From a Newtonian perspective:
- F = ma
- F dx = ma dx
- E = m (dv/dt) dx
- m = (E dt) / (dv dx)
Mass is directly proportional to time, if time slows down then mass goes down or decreases.
From a Relativistic perspective:
- Approaching c, time slows down and mass goes up or increases. Mass is inversely proportional to time.
Irrespective of what speed you are travelling, the fundamental relationship between mass and time should remain. So is mass directly or inversely proportional to time?