Gravity is a ubiquitous part of our daily lives — whether we’re being tragically brought to our knees after tripping on the rug or gleefully jumping from a swing’s apex. But despite how common the experience of gravity is, it remains one of physics’ least understood fundamental forces.
But now, thanks to research published Wednesday in the journal Nature, physicists may be closer than ever to reconciling this capricious force with other big, mysterious players like quantum mechanics, string theory, and dark matter.
What’s new — As it turns out, all they needed to do was build, what the researchers are calling, the tiniest ever laboratory-created gravitational field. By measuring the forces between two tiny masses on a tiny, torsion balance the researchers were able to verify that Newtonian gravity holds strong even at barely perceptible levels.
Why it matters — Demonstrating that it’s possible to detect super-tiny gravitational fields in the lab could be huge for scientists trying to learn how Newtonian gravity (that, is the kind of gravity we experience every day) could possibly play nice with the strange, tiny world of quantum mechanics, which governs the interactions of tiny particles. Such a reconciliation could help physicists reach their own Holy Grail: a grand unifying theory that would finally describe gravity in every possible scenario, from planets to quarks.
Here’s the background — It’s a tale as old as time: Isaac Newton was sitting under an apple tree in roughly 1665 when he was (allegedly) struck on the head by a falling apple and — eureka! — he suddenly saw a connection between the gravitational force pulling an apple down from a tree (in this case, Earth’s gravity which is 9.8 meters per second squared, or little g) and the gravitational forces pulling on planets in our solar system to form well-controlled rotations.
This larger gravitational force between two bodies relies on something called the gravitational constant that is used to calculate the relative gravitational force between two objects. The equation describing this force essentially states that bigger objects have more pull but the farther away they are from each other, the less pull they have.
Newton’s gravitational constant works pretty well when describing big things on Earth or in space that we can see with the naked eye, but as scientists in the 20th and 21st century began to expand these ideas to the still-young field of quantum mechanics, things began to quickly break down.
Apart from gravity, the universe is governed by three other fundamental forces: the weak, strong, and electromagnetic forces. These three forces all play nicely into a quantum view of our world, but Newton’s gravity struggles to explain what happens at these incredibly small scales.
Gravitational theories like string theory (which proposes gravity exists in multiple dimensions) and dark matter (which describes invisible, gravity-generating parts of the universe), as well as Einstein’s theory of General Relativity, are all attempts at reconciling Newtonian and quantum gravity. But so far, physicists have struggled to mend this relationship.
That’s where this new German study comes into play.
By investigating the force of gravity using less than 100-milligram gold masses (equivalent to the weight of just four houseflies), this team of researchers is taking the study of gravity even closer to the quantum realm and could finally help uncover the missing link between these two governing theories.
What they did — In their lab, the research team created a miniature torsion balance, which works by attaching one mass to either end of a horizontal rod suspended vertically by a hanging wire. Earth’s gravity is counteracted upon by the vertical hanging string, leaving just the gravitational interaction between the two masses themselves as the measurable phenomena.
To measure the incredibly small force these two objects would enact upon each other, the researchers bounced a laser beam off a mirror into a detector which calculated motion via the displacement of the laser beam.
Because gravity is the weakest of the four fundamental forces and because these point masses were so tiny, the team had to be extra careful to eliminate as much extra noise (e.g. traffic vibration from nearby roads as possible) by taking measurements during relatively quiet times and using special rubber damping equipment to absorb vibrations.
The team remarks in their study that measurements “taken during the seismically quiet Christmas season” were particularly important.
What they discovered — After 350 hours of observations, the researchers reported that they were able to detect an incredibly tiny gravitational force between their two objects of just 9 x 10^-14 newtons (where a Newton is a unit of force equivalent to kg⋅m⋅s^−2.) For comparison, the gravitational force between the Earth and Moon is roughly 2 x 10^20 newtons.
Using their system the team was also able to remeasure big G itself and derived a value only nine percent off the universally agreed to value of 6.67 × 10^-11 newton-metre^2-kilogram^−2. This is strong evidence that Newton’s theory of gravity can still persist even at incredibly small scales — confirming present models.
What’s next — This first-ever discovery of Newtonian gravity at such a small scale potentially opens the flood gates for researchers to explore even tinier and tinier sources to better understand if this force still holds on the quantum scale, but before they can do that, there are some obstacles that still need to be overcome.
For example, as masses get smaller, so do their gravitational fields, making it even more difficult to differentiate them from ambient vibrations or noise. In addition to making labs even more soundproof, the researchers write that even fine-tuning aspects of their torsion system (such as reducing damping from the materials themselves) will be crucial.
It’s a big ask, but achieving it could transform physics as we know it forever.
Abstract: Gravity is the weakest of all known fundamental forces and poses some of the most important open questions to modern physics: it remains resistant to unification within the standard model of physics and its underlying concepts appear to be fundamentally disconnected from quantum theory. Testing gravity at all scales is therefore an important experimental endeavour. So far, these tests have mainly involved macroscopic masses at the kilogram scale and beyond. Here we show gravitational coupling between two gold spheres of 1 millimetre radius, thereby entering the regime of sub-100-milligram sources of gravity. Periodic modulation of the position of the source mass allows us to perform a spatial mapping of the gravitational force. Both linear and quadratic coupling are observed as a consequence of the nonlinearity of the gravitational potential. Our results extend the parameter space of gravity measurements to small, single source masses and low gravitational field strengths. Further improvements to our methodology will enable the isolation of gravity as a coupling force for objects below the Planck mass. 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.