The second postulate of Special Relativity states:

The speed of light in a vacuum is the same for all observers, regardless of their motion relative to the source.

Now imagine the observer that is looking at the light which is traveling away from him in a direction he is looking. In this case, the light should look somewhat like a "dot" to such observer - not really moving at any direction for him. However according to postulate above the light should move at the speed of c (if we are in vacuum to be precise).

What is wrong with this reasoning? Or is it just some kind of exception from the rule?

  • 1
    $\begingroup$ You only see light when it actually hits your eye. $\endgroup$
    – Řídící
    Mar 24, 2014 at 20:58
  • $\begingroup$ My guess is that in your reasoning you "projected out" the dimension in which light was moving. How do you measure the speed of the "dot" if don't have any clue what distance it moves in a given time ? $\endgroup$
    – Nick
    Mar 24, 2014 at 21:04
  • $\begingroup$ Thanks Nick, I think your comment is closest to the answer after all. As I understand it now, the light is still going at the speed c for the observer, he is just not able see that direction. What helped me to understand was the idea of something moving at the speed (v,0,0) in some frame of reference (coordinate system). Then consider observer standing in the origin of such coordinate system. He wouldn't see the actual movement, but that wouldnt change the fact that the object is moving at the speed of v relative to his frame of reference. $\endgroup$
    – user43187
    Mar 29, 2014 at 18:38
  • $\begingroup$ You have to consider the observer's "reference frame", not her point of view $\endgroup$
    – Henry
    Mar 30, 2014 at 11:19

2 Answers 2


"Now imagine the observer that is looking at the light which is traveling away from him in a direction he is looking."

"What's wrong with this reasoning?"

Well, this observer will see nothing - not even a point. It's just physically impossible to see light moving away from you.


The SR postulate is talking about an "observer". Your problem is that your question is failing to describe an observation process. The "photon of your look" will never be reflected by the photon you want to observe. By consequence, the way you described will never permit you to observe a photon.


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