Look carefully, and you will see clocks all around you: The plant wilting on your desk tells you that a week has passed since you watered it; the setting Sun tells you that another day has gone by; your smartphone tells time with its GPS-calibrated clock set by pinging satellites in space. Although we tend to experience time on the scale of seconds to years, scientists have developed ways to measure time at the level of 1 part in 1018 by using optical atomic clocks, which determine time by probing atomic resonances with lasers. That’s equivalent to measuring the distance from Earth to the Moon with an uncertainty of less than half a nanometer. Such extremely precise clocks offer new ways for physicists to, for example, test general relativity, search for dark matter, and probe variations in fundamental constants.

To do many of those experiments, researchers look for unexplained...

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