Fly near light speed for a year. Come back. Everyone you knew is old or gone. How much younger are you?
Einstein's special relativity says the faster you move through space, the slower you move through time. The time dilation factor is gamma = 1 / sqrt(1 - v²/c²). At 90% the speed of light, one year on your spaceship equals 2.29 years on Earth. At 99.9%, one ship year equals 22 Earth years.
This isn't science fiction. GPS satellites have to correct for relativistic time dilation every single day, or your navigation would be off by kilometers within hours. The satellites' atomic clocks run slightly faster because they're farther from Earth's gravity well, and slightly slower because they're moving fast. The two effects partially cancel, but not completely.
Astronauts on the ISS age about 0.005 seconds less per year than people on Earth. After six months in orbit, they're technically 0.0025 seconds younger. Not enough to notice, but enough to prove Einstein right.
Einstein's special relativity says: the faster you move through space, the slower you move through time. This isn't science fiction — it's measured every day on GPS satellites. Their clocks run slightly slower due to their speed, and slightly faster due to weaker gravity at altitude. If engineers didn't correct for relativity, GPS would be off by kilometers within hours.
The effect is tiny at everyday speeds. Fly around the world at 900 km/h, and your clock slows by about a nanosecond. But get close to the speed of light, and the math gets dramatic. Travel at 99% the speed of light for one year, and when you come back to Earth, 7 years have passed. At 99.999%, one year on the ship equals 223 years on Earth.
This is the twin paradox. One twin stays home, the other travels at near-light speed. The traveling twin comes back younger. It's not a paradox — it's just how spacetime works. Time isn't a universal clock. It depends on how fast you're moving and how deep you're in a gravity well. Your second is not my second.
Moving clocks run slow. At 99% the speed of light, 1 year on the ship = ~7 years on Earth. This is real and measured daily on GPS satellites.
Yes — the twin paradox. Travel at near-light speed for a year, return, and decades have passed on Earth. It's one-way travel.
At everyday speeds (1000 km/h), the effect is nanoseconds. You need a significant fraction of light speed to feel it.
Yes. GPS satellites have to correct for it every day. Their clocks run 7 microseconds slower per day due to speed, and 45 microseconds faster due to weaker gravity. Net effect: 38 microseconds faster. If engineers didn't fix this, GPS positions would be off by 10 km per day.
The traveling twin. Because they accelerated (turned around), they experienced less time. The staying twin aged more. This isn't a paradox — it's just that acceleration breaks the symmetry between the two observers.
One year on the ship equals 223 years on Earth. Travel to the center of the galaxy (26,000 light-years) would take about 11 years ship time. Come back, and 52,000 years have passed on Earth. Everyone you knew is gone.