The twin paradox
One twin flies to a star and back. She comes home younger than her brother.
Paul Langevin, 1911, from Einstein's special relativity (1905)
Ira and Ishaan
Ira and Ishaan are twins, born on the same day in Bengaluru. On their twentieth birthday, Ira climbs aboard a starship and heads for a nearby star at 80 per cent of the speed of light. Ishaan waves from the launch pad and goes back to college.
To Ishaan, Ira's trip there and back takes 10 years. He graduates, gets a job, moves house twice. On their thirtieth birthday he goes to the spaceport to meet her.
Ira steps out looking barely older. By her ship's clocks, her calendar and her own heartbeat, only 6 years have passed. She is 26. Her twin brother is 30.
"Hang on," says Ishaan. "From your point of view, I was the one zooming away at 80 per cent of light speed. My clock should have run slow. Why aren't I the younger one?"
The puzzle
In relativity each twin sees the other's clock running slow. So why does only one of them end up younger?
See it for yourself
What's really going on
The two twins are not in the same situation. Ishaan stays in one steady frame of reference the whole time. Ira has to fire her engines to leave, turn round at the star and slow down at home. Only she changes direction, and that turnaround is what breaks the symmetry. When you work through the maths of her changing frames, both twins agree that she has aged less.
The effect is real and measured. In 1971, Joseph Hafele and Richard Keating flew atomic clocks round the world on airliners and found they disagreed with clocks on the ground by tens of billionths of a second, as relativity predicted. GPS satellites have to correct for these time effects every day.
Think about it
At 99 per cent of the speed of light, how much would Ira age on the same 10-year trip? Try it in the animation.