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

Do photons effectivley travel instantaneously?

  1. Nov 23, 2003 #1
    Do photons effectivley travel instantaneously? If not then the question posed by Einstein "If I travel at the speed of light and hold a mirror in-front of my face, do I see a reflection?" has the answer, no!

    Although I'm not positive why but would say that because your in the same frame of reference as the mirror that you do see a reflection.

  2. jcsd
  3. Nov 23, 2003 #2


    User Avatar
    Science Advisor
    Gold Member

    Photons travel at c, they don't have a rest frame.
  4. Nov 23, 2003 #3
    Just for the record, here is what Einstein wrote.

    Wenn ich einem Lichtstrahl nacheile mit der Geschwindigkeit c (Lichtgeschwindigkeit im Vacuum), so sollte ich einen solchen Lichtstrahl als ruhendes, raümlich oszillatorisches elektromagnetisches Feld wahrnehmen. So etwas scheint es aber nicht zu geben, weder auf Grund der Erfahrung noch gemäss den Maxwell'schen Gleichungen.

    Here is an English translation.

    If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as a spatially oscillatory electromagnetic field at rest. However, there seems to be no such thing, whether on the basis of experience or according to Maxwell's equations.

    If he were traveling at c behind the beam of light, then he would not be catching up to it. He said "nacheile", which means "lag". So it should be "If I lag a beam of light...". He didn't say "gehe mit" (go with) or "reise mit" (travel with) that beam of light. So he hasn't explained just how he thought he could experience those (for him) stationary oscillations, and he hasn't explained just how he might have transformed those Maxwell equations to that moving frame of reference. But this is just a remembrance of something from his early thoughts, before his theory of relativity. It would have been about 1895 and (he says) he was sixteen.

    I think he agrees with the topic originator. It would be just like standing still (relativity principle), so the light beam can't be at rest even in this frame of reference. Relativity theory won't permit this (traveling at lightspeed)to be a frame of reference at all.

    *autobiography from Schilpp Library of Living Philosophers
    Last edited: Nov 23, 2003
  5. Nov 28, 2003 #4
    Thanx guys, I've read a bit more on the subject, and if he couldnt see his image then as I understand it he would know the speed he was moving at without having to look outside his own frame. This violates the principle of relativity. Makes sense!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook