- #1

etotheipi

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## Main Question or Discussion Point

I will refer to the example given in 'On the electrodynamics of moving bodies' concerning a rod moving in a coordinate system, in which a beam of light is sent from one end of the rod to the other and is then reflected back.

Usually when calculating relative velocities, we may simply consider the object the velocities are relative to to be at rest and go from there, so for example if an object A is at 3ms-1 and object B is at 5 ms-1, the relative velocity of B from A (taking A to be at rest in its frame of reference) is 2ms-1.

In the case of the rod, from the perspective of an observer in the stationary frame, a beam of light of velocity c travelling in the same direction of the rod of velocity v is given a relative velocity to the rod of c - v, in accordance with the relative velocity equation. However, I don't quite understand this in this context since if we consider the rod to be at rest - as we usually do when imagining relative velocities - the speed of light relative to it is c - v, and not c as implied by the axioms of special relativity.

My question is whether, in the stationary rest frame, when we calculate relative velocities are we 'still in' the stationary rest frame? That is, it is quoted in many places that the time taken for the beam to reach the end of the rod is (c-v)/L implying a relative velocity of (c-v) and length of L, however the notion of a relative velocity when we are in another reference frame is slightly confusing to me.

Usually when calculating relative velocities, we may simply consider the object the velocities are relative to to be at rest and go from there, so for example if an object A is at 3ms-1 and object B is at 5 ms-1, the relative velocity of B from A (taking A to be at rest in its frame of reference) is 2ms-1.

In the case of the rod, from the perspective of an observer in the stationary frame, a beam of light of velocity c travelling in the same direction of the rod of velocity v is given a relative velocity to the rod of c - v, in accordance with the relative velocity equation. However, I don't quite understand this in this context since if we consider the rod to be at rest - as we usually do when imagining relative velocities - the speed of light relative to it is c - v, and not c as implied by the axioms of special relativity.

My question is whether, in the stationary rest frame, when we calculate relative velocities are we 'still in' the stationary rest frame? That is, it is quoted in many places that the time taken for the beam to reach the end of the rod is (c-v)/L implying a relative velocity of (c-v) and length of L, however the notion of a relative velocity when we are in another reference frame is slightly confusing to me.