to Mount Everest[13][note 1] has a Schwarzschild radius much smaller than a nanometre.[note 2] Its average density at that size would be so high that no known mechanism could form such extremely compact objects It seems that at energies available to us they are basically either virtual or non-existent. This contradicts the common notion that cosmic rays create microbhs occasionally, but I guess we have to wait for a physicist to clarify this.
  - jfengel 5y ago
    In theory, it might have made a black hole. It would have lasted a ridiculously small period of time, but be quite recognizable by the energy it gave off. Instead of the usual decay patterns, it would give off a spray just of photons, like a black body at a recognizable (and very high) temperature. We didn't see that, and in fact theory predicted that it was insanely unlikely that we would. But there's nothing wrong with the possibility of a black hole much, much, much smaller than a gram, with a radius smaller than the Planck length. If we had seen it, it would have been insanely informative. But it wasn't ever gonna happen.
    - morei 5y ago
      Do you know what theory that might be? The difficulty in producing a black hole is getting the energy density high enough. We have no known mechanism to get an energy density that's even close the right order of magnitude. Maybe you meant that in theory quantum fluctuations might do it? Unfortunately, this is really a non-answer. The probability is so ridiculously low that it's not practicably distinguishable from zero. (It's _vastly_ more likely that every measurement ever taken and that _will_ be ever taken is wrong, than that the event actually happened).