WEBVTT

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Imagine two precise clocks in the same room.

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Raise one by a single step.

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According to general relativity, the higher
clock should tick slightly faster.

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The change is too small to notice on an ordinary
clock.

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In 2010, NIST researchers raised one of two
optical clocks by thirty-three centimeters.

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That is about a foot.

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They compared the ticks and measured the
expected difference.

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The experiment used aluminum ions to keep time.

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In 2022, JILA researchers went smaller.

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They compared the ticking across different
heights inside one cloud of strontium atoms.

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The separation was about a millimeter.

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A tiny height difference can leave a measurable
difference in time.