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Vienna quantum physicists measure the smallest gravitational force to date
Scientists have demonstrated gravitational coupling between two gold spheres of one millimetre radius, thereby entering the regime of sub-100- milligram sources of gravity
R
esearchers at the University of Vienna and the Austrian Academy
of Sciences have measured the gravitational field of a gold sphere just two millimetres in diameter, using a highly sensitive pendulum. According to the researchers, this is the smallest gravitational force ever measured.
The experiment opens up
new possibilities for testing the laws of gravity on previously unattained small scales. The results were published in March in Nature. Gravity is the weakest of all
known forces in nature – and yet it is most strongly present in our everyday lives. In a vacuum, all objects near the Earth’s surface fall with
28 Electro Optics April 2021
the same acceleration: their velocity increases by about 9.8 m/s every second. The strength of gravity is determined by the mass of the Earth and the distance from the centre. On the Moon, which is about 80 times lighter and almost four times smaller than the Earth, all objects fall six times slower. On a planet of the size of a ladybird, objects would fall 30 billion times slower there than on Earth. Gravitational forces of this magnitude normally occur only in the most distant regions of galaxies to trap remote stars. Testing gravity at all scales is an important experimental endeavour. So far, these tests have mainly involved macroscopic masses at the kilogram scale and beyond.
A team of quantum physicists
led by Markus Aspelmeyer and Tobias Westphal of the University of Vienna and the Austrian Academy of Sciences have demonstrated gravitational coupling between two gold spheres of one millimetre radius, thereby entering the regime of sub-100-milligram sources of gravity. To do so, the researchers
drew on a famous experiment conducted by Henry Cavendish at the end of the 18th century. During the time of Isaac
Newton, it was believed that gravity was reserved for astronomical objects such as planets. It was not until the work of Cavendish (and Nevil Maskelyne before him) that it was possible to show that
“Testing gravity at all scales is an important experimental endeavour”
The tiny pendulum is suspended from a thin glass fiber and feels the gravitational force of the millimeter-large gold ball
objects on Earth also generate their own gravity. Using a pendulum device, Cavendish succeeded in measuring the gravitational force generated by a lead ball 30cm tall and weighing 160kg in 1797. A so-called torsion pendulum – two masses at the ends of a rod suspended from a thin wire and free to rotate – is measurably deflected by the gravitational force of the lead mass. Over the coming centuries, these experiments were further perfected to measure gravitational forces with increasing accuracy. The Vienna team has
picked up this idea and built a miniature version of the Cavendish experiment. A two-millimetre gold sphere weighing 90mg serves as the gravitational mass. The torsion pendulum consists of a glass rod four centimetres long
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Tobias Westphal
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