Doc Al
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From the 'moving' frame's viewpoint, the motion of the backdrop means nothing. They have a perfectly good and stationary frame in which to do their measurements--their own. Of course, being smart fellows they are well aware that the 'stationary' frame measures a different travel distance for the photon. But don't forget that according to the moving frame the clocks used to measure the time interval in the stationary frame are not even synchronized. So who made the 'error'?jmallett said:OK, so I am going wrong somewhere, this is how I saw it as an error:
What the stationary observer sees is that the moving observers did not measure the actual length of the rod. Suppose there is a backdrop on the other side of the rod from the stationary observers (think of the rod going down a tunnel). The stationary observers see the photon emitted at the front of the rod against the backdrop, watch the rod move forward and the photon move "backward" until it reaches a coordinate where the end of the rod is going to be when it meets the photon. From the stationary observers point of view they definitely did not measure the rod's length, did they ? They measured a distance between two points that they observed and, if they had quick enough eyesight would see the front of the rod has already passed the reference point for the start of the measurement.
Again, there's no error. A stationary observer who knows the value of c and v and relativistic physics can translate measurements made in one frame into measurements made in another.Now a stationary observer who knows the value of c and v can use simple math to determine the error in the measurement made my the moving observers.
From their viewpoint--which is perfectly legitimate--the photon did travel the length of the rod!The moving observers, because they don't recognize the stationary frame of reference, simply assume that the photon traveled the length of the rod.
You still are hung up thinking that one reference frame is the 'correct' one. Either one is perfectly OK to use.It didn't because by the time the photon reached the back end the back end had moved (they just didn't know it because they were completely absorbed by their own frame of reference).
They don't! In relativity, 'relative speed' means the speed one thing has as measured in the frame of the other. From the frame of the rod, the photon moves at speed c. (What's interesting is that the photon moves at speed c with respect to both frames!)What worries me is how does the photon and the end of the road move relative to each other at faster than the speed of light ?
No. Again, viewing things from the frame in which the rod is at rest, the photon moves at speed c, like usual. However, it's also true that viewed from the 'stationary' frame the rod is moving towards the incoming photon, thus the distance between them--according to the stationary frame--is closing at greater than the speed of light. So what?because when we think of either of those points being the "fixed" point and the other the moving one they are now seeing the other travel faster than the speed of light.