Hawking radiation

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Hawking radiation is the means by which black holes decay. It is named for Stephen Hawking.

Space - that's all space, the void between galaxies and equally the space within and between the atoms of your body - is not empty. There is a constant creation/extinction of virtual subatomic particles (particle here can include energy as well). These particles are created in particle-antiparticle pairs, which subsequently annihilate each other in order to respect the conservation of energy. This is known as vacuum fluctuation and it's all to do with quantum physics.[1].

If such a fluctuation occurs at the event horizon of a black hole, it is possible during the very brief time that the pair exists, that one of them might be pulled into the black hole, while the other escapes. This second particle becomes real, while the former remains virtual and has to assume negative mass/energy, due to the already mentioned conservation of energy.

  1. There is thus a negative effect on the overall mass of the black hole.
  2. The smaller the black hole is the faster it radiates.
  3. The temperature of the radiation emitted by this effect is, for small objects up to some hundred stellar masses, extremely low (< 1 x 10-9 Kelvin). More massive holes as those existing on the centres of galaxies will have even lower temperatures, and of course that means a really low power output, thus naturally a very difficult time attempting to detect said radiation.
  4. Tiny black holes will evaporate very fast, and the less massive they are the more energetic that will be the radiation emitted by them.
  5. All of the above means black holes will not begin to radiate until the temperature of the surrounding space (read: the cosmic microwave background, in absence of matter infalling into them) are lower than the holes' one, something which will take quite a long time.

This theory is not universally accepted[2] because as noted above its the detection of Hawking radiation is way beyond our technologies, but is a possible route whereby the universe will end as a low temperature sea of photons.

Evaporation time[edit]

The time that it would take a black hole to completely evaporate via Hawking radiation depends entirely on its mass:

t_{\operatorname{ev}} = \frac{5120 \pi G^2 M_0^{3}}{\hbar c^4} \;

Thus, a black hole with one Solar mass (2 x 1030 kg) would last for 2 x 1067 years, while a black hole with a mass of 1 gram would last for 8 x 10-26 seconds.

What happens next[edit]

The ultimate fate of the black hole is not currently known, since in the very last stages of its evaporation things go both quantum and extremely hot, and alternatives range from it leaving a small residue to simply disappearing in a burst of radiation.

See also[edit]


  1. Hawking radiation Classically, black holes are black. Quantum mechanically, black holes radiate, with a radiation known as Hawking radiation, after the British physicist Stephen Hawking who first proposed it. (University standard article)
  2. Do black holes radiate? (Also University standard article)