Gravitons are the particles that make up gravity. More than a century ago, Einstein explained that gravity changes in space and time. Gravity has shown surprising effects, like time slowing down, gravitational waves, and black holes.
But gravity is unique: we only know about its basic version, while other forces are explained by quantum theory. Scientists have long sought to combine gravity with quantum mechanics, but this remains an unsolved puzzle.
Certain single indivisible particles are expected to occur in any quantum theory of gravity. These elusive particles have been named as gravitons. Gravitons can be considered as building blocks of gravity.
The gravitational waves that travel through Earth from huge cosmic events are thought to be made of countless gravitons. Big detectors like LIGO have confirmed the existence of these waves. However, no one has detected a graviton, and spotting one was once considered impossible.
A team led by Stevens physics professor Igor Pikovski has proposed a way to detect single gravitons in a quantum sensing experiment. Their method involves coupling an existing physical detection technology called an acoustic resonator, basically a heavy cylinder, with improved energy state-detection methods (also known as quantum sensing).
This solution is similar to the photoelectric effect. Like electromagnetic waves, gravitational waves interact with matter in discrete steps. Energy is exchanged in single units, or gravitons, which are absorbed or emitted one at a time.
But how do we detect them?
Postdoctoral researcher Sreenath Manikandan said, “We need to cool the material and then monitor how the energy changes in a single step, and this can be achieved through quantum sensing.”
First-year graduate student Germain Tobar said, “By observing these quantum jumps in the material, we can deduce that a graviton was absorbed. We call it the ‘gravito-phononic effect.’”
One idea from the team is to use data from LIGO, the U.S. observatory that has confirmed gravitational waves. While LIGO is excellent at detecting gravitational waves, it can’t catch single gravitons. However, by cross-correlating LIGO’s data with our proposed detector, we might be able to isolate and detect individual gravitons.
The team did some math and creativity. They also used recent technological advancements to their advantage. Things have changed recently: Scientists are now observing quantum effects in large objects. Pikovski saw that these macroscopic quantum objects are perfect for detecting single graviton signatures because they interact more strongly with gravity and show energy changes in discrete steps.
The team is designing an experiment using data from gravitational waves measured on Earth, like those from the 2017 neutron star collision. They calculate the best conditions to maximize the chance of absorbing a single graviton.
They found that the measurements could be done by using a device similar to the Weber bar. Weber bars are thick, heavy (up to a ton) cylindrical bars named for their inventor. Because of the emergence of optical-based detection technologies, these bars are less likely to be used in recent studies. That’s because they can absorb and emit gravitons—in direct analogy to what Einstein coined the “stimulated emission and absorption” of photons, the smallest building blocks of light.
A new quantum detector would be cooled to its lowest energy and then made to vibrate slightly by passing gravitational waves. Extremely sensitive energy sensors could potentially detect these vibrations as discrete changes or quantum jumps, which would signal the presence of a single graviton.
However, there’s a challenge: the technology needed to detect these gravitons has yet to be created.
Tobar said, “Quantum jumps have been observed in materials recently, but not yet at the masses we need. But technology advances rapidly, and we have more ideas on making it easier.”
Thomas Beitel, a Graduate student, said, “We’re certain this experiment would work. Now that we know that gravitons can be detected, it’s added motivation to develop the appropriate quantum-sensing technology further. With some luck, one will be able to capture single gravitons soon.”
