Relativity with a human touch

I make it 90 billionths of a second over 80 years

In the famous twin paradox, a sibling who journeys in a fast-moving spacecraft will return home younger than the sibling who remained on Earth. While this apparent slowing of time occurs whenever a body is set in motion, it had been much too small to be detected for movement on a human scale.

But now physicists in the US have used two of the world’s most accurate optical clocks to see this and other relativistic effects at speeds and distances on a human scale. The team has seen time slow down in a clock moving less than about 35 km/h relative to its twin. It has also showed that time speeds up in a clock that is hoisted a mere 33 cm above the other.

James Chin-Wen Chou and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado used two clocks – each based on just one aluminium ion – to do their time dilation experiments. The first such clock was unveiled by the team earlier this year and has the ability to remain accurate to within one second in 3.7 billion years and the second has a similar accuracy.

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