>the speed of light does not varyyes
>>11657164>At which speed do they move relative to each other?In what frame? That's the whole point of SR. A person sitting on the ground who looks at two photons moving in opposite directions observes the distance between them to change at rate c. From reference of a photon, the question doesn't make sense.
Special relativity makes two basic assumptions about the universe, and these assumptions in combo with some techniques from naive newtonian mechanics you get time dilation and length contraction.
>If S is an inertial frame, than any S' with a constant velocity relative to S is also an inertial frame (this shouldn't surprise you)>The speed of light is a fixed constant c in all inertial frames (this is surprising, but is supported by experiments including michelson-morley. the constant c is contained directly within maxwell's equations and its value has been known experimentally for over 200 years)Imagine an inertial frame S, the ground. There is a train called frame S' is moving at constant speed V in a straight line, say, the x-axis. At some position and time (x,t) in S after it crosses the origin, a light turns on inside the traincar. In classical mechanics, we can say that the event in S' reference from is at point x'=x-Vt and t'=t. This shouldn't be surprising, this is 16th century physics. In relativity, however, we have and .
