Published online 23 September 2010 | Nature | doi:10.1038/news.2010.487

Corrected online:

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As Einstein predicted, a slow drive or a step up a ladder is enough to warp time.

Clocks run slower as speed increases, and faster as gravity weakens. W. Tiller/iStockphoto

Albert Einstein's theories of relativity, which predict that relative speed and gravity affect the passing of time, have never been easy to bring home to the general public. In the early 1970s, scientists demonstrated relativity by putting synchronized atomic clocks on jumbo jets that flew eastwards and westwards around Earth. The westbound plane — the one flying against Earth's rotation — gained time compared with a fixed reference clock on the ground. But this wasn't exactly an everyday scenario.

Chin-wen Chou and his colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, have now demonstrated Einstein's theories on more mundane scales. In tests of the special and general theories of relativity, the NIST researchers show that time speeds up if you climb just one rung up a ladder, and slows down if you travel at just 36 kilometres per hour. Their results are reported in Science this week1.

Holger Müller, a physicist at the University of California, Berkeley, says that the study shows that relativity is no longer confined to experiments working with huge speeds and distances. "This is mainly a grand technological feat, but has an almost philosophical component," he says. "It shows that relativity is something tangible."

The NIST researchers' study depends on its clocks, which are among the most accurate in the world. Standard atomic clocks, which have been around since the 1950s, pass microwaves of a specific frequency onto atoms, which respond by jumping to a higher energy level. The signal from this jump feeds back to the microwave source, prevents its frequency from deviating, and so keeps the 'ticks' of the clock accurate to one part in 1015 or more.

Chou and his team used an optical clock invented in 2005. This uses laser light, which has a frequency some 100,000 times higher than microwaves. Optical clocks are thus tens or hundreds of times more accurate than microwave clocks — NIST's loses less than one second in three billion years.

Playing with time

General relativity states that time speeds up for objects as gravity weakens. To demonstrate this, Chou and his colleagues raised one optical clock 33 centimetres above another. The slightly lower gravity at that height meant that compared with the reference clock, the raised clock ticked with a fractional boost in frequency of 4 × 10–17, equivalent to a gain of 90 billionths of a second over 79 years.

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To demonstrate special relativity, which says that time slows down for moving objects, the researchers jolted the single atom in their optical clock so that it oscillated at relative speeds of less than 10 metres per second, or 36 kilometres per hour. This time, the clock's ticks seemed to drop by a fractional frequency of almost 6 × 10–16.

Chou says that his group's work should not be compared with previous more accurate experiments that have specifically tried to test relativity's predictions. But, he says, it "reminds people that the effects of relativity are actually experienced in their daily lives, not just by scientists".

The ability to detect small changes in gravity could have applications in geophysics and hydrology. According to Gerald Gwinner, a physicist at the University of Manitoba in Winnipeg, Canada, this might help scientists to track how mass — for example, in the form of water — is redistributed owing to climate change. "They could build a network that checks in real time the level of gravity around the globe," he says.

Corrected: The penultimate paragraph has been corrected to reflect that previous experiments tested the theory of relativity with more accuracy, but on larger scales.