As Photon said, you can't travel at 100% the speed of light. You can only approach the speed of light, meaning 99.9% (repeating).
If you read "Relativity: The Special and General Theory" it explains it all pretty simply.
Again, as photon said, the faster you go, the slower time passes for you.
This has been proven by synchronizing an atomic clock on a very fast jet with one on the ground. Fly the jet around at high speeds for a long period of time and eventually you will see a difference between the two clocks because time has slowed down for the clock on the jet relative to the time for the clock on the ground.
The same is true for astronauts because the orbit the earth at 17,000mph for months at a time. They have basically time traveled a fraction of a second into the future because their clocks slow down relative to time on earth.
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