On Thu, Feb 20, 2014 at 12:25 PM, H Veeder <hveeder...@gmail.com> wrote:
> John, > > Eric is right about the constancy of c being a *postulate* from which > time-dilation and length contraction are derived. > However, that doesn't discount your thought experiments as a way of > probing the coherence of SR. > > Imagine two friends with synchronized watches. One friend boards a train > and zips away for a time at near c and then gets off and walks back to his > friend > so that they can compare the time on their watches. Which watch is ahead? > > Using the principles of SR I can come up with contradictory answers. > Yes, but the difficulty with arguing this point is that SR argues that the experience of the 2 friends is asymmetric, and the 2 friends can't communicate the rate of passage of time in an effective manner while travelling (since there is no instantaneousness communication) and if they do try to communicate while the distance between them grows there will be an apparent distortion of time as each second that passes the time it takes for any message to be sent between them grows slowing the apparent rate of time. This muddies the water enough that it can just be called a paradox, and confusing but move on.. But if there is communication that is orthogonal to the direction of travel, this is not effected by such concerns, and near instantaneous constant delay communication is possible. And time dilation simply can not be reconciled in this manner. To repeat the thought experiment, let's say there is a 3rd friend, he is on the ground to the side of the track, he can see the watch of both the friend on the train and the one at the station. And they can all see his watch too. If the chap on the train really has time slow, then this 3rd friend would see it, and he would notice that train guys watch almost stopped ticking. But if this is so the friend on the train would have to notice the 3rd friends watch tick faster than his since they are able to easily observe each other without distortion. They can't both watch the other stay fresh faced while they grow old because the train could stop (or the 3rd friend could hop on) in an instant, both can't see the other suddenly age rapidly. If the friend on the train looked back at the friend on the platform he would find SR prediction met, the watch of his platform friend would appear to have almost stopped and visa versa, and the platform friend would see train friends watch almost stop ticking. But if there is no preferred reference frame, both would have to agree that the 3rd friends time piece is keeping sync with theirs. With the orthogonal communication (which can be for a longer time the further the 3rd friend if from the track, or forever if he is in the centre of a circular track) this ruse can not be continued. Time dilation without a preferred frame is not possible, and time dilation with a preferred reference frame is not SR. John
