I have a thought experiment that may be of interest to some of you. If this has been discussed before, please direct me to the discussion. Thanks! The experiment involves a beam generator consisting of a laser, a parametric downconverter cut for operation at the 2nd harmonic, and mirrors to direct the output in opposite directions. The laser is ultraviolet, the downconverter output is green. If you are not familiar with downconverters, the basic idea is that one photon goes in and two come out in slightly different directions but with the same total energy. At the 2nd harmonic they each have half the frequency of the incoming photon. Now, put the generator in a spaceship, with the beams directed fore and aft. Accelerate the ship to 1/3 c and run it past an observer. The observer observes a blue beam directed toward him and a red beam directed back along the ship's path. Lorentz transformation applied to the perceived wavelength of the downconverted light: the forward beam is blue-shifted, the backward beam is red-shifted, from the observer's perspective. Now let us have a second generator, stationary with respect to the observer, but it uses an asymmetric downconverter at the 3rd harmonic, so the output is naturally blue and red with the blue photon twice the frequency of the red photon. The sum of their frequencies is still equal to the frequency of the incoming ultraviolet photon (conservation of energy). Is there any way for the observer to distinguish which beams came from the stationary generator and which from the moving generator, if the observer is not able to see the generators themselves? The reason this is of interest, is another relativistic effect combined with a quantum effect. Parametric downconverters produce pairs of photons that are polarization entangled. Because they are entangled, if you measure one photon, its partner also collapses into a definite state (Einstein's "spooky action at a distance"). Now draw a worldline between the two collapse events. If you are at rest with respect to the source of the photons, then that worldline is "flat" and the events are perceived as simultaneous. If you are in motion with respect to the source, then a Poincare rotation applies to your perception of that worldline: if the source is approaching you, then the near collapse (of the "blue" photon's state) is rotated slightly into the future, and the far collapse (the "red" photon's state) is rotated slightly into the past, as far as your perception is concerned. You observe the "red" photon collapsing before the "blue" one does. Finally, and with apologies for the lengthy setup, here's my point (and my real question). If one cannot distinguish whether the redshift/blueshift of a pair of beams of light is due to motion, or due to the nature of the downconverter, then should there not be a Poincare rotation of the worldline between the collapse events, in either case? And is there an experiment that could verify (or falsify) this question?