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
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