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.
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 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 sea level.
"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 proved by studying what happens to clocks strapped aboard jets streaking across the sky or perched on rockets miles 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 infinitesimally small.
In its paper, the team reported that the second hand of a clock positioned about two-thirds of a mile above an identical clock near 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 would fill 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 — more than a million billion times per second.
That's about 100,000 times as fast as 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.
Chou said that three developments allowed this precision: recent improvements in laser technology; the development of optical frequency combs, which count a laser's "ticks"; and advances in quantum information science, which make it possible to use the extremely stable aluminum ion to calibrate the laser.
Caltech professor Sean Carroll, 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. "I don't know what the applications will be," he said, "but they will be crucial."
If improved further, Chou said, the clock might be able to help scientists perform a variety of tasks, including measuring the changes in Earth's gravitational field at different places on the planet.
Even if the work does not break any ground in our understanding of fundamental physics, it remains the kind of elegant feat that can make scientists swoon.
"This is beautiful!" said Drexel University physics professor Dave Goldberg, who, like Carroll, was not involved in the experiments.
"Einstein was right once again!" Goldberg said. "This is a beautiful testament to the precision that we're capable of."