Scientists Might Achieve the Impossible and Actually *See* Gravity
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The graviton, the hypothetical “force carrier” for gravity, is the only fundamental force carrier that has yet to be detected.
A new study suggests that a proposed graviton detector that uses 4,000-pound aluminum cylinders as acoustic resonators could be used in conjunction with Laser Interferometer Gravitational-Wave Observatory (LIGO) to detect single gravitons.
Although an ambitious plan, the experiment needs to develop the quantum sensors required to detect energy level changes in the supercooled material, which won't be easy.
So … where are all the gravitons?
Disregarding the fact that it sounds more like a Decepticon than a theoretical particle, the long-hypothesized graviton is the only missing “force carrier” in today’s Standard Model of particle physics.
As their name suggests, force carriers are like the messages exchanged between particles. The most well-known of this quantum family is the photon, which is the force carrier of electromagnetism, but the strong and weak nuclear forces also have the gluon and w and z bosons as force carriers, respectively. Gravity is the only one of the four fundamental forces whose force carrier, the graviton, has never been directly observed.
Because of physicists’ inability to square quantum theory with general relativity, this has led some to question whether gravity even acts like the other fundamental forces and if the graviton itself even exists. However, scientists haven’t ceded the field quite yet, because if gravitons did exist, they’d likely be exponentially more difficult to detect—by some measurements, they’re a billion, billion, billion times lighter than an electron at around 6 × 10−32 electron-volts.
Difficult... but not impossible.
At least that’s the thinking behind a new paper from scientists at Stockholm University and New Jersey private research university Stevens Institute of Technology. In this new study published in the journal Nature Communications, the researchers argue that a new quantum-sensing technique might just pull it off. Drawing inspiration from Albert Einstein’s Nobel Prize-winning work on the photoelectric effect, which essentially discovered the existence of the photon, this detection method would similarly hunt for gravitons in gravitational waves.
“Our solution mimics the photoelectric effect,” Stockholm University Ph.D student Germain Tobar, a co-author of the study, said in a press statement, “but we use acoustic resonators and gravitational waves that pass Earth. We call it the ‘gravito-phononic’ effect.”
The researchers contrast these acoustic resonators with the technique used by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which detected the first gravitational waves back in 2015. Where LIGO uses lasers to measure changes in distance as a passing gravitational wave distorts spacetime (all the way down to 1/10,000th the width of a proton), this new research instead relies on massive, vibrating objects.
“If we use heavy cylinders that resonate with the waves,” postdoctoral researcher at the Nordic Institute for Theoretical Physics (Nordita) Sreenath Manikandan, also a co-author of the study, said in a press statement, “then for a sufficiently strong wave some energy can get deposited. The trick is to use quantum sensing to observe single quantum jumps in energy whenever single gravitons are absorbed or emitted.”
These cylinders would be 4,000-pound bars of aluminum chilled to just above absolute zero (we are talking about quantum sensing, after all). Once a gravitational wave passes through this massive detector, each oh-so-subtle vibration detected by a change in energy level would be a graviton.
To pull this off, the team does get a big boost from existing gravitational wave detectors. Although LIGO is good at detecting gravitational wave events, it can’t single out one specific graviton, so this ‘gravito-phononic’ effect experiment will cross-reference events detected by LIGO to see if their detector sensed any possible gravitons. For such gravitons to be detected, the team admits that it needs a pretty energetic gravitational wave, but their calculations suggest that one such wave created by a neutron-star merger in 2017 could’ve created enough gravitons for a high probability of absorption.
The biggest hurdle between now and finally closing the book on the physics bestseller Mystery of the Missing Graviton is building the quantum sensors themselves, which currently don’t exist. But this preliminary work suggests that once they’re in place, solving one of physics’ biggest unknowns could be within our grasp.
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