But we don't really think that speeds add linearly. if you drive 80 kph North, and I drive 60 kph East, no one would say we are separating at 140 kph. we would

But we don't really think that speeds add linearly. if you drive 80 kph North, and I drive 60 kph East, no one would say we are separating at 140 kph. we would use Pythagoras to get the right number. We just don't realize we live in a 4D space where time is one dimension, so we think driving in opposite directions is a special case where we can add the speeds. but there is the fourth dimension, and we have to use an equation very similar to Pythagoras to get the right value. I know you must know this, this is more for the benefit of the reader.