[whatshisface]

Having thought about it, I'm not sure if I agree. Light can reflect off of transparent crystals, which do not have the right electron energy levels for absorption (that's why they're transparent.) It's possible that the Feynman diagram of a reflection might involve the photon "going away and another one replacing it" (I don't know what the diagram looks like), but Feynman diagrams cannot be interpreted as sequences of events. Instead, they describe an instantaneous flow of amplitude between different quantum states. I would only go along with the absorption and re-emission interpretation if there was always a time delay between the two steps (which there cannot be if the mirror does not have any energy level deltas suitable for storing the energy).

The other side of the debate would be that if the mirror is moving towards or away from the light, the reflection will be Doppler-shifted to a higher or lower frequency. Does this mean that the reflected photons are not the same photons as the incident photons, or does it mean that the same photons have had their energy changed? I think there is no meaningful distinction because every two particles with the same name in quantum mechanics are identical anyway. There's no telling which are which. If I showed you a photon, then took it back and showed you another, you would never be able to tell whether I had opened the same box twice or if I had taken the old one out and captured a new one from my desk lamp.

[chemeng]

They are not the same photons. They are emitted by the electron changing energy states after excitation of the incoming photon. But your intuition that there is some quantum weirdness is correct. Quantum Electrodynamics is the theory you're looking for. It unifies quantum mechanics and special relatively in the context of light/matter interaction. Given you seem to be familiar with Feynman. You're in for a treat! His book on QED is meant to be an accessible explanation.[1]

[1]https://en.m.wikipedia.org/wiki/QED:_The_Strange_Theory_of_L...

[whatshisface]

Hydrogen atoms can't store photons that do not have the same wavelength as any of its spectral lines. Nonetheless, hydrogen gas has an index of refraction that is a continuous function of frequency, and as a result it can reflect light of any wavelength. Therefore, hydrogen does not store any energy for any length of time in the course of typical reflection. Even if the Feynman diagram for reflection looks like a combination of absorption and re-emission, if the absorption and emission process can't occur at separate times, then it is not right to say that one is followed by the other.

[boomboomsubban]

Isn't hydrogen gas almost exclusively H2, giving it plenty of possible electrons to interact with photons?

[whatshisface]

That's a good catch, but H2's spectral lines are still discrete. It's not that gasses can't absorb photons, it's that they can only do so at very specific frequencies.

To simplify the argument, you could use Helium instead. Here is a (discrete!) list of its spectral lines: https://physics.nist.gov/PhysRefData/Handbook/Tables/heliumt...

And here is a chart of its (continuous!) index of refraction: https://refractiveindex.info/?shelf=main&book=He&page=Mansfi...

[BeetleB]

You didn't address the concern. A given atom cannot absorb any photon. They can only absorb photons of certain energy levels/wavelengths. Electrons can occupy only certain energy levels. So they can only absorb photons that match the difference if two energy levels.

[empath75]

> They are not the same photons.

How do you define "the same" for two photons?

[tsbinz]

This is the question that should be higher up in the thread. This whole discussion makes no sense. Photons carry no hidden identity.

[zamadatix]

A photon doesn't need to carry a UUID for people to conceptualize a difference between a photon going through free space and being absorbed and re emitted. The parent comment asks the right question, what is being meant by "the same" is required to know before being able to say the discussion makes no sense.

Taking your first literal interpretation as the only way the discussion could have value is no more helpful than this exchange: "I'm going to the store, do you want the same hot dogs we have in the fridge?" "That doesn't make sense, the store can't have the same hot dogs we have in the fridge".

[izietto]

> "I'm going to the store, do you want the same hot dogs we have in the fridge?" "That doesn't make sense, the store can't have the same hot dogs we have in the fridge"

Yeah but hot dogs belong to classical mechanics, while photons to quantum mechanics, and in quantum mechanics you can actually have the same hot dogs in different places, until you observe them

[zamadatix]

That a photon can be in superposition is even a better example than my hot dog example. Nobody is impressed if you can find a corner case interpretation that makes the question sound dumb they are impressed if you can try to figure out what the person really wanted to know and answer it to the best of current understanding.

In the hotdog example the point is the person is actually trying to figure out if you want them to buy you more hotdogs not whether or not the exact same hotdogs can be found at the store and fridge at the same time. In this actual discussion about the photon it's not about whether or not the photon actually carries an ID card with it it's about what actual interactions are happening to photons when the light is reflected or refracted in different ways.

Sometimes what a person is trying to figure out may not line up with how things actually work, in fact take any science and add 100 years of knowledge and pretty much all discussions are this way. That does not mean it was nonsensical to talk about it just means the details are more exact and complicated than the asker knew how to phrase (and again, it's always like more exact and complicated than any of us know as we don't know everything there is to know about the universe).

[MrPatan]

There is no classical mechanics, only quantum mechanics. Or are you suggesting it's an optimization of the simulation or something?

[Enginerrrd]

>The parent comment asks the right question, what is being meant by "the same" is required to know before being able to say the discussion makes no sense.

This is true, but I think the comment you are replying to is correctly identifying a fundamental conceptual misunderstanding of the comments preceding it. In the sense that they likely mean, it's not really a valid question. Sort of like the "are you still beating your wife?" question. Asking it is making incorrect assumptions.

[zelphirkalt]

Who knows, maybe they actually do, but we do not know it yet. I mean, how do you know they do not?

[rosybox]

The absence of evidence doesn't make an idea more plausible, it makes it less plausible.

I can't disprove that a celestial teapot is circling the sun between Earth and Mars, so how do you know it isn't?

https://en.wikipedia.org/wiki/Russell%27s_teapot

[zelphirkalt]

Less plausible != impossible.

"Knowing" for me means, that there is no way, that we can be wrong about something. Otherwise it's only "believe", not "knowledge".

It would be good to say the following, instead of "it is so": "So far experiments did never show any sign of identity of photons." (Were there any such experiments?) Oh and what about quantum entanglement of photons? Could that not be interpreted as a sign of identity?

[mhh__]

Modern physics is built on the idea of fundamentally indistinguishable particles.

We may not know but we can only know what we can see and what we can see is explained by what we know.

[ngvrnd]

it's all the same photon

[hanoz]

For those missing the reference: https://en.m.wikipedia.org/wiki/One-electron_universe

[mr_toad]

I wonder if there’s an antimatter universe going backwards in time from the Big Bang.

[viklove]

But that's just not true. The electron will only get excited if the incoming photon has a very specific wavelength, so most of the light that hit your house's walls, for example, is reflecting. The photoelectric effect is real, but it only applies in specific circumstances...

[chemeng]

There will be a transfer of energy to the system if there is an interaction, but will only absorb the incident photon at specific energies. In the case of reflection in this classical view, it simply re-emits the energy back out as there are no valid energy transitions. The reality is far more complex and this thread is mixing multiple theories together, wave and particle views and simultaneously comparing single atom mechanics with those of lattices/solids. This is all causing a lot of confusion that's hard to address in comments.

[ggggtez]

I think this is called, "knowing enough to be dangerous". You clearly have some understanding of light, but you are missing some key facts that would lead you to the correct understanding.

>Light can reflect off of transparent crystals

No, the point of the above post is that a photon is not "reflected", but captured and re-transmitted by photo-electric effect.

Light travels at maximum speed C, only when in a vacuum. Otherwise it travels through a medium. We can see that the different speeds of light cause dispersion such as when it enters and leaves a clear prism (changing mediums, thus spreading out the different speeds/wavelengths of light). Since we know that light is "traveling in a medium" when it travels in air or in the prism, then what do we mean by this? It is traveling slower, so it must have some information about the medium...

So how does the light "know" it's in a material, without interacting with the atoms of the material?

Of course, it couldn't know. It is interacting with the material. The behavior of the particle is fully explained by the photo-electric effect.

[whatshisface]

Absorption and emission are not the only two processes by which light can interact with matter, reflection is another. The argument is that if the material can't store the energy then you can't really call the event an absorption. Feynman diagrams aren't sequences of events so just because you see the photon coming to, disappearing, and then departing an electron doesn't mean it happens in that order.

[anonytrary]

> Light can reflect off of transparent crystals

If light is reflecting off of something, it's precisely because that thing isn't transparent[0].

> if the mirror is moving towards or away from the light, the reflection will be Doppler-shifted to a higher or lower frequency.

This does not refute OP's assertion. If the photon is absorbed and a "different"[1] one emitted, the emitted photon is still emitted from a reference frame that is moving with some velocity, therefore the emitted photon will still be Doppler-shifted... The photon is still emitted at the correct energy level. The photon appears to have higher energy in the stationary reference frame.

[0] Disregarding human-centric definitions that have to do with visible spectrum.

[1] Thinking of the photons as "different" doesn't actually mean much when it comes to quantum mechanics; they're indistinguishable bosons.