Quick question: Is light a form of matter?

I was always under the impression that light is not a form of matter, but rather something else. Is this true? Or am I wrong? Also, if you have the time, could you point out some other things that aren’t matter. Thanks.

I Am Not A Physicist, but my (no doubt flawed) understanding is that light is both matter and energy depending on how you look at it. And strangely, how you look at it influences how it behaves.


One working definition is that all the fundamental fermions and any composite particles made up of those fermions is matter. This would be quarks, charged leptons (electron, muon, tau), and neutrinos, plus composite objects like protons, neutrons, and (say) a brick. Photons would not be matter under this definition. But, “matter” hasn’t proven itself as a useful enough term to earn a specific definition, and there are numerous ways that the word “matter” actually gets used, depending on context.

(Or, in other words: the scientific community at large doesn’t assign any specific definition to the word “matter”. As long as you define what you mean in any given context, though, you’re okay to use it as a word.)

I rather suspect he means something like “does it have a physical presence, a reality, or is it just pure energy without form”.

Light exerts force like matter.


The particle-wave duality exists for electrons, too. But we tend to consider electrons part of matter.

I thought matter was generally defined as everything from atoms up - once you start talking about fundamental particles in their own right, the term ‘matter’ (which I understand to be a general one, with chief utility in talking about the composition of physical objects) doesn’t seem so useful.

Does light occupy volume, even?

That’s one definition, though like others it has problems at the extremes. e.g. it implies that a neutron star contains no matter, even though they are typically about 1.5 times the mass of our sun.

There are two fundamental types of particles: fermions, and bosons. There is a very intuitive distinction between them owing to the fact that fermions can emit/absorb bosons, but bosons cannot emit/absorb fermions. What this means is that fermions can be said to ‘interact’ via the exchange of bosons. So there is a sense in which bosons are ‘messenger’ particles, and fermions are ‘matter’ that interacts with other matter through the exchange of bosons. Light is an example of a boson, a messenger particle. It’s role in life is to go back and forth between fermions (like electrons), and mediate how they interact with each other. Ultimately the word ‘matter’ it is just an arbitrary definition, but I’ve found that generally the term is used by particle physicists to refer to fermions as opposed to bosons.

The problem with this distinction is that you can combine fermions to get bosons, like mesons or Cooper pairs, for instance, or much more complicated things, like (famously) rubidium-87 nuclei, which one would probably ordinarily like to think of as matter… So a hard-and-fast divide is hard to come by.

That said, when it comes to elementary, rather than composite particles, I’d tend to think of fermions as matter, as well, and thus, would not count the photon as a matter particle.

Incidentally, I remember reading about an approach to describe the photon as a bound state of neutrinos (by de Broglie?), but in the end, it didn’t work out…

I suppose you think “Seasons Greetings” are matter? The war against Christmas has officially extended to the world of physics.

Does having mass factor into the definition at all?

I thought that it was that the force particles are not matter and have no mass (even if the Higgs boson exists and is what gives the property of mass to other particles). Force particles are all bosons but not all bosons are force particles. Photons, gluons, w and z particles, and if every proven to exist, gravitons and/or Higgs, are force particles, not matter. Mesons are bosons but not force particles, have mass, and are matter, for example.

Ws and Zs do have mass (at least on energy scales on which electroweak symmetry is broken), however, and neutrinos were originally thought to have none, and one of them still might not have mass, even with neutrino oscillations.

Huh. Did not know that.

How about then defining that anything that is not a force particle is matter and force particles are not? It seems to capture common usage even among physicists …

At the subatomic level you get to use the cool quote from Raymond Hall, Fermilab: " Stuff is made of particles. Therefore, particles can not be made of stuff."

(I may have found the quote here on the board. But when I search to see, it fills the memory and errors out, so I can’t give attribution.)

Light (and electromagnetic radiation in general) is made up of photon, and differs fundamentally from leptons (electrons, muons, and the like), hadrons, andthe like. They have no rest mass and travel at the speed of light, which sets them apart from most other elementary particles (neutrinos have a small mass, and don’t travel at lightspeed. Gravitons, if they exist, are massless)

Photons, despite having no rest mass, can nevertheless carry momentum and exert force. Light clearly interacts with matter, but in an odd way – since it carries no electric charge, it doesn’t directly interact with electric or magnetic fields (electro- and magneto- optics effects are really due to the electric or magnetic field interacting with the medium through which the light passes, or by which it is absorbed or emitted). There is a Feynman diagram for the interaction of light with light, but it requires each photon to create virtual particles which then interact with each other via more standard interactions, and the probability of such interaction is extremely small. For practical purposes, it doesn’t exist – you can send extrmely powerful laser beams through each other in vacuum and not see any interaction.
By most common-sense definitions, then, light isn’t matter. In order to get some aspect of it to act like matter, you have to create a pathological situation. In ordinary human experience – even in most human laboratory experience, light is a very different animal.

I would, but …


they don’t matter.

[cue music]

There is no such thing as “pure energy without form” except in bad science fiction.

As Pasta notes, modern particle physics and the Standard Model does not have a rigorous definition of “matter”, unless by that you mean fermions (particles that obey Fermi-Dirac statistics and thus, obey the Pauli exclusion principle). Photons do occupy a space in the sense that it consists of a field, but the same can be said for fermions as well. In fact, all matter and energy are, at least from the point of view of quantum field theory, just fields of quantized energy that interact by a defined algebra.