ThereвЂ™s another wrinkle here worth mentioning: time dilation as a result of gravitational effects. But if youвЂ™re trying to reach an exoplanet 10 to 50 light-years away and still make it home before you yourself die of old age, youвЂ™d have to be moving at close to light speed. Exactly how much younger depends on exactly how fast the spacecraft had been moving and accelerating, so itвЂ™s not something we can readily answer. So in your question, an astronaut returning from a space journey at вЂњrelativistic speedsвЂќ (where the effects of relativity start to manifest-generally at least one-tenth the speed of light) would, upon return, be younger than same-age friends and family who stayed on Earth. This has also been confirmed in other physics experiments (e.g., fast-moving muon particles take longer to decay). The relative motion actually had a measurable impact and created a time difference between the two clocks. The 1971 Hafele-Keating experiments proved as much, when two atomic clocks were flown on planes traveling in opposite directions. Unlike the Twin Paradox, time dilation isnвЂ™t a thought experiment or a hypothetical conceptвЂ“вЂ“itвЂ™s real. If youвЂ™re able to travel near the speed of light, the effects are much more pronounced. The clock in motion will tick more slowly than the clocks weвЂ™re watching on Earth.

Time is measured differently for the twin who moved through space and the twin who stayed on Earth.

The faster you move through the three dimensions that define physical space, the more slowly youвЂ™re moving through the fourth dimension, timeвЂ“вЂ“at least relative to another object. Time dilation goes back to EinsteinвЂ™s theory of special relativity, which teaches us that motion through space actually creates alterations in the flow of time. This is precisely the scenario outlined in the famous thought experiment the Twin Paradox: an astronaut with an identical twin at mission control makes a journey into space on a high-speed rocket and returns home to find that the twin has aged faster. Time dilation is a concept that pops up in lots of sci-fi, including Orson Scott CardвЂ™s EnderвЂ™s Game, where one character ages only eight years in space while 50 years pass on Earth.

No gravity in space shuttle