Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Absolute speed of light

  1. May 30, 2010 #1
    Consider this thought experiment. Second postulate of SR (invariance of c) says that the speed of light is measured the same for all inertial observers regardless of their relative KE.

    To better understand what that means I try to imagine how would that look from a perspective of the moving light? I mean when we symmetrically inverse the postulate to the light's point of view instead of the perspective of the inercial observers.

    Would that mean that from the light's perspective everything with a rest mass would either fly away or getting closer with exactly speed of c and that light basically ignores their relative kinetic energies?
  2. jcsd
  3. May 30, 2010 #2


    User Avatar
    Homework Helper

    The short answer is that an observer riding a photon is not an allowed inertial frame. There are quire a few threads on this topic on this forum.
  4. May 30, 2010 #3
    Yes, I know. That's why I called it a thought experiment. It's not my intention to argue whether it can exist or not. I just want to know if I understand the law of invariance of c correctly in this inverse situation.
  5. May 30, 2010 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Non physical thought experiments cannot possibly yield physical results.
  6. May 30, 2010 #5


    User Avatar
    Homework Helper

    If there is no inverse situation then you cannot hope to draw a correct conclusion from such a thought experiment within the realm of relativity.
  7. May 30, 2010 #6
    The situation is not defined at that frame. But so is not in a singularity. That doesn't mean there is not any physical reality at that frame.

    But Ok, if you stop thinking and refuse to do it even as a fun hypothetical excersise only because current theory says "there are lions out there" then I can understand. But with this attitude we can never hope for a new theory that will elaborate a map of this now empty and forbidden place. And we need this new theory exactly because these frames are not defined!
  8. May 30, 2010 #7
    It is possible to construct a non-inertial frame that does what you are suggesting, but you won't be able to draw conclusions from it.

    Why would we need or want to make a physical theory for nonphysical situations?
  9. May 30, 2010 #8
    It seems to me that you're wondering about how things would be measured using the grid/clocks in the light's (hypothetical) rest frame. But such a frame conflicts with the second postulate. As I understand it, all of the particles in nature fall into one of two classes: (1) particles that are always observed (from every inertial frame) as traveling slower than c, and (2) particles that are always observed as traveling at speed c. To that extent, the idea of "observing" a photon in its "rest frame" is contradictory to the physical reality.
  10. May 30, 2010 #9
    For the sake of learning, maybe? Getting an insight why exactly these situations are "nonphysical and if they indeed are"?

    I have read quite a few threads on the topic of the "rest frame of light" on this forum and the general consensus was that the current theory cannot prove that the situation is indeed nonphysical. It just doesn't have any means to describe the situation. And that is a difference.

    That's why I want to leave this debate behind and just temporally explore an option where it is a physical situation (albeit undefined) and see what consequences it would have. And what would be consequences of those consequences. Maybe the final conclusion will be that it's indeed useless or physically impossible. But I don't want to judge before I explore it.
  11. May 30, 2010 #10
    First you would have to answer: What does another photon look like in one photon's rest frame? An answer to this question, unless we part with the second postulate of SR, will result in a paradox or in infinitely many equally valid disagreeing calculations.
    Last edited: May 30, 2010
  12. May 30, 2010 #11
    Espen, I suggest you to see https://www.physicsforums.com/showthread.php?t=398812&page=3" where are some reasonable speculations of how it could look like, including mine.

    But in my original situation I do not consider this situation. I would just like to explore the situation of photon vers rest mass objects.
    Last edited by a moderator: Apr 25, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook