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Comment by inlineint | Hacker News Reader

> Now, however, other types of nanolight are on the rise. Some have a rare ability to absorb lots of low-energy photons and combine the energy into a handful of high-energy photons

Doesn't it violate the second law of thermodynamics?

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Adding two 1 eV photons in a two-photon upconverter does not give you a single 2 eV photon. Some of the energy of the upconverted photons is lost as heat, so thermodynamically it's okay.

Yes, you are right. Each of these nanoparticles should also radiate heat radiation or heat the environment near it in other way, but it is not clear from the citation.

No, because you're not gaining anything. Lots of energy goes in (in the form of lots and lots of photons having not so much energy each) and lots comes out (in the form of many fewer photons, each with a larger amount of energy), but you don't get more total energy out than in. It's not conceptually different from, say, trickle charging a LiPO battery for a week from dollar-store AAAs, then using it for a few glorious seconds flying a drone that couldn't have even lifted the AAAs. You're still using the energy from the AAAs, and suffering losses, but you're still doing something you couldn't have done with the AAAs alone. The mechanism is different - very, very different - but the idea's the same.

You are talking more about the first low of thermodynamics (conservation of energy law) than the second. The problem is that the processes you described radiate heat, but it is not explicitly stated that the process of combining of a few photos with low energy to photons with high energy does the same.

I'd like to point out two-photon absorption, in which two photons incident on something at the same time are absorbed like a single photon with the total energy of both:

https://en.wikipedia.org/wiki/Two-photon_absorption

(Too bad that article doesn't mention thermodynamics.) It has wonderful uses like https://en.wikipedia.org/wiki/Two-photon_excitation_microsco...

You can shine red light (low-energy photons) on a photovoltaic cell and use the resulting electricity to power a blue light LED (high-energy photons), so this process does not violate any physical laws, I guess?

It is possible because the temperature of a photovoltaic cell is lower than the temperature of red light radiation, so the entire system radiates both higher-energy photons (blue light) and lower-energy photons (infra-red heat radiation).

Not if

  original_number_of_photons * original_energy_per_photon >= final_number_of_photons * final_energy_per_photon

The quote explicitly says that there's a lot of the original photons and a few of the resulting photons, so the increase of energy per photon would be compensated, right?

The expression can't be an equality, otherwise it is possible to use such particles to transmit heat from a radiating black body with lower temperature to a radiating black body with higher temperature using a filter from these particles.

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