Even coherent gravitational waves produce graviton noise, but as Dyson also found its far too small to ever measure. This is because the jitter created as the detector absorbs gravitons is exquisitely balanced with the jitter created when it emits gravitons, said Wilczek, who had hoped that their calculation would lead to a bigger noise for coherent states. It was a little disappointing, he said.

Undeterred, Parikh, Wilczek and Zahariade examined several other types of gravitational waves that Dyson did not consider. They found that one quantum state in particular, called a squeezed state, produces a much more pronounced graviton noise. In fact, Parikh, Wilczek and Zahariade found that the noise increases exponentially the more the gravitons are squeezed.

Their theoretical exploration suggested against prevailing wisdom that graviton noise is in principle observable. Moreover, detecting this noise would tell physicists about the exotic sources that might create squeezed gravitational waves. They are thinking about it in a very serious way, and theyre approaching it in a precise language, said Erik Verlinde, a theoretical physicist at the University of Amsterdam.

We always had this image that gravitons would bombard detectors in some way, and so there would be a little bit of jitter, said Parikh. But, Zahariade added, when we understood how this graviton noise term arises mathematically, it was a beautiful moment.

The calculations were worked out over three years and are summarized in a recent paper. The paper describing the complete set of calculations is currently under peer review.

Yet while squeezed light is routinely made in the lab including at LIGO its still unknown whether squeezed gravitational waves exist. Wilczek suspects that the final stages of black hole mergers, where gravitational fields are very strong and changing rapidly, could produce this squeezing effect. Inflation a period in the early universe when space-time expanded very rapidly could also lead to squeezing. The authors now plan to build precise models of these cosmological events and the gravitational waves they emit.

This opens the door to very difficult calculations that are going to be a challenge to carry through to the end, said Wilczek. But the good news is that it gets really interesting and potentially realistic as an experimental target.

Rather than looking to quantum sources in the cosmos, other physicists hope to see graviton noise directly in the bubbling vacuum of space-time, where particles fleetingly pop into existence and then disappear. As they appear, these virtual particles cause space-time to gently warp around them, creating random fluctuations known as space-time foam.

This quantum world might seem inaccessible to experiment. But its not if the universe obeys the holographic principle, in which the fabric of space-time emerges in the same way that a 3D hologram pops out of a 2D pattern. If the holographic principle is true, quantum particles like the graviton live on the lower-dimensional surface and encode the familiar force of gravity in higher-dimensional space-time.

In such a scenario, the effects of quantum gravity can be amplified into the everyday world of experiments like LIGO. Recent work by Verlinde and Kathryn Zurek, a theoretical physicist at the California Institute of Technology, proposes using LIGO or another sensitive interferometer to observe the bubbling vacuum that surrounds the instrument.

In a holographic universe, the interferometer sits in higher-dimensional space-time, which is closely wrapped in a lower-dimensional quantum surface. Adding up the tiny fluctuations across the surface creates a noise that is big enough to be detected by the interferometer. Weve shown that the effects due to quantum gravity are not just determined by the Planck scale, but also by [the interferometers] scale, said Verlinde.

If their assumptions about the holographic principle hold true, graviton noise will become an experimental target for LIGO, or even for a tabletop experiment. In 2015 at the Fermi National Accelerator Laboratory, a tabletop experiment called the Holometer looked for evidence that the universe is holographic and was found wanting. The theoretical ideas at that time were very primitive, said Verlinde, noting that the calculations in his paper with Zurek are grounded on the more in-depth holographic methods developed since then. If the calculations enable researchers to precisely predict what this graviton noise looks like, he thinks their odds of discovery are better although still rather unlikely.

Zurek and Verlindes approach will only work if our universe is holographic a conjecture that is far from established. Describing their attitude as more of a wild west mentality, Zurek said, Its high risk and unlikely to succeed, but what the heck, we dont understand quantum gravity.

By contrast, Parikh, Wilczek and Zahariades calculation is built on physics that few would disagree with. We did a very conservative calculation, which is almost certainly correct, said Parikh. It essentially just assumes there exists something called the graviton and that gravity can be quantized.

But the trio acknowledge that more theoretical legwork must be done before its known whether current or planned gravitational wave detectors can discover graviton noise. It would require several lucky breaks, said Parikh. Not only must the universe harbor exotic sources that create squeezed gravitational waves, but the graviton noise must be distinguishable from the many other sources of noise that LIGO is already subject to.

So far, LIGO hasnt shown any signs of physics that breaks with the predictions of Einsteins general relativity, said Holz, who is a member of the LIGO collaboration. Thats where you start: General relativity is amazing. Still, he acknowledges that gravitational wave detectors are our best hope for making new fundamental discoveries about the universe, because the terrain is completely uncharted.

Wilczek argues that if researchers develop an understanding of what graviton noise might look like, gravitational wave detectors can be adjusted to improve the chances of finding it. Naturally, people have been focusing on trying to fish out signals, and not worrying about the interesting properties of the noise, said Wilczek. But if you have that in mind, you would maybe design something different. (Holz clarified that LIGO researchers have already studied some possible cosmic noise signals.)

Despite the challenges ahead, Wilczek said he is guardedly optimistic that their work will lead to predictions that can be probed experimentally. In any case, he hopes the paper will spur other theorists to study graviton noise.

Fundamental physics is a hard business. You can famously write the whole thing on a T-shirt, and its hard to make additions or changes to that, Wilczek said. I dont see how this is going to lead there directly, but it opens a new window on the world.

And then well see what we see.

This article was reprinted onWired.com.

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Gravitons Revealed in the Noise of Gravitational Waves - Quanta Magazine