Optical atomic clocks
An optical atomic clock keeps time with ultracold atoms such as strontium. The best are now steady enough to register gravity changing across a single millimetre.
General relativity says a clock higher in Earth's gravity runs faster: a kilometre up, it gains about 3 seconds in a million years. Atomic clocks had checked this over heights from 30 centimetres to thousands of kilometres. Then one experiment saw the effect inside a millimetre-scale cloud of ultracold strontium. At that sensitivity a clock doubles as a ruler. Two optical clocks 457 kilometres apart measured the difference in gravitational potential between their sites to within the equivalent of 27 centimetres of height.
Sources
- Grotti et al., Long-distance chronometric leveling with a portable optical clock (arXiv)
- Bothwell et al., Resolving the gravitational redshift within a millimeter atomic sample (arXiv)
- NIST, Relativity and Optical Clocks
- Aeppli et al., A clock with 8×10^-19 systematic uncertainty (arXiv)
- RIKEN, Scientists use the Tokyo Skytree to test Einstein's theory of general relativity
- University of Tokyo, Test of General Relativity by a Pair of Transportable Optical Lattice Clocks
- Dimarcq et al., Roadmap towards the redefinition of the second (arXiv)
- Hilton et al., Demonstration of a Mobile Optical Clock Ensemble at Sea (arXiv)
- ScienceDaily, Pair of aluminum atomic clocks reveal Einstein's relativity at a personal scale
wormhole
Yes, by an amount nobody could feel. Researchers measured the shift across a height of 33 centimetres, about two steps of a staircase, which means a person standing higher is ageing slightly faster. Added up over a 79-year life, the difference comes to roughly 90 billionths of a second.
wormhole
Watching the ground move. Clocks that can resolve about a centimetre of height could follow the swelling of an active volcano or the deformation of the crust. They could also fix the reference level from which every other height is measured.
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