Exponential increase in time dilation is mirrored in the relationship between apparent mass (as measured by an external observer) and velocity as we approach c. In other words, mass tends toward infinity as you approach the limit c, effectively setting a universal (multiversal? teehee) speed limit since an object of inifinite mass would take infinite energy to move — impossible! Since photons have no mass, they can travel at 100% the speed of light, and time dilation is total for them: time doesn't exist for photons until they are slowed by gravity, air, or other things. Wow! Time dilation calculator