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

Special Relativity Question

  1. Dec 26, 2004 #1
    So imagine you are staring at a two seperate photons. Instantaneously, you jump into the frame of one of the photons. Would you then observe the other photon moving at c? They wouldn't appear at rest, right? Is there something fundamentally wrong with my set up that gets in the way of thinking about this problem? My SR knowledge is weak (only worked through the kogut book) and when my intuition and physics knowledge fails me, I've got nowhere to turn (but here!)
  2. jcsd
  3. Dec 26, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Photons don't have rest frames.

    However, if you insist on taking limits, or whatever, to make sense of it as best as you can, you'll find that everything in the observable universe gets collapsed into a plane... but yes, other photons still travel at c.

    Paradoxically, so could another photon that happened to be at the same place and moving in the same direction as the first.

    addendum: the procedure I mentioned is merely for "constructing" an "inertial frame" whose relative velocity to a real inertial frame is c.
    Last edited: Dec 26, 2004
  4. Dec 26, 2004 #3
    Why don't they have rest frames ?
  5. Dec 26, 2004 #4

    Because if they did, photons would not be traveling at c in that frame. And photons travel at c in all frames.
  6. Dec 26, 2004 #5


    User Avatar
    Science Advisor
    Homework Helper

    There are some words which make the first phrase incorrect.It should have been:"Because if they did,they would not be travelling at "c" in any frame,therefore would have nonzero rest mass.And photons travel at "c" in all frames".

  7. Dec 27, 2004 #6
    Thanks. That clears things up a lot.
  8. Dec 27, 2004 #7
    If the correct statement is "they would not be travelling at c in any frame" and I say "they would not be travelling at c in that frame" I don't see why my statement is "incorrect". Incomplete, maybe. But why incorrect?
  9. Dec 27, 2004 #8


    User Avatar
    Science Advisor
    Homework Helper

    To me,in science there's no room for "approximation".We'd like to call them 'exact sciences'.A definition and in general any statement which is incomplete is incorrect.That's the way i see it. :wink:
    Your remark left the door open for the statement that it could have been true for other reference frames and not for the one u mentioned.My remark excluded that possibility.
    Awkwardy,though being called "theory of relativity",i don't see anything "relative" in it.It is a very rigurous theory.

  10. Dec 28, 2004 #9


    User Avatar
    Science Advisor
    Gold Member

    Photons do not have a valid reference frame. Division by zero.
  11. Dec 29, 2004 #10


    User Avatar
    Science Advisor
    Gold Member

    Hmm, but surely you're being circular here (in the post before this post) you're essentially stating that photons don't have reference frames because they travel along null worldlines and partilces that travel along null worldlines don't have refrence frames. Whilst that is true it's unsatisfying as it doesn't tell us why particles with null worldlines don't have reference frames.

    I don't see anything wrong with Jdavel's answer (except perhaps that the fact the 2nd postulat eapplies to inertial refernce frames only was glossed over), as it shows that for a photon to have a reference frame is in direct conflict with SR.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for Special Relativity Question Date
I A question about the relativistic energy dispersion relation Feb 20, 2018
B Special relativity equation question Jan 8, 2018
B Special relativity equation question Jan 7, 2018
B Question about the light Jun 2, 2017
I Two orbiting clocks question May 11, 2017