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Twin paradox
Intermediate
The travelling twin ages less.
One twin travels to a star and returns, while the other stays home; the traveller ages less. The asymmetry is that the traveller accelerates (changes inertial frame).
At 0.8c for a 8-light-year round trip: 10 years at home, 6 for the traveller.
Verified with atomic clocks on aircraft.
In more detail
In the twin paradox, one twin travels at high speed and returns younger than the twin who stayed at home. The asymmetry comes from the acceleration needed to turn around.
Real-world uses
- It illustrates the consequences of time dilation.
- Astronauts on high-speed journeys experience small time differences.
- The twin paradox is used to illustrate how acceleration and changing frames break the symmetry between two travellers.
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Frequently asked questions
Why are the twins not symmetric?
Because the travelling twin changes inertial frames, which the stay-at-home twin does not.
Is the effect large in practice?
Only at speeds close to light, so for normal travel the difference is tiny.
Who ages less in the twin paradox?
The twin who travels, because their path is not inertial, so their proper time is shorter than the stay-at-home twin's.
Related topics in Relativity
Lorentz transformations · Velocity addition · Muon decay evidence · Spacetime diagrams · Spacetime interval · Light cones and causality