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Could Hawking radiation be detected with instruments available today? If not, what sort of instruments would be required to observe it? Would there be any way to confirm through observations the existence of Hawking radiation?

I'm primarily asking about techniques that we could reasonably implement with current technology or something that could be implemented in the near future, but if you need to get close to a black hole to be able to observe the radiation and couldn't do it from a spacecraft in Earth or solar orbit, then that would be the answer.

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    $\begingroup$ I'm mostly thinking about things that we could reasonably implement with foreseeable technology. But if you need to get close to a black hole to be able to observe the radiation, then that would be the answer. $\endgroup$ – usernumber Feb 25 at 11:32
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    $\begingroup$ Thanks! I've edited your post to reflect that. Have a look and feel free to edit further. Sometimes people are quick to vote before reading comments, so it's best to implement all clarifications back in the original post. $\endgroup$ – uhoh Feb 25 at 11:35
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No. Problems:

  1. Hawking radiation would have very small temperature of $10^{-6} M_{sun}/M_{BH}$ Kelvins. Primordial black holes below 1kg already did evaporate, so there would be no sources from small black holes. The only hope is artificial black holes created by CERN.
  2. Falling matter inside black hole would expirience enormous amount of acceleration leading to emittance of photons (particularly X-rays). So you would first are required to cut out this noise.
  3. Black holes are surrounded by ergosphere consisting of hot plasma, which effectively blocks all photons below some critical temperature. For example, we have our radiowaves blocked in the same way by ionosphere of Earth.
  4. (Reasons above were not enough?) There is other quantum-mechanical mechanisms of creating particle-antiparticle pairs which potentially lead to heavy emittance of antiparticles, mostly positrons from black holes. So you would somehow need to shield from positrons, and the photons coming after $e+e^{+}\rightarrow \gamma + \gamma$ kind of reactions.

You would be required somehow prepare black hole for investigation by removal surrounding physical layers from ergosphere, stopping its rotation (really impossible?). I don't know of any "surgical" instruments available today for such task ("surgical" in a sense that we will need to modify our black hole).

Our measuring instruments are totally capable, though. We also know where black holes are located. Tiny problem of communication still persists. But I think scientists would be perfectly fine to wait few millions of years for station to return. Also there is problem from the field of mechanics. It is enormously hard to cross ways with gravitationally strong object, because your trajectory (hyperbolic type) would intersect with a lot of other trajectories (of elliptic type). But I did not list it, counting on humankind getting smarter with this kind of things in a few centuries.

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  • $\begingroup$ +1 very nice answer! Can you add a citation or link supporting the $10^{-6} M_{sun}/M_{BH}$ Kelvins? Take-home point; we should pray for artificial black hole creation at CERN ;-) $\endgroup$ – uhoh Feb 26 at 3:27
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    $\begingroup$ @uhoh The Hawking temperature is actually colder than that, almost $6.17\times 10^{-8}M_\odot/M_{BH}$ K. $\endgroup$ – PM 2Ring Feb 26 at 7:45
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    $\begingroup$ Why do you need (or want) to stop the BH's rotation to measure its temperature? What is the relevance of surgical instruments? What measuring instruments do we have that can detect blackbody radiation of a temperature less than $10^{-7}$ kelvins, with a luminosity under $10^{-28}$ watts, especially considering that the Hawking radiation from a stellar BH would be swamped by the CMBR that's over 10 million times hotter? $\endgroup$ – PM 2Ring Feb 26 at 7:54
  • $\begingroup$ @PM2Ring charged black holes known for Schwinger mechanism of heavy positron production; "surgical" is metaphorical meaning describing our approach $\endgroup$ – sanaris Feb 26 at 15:45
  • $\begingroup$ Sorry, I don't see the relevance of the Schwinger effect here. We were talking about spinning black holes, not charged ones. FWIW, it's expected that most (non-primordial) black holes have a high spin parameter but negligible charge, certainly not the kind of electric field intensity required for the Schwinger effect to become noticeable. $\endgroup$ – PM 2Ring Feb 26 at 15:58
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In principle, yes.

The main sticking point is that the emission of Hawking radiation become weaker for bigger black holes. Black holes from the collapse of stars always have masses of at least a few solar masses. At these masses Hawking radiation is so weak that we have no hope measuring it.

However, it is also possible for black holes of almost any mass to be formed in the early universe. These are called primordial black holes. Exactly how many and at what masses these are formed depend on various (unknown) aspects of physics in the early universe.

If light enough these black holes would emit sufficient Hawking radiation to evaporate on time scales short that the age of the universe, producing bright flashes at the end of their life as evaporation speeds up. The Hawking radiation from these evaporating black holes in principle would the visible to our instruments.

The fact that we don't see Hawking radiation from evaporating black holes, puts bounds on the number of ultralight primordial black holes in the appropriate mass range.

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The lifetime of a black hole to evaporation by Hawking radiation scales as the cube of its mass. If sufficiently low-mass black holes are created then they can evaporate on a timescale shorter than the age of the universe. As they do so, their mass decreases and their evaporation timescale shortens - hence the rate at which they evaporate increases dramatically as they approach the ends of their lives.

The final few seconds of a $<5 \times 10^{14}$ g black hole could take place in the universe today. The observed signature of such an event is disputed and depends on the nitty gritty details of particle physics and the spin of the black hole. However, it is entirely possible that there will be a short burst of high energy gamma ray radiation (photons with energies $>1$ GeV) during the final 100 seconds or so of the black hole's life (e.g. Ukawatta et al. 2016).

The problem seems to be that for the luminosity to ramp up to something that might be detectable, you really need to be talking about the last $\sim 1$ second of the black hole's life, where the temperatures are so extreme that it isn't photons that are emitted, but massive particles that may subsequently fragment and decay producing gamma rays with a spectrum of energies. There is considerable uncertainty about what sort of particles are emitted by the black hole and what they decay into.

This is serious science that is being taken seriously. The paper above by Ukawatta et al. predicts that if nothing is detected by a search using gamma ray detectors like HAWC, that upper limits on the density of evaporating black holes of $\sim 10^{4}$ pc$^{-3}$/year could be placed.

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