# Quick question: Is light a form of matter? **URL:** \ **Category:** Factual Questions\ **Created:** [June 2, 2011, 2:59am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040 "2011-06-02T02:59:09Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![RadicalPi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/radicalpi/32/9806_2.png) [@RadicalPi](https://boards.straightdope.com/u/RadicalPi)\ **Post date:** [June 2, 2011, 2:59am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/1 "2011-06-02T02:59:09Z")
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. ---
**Author:** ![inkling](https://avatars.discourse-cdn.com/v4/letter/i/f17d59/32.png) [@inkling](https://boards.straightdope.com/u/inkling)\ **Post date:** [June 2, 2011, 4:13am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/2 "2011-06-02T04:13:29Z")
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. > **[Double-slit experiment](https://en.wikipedia.org/wiki/Double_slit_experiment)** > > In modern physics, the double-slit experiment demonstrates that light and matter can satisfy the seemingly-incongruous classical definitions for both waves and particles, which is considered evidence for the fundamentally probabilistic nature of quantum mechanics. This type of experiment was first performed by Thomas Young in 1801, as a demonstration of the wave behavior of visible light. At that time it was thought that light consisted of either waves or particles. With the beginning of moder ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [June 2, 2011, 4:23am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/3 "2011-06-02T04:23:52Z")
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.) ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [June 2, 2011, 6:18am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/4 "2011-06-02T06:18:22Z")
I rather suspect he means something like “does it have a physical presence, a reality, or is it just pure energy without form”. ---
**Author:** ![Try2B\_Comprehensive](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/try2b_comprehensive/32/65_2.png) [@Try2B\_Comprehensive](https://boards.straightdope.com/u/Try2B_Comprehensive)\ **Post date:** [June 2, 2011, 6:29am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/5 "2011-06-02T06:29:17Z")
Light exerts force like matter. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 2, 2011, 7:20am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/6 "2011-06-02T07:20:54Z")
> [@RadicalPi](#): > > Also, if you have the time, could you point out some other things that aren’t matter. Thanks. Ghosts Ideas Music Christmas ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [June 2, 2011, 8:13am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/7 "2011-06-02T08:13:58Z")
> [@inkling](#): > > 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. > > [Double-slit experiment - Wikipedia](http://en.wikipedia.org/wiki/Double_slit_experiment) The particle-wave duality exists for electrons, too. But we tend to consider electrons part of matter. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 2, 2011, 9:36am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/8 "2011-06-02T09:36:15Z")
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? ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [June 2, 2011, 10:15am UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/9 "2011-06-02T10:15:13Z")
> [@Mangetout](#): > > 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. 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. ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [June 2, 2011, 12:30pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/10 "2011-06-02T12:30:06Z")
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. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [June 2, 2011, 1:22pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/11 "2011-06-02T13:22:52Z")
> [@Pasta](#): > > 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.) 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. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [June 2, 2011, 1:33pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/12 "2011-06-02T13:33:27Z")
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… ---
**Author:** ![samjones](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@samjones](https://boards.straightdope.com/u/samjones)\ **Post date:** [June 2, 2011, 1:44pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/13 "2011-06-02T13:44:29Z")
> [@Mangetout](#): > > Christmas I suppose you think “Seasons Greetings” _are_ matter? The war against Christmas has officially extended to the world of physics. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 2, 2011, 2:17pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/14 "2011-06-02T14:17:28Z")
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. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [June 2, 2011, 2:37pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/15 "2011-06-02T14:37:22Z")
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. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 2, 2011, 2:55pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/16 "2011-06-02T14:55:46Z")
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 … ---
**Author:** ![Yllaria](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yllaria/32/3452_2.png) [@Yllaria](https://boards.straightdope.com/u/Yllaria)\ **Post date:** [June 2, 2011, 3:04pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/17 "2011-06-02T15:04:17Z")
> [@Mangetout](#): > > . . . - 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. . . 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.) ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 2, 2011, 3:14pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/18 "2011-06-02T15:14:29Z")
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. ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [June 2, 2011, 3:16pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/19 "2011-06-02T15:16:04Z")
> [@RadicalPi](#): > > if you have the time, could you point out some other things that aren’t matter I would, but … [sunglasses] they don’t matter. [cue music] ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [June 2, 2011, 3:19pm UTC](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040/20 "2011-06-02T15:19:53Z")
> [@Askance](#): > > I rather suspect he means something like “does it have a physical presence, a reality, or is it just pure energy without form”. 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. Stranger [Next page](https://boards.straightdope.com/t/quick-question-is-light-a-form-of-matter/584040.md?page=2)