Assuming we could build a spaceship that will accelerate an astronaut close to the speed of light, only for them to return a few hours later (in the astronaut's time frame), through a quirk of relativity it's possible that thousands of years would have passed on Earth. Therefore, the superfast spaceship will have become a time machine! Want to go further into the future? No problem! Fly the spaceship even faster.
(Keep in mind that it's still impossible - according to our current knowledge of space, time and good ol' fashioned physics - to travel faster than the speed of light, but traveling at any fraction of the speed of light is still allowed in physics. The engineering of such a machine, on the other hand, would require some pretty epic propulsion technology behind it.)
ANALYSIS: Turning a Hot Tub into a Time Machine
"If you go fast, your clock runs slow relative to people who are still. As you approach the speed of light, your clock runs so slow you could come back 10,000 years in the future," he said.