A team of physicists at Stockholm University has presented a breakthrough proposal: a method to detect individual gravitons. Interestingly, until now, “these elusive particles were thought to be too difficult to observe.”
For context, gravitons are theoretical particles believed to be the building blocks of gravity. Scientists have long been trying to bridge the gap between gravity and quantum mechanics.
“If we believe in quantum theory, even gravity must be made of tiny quantized particles – gravitons,” say the scientists.
But “single gravitons almost never interact with anything. As they pass through space, they lose almost all of their matter. Their detection seemed impossible,” the researchers added in a press release.
Proposed solution
Now a research team, led by Stevens physics professor Igor Pikovski, “has worked out how to build a single graviton detector” that would allow the detection of gravitons.
Their method takes advantage of recent advances in quantum sensing and the study of macroscopic quantum objects.
These objects, large enough to be seen with the naked eye but exhibiting quantum behavior, are ideal for detecting gravitons due to their strong interaction with gravity.
The team’s design builds on existing technologies such as acoustic resonators and Weber rods.
The detector works by cooling a macroscopic quantum object to its lowest energy state and then exposing it to gravitational waves.
“We have to cool the material and then monitor how the energy changes in a single step, and this can be done using quantum sensing,” explained postdoctoral researcher Sreenath Manikandan.
“Gravito-Phonon Effect”
Scientists hypothesize that when a graviton interacts with an object, it causes a discrete change in its energy, a phenomenon they call a “quantum jump.”
This distortion is usually too small to notice, but by carefully tracking these quantum jumps, the team believes they can pinpoint the absorption of a single graviton.
This “gravito-phonon effect” mirrors the photoelectric effect, where light interacts with matter in discrete steps, or photons.
“Our solution mimics the photoelectric effect, but we use acoustic resonators and gravitational waves that pass around the Earth,” said PhD student Germain Tobar. “We call it the ‘gravito-phonon’ effect.”
Use of LIGO data
To increase their chances of detecting gravitons, the team proposes to use data from the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO is known for its breakthrough detection of gravitational waves, but it cannot directly detect individual gravitons.
According to the team, detecting a single graviton requires extremely energetic gravitational waves. This is because the interaction between a single graviton and matter is expected to be incredibly weak.
Besides, we can’t just create gravitational waves whenever we want in the lab. They are produced by massive cosmic events such as collisions of black holes or neutron stars.
However, by carefully comparing the LIGO data with the proposed detector measurements, the researchers believe they can isolate and confirm the signature of a single graviton interaction.
“We can solve both problems by using existing gravitational wave observatories,” says PhD student Thomas Beitel.
“We are waiting for LIGO to detect a passing gravitational wave and watch as it simultaneously creates quantum jumps in our detector.”
The search for a unified theory
While the theoretical framework of the experiment is robust, the technology to detect these gravitons remains a huge challenge. The desired level of quantum sensing capabilities has yet to be fully developed.
That said, the team is optimistic and is now focusing on designing a specific experiment using gravitational wave data detected on Earth.
Research into the detection of gravitons has been going on for over a century. Einstein’s theory of relativity revolutionized our understanding of gravity, describing it as the curvature of space-time.
However, gravity remains the only fundamental force that has not been fully explained by quantum theory. A successful detection of the graviton would be a significant step towards a unified “theory of everything”.
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