# Time dilation and relativity paradox? [duplicate]

I've come across a weird paradox that I can't answer, I will explain it via the following thought experiment:

There is a space-train and an observer 1 light year apart with synchronised clocks. The train travels to the observer at just below the speed of light. In the reference frame of the observer the train is subject to time dilation and length contraction. When it arrives its clock reads 1 second as its time was basically standing still and the observers clock reads one year. From the trains reference frame it sees the 'observer' travelling towards the train at near light speed, so the train should expect to read the observers clock as 1 second and the trains clock as a year as the train sees the observer to have undergone time dilation. So what do the clocks actually read and why? The only way I can imagine the paradox with relativity not occurring is if both clocks read the same otherwise you could tell what speed you were going?

• This is exactly the twin paradox
– Jim
Aug 18, 2015 at 15:44
• There is no way for the space-train and an observer 1 light year apart to synchronise their clocks, so none of the rest of your argument follows. Aug 18, 2015 at 15:45
• possible duplicate of How is the classical twin paradox resolved?
– Jim
Aug 18, 2015 at 15:46
• @JohnRennie They can be reasonable synchronized to within acceptably measurement precision. You can't be 100% sure they are synchronized, but you can be sure enough that an engineer would put their stamp of approval on the experiment (if you care about such things)
– Jim
Aug 18, 2015 at 15:49
• More like a duplicate of this Aug 18, 2015 at 17:19