Here’s something that needed to be explicitly pointed out to me: there’s a direct connection between the fact that simultaneity is observer dependent and that length is observer dependent. The reason is that measuring the length of a rod boils down to finding two positions (the two ends of the rod) **at the same time**. If two observers don’t agree on simultaneity, i.e. when two events occur at the same time, then they won’t agree on the two positions to use for measuring the length.

While I’m on the subject of length contraction, I’d like to point out that the length an observer assigns to a moving object is not necessarily what the observer sees when looking at the object (or equivalently, the length of the object as measured from a photograph taken by the observer). That’s because Continue reading “Relativistic Length”