Relativity on the human scale

Updating Einstein’s Universe and Magic Universe

Relativity on the human scale

The most gratifying physics I’ve seen for a while comes in today’s Science magazine, from James Chin-Wen Chou and his colleagues in the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colorado. They detect well-known effects of relativity on the rate of time passing, but now on the scale of ordinary human activities.

Standard atomic clocks employ microwaves to ensure their regularity, but Chou’s team used laser light in a pair of aluminium-27 optical clocks (invented in 2005), which gives about 100 times better accuracy. In one experiment, they used an electric field to jiggle the aluminium ion at the heart of a clock and showed that time passed more slowly in accordance Einstein’s Special Relativity theory, about the effect of motion on time. The effect of atomic motion as slow as 8 metres per second (about 30 km/h) was detectable.

Raising a clock makes it run a little faster. Credit: Chou et al., Science, 24 September 2010 – see reference.

Especially pleasing for me was another experiment, in which one clock was jacked up just 33 cm relative to the other. The clock gaining height ran faster because it was further from the Earth’s centre of gravity, and the gravitational field was slightly weaker, in accordance with General Relativity. As the change in clock rate was only about 40 parts in a billion billion (1018), its detection was a tour de force for the NIST team.

This effect of altitude on time was the key to the efforts by Martin Freeth of BBC-TV and me to make Einstein’s theory of gravity, General Relativity, comprehensible to the public, in our film “Einstein’s Universe” (1979).

At a critical stage in the film, after dealing with gravity’s effects on time near the Sun and black holes, we came back to Earth. Archival footage showed a US naval aircraft flying around and around Chesapeake Bay in 1975-76, carrying an ensemble of three caesium and three rubidium atomic clocks. Laser flashes compared the time it recorded with the time-keeping of an identical ensemble of clocks on the ground.

Here’s a fragment from the ensuing transcript, where we used the confirmation of gravity’s effect on time to explain to Peter Ustinov – our “intelligent layman” — why, in Einstein’s theory, Newton’s apple fell to the ground.

CALDER: The main effect on time related to the aircraft’s height. At 35,000 feet the airborne clocks gained about three billionths of a second every hour, and each flight lasted about 15 hours. Five flights like that accurately confirmed the effect of gravity on time. So Einstein’s account of how the world works triumphs yet again.

EINSTEIN (spoken by Peter Ustinov): “To punish me for my contempt for authority, fate made me authority myself.”

CALDER (holding an apple as he speaks to Ustinov in the McDonald Observatory garden): What’s true of atoms in atomic clocks is also true of atoms in ordinary objects, like an apple. And perhaps we could draw some of these threads together by telling how, in a timeshell, starting at the top of a tree and moving into a timeshell lower down, an apple manages to accelerate in the way that’s so familiar. (continuing over animation) It’s moving into shells, very fine shells, of ever-slowing time. Its atoms are operating more slowly. It seems to be losing internal energy which has to reappear in some new form, and the form it takes is energy of motion. So the apple is going faster and faster as it moves down into slower and slower zones of time, until finally it hits the ground and that energy of motion is destroyed.

USTINOV (in the garden): Well there are just, Nigel … just two little points I’d like to clarify before we all go further into this adventure. It seems to me that the apple has acquired such a particular status with Newton that perhaps one ought to realise, for uninitiated agriculturalists, that pears and grapes and in fact people are subject to the same laws. That pears are not exempt.

CALDER: Exempt from the action of gravity? Well certainly not, because especially since Einstein the emphasis in present thinking is that gravity affects everything in just the same way, and in the case of people our atoms also are affected in their rate of operation, according to whether we’re living down in the valley or up on the mountain.

The update for my book Einstein’s Universe, which accompanied the TV show, belongs in Afterword 2005: The Melting Pot, after another update about Gravity Probe B, a Stanford space experiment devised to detect the dragging of spacetime by the rotation of the Earth.

Two updates for Einstein’s Universe

Technical and analytical difficulties delayed the results from Gravity Probe B, but by 2009 the team was able to report that the Earth’s rotation drags around the fabric of spacetime in the planet’s vicinity at roughly 80 milliarcseconds per year, compared with 75 predicted by General Relativity – which is all but a bull’s-eye, given the limits of experimental error. The predicted rate, by the way, translates into one complete rotation of spacetime around the Earth in 1.6 million years.

By 2010, the basic relativistic effects of motion and gravity were detectable on a human scale, using atomic clocks of astonishing precision. Chin-Wen Chou and his colleagues at the US National Institute of Standards and Technology reported that they had detected the slowing of time for an atom moving at 30 kilometres an hour, and the speeding of time for one atomic clock raised just 33 centimetres relative to another. This corresponds neatly with the description of the apple on the tree, waiting to fall, given at the end of Chapter 7.

Update for Magic Universe

A paragraph in the story “Gravity: did Uncle Albert really get it right?” reads:

Space and time are neither cartographic abstractions nor God-given absolutes, but the pliable carriers of the force of gravity that Newton craved. On the Earth, gravity is slightly stronger at the foot of a tree than in the branches, which are farther from the planet’s centre. Time and light both travel more slowly for a fallen apple than they did when it hung from the tree, so it possesses less inherent mass-energy. Where did the energy go? By moving downwards, in the direction of increasing gravity, the apple was able to convert it first into energy of motion and then into a bruising thud.


The faster passage of time up in the apple tree was beautifully confirmed by experiments with improved atomic clocks at the US National Institute of Standards and Technology, reported in 2010. These confirmed a detectable acceleration for one clock lifted a mere 33 centimetres relative to another clock.


C.W. Chou, D.B. Hume, T. Rosenband and D.J. Wineland , ‘Optical Clocks and Relativity,’ Science, 329, 1630, 2010

Einstein’s Universe” produced by Martin Freeth of BBC-TV and written by Nigel Calder, BBC and WGBH, 1979

N. Calder, Einstein’s Universe, 2005 edition, p. 176, Penguin, 2005

N. Calder, Magic Universe, pp. 352-3, Oxford UP, 2003


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