LOS ANGELES — Among the oft-repeated predictions of Albert Einstein's famous theory of relativity is that if a twin travels through the cosmos on a high-speed rocket, when he returns to Earth he will be noticeably younger than the twin who stayed home.
Now physicists have demonstrated that the same is true even if the traveling twin is merely driving in a car about 20 mph. But in that case, when the twin gets home from the grocery store, he is only a tiny fraction of a nanosecond younger, according to a report in Friday's edition of the journal Science.
"During your daily life, you experience relativity," said James Chin-Wen Chou, a postdoctoral researcher at the National Institute of Standards and Technology in Boulder, Colo., who led the experiments. "This (makes) science resonate with regular people."
In the past, the phenomenon known as time dilation has been proven by studying what happens to clocks strapped aboard jets streaking across the sky or perched on rockets above Earth.
But the rules of relativity are always in force, even over the smallest distances and at the slowest speeds. The difference is that the effect is infinitesimal.
In their paper, the NIST team reported that the second hand of a clock positioned about two-thirds of a mile above an identical clock near the Earth's surface will speed up only enough to tick out three extra seconds over the course of a million years.
Chou and his colleagues were able to observe an even smaller time dilation — in clocks separated by a just a foot — because they built a precise timepiece.
Their atomic clock, which fills a large dining room table, works by calibrating the frequency of a laser to that of an aluminum ion. The oscillations of the laser are the equivalent of a traditional clock's ticks, but they occur far more rapidly — over a million billion times per second.
That's about 100,000 times faster than the tick rate of the microwave-based atomic clocks that currently set the time standard in the United States.
"We are able to divide time into finer chunks," Chou said.
In the elevation experiment, the aluminum-based atomic clocks let the researchers measure a difference of approximately 90 billionths of a second over a human's 79-year lifetime.
Sean Carroll, a professor at the California Institute of Technology who studies theoretical physics and cosmology, said the observations of time dilation were "cool, but the underlying advances in technology that let this happen" were more significant.
If improved, Chou said, the clock might be able to help scientists perform a variety of tasks, including measuring the changes in the Earth's gravitational field at different places on the planet.