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 that stayed home.

Now physicists have demonstrated that the same is true even if the traveling twin is merely driving in a car about 20 miles per hour. 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.

The reverse is often said to be true for a twin who spends time high on a mountaintop — general relativity predicts that time passes more quickly at greater altitudes because objects don’t feel the Earth’s gravity quite as strongly. But the physicists found that a twin who lives just about a foot above sea level will age ever-so-slightly faster than a twin living at the beach, too.

“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 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.