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One thing to remember with photons is that while speed is consistent, intensity isn't.

Yes of course these are not perfect processes. Otherwise an undersea earthquake would create a tsunami on all of its coastlines. Energy is lost or made non-coherent in a variety of imperfections, including heat and scattering. In the case of ocean waves, they are typically created and recharged by the wind as they move along.

I mostly wanted to make the point that a wave is not displacing its medium, but simply moving through it. A wave is nothing but water and energy. There's nothing there displacing the water to do work. The water moves around due to the energy, but in a way which is generally neutral in terms of work actually done. A wave hitting the shoreline really just transfers from moving through the water to moving through the land.

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It's only a paradox due to misunderstanding what is happening.

It's not the motion or velocity that causes the time dilation, it's the acceleration. The twin on the ship experiences acceleration (which is absolute, not relative) while the Earth-bound twin does not.

Once you take this into account, the paradox is easily solved.

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>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.

So how does that coincide with the entanglement of two particles.?
These two particles are identified for sure....

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There's some good info here but also a lot of misleading or easily misread info.

You're correct on the fungibility of photons. Nothing to add there.

But you are dangerously vague for the rest.

> The easiest to understand model is the one you mentioned- and it does work.

Careful. The confusion you describe (even among physicists) comes at the root from not being specific on what we mean by absorption and emission, and unless you clarify you have not helped the situation. Let me try to be extremely clear about the possible models.

1. Classical scattering of waves. Here, there is a main EM wave which can partially scatter off atoms, since they are charged -- and importantly it's a collective scattering from all of the atoms in the path. The superposition (combination) of the original wave and the small scattered waves leads to a new, slower wave, with the same frequency as the original. Note that at no point is the incoming wave completely extinguished (absorbed), it's only ever a partial effect. This model works mathematically.

2. Classical scattering of particles. Here, you imagine a billiard ball photon travelling along that happens to hit an atom and be absorbed. For a moment there is no more photon anywhere. Then the atom relaxes, and a photon is re-emitted to continue on its journey. This model does not work mathematically in any way to explain the speed of light in a medium. You cannot assign the slowing down of light to a random time delay between classical particle absorption and emission nor to a particle taking a 'longer zigzag route' through the medium.
   When people ask this question on the internet, it's usually with a foundation of only classical mechanics, not quantum mechanics, and so this is usually what they are imagining when they say "absorption and re-emission" and it is wrong.

3. Quantum mechanical scattering. This one is tricky to understand without at least an intro to quantum mechanics but fundamentally we are scattering probabilities, not whole particles. There are several ways to do the maths -- you can consider the propagation, partial scattering, and interference of the photon wavefunction as it interacts with virtual energy levels in the atoms (looks very similar to the classical wave math -- superpositions of the original and partially scattered probability waves). Virtual energy levels are guaranteed to be unstable and exciting the atom to one is fundamentally different to exciting to a stable energy level. Or you can consider a path integral approach like that of Feynman (again, summing probable paths, not definite or discrete ones), or you can start calculating the collective excitation of the photon and all neighbouring atoms as a dressed quasiparticle with new properties (specifically, mass -- so travels less than c). The important thing here is that all the behaviour is collective, never discretely with a single atom -- and at no point is the photon (with its original frequency and direction) completely gone. When physicists say "absorption and re-emission" they are often thinking of this model because we use the same terms for classical scattering and scattering of a QM wavefunction -- but that does not mean it is the same model as 2) above. It is not, it is fundamentally different.

> 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

Your answer doesn't work. It is true that "excitations are always discrete" is an oversimplification -- the existence of black-body radiation proves that. But you don't get to claim that all materials absorb at all frequencies and therefore slow light through absorption and reemission, without also explaining why the transparency of a material doesn't have anything to do with the speed of light through it (cf. glass at optical wavelengths). Again this confusion you introduced comes from not clearly differentiating between the QM scattering of probabilities -- which may involve virtual energy levels and whole lot of behind-the-scenes stuff -- and complete, classical absorption of a photon to a new stable energy level. The real answer is simply that you don't need stable energy levels (which cause the material to become opaque) to exist to do the QM scattering math. Though transitions to virtual energy levels are low probability and necessarily temporary, their collective sum, including the original photon as well, gets us to where we want to go.

I work in optical engineering and am not an expert scientist in the nature of light. Our professional society SPIE held conferences (titled nature of light: what is a ohoton) over the last several years ( i believe there were 7 sessions over 7 years) where the experts argued out the nature of light and there is still disagreement amongst them on what a photon is and whether it exists.

Meanwhile, solving electromagnetic field components in the presence of optical components similar to Feynmans approach, whether it be a simple lens or photonic integrated circuits, well models actual behavior in all practical cases encountered with modern optical devices that I am aware of. I haven't seen any benefit to switching to a photon emission based model, especially when the experts cant even agree on the fundamental nature of the photon.

Well here on earth, is it possible to lift anything with 100% efficiency? Shouldn't there be some loss of energy

Well i think he's asking about the possibility that ancestral biological structures might have been very similar. Like say ancient vacuoles maybe existing on their own and using RNA as a means of building other vacuoles. The RNA strands might have been extremely simplistic by today's biological standards.

Then you have other extremely simplistic structures that originated somewhere else, using RNA as means of doing extremely simple and menial tasks. Then these proliferate and eventually come into contact with each other and by coincidence form more complex relationships as single structures over time, kind of how the mitochondria is theorized to have become a part of the cell millions of years later.

When they combine maybe it would be difficult to determine that they were two distinct lines of life because they used the same means of relaying genetic information.

Great explanation! I never knew there were so many competing explanations for light slowing down in a medium. I’d always just thought of it as the mass of the material warping space a little, meaning that the photon is effectively travelling further. Of course I never studied physics properly.

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That was very informative and probably what I was looking for. Thanks a lot!

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Amazing reply. Thank you.

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Beautiful, thank you

What about turtles? Isn't their shell intimately related to their spine & sternum?

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For the "why" to this explanation, it comes from the change to the permittivity and permeability of the material.

https://en.m.wikipedia.org/wiki/Relative_permittivity

Here on earth, a ball resists being lifted due to gravity. It also returns all that energy when it's dropped again.

A wave that lifts a bunch of ping pong balls on it's leading edge, then drops them on it's trailing edge where the energy is returned to the wave, has done net zero work.