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Measuring gravity with an atomic fountain

What Galileo did with cannonballs has now been done with atoms. By watching atoms fall, Achim Peters and colleagues at Stanford University, Stanford, California have been able to measure the pull of gravity with unprecedented accuracy.

Whether or not Galileo did indeed drop his lethal projectiles from the Leaning Tower of Pisa, he was the first to challenge Aristotle?s dogma that heavy objects fall faster than light ones. Galileo showed that they all fall at the same rate (if air resistance is negligible), increasing their speed each second by nearly ten metres per second. This ?acceleration due to gravity? is determined by the Earth?s mass. On a more massive planet, gravity would pull more strongly.

Physicists have since improved many times on the accuracy of Galileo?s experiments; but the strength of gravity is still among the worst-characterized of nature?s fundamental quantities. And there are several reasons for wanting still greater precision. For example, a better quantification of gravity might enable very accurate tests of Einstein?s theory of general relativity.

Now Peters? group report in the 26 August issue of Nature that they have measured the acceleration induced by gravity to an accuracy of three parts in a billion - equivalent to measuring the width of Ireland from east to west with an accuracy of within one millimetre. They have done so by creating a fountain of caesium atoms and watching them fall.

It has to be about the coldest fountain in the world. The effective temperature must be just ten billionths of a degree above absolute zero to enable the high-precision measurements to be made - if the atoms were warmer and jiggling about more, crucial information would be washed out. Peters? team cooled the atoms using a technique called ?laser cooling?. They surrounded the atoms with lasers, whose intense beams created an ?optical trap? at the point where they crossed. Within this light field the atoms were like flies trapped in molasses - barely able to move in any direction.

This ultracold pool of caesium atoms was then propelled upwards out of the optical trap by another laser beam so that they could fall gracefully under the Earth?s gravity.

Because they are so small, atoms can exhibit quantum-mechanical wave-like behaviour - but the consequences of this get smeared out unless the atoms are very cold. In the frigid fountain, the upward-travelling ?atom wave? can interfere with the downward-travelling wave, and the interference can be detected by probing the slow-moving jet with two successive pulses of laser light.

So detecting the interference pattern is a little like throwing a ball in the air with a certain velocity and calculating the strength of the gravitational field by measuring how long it takes to fall back into your hand. But the atom-fountain method is many times more accurate. The researchers were even able to see their measurements rise and fall with the tides, reflecting the change in the gravitational field owing to the motion of the Moon.

And to check that the atoms were behaving just like falling balls, the researchers used a standard instrument to make a very accurate measurement of the gravitational field as experienced by everyday objects. The results coincided with those from the atom fountain. So they should, you might say - but it is not obvious that quantum particles should experience gravity in the same way as cannonballs, and previous comparisons have suggested that indeed there might be some difference. The new results show that such a discrepancy does not seem to be fundamental, but an artefact of the earlier experiments.

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Ball, P. Measuring gravity with an atomic fountain. Nature (1999). https://doi.org/10.1038/news990826-1

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