Are there any massless particles?

Category: Physics      Published: October 23, 2023

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University

Yes, there are indeed physical particles that have zero mass. The particles that are massless are: the gluon, the photon, and the graviton. Their properties are summarized in the table below.

Gluons are the massless particles that hold together atomic nuclei. They also hold together neutrons, protons, and other hadrons. In other words, gluons are what deliver the strong nuclear force. They do this by acting on particles that have color charge (which has nothing to do with literal color or electric charge).

Photons are the smallest possible bits of light. All forms of light are composed of photons, including radio waves, microwaves, terahertz waves, infrared waves, visible light, ultraviolet rays, x-rays, and gamma rays. This means that light does not have mass. Photons are what deliver the electromagnetic force. They do this by acting on particles that have electric charge or magnetic moment.

Gravitons are the massless particles that deliver the gravitational force. Gravity acts most strongly on objects with mass, but also acts on all other objects, including light. However, gravitons are only hypothetical at this point.*

*Although graviton particles are currently hypothetical, gravitational waves have been confirmed experimentally to exist and to be massless. See below for a longer explanation.

**Photons cannot directly interact with each other through the electromagnetic force because they do not possess electric charge or magnetic moment. However, photons can indirectly interact with each other through intermediate particles and through gravitational effects.

Neutrinos were long thought to be massless but are now known to have mass. Also, some quasiparticles are massless, including acoustic phonons (the constituents of sound waves), Weyl fermions, and some plasmons. However, quasiparticles are not genuine particles on the fundamental level and therefore will not be considered here. Massless particles have the following general properties.

General Properties of Massless Particles

1. They can exist despite having no mass because they have energy.
2. They can never be motionless in any valid reference frame.
3. They must always travel at the speed of light, in all valid reference frames.
4. They have zero electric charge.
5. They have zero weak charge.
6. They have integer-valued spin.
7. Each is its own antiparticle.

Currently, gravitons are only hypothetical. There is not yet any scientific evidence that gravitons exist. Furthermore, there is not even a proven theoretical framework that predicts or describes gravitons. However, gravitons are not as wildly speculative as they may sound. The existence of gravitational waves has been confirmed experimentally and has been successfully predicted by Einstein's theory of general relativity. If they exist, free gravitons would simply be the particle constituents of gravitational waves. Even if gravitons end up not existing, gravitational waves certainly exist and are certainly massless.

Imaginative artistic depiction of a massless particle. Public Domain Image, source: Christopher S. Baird.

The bulk, free state of gluons is a quark-gluon plasma. These plasmas are incredibly rare and short-lived. Gluons are usually only found inside atomic nuclei and inside protons, neutrons, and other hadrons. Because gluons are essential for the stable formation and existence of protons, neutrons, and atomic nuclei, they are essential for the existence of all life and all other objects made of atoms.

In contrast to gluons, photons are typically present everywhere, even in deep space. Even if visible-light photons are not present (such as in an empty, sealed, opaque box), there are always some infrared photons, terahertz photons, and/or radio-wave photons present that were created by the thermal radiation process. For instance, the walls of an empty, sealed, opaque box will emit thermal radiation photons into the box. The bulk, free state of photons is a freely-traveling light wave, which can also be called an electromagnetic wave. More complicated photon-field states include electrostatic fields, magnetostatic fields, and polaritons. The most significant effects on humans from the photons that are present everywhere is the simple heating of tissues and the enabling of vision.

Gravitational waves can be present anywhere, but they are exceptionally weak and have no effect on daily life. Gravitational waves are the bulk, free state of gravitons. Other states are the static and pseudo-static gravitational fields that cause everday objects to fall and also cause the formation of moons, planets, stars, solar systems, galaxies, and so forth. Such gravitational fields are present everywhere (except, perhaps, in cosmic voids) and are strong enough to dictate the formation and motion of all astronomical-scale objects. Humans experience the static-gravitational-field state of gravitons every day. This is what we usually mean when we say "gravity." Gravitational effects fundamentally arise from spacetime curvature. This is similar to how electromagnetic effects fundamentally arise from curvature of the quanutum electromagnetic field.

The known massless particles have the special role that they deliver the fundamental forces. As mentioned above, gluons deliver the strong nuclear force, photons deliver the electromagnetic force, and gravitons deliver the gravitational force. Interestingly, the last fundamental force, the weak nuclear force, is delivered by particles that do have mass. These particles are called weak bosons.

The massless particles deliver the fundamental forces that hold things together. Click on the image to enlarge it. Public Domain Image, source: Christopher S. Baird.

The strong nuclear force is what binds quarks together to form protons, neutrons, and other hadrons, and binds together neutrons and protons to form atomic nuclei. The strong nuclear force is what enables objects with color charge to interact. (Again, this has nothing to do with literal color or electric charge. A better name would be "strong nuclear force source.") Interestingly, gluons themselves have color charge. This means that gluons can directly interact with each other through the strong nuclear interaction. In other words, gluons are directly self-interacting. This actually has profound consequences, such as causing quark confinement.

The electromagnetic force is what binds electrons and nuclei together in atoms, binds atoms together to form molecules, and binds molecules together to form larger objects, such as rocks, plants, animals, and mountains. The electromagnetic force is what enables objects with electric charge or magnetic moment to interact with other objects with electric charge or magnetic moment. It is also the force that gives rise to almost all macroscopic non-fundamental forces such as friction, drag, tension, wind forces, and normal forces. Photons themselves do not have electric charge or magnetic moment. This means that photons cannot directly interact with each other through the electromagnetic interaction. However, photons can indirectly interact with each other through the involvement of intermediate particles. Also, photons can interact gravitationally with each other because they have energy and travel through spacetime. However, these effects are exceptionally weak.

The gravitational force is what binds matter together to form moons, planets, and stars; binds moons, planets, and stars together to form solar systems; binds solar systems together to form galaxies; binds galaxies together to form galaxy groups; binds galaxy groups together to form clusters; binds clusters together to form superclusters; and binds superclusters together to form cosmic filaments. The gravitational force is what enables objects with mass to interact with each other. Furthermore, because the gravitational force fundamentally involves the warping of spacetime itself, it enables all objects to interact with each other, whether they have mass or not. For instance, massless photons have been confirmed to experience gravitational effects from astronomical objects. These effects include the gravitational lensing of light, the gravitational redshifting of light, and the gravitational trapping of light inside a black hole.

Furthermore, gravitational waves themselves are affected by gravitational effects. These effects include the gravitational lensing of gravitational waves, the gravitational redshifting of gravitational waves, and the scattering of gravitational waves off of other gravitational waves. This means that, if gravitons exist, gravitons are able to directly interact with each other through gravitational interactions. In other words, gravitons are expected to be directly self-interacting.

Topics: gluon, graviton, gravity, mass, massless particle, photon