Measuring the heat capacity of condensed light

Measuring the heat capacity of condensed light
Prof. Martin Weitz's team at the Institute of Applied Physics of the University of Bonn has measured the temperature of a gas of light, when it condenses. The thermometer should be understood only figuratively, because, the temperature was determined by recording the wavelengths of the light. Credit: Tobias Damm

Liquid water is a very good heat storage medium – anyone with a Thermos bottle knows that. However, as soon as water boils or freezes, its storage capacity drops precipitously. Physicists at the University of Bonn have now observed very similar behavior in a gas of light particles. Their findings can be used, for example, to produce ultra-precise thermometers. The work appears in the prestigious technical journal Nature Communications.

Water vapor becomes liquid under 100 degrees Celsius – it condenses. Physicists speak of a phase transition. In this process, certain thermodynamic characteristics of the water change abruptly. For instance, at a single stroke, twice as much heat energy can be stored than in the gaseous state.

Light consists of tiny indivisible portions, the photons. Under certain conditions, they, too, can condense, if they are cooled enough. Many thousands of these light packets then suddenly fuse into a kind of super-photon with unusual characteristics – a so-called Bose-Einstein condensate.

Photon gas also changes heat storage characteristics abruptly

The physicists at the University of Bonn have now been able to show that the photon gas at this phase transition behaves according to the theoretical predictions of Bose and Einstein: Similar to water, it abruptly changes its capacity, meaning the ability to store thermal energy. "This behavior was already known from condensed atoms", explains Prof. Dr. Martin Weitz of the Institute of Applied Physics. "However, this is the first time that this phenomenon has been demonstrated for a condensate of light".

Atoms, too, form a Bose-Einstein condensate, when they are cooled greatly and enough of them are simultaneously concentrated in a small space. They then suddenly become indistinguishable: They act like a single giant atom. Twenty years ago, physicists already demonstrated that the of atoms suddenly changes at this phase transition. How strong this change is, however, can be measured only imprecisely for atoms. "In our condensate, this can be done substantially better", emphasizes Dr. Jan Klärs, who has since moved from Bonn to ETH Zurich.

Measuring the heat capacity of condensed light
At the measurement apparatus: Prof. Dr. Martin Weitz, Tobias Damm, David Dung, Dr. Julian Schmitt, and Dr. Frank Vewinger of the Institute of Applied Physics of the University of Bonn. Credit: Volker Lannert/Uni Bonn

The heat capacity of a material is calculated from the energy needed to heat it by one degree. Usually this is done by measuring the temperature of the substance before and after adding a defined amount of energy. However, the temperature of a gas of light can not be measured with a thermometer; but that is also not necessary. "In order to determine the temperature of the gas, it is only necessary to know the different wavelengths of the – the distribution of its colors", says Klärs. And this can be determined with extreme precision with the methods available today.

"Our findings for the change in the heat capacity at the transition from photon gas to Bose-Einstein condensate match the theoretical predictions exactly", explains Tobias Damm of the Institute of Applied Physics. "The precision of this method is so high that it is very suitable for precision measurement of certain natural thermodynamic constants".

The heat content of the photon gas changes not only upon condensation to a super-photon, but also continuously with the ambient temperature. The Bonn physicists therefore hope that their findings can also be used to build high-precision thermometers.


Explore further

The first model for capturing and condensing light under realistic conditions

More information: Tobias Damm et al. Calorimetry of a Bose–Einstein-condensed photon gas, Nature Communications (2016). DOI: 10.1038/NCOMMS11340
Journal information: Nature Communications

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Apr 19, 2016
gas of light? no, waves are not particles! the slit experiment with particles is the same experiment with waves. Particles emit waves, therefore we should see wave properties, OK? Waves may cause particles to move. So, no, you may focus light but condensation is really, really, stupid. It should not take a 100 years to know Dr. E, the standard model, etc. are complete nonsense. The speed of light is not constant. Just think about how fast a wavelet passes and what the initial wavelength is. Don't try magic, like changing space and time. Not necessary. Lambda Nu is constant but the speed may be from 0 to infinity. Duh!

https://drive.goo...4dWFJSDA

Apr 19, 2016
gas of light?

Instead of spending 5 minutes typing out BS you might just have spent 5 seconds on wikipedia to find out what a photon gas is fiirst.

Apr 19, 2016
Stop the train everyone! Hyperfuzzy and his google doc has solved all of physics in just 13 short pages. He's got it all: Gravity, Light Speed, Big bang, Antimatter, Standard Model, Dark Matter, Wave Particle Duality.

There's even a few mathsy looking things in there too: some Greek letters, some big Greek letters, some arrows, the word "Isomorphism". This killer proof hinges on the miraculous undefined function f(+,-) --> mass, gravity.

Move over Einstein, we have entered the age of Rufus G. Warren, Jr.

Apr 20, 2016
Stop the train everyone! Hyperfuzzy and his google doc has solved all of physics in just 13 short pages. He's got it all: Gravity, Light Speed, Big bang, Antimatter, Standard Model, Dark Matter, Wave Particle Duality.

There's even a few mathsy looking things in there too: some Greek letters, some big Greek letters, some arrows, the word "Isomorphism". This killer proof hinges on the miraculous undefined function f(+,-) --> mass, gravity.

Move over Einstein, we have entered the age of Rufus G. Warren, Jr.

Thanks, but come on guys. The function of the +&- is meant to cover self assembly. Anyway, if you reject, stay ...

Apr 23, 2016
Did zephyr write this article?

Apr 23, 2016
Do you guys also believe in ghosts, angels, fairies, leprechauns, magic, gluons, particle/wave duality, bending space and time, crashing protons and something called a higgs to define the undefinable. You guys are silly. A single point in a non-space that creates the world? This makes sense to you? Well, guess I must be the dumb one in the room!

Apr 23, 2016
This comment has been removed by a moderator.

Apr 23, 2016
This comment has been removed by a moderator.

Apr 23, 2016
It's like zephyr enjoys getting banned and getting dozens of posts deleted all at once.

It's like writing in the sand. It's like writing on the sidewalk with chalk. It's like spelling your name in alphabet soup. It's like etch-a-sketch.

It's like making a big question mark as you pee in the snow.

Apr 23, 2016
A field of photons that get bigger or heavier or more energetic over time not based upon Maxwell; but ... who writes this $hit? Yes, atoms and electrons and protons can do this. How many states do you wish to activate? Don't forget mirror imaging and the price of instability. By the way, you can read the spectrum if and only if you can see it all, so your ...

Apr 23, 2016
This comment has been removed by a moderator.

Apr 23, 2016
This comment has been removed by a moderator.

Apr 23, 2016
It's like zephyr enjoys getting banned and getting dozens of posts deleted all at once
It's clear for me, that for layman people who don't know about all of this the above posts must sound like pure nonsense: massive photons condensing inside the resonator: which sorcery is this all about? What the heck this guy''s talking about? Lets delete him for sure! Anyway, I'm archiving all my posts and recycling the same comments to another forums and news servers at the same moment - so I actually don't care. Most of people don't read old articles anyway, the discussions bellow them the less.

Transmission eq?

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