Hello, I just thought of something that looks like a paradox to me. Suppose you have a device which can detect a light source's frequency, and will explode if the frequency exceeds a threshold value [itex]f_t[/itex]. Place this device stationary wrt the light source. Now, suppose there are two observers, one stationary, and one moving towards the light source fast enough at constant speed. By the Doppler effect, the moving observer will see the frequency larger than [itex]f_t[/itex], and so the device should explode in his frame of reference. But the stationary observer sees a frequency less than [itex]f_t[/itex], so the device does not explode in his reference frame. I don't even think the effects of special relativity would be relevant here. You could set up the light source to emit at a frequency [itex]f_t - \epsilon[/itex] (for some [itex]\epsilon > 0[/itex]), and have the device be sensitive enough to detect the variation of frequency that occurs by moving with some reasonable speed v << c that is sufficient to cause a frequency shift of [itex] 2 \epsilon [/itex].