Clocks higher up tick faster—and the difference fits on a table
Two optical clocks separated vertically by 33 centimetres tick at measurably different rates because gravitational potential differs with height. The effect was measured directly on a laboratory scale in 2010 and has since been resolved across a millimetre-scale atomic sample.
General relativity predicts that a clock higher in a gravitational field accumulates slightly more proper time. Aluminium-ion clocks with fractional accuracy near 10⁻¹⁷ resolved the difference across a third of a metre; later strontium lattice clocks resolved it within one atomic ensemble. Over a human lifetime, the head-to-feet difference is only tens of nanoseconds. The measurement concerns elapsed time along different worldlines, not biological ageing mechanisms.
Relativity is not confined only to black holes or distant satellites. Clocks are now precise enough to sense centimetre-scale gravitational potential, turning timekeeping into a practical geodetic measurement on Earth.
Invert the relationship and a clock measures gravitational potential, which means height. Chronometric levelling now resolves elevation differences of roughly a centimetre over long baselines, without a line of sight, which is difficult by any other method across continents. Transportable optical clocks have been carried into road tunnels to compare against geodetic surveys.
The caesium microwave standard that defines the second is a hundred times less stable than the best optical clocks. A redefinition around an optical transition is being prepared. When it lands, the base unit of time will be pinned to a physical system whose rate depends measurably on where you put it — which will have to be written into the definition.
Does this prove that a person’s head biologically ages faster than their feet?
No biological ageing was measured. The result is exactly about clock rates and proper time. The popular phrase is a useful scale comparison but becomes misleading when it implies a demonstrated difference in tissue senescence.
Nothing on the physics; the objection is to the framing, and it is correct.
Which of your intuitions survive only because your instruments are coarse?
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