"We were interested in what the simplest clocks are to explore the question of what time is," said researcher Holger Muller, a physicist at the University of California at Berkeley. "If you say that, say, you can't measure time with less than two particles, does that mean that anything below two particles doesn't experience time at all?"
The researchers theorized it was possible to create a clock made up of just one particle. To understand, one starts with Einstein's famous equation E=mc2, which showed that matter can be converted to energy and vice versa. One consequence of this, called de Broglie's matter-wave hypothesis, suggests that matter can also behave like waves. As such, a particle of matter can in principle behave like a wave that oscillates in a regular manner, thus acting like a clock.
"We've shown that one single particle really can measure time," Muller told LiveScience.
The problem with making a clock from a particle of matter is that the frequency at which it oscillates "should be so high that one should never be able to measure it," Muller said. To get over this hurdle, the scientists relied on a phenomenon known as time dilation, another consequence of Einstein's theory of relativity. This suggests that as objects move away from and back to a location, they experience less elapsed time than objects that stayed at that location the entire time.
The researchers recreated this phenomenon using lasers on cesium atoms. "We essentially split an atom into two halves, and had one stay where it is and the other go forward and come back," Muller said. "A tiny, tiny bit less time elapsed for the half that moved, so it oscillated less."
The fact that one half of the atom oscillated less than the other meant that when these halves are reunited, they did not recombine perfectly, but interference occurred that the scientists could measure. By knowing the size of this discrepancy and the extent to which the researchers disturbed the atom, the researchers could deduce the original frequency at which the atom oscillated.