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What Are Gravitons?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Does Gravity Have a Particle?

How often do we hear about the impossible struggle of unification of relativity to quantum mechanics? I ponder it frequently and have read much research on it. Theories stemming from string theory to loop quantum gravity all postulate potential solutions but also present their own problems. While I do like aspects of both, there is another option that is also gaining ground: What if gravity, like the other forces, has a particle? Enter the graviton.

Some Gravity Matters

It’s not like gravity hasn’t had its own issues over the years. Newtons gravity provided mathematical underpinnings to the mechanics of gravity, but it also implied an instantaneous exchange of gravitational information, which clearly isn’t right. Somehow, changes to an objects distance from another or either masses being fluctuated impacted the force between them, but a time delay must occur as the information spreads outward. This idea was a partial clue to the theory of general relativity, which merged and space and time into one construct (De Rham 31-2).

But while relativity was taking flight, another revolution was happening: Quantum Mechanics, or the reduction of the atomic world into clouds of probabilities, entangled states of existence, and a loss of classical mechanic confidence. It extended the idea of force carriers, or bosons, that had started with photons and extended them to the strong and weak nuclear forces. And so, we ended up with photons, W and Z bosons, and gluons, and that was that, right? (33)

Relating Back to Bosons

So why wouldn’t gravity be the same and have a boson to correspond to it? Well, for starters the mass of a boson is inversely related to the range of the force. Heavier things are not as easy to apply forces to for sustained periods, and smaller objects can have greater forces exerted on them with meaningful behavior.

But gravity is exerted over huge ranges, and even though it is the weakest of the four forces it acts at the largest scale of the four as well. This implies that if a graviton exists, then its mass must be incredibly small perhaps even massless like a photon (Ibid).

Well, as we presume photons to be. Experiments have shown that if a photon does have mass, it must be less than 10^-54 kg. Similar math points to a graviton with a mass less than 10^-58 kg. That mass tells us something about their speed and therefore how they propagate through space.

Many different models arise because of this, with different implications. One of which implies graviton movement would result in negative energy, which in short would be destructive and very, very bad for the Universe. Obviously, we are all still here so we should take that data point as a counter to that model. It’s reasonable to, I suspect. Are there other (less obvious) observational methods for detecting them? (Ibid)

Signals in the Dark

Part of the problem in trying to spot gravitons is the scale they operate on the Planck scale, where the absolute minimum values for things such as mass and length exist. To look at that scale in search of gravitons would “be so massive that it would collapse into a black hole.” Yikes. So instead of directly looking for them, we can parse them out from other sources hopefully.

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One such possibility is through LIGO, the gravity wave detector. Maulik Parikh (Arizona State University), Frank Wilczek (MIT) and George Zahariade (Arizona State University) have found that if gravitons exist, then a gravity-wave event would leave behind a “quantum noise” due to the inherent quantum nature of spacetime at such a small scale (Lewton).

This noise would leave a signal imprint upon matter in a transversal (up/down) motion as opposed to the gravity waves themselves which are longitudinal (left-right). Normal gravity waves should produce this, but because the absorption and emission of gravitons would be so nearly identical the signal would be practically canceled out.

However, if your gravity waves were in a squeezed state the balance would be out of whack and thus more detectable. The only catch here is that squeezed gravity waves are an unknown – they may, or may not, exist. It would require a wildly fluctuating system like a black hole merger to accomplish it (Ibid).

Work by Douglas Singleton (California State University) is also promising. He noted the light nature of the graviton and got to wondering if it would be easier to spot them if they were clumped together. It’s like trying to measure the width of a piece of paper.

Instead, just gather a group of them, know the number of pages, and you can math it out. In this case, our grouping would potentially be around a black hole merger, and Singleton has been able to demonstrate that, provided gravitons find themselves in a chaotic enough scenario, can decay into photons, and vice versa (Wood).

Spot photons from a merger with no clear source and you may have an indirect detection on your hands. Raymond Sawyer (University of California) developed the extension of Singleton’s graviton-photon conversion and looked at the mechanics of a black hole merger, realizing it was the perfect event to make the switch happen. The only catch? The photons generated would be weak radio waves and thus hard to detect. Hey, no one said rewriting physics was going to be easy, right? (Ibid)

Gravitons: A Closer Look at Gravity

Works Cited

De Rham, Claudia. “Rethinking gravity.” New Scientist. New Scientist Ltd., 11 Jul. 2020. Print. 31-4.

Lewton, Thomas. “How the Bits of Quantum Gravity Can Buzz.” Quantamagazine.com. Quanta, 23 Jul. 2020. Web. 01 Sept. 2020.

Wood, Charlie. “Glowing Gravitons.” Scientific American. Jul. 2020. Print. 16.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley

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