Gold-standard gravitational measurements

Physicists have recorded the smallest gravitational field ever measured, showing that Newton’s law of gravity holds even for small masses.

The new study, published in Nature, may help us explore fundamental physics such as the nature of dark matter, or the relationship between gravity and quantum physics.

Though we’re familiar with the effects of gravity in our everyday lives, it is the weakest and least understood of the four fundamental forces. Physicists aren’t even sure how it fits into the standard model of physics, or how it can be reconciled with quantum physics.

While recent observations of massive astrophysical objects – like detecting gravitational waves from a black hole merger – have improved our understanding on a large-scale, including testing Einstein’s theory of general relativity, small-scale studies here on Earth are important too.

In this new study, led by quantum physicist Markus Aspelmeyer from the University of Vienna, Austria, researchers tested the coupling force between two golden spheres with masses of about 90 milligrams – equal to the mass of four houseflies.

“To our knowledge, this is the smallest single object whose gravitational field has been measured,” the authors write in their paper.

The experimental set-up wasn’t easy. Testing gravity requires a highly controlled environment without the “noise” of other forces stronger than gravity, so the team connected one of the spheres to a vacuum chamber to minimise seismic and acoustic effects, and blocked electrostatic forces with a gold-plated aluminium screen. The other sphere was then brought within a couple of millimetres.

The test confirmed the classical Newtonian physics that we all learn in high school: the gravitational force between the spheres is dependent on their masses and distance.

“The detection of such a minuscule gravitational signal is itself an exciting result, but the authors went even further by determining a value for G from their experiment,” notes Christian Rothleitner from the National Metrology Institute of Germany, in an accompanying article in Nature.

G is Newton’s gravitational constant, crucial to calculating gravitational force. While experimental measurements of other fundamental constants have improved and converged over time – like that of the speed of light – this hasn’t happened for G.

Aspelmeyer and team provide a new estimate for G, which differs from the internationally agreed value by about 9%.

But this, Rothleitner notes, is “a small amount, given that the experimental uncertainties of their system have not yet been optimized for precise measurements of G”.

The next step is to improve the sensitivity of the experimental set-up and begin to measure even smaller masses.

These kinds of observations could help physicists measure gravitational fields that are so weak they could enter the quantum regime.

“This work opens the way to the unexplored frontier of microscopic source masses, which will enable studies of fundamental interactions and provide a path towards exploring the quantum nature of gravity,” the authors write.

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More about gravity:

• Gravity tested with quantum spin
• New theory of ‘dark gravity’ passes first test
• A step closer to a theory of quantum gravity