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The Twin Paradox Explained
One twin stays on Earth; the other flies to a distant star and back at close to the speed of light. When they meet again, the traveller is younger. This is the famous twin paradox of special relativity.
Why the traveller ages less
Moving clocks run slow by the Lorentz factor γ = 1/√(1 − v²/c²). Every biological process is a kind of clock, so the travelling twin ages by Δτ = Δt/γ while Δt passes on Earth.
So why is it called a paradox?
From the ship, it is Earth that seems to be moving, so shouldn't the Earth twin age less? The answer is that the situation is not symmetric. The Earth twin stays in one inertial frame the whole time. The traveller has to turn around to come home, switching from the outbound frame to the inbound one. That switch changes which events the traveller counts as "now" back on Earth, and it is where the extra Earth years appear in their accounting.
Worked example
At 95% of light speed, γ ≈ 3.2. If the round trip takes 60 years by Earth's calendar, the traveller ages only 60/3.2 ≈ 19 years. Twins who left at 20 meet again at ages 80 and 39.
Has it been tested?
Yes, on a smaller scale. In 1971 atomic clocks flown around the world on airliners came back out of step with clocks on the ground, by exactly the amount relativity predicts.
Frequently asked questions
Why does the travelling twin come back younger?
Moving clocks, including biological ones, run slow by the Lorentz factor γ. The traveller spends the trip at high speed, so less time passes for them.
Why isn't the twin paradox symmetric?
Only the traveller changes inertial frame by turning around. The Earth twin stays in one frame, so the two twins' situations are physically different.
How much younger would the traveller be?
It depends on the speed. At 95% of light speed (γ ≈ 3.2), a 60-year round trip by Earth clocks takes only about 19 years for the traveller.