Einstein used Brownian motion to deduce the existence of atoms, which bombard the microscopic particles. “We were always inspired by Brownian motion,” said Parikh, referring to the random jiggle and shake of microscopic particles in a fluid. Unlike Dyson, whose broad-brush calculation focused on a single graviton, they considered the effects of many gravitons. “That’s like asking: How can a surfer on a wave tell just from the motion that the wave is made up of droplets of water?” said Parikh.
With this in mind, the authors asked whether gravitational wave detectors are, in principle, sensitive enough to see gravitons. Just as light rays can be pictured as a well-behaved collection of photons, gravitational waves - ripples in space-time created by violent cosmic processes - are thought to be made up of gravitons. Gravitons are thought to carry the force of gravity in a way that’s similar to how photons carry the electromagnetic force. “There’s nothing like actual experimental results to focus the attention,” said Wilczek. Indeed, neither Wilczek, Parikh nor George Zahariade, a cosmologist at Arizona State and the third co-author, took the possibility seriously until after the 2015 discovery of gravitational waves by LIGO. “There’s a kind of default consensus that it’s a waste of time to think about quantum effects and gravitational radiation,” said Frank Wilczek, a Nobel Prize-winning physicist at the Massachusetts Institute of Technology who was a co-author with Parikh on the new paper. The Jitter of the Waveĭyson’s 2013 calculation convinced many people that gravitational wave detectors were, at best, impractical probes for learning about quantum gravity. By considering how gravitons interact with a detector en masse, they have given a solid theoretical footing to the idea of graviton noise - and taken physicists one step closer to an experimental proof that deep down, gravity plays by the rules of quantum mechanics. Īnd while it’s still unclear if existing or even future gravitational wave observatories have the sensitivity needed to detect this noise, these calculations have made the near-impossible at least plausible. “We’ve found that the quantum fuzziness of space-time is imprinted on matter as a kind of jitter,” said Maulik Parikh, a cosmologist at Arizona State University and a co-author of one of the papers. “And the quantum gravity stuff is happening right at the center of this - so that’s too bad.”īut recently published papers challenge this view, suggesting that gravitons may create observable “noise” in gravitational wave detectors such as LIGO, the Laser Interferometer Gravitational-Wave Observatory. “The problem with black holes is that they’re black, and so nothing comes out,” said Daniel Holz, an astrophysicist at the University of Chicago. “It appears that Nature conspires to forbid any measurement of distance with error smaller than the Planck length,” said Freeman Dyson, the celebrated theoretical physicist, in a 2013 talk presenting a back-of-the-envelope calculation of this limit.Īnd so gravitons, according to conventional thinking, might only reveal themselves in the universe’s most extreme places: around the time of the Big Bang, or in the heart of black holes.
Unfortunately, any measuring device capable of directly probing down to this “Planck length” would necessarily be so massive that it would collapse into a black hole. The world of gravitons only becomes apparent when you zoom in to the fabric of space-time at the smallest possible scales, which requires a device that can harness truly extreme amounts of energy.
But they are notoriously hard - perhaps impossible - to observe in nature. These hypothetical elementary particles are a cornerstone of theories of quantum gravity, which seek to unify Albert Einstein’s general theory of relativity with quantum mechanics. Many physicists assume that gravitons exist, but few think that we will ever see them.
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