Physicists Have Measured The Smallest Gravitational Field Yet

Artist impression of an exaggerated gravitational space-time warp around the small gold ball used in this experiment. The ball sits onto a eurocent. Image Credit: Arkitek Scientific

Scientists at the University of Vienna have now measured the smallest known gravitational field. As reported in Nature, researchers have measured the gravitational pull of two gold spheres about 2 millimeters (0.079 inches) in diameter with a mass of around 90 milligrams (0.003 ounces). This phenomenal work has opened a new window in humanity’s understanding of gravity.

Newton's interpretation of gravity is still useful in a lot of places. Einstein’s work has pushed the envelope even further, and it has been proved correct in explaining this force in the most extreme environments – such as around black holes or merging neutron stars. But what physicists haven’t managed to do yet is combine gravity with quantum mechanics.

A possible answer to this could come from studying the gravity of very small things – but this is easier said than done. Previous experiments mostly focused on measuring the gravitational interactions between large masses, such as the Earth, and tiny masses. This is the first example of gravitational field measurement between two objects with very small masses.

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“Looking for the gravitational field of small masses is quite interesting because we don’t actually know how and if the laws of gravitation actually hold for them,” co-author Hans Hepach told IFLScience.

The researchers explained that it is currently unknown if and where gravity behaves differently, although some theoretical work suggests that values around the Planck mass (22 micrograms, or the mass of a flea egg) should already show some deviation from the standard laws of gravity. This work is not probing such small masses yet, but the researchers light-heartedly point out that “you need to start somewhere.”

“If theories such as dark matter and dark energy have some truth to them, then at some level and at least at very short distances the regular laws [of gravity] as we know them, would have to be modified,” senior author Professor Markus Aspelmeyer told IFLScience.

Torsion setup to measure gravity
Gravity is measured between two gold masses that are brought close to each other using a torsion pendulum. Image Credit: Tobias Westphal, University of Vienna

This important milestone in studying gravity could not have been possible without an incredible experimental setup. Gravity is very very weak – we experience it as a strong force because the Earth is huge compared to us. However, in absolute terms, it is over a trillion trillion trillion times weaker than the electromagnetic force.  

So measuring this tiny attraction between small things requires tackling many sources of experimental noise. These vary from electrostatic forces to seismic tremors, and even standing too close to the experiment had an effect. The researchers add that the effect of runners taking part in the Vienna marathon was also picked up by the experiment.

It was difficult to overcome these challenges, but the team is already taking new ones on board.

“We are already working on our next measurement, where we will reduce the mass by a factor of one thousand” co-author Jeremias Pfaff explained to IFLScience.

This will require understanding even better how environmental influences affect the experimental setup so that the effects of gravity can be accurately isolated.

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