Hi all, I have been interested in the Theory of Special Relativity for the past few years, and there is something I'd like to discuss. Somebody else may be discussing this elsewhere but if there is I haven't been able to find them. I have never felt comfortable with time dilation due to velocity as inferred from the light clock thought experiment (for those unfamiliar with this thought experiment, see 'Simple inference of time dilation due to relative velocity' at http://en.wikipedia.org/wiki/Time_dilation). When thinking about this experiment, it occurred to me that it was impossible for the observer moving past the clock to see the light pulse travelling between the two mirrors. Surely any light that the observer sees is different to this light pulse. I thought that this might just be a shortcoming of that particular experiment, so I went to as pure a source as I could find - the English translation of 'Relativity - The Special and General Theory' at Bartleby Online (http://www.bartleby.com/173/). I could have gone purer but I really didn't want to spend time learning German when there are plenty of people out there who can do it better than me anyway. I was surprised to find that this oversight seems to exist there as well. In Chapter 7 'The Apparent Incompatibility of the Law of Propagation of Light with the Principle of Relativity', Einstein describes a thought experiment where the ubiquitous railway carriage is travelling parallel to a ray of light, and tries to explain why the light must appear to have the same speed to both the observer on the railway carriage and the observer on the embankment. The answer Einstein gives is that time slows down as an observer approaches the speed of light, but surely this contradicts the principle of relativity where nobody is 'really moving'. As long as there is no net force acting on an observer (ie. no acceleration), all observers can apply the same laws. This means that time should slow down equally for the observer on the railway carriage and the observer on the embankment. I think the answer to the problem of incompatibility lies in the fact that the observer on the railway carriage could never see that ray of light that is travelling parallel to him, because that would require another ray of light to travel from the first ray to the carriage. The only observer that would see the first ray of light would be someone standing next to the railway track in the path of that ray. And it's a good thing that there is no air in this experiment, otherwise the turbulence created by a supersonic train would surely wipe this poor observer from existence. Therefore, the observer on the carriage could never judge the speed of that ray of light and there is no incompatibility - each individual ray of light can only be observed by one observer, and therefore time dilation as described in these instances can't exist. To state it a little more succinctly - light and the constant speed it travels in a vacuum is required for special relativity to apply, so surely we can't apply the principles of relativity to part of the system that supports it? Any comments, questions or derisions are welcome. Cheers.