If you and I are in separate space ships both traveling fast, there’s literally no possible way to determine if I am staying still and you are moving, or if you are staying still and I am moving, or if we are both moving. Also, from your perspective time will flow normally for you and slower for me, but from my perspective time will flow normally for me and slow for you.
Basically, everything, including time, is completely relative.
Space fact #5
Published
I May or May Not be Steve Rogers 