Since the laws of physics determine how a clock works, it must work the same way for all observers. Hence if two clocks agree for one observer they must agree for all observers. The light clock slows down, and all clocks agree with the light clock, therefore all clocks slow down. If all clocks slow down, then physical time, which is defined by clocks, slows down. Experimental verifications include the fact that muons reach the surface of the Earth before decaying, and the flying of atomic clocks in aircraft.
Time dilation implies that time travel into the future is possible by going fast.
Therefore a 10 year round trip at this speed, by Earth clocks, would last only 5 years of astronaut's time. The astronauts would therefore take 5 years to travel 10 years intothe Earth's future. Relativity of simultaneity is another consequence of the constancy of the speed of light. Events which are simultaneous for one observer, but seperated in space, are not simultaneous for another observer. The conclusion of time dilation and the relativity of place and of simultaneity is that one person's space is another's time, and vice versa.