I have read through here and the consensus seems to be that there are a lot of "educated guesses" as to what happens and why.
One pretty much universal thing that seems to be constant is the fact that once photons interact with mass, the journey is over and any propagation is "decided and implemented by the media in witch the original photon or photons interacted with and that the original ones that initiated this great debate no longer exist because once interaction happens, collapse happens.
What if.... you could CATCH a beam of light and study it and when you are done with it simply open your hand and let it continue along its journey just as if you never "caught" it? According to everything here this does not even compute.
What I DON'T see in ANY of these answers is the discovery and breakthrough that was made in 1999 and then again in 2001 by Lene Hou. In 1999 she slowed a beam of light from its 186,000 miles per second down to around 38 miles per second.... and then in 2001 was able to STOP the beam of light.... beam of light traveling at 0mph and still existing. This does not mean like you are pointing a laser at a wall and when the beam hits the wall it STOPS... because when she was done with the light beam.... the SAME beam once again continues on its way without losing any of the "information" of the beam.
Let that sink in for a second....
In order to "understand" light and the processes therein... the FIRST logical step to doing so would be to STOP light and study it. Until we can do this... all we have is "best guesses". I mean how can you even START to study and form any real conclusions about light and the processes that it operates on when light itself can only exist (because the particles that light is MADE of) at light speed? I see a lot of "this law" and "that law" being thrown around that are based on observing the "after the fact" effects of the interactions of these particles with the "real world" and numbers or math that is created to build a "model" that tries to explain why these interactions happen. I mean that is pretty much what we go off of when wrapping our brains around this stuff and when we observe a new "effect" from the interaction of these things traveling at light speed with ordinary "matter" we get a new math formula that tries to create a "model" of the processes that "make it happen".
This very thing is the exact reason why the original Question even exists... and there are answers to it that display all these "models" created by people trying to make sense of these interactions of things we can't see and the tings we can see and the results of that.
You can watch a worm enclose itself inside a sack it created and come out of that sack as a winged creature... and then conjure up a reason it happened or why it happened or how it happened... but you will never have a concrete answer to these "ideas" until you find it happening and look inside the sack and watch it happen.
If photons only exist at light speed that means light only exists at light speed and therefore we can only GUESS (though it might be a very good educated guess, it is still a GUESS nonetheless) about the actual process that these particles run on. All the numbers and formulas in the world will never truly reveal the true workings happening and they all are representations of "models" of what we THINK is happening.
This is why I say the first logical step to truly understanding the nature of these particles is to catch them and MAKE them exist on a scale WE can watch them and study them.
This was done like 24 years ago using a Bose-Einstein Condensate by one of only a HANDFUL of people in all of history that has even managed to CREATE one. In this environment was she able to literally "CATCH" a light beam and STOP it, and then release it back into "nature" to go on its merry way without "destroying" the information contained in the Quantum system.
Since then she has also managed to TRANSFER that Q-info into not only one but TWO atom forms using the environments created by BEC.
All of this does not "answer" the original question but the "answer" DOES lie within the things she is doing because we can now slow light down to OUR scale and give us the ability to OBSERVE it without "corrupting" it with the ability to catch-release it. This information I am writing about... if it were written the day she actually did this... the ink and paper it was writtin on... is essentially old enough to be classified as an "antique".
Her work has proven that photons CAN exist while traveling at slow speeds and CAN exist while not moving at all and then released back into its "natural habitat" and its speed of c "life".
The real world applications of these very discoveries are being pursued to create a method of Quantum Encription using the ability to not only catch the light and release it... but to catch it, then manipulate it before releasing it , thus opening the door to literal encryption of the information contained in the light so that it can again be manipulated to "decrypt" the infomation back to its original state.
Also... Lene Hou was initially denied any funding for ANY of her experiments because she was one of those "Theoretical Physicists" and the experimentation she suggested would be too complex and complicated for her to do... So they denied her funding... She could never hope to do any real "Applied Physics"...
This is where Lene Hou talks about using 2 BEC to capture light and creating a "imprint" of the information contained in the wave, then moving it and recreating the original beam from the imprint and then releasing the beam from a new location. - https://www.photonics.com/Articles/Light_Changed_to_Matter_Then_Stopped_and_Moved/a28520
These guys manipulated light using temprature and splitting photons off that get caught in their wells.
"A beam of light is thrown back and forth between two mirrors. During this process, the photons collide with dye molecules located between the reflecting surfaces. The dye molecules "swallow" the photons and then spit them out again. "The photons acquire the temperature of the dye solution," says Weitz. "In the course of this, they cool down to room temperature without getting lost."
By irradiating the dye solution with a laser, the physicists increase the number of photons between the mirrors. The strong concentration of the light particles combined with simultaneous cooling causes the individual photons to fuse to form a "super photon," - https://www.sciencedaily.com/releases/2019/11/191114141246.htm