A gas made from light becomes easier to compress as you squash it

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.

[0] https://en.m.wikipedia.org/wiki/N%C7%83xau_ǂToma

Potentially a white hole

> 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

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

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.)

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.)

That would create a Kugelblitz, a black hole made of light.

No, I don’t think that makes sense?

It would either just make a black hole, or, cause no issue?

Sounds very interesting, seems the moral option.