If the distance between you and another person is changing,
(you are in motion relative to each other, and it doesn't matter
how fast), and you both have clocks, then ...
-- you will say that your clock and your heartbeat are running normally
but his his clock and heartbeat are running slow;
-- he will say that his clock and his heartbeat are running normally
but your clock and heartbeat are running slow.
You'll both be correct.
