Albert Einstein's assertion（断言，声明） that there's an ultimate speed limit -- the speed of light -- has withstood countless tests over the past 100 years, but that didn't stop University of California, Berkeley, postdoc Michael Hohensee and graduate student Nathan Leefer from checking whether some particles break this law. The team's first attempt to test this fundamental tenet（原则，信条） of the special theory of relativity demonstrated once again that Einstein was right, but Leefer and Hohensee are improving the experiment to push the theory's limits even farther -- and perhaps turn up a discrepancy（矛盾，相差） that could help physicists fix holes in today's main theories of the universe.

"As a physicist, I want to know how the world works, and right now our best models of how the world works -- the Standard Model of particle physics and Einstein's theory of general relativity -- don't fit together at high energies," said Hohensee of the Department of Physics. "By finding points of breakage in the models, we can start to improve these theories."

Hohensee, Leefer and Dmitry Budker, a UC Berkeley professor of physics, conducted the test using a new technique involving two isotopes of the element dysprosium. By measuring the energy required to change the velocity of electrons as they jumped from one atomic orbital to another while Earth rotated over a 12-hour period, they determined that the maximum speed of an electron -- in theory, the speed of light, about 300 million meters per second -- is the same in all directions to within 17 nanometers per second. Their measurements were 10 times more precise than previous attempts to measure the maximum speed of electrons.

Using the two isotopes of dysprosium as "clocks," they also showed that as Earth moved closer to or farther from the sun over the course of two years, the relative frequency of these "clocks" remained constant, as Einstein predicted in his general theory of relativity. Their limits on anomalies in the physics of electrons that produce deviations from Einstein's gravitational redshift are 160 times better than previous experimental limits.

The UC Berkeley physicists and colleagues at the University of New South Wales in Sydney, Australia, who provided crucial theoretical calculations, published their results this week in the journal Physical Review Letters.