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Comment by brimble | Hacker News Reader

> To do this, we stored light particles in a tiny box made of mirrors," explains Dr. Julian Schmitt of the IAP, who is a principal investigator in the group of Prof. Dr. Martin Weitz. "The more photons we put in there, the denser the photon gas became."

This seems like the kind of clever but also crazy thing a kid would come up with based on limited information about the world, but that definitely doesn't work.

There's got to be more to it than this description... right? Or this is one of those things where a physicist said "mirror" but actually means something totally different that no normal person would call a mirror, as physicists seem to do often and which causes so very much misunderstanding of physics?

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Good question; had to go to the supplementary materials of the paper in Science [1]. It is called a Bragg Mirror [2], which basically seems like a special coating consisting of layers of alternating refractive index. I don't know the details but this isn't _necessarily_ more exotic than, say, the antireflection coatings on eyeglasses or photographic optics. The supplementary material states that the mirror is 99.998% reflective in the wavelength under study.

[1] https://www.science.org/doi/10.1126/science.abm2543 [2] https://en.wikipedia.org/wiki/Distributed_Bragg_reflector

What’s the difference between photons bouncing in a box, and a gas? …no, really?

There’s a lot of them, starting with the fact that gas particles have mass, and photons generally don’t. However, confinement causes mass. The gas particles are in a sense confined even when floating free (by other fields), and photons are only confined when they bounce off the walls, however-

They do that a lot. As you zoom out, the phenomena start to look alike.

Sure, I'm more hung up on the "just shoot some photons into a mirrored box and they'll bounce around forever". That... can't possibly be very close to what they actually did, right? This is one of those cases in which the description that makes it to the public has almost nothing to do with what actually happened, surely? Or did they really do that, and if so, how? It seems like it'd fail for a hundred different reasons. That's more interesting than the phenomenon, to me.

I watched the video and nothing they showed looks like a mirrored box.

A mirror is an object that reflects an image / light / photons. There are mirrors with different reflectiveness and different construction. In the video the prof says they are using a "highly reflective nanostructured mirror", so it's obviously not exactly your silvered glass bathroom variety mirror, but in principle it's the same: a surface that reflects almost all photons, and absorbs or transmits almost none.

The box is also not necessarily like your average shoe box, I'd guess it's much smaller (millimeters), show it's not shown on the video directly, up close.

Seems like the ultimate size would be limited by the reflectivity coefficient too. The speed of the photons makes for a whole lot of reflection events. The stats take over from there.

I found the idea of this experiment strangely compelling too.

This stuff is hard to begin to understand for most people I would imagine. That is generally true for me. For me, that all translates into an fun and interesting mode of conceptual thought akin to what happens with good sci-fi.

Mixing the concepts of gasses and light is novel to me as well, and I suppose that is what makes this one compelling.

I wonder whether this idea might be combined with how we have slowed the travel speed of light down many orders? Perhaps the materials forming the mirror box do that?

I am going to read this one again (during more normal waking concentration hours) and follow a few of the science rabbit holes the references will surely contain.

Well, photons are bosons (spin 1 not 1/2) as result they have very different statistics. You can get population inversion (e.g. a laser or bose condensate) where many of the bosons all sit in a single energy level. That's not possible for fermions due to the fermi exclusion principal (e.g. electron orbital shells).

> confinement causes mass

So the Pythagoreans were on the right track!

> However, confinement causes mass

That's interesting, can you elaborate? Or point to some link that discuss this

https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalenc... gives some hints:

> A simple example of an object with moving parts but zero total momentum is a container of gas. In this case, the mass of the container is given by its total energy [...] Similarly, even photons, if trapped in an isolated container, would contribute their energy to the mass of the container. Such extra mass, in theory, could be weighed in the same way as any other type of rest mass, even though individually photons have no rest mass.

i.e., it's a consequence of energy-mass equivalence, which applies equally to isolated systems as it does individual particles.

>This seems like the kind of clever but also crazy thing a kid would come up with based on limited information about the world, but that definitely doesn't work.

Now I remember this- I wanted to know what the 'true color' (ha) of a mirror would be, and imagined a sphere of mirrors. However I could not wrap my head around how one would keep the light from bouncing off themselves, hiding away the 'true color' again. Haha.

I see we had a similar childhood. I have sketches somewhere from when I was around 11-12 describing such a device.

You might enjoy this VSauce video, "What color is a mirror?" https://youtu.be/-yrZpTHBEss

Can confirm, did think of this as a kid. Also wanted the box to have a hole that could be opened to release all those stored up photons in the form of a deathray.

Hey me too xD

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