Hello all, having decided that I wish to apply to Oxford to study Physics (as well as Imperial and, I am still considering Natural Sciences at Cambridge), I have been informed that extra reading and independent study would be advised, so I'm delving into the world of relativity (I'm a first year AS student so we've done nothing on it yet). I've stumbled upon an explanation of why time dilation happens but the explanation seems odd to me. We've been told that time dilation happens in GCSEs etc but never had any sort of explanation as to why. This analogy (I don't think it's a proof but I haven't seen it referred to as a proof or an analogy so I'm assuming it's an analogy) doesn't appear to prove anything to me... Say you have a stationary light clock with the photon bouncing (I'm presuming you all know the experiment I;m talking about - http://galileoandeinstein.physics.virginia.edu/lectures/srelwhat.html has it around half way down if you don't). The photon will bounce off one mirror at the angle it hits it - so straight back off the mirror. If you then move the mirror fast enough, to say the right, then the photon will have bounced off one mirror but the other mirror will now be to the right of the photon's path before the photon has arrived and the photon will not reflect back to the other mirror, it will just continue off into space because there is no mirror to reflect, so how can the photon take a "longer path" back to the mirror. Is this an example of the analogy breaking down or am I missing some knowledge of how photons move? Does the photon reflect off a surface at an angle if the surface is moving? Or is it just a flaw in the analogy? I accept that time dilation happens - experimental data proves it as such, but is there an analogy which would actually explain this phenomenon without any flaws? Is this actually a flaw or is it my lack of knowledge? Thanks in advance.