Hawking theorized that a black hole must radiate and therefore lose mass (Hawking radiation). According to classical relativity though, nothing can escape a black hole, the hawking radiation would just fall back inside the black hole. Therefore how can one measure Hawking radiation or will this hypothesis remain a hypothesis forever?

  • 7
    $\begingroup$ "nothing can escape a black hole" is only true of stuff inside the event horizon. Hawking radiation arises from "vacuum fluctuations [which] cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes." $\endgroup$ Dec 31, 2012 at 18:49
  • $\begingroup$ @RedGrittyBrick neatly sums up why Hawking radiation does not fall back into the black hole. One can measure Hawking radiation the way one measures any other electromagnetic radiation. In order to identify such radiation as Hawking radiation, you would need to measure the spectrum of a suspected black hole and compare the observed spectrum to that theoretically predicted for Hawking radiation. $\endgroup$
    – KDN
    Jan 1, 2013 at 1:25
  • 2
    $\begingroup$ @adeetya the idea that Hawking radiation is due to one member of a virtual pair crossing the event horizon is just a metaphor. See physics.stackexchange.com/q/30597 for more details. $\endgroup$ Jan 1, 2013 at 9:08
  • $\begingroup$ Possible duplicate: physics.stackexchange.com/q/11576/2451 $\endgroup$
    – Qmechanic
    Jan 11, 2013 at 17:18

1 Answer 1


The conceptual hang-up you have with Hawking radiation falling back into a black hole is a misunderstanding. You're right that classically "nothing can escape a black hole". The trick to Hawking radiation though is that it forms on the horizon of the black hole and saps energy from it. The radiation is not escaping from the black hole, it is stealing energy from the black hole to emit radiation from just beyond the event horizon. The Wikipedia article you linked to spells this out in some detail.

That being said, your question is about how we could detect Hawking radiation from a black hole. Given current technology we can't. Astrophysical black holes (massive ones) emit such low energy radiation that the cosmic microwave background radiation drown it out. See black hole thermodynamics for more information about this.

Our best bet for observing Hawking radiation is to observe a micro black hole. We can't make these in the lab yet (perhaps eventually we will have the technology to do so) but there are models for the big bag that predict that they were created early in the universe and are now old enough to have observable radiation / be evaporating. If this prediction is correct then the Fermi Gamma-ray telescope could possibly detect Hawking radiation from an evaporating micro black hole.


Not the answer you're looking for? Browse other questions tagged or ask your own question.