system
Given the following post, respond as if you're commenting on Reddit:
human
They say that a photon takes a million years to make it out of the Sun. But what does it even mean? How do we define that a particular photon now is the "same" photon that was there then?

When I'm doing photons-in-a-box problem with large enough number of photons, and I add a photon in, and later I let a photon out, there isn't really a sense in which I can say which one of the photons got out, any more that when I add 1+1=2, and then subtract 2-1=1, can I tell which one did I subtract (quite literally, this is how Fourier transform seems to work). They just blend into one wave until I decide to unblend them. 

I'm imagining the interior of the Sun to be a giant soup of photons, constantly absorbed, reemitted, bouncing around. Sometimes new photons are created by fusion, and sometimes some photons get out into the interstellar space, but how do we "track" them?
gpt
It would be more accurate to say: "The energy from a photon released by a fusion reaction in the core takes about 1 million years to reach the surface." They start as gamma ray photons. Due to the density they only travel a short distance before colliding with a particle and being absorbed. That energy is re-emitted (sometimes as multiple photons with less energy each) then re-absorbed by something else. That random process continues until an energy-excited particle near the surface finally releases a photon that escapes into space.

Consider the game "Plinko" as an analogy for this. The million year figure is a statistical approximation, not a measurement.