In a recent thread, a discussion developed on the subject of how we observe light and how this affects our understanding of SR. I called attention to the famous light clock diagrams: http://home.comcast.net/~peter.m.brown/sr/image_gif/sr05-im-01.gif [Broken] In my view, the problem here is not with the resulting formulas, nor with time dilation, nor with the assumptions of SR, but that the light clock diagrams often lead to logical paradoxes, depending on how you interpret the problem, and I believe this happens because these diagrams are paradoxical to start with, and I would like to hear other opinions on this subject. From where I stand, the problem with the diagrams is very simple: You can't detect light at a distance. Detection of EM waves (or photons) in SR is strictly a local event. You can't be aware of light moving in any direction other than straight into your eyes (or detectors). So how can a non-local observer see those light rays? They are bouncing back and forth between the two mirrors, and anywhere else. Suppose those are laser beams (so they don't radiate spherically). From the stationary frame, they go straight up and straight down, cross the distance between the mirrors at the speed of light and the time is proper time, so everything's fine. From the point of view of a distant observer, if you follow the logic in the diagrams, you could either deduce a change in the speed of light or time dilation. Since every experiment shows us that light always travels at c, we need time dilation to explain the angular light beams in the moving system. First, if you assume that light was emitted at an angle, how does the emitter know the correct angle of emission? Second, how can those light paths be part of anyone's data? If they reach the mirror, they don't reach the observer. We can assume that each mirror gives off a light signal every time it reflects the beam, and a local observer would measure the speed of the light as c, and no time dilation would be noticed. If a distant observer receives those light signals, than he must do the transforms with that light, and not the light that is being reflected inside the light clock. If you do that, than you will achieve the values for time dilation and length contraction before you diagram the light clock's beams, and when you finally get to diagram those beams, you will diagram them just like the local/stationary observer would. Light would never be diagramed at those angles and nobody would even consider that light could go above c, or that it had to experience time dilation. Isn't it weird to claim that the light has been time dilated? We use light to measure time dilation and length contraction on other things, so how can you time dilate the light itself? Here is a very thorough and standard analysis: http://physics.syr.edu/courses/modules/LIGHTCONE/LightClock/ I look for hearing different opinions on: - How can we correctly diagram undetected light like that? How does the emitter adjust the angle of emission? - Conversely, how can light be emitted at an angle if it's speed is not affected by the motion of the emitter? - Shouldn't we apply SR transforms primarily with light that is actually observed and than use that information to diagram the interior of the light clock? - Isn't diagraming the light clock in the moving frame like that illegal? Aren't those vectors purely imaginary?