Science Puzzle
The Relativistic Twins
Twin A stays on Earth. Twin B boards a rocket and travels to a distant star at nearly the speed of light, then returns. When Twin B gets back, both twins check their clocks and their ages.
Twin A has aged 30 years. Twin B has aged only about 4 years. Who was right about time passing, and how can they have aged differently if they are the same person?
The Answer
Both are right about their own clocks, and they really are now different ages. This is time dilation, a real and experimentally verified consequence of special relativity.
Time is not a fixed background that passes at the same rate everywhere. Moving clocks run slow relative to stationary ones. At 99% of the speed of light, a clock ticks at about 14% of its normal rate. Twin B genuinely experienced less time during the journey.
The paradox is often stated as: each twin sees the other moving, so why is not the effect symmetric? The answer is that Twin B is the one who actually accelerated and decelerated. The two situations are not equivalent. The twin who changed velocity is the one who ages less.
Time dilation is not a thought experiment: GPS satellites must correct for it every day or navigation errors would accumulate rapidly.
The principle: Time dilation. Moving clocks run slow relative to stationary ones. Near light speed, the effect is dramatic and real, not apparent. The twin who accelerated genuinely ages less.