Gravitons are hypothetical elementary particles that are said to be the cornerstone of quantum gravity theories. Many physicists believe the existence of gravitons, but some doubt that it can ever be observed in a natural world.
These particles unify the general theory of relativity of Albert Einstein and quantum mechanics. To see the world of gravitons requires a device that can harness a great amount of energy. However, that too seems impossible to accomplish because that device must be so massive that it would collapse into a black hole.
Celebrated physicist Freeman Dyson said, "it appears that Nature conspires to forbid any measurement of distance with error smaller than the Planck length."
Many scientists believe that gravitons only exist in the most extreme places in the universe, such as when the Big Bang happened or in black holes. But recently published studies challenge this view, which suggests that gravitons create noise in gravitational wave detectors.
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Graviton Noise
Nobel Prize winner and study co-author Frank Wilczek said that no one took Dyson's 2013 calculation as a practical way to learn about quantum gravity.
Together with his co-authors Maulik Parikh and George Zahariade, both cosmologists at Arizona State University, they discovered that Dyson's calculations might be useful in their 2015 discovery of gravitational waves using the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Gravitons are thought to behave like photons, wherein they made up the gravitational waves. In principle, gravitational wave detectors that are sensitive enough to see gravitons can be used to observe them.
The scientists considered the effects of many gravitons inspired by the Brownian motion or the random jiggle and shake of particles in a fluid. Einstein used this principle to prove the existence of atoms. Consequently, the scientists thought that the collective behavior of many gravitons might somehow reshape the gravitational wave.
Gravitational wave detectors can be two masses separated by some distance. The gravitational wave increases or decreases as it stretches and squashes the space between the time when it passes through the gravitational wave detector. Adding gravitons to the mix will change the usual pattern of ripples in space-time.
The masses randomly jitters as the device absorbs and emits gravitons. That jitter is called the graviton noise. LIGO is designed to search for conventional gravitational waves and neutron stars as they spiral around each other and collide.
The scientists' theoretical exploration reveals that in principle, graviton noise can be observed and detecting this noise would tell the exotic sources that might create squeezed gravitational waves.
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The Hologram Principle
Physicists preferred to look for quantum sources in the bubbling vacuum of space-time, where particles appear and disappear instantly, rather than in the cosmos. As gravitons appear, they cause space-time to warp around them gently and creates the space-time foam.
This quantum world might be accessible if the universe obeys the "holographic principle," wherein the space-time emerges similarly to a 3D hologram that appears out of a 2D pattern.
The effects of quantum gravity can be amplified into LIGO experiments if the holographic principle is true. The recent research by theoretical physicists Verlinde and Kathryn Zurek from California Institute of Technology, suggests using LIGO or any sensitive interferometer to observe the bubbling vacuum surrounding the instrument.
If the scientist's assumptions about the holographic principle are true, graviton becomes an experimental target for LIGO, and if the calculations allow them to precisely predict what graviton noise looks like, the odds of discovering it are better although still rather unlikely.
