The twin paradox
Set the speed of the spaceship and look at the age of both twins on the return.
The experiment starts as soon as the page has loaded (JavaScript is required).
What just happened (read it after the experiment)
According to Einstein’s relativity, time passes more slowly for a clock that moves fast. At 99.5% of the speed of light, each year for the traveler is worth about ten years on Earth.
It isn’t just a theory: the clocks on GPS satellites gain 38 microseconds a day compared with ours, through the combined effect of their speed and gravity. Without correction, your GPS would drift by more than six miles a day.
Why does the traveler age less, and not the twin who stays home? Because the traveler is the one who accelerates and turns around: the two situations are not symmetrical.
More experiments
Sources
- Albert Einstein, “Zur Elektrodynamik bewegter Körper,” 1905.
- Neil Ashby, “Relativity in the Global Positioning System,” Living Reviews in Relativity, 2003.