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Entropy of photons

  1. Jul 20, 2004 #1
    photons are not affected by time or entropy, but are governed by some unknown energy deeper in the implicate here are my reasonings:
    According to the thermodynamic arrow of time, disorder for every thing
    is always increasing. entropy is directly related to time and they cannot go backwards.But as you aproach the speed of light time starts to slow down. Therefore entropy decreases. This explains why decay in matter happens slower when approaching the speed of light. Now once you reach the speed of light, the border of past and future, time would stand still. This would mean that something with speed C ( obviously a massless particle) could travel through space (according to observation), but some how be suspended in time with zero entropy. Is this possible?

    its just a thought. please respond to point out mistakes in reasoning.
    Last edited: Jul 21, 2004
  2. jcsd
  3. Jul 21, 2004 #2
    okay this is a point that i have been trying to push for a long time... i agree with you on most parts, except the speed of light being the border of past and future...

    observer "A" inside bubble traveling at speed of light would feel time go by normaly.
  4. Jul 21, 2004 #3
    okay this is a point that i have been trying to push for a long time... i agree with you on most parts, except the speed of light being the border of past and future...

    observer "A" inside bubble traveling at speed of light would feel time go by normaly.
    observer B would see observer A frozen in time... so time doesn't slow for objects travelling that fast, it is just observed as slower...
  5. Jul 21, 2004 #4
    right. it is of course all relative. but the speed of light is the only constant. special relativity conveys that when we dicuss velocity we must specify what is doing the measuring. in this case ( and all cases for that matter) we are. The speed of light is the only thing that is stead fast enough to compare any thing to. unlike everything else with a velocity, you can never catch up to a photon no matter how fast you move towards it. because it is always moving away at that constant speed. I can only speculate what would happen if somthing were to go faster than the speed of light( without being observed because we know that changes everything) but one would assume on logic alone that time would reverse, opening up all sorts of paradoxes. now lets take the photon which is always traveling that speed. there is some kind of under lying force, mabye a projection from a higher dimensional reality that defines the speed of light as a barrier in this 4 dimensional reality. mabye were talking about a non local infinite dimension trying to give us some kind stability so we can go about our happy lives. or mabye I just need to get some sleep! any way thanks for responding. this stuff has been on my head for a while and being a high-school drop out musician I dont get much feedback from my friends.
  6. Jul 21, 2004 #5


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    Just as a friendly suggestion, try this rather in the relativity forum.

    I certainly agree with this. One of the fundamental pieces of evidence that we use to test the time dilation is based on atomic clocks.

    The light cone is the border of future and relative present, not past and future (except at the apex). Time does not stand still for either the observer who sees the traveling at the speed of light nor the observer who is doing the traveling. Time ticks by normally for the first observer (otherwise speed would not mean anything). Time ticks by all at once for the light speed particle. The lightspeed particle does not experience any ellapsed time on its journey, regardless of how far, because even the greatest distances in the universe contract to zero as far as the lightspeed observer is concerned. Going at light speed, the observer would see the entire remaining future of the universe wizz by immediately and come to its end.

    Yes. A photon is absorbed immediately after it is emitted, as far as it's concerned. We observe some transit time only because our difference in speed is c. The life of a photon is very abrupt. Remember, though, that to the photon, it doesn't travel any distance either, so why should the trip take any time? This is how the causal structure of the universe is glued together by massless bosons.
  7. Jul 21, 2004 #6
    I am not a physicist, but I would think that the RATE at which entropy increases for a particle would get closer to zero as the particle's velocity gets closer to c as observed by a distant observer.
  8. Jul 21, 2004 #7
    :redface: yes you are right . the RATE of entropy would slow down and then reach 0 when the speed of light is achieved. thanks for the input.
    Last edited: Jul 21, 2004
  9. Jul 22, 2004 #8
    here's a question... do you guys REALLY think that time would go backward upon breaking the speed of light? does that make any sense?

    i think that once you reach the speed of light, you go "forward" in time for the same reason that a photon seems to exist forever.... a million years would go by around you as you travelled at that speed in your few short microseconds...
  10. Jul 26, 2004 #9


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    You all need to stop discussing personal theories in the mainstream physics forums here. You all know the rules.

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  11. Jul 26, 2004 #10


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    Your reasoning isn't bad, emortis9, but its a lot simpler than that: entropy doesn't apply to a photon in motion because its energy is constant (disregarding doppler shift).
  12. Jul 29, 2004 #11
    Try Mr. Tomkins in Paperback by G. Gamow. .. he postulates the universe if c was smaller and h-bar larger. Thinking about extremes does help somewhat, but we can only ever really accelerate at g before feeling rough!!
  13. Jul 31, 2004 #12

    So if the energy of something is constant, then entropy will not be a factor?
    Or is this only possible with massles particles that travel at C?
  14. Aug 1, 2004 #13
    The total energy of our universe is constant, yet entropy certainly plays a role.

    As remarked, it doesn't make sense to discuss the entropy of a "single" photon. Entropy S applies to large numbers (distributions) of photons, like in Stephan-Boltzmann theory of radiation from an object (star, lamp, etc), which successfully predicts that S/V = a T^3, with the volume V of the object, a~10^-15 is a constant in S.I., and T is the temperature. As the object cools, its entropy decreases and the emitted photons are absorbed by other surrounding S-B objects, which are also radiating and loosing entropy.

    (n.b., the total entropy of the universe is still always increasing with time since the temperature becomes more and more uniform across the universe).

    This is all for the case of an expanding universe though.
    Last edited: Aug 1, 2004
  15. Aug 1, 2004 #14


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    Correct. Entropy is essentially wasted energy. As such, it only arises through energy conversions. Have you ever looked at a steam table?
    Right. The universe is a system, not an object.
  16. Aug 3, 2004 #15


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    What is the difference between an object and a system in your opinion?
  17. Aug 4, 2004 #16


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    An object is inside of (or part of) a system. Take a steam generator, for example: The system is composed of tanks, boilers, pumps, pipes, etc. There are complicated interactions and obviously, entropy loss when operating the steam generator.

    But a system could also be a rock sitting on a table (both the rock and the table are part of the system). Since the rock isn't moving, no energy is being transferred and thus no entropy generated by the rock-table system.
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