photon-photon scattering is a thing but it's very weak.

This is unfortunately not what I mean, but thanks anyway. Technically speaking I am talking about the energy contribution in the Hamiltonian integral of a system of multiple identical particles that arises from the requirement that the total wave function must not change its sign upon exchange of particle labels (in the case of bosons). Does this exist for photons?

[removed]

Anywhere that you cannot afford to shut down the cooling you will have more than one cooling tower so you can continue operation if a tower is out of service. (eg Hospital, Critical computer centre, Critical Process)

[removed]

[removed]

I don't know if this completely what you mean, but laser works by stimulated emission of photons, since they like to be in the same quantum state.

[removed]

[removed]

Yep, the water used in the cooling tower is the perfect place for bugs to grow and many commercial HVAC systems don’t have HEPA/UV treatment for the air handlers. You can treat the cooling tower water with quats to suppress bacterial/viral growth. Also, the condensate that forms in the air handlers is also ideal for bugs to grow.

The movement in HVAC is to have separate air handlers and only one cooling tower.

One thing I am wondering after reading your (really good) reply: Is there exchange interaction between photons?

I‘m a theoretical chemist and well familiar with exchange interactions between electrons, in magnetic materials, and in superfluid helium-4. Is there an equivalent attractive force between photons since they are bosons?

Astronauts who went to the moon actually did live fractionally longer than people who stayed on Earth because of the speed they traveled at. Its not measurable in this case but it can, I believe, be observed in the behavior of relativistic jets from stars.

[removed]

[removed]

>It is my understanding that the light is slowed through the medium because photons are absorbed and then re-emitted repeatedly.

That doesn't line up with what we observe. If an object absorbs a photon, it doesn't "remember" the direction the photon came from and then emit a new photon continuing along the same path - it emits a photon in a completely random direction. Yet we see light travel in a straight line, only bending when moving between mediums.

I don't think you can get a satisfactory explanation for why light slows down by thinking about photons. Light is an electromagnetic wave that exerts a force on the electrons in a material, the electrons oscillate and produce their own electromagnetic waves; do the maths and you find that the sum of all of these waves is a single slower moving one.

[removed]

[removed]

[removed]

This informative and seems to be a well informed response. On the other hand this sounds exactly like what ChatGPT3 would say when prompted with "How do you clean inaccessible ducts?" Which is maybe a compliment.

[removed]

So, first answering your main question- elementary particles are all fungible. That means, they are truly identical, and they are impossible to label. So, if a photon is absorbed and then remitted, it doesn't really make sense to say "is it the same photon or a different one?" There aren't really "same" or "different" photons, there's just photons, unlabeled.

And it's not just photons. Any time you have a particle collision which results in some different elementary particles (like the ones from particle accelerators), if one of the products and reactants are the same elementary particle, you can't answer "is this the same or a different particle?" It's a particle. That's all you can say.

Now, to get into the can of worms you opened, and probably didn't even know it. It is this line:

> It is my understanding that the light is slowed through the medium because photons are absorbed and then re-emitted repeatedly.

I always say, if you want to get some physicists to fight, ask them why light propagates slower through a non-vacuum. You'll get a different answer from each one, and they will pretty aggressively defend their position and discount the others. I always find it fascinating, because it seems like a pretty simple question (why does light travel slower in a non-vacuum?) but the answer is quite complex, and our models for it all work, but tell slightly different stories.

The easiest to understand model is the one you mentioned- and it does work. The most common complaint is that an atom can only absorb very specific wavelengths, but light of all wavelengths is slowed down by materials. But, this is handled by understanding that collections of particles will have nearly an infinite number of modes of excitation- you can cause groups of particles to vibrate or rotate, you can cause vibrations between groups of 2 particles, or groups of 3 (or 4, or 5....). There's a ton of different excitation modes, and for a dense medium, you can absorb and re-emit any wavelength of light.

Other people will express a model where light actually takes many paths through the medium, and that superposition actually results in it appearing as if the light is traveling under 'c'. Still others will talk about how photons become a quasiparticle when in a dense medium, and that particle doesn't travel at 'c'. And I'm sure there are others out there. All of these explanations "work" and I won't say one is right over the other.

[removed]

[removed]

Quite good and thorough answer. This only works for mixing liquids with the same (or close enough) chemical compositions though. With two different liquids reactions might take place though, altering the final amount of heat dramatically. The enthaply of mixing might be positive or negative as well. Therefore, mixing two different solutions at the same temperature can consume or produce heat as well.

I also definitely did not start a chimney fire.

It was a dryer lint fire.