STEPHEN HAWKINGS : A LIFE IN SCIENCE
WROTE BY MICHAEL WHITE AND JOHN GRIBBINS
Two velocities, for example, can never add up to give a relative velocity greater than c. An observer may see two other observers approaching each other on a head on collision course, each travelling at the speed of 0.9c in the first observers reference frame; but the measurements carried out by either of those two fast moving observers will always show that the other one is travelling at a speed less than c, but bigger(In this case)than 0.9c.
The reason why velocities add up in this strange way has to do with the way both space and time are warped at high velocities. In order to account for the constancy of speed of light, Einstein had to accept that moving clocks run more slowly than stationary clocks and that moving objects shrink in the direction of their motion. The equations also tell us that moving objects increase in mass the faster the go.