New ideas in low-energy tests of fundamental physics
A popular alternative to dark energy in explaining the current acceleration of the universe discovered with type Ia supernovae is modifying gravity at cosmological scales. But this is risky: even when everything is well for cosmology, other fundamental and experimental aspects of gravity must be checked in order for the theory to be viable. The successes of modified gravity and its challenges, which have generated a large body of literature in the past ten years, will be reviewed.
Precision atom interferometry is poised to become a powerful tool for discovery in fundamental physics. Towards this end, I will describe recent, record-breaking atom interferometry experiments performed in a 10 meter drop tower that demonstrate long-lived quantum superposition states with macroscopic spatial separations.
I will discuss experiments we are conducting for precision tests of gravitational physics using cold atom interferometry. In particular, I will report on the measurement of the gravitational constant G with a Rb Raman interferometer, and on experiments based on Bloch oscillations of Sr atoms confined in an optical lattice for gravity measurements at small spatial scales and for testing Einstein equivalence principle.
Satellite geodesy uses the timing of photons from satellites to determine the Earth’s time varying shape, gravity field, and orientation in space, with accuracies of 100 seconds, corresponding to speeds
I will discuss present limits on the variation of the fine structure constant and the electron to proton mass ratio from the astrophysical data on the spectra from the interstellar gas medium. The emphasis will be made on the infrared and microwave spectra. Such spectra may be 2 - 3 orders of magnitude more sensitive to the variation of constants than optical spectra.
Official U.S. time is currently realized by an ensemble of commercial cesium-beam atomic clocks and hydrogen masers. Cesium-fountain devices presently serve as ultimate frequency references and help to define the SI second. The present quandary is: these microwave-based standards are rapidly becoming outmatched by new optical atomic frequency references---by a factor of 1,000 in stability, and perhaps a factor of 100 in accuracy. I will survey the ongoing optical atomic clock projects at NIST and highlight related work in optical time and frequency measurement and transfer.
We report frequency comparison of two Sr optical lattice clocks operated at cryogenic temperature to dramatically reduce blackbody radiation shift. After 11 measurements performed over a month, the two cryo-clocks agree to within (-1.1±1.6)×〖10〗^(-18).
Current status of a frequency ratio measurement of Hg/Sr clocks and a remote comparison of cryo-clocks located at Riken and University of Tokyo will be mentioned.
The precision of atomic clocks continues to improve at a rapid pace: While caesium clocks now reach relative systematic uncertainties of a few 10-16, several optical clocks based on different atomic systems are now reported with uncertainties in the 10-18 range. This variety of precise clocks will allow for improved tests of fundamental physics, especially quantitative tests of relativity and searches for variations of constants. Laser-cooled and trapped ions permit the study of strongly forbidden transitions with extremely small natural linewidths and long coherence times.