Time dilation seen at just 10 m/s

There’s no reason to think that the lengthening of time predicted by the theory of relativity does not hold even for everyday speeds, but the effect is so minuscule that it has taken two of the world’s most accurate optical clocks to measure it. Trapped at the heart of each clock is an aluminum ion: Displacing the ion in one clock just slightly from the trap’s center induces a relative average speed difference between the two clocks’ ions of 10 m/s (22 mile/hr). The resulting fractional frequency difference is on the order of 10⁻¹⁶. That small shift was measured recently by James Chin-Wen Chou and his colleagues at NIST in Boulder, Colorado, using NIST’s newest optical clock, whose accuracy is 8.6 x 10⁻¹⁸, and a slightly less accurate older clock. The group also measured the frequency difference between the clocks’ ions when one clock was raised by 33 cm relative to the other. The measurements not only demonstrate the high performance reached by optical clocks but also show that they may play an important role in geodesy, the measurement of Earth’s gravitational potential. The time keepers might be sensitive to elevation changes as small as 1 cm if they can attain the current goal of 10⁻¹⁸ accuracy. Such measurements would complement those of satellite-borne instruments, which also have 1-cm sensitivity but average over large areas of Earth’s surface. (C. W. Chou et al., Science 329, 1630, 2010 .)—Barbara Goss Levi