The reason measurements of velocity are relative is because the measurer has its own non-zero velocity that affect the measurement. A car has a total velocity larger than what we measure on the surface because the observer isn't taking into account his own movement as the Earth spins. If you could know your own velocity perfectly, you could calculate the velocity of everything else. The problem is that to know your own velocity you must measure it from another reference frame, which also isn't sure of its own velocity. We need a constant reference frame to find absolute velocity.
Light might be that constant reference frame.
Light moves at a constant speed; its not affected by the movement of its source, only the direction of travel. If you wanted to know whether or not your spaceship was moving, you could aim a laser beam at the wall and see if it missed the mark; which it would only do if the spaceship itself was moving. If you came to the point where the laser would always hit its target perfectly (and I mean, perfectly), then your spaceship would be AT REST, RELATIVE TO THE UNIVERSE ITSELF. Any movement external observers measured from your spaceship would be caused by their velocity relative to the universe, not yours. You would experience no time dilation.
Why?
Light is a wave in the electromagnetical field. It has no mass and is moving at the maximum speed the universe allows. Light moves at C, relative to the universe itself. For this reason, it can be used to measure absolute velocity with some calculations and knowledge of this principle.
