Gravity tested with quantum spin

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Italian astronomer and scientist Galileo Galilei performs his legendary experiment – dropping a cannonball and a wooden ball from the top of the Leaning Tower of Pisa, circa 1620.
Credit: Hulton Archive/Getty Images

A new experiment, based on measuring the free fall of rubidium atoms in a vacuum, confirms that atoms of different quantum spin experience identical acceleration due to gravity. 

The result is important in the quest to unify general relativity with quantum mechanics, and may already rule out some proposed theories of quantum gravity.

Resolving the mismatch between the two great pillars of physics – the theory of gravity, and the theory of matter – is probably the grandest challenge in contemporary physics.

Einstein’s theory of general relativity tells how gravity arises from mass bending space and time. The theory describes the universe on the largest of scales, from the orbits of the planets to the rotation of galaxies to the Big Bang itself.

Quantum mechanics, on the other hand, describes the microscopic world of particles and how they join together to make the matter around us.

The problem is these two ideas don’t seem to mesh. The very large and the very small seem to play by different rules.

Now, a new breed of experiments is allowing physicists to measure the force of gravity at the scale of quantum objects and so test, for the first time, some of the theories proposing to bridge the chasm between gravity and quantum mechanics.

In the new work, a team of Chinese scientists from Huazhong University of Science and Technology in Wuhan has compared the acceleration of rubidium atoms due to gravity and found it to be identical regardless of the orientation of the atom’s spin.

This research is published in Physical Review Letters.

At heart, this experiment is a test of the equivalence principle, which says the acceleration due to gravity is identical for any object.

Tests of this principle have been performed in various guises over the centuries, from renaissance Europe to the surface of the moon.

One of the most famous images in all of physics is that of the Italian scientist Galileo Galilei, atop the leaning Tower of Pisa, letting go of two metal balls of different masses to show they fell at the same rate. Although this account may be apocryphal, Galileo certainly did describe an experiment rolling balls down a slope, which showed the same thing.

And in 1971, Commander David Scott famously tested the equivalence principle by dropping a hammer and feather at the same time, while standing on the moon.

Although the equivalence principle is central to general relativity, many quantum theories of gravity, which attempt to describe gravity using quantum mechanics, predict that the equivalence principle could be violated. 

In particular, some quantum properties, such as the spin of an atom, might affect free fall the theories say.

To test this, the Chinese team, led by physicist Zhong-Kun Hu, set up an intricate experiment, which measured the rate of free fall of atoms of rubidium.

The experiment is based on atom interferometry, which exploits the wave nature of atoms to monitor their motion extremely precisely.

First, the team isolated and cooled a collection of rubidium atoms to few millionths of a degree above absolute zero.

The atoms started out at the bottom of a tube that had been emptied completely of air.

The team then pointed a laser beam from below and using the light to give the cold atoms a kick, propelling them upwards in the tube. But what goes up, must come down. This set up a “fountain” of atoms, rising and falling.

The scientists found that the free fall acceleration of the rubidium atoms with opposite spins agreed to within one part in 10 million.

In the past decade, similar experiments have already verified universality of free fall for different atoms, and for different isotopes of the same element.

But this is the first time gravity has been tested in terms of quantum spin. It means that several exotic theories which had predicted a significant interaction between quantum spin and gravity will have to be modified, or thrown out.

Back to the drawing board.

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