Researchers observe stationary Hawking radiation in an analog black hole | Hacker News Reader

Researchers observe stationary Hawking radiation in an analog black hole

phys.org

40 pointsdnetesn17 comments

I was confused as to how the researchers created a black hole in a lab, which I hadn't heard of being done before. As far as I can tell, an "analog" black hole is different from a normal black hole in that it doesn't use gravity and only works on acoustic waves or electromagnetic waves, depending on the type [1]. That said I don't really get what's going on.

I'd be curious if someone who actually knows what they're talking about could try to clarify how the researchers have a black hole here.

EDIT: There's a link[2] to another article at the bottom that describes how the researchers are simulating a black hole. My bad for not catching this earlier.

[1]: http://www.phy.olemiss.edu/~luca/Topics/bh/analog.html [2]: https://phys.org/news/2014-10-mimic-hawking-lab.html

Yes, I think it might have been more straightforward to say “Researchers observe stationary Hawking radiation in ‘acoustic black hole’” or something to clarify to laypeople that this is a mathematically analogous but physically very different experiment. I don’t think it’s too difficult to conceptualize that sound waves behave similarly to light waves and that properties of black holes can be “simulated” with sound.

Making a microscopic black hole on the surface of the Earth was #3 on the list of `How to Destroy the Earth`[1], also on HN today. For the first half of the article I was musing at the connection (and implausibility of reading about a research team actually completing the feat).

[1] https://news.ycombinator.com/item?id=26204892

The smaller the black hole, the faster it evaporates. But having a planet around would instantly make it larger.

Sufficiently small non-charged blackholes would have minimal interaction cross sections with any matter. If such a thing was made with an accelerator, it as well as any material it interacted with would likely fly through the planet and out into interstellar space. Even managing to detect that it had been created would be a substantial challenge.

I suppose for that its Schwartzschield radius should be much smaller than a typical atom?

aye - the Schwarzschild radius of the entire planet is only 8.87 mm. The radius of a 70kg human is 10^12th times smaller than a hydrogen atom. Presuming it is even physically possible for atom massed black holes to exist - hypothetical hydrogen atom black hole would have a radius 10^41 times smaller than a hydrogen atom.

Ultimately the physics of such objects are not well defined, the radius of a hydrogen atom or a proton is defined in terms of electric potentials, calculating the interaction cross section between the quarks in a proton and a hypothetical atom massed black hole is completely ill-defined. At these scales the strong force would dominate - and you'd run into a number of uncomfortable problems which either require hawking radiation to exist or for GR to not conserve energy ( which it doesn't in the classical theory )

but all that being said, whatever collisions created this black hole would surely have left it with a velocity that's a very large fraction of the speed of light.

So here's just my understanding using basic physics: That kind of gravitational pull can't come from nowhere.

If you start from the smallest theoretically possible black hole of about 22 micrograms, it should have the gravitational pull of just that - 22 micrograms. Suppose it doesn't just immediately disappear, it won't even have gravitational pull to move a feather next to it.

You could probably even start wadding the feather at it and it wouldn't do much to it, because I don't think that's enough force to even break it apart. If you put a grain of sand on a table you don't expect the gravitation of that grain of sand to break the table apart either.

Maybe it'll manage to take in some surrounding air? But in that case you'd have all the time in the world to build a vacuum chamber around it - if you're worried. Likely the black hole would need to be extremely lucky for any (air) molecule to come close enough that it can pull it in, since it'll be a tiny thing in a sea of mostly nothing.

In any case, I suppose that in reality the black hole would just dissipate quicker than it could take in mass - because black holes need a lot of new mass to keep going considering the amount of energy/mass they spit out. And only black holes made from a lot of matter (i.e. stars) appear to have the gravitational pull to feed themselves and stay alive, since that's all we can observe in nature.

Anyways, that's just using a lot of basic physics that maybe don't even apply to black holes in that way.

The problem is that you can't really push a black hole. As long as there is a faint gravitational pull, it will keep coming closer to the mass, until it's able to touch and consume it.

The only hope that it would evaporate sooner.

Isn't the lifespan of microscopic black holes also... Microscopic?

Yes, and then they go out in a flash.

What about electrically charged black holes?

I should add that that the mass/energy leaving black holes is way smaller than I assumed (just Hawking Radiation?). Maybe someone with an actual understanding of the topic could chime in.

About the only information about theoretical small black holes was this quote: "But if you happened to turn the Eiffel Tower into a black hole, it would evaporate in only about a day. I don't know why you would, but there you go."[1]

[1]: https://public.nrao.edu/ask/the-life-cycle-of-a-black-hole/

60 symbols have a good video on analog black holes https://m.youtube.com/watch?v=kOnoYQchHFw

Based on [0], I think the "inner horizon" the scientist refers to is the Cauchy horizon[1]. It's unclear to me what connection that has with Hawking radiation, though.

[0] https://old.reddit.com/r/askscience/comments/5uzg51/what_is_...

[1] https://en.wikipedia.org/wiki/Cauchy_horizon

I think the refer to the concept of regular black holes. It is a concept where the BH has two horizons, and within the inner one the metric is no more Schwarzschild-like but something more "regular" or Minkowskian again. This model is widespread in effective quantum gravity theories.

I haven't heard of those. Do you know of a layman-accessible source?

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