Physics · the experiment
Read the tide in time
Optical clocks 800 km apart, a fibre linking them, the Moon and the Sun deforming the Earth. Four questions, from beginner to expert, on what this network measured.
1 · Say what the clocks tracked
Beginner
What did these fibre-linked optical clocks track for the first time?
2 · Turn stability into height
Raised by one metre, a clock changes its rate by about 1 part in 10¹⁶. To follow the tides, a comparison must be stable to about 1 × 10⁻¹⁸ within a few hours.
Intermediate
What height difference does a stability of 1 × 10⁻¹⁸ correspond to?
3 · Explain two different tides
Between Turin and Braunschweig, the tide differs mainly in strength. Between Teddington and Braunschweig, mainly in timing.
Expert
Where does this difference between the two pairs come from?
4 · Predict the null test
Two clocks side by side in Turin: their tidal difference is about 10⁻²². The filter is nevertheless told to look for the tide of an 800 km baseline.
Expert
What should the filter find if the analysis is sound?
Tide read
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