Will physics one day prove that gravitons are real?
Will physics one day prove that gravitons are real?
By Don Lincoln | Published: 2025-01-09 15:21:00 | Source: Hard Science – Big Think
Sign up for Big Think on Substack
The most surprising and impactful new stories delivered to your inbox every week for free.
In 2009, British journalist Ian Betteridge wrote article He emphasized the saying that any title that ends with a question mark can be answered with a question mark no. It has come to be called Betteridge’s lawbut this article will show that, as with many other proverbs, the truth becomes more ambiguous once it is applied to real-world situations.
Gravitons are theoretical subatomic particles that some scientists believe transmit the force of gravity. If it exists, it is very small and has no mass or electrical charge. To generate gravity, the thinking goes, gravitons jump from one massive object to another, bringing the objects closer together. Although there is no experimental evidence for gravitons, they remain a respected concept in the world of professional physicists.
But why do most physicists doubt that gravitons are real even if they lack the hard evidence needed to support this idea? To answer this, we first need to understand our best current theory of gravity, as well as some lessons from modern quantum theory.
The first professional theory of gravitation was proposed in the 1680s by Isaac Newton. His published equations proved to be accurate enough that NASA still uses them today to predict the motion of space probes. However, Newton’s ideas have a major conceptual problem: they don’t really explain anything. They just say that massive objects attract each other, and his equations make this statement quantitative.
The situation changed in the early 20th century when Albert Einstein published his general theory of relativity, which remains the most accurate theory of gravity to date. This theory says that matter distorts the shape of space itself. Just as the path of a car follows the curviest terrain of a road, the motion of stars and planets are examples of objects that follow the ridges and curves of curved space.
Einstein’s theory makes predictions similar to Newton’s theory, although Einstein’s theory is more accurate in situations where gravity is strong. General relativity has also been thoroughly tested and shows an accurate description of gravity under most circumstances.
“Most conditions” are not covered everyone Circumstances, though. When Einstein’s theory is applied to the world of atoms and electrons under the microscope, it fails completely. The theory predicts infinities, and infinity is the hallmark of a scientific model being pushed beyond physical reality into mathematical abstraction. As such, scientists have not yet developed a theory of gravity that can be applied in the world of very small objects. However, they have coined an alternative name for such a model: “quantum gravity.”
Scientists have also developed quantum-level explanations for the other three known fundamental forces: electromagnetism, and the strong and weak nuclear forces. Explanations of these forces are generally called “quantum field theories”, and unlike quantum gravity, their validity has been verified by definitive evidence.
Since there is no accepted theory of quantum gravity, scientists are free to let their imaginations run wild. One common approach is to combine quantum gravity with successful quantum field theories. The idea is that if a technique successfully manipulates other known fundamental forces, perhaps the same approach describes quantum gravity as well.
One common feature of successful quantum field theories is that they all require one or more subatomic particles to mediate the force 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 molecules together. The third quantum force, the weak nuclear force, requires two particles, euphemistically called W and Z bosons.
Given that quantum theories of other forces all require a particle to transport them, scientists believe this is likely to be true for gravity as well. Although the force of a gravitational particle has never been observed, scientists have named it a “graviton.”
Since scientists know a lot about gravity, they can guess many possible properties of gravitons. Since the gravitational scale is infinite, gravitons must be massless. Since gravity ignores electricity, gravitons must be electrically neutral. Finally, since gravity is an attractive force, the graviton must have a subatomic spin of 2 (in contrast to other force-carrying quantum particles, which all have a subatomic spin of 1).
While assuming particle nature is all well and good, until these predictions are verified experimentally, there is no reason to take them seriously. So, what are the chances of discovering gravitons?
The currently known laws of physics provide a sense of how difficult it is to verify a theory. For example, the force of gravity is very weak compared to other known forces. While the number depends a bit on the conditions under which you test it, it turns out that gravity is in the ballpark of 1040 Times weaker than electromagnetism. This weakness makes it difficult to detect gravitons. In any conceivable measurement involving subatomic particles, the electromagnetic force would dominate and mask any gravitational effects.
To generate a significant gravitational force, many atoms need to work together to create this force. An advantage of this approach is that atoms are electrically neutral, so the electromagnetic force would be zero. However, once you have many atoms working together, you no longer experience quantum volumes, and therefore no longer experience quantum gravity.
In fact, if the simplest ideas of quantum gravity were applied, scientists may never be able to verify the validity of the theory of quantum gravity. On subatomic scales, the force may simply be too subtle to be observed at all.
This brings us back to the question at the top of the article: Will physics one day prove that gravitons are real? There is certainly good reason to believe they might do so. There must be a theory of gravity that works on quantum scales, and all other quantum theories require particles to generate their own force. On the other hand, there is currently no experimental evidence to confirm or deny the existence of gravitons.
On the plus side, there is no fundamental barrier to testing quantum gravity, which means that some future methods may be able to answer this question. However, in the foreseeable future, this question cannot be answered with a simple yes or no. The only honest answer is maybe.
Sign up for Big Think on Substack
The most surprising and impactful new stories delivered to your inbox every week for free.
ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ
