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Experiment with torsion pendulum
Photo: Tobias Westphal
Quantum physicists have measured the mass attraction between gold spheres measuring just two millimeters.
For their experiment, experts from the University of Vienna used a special pendulum that had to be shielded from external disturbances with considerable effort.
The result was a new record: the weakest proven gravitational force to date.
The experts now hope to track down the quantum effects of gravity with the help of significantly smaller masses.
"Gravitation is the weakest of all known natural forces - and it poses one of the most important open questions in modern physics," write Tobias Westphal and his colleagues in the journal "Nature".
The problem: The naturally always present attraction between two masses has not yet been able to be reconciled with the standard model of particle physics.
Limit quantum physics
This standard model describes all natural forces - such as electromagnetism and the interaction between elementary particles.
Quantum physical effects are also part of this - but the gravitation between masses cannot yet be described or explained on the basis of these.
Physicists are therefore trying to investigate the attraction of ever smaller masses - and ultimately to penetrate into areas in which quantum effects play a role.
There they hope to come across previously unknown phenomena that could ultimately lead to a unified theory of all natural forces.
So far, however, it has only been possible to measure the forces of attraction between masses up to around one kilogram.
In contrast, the gold spheres used in the current experiment weigh just 90 milligrams.
The specialists from Vienna attached the tiny objects to a so-called torsion pendulum.
Experiment with a rotatable pendulum
This consists of a four centimeter long and half a millimeter thick glass rod, which is suspended from a glass fiber with a diameter of a few thousandths of a millimeter.
Gold balls are attached to both ends of the rod.
Very close to one of these spheres is another one - and the force of attraction between these two spheres twists the pendulum.
The twisting of the pendulum wire - called torsion - then balances out this force.
It is not initially to be recorded.
However, if the experts move a third gold ball, the pendulum begins to swing due to its gravitational effect.
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The extremely weak force of attraction between the balls can be determined from the measurement of these vibrations with the aid of a laser beam.
However, this can only be done reliably if no other forces influence the movement.
Problem with pedestrians and trams
"The largest non-gravitational effect in our experiment comes from seismic vibrations that are generated by pedestrians and tram traffic around our laboratory in Vienna," says co-author Hans Hepach.
"We therefore received the best measurement data at night and during the Christmas holidays, when there was little traffic."
Overall, the result of the measurements deviates from the theoretical value by around nine percent.
"But we can fully explain this deviation through the known systematic uncertainties of our experiment," the experts say.
Your goal is now to let the masses of the spheres get smaller and smaller until quantum effects play a role.
There is still a long way to go before then.
Physicists consider a mass of around a hundredth of a milligram to be the critical mass - the so-called Planck mass - for the occurrence of quantum phenomena in gravitation.
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jme / dpa
