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.
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.
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.
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.)
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?).
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):
> 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.)
> 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
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....
While general relatively is just as happy to make a black hole from light as from matter, the exchange ratio is E=mc^2, and c^2 is big in a way that makes the difference between Elon Musk’s net worth and the pay Nǃxau ǂToma received for for The Gods Must Be Crazy [0] seem insignificant in comparison.
A 1mm^3 black hole would need about 1.043×10^28 gigawatt hours of light, or about what the sun emitted in total over the last 3.1 million years.
Which is a fun project you can do at home on your kitchen counter and has mystified scientists since 1934. Some theories suggest it is a fusion reaction and that there is a plasma hotter than the sun forming inside the bubble.
I'm wondering what would happen if they could manage to compress the light as it formed. Maybe it could sustain the light for longer periods of time or increase its intensity? It would probably be difficult to time a sub-nanosecond compression of this kind, though.
25 comments
[ 0.19 ms ] story [ 58.6 ms ] threadIf 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.
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.
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.
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):
> 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.)
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
This is the force used in the idea of a light sail. https://en.wikipedia.org/wiki/Solar_sail
It's also used in optical tweezers. https://en.wikipedia.org/wiki/Optical_tweezers
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....
Asking for a friend.
It would either just make a black hole, or, cause no issue?
If it was a gas made of matter, and it became easier to compress the more we compressed it, that'd be a recipe to create a black hole.. (?)
I’m pretty sure the idea is that a kugelblitz would be a black hole not a white hole?
A 1mm^3 black hole would need about 1.043×10^28 gigawatt hours of light, or about what the sun emitted in total over the last 3.1 million years.
[0] https://en.m.wikipedia.org/wiki/N%C7%83xau_ǂToma
https://youtu.be/v3hd3AI2CAA
https://hackaday.com/2019/09/06/capture-a-star-in-a-jar-with...
Which is a fun project you can do at home on your kitchen counter and has mystified scientists since 1934. Some theories suggest it is a fusion reaction and that there is a plasma hotter than the sun forming inside the bubble.
I'm wondering what would happen if they could manage to compress the light as it formed. Maybe it could sustain the light for longer periods of time or increase its intensity? It would probably be difficult to time a sub-nanosecond compression of this kind, though.