Study: experiments that can demonstrate that the ‘quantumness’ of gravity is feasible

Gravity permeates every part of our world, pulling us toward Earth and connecting the solar system, the galaxies, and the universe through which our planet slides.

But to think that gravity, an invisible force, could exhibit the same quantum properties as the most minuscule parts of matter is one of the wildest ideas in physics – and one that is fiendishly difficult to test.

Now a team of physicists from the Netherlands and Germany claim they have devised exactly the experiments that could reveal whether gravity is essentially a quantum force, whose properties are shaped on the smallest possible scales, or a classical, geometric one like Einstein envisioned had.

Three of the four fundamental forces of nature can be described by quantum mechanics, but not yet by gravity – the weakest of them all. Finding out whether gravity is classical or quantum in nature could help reconcile these apparent differences, as well as solve some of gravity’s currently unsolved mysteries.

Typically, quantum mechanics is associated with the smallest parts of our world where we see quantum effects most clearly: between subatomic particles such as electrons, packets of light called photons, whole atoms, and even molecules.

But theoretically, all objects – from planets to solar systems, entire galaxies and possibly the entire universe – could be described by the laws of quantum mechanics, which exist as a spectrum of possibilities before they are observed.

However, as objects get bigger and gravity increases, they tend to shed their quantum characteristics and move in the classical direction – which is how we came to know them.

However, if heavier masses with stronger gravitational fields could behave like a quantum particle, which can exist in several states at once and become entangled, then it would be insanely difficult to detect. That’s because quantum entanglement, so far the test for “quantumness,” is incredibly fragile, collapsing even at subatomic levels with the slightest disturbance.

“The extreme weakness of the gravitational interaction, compared to all other forces, has left us in the bitter situation of not yet being able to investigate its ultimate nature,” says Ludovico Lami, a mathematical physicist at the University of Amsterdam. and colleagues write in their newspaper.

In previous experiments looking for quantum entanglement between masses and their gravitational fields, the heaviest object physicists have seen slipping into an entangled state is more than a trillion times lighter than the smallest mass for which a gravitational field has been detected. This is still far too small to generate a “noticeable gravitational field,” the scientists say.

To break this impasse, Lami and colleagues have instead proposed a series of experiments that could reveal the “quantumness” of gravity without generating or having to detect any entanglement.

The experiments would involve two torsion pendulums, similar to the ones that English scientist Henry Cavendish used in 1797 to first measure the strength of gravity, along with some shields, mirrors and lasers.

These experiments remain entirely theoretical for now, but that didn’t stop Lami and colleagues from calculating the upper limits of certain experimental signals for quantumness that gravity, if classical, should not be able to overcome.

“We have carefully analyzed the experimental requirements necessary to implement our proposal in actual experiments,” explains Lami, “and have come to the conclusion that, although a certain degree of technological progress is still needed, such experiments will soon can really be within reach.”

However, the researchers made some assumptions in their calculations that are hotly debated. There are also further suggestions for other tabletop experiments that could detect quantum gravity.

The research was published in Physical Assessment X.

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