[HN Gopher] A gas made from light becomes easier to compress as ...
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A gas made from light becomes easier to compress as you squash it
Author : chriskanan
Score : 47 points
Date : 2022-03-25 11:11 UTC (2 days ago)
(HTM) web link (www.newscientist.com)
(TXT) w3m dump (www.newscientist.com)
| palisade wrote:
| I wonder what would happen if you used this technique to compress
| sonoluminescence?
|
| 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.
| MrYellowP wrote:
| I've once heard that we could theoretically destroy the universe
| if we just crammed enough photons into a tiny enough space. Would
| this help?
|
| Asking for a friend.
| fermuch wrote:
| That would create a Kugelblitz, a black hole made of light.
| louwrentius wrote:
| Sounds very interesting, seems the moral option.
| drdeca wrote:
| No, I don't think that makes sense?
|
| It would either just make a black hole, or, cause no issue?
| rmbyrro wrote:
| My initial reaction: wow, did we find a way to create a mini
| black-hole? Well, it ain't quite a _matter-based_ gas...
|
| 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.. (?)
| ben_w wrote:
| 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 N!xau |=Toma received for _for The
| Gods Must Be Crazy_ [0] seem insignificant in comparison.
|
| A 1mm^3 black hole would need about 1.043x10^28 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_|=Toma
| throwawaycities wrote:
| Potentially a white hole
| yccs27 wrote:
| Original paper: https://arxiv.org/abs/2112.12787
| willis936 wrote:
| Any chance of mankind mankind making a pico-scale kugelblitz?
|
| https://youtu.be/v3hd3AI2CAA
| sjmm1989 wrote:
| 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.
| jlokier wrote:
| > 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
|
| 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
| sjmm1989 wrote:
| 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....
| alecst wrote:
| 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.)
| civilized wrote:
| Light gas compresses easily because photons are bosons. Most
| ordinary matter is fermionic.
| yccs27 wrote:
| Some atoms also behave as bosons (specifically those
| consisting of an even number of elementary fermions), so that
| cannot be the real difference.
| wrycoder wrote:
| Except for electron degeneracy pressure.
| gus_massa wrote:
| 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 wrote:
| 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.
| [deleted]
| yccs27 wrote:
| 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 wrote:
| 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]
| ars wrote:
| Previous discussion:
| https://news.ycombinator.com/item?id=30803455
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