Reader Scott writes in with a good question that I’ve heard
posed by several people in various places, because it really
goes to the core of the counterintuitive nature of relativity.
Okay, let’s assume I’m in a spacecraft moving at a speed of c-1
m/s. That’s one m/s less than the speed of light. Would I be
able to move from the back of the ship to the front at 1 m/s?
Would I even be able to survive at that speed? Here’s my reply,
extended a bit to include the relevant equations and go into a
little more detail. The derivations will all wait until later:
Yes indeed. The question at issue is your speed with respect to
a particular reference frame. Let’s pretend that you’re on an
airplane flying at 500 mph with respect to the ground. There’s
nothing stopping you from walking forward at 1 mph with respect
to the plane. And that also happens to mean that you’re walking
at 501 mph with respect to an observer on the ground. Assuming
the plane ride is nice and smooth and the windows are closed,
you wouldn’t even be