# Light and entropy

## Main Question or Discussion Point

Ok so the thread I was looking at is closed and I cannot pose this question there. The question was posed if light will travel forever or if it will degrade over time and the answers that were given were a bit incomplete. Here is a link to that thread https://www.physicsforums.com/showthread.php?t=406527 This thread is pretty old, probably why its closed. I am new to the forums and this may be a redundant question but the short answer to the question was that it will travel forever without really saying why that is. Why would the energy of a photon not degrade or disperse as a result of increasing entropy with no other interactions on that photon?

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ZapperZ
Staff Emeritus
Ok so the thread I was looking at is closed and I cannot pose this question there. The question was posed if light will travel forever or if it will degrade over time and the answers that were given were a bit incomplete. Here is a link to that thread https://www.physicsforums.com/showthread.php?t=406527 This thread is pretty old, probably why its closed. I am new to the forums and this may be a redundant question but the short answer to the question was that it will travel forever without really saying why that is. Why would the energy of a photon not degrade or disperse as a result of increasing entropy with no other interactions on that photon?
But this could be turned around also. We know that entropy is really a statistical process, and therefore, is an outcome of many, many interactions. If a photon undergoes NO interaction, then you have to tell us why there should be a change in entropy of that photon. I find the assertion that there should be an entropy change for such an isolated system LESS intuitive than no entropy change.

Zz.

jfizzix
Gold Member
Two points:
First:
The entropy of a physical system is only really well defined if that system is in thermodynamic equilibrium. If you wanted to push entropy to the limit, you can also correctly define it as proportional to the average number of bits needed to completely describe the kind of system in question.

In that sense, the entropy of a photon would not increase until it interacts with something.

Second:
We don't know for sure one way or the other. Our best theories tell us that low energy photons are only destroyed when absorbed. So if nothing interacts with the photons, they will keep moving forever. but this doesn't have to be the case.

If the photon had an eensy weensy, but nonzero mass, our theories tell us that the photon would be unstable and have to decay into something eventually, even if it takes a very long time to do so.

Hope that helps:)

I think I should have said "photons" instead of "a photon" or just light in general. After doing a little searching I found my answer and have a much clearer understanding of why this photon would not be subject to entropy in an isolated system.

Can you assert anything with a question?

as·ser·tion
əˈsərSHən/Submit
noun
1.
a confident and forceful statement of fact or belief.
"his assertion that his father had deserted the family"
synonyms: declaration, contention, statement, claim, opinion, proclamation, announcement, pronouncement, protestation, avowal;

I will explain why i did ask the question however. Intuition would tell you that if you are observing this photon (in theory) travel for billions of years it would have to lose some energy as it goes just from a time aspect. I didn't take into account that this proton is not subject to time since it is at the cosmic speed limit and that there is no time for the photon to actually lose energy from its perspective. So... in an isolated system there would not be an increase in entropy.

Thanks Zapper for your intriguing insight into this matter.

ZapperZ
Staff Emeritus
I will explain why i did ask the question however. Intuition would tell you that if you are observing this photon (in theory) travel for billions of years it would have to lose some energy as it goes just from a time aspect. I didn't take into account that this proton is not subject to time since it is at the cosmic speed limit and that there is no time for the photon to actually lose energy from its perspective. So... in an isolated system there would not be an increase in entropy.
Unfortunately, this is faulty reasoning.

You can't transform to the photon's reference frame and apply our physics. So saying photon is not "subject to time" is wrong.

Secondly, something doesn't lose energy even if it just sits there doing nothing. There has to be an interaction. You can replace this with an electron, or a neutrino. If it doesn't interact, it might as well be in total isolation, no matter at what speed.

So here, the unchanging entropy has nothing to do with light/photon moving at c.

Zz.