If Gravitons Exist, this Experiment Might Find Them

There are four fundamental forces in the Universe; strong, weak, electromagnetic and gravity. Quantum theory explains three of the four through the interaction of particles but science has yet to discover a corresponding particle for gravity. Known as the ‘graviton’, the hypothetical gravity particle is thought to constitute gravitational waves but it hasn’t been detected in gravity wave detector. A new experiment hopes to change that using an acoustic resonator to identify individual gravitons and confirm their existence. 

The four fundamental forces of nature govern the Universe. Gravity is one that many people are familiar with yet we do not fully understand how it works. Its effects are obvious though as the attraction between objects with mass. It keeps the planets in orbit around the Sun, the Moon in orbit around the Earth and us pinned to the surface of planet Earth. One of the earliest attempts to describe it was  from Isaac Newton who stated that gravity was proportional to the mass of objects and inversely proportional to the square of the distance between them. Even at the largest scale of the cosmos it seems to be essential for the structure of the Universe. 

One of the challenges with gravity is that, unlike the other fundamental forces, it can only be explained in a classical sense. Quantum physics can explain the other three forces by way of particles; the electromagnetic force has the photon, the strong nuclear force has the gluon, the weak nuclear force has the W and Z bosons but gravity has, well nothing yet. Other than the hypothesised graviton. The graviton can be thought of as the building block of gravity much as bricks are the building blocks of a house or atoms the building blocks of matter. 

Detectors like LIGO the Laser Interferometer Gravitational-Wave Observatory, can detect gravity waves from large scale events like mergers of black holes and neutron stars yet to date, a graviton has never been detected. That may soon be about to change though. A team of researchers led by physics professor Igor Pikovski from the Stevens Institute of Technology suggests a new solution. By utilising existing detection technology, which is essentially a heavy cylinder known as an acoustic resonator, the team propose adding improved energy state detection methods known as quantum sensing. 

The proposed solution, explains Pikovski “is similar to the photo-electric effect that led Einstein to the quantum theory of light, just with gravitational waves replacing electromagnetic waves.” The secret is the discrete steps of energy that are exchanged between the material and the waves as single gravitons are absorbed. The team will use LIGO to confirm gravity wave detections and cross reference with their own data. 

The new approach has been inspired by gravity wave data that have been detected on Earth. Waves detected in 2017 came from a collision event between two city-sized super dense neutron stars. The team calculated the parameters that would facilitate the absorption probability for a single graviton. 

The team began thinking through a possible experiment. Using data from gravitational waves that have previously been measured on Earth, such as those that arrived in 2017 from a collision of two Manhattan-sized (but super-dense) faraway neutron stars, they calculated the parameters that would optimise the absorption probability for a single graviton. Their development led to devices similar to the Weber bar (thick, heavy 1 ton cylindrical bars) to allow gravitons to be detected. 

The bars would be suspended in the newly designed quantum detector, cooled to the lowest possible energy state and the passage of a gravity wave would set it vibrating. The team then hope to be able to measure the vibration using super-sensitive energy detectors to see how the vibrations changed in discrete steps, indicating a graviton event. 

It’s an exciting time for gravity based physics and we are most definitely getting closer to unravelling its mysteries. Unfortunately though, the super-sensitive detectors are not available yet but according to Pikovski’s team, they are not far away. Pikovski summed it up “We know that quantum gravity is still unsolved, and it’s too hard to test it in its full glory but we can now take the first steps, just as scientists did over a hundred years ago with quanta of light.”

Source : New research suggests a way to capture physicists’ most wanted particle — the graviton

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