clocks can now see time run differently across a millimeter

How I got here

Started with

How small a height difference can an atomic clock detect?

Ended at

Geodesy

The Trail

1. “atomic clock time dilation millimeter jila 2022” (google.com)
2. Atomic clock (en.wikipedia.org)
3. Optical lattice clock (en.wikipedia.org)
4. Gravitational time dilation (en.wikipedia.org)
5. Strontium (en.wikipedia.org)
6. Geodesy (en.wikipedia.org)

What I came away with

In 2022 a JILA group measured the gravitational redshift across a single cloud of strontium atoms about a millimeter tall; a decade earlier you needed to lift a clock about a third of a meter. Optical clocks are now good enough to act as height sensors: compare two of them and general relativity tells you their difference in elevation. It's a big reason the SI second is expected to be redefined around optical transitions.

Keep going

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