Light & Entropy: Will Photon Energy Last Forever?

  • Context: Graduate 
  • Thread starter Thread starter Igottaknow
  • Start date Start date
  • Tags Tags
    Entropy Light
Click For Summary

Discussion Overview

The discussion centers around the question of whether light, specifically photons, will travel indefinitely without degrading or losing energy over time, particularly in the context of entropy and interactions. Participants explore theoretical implications, the nature of entropy, and the conditions under which photons exist.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question why photons would not degrade or disperse due to increasing entropy if they undergo no interactions.
  • It is proposed that the entropy of a physical system is well-defined only in thermodynamic equilibrium, suggesting that a photon's entropy does not increase until it interacts with something.
  • One participant notes that current theories indicate low energy photons are only destroyed upon absorption, implying they would continue moving indefinitely if not interacted with.
  • Another viewpoint suggests that if photons had a nonzero mass, they would be unstable and eventually decay, introducing uncertainty into their longevity.
  • One participant reflects on their initial intuition that photons would lose energy over time, later clarifying that from the photon's perspective, it does not experience time, thus not subject to entropy changes in isolation.
  • Counterarguments are presented, asserting that one cannot apply standard physics in the photon's reference frame and that energy loss requires interaction, not merely the passage of time.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between photons, entropy, and energy degradation. There is no consensus on whether photons can truly travel indefinitely without any change in their energy or entropy.

Contextual Notes

Participants highlight limitations in understanding entropy as it relates to isolated systems and the implications of interactions on photon behavior. The discussion reflects ongoing uncertainty regarding the fundamental nature of photons and their interactions.

Igottaknow
Messages
14
Reaction score
0
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?
 
Science news on Phys.org
Igottaknow said:
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.
 
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.
 
Igottaknow said:
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.
 

Similar threads

Graduate Is there an effective life time of the universe where it will reach max entropy?
  • · Replies 21 ·
Replies
21
Views
4K
High School Red shifted tired light doubts
  • · Replies 10 ·
Replies
10
Views
2K
Graduate Questions about entropy, normal force, and the human body
  • · Replies 3 ·
Replies
3
Views
2K
High School Why do photon polarization experiments show similar outcomes in different situations?
  • · Replies 11 ·
Replies
11
Views
3K
Graduate Unpacking Entropy Bounds and Their Violations
  • · Replies 4 ·
Replies
4
Views
1K
High School Acceleration of time due to entropy
  • · Replies 1 ·
Replies
1
Views
2K
High School Dark Energy, Dark Matter and the Speed of Light
  • · Replies 23 ·
Replies
23
Views
4K
Undergrad Revisiting the Light Clock
  • · Replies 25 ·
Replies
25
Views
2K
Undergrad Ultra-high energy photon interactions with matter
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Why is Photon Energy Quantized in Terms of Sine Wave Frequency?
  • · Replies 78 ·
3
Replies
78
Views
7K