Focus: Testing Relativity Using Earth’s Motion
Many theories that go beyond the standard model of particle physics, such as string theories, predict violations of relativity. Now a team has searched for two types of such violations at an unprecedented precision using a new technique involving the element dysprosium, as they report in Physical Review Letters. The team looked for long-term variations in the energy of a specific atomic transition and found no violations. But they say refinements of their experiment may eventually lead to measurements times more precise than current experiments—opening up a new avenue to probe for signs of physics beyond the standard model.
Two of the pillars of 20th century physics, general relativity and the standard model of particle physics, rely on some core assumptions about the nature of space and time. For example, one aspect of so-called Lorentz invariance is that the speed of light is universal and is the maximum attainable speed (MAS) for objects traveling in any direction. If this speed limit were lower in one direction than in others, an object traveling in that direction would require more energy to be accelerated to a given speed than it would in other directions. So traveling in that direction at a given speed would represent a larger kinetic energy. And if another principle of relativity called local position invariance were violated, then the kinetic energy would also depend on the object’s location in a gravitational field. Any violations of these principles may hint at phenomena explained only by theories beyond the standard model.
Previous searches for such violations have included attempts to detect whether the speed of light changes with direction and studies of radiation emitted by high-energy electrons from space, among many others. Now Michael Hohensee of the University of California, Berkeley, and his colleagues have used a new approach: They measured the transition energy between two electron energy states (which they call and ) of dysprosium and looked for any variation in its value due to the earth’s orientation, position, or direction of motion, during a period of two years.
The team chose dysprosium because it has a pair of closely-spaced energy states that involve orbitals where electrons travel at very different speeds. Because of this speed difference, a change in the electron’s kinetic energy due to a change in the atom’s orientation would affect the two states very differently. The researchers illuminated a beam of dysprosium atoms with two laser beams to excite them to state (via another state) and then drove the transition to with a precisely calibrated microwave beam. To measure the transition energy, they found the microwave frequency most effective at driving the transition. The team repeated the experiment many times over a period from 2010 to 2012.
The orbitals of the atoms were oriented to some extent by the polarization of the exciting laser beams. So if the electrons’ kinetic energy depended on their direction of motion, the team would have seen a daily oscillation in the transition energy from the earth’s rotation. Similarly, if there were any effect from the earth’s position in the sun’s gravitational field (violating local position invariance), there would have been an annual oscillation.
There are many different ways to violate Lorentz invariance, so researchers in the field have developed a standard set of parameters to characterize different types of violations. Hohensee and his colleagues measured eight of the nine parameters that describe any dependence of the electron’s maximum attainable speed on the speed and direction of the lab’s reference frame. They significantly improved limits from previous experiments for four of them, one by a factor of 10. Their new limits on local position invariance for electrons are 160 times more precise than previous ones.
But the most significant advance isn’t the measurements themselves—it’s the new technique using dysprosium, says Alan Kostelecky of Indiana University in Bloomington. The experiment wasn’t optimized (it was originally set up for another purpose), so with more data and a better-tuned apparatus, the researchers say they may be able to reach precisions of one part in , 1000 times better than their current results. “The prospective improvements are sufficiently dramatic that they offer a real discovery potential,” says Kostelecky.
Marcus Woo is a freelance science writer in the San Francisco Bay Area.