History of Hawking radiation
History of Hawking radiation
The subject of Hawking radiation became important in the 1970s, when it was realised that the effect, derived using arguments from thermodynamics and quantum mechanics, was incompatible with the predictions of C20th textbook GR and with John Wheeler's popular 1960s notion of the utterly black black hole.
Although the existence of a counterpart effect under C19th ballistic emission theory is trivially obvious, the subject does not seem to have been publicly explored prior to the 1970s, probably because the strength of the effect is usually so vanishingly weak that anybody evaluating it would have been likely to conclude that it was of no experimental or theoretical importance.
- John Wheeler champions the idea of the totally non-radiating GR1960 body whose temperature is zero, and which appears to present a feature a one-way surface with a surrounding gravity-well - a black hole. It is proved mathematically and that nothing that happens within r=2M can affect the outside universe, by definition.
- Stephen Hawking argues that the black hole radius can only ever stay the same or increase, and can never decrease. Black holes really are immortal, and nothign can ever, ever get out of them.
1971: Black hole entropy
- Jacob Bekenstein takes the apparent correspondence between black hole area increase and entropy, and proposes that the relationship is not a coincidence, but is fundamental.
- However, relating the surface area to entropy also relates the surface gravity to temperature (Thorne 1994), so Bekenstein's connection suggests that black holes have a positive temperature, and therefore radiate. However since black holes obviously can't radiate, this aspect of the model is not taken literally.
1971: Rotating black holes radiate
- Yacov Borisovich Zel'dovich suggests that spinning black hole should radiate (Thorne 1994). The spinning hole drags nearby light and particles, and when energy passes through the region and creates particle-pairs which would normally have vanishingly-short lifetimes before they mutually self-annihilate, the shearing effects of the spinning hole's gravitational field could wrench the two members apart, with one member of the pair being thrown free.
- A spinning hole should therefore be throwing off particles in its direction of spin, in a manner reminiscent of he spatters of dirt thrown off by a spinning muddy bicycle wheel.
1974: ALL black holes radiate
- Stephen Hawking takes Zeldovich's idea and reformulates it, announcing that in fact all black holes have to radiate and shrink if unfed (contradicting his own previous proof).
- The idea is received badly. Hawking presents it at a conference organised by black hole expert John Taylor, at which point Taylor declares that the idea is "absolute rubbish" (or words to that effect), and halts the meeting, with attendees given no chance to ask questions. Hawking submits the idea to Nature as a "Letters to the Editor" submission rather than as a fully peer-reviewed paper (which would almost certainly be rejected). Taylor says that it shouldn't be published, but a second referee disagrees, and Nature prints the piece.
- The letter causes a certain degree of turmoil – in 1973, Charles Misner, Kip Thorne and John Wheeler (MTW) have just published their textbook masterwork Gravitation, asserting the supremacy of GR1960, and saying that any competitor theory is not to be considered even worthy of testing unless it meshes with quantum mechanics ... but according to Hawking's calculations, GR1960 itself doesn't mesh with QM, and is therefore, according MTW's own criteria, not to be considered a credible theory.
- Similarities are suggested between the behaviour of black hole horizons and those of acoustic or sonic horizons, leading to papers on sonic black hole or dumb hole analogies with QM behaviour. The acoustic version of Hawking radiation is explored in more detail, having been previously almost ignored because of its tiny magnitude. Suggestions are made that the acoustic version of the effect may actually be measurable for the first time using Bose-Einstein condensate (BEC)
- It is pointed out that C18th dark stars would have emitted radiation apparently indistinguishable from Hawking radiation, that cosmological horizons also emit radiation that could be considered a special case, 'cosmological Hawking radiation, and that although the two cases are believed to be physically different, nobody seems to have an actual method of telling them apart, as the classical version can generate the "traditional" pair-particle description of HR as a projectional artefact.
Into the Twenty-First Century
- Matt Visser publishes a paper on acoustic metrics in ~1998, bringing attention to their Lorentzlike relationships and support for Hawking radiation. This is followed by a book, a significant number of papers by Visser and other researchers, and large review paper, periodically updated, on acoustic metrics as a "toy model" for examining the properties of a future theory of quantum gravity. A tentative suggestion is made in the conclusions, of the idea that this research might be representing something more than a mere toy model.
- Although Hawking radiation seemed like "alien" behaviour when it was first suggested, as of June 2016 it seems to be default behaviour in pretty much every field of physics except special relativity, and GR1960 and other SR-based gravitational models. It also seems to be required by thermodynamics and information theory, and would seem to be a feature of acoustic-metric based approaches.