Quick question: Is light a form of matter?

As has been noted ‘matter’ is not rigourously defined enough in physics to give a definitive answer.

I’d say most of the time you see the term ‘matter’ used generally it’s definition, on balance of probabilty, won’t include light, for example ‘matter-filled universe’ vs radiation-filled universe’ in general relativity. However seeing ‘matter’ used in a way that includes light or is ambiguous as to whether light is included is common enough as to be completely unremarkable.

I would scratch that sentence, replacing it with “There is no clear common usage among (particle) physicists.”

Just for kicks: another use for the word “matter” is as an antonym for “antimatter”. This can specific, referring only to baryons (versus antibaryons), or it can be more general, referring to electrons (vs. positrons), quarks (vs antiquarks), etc. Since a photon is its own antiparticle, it can’t be catergorized as “matter” or “not matter” under this usage (as “not matter” would incorrectly imply “antimatter”, in the same way that “Joe is not left-handed” implies that Joe is right-handed.)

But, again, this is just another of many context-dependent usages you can find in ithe wild.

The question might be quick, but generally there are no quick answers in physics if you have enough physicists in the room.

Eh, it’s not really a physics question. If the question were “Is a photon a fermion?”, or “Does a photon have mass?”, then there’d be a quick answer (no, in both cases). But “Is light matter?” is a terminology question, not a physics question, and terminology is squishier than physics.

Why isn’t “Is a photon a fermion?” a question of terminology? It just seems that the definition of “fermion” has been nailed down much better than that of “matter”.

The properties of a fermion (half-integer spin, obey canonical anticommutation relations algebra) are well established, and a photon does not satisfy this definition. Matter, on the other hand is a term that “everybody knows”, but isn’t really well defined in a particle physics sense, and indeed, may not be a fundamentally sensible question.


In elementary school I was taught that matter is defined as anything that has mass and takes up space. Of course “mass” was defined as a measure of the amount of matter in something, so clearly there were some terminological issues going on. I’m not sure what “takes up space” means (does an electron take up space? how much?) but I thought that having rest mass was more or less definitional.

Did I learn * anything* in elementary school that was true?

“Takes up space” can be interpreted as a quick and dirty statement of the Pauli Exclusion Principle, I suppose.

What about e=mc[sup]2[/sup]?

If light exerts force, then it has/is energy. If it has/is energy, frankly that is the equivalent of a certain amount of matter. Or mass anyway- mass in the space of a Cartesian point?

A better question might be ‘is no-matter possible?’ or maybe ‘is there any such thing as empty space?’.

None of these statements make any sense.

All of space is filled with fields, and even isolated vacuum is hypothesized to contain a foam of virtual particles being constantly generated and annihilated at energy levels too small to detect normally and for intervals that are not observable.


I had assumed that it did have a specific definition, the one you just said about the quarks and the leptons, but it seems that I was wrong. It seems that the term “matter” is a sort of catchall with a meaning that depends on what you’re trying to do. Thanks to you and the others for pointing that out to me. Ignorance successfully fought.

This raises another question for me, though. If we don’t really know what matter is, how do we know what antimatter is? Is there a clear way to distinguish the two, or is it just that the particles that we loosely call matter are just the ones that happen to be common and antimatter is the stuff that isn’t? Or is this another thing that doesn’t come up enough to be useful in physics. Like that two particles are either antiparticles or not, and the “team” you would put them on (the Regulars versus the Antis) isn’t really important. But maybe it is, since the lack of antimatter is, if I’m not mistaken, something that needs to be explained that hasn’t been explained yet.

No, this is not what I meant. I was wondering if things like dark matter or antimatter or energy or some other things along those lines were considered to be matter, but I didn’t want to bring them up and embarrass myself.

There are many particle-antiparticle pairs for which one is significantly more common than the other, but then again, there are also many such pairs for which both are equally common. Where things get especially confusing is with the mesons, which consist of a quark and an antiquark. Everyone agrees that the pi+ (an up quark and an antidown quark) and the pi- (a down quark and an antiup quark) are antiparticles of each other, but how do you say that one’s the “particle” and the other the “antiparticle”? The usual convention in such cases is to call the negatively-charged one the “antiparticle”, but that’s just a convention, and anyway there are also some neutral ones.

In any context in which one is dealing with dark matter, it would be considered “matter”, even though it might or might not be fermions. Antimatter, it depends on the context, and most forms of energy would usually not be described as “matter”.

Unless of course “dark matter” turns out to actually be antimatter oriented in the extra curled up dimensions in such a way that it is only partly observable by “matter” which turns out orient itself differently in those dimensions - in such a case gravity would be the only interaction able to spill through the multiple dimensional space.

That’s what you always say!


Sooner or later that concept will catch on! And I am just gonna keep at it!

Be careful with that, since we really don’t know what <i>anything</i> is. Pick nearly any topic at all, and you can say we don’t really understand it 100%, since no concept is imperfect and all have limits, and there are some slightly fuzzy areas and unknowns. Wait a few hundred years, and we’ll have much better understanding. But for most practical purposes, we already understood matter a hundred years ago.

Also, think what happens whenever we ask questions about “x” but without providing our working definition of the word “x.” It’s a recipe for endless arguments and clouds of confusion. Paraphrasing Feynman: if nobody agrees on what word “Wakalixes” means, then it’s not a good idea to start extensive discussions about “Wakalixes.”
But yes, the “regulars” are simply the ones which are common, and the “antis” don’t survive long unless they can be kept in a good vacuum and prevented from contacting any “regulars.” You can make anti-hydrogen gas using anti-protons and positrons, but you can’t store it in a jar.

Further thought about original post: In grade school we learn about matter and energy. We’ll possess a solid understanding… as long as we stay away from more advanced classes!

Also, is a magnetic field a form of matter? How about an electric field? Are radio waves a form of matter? So matter can travel right through walls, right through plastic and glass? (After all, light is also waves of EM fields.) Or going farther, is sound a form of matter? Sound after all is quantized as phonons. Are waves on a rope a form of matter?

yes and no. If you mean wavelength that light has and how it varies then no.

But the light is particles reflecting off objects at different wavelenths creating colour as each colour is a differnt wavelength in the visible spectrum. But it is made of photons which are particles. That is why the mere viewing of an experiment on the sub-atomic level changes its aoutcome.

That’s not really right. You’re referring to the ‘quick-and-dirty’ explanation of Heisenberg uncertainty (‘Heisenberg’s microscope’), which was indeed reportedly how it was first derived, but it is actually much more fundamental than that – for any observables that don’t commute with each other, it’s the case that they can’t be measured to arbitrary accuracy simultaneously. This isn’t just an effective experimental limitation, as it would be in the ‘disturbing photon’ thought experiment, but rather, it’s physically fundamental – the information just does not exist.

Also, this whole ‘observation changes the outcome’ is a poorly popularized version of the idea of wave-function collapse – that from a superposition of many possible states for a system to be in, measurement selects a specific one to become ‘actualized’. This is in itself a somewhat contentious concept, with long standing debates raging over whether or not it actually happens, is a real, physical process or just something akin to the Bayesian updating of outcome probabilities when new knowledge about the system is gained, but the essence of it is that a system has a well-defined, unique state only in the context of measurement – so it’s not that ‘observation changes the outcome’, but rather that observation (which, btw, doesn’t imply anything like a conscious, human, or even (very) macroscopic observer) ensures the existence of any unique outcome at all.

When i said that the observation changes the outcome what i was refering to is that for us to view anything we need photons as it photons bouncing off particles that create an image for a human eye. But these photons have the POTENTIEL although not always to change the outcome of an experiment.

Except one can do interaction-free measurements (the most famous form of which is the so-called bomb tester experiment) in quantum mechanics, such that, for instance, one can build up an image of an object without ever having any photons interact with it; yet even here, Heisenberg uncertainty is preserved. But this is rather tangential to the topic at hand.