You've probably heard it said that the fourth dimension is time. This idea is especially popular with SF authors when writing time travel stories. But what does this mean? And what are the implications?

Forgetting about time for a while, in ordinary life we deal with three dimensions: left-right, forward-back and up-down. What this means is that every material object and every location in the universe can be described mathematically using only three numbers. When we measure a material...