How gravitational waves can lead us to the holy grail of modern physics

It keeps you grounded, makes apples fall from trees and directs the cosmic dance between celestial bodies like the earth, sun and moon. However, gravity is perhaps the most unfortunate force in modern physics.

In recent decades, the search for the true nature of gravity has been shown to be the holy grail of physics. Whoever truly understands gravity takes a steady step towards a deeper understanding of the reality around us.

Against this background, three prominent theoretical physicists, including Nobel Prize winner Frank Wilczek, have launched a tantalizing new idea. In the prestigious Physics Journal physical review messages They write that the deeper nature of gravity may be hidden in so-called gravitational waves, the vibrations of space and time that arise, among other things, when heavy objects such as black holes collide in the depths of the universe.

Fundamental forces of nature

First take a step back. Physics distinguishes four fundamental forces of nature. In addition to gravity, this is the electromagnetic force responsible for light and electricity, among other things, the strong nuclear force that ensures that atoms do not disintegrate, and the weak interaction that causes heavier particles to decay into lighter ones. grains.

And while the last three may seem esoteric, it’s these factors—not the familiar gravity—that physicists understand best. They are described in the so-called Standard Model of particle physics, the mathematical model that captures all the building blocks of reality and the forces associated with them into a formula that fits in a T-shirt or coffee mug.

In this description, each force has a “force-carrying particle” – a boson, in technical terms – that makes the force work in practice. As for the electromagnetic force, this is, for example, a photon, a light particle. This is why many physicists believe that gravity must also contain such a boson. And although no one has seen this particle before, it does indeed have a name: the graviton.

“In fact, no one doubts that such a graviton really exists,” says theoretical physicist Eric Verlinde (University of Amsterdam). “But as in science, you’ll want to see it for a while.” This is where Wilczyk and colleagues’ new paper comes in. According to them, it should be possible to find the signature of gravitons in gravitational waves.

Noise behind the waves

Back to this standard form. Above all, this uses quantum theory, the set of natural laws that describe the crazy behavior of the particle world. Only: a quantum theory of gravity does not exist yet. One of the main reasons is that the level of energy at which the quantum effect on gravity begins to appear is so ridiculously high that this rarely happens in practice. Except for cosmic collisions that make space and time tremble so that you can measure the gravitational waves generated on Earth.

“These physicists — I myself sometimes work with first author Molek Baric, with whom I discussed this idea before — have a very interesting idea: How many gravitons would be present in such gravitational waves and how would you measure them?” says Verlind. , who was not directly involved in the research.

In the quantum world, reality at the most complex level is not smooth, but grainy – a vibrant environment completely alien to our senses, where particles flash in and out of reality. According to the new article, these quantum fluctuations should cause a characteristic behavior of the graviton whose signature remains in the measured gravitational waves. “It has to appear as a distinct kind of noise,” Verlind says. If you measure exactly this noise in several detectors at the same time, you can be sure that it is the signal you are looking for.

According to the authors, the signals should be visible in current Ligo gravitational wave detectors, at two locations in the United States, and Virgo in Italy. However, Verlinde has doubts about whether you can make such a difficult measurement. I think there is only a small chance that something like this will work. I think the chance is greater with the planned new generation of gravitational wave detectors, which can be measured more accurately.

He emphasizes that this does not mean that experimenters should not research. “If you are spontaneously trying to find these signals, you develop methods that can make measurements with existing detectors more accurate,” he says.

hard evidence

It is very rare to test theoretical ideas about quantum gravity in experiments. Whereas experiences are the cornerstone of our physical knowledge: only when you measure something in the real world do you know for sure that it exists. “That’s why I’m part of a consortium with Barrick, among others, thinking about measurable signals from quantum gravity,” Verlind says. We believe such signals are detectable. More and more people are taking it seriously now. Finding the expected noise behind gravitational waves is a good first step, Verlind says. “This will be the first definitive proof that gravity is in fact quantum mechanical,” he says.

If so, it is only a matter of time until physicists can finally complete the formula on their cups and T-shirts with an accurate description of gravity.