Atomic clocks in space


Experiments in space offer the opportunity of performing very sensitive searches for relativity violations. Among the possibilities are experiments on a satellite comparing the rates of two atomic clocks as their orientation changes. Traditional Earth-based experiments comparing atomic clocks (see the next animation) are sensitive only to effects depending on the Earth's rotation axis and 24-hour period, which are fixed. Conducting experiments in space allows much greater flexibility in the rotation axis and periodicity, as well as access to higher speeds in some cases. Future missions on satellites or the International Space Station could compare atomic transitions with enough sensitivity to detect possible minuscule violations of Lorentz and CPT symmetry.

The orbit of a satellite is illustrated in the first animation. The Earth is seen to move about the Sun along the green ellipse. The satellite is represented by the white square orbiting the Earth. Perturbations cause the satellite orbit to precess, so the motion isn't a simple ellipse relative to the Earth. Note that the periods of the satellite and Earth motions are not to scale.

The white arrow shows the satellite orientation and hence the orientation of the atomic clock. The red arrows represent the quantities (technically, the vacuum expectation values) that produce a directionality for empty space. As the satellite orbits the Earth, the orientation of the atomic clock changes relative to the red arrows and so the clock's rate changes (see the second animation). The straight white lines emerging from the Sun represent the axes of a Sun-based system of celestial-equatorial coordinates (a standard coordinate system for reporting experimental results).

The effect of Lorentz and CPT violation on an atomic clock placed in orbit is illustrated in the second animation. It shows an atom and some of the atom's energy levels, all part of an atomic clock on a space satellite.

The atom is colored in proportion to the frequency of light given by the level separation. As the satellite orbits the Earth, the orientation of the atomic clock changes. This modifies the level spacings of the atom, which produces a different frequency of light and hence a different rate for the atomic clock. Beats can be seen in the cyclic variation of the clock rate. These result from a combination of the orbital frequency of the satellite and a double-frequency component arising from the high speed of the satellite.
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