Hawking Radiation Mimicked In the Lab

Annanag writes *Nothing* escapes a black hole, right? Except 40 years ago Stephen Hawking threw a spanner in the works by suggesting that, courtesy of quantum mechanics, some light particles can actually break free of a black hole's massive pull. Then you have the tantalizing question of whether information can also escape, encoded in that so-called 'Hawking radiation'. The only problem being that no one has ever been able to detect Hawking radiation being emitted from a black hole. BUT a physicist has now come closer than ever before to creating an imitation of a black hole event horizon in the lab, opening up a potential avenue for investigating Hawking radiation and exploring how quantum mechanics and general relativity might be brought together.

Comments

Sound waves as quantum particles?

[Dan East]

This stuff isn't my strong suit at all, but I'm having a hard time grasping how sound waves can behave like subatomic particles in this way.

Pairs of sound waves pop in and out of existence in a laboratory vacuum, mimicking particle-antiparticle pairs in the vacuum of space.

Sound is a wave through some medium, so how can they pop into existence in a vacuum? Are particles of some kind (and what are they? Hydrogen atoms? Helium?) popping into existence long enough for them to physically interact with one another so a physical wave can propagate from one particle to another before they pop back out of existence, and thus "sound waves" are appearing?

All this is pretty amazing to me, but the amount of complexity involved (using dual event horizons to reflect the waves back and forth to amplify the audio signal because its so weak, etc) sure would leave a lot of room to screw something up along the way. Seems the signal to noise ratio would be pretty bad.

Re:Sound waves as quantum particles?

[Anonymous Coward]

Also, slashdot,
"Hawking Radiation Mimicked In the Lab"

and ... a physicist has now come closer than ever before to creating an imitation of a black hole event horizon in the lab, opening up a ---> potential --- avenue for investigating Hawking radiation ...

The fuck slashdot?
Would you say that the title is kinda, sorta, misleading?

Re:Sound waves as quantum particles?

[Anonymous Coward]

"I'm having a hard time grasping how sound waves can behave like subatomic particles in this way."

It's done with phonons -- quantised fluctuations that in the classical limit are sound waves.

"Sound is a wave through some medium, so how can they pop into existence in a vacuum? Are particles of some kind (and what are they? Hydrogen atoms? Helium?) popping into existence long enough for them to physically interact with one another so a physical wave can propagate from one particle to another before they pop back out of existence, and thus "sound waves" are appearing?"

No, it's literally that pairs of phonons can be produced from the "sound vacuum" in the same way that pairs of photons can be produced in a normal vacuum. If you like, you can think of it as quantised shifts in the structure of the quantum fluid (superfluid helium or a Bose-Einstein condensate, or what have you) generating these phonons. It's not, strictly speaking, true but at least it's a physical picture.

"Seems the signal to noise ratio would be pretty bad."

I'm a long time out of this field -- I did my Masters in acoustic holes but that was a long time back -- but I'd also expect signal to noise to be pretty lousy. However, any signal at all would be awesome.

Re:Sound waves as quantum particles?

[Khashishi]

I think "vacuum" in the article means the Bose-Einstein condensate with no phonons, which is the analog to the true vacuum and not actually a vacuum.

Re:Sound waves as quantum particles?

[devent]

The cold temperature also ensures that the fluid, known as a Bose-Einstein condensate, provides a silent medium for the passage of sound waves that arise from quantum fluctuations.

The sound waves arises from quantum fluctuations, and those sound waves can propagate through the medium because it is a Bose-Einstein condensate, i.e. it have almost(?) zero resistance to sound waves. So that experiment is acutally measuring quantum fluctuations.

Re:Sound waves as quantum particles?

[alexander_686]

Here is a old article, from 2009, but I found it helpful.

http://www.economist.com/node/...

It was also a plot point for the Big Bank Theory - season 6 - where Lenord goes off in a ship in find such things.

Re:Sound waves as quantum particles?

[rjh]

You can thank wave-particle duality. In the quantum world, if something has an existence as a wave, it must also have an existence as a particle. Sound waves have a particle analogue: phonons.

Also remember: in quantum mechanics there's no such thing as a vacuum. Virtual particles spring into existence constantly, making it possible to interact with vacuum in a number of really surprising ways.

Re: All I can say to that is...

[Anonymous Coward]

I can't hear you. I am on the other side of the black hole

Mimicking a theory, not a phenomenon

[RWerp]

Because of the scale of the experimental setup, it is quite obvious that no gravitational effects are involved. Hence, there is no possibility for this experiment to recreate phenomena at the intersection of quantum mechanics and general relativity. What the Steinbauer does is he replicates a particular model of the black hole. If his setup works, fine, but it doesn't prove a single thing about how black holes behave - because he did not create one.

Re:Mimicking a theory, not a phenomenon

[Anonymous Coward]

It tells us how horizons behave. The production of Hawking radiation in a gravitational black hole relies (and relies only) on the presence of a horizon. In an acoustic hole, we've got a horizon for phonons, rather than for photons, but it's still a horizon. The actual structure of the geometry in the simplest cases is Schwarzschild, but one can play some interesting games to get a more complicated setup which is more usable - and in any event, it also exhibits a horizon. Therefore, while the effective field theory describing the phonons holds, and while the system exhibits a horizon, any observation of Hawking radiation will directly test the processes by which we believe Hawking radiation is produced in actual gravitational holes. It's based on basically the same physics. What it *won't* tell us is anything about the backreaction of Hawking radiation on a gravitational hole, because while the kinematics of an analogue hole are the same as a gravitational hole (at an unperturbed level), the dynamics are completely different. Further, the system will eventually produce enough Hawking radiation that the condensate will be depleted to an extent that the analogy is no longer valid even at a background level. However, while the analogy is valid -- and that can be quantified and therefore controlled -- we can still exploit it. And when the analogy isn't valid we're still learning useful things about the behaviour of supercold fluids.

Re:Mimicking a theory, not a phenomenon

[RWerp]

How is this based on the same physics? Do you mean that theories are the same?

Re:Mimicking a theory, not a phenomenon

[Anonymous Coward]

When it comes to the derivation of Hawking radiation, surprisingly, yes. It relies on there being a quantum vacuum for some type of particle (in gravitational radiation, photons; in the analogue case, phonons), and it relies on there being a horizon (in the gravitational case this is an event horizon; in the analogue case it's an acoustic horizon). It also relies on the analogue medium producing phonons of the right form -- a form where (in the appropriate regime) the phonons have a quantum theory that acts like photons do. In particular, you have to have a medium where the phonons have a "squeezer" Hamiltonian in the vicinity of the acoustic hole, meaning that pair production will happen. The derivation of Hawking radiation from this point takes the same form in both a gravitational hole and an acoustic hole. Of course, when the conditions change, as they inevitably will, the analogy breaks down but in its regimes of validity there's no issue, and we can quantify the extent to which the analogy is holding.

Make sense if pinch instead of hole

[advid.net]

We should pay more interest to the idea that black holes may not exist like we portray them since the word came out:

Black holes do not exist
Jean-Pierre Petit
04/2014

ABSTRACT We reconsider classical features of Schwarzschild and Kerr metrics, which are the fundamental basis of the black hole model, through new space and time coordinates which transform the object into a space bridge linking two folds of the [...]

Parer available for download

A what?

[ArcadeMan]

Nothing escapes a black hole, right?

You mean a Hawking Hole, right?

Re:A what?

[tehlinux]

That's a hole in nothing with monsters in it. Also known as a Fry Hole.

How Would Hawking Radiation Dissolve a Black Hole?

[31415926535897]

In all my years of reading and thinking about black holes, one question I've got about HR his how it would actually end up causing the decay of a black hole. From what I understand, HR is the spontaneous creation of matter and anti-matter in space that would normally annihilate itself (allowed by QM theory)--the key difference is that this event can happen at the edge of the event horizon. With some positive probability, the anti-matter will be created within the event horizon radius, but the matter will remain outside and escape. When you look at the whole system then, the anti-matter will annihilate matter within the black hole (causing it to "dissolve") and the matter will remain outside the clutches of the black hole.

I'm sure I'm describing it very simplistically, but I believe my question after that should work for all systems that are analogous:

How is the HR process not symmetric? Whatever would cause the dissolution of the black hole--how would the same process happening in reverse (matter falling into the black hole and anti-matter escaping) not cause equilibrium to be maintained?

event horizon?

[Anonymous Coward]

Didn't we just decide black holes don't have those?

Re:event horizon?

[Khashishi]

Science isn't so clear cut. The question isn't settled yet, and probably won't be settled until we have a clear theory of quantum gravity.

Re:How Would Hawking Radiation Dissolve a Black Ho

[Anonymous Coward]

One thing is off in your explanation: it is not anti-matter that falls into the black hole. Anti-matter still has positive net energy (âoeweightâ), so throwing antimatter into a black hole will make it more massive, not less massive. The actual idea is that a of a negative-energy particle (the total energy of the virtual pair is zero, so if one particle becomes real, with positive energy, the other had to have negative net energy) falling in, *not* a matter anti-particle (like a positron). Quoth wikipedia:

Antiparticles should not be confused with virtual particles or virtual antiparticles.

The equilibrium is not maintained because if a virtual particle outside the event horizon becomes real, it will always end up having positive net energy: real negative-energy particles do not exist.

I never did get this...

[Charliemopps]

I never did get this... Hawking radiation doesn't "Escape" a black hole. In empty space, there is a constant seething foam of particle-antiparticle pairs that get created all the time. Normally these pairs immediately collide with one another, or their neighbors, and obliterate each other so they are mostly undetectable. With a blackhole you have an event horizon. One side of which is inescapable, the other side is escapable. It stands to reason, that along this line these particle-antiparticle pairs would get created with one inside the horizon and the other outside of it. Resulting in a net increase in the number of particles created. Nothing "Escaped" at all.

Re:Dear Scientists

[ultranova]

Dear Scientists, Please stop trying to create black holes. I would prefer not waking up in the middle of some random night feeling a tug on my body and seconds later falling into the aforementioned black hole to my death.

Why hasn't this been made into a movie already?!?

A black hole created at, say, Large Hadron Collider, would fall into Earth's center and then continue onto the other side, rising to the surface only to fall again, in an essentially chaotic orbit, snapping a person here and another there, slowly growing as it ate Earth's innards like a hookworm, surrounded by a growing spiral of white-hot remains of its feast. It's perfect material for a horror movie, or a disaster one, or an artistic one, a monster movie, or a Michael Bay explosion fest.

Re:Dear Scientists

[ultranova]

Closing scene is the last few rocket ships barely escaping Earth as it disappears, with innumerable other rocket ships not making it.

Collapses, not disappears. The hole is hollowing out the insides of the planet, and when enough is gone, the no-longer-supported crust cracks into pieces that fall in as magma sprays everywhere. And of course we have terrible earthquakes and volcanic activity leading up to the final doom.

Obviously none of it has to be scientifically accurate, so pedants please don't start trying to pick the plot apart, because it's just too easy pickings.

Well, scientifically speaking the entire mass of the Earth would only create a hole the size of a centimeter or so, so it'd take a long time for the entire planet to plunge down that drain. The "superball of death", on the other hand, could easily trigger massive quakes and tides at every bounce due to the tidal forces, despite the hole itself being very small.