Bottom line: For decades, physicists have been trying to solve one of the greatest mysteries in science: how gravity operates at the smallest scales governed by quantum mechanics. While we have theories explaining gravity's effects on large objects like planets and stars, we still don't fully understand how it functions at the subatomic level.
The leading idea is that gravity arises from the exchange of hypothetical "graviton" particles, much like electromagnetism arises from the exchange of photons. However, gravitons have always been considered too difficult to observe because they interact with matter very weakly, similar to neutrinos.
Recently, a team of researchers led by Igor Pikovski published a paper in Nature Communications demonstrating how gravitons might be experimentally detectable using quantum sensing techniques. In other words, scientists may soon be able to "see" gravity.
Pikovski's team realized they could adapt an old physics concept – the photoelectric effect, first explained by Einstein in 1905 – to detect gravity. Einstein theorized that light is composed of tiny, indivisible packets called photons. He used this idea to explain the photoelectric effect, predicting that energy is exchanged between light and matter only in discrete amounts. Despite initial resistance, this theory ultimately proved to be revolutionary.
"Our solution mimics the photoelectric effect, but we use acoustic resonators and gravitational waves that pass Earth," explained PhD student Germain Tobar, a co-author of the study. "We call it the 'gravito-phononic' effect."
Here's how it would work: take an extremely massive cylinder made of 4,000-pound aluminum bars and cool it down to its lowest quantum energy state. When an energetic gravitational wave passes through, it should slightly distort the cylinder, alternately stretching and squeezing it.
By monitoring the cylinder's vibrations, the researchers predict that occasional tiny "quantum jumps" in its energy could be detected – each representing the absorption or emission of a single graviton from the passing gravitational wave.
The only catch is that meaningful readings would only occur from events producing exceptionally strong gravitational waves. Thus, the researchers will need to rely on significant events like the famous 2017 neutron star collision, which should provide more than enough gravitons to have a reasonable chance of observing this effect. They also plan to use existing gravitational wave observatories to enhance detection.
"We wait until LIGO detects a passing gravitational wave and observe how it produces quantum jumps in our detector at the same time," explained Thomas Beitel, another co-author. LIGO stands for "Laser Interferometer Gravitational-wave Observatory" and is currently the world's largest gravitational wave observatory.
Even with this support, the researchers acknowledge that their idea represents an extraordinary technical challenge, pushing quantum sensing to work with larger masses than ever before.