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.
[1]https://en.m.wikipedia.org/wiki/QED:_The_Strange_Theory_of_L...
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...
How do you define "the same" for two photons?
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".
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
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).
As humans living in the classical world, we hear that and think "oh, that just means we don't know what's really going on, it either bounces or re-emits, it just looks the same to us." But no - reality is just an interaction of quantum fields and we're putting these human stories on top of it. It's like debating whether or not submarines swim.
When you encounter an XY problem question/statement only stating "this doesn't make any sense. Computers are very complicated and don't work like that in the first place" is useless to everyone except maybe a comment or that wants to boost their ego. Taking the XY question, explaining why it's wrong, and trying to explain how things actually work at a deeper level is helpful. I think your comment about them being field interactions and needing to think about them that way is a good start on that kind of answer rather than the ones above.
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.
I can't disprove that a celestial teapot is circling the sun between Earth and Mars, so how do you know it isn't?
"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?
We may not know but we can only know what we can see and what we can see is explained by what we know.
>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.
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.
> Even in a vacuum you can send a photon in one end and see one on the other and not really know that the one going in was “the same” as the one going out.
I don't really care, if _I or current physicists_ cannot _distinguish_ them, when I ask, whether they _are_ the same. I would rather have an answer like as follows:
"We do not know this yet. We do not have the means of telling, whether it is really the same photon or not. However, even if they were not the same, it would make no difference (according to our current understanding of physics!), as their effect on the surroundings would still be the same, because ..."
Geometrical optics is really sufficient here, it explains it perfectly, and is much more intuitive than other more complex models.
This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 degrees.) If photons we’re distinguishable, then swapping two photons would swap them, and the resulting state would be different. The matters from a statistical perspective. Look up Bose-Einstein statistics if you’re interested.
As for whether photons are absorbed and re-emitted or merely reflected, this surely depends on the surface on question.
In contrast, anything phosphorescent most definitely absorbs and re-emits, sometimes hours later. Imagine glow-in-the-dark pigment.
Physics is all about choosing an appropriate model of the world that captures the detail you need without being more complicated than needed. Is a person on a playground swing a pendulum, or is it a bag of molecules sitting on a flexible piece of rubber suspended by a bunch of rusty metal links from a flexible steel bar that is, in turn, supported by many geological layers in the Earth’s crust?
Do you mean there are two kinds? Or if you swap any two?
This is why two fermions can’t be in the same state: if you swapped them, you’d be back where you started, but that somehow also had to phase shift 180 degrees, and the only complex amplitude that is the same as its own negation is zero.
What about total internal reflection? Why does the light travel through the entire body of a glass prism without issue and then suddenly get absorbed and reemitted at the surface? Glass (and water) are transparent specifically because they don't absorb photons in the visible spectrum, so how is it that they absorb and reemit?
This example is wrong, but useful, and it's (probably) less wrong than what you've been talking about so far. At the very least, it's more useful.
Also this one:
Richard Feynman's "QED" is a great read on this.
http://www.vega.org.uk/video/programme/46
This explanation starts at about 30 minutes, but I highly recommend watching from the beginning.
http://www.vega.org.uk/video/subseries/8
1. A gentle lead-in to the subject, Feynman starts by discussing photons and their properties. 2. What are reflection and transmission, and how do they work? 3. Feynman diagrams and the intricacies of particle interaction. 4. What does it mean, and where is it all leading?
The site has many other science videos too:
Which is, in fact, what happens. Solar sails exploit this fact:
That is, the equal and opposite direction, exactly as predicted by conservation of momentum.
It's less hand-wavey. Also, the reflectivity of water is very low at low angles of incidence, rising to 100% at normal angles. Less light reaches the wetted material. https://en.wikipedia.org/wiki/File:Water_reflectivity.jpg
Purple is an artifact of our minds, for example. Does not exist as a distinct wavelength of light.
The object absorbs, does not absorb, emits, whatever wavelengths of light. To us, those have colors, where they are in the visible spectrum.
Take us out of the equation, and?
For example.. if I'm looking at a star, and thinking it's the 'same' photon that came off of that star billions of years ago... that might be true in the vacuum of space, but since it's passed through the medium of the atmosphere, does that mean it's constantly been absorbed/new photons being emitted?
That's how electrons and photons interact. It doesn't make sense to say that they are different photons or the same ones, since you can't observe the interaction.
For anything opaque, you should expect this, because otherwise how could you see something's color if you only saw the same photons as the light source it reflected?
https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality#...