A graviton is a hypothetical particle that would carry the force of gravity, much like a photon carries the force of electromagnetism. It has never been detected, but physicists have strong theoretical reasons to believe it exists. If gravity works the way the other fundamental forces do at the quantum level, gravitons are the particles responsible for transmitting gravitational attraction between objects.
How Gravitons Fit Into Particle Physics
Three of the four fundamental forces in nature have known carrier particles. Electromagnetism has the photon. The strong nuclear force (which holds atomic nuclei together) has gluons. The weak nuclear force (responsible for certain types of radioactive decay) has the W and Z bosons. Gravity is the odd one out. It’s the only fundamental force without a confirmed quantum carrier, and the graviton is the proposed particle that would fill that gap.
In quantum field theory, the graviton is expected to be massless, which lines up with the fact that gravity has unlimited range. Just as photons are massless and electromagnetism reaches across the entire universe, a massless graviton would allow gravity to do the same. The graviton is also predicted to have no electric charge and a property called spin-2, which is higher than any confirmed force-carrying particle. Photons, for comparison, are spin-1.
Why Spin-2 Matters
Spin in particle physics isn’t literal rotation. It’s a quantum property that determines how a particle interacts with other matter and how it behaves under certain mathematical transformations. The reason physicists are confident the graviton must be spin-2 comes down to the nature of gravity itself. Gravity’s source is energy and momentum (described technically by something called the stress-energy tensor), and the mathematics require a spin-2 particle to properly reproduce the behavior we observe in general relativity. A spin-1 particle, like the photon, would produce a force where like charges repel. But gravity is purely attractive between masses, and only a spin-2 carrier reproduces that.
This prediction is so specific that if a massless spin-2 particle were ever discovered, physicists would identify it as the graviton almost by definition.
What Gravitons Would Actually Do
In the same way that photons are exchanged between charged particles to create electromagnetic effects, gravitons would be exchanged between anything that has energy or mass to create gravitational effects. When Earth pulls you toward the ground, the quantum description of that interaction involves a constant exchange of gravitons between your body and the planet.
One particularly interesting theoretical prediction involves quantum entanglement. Physicists have shown mathematically that if two masses are placed near each other, the quantum nature of gravitons exchanging between them could entangle the two objects. This means the masses would become correlated in ways that have no classical explanation. Importantly, a purely classical gravitational field (one without gravitons) cannot produce this entanglement. This distinction has become a key idea for potentially confirming that gravity is truly quantum in nature, even if individual gravitons remain invisible.
Gravitons in String Theory
String theory offers a different way to think about gravitons. In this framework, all particles are tiny vibrating strings of energy, and the type of particle depends on how the string vibrates. Gravitons appear naturally in string theory as a vibration mode of closed strings, which are loops with no endpoints. This was actually one of string theory’s earliest and most compelling features: gravity wasn’t added by hand but emerged automatically from the math. A graviton in string theory is a massless spin-2 particle, matching the predictions from standard quantum field theory.
How Fast Gravitons Would Travel
Because gravitons are predicted to be massless, they should travel at exactly the speed of light. This prediction got strong indirect support in 2017, when detectors picked up gravitational waves from two colliding neutron stars (an event called GW170817) at nearly the same instant a gamma-ray burst arrived from the same source. The gravitational signal and the light signal traveled for about 130 million years and arrived within seconds of each other, showing that gravitational waves move at the speed of light to within one part in a quadrillion. Combined with additional observations from black hole mergers, the measured speed of gravitational waves falls within 97% to 101% of the speed of light at 90% confidence.
This doesn’t prove gravitons exist, but it confirms that whatever carries gravity behaves exactly as a massless particle should.
How Heavy Could a Graviton Be?
If the graviton has any mass at all, it would be extraordinarily small. The best current upper limit, derived from measurements of the cosmic microwave background, puts the graviton’s mass at less than 5 × 10⁻³² electronvolts. For perspective, an electron (already one of the lightest known particles) has a mass of about 511,000 electronvolts. So even if the graviton isn’t perfectly massless, its mass would be at least 37 orders of magnitude lighter than an electron. Most physicists expect it to be truly massless, but these experimental bounds keep tightening the constraints.
Why We Haven’t Detected One
The core problem is that gravity is extraordinarily weak compared to the other fundamental forces. It’s roughly 10³⁶ times weaker than electromagnetism. This means gravitons interact with matter so feebly that detecting a single one is, with current technology, essentially impossible.
The idea of quantizing gravity dates back to 1935, when physicist Matvei Bronstein first worked out a linearized quantum theory of gravity that produces gravitons in direct analogy to how quantizing electromagnetism produces photons. The math works in principle. You can calculate the rate at which an atom would emit a graviton using the same tools physicists use to calculate photon emission. The problem is that the resulting emission rates are vanishingly small. An atom transitioning between energy levels emits photons readily, but the probability of it emitting a graviton during the same transition is so tiny it would take longer than the age of the universe to observe.
A 2024 paper in Nature Communications explored whether quantum sensing techniques could eventually detect single gravitons. The approach would involve watching for the telltale quantum jumps in a carefully prepared detector system, similar to how physicists detect individual photons. The theoretical framework is sound, but the engineering challenges are staggering. No experiment currently planned can achieve the sensitivity needed.
Gravitational Waves Are Not Gravitons
It’s worth clarifying a common point of confusion. Gravitational waves, which LIGO and Virgo have been detecting since 2015, are ripples in spacetime produced by massive accelerating objects like merging black holes. These are real, confirmed phenomena. But detecting gravitational waves is not the same as detecting gravitons. A gravitational wave is a classical phenomenon, like an ocean wave. Gravitons would be the quantum units making up that wave, analogous to individual water molecules. The waves LIGO detects contain an incomprehensibly large number of gravitons, but the detectors measure the collective wave, not individual particles.
Until someone finds a way to detect single gravitons or observe an unmistakably quantum effect of gravity (like gravitationally induced entanglement between masses), the graviton remains one of the most well-motivated but unconfirmed predictions in all of physics.