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In case you missed it: Using counterpoised lasers and some very sprightly lithium ions, scientists in Germany have recently confirmed, with unprecedented accuracy, what we all somehow suspected—clocks really do run slower when accelerated close to the speed of light (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.113.120405). In a related experiment, scientists also observed a long-elusive spectral line that offers a crucial, previously unavailable tool for testing another fundamental physical theory, strong-field quantum electrodynamics, or QED (Phys. Rev. A, doi: 10.1103/PhysRevA.90.030501).
Special relativity’s “time dilation” prediction holds that clocks moving relative to an inertial reference frame will appear, from that inertial observation point, to tick slower than clocks at rest. For a new test of that long-established principle, the German team used the fluorescent transition frequencies of lithium (Li+) ions—accelerated to one-third the speed of light in the Experimental Storage Ring (ESR) in Darmstadt—as the moving “clock” in the experiment. The scientists then aimed two lasers at the moving ion stream, one pointing in the direction of ion motion and the other pointing in the opposite direction. The lasers act as an energy source to stimulate the moving ions to fluorescence, which in turn is measured using photodetectors.
The team tuned the frequency of each of the two lasers until the fluorescence signal from the ions reached its maximum, an indication that each laser was at the resonant frequency for the moving lithium ions. The combination of the frequency shifts of the two lasers, relative both to each other and to the known resonance frequency of the Li+ transition at rest, agreed with the time dilation prediction to two parts per billion. That’s approximately four times as accurate, according to the researchers, as in previous experiments—a result that should hearten Einstein’s ghost.
A new tool for testing strong-field QED
Scientists from the same lab used a similar setup at the ESR to make an observation that could advance high-precision tests of another fundamental theory: QED. While generally extremely well established, QED still has some untested predictions in the large magnetic fields that dominate around heavy atomic nuclei. One reason has been the long-standing problem of getting some actual observables against which to test the theory—in particular, the energies of specific electron transitions in “lithium-like” ions that are sensitive to QED effects.
To nail down one such parameter, the research team accelerated ions of bismuth-80 (Bi80+) to more than 70 percent of the speed of light, used lasers to excite the moving ions, and adjusted the laser frequency to coax out an observation of the M1 hyperfine transition spectral line. It was the first observation of that spectral line in a “lithium-like” ion using laser spectroscopy, after multiple efforts spanning 13 years. And, according to the team, it provides a new reference experimental value to use in testing as-yet-unverified predictions of QED theory in the strong magnetic field of a highly charged, heavy nucleus.