3 ms·

So I read the original article about this to some extent, and something just doesn't sit right with me. If it becomes 'easier' to compress as you squash it, I

by sjmm1989 5y ago

So I read the original article about this to some extent, and something just doesn't sit right with me. If it becomes 'easier' to compress as you squash it, I assume there is some level of resistance before they attempt to squash it, right? Something measurable right? Because the more I think about this, the more I want to say "Well of course a photonic gas is easier to squish when applied pressure. It's light, it likes to spread out... and fill spaces..." Is that just me? Cause this 'discovery' feels very much like a 2+2=5 kind of scenario. Something seems wrong. Perhaps its me, but still... can't shake this feeling. I don't know... Just seems like something is off.

civilized 5y ago
Light gas compresses easily because photons are bosons. Most ordinary matter is fermionic.
- yccs27 5y ago
  Some atoms also behave as bosons (specifically those consisting of an even number of elementary fermions), so that cannot be the real difference.
  - gus_massa 5y ago
    IIUC the effect in bosons was already known, and the new interesting part is that they were able to measure it in a photon gas experimentally: From the abstract of the paper: > For gases of material particles, studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases. [i.e. a similar result is known for atoms] > Here 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 is carried out in a nanostructured dye-filled optical microcavity. [i.e. this is a new experimental result] > We observe signatures of Bose-Einstein condensation at high phase-space densities in the finite-size system. [i.e. the photons are bosons as expected] Just in case, this is not a dismissive comment. I'd like to add that it's amazing and weird that they can measure this.
    - yccs27 5y ago
      Oh wow, thank you for the clarification! If I understand it correctly then, the divergent compressibility is a consequence of Bose-Einstein condensation? It definitely is exciting to see light show this effect. I wonder how much the difference between light and matter (due to very different dispersion relations) has an influence on the BEC properties like compressibility. I guess that's what they are trying to find out.
  - wrycoder 5y ago
    Except for electron degeneracy pressure.
  - deleted 5y ago
    [deleted]
yccs27 5y ago
My main question is: Did the number of photons stay constant throughout the isothermal compression? If it didn't, that would basically explain everything. (I wouldn't rule out other effects though.)
- throwawayninja 5y ago
  If they're escaping, it raises interesting questions about where they might be going / possibly merging? What a strange view of the universe if two particles under pressure could combine and have different attributes (less mass?).
  - deleted 5y ago
    [deleted]
alecst 5y ago
For an ideal gas in a 3d box, the "equation of state" that relates P(ressure), T(emperature), and V(olume) is PV = nRT, where R is a constant and n is the number of particles. You can rewrite this as: P = nRT/V Compressing the gas (by shrinking V) while keeping n constant will result in a higher pressure P. You can imagine a cylindrical piston, where the pressure is the force per unit area it takes to hold the piston down. The important thing to remember is that this equation describes an ideal gas in a 3d box. An ideal gas is made of tiny particles that only interact with the piston's walls, but not with each other. This is an approximation to reality, but a decent one when the gas is low density. Weird things can happen when particles interact, and when the dimensions are different. The equation of state above is one example of how P, V, and T can be related, but if the particles interact a little more the equation of state can change. And if they interact a lot more (by, say, becoming a liquid) the equation of state can change again. This paper is talking about photons in a 2d box (an optical trap), and in part talks about measuring/confirming its equation of state -- the relationship between P, T, and V. These particles have a peculiar kind of interaction, where the photons don't really interact unless they're in a special state. I'm gonna quote the relevant part of the paper (from https://arxiv.org/pdf/2112.12787.pdf https://arxiv.org/pdf/2112.12787.pdf): > It is well understood that as the thermal wave packets spatially overlap the classically expected decrease in compressibility with density (it is harder to compress a dense gas than a dilute one) is replaced by a compressibility increase stemming from the quantum-statistical occupation of low-lying energy levels, reducing the energy cost for compression as compared to the classical gas case. In the extreme high-density limit of an infinite-size deeply degenerate gas, bosons can be added to the system at essentially vanishing energy cost... My translation: it's normally harder to compress a normal gas the more you squeeze it, but for a photon gas it's different. Because photons are bosons, as you compress them (or cool them), they tend to group together in a special configuration. That special configuration is called a Bose-Einstein Condensate (BEC). In a BEC, a meaningful fraction photons pile into the ground state. (This is what the paper calls "degeneracy" -- quantum particles being in the same energy state.) (According to the paper this is NOT possible when the 2d configuration is "infinite", but does happen in some cases when the trap is finite, as is the case for a real experiment.) To say more than this would be tricky and take an expert, which I am not. But I think this might illuminate some of the subtler aspects of the experiment which may take away some of the uneasiness that you're feeling. But I still think the uneasy feeling is justified: when you get into quantum thermodynamics some things become a little trickier to reason about, as intuitions about pressure, volume, and temperature begin to break down somewhat. Edit: as a final clarification, I think they try to keep the number of photons constant: > To maintain a steady-state photon number inside the cavity, continuous pumping is required to compensate losses from mirror transmission. A reasonable intuition might be: as the number of photons in the ground state (N_0) increases, the remaining photons that can provide significant pressure (N) reduces. (I am unsure about this, because I don't know how much pressure ground-state photons contribute.)
jlokier 5y ago
> If it becomes 'easier' to compress as you squash it, I assume there is some level of resistance before they attempt to squash it, right? Something measurable right? Yes, there is a measurable force resisting compression, called radiation pressure. (You could reasonably call it photon pressure or light pressure, meaning the same thing.) https://en.wikipedia.org/wiki/Radiation_pressure https://en.wikipedia.org/wiki/Radiation_pressure This is the force used in the idea of a light sail. https://en.wikipedia.org/wiki/Solar_sail https://en.wikipedia.org/wiki/Solar_sail It's also used in optical tweezers. https://en.wikipedia.org/wiki/Optical_tweezers https://en.wikipedia.org/wiki/Optical_tweezers
- sjmm1989 5y ago
  Alright, fair enough. I know about these things on some level, so that's making more sense now. Maybe it's just the way they wrote it, but it felt very much to me like it was some sort of sly joke. A gas that spreads easier the more you force it. Hmmm....

civilized 5y ago

Light gas compresses easily because photons are bosons. Most ordinary matter is fermionic.

yccs27 5y ago
Some atoms also behave as bosons (specifically those consisting of an even number of elementary fermions), so that cannot be the real difference.
- gus_massa 5y ago
  IIUC the effect in bosons was already known, and the new interesting part is that they were able to measure it in a photon gas experimentally: From the abstract of the paper: > For gases of material particles, studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases. [i.e. a similar result is known for atoms] > Here 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 is carried out in a nanostructured dye-filled optical microcavity. [i.e. this is a new experimental result] > We observe signatures of Bose-Einstein condensation at high phase-space densities in the finite-size system. [i.e. the photons are bosons as expected] Just in case, this is not a dismissive comment. I'd like to add that it's amazing and weird that they can measure this.
  - yccs27 5y ago
    Oh wow, thank you for the clarification! If I understand it correctly then, the divergent compressibility is a consequence of Bose-Einstein condensation? It definitely is exciting to see light show this effect. I wonder how much the difference between light and matter (due to very different dispersion relations) has an influence on the BEC properties like compressibility. I guess that's what they are trying to find out.
- wrycoder 5y ago
  Except for electron degeneracy pressure.
- deleted 5y ago
  [deleted]

yccs27 5y ago

My main question is: Did the number of photons stay constant throughout the isothermal compression? If it didn't, that would basically explain everything. (I wouldn't rule out other effects though.)

throwawayninja 5y ago
If they're escaping, it raises interesting questions about where they might be going / possibly merging? What a strange view of the universe if two particles under pressure could combine and have different attributes (less mass?).
- deleted 5y ago
  [deleted]

alecst 5y ago

For an ideal gas in a 3d box, the "equation of state" that relates P(ressure), T(emperature), and V(olume) is PV = nRT, where R is a constant and n is the number of particles. You can rewrite this as: P = nRT/V Compressing the gas (by shrinking V) while keeping n constant will result in a higher pressure P. You can imagine a cylindrical piston, where the pressure is the force per unit area it takes to hold the piston down. The important thing to remember is that this equation describes an ideal gas in a 3d box. An ideal gas is made of tiny particles that only interact with the piston's walls, but not with each other. This is an approximation to reality, but a decent one when the gas is low density. Weird things can happen when particles interact, and when the dimensions are different. The equation of state above is one example of how P, V, and T can be related, but if the particles interact a little more the equation of state can change. And if they interact a lot more (by, say, becoming a liquid) the equation of state can change again. This paper is talking about photons in a 2d box (an optical trap), and in part talks about measuring/confirming its equation of state -- the relationship between P, T, and V. These particles have a peculiar kind of interaction, where the photons don't really interact unless they're in a special state. I'm gonna quote the relevant part of the paper (from https://arxiv.org/pdf/2112.12787.pdf https://arxiv.org/pdf/2112.12787.pdf): > It is well understood that as the thermal wave packets spatially overlap the classically expected decrease in compressibility with density (it is harder to compress a dense gas than a dilute one) is replaced by a compressibility increase stemming from the quantum-statistical occupation of low-lying energy levels, reducing the energy cost for compression as compared to the classical gas case. In the extreme high-density limit of an infinite-size deeply degenerate gas, bosons can be added to the system at essentially vanishing energy cost... My translation: it's normally harder to compress a normal gas the more you squeeze it, but for a photon gas it's different. Because photons are bosons, as you compress them (or cool them), they tend to group together in a special configuration. That special configuration is called a Bose-Einstein Condensate (BEC). In a BEC, a meaningful fraction photons pile into the ground state. (This is what the paper calls "degeneracy" -- quantum particles being in the same energy state.) (According to the paper this is NOT possible when the 2d configuration is "infinite", but does happen in some cases when the trap is finite, as is the case for a real experiment.) To say more than this would be tricky and take an expert, which I am not. But I think this might illuminate some of the subtler aspects of the experiment which may take away some of the uneasiness that you're feeling. But I still think the uneasy feeling is justified: when you get into quantum thermodynamics some things become a little trickier to reason about, as intuitions about pressure, volume, and temperature begin to break down somewhat. Edit: as a final clarification, I think they try to keep the number of photons constant: > To maintain a steady-state photon number inside the cavity, continuous pumping is required to compensate losses from mirror transmission. A reasonable intuition might be: as the number of photons in the ground state (N_0) increases, the remaining photons that can provide significant pressure (N) reduces. (I am unsure about this, because I don't know how much pressure ground-state photons contribute.)

jlokier 5y ago

> If it becomes 'easier' to compress as you squash it, I assume there is some level of resistance before they attempt to squash it, right? Something measurable right? Yes, there is a measurable force resisting compression, called radiation pressure. (You could reasonably call it photon pressure or light pressure, meaning the same thing.) https://en.wikipedia.org/wiki/Radiation_pressure https://en.wikipedia.org/wiki/Radiation_pressure This is the force used in the idea of a light sail. https://en.wikipedia.org/wiki/Solar_sail https://en.wikipedia.org/wiki/Solar_sail It's also used in optical tweezers. https://en.wikipedia.org/wiki/Optical_tweezers https://en.wikipedia.org/wiki/Optical_tweezers

sjmm1989 5y ago
Alright, fair enough. I know about these things on some level, so that's making more sense now. Maybe it's just the way they wrote it, but it felt very much to me like it was some sort of sly joke. A gas that spreads easier the more you force it. Hmmm....