Physicists create compressible optical quantum gas

[beecee]

https://phys.org/news/2022-03-physicists-compressible-optical-quantum-gas.html

Researchers at the University of Bonn have created a gas of light particles that can be extremely compressed. Their results confirm the predictions of central theories of quantum physics. The findings could also point the way to new types of sensors that can measure minute forces. The study is published in the journal Science.

If you plug the outlet of an air pump with your finger, you can still push its piston down. The reason: Gases are fairly easy to compress—unlike liquids, for example. If the pump contained water instead of air, it would be essentially impossible to move the piston, even with the greatest effort.

Gases usually consist of atoms or molecules that swirl more or less quickly through space. It is quite similar with light: Its smallest building blocks are photons, which in some respect behave like particles. And these photons can also be treated as a gas, however, one that behaves somewhat unusually: You can compress it under certain conditions with almost no effort. At least that is what theory predicts.

extract:

"The rule is usually as follows: The denser a gas, the harder it is to compress. This is also the case with the plugged air pump—at first the piston can be pushed down very easily, but at some point it can hardly be moved any further, even when applying a lot of force. The Bonn experiments were initially similar: The more photons they put into the mirror box, the more difficult it became to compress the gas.

However, the behavior changed abruptly at a certain point: As soon as the photon gas exceeded a specific density, it could suddenly be compressed with almost no resistance. "This effect results from the rules of quantum mechanics," explains Schmitt, who is also an associate member of the Cluster of Excellence "Matter and Light for Quantum Computing" and project leader in the Transregio Collaborative Research Center 185. The reason: The light particles exhibit a "fuzziness"—in simple terms, their location is somewhat blurred. As they come very close to each other at high densities, the photons begin to overlap. Physicists then also speak of a "quantum degeneracy" of the gas. And it becomes much easier to compress such a quantum degenerate gas".

more at link...................................

the paper:

https://www.science.org/doi/10.1126/science.abm2543

Compressibility and the equation of state of an optical quantum gas in a box:

Abstract:

The compressibility of a medium, quantifying its response to mechanical perturbations, is a fundamental property determined by the equation of state. For gases of material particles, studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases. We demonstrate a measurement of the compressibility of a two-dimensional quantum gas of light in a box potential and obtain the equation of state for the optical medium. The experiment was carried out in a nanostructured dye-filled optical microcavity. We observed signatures of Bose-Einstein condensation at high phase-space densities in the finite-size system. Upon entering the quantum degenerate regime, the measured density response to an external force sharply increases, hinting at the peculiar prediction of an infinite compressibility of the deeply degenerate Bose gas.

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

 

OK, can someone explain to me the full consequences of this? Particularly the "the photons begin to overlap" phrase in the abstract, and the  "quantum degeneracy" reference.

Is this analogous to the EDP, and NDP that precede a BH formation?

Edited March 29, 2022 by beecee

[Genady]

31 minutes ago, beecee said:

Is this analogous to the EDP, and NDP that precede a BH formation?

I don't think so. EDP and NDP resist compression because electrons and neutrons are fermions and obey Pauli exclusion principle. But photons are bosons and happily occupy the same state.

[joigus]

48 minutes ago, beecee said:

OK, can someone explain to me the full consequences of this? Particularly the "the photons begin to overlap" phrase in the abstract, and the  "quantum degeneracy" reference.

Is this analogous to the EDP, and NDP that precede a BH formation?

"The photons begin to overlap" means that the photons, bosons that they are, begin to favour forming a common state (Bose condensation). The thing going on in BH's is the opposite quantum-degeneracy force: neutrons are fermions, so they cannot be in the same quantum state. In the case of BH formation, quantum-degeneracy force is overcome by gravitation.

Edited March 29, 2022 by joigus
minor correction

[beecee]

8 minutes ago, Genady said:

I don't think so. EDP and NDP resist compression because electrons and neutrons are fermions and obey Pauli exclusion principle. But photons are bosons and happily occupy the same state.

Thanks, I am aware of the "Pauli exclusion principle" but may have misunderstood it. I was sort of under the impression that the NDP did overcome the Pauli exclusion principle. So I am wrong on that count?

[joigus]

"Degeneracy" in quantum mechanics means that states that differ in the values of other quantum numbers have the same energy.

[beecee]

1 minute ago, joigus said:

"The photons begin to overlap" means that the photons, bosons that they are, begin to favour forming a common state (Bose condensation). The thing going on in BH's is the opposite quantum-degeneracy force: neutrons are fermions, so they cannot be in the same quantum state. In the case of BH formation, quantum-degeneracy force is overcome by gravitation.

Ahaaa, so in effect gravity does the "overcoming" thanks. (just a limit to what I thought I understood)

[Genady]

Just now, beecee said:

Thanks, I am aware of the "Pauli exclusion principle" but may have misunderstood it. I was sort of under the impression that the NDP did overcome the Pauli exclusion principle. So I am wrong on that count?

Yes, it did not. But because neutrons are heavier, they can be squeezed into smaller spaces.

3 minutes ago, beecee said:

Ahaaa, so in effect gravity does the "overcoming" thanks. (just a limit to what I thought I understood)

Gravity does not overcome the exclusion principle either. But it is strong enough to push the neutrons to higher energy levels, which is the way to obey the Pauli.

[joigus]

26 minutes ago, Genady said:

Yes, it did not. But because neutrons are heavier, they can be squeezed into smaller spaces.

Gravity does not overcome the exclusion principle either. But it is strong enough to push the neutrons to higher energy levels, which is the way to obey the Pauli.

I think energy itself is not enough to "overcome" Pauli's exclusion principle, and entropy must be playing a strong role. If all those neutrons tried to jump to the same gravity-excited level, they would still be subject to the PEP. It's (probably) because black holes have enormous entropies that this is possible, I think.

Just now, joigus said:

I think energy itself is not enough to "overcome" Pauli's exclusion principle, and entropy must be playing a strong role. If all those neutrons tried to jump to the same gravity-excited level, they would still be subject to the PEP. It's (probably) because black holes have enormous entropies that this is possible, I think.

By "overcome Pauli's exclusion principle" I mean to create enough entropy that quantum degeneracy is broken.

12 minutes ago, joigus said:

By "overcome Pauli's exclusion principle" I mean to create enough entropy that quantum degeneracy is broken.

Sorry, you're right. Quantum degeneracy is never broken. To be more precise: that there are so many close-by states available that, entropy being so high, quantum degeneracy doesn't have a fighting chance.

A similar argument is sketched here: 

https://physics.stackexchange.com/questions/93988/does-black-hole-formation-contradict-the-pauli-exclusion-principle

Tell me what you think.

[Genady]

45 minutes ago, joigus said:

Quantum degeneracy is never broken. To be more precise: that there are so many close-by states available that, entropy being so high, quantum degeneracy doesn't have a fighting chance.

I think this is right. The energy of squeezing goes into pushing neutrons to occupy more and higher energy levels.

But, back to the OP, why would the photons resist compression?

[Genady]

4 hours ago, Genady said:

I think this is right. The energy of squeezing goes into pushing neutrons to occupy more and higher energy levels.

But, back to the OP, why would the photons resist compression?

Actually, not "more", and "higher energy" but not "higher levels". They occupy the same energy level numbers, but as the available space gets smaller, the separation between the levels gets larger, i.e. the same n-th level has a higher energy. This is how I understand it qualitatively.

Edited March 30, 2022 by Genady

[joigus]

9 hours ago, Genady said:

But, back to the OP, why would the photons resist compression?

The photons wouldn't resist compression; they would suddenly fall down into the same degenerate state. They'd form a Bose condensate.

It's a "do the opposite" story. Fermions hate degeneracy; photons just love it.

[swansont]

11 hours ago, Genady said:

But, back to the OP, why would the photons resist compression?

You can’t separate the photons from the interaction with the mirrors. If you have a photon in a cavity, there is a 2p momentum exerted on reflection, which happens in some time t. As the cavity gets smaller, you reflect more often, so t gets smaller and the force goes up. You have to push harder to compress it.

[Genady]

1 hour ago, swansont said:

You can’t separate the photons from the interaction with the mirrors. If you have a photon in a cavity, there is a 2p momentum exerted on reflection, which happens in some time t. As the cavity gets smaller, you reflect more often, so t gets smaller and the force goes up. You have to push harder to compress it.

Thanks. So, it is just the increasing pressure. I think what confused me was that they didn't mention pressure, only compression.

[exchemist]

8 hours ago, swansont said:

You can’t separate the photons from the interaction with the mirrors. If you have a photon in a cavity, there is a 2p momentum exerted on reflection, which happens in some time t. As the cavity gets smaller, you reflect more often, so t gets smaller and the force goes up. You have to push harder to compress it.

That makes sense, but in that case why does the radiation pressure almost disappear as the Bose-Einstein condensate forms? Are we saying the photons all fall into a particle-in-a-box ground state, in which they have only zero point momentum, or something? So, paradoxically, compression "cools" them?

 

 

Edited March 30, 2022 by exchemist

[Genady]

40 minutes ago, exchemist said:

That makes sense, but in that case why does the radiation pressure almost disappear as the Bose-Einstein condensate forms? Are we saying the photons all fall into a particle-in-a-box ground state, in which they have only zero point momentum, or something? So, paradoxically, compression "cools" them?

 

 

I don't think the pressure disappears. It rather stops increasing after some density is achieved. So after that you can keep compressing it without pushing any harder.

[beecee]

38 minutes ago, Genady said:

I don't think the pressure disappears. It rather stops increasing after some density is achieved. So after that you can keep compressing it without pushing any harder.

I think that is what is said/claimed in the following.......

"The rule is usually as follows: The denser a gas, the harder it is to compress. This is also the case with the plugged air pump—at first the piston can be pushed down very easily, but at some point it can hardly be moved any further, even when applying a lot of force. The Bonn experiments were initially similar: The more photons they put into the mirror box, the more difficult it became to compress the gas.

However, the behavior changed abruptly at a certain point: As soon as the photon gas exceeded a specific density, it could suddenly be compressed with almost no resistance. "This effect results from the rules of quantum mechanics," explains Schmitt, who is also an associate member of the Cluster of Excellence "Matter and Light for Quantum Computing" and project leader in the Transregio Collaborative Research Center 185. The reason: The light particles exhibit a "fuzziness"—in simple terms, their location is somewhat blurred. As they come very close to each other at high densities, the photons begin to overlap. Physicists then also speak of a "quantum degeneracy" of the gas. And it becomes much easier to compress such a quantum degenerate gas".

[exchemist]

58 minutes ago, Genady said:

I don't think the pressure disappears. It rather stops increasing after some density is achieved. So after that you can keep compressing it without pushing any harder.

OK, so that would equate to pushing a progressively increasing proportion of photons into a condensate phase, rather than suddenly reaching a threshold, like a lambda point, at which a bulk transition of the whole system occurs, into a condensate phase.

However from the phrasing in the description I'm left wondering which of the two it is.  I see that, rather than compressing the photons into a smaller space, what they did was add more photons, increasing the density that way instead. The suggestion is that the compressibility suddenly dropped, as if a lambda point was reached at a certain density.

Edited March 30, 2022 by exchemist