Lorentz symmetry is a cornerstone of modern physics and lies at the foundation of quantum field theory (QFT) and Einstein's theory of general relativity, the two most successful theories in physics which together describe the four fundamental forces of nature. However, the inability to incorporate gravity as describe by general relativity into the QFT standard model of partical physics, which very successfully combines the electromagnetic, strong and weak interactions, has lead to the developement of alternative so-called Grand Unified Theories (GUTs) or theories of quantum gravity. Since many of these theories break Lorentz symmetry at some small level, experimental searches for Lorentz-violating effects could help shed light on new physics beyond the standard model and provide clues as to the nature of quantum gravity. Parameterization of such effects within the Lorentz-violating Standard Model Extension (SME) developed by Alan Kostelecky has allowed direct comparison of many experiments, ranging from table-top precision measurements to observations of ultra-high energy cosmic and gamma rays to astrophysical observations.
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We use ultra-high precision techniques involving polarized atomic spin to test Lorentz symmetry. The presence of Lorentz violation would appear as an effective field felt by the atoms. Presumably, this field acts as a cosmically fixed background which, from the point of view of our Earth bound experiment, fluctuates with a sidereal period as the Earth rotates in it.
An alkali metal-noble gas co-magnetometer is used in our expereiment to very sensitively measure fields which couple to atomic spin while suppressing magnetic field interactions. The following simulation demonstrates magnetic field cancelation in a potassium - helium-3 co-magnetometer. During the transients, the polarized helium atoms align to an applied magnetic field, thereby cancelling the effect on the potassium atoms which return to their original position.
Very stringent limits on rotation and boost Lorentz violation has already been determined using measurements conducted in Princeton with this experimental setup . However, a significant systematic effect limiting further improvements is the gyroscopic pick-up of the Earth's rotation. The effective signal measured by the atoms is more than 10,000 times larger than the measured Lorentz violating limit. In order to overcome this systematic effect, we have moved our appartus to the Amundsen-Scott South Pole Station. At the South Pole, the effects of the Earth's rotation are almost completely suppressed, and it is anticipated that our precision can be improved by at least two orders of magnitude.
The apparatus was installed in the Cryo facility at the Amundsen-Scott South Pole Station during January through mid-February 2013. Pictured with the apparatus below is Postdoc Marc Smiciklas(right) and the wintering over Research Associate Andrew Vernaza(left). The apparatus is currently operational and collecting data over the Austral Winter.
Following is a comparision of the gyroscopic pick-up of the Earth's rotation measured in Princeton and at the South Pole.
Please refer to the following slides and papers for more information.