According
to the special theory of relativity, all laws of nature
are the same in
reference frames that move at constant speeds.
That means that there's no test or measurement you can do to
detect whether you're moving at a constant speed or how fast.
And THAT means that there's no such thing as "who's REALLY
moving" and "who's REALLY at rest". There's no such thing as
"real" motion, only motion relative to something else.
