Please Register or Log in to view the hidden image! I think MacM is onto something here. In the embankment frame, the color filter at the front of the train is receiving redshifted light. Perhaps the time-dilated, length contracted atoms in the filter would cause the filter's bypass frequency to be redshifted as well, and therefore the light would still pass through. However, in the embankment frame, the color filter at the rear of the train is receiving blueshifted light. There is no difference in the way the atoms in this filter are time-dilated and length contracted compared with the atoms in the filter at the front of the train. Therefore, I do not understand why the blueshifted light would pass through the filter at the rear of the train. If the time dilation and length contraction cause the bypass frequency of one filter to become lower, why would these same effects cause the bypass frequency of the other filter to become higher? The only other thing that I can conceive of which applies to the rear of the train in a different way than it applies to the front of the train is the loss-of-simultanaety (in the embankment frame, clocks are not synchronized properly across the length of the train). However, I do not see how this could be the solution for the two filters passing different frequencies of light. Up until now, MacM has not made a very compelling argument for the "different photon" theory, but he just might have scored some bonus points here. What would the SR explanation be for this "relativistic, dopper-related, filter-frequency-shift" phenomenon?