In 2009, the British journalist Ian Betteridge wrote an article in which he asserted his maxim that any headline ending in a question mark can be answered, “No.” This maxim has come to be called Betteridge’s law. This article will demonstrate that, as with all proverbs, the real-world situation can be much murkier.
Gravitons are theoretical subatomic particles that some scientists think transmit the force of gravity. If they exist, they are vanishingly small and have no mass or electric charge. To generate gravity, the thinking goes, gravitons jump from one massive object to another, drawing them ever closer to one another. Yet despite no experimental evidence showing that gravitons exist, they remain a respectable concept in the world of professional physicists.
So, why do most physicists suspect that gravitons are real even if they lack the confirmatory evidence to support that belief? To understand the reason requires that we understand both our best current theory of gravity, as well as some of the lessons of modern quantum theory.
The first professional theory of gravity was proposed back in the 1680s by Isaac Newton. His published equations have proven accurate enough that NASA still uses them today to predict the motion of their space probes. However, there is one big conceptual problem with Newton’s ideas: They don’t really explain anything. They just say that massive objects attract one another, and his equations make that statement quantitative.
The situation changed in the early 1900s when Albert Einstein published his general theory of relativity, which remains the most accurate theory of gravity devised thus far. His theory says that matter distorts the shape of space itself. Just as the path of an automobile follows the terrain of even the curviest road, the motion of the stars and planets are examples of objects following the bumps and curves of bent space.
Einstein’s theory makes similar predictions to Newton’s in most cases, although Einstein’s is more accurate in situations where gravity is strong. General relativity has also been thoroughly tested and appears to be an accurate description of gravity under most conditions.
However, “most conditions” doesn’t cover all conditions. When Einstein’s theory is applied to the submicroscopic world of atoms and electrons, it utterly fails. The theory predicts infinities, and infinity is the signature of a scientific model being pushed beyond its realm of physical reality and into that of mathematical abstraction.
In other words, scientists have yet to develop a theory of gravity that applies in the world of the ultra-small. They have, however, drafted a placeholder name for such a model. They call it “quantum gravity.” Scientists have also formed quantum-level explanations for the other three known fundamental forces: electromagnetism, and the strong and weak nuclear forces. The explanations for these forces are generically called “quantum field theories,” and unlike quantum gravity, they have been validated with confirmatory evidence.
Because no accepted theory of quantum gravity exists, scientists are free to indulge their imaginations. One common approach is to bootstrap quantum gravity to the successful quantum field theories. The idea is that if a technique had success for the other known fundamental forces, then perhaps the same approach will describe quantum gravity, too.
One shared feature of the successful quantum field theories is that they all require one or more subatomic particles to mediate the force that they describe. The electromagnetic force is transmitted by the exchange of photons between subatomic particles (a photon is a particle of light). The strong nuclear force is transmitted by a particle called a gluon, so named because it “glues” particles together. The third quantum force, the weak nuclear force, requires two particles, the prosaically named W and Z bosons.
Given that the quantum theories for the other forces all require a particle to transmit them, scientists believe it is likely that this will be true for gravity, as well. While the force-particle of gravity has not been observed, scientists call it a “graviton.”
And since we know a lot about gravity, we can surmise many of the graviton’s potential properties. Because the range of gravity is infinite, gravitons must be massless. Because gravity ignores electricity, gravitons should be electrically neutral. And because gravity is an attractive force, the graviton should have a subatomic spin of 2. This is in contrast with the other quantum force-carrying particles, all of which have a spin of 1.
While surmising the nature of a particle is all well and good, until such predictions are validated by experiment, there is no reason to take them seriously. So, what are the prospects for detecting gravitons?
The currently known laws of physics can give us a sense of how difficult it will be to validate the theory. For instance, the force of gravity is extraordinarily weak compared to the other known forces. While the number depends a bit on the circumstances under which you test it, it turns out that gravity is in the ballpark of 1040 times weaker than electromagnetism. It is this weakness that makes it so difficult to detect gravitons. In any conceivable measurement involving subatomic particles, the force of electromagnetism dominates, meaning that any gravitational effects are rendered hidden.
To generate an appreciable gravitational force, one needs many atoms working together to create the force. A bonus of this approach is that atoms are electrically neutral, which means that the electromagnetic force is zero. However, once you have many atoms working together, you are no longer probing quantum sizes and therefore no longer testing quantum gravity.
In fact, if the simplest ideas of quantum gravity apply, it may be that we will never be able to validate the theory of quantum gravity. On subatomic scales, the force is simply too minute to ever be observed.
So, returning to the question headlining this article, Are gravitons real? There is certainly ample reason to believe that they should be. A theory of gravity that works on quantum-size scales must exist, and all other quantum theories require particles to generate the force. On the other hand, there is no experimental evidence to confirm or falsify gravitons.
On the upside, there is no fundamental barrier to testing quantum gravity, meaning that some future approach may be able to answer the question. However, for the foreseeable future, the question of whether gravitons are real cannot be answered with neither a “yes” nor “no.” The only honest answer is “maybe.”
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The post “Will physics ever prove that gravitons are real?” by Don Lincoln was published on 01/09/2025 by bigthink.com