Ever since he was a kid growing up in Germany, Holger Müller has been asking himself a fundamental question: What is time? That question has now led Müller, today an assistant professor of physics at the University of California, Berkeley, to a fundamentally new way of measuring time.

Taking advantage of the fact that, in nature, matter can be both a particle and a wave, he has discovered a way to tell time by counting the oscillations（振动） of a matter wave. A matter wave's frequency is 10 billion times higher than that of visible light.

"A rock is a clock, so to speak," Müller said.

In a paper appearing in the Jan. 11 issue of Science, Müller and his UC Berkeley colleagues describe how to tell time using only the matter wave of a cesium atom. He refers to his method as a Compton clock because it is based on the so-called Compton frequency of a matter wave.

"When I was very young and reading science books, I always wondered why there was so little explanation of what time is," said Müller, who is also a guest scientist at Lawrence Berkeley National Laboratory. "Since then, I've often asked myself, 'What is the simplest thing that can measure time, the simplest system that feels the passage of time?' Now we have an upper limit: one single massive particle is enough."

While Müller's Compton clock is still 100 million times less precise than today's best atomic clocks, which employ aluminum ions, improvements in the technique could boost its precision to that of atomic clocks, including the cesium（铯） clocks now used to define the second, he said.

"This is a beautiful experiment and cleverly designed, but it is going to be controversial and hotly debated," said John Close, a quantum physicist at The Australian National University in Canberra. "The question is, 'Is the Compton frequency of atoms a clock or not a clock?' Holger's point is now made. It is a clock. I've made one, it works."

Müller welcomes debate, since his experiment deals with a basic concept of quantum mechanics -- the wave-particle duality of matter -- that has befuddled（使迷惑） students for nearly 90 years.

"We are talking about some really fundamental ideas," Close said. "The discussion will create a deeper understanding of quantum physics."

Müller can also turn the technique around to use time to measure mass. The reference mass today is a platinum-iridium cylinder defined as weighing one kilogram and kept under lock and key in a vault in France, with precise copies sparingly dispersed around the world. Using Müller's matter wave technique provides a new way for researchers to build their own kilogram reference.