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Is this article confusing the notion of relativistic mass with rest mass?[edit]

We have this sentence,

"By contrast, massless particles, such as photons..." which uses the modern definition of mass where photons are mass-less. But then there is this "For example, mass is a conserved quantity, which means that its value is unchanging through time, within closed systems" which is using the relativistic notion of mass, which in my education we learned is a depreciated way of looking at things.

These sentences seem to me to be a direct contradiction. If mass is truly conserved then we have to consider the photon to be a massive particle. This is the old way of looking at things, were distinctions are made between rest mass and relatistic mass. If we consider the photon to be mass-less, as it is usually defined in modern times, then mass is not conserved.

Per Noether's theorem, symmetries produce conserved quantities like energy and momentum. There is no symmetry that entails conservation of mass, is there?

ModusPwnd (talk) 23:58, 20 May 2013 (UTC)ModusPwnd

There are two types of energy and two types of mass. Rest mass and invariant mass (together constituting one type) are both conserved (they stay the same for any observer, though time). They are also invariant (their value is the same for any observer). Photons have no rest mass, although one or more photons moving in different directions do have an invariant mass, as a system (even though each is individually massless). Divide any of these quantities that have mass by c^2 to get rest energy (of course for single photons you divide zero by c^2 and get a rest energy of zero).

The other type of mass, relavistic mass, is simply E/c^2 and is deprecated because you might as well use total energy. Single photons do have relativistic energy and mass. Relavivistic mass is also conserved in time for any given non-accelerated observer (which is what "conserved means), even though different inertial (galilean reference frame) observers disagree as to its absolute value (for a single photon it can be anything you like).

It's difficult for many people to get used to the idea that a single photon added to a box increases its (rest) mass on a scale, even though the photon itself has no mass. But that's because you're weighing the photon-box system, which has zero momentum, and so its rest energy is equal to its total energy and its rest mass is equal to its relativistic mass. These matters are more deeply explained at mass-energy equivalence.

And by the way while mass is a conserved quantity, MATTER is not. Which is the whole point of addressing this in this article. If you have an electron annihilate a positron in a box, the two (massless) gamma rays will bounce around inside the box and its mass (on the scale) won't change! That's because as a system, the two gamma rays still have two electrons' worth of mass! WHich you can weigh when they are trapped. But of course, they are no longer matter. SBHarris 00:27, 21 May 2013 (UTC)

But a scale doesn't measure mass, it measures force. The photon's energy contributes to the gravitational force. They have no mass. Mass-energy equivelence doesn't mean energy is mass, it means they are equivalent. This is how I was taught and this is how I teach my students. There is no conservation of mass, that is an approximation that does not strictly hold. If it did, there would have to be a symmetry associated per noether's theorem, but there is not... is there? Photon's are massless. A system of photons is massless. They are subject to the gravitational force because of their energy. Otherwise, you are using depreciated relativistic mass and should not call photons massless.

Some links From Harvard-Simithsonian center for astrophysics; From the Physics Forums;

From Urbana-Champaign;

Though I think that Urbana-Champaign link may be making the same confusion between relativistic and rest mass. ModusPwnd (talk) 16:50, 21 May 2013 (UTC)

The measurement of the box of two photons doesn't have to be with a scale, it can be by shaking it (as in zero-g mass measurement). The box has more inertia. And it gravitates more, which is why it weighs more on a spring scale, too. There is no measurement you can do on the outside of box to see if it holds an electron and positron, or the two gammas from their annihilation. And there's a basic reason for this.

Two photons (so long as not moving in the same direction) are not massless but DO have mass, and not just "relativistic mass". See [1] but this is a standard student exercise in SR. This mass (again) is not the deprecated Tolman "relavistic mass" (although it happens to be equal to it, for reasons to be explained). It is (also) invariant mass which is system mass, which is what you weigh or measure. And what has inertia and gravitates. And constructs the Landau-Lifschitz pseudotensor of the system, and so on. In that sense it is "real" and is the only type of energy you can use to construct new particles from in experiments, and so on.

Unlike the energy of a SINGLE photon, which doesn't really have a unique value (depends on who is looking) the mass of two photons has a minimum value which cannot be reduced any further by reference frame-change. This value is in the COM frame, which is the frame where momentum is zero, which means both photons have the same energy (momentum) and are traveling in exactly the opposite direction. In this frame, their invariant mass (and relativistic mass) is 2E/c^2 where E is the energy of either photon. In any other frame the total E is larger, but the invariant mass stays the same (of course) because such a system has net momentum which subtracts from the energy in the E^2 - p^2 = m^2 relation. In the COM frame, E=m. Hence the utility of the COM frame-- relativistic mass, energy, and system invariant mass and energy are all the same there. Relativistic E and m differ when seen in other frames. But they can't be used to make new particles (energy transformed to matter). Only invariant mass can be used for that.

The massiveness of massless fields is a good thing, since most of reality requires it. The equivalence of relativistic mass and invariant mass in the COM frame comes in handy in measuring the mass of systems like-- hadrons. These are composed of massive (but fairly light) quarks moving very fast, but bound in a "box" of a hadron particle (like a neutron or proton). Only 2% of the mass of the neutron or proton you weigh is the rest mass of the quarks inside. The rest (exactly as with the photons in the box above) is the mass of massless fields (composed of photons or gluons), or kinetic energy (which of course has relativistic mass but no REST MASS, since it's never seen in the given particle's rest frame). So 98% of the simple mass of "ordinary matter" is actually due to massless particles or massless kinetic energy of massive particles. But it all shows up as "rest mass" of the confining particles because we measure this in the coumpound particle (system) rest frame, where relativistic mass IS THE SAME AS invariant system mass. Same value. Cool, eh? Therefore: Massless particles add rest mass to systems. Make your students say this many times.

I haven't yet checked out the websites you provided above (no time this second) but I will. These websites and many science teachers, however, screw this all up. Some of the fault of this goes to Tolman and his wacky idea of relativistic mass to keep E=mc^2 always "true." But the rest of the problem is that matter gets confused with mass, as in our box of matter-antimatter or gamma rays. There we have matter disappearing, and matter appearing, and vice versa, but the invariant mass of this system does not change through it all. And if it were confined, both photons and matter would weigh out as rest mass, which also would not change as one was turned into the other.

Blow up an atom bomb in a superstrong box and does its mass change? Not until you let out the light and heat. Until then, it just stays very hot and its mass is constant (shake it and it has the same inertia, weigh it and it weighs the same-- even though full of plasma). But let the light and heat out (cool the particles down, as Feynman would say), and they transfer mass to whatever absorbs them, so you see that the total mass is conserved. Not only relavitistic mass but also invariant mass. And yes, these are both types of energy and so are subject to the energy-time symmetry, as advertised by Noether.

By the way, Taylor and Wheeler in their book on SR, _Spacetime Physics_ get all this exactly as explained above. So this is not some wacky theory of mine own. It's just that (as explained) a lot of even science teachers have not gotten it straight. It's not easy. SBHarris 23:22, 21 May 2013 (UTC)

Well, thanks for the words. I'm still not convinced that this is the best way to look at it, but I do see your points. "There is no measurement you can do on the outside of box to see if it holds an electron and positron, or the two gammas from their annihilation. ".. why does it have to be on the outside? You can open the box. This is how I would determine how much of the inertia/gravitational force is due to massless energy of photons or the energy of mass. There is no need to be able to determine this from a closed box. Shaking is interacting with the system just as opening it is. I would shake the box, note the inertia or weigh the box and note the gravitational force and compute the energy. Then I look inside, see positronium and note the mass. Then I close the box, allow the positronium to decay and shake it and weigh it again. Same inertia, same gravitational force. The energy is conserved. Open the box, no massive particles. Mass is not conserved. Seems simple enough to me.
As you describe, binding energy is more complicated since its certainly associated with matter unlike photons. Would you note the binding energy of the positronium as massive or massless energy? That is a matter of taste I suppose, depending on what you are doing. I wouldn't consider positronium to be matter. Even a heavy atom is tough to call matter, IMO. I think of it as necessarily macroscopic thing, but then it doesnt really matter since matter is not really a scientific term anyway, unlike mass. ModusPwnd (talk) 01:56, 22 May 2013 (UTC)

The problem with your method for defining "mass" is that it discounts "stuff" in the box that acts exactly like any other mass, just because you don't want to call it mass. And it forces you to conclude that the 98% of a neutron and proton (and thus an atom) that isn't massive quarks, is NOT mass. Which means that 98% of the what we weighed (or otherwise determined) was the mass of you and me isn't actually "mass." I opened up the box of my nucleons and found that 2 kg of me is real mass and 98kg of me is fake mass due to being massless field? Now what? This seems not a very good definition for mass. What's the point of it? The whole idea of mass is that it's anything that acts like mass. SBHarris 03:14, 22 May 2013 (UTC)

Not exactly like any other mass. Photons are mass-less and as such they have specific characteristics. Once they are absorbed they are no longer photons. Per the interpretation you put forth, photons should not be called massless right? To be consistent with the description you have described, we need to change the description of the photon and call it a massive particle. How is the description you put forth different than just embracing the notion of relativistic mass and saying that the photon is a massive particle? Whats wrong with defining bound particles to have the mass of their constitutions and their binding energy and considering photons to be massless and thus mass is not conserved? A free electron has a different mass than a bound electron. Does it seem to ad-hoc? I guess that is the weakness of all interpretations. But that is the interpretation I think my education centered around. ModusPwnd (talk) 04:15, 22 May 2013 (UTC)

If you will read [2] you will see that TWO photons do have a mass, and it's not relativistic mass. Nor are they required to be bound. This is real physics; read the paper. There's a lot of discussion of it on the Physics Forum, which tracks what we've said: look at Pervect and Hillman's answers: [3] The fact that two photons has a mass does not mean that ONE photon in free space has a mass. ONE free photon is massless.

Looking at binding energy makes the problem we consider more complicated because binding energy is (by definition) lost from the system, so energy is not conserved when that happens. If you could recover the binding energy from where it went (to a second system B) you would find it equalled the lost energy in system A. Lost binding energy makes mass smaller because energy is lost, and its mass along with it.

A gedanken is helpful. Take a box with 26 free protons and 30 free neutrons and measure its mass. Now. allow the nuucleons to fuse into an iron nucleus (Fe-56). This takes overcoming the EM potentials ala nuclear fusion, but pretend we do it by tunnelling with no energy input. The result is a Fe-56 nucleus which has 99% of the mass of the protons+neutrons. The 1% of mass is now present as gamma rays, given off when the nucleons combined. That is binding energy. So long as you don't let the gammas out of the box, the mass of the box is unchanged. If you let them out into box B, the mass decreases by 1% of the mass of Fe-56, and that's the binding energy. But the mass of box B increases by exactly as much, when it absorbs the gammas, so the mass of binding is/was conserved. It just moved from here to there. In general that's always true. Though occassionally forgotten when the mass is not kept track of. SBHarris 20:45, 22 May 2013 (UTC) From the Physics Forums; You quoted this thread above and I promised to read it. Did you read to the end? Tom.stoer said the same thing I have said. Invariant mass is conserved, period. The sum of rest masses is not conserved, but nobody ever said it was. Invariant mass is not relativistic mass, it's something else entirely. For systems, invariant mass corresponds to system rest mass, in fact. SBHarris 02:39, 23 May 2013 (UTC)

Any possibility of a better, clearer Introduction? Rewrite recommended.[edit]

I find the Introduction, at best, confusing. I am a chemist with some partial understandings of this topic - the entire article is definitely from the physics point of view, obviously. Here is my 2 cents. I tried to confine my comments to the introduction, but find that its weaknesses spill over into the other sections. It is way way too long! The last paragraph seems to be →almost← adequate as an introduction. It recapitulates some of the excess verbiage of the first several paragraphs, but misses some important things (as does the entire intro, I think). One of the first problems I have with the introduction is that is simply not unified. I think it is necessary introduce the "definitions" section along with the explanation that the word is used in different scientific disciplines to mean different things, its use is not consistent, and that there are no accepted definitions of the word (IS it a term?). I argue that there are not 5 definitions (meanings). I think that the section on Relativistic Matter should be removed. Mass (as a substance) is far more likely to be what is meant by the RARE use of the term in discussions in Special Relativity. BUT, I argue that there needs to be a section on Cosmological Matter (possibly including sub-sections on ordinary and dark matter, (bosonic matter covered here if not elsewhere)). Luminous and Nonluminous matter need explanation. Based on the discussions on this page, there are many much more competent than I to do this.

I came to the article looking for the best current value for the amount of ordinary matter in the observable Universe. I am looking for a quantitative mass guesstimate. That should be, it seems to me, a basic piece of information given here. The pie chart has luminous, nonluminous, and dark matter and also dark energy. This really needs to be explained. Where are the photons? Are they all included in the 0.005% radiation component of luminous matter? Just because W/Z bosons have a short life-time, why are they ignored? At any one "instant", is the energy they constitute insignificant? I think the article confuses the levels of abstraction between the Gauge Field paradigm and the Atomic paradigm. NO discussion of matter should ignore energy, yet this article seems to avoid it. Atomic matter is not "usually" composed of electrons, protons, and neutrons - it is ALWAYS composed of them! This is basic chemistry. Yet, I saw nothing about the elements, and a very weak exposition about the building blocks of the Universe (from the chemical pov). Discussing neutrinos, etc. is more appropriate for the sub-atomic, NOT the atomic, paradigm.

So there are four forces in nature (standard model only accounts for 3, right?) Matter and the 3 forces (together with gravity??) needs to be discussed. As far as I understand it (poorly) ONLY the electromagnetic force/field has any significant →non-atomic← contribution to the energy of the Universe. Perhaps neutron stars must be discussed? The article mentions the types of matter, but is inconsistent and not unified. What we see is atomic matter? We can not "see" neutron stars? (Quark-gluon stars?) Degenerate matter?

Basic problem is that the article doesn't have a single voice. All of its problems are tractable. Where in the article does it "explain" the difference between matter and the energy it contains? I understand that the concept of matter as something different from matter+energy is subtle and can't be consistently defined (it varies depending on context). Where is that explained here? — Preceding unsigned comment added by (talk) 21:44, 1 June 2013 (UTC)


The article doesn't speak with a single voice since long, long ago, somebody from the physics community decided that they "knew" what matter was, and what it was, was fundamental fermions (quarks and leptons). They then wrote the article that way. The problem being that the "guage fields" of these leptons (made of virtual bosons like gluons), are reponsible for 98% of the mass in nucleons, and thus 98% of the mass of "ordinary matter" (stuff made of chemical elements). So it was all confusing. The die-hard finally said he was fine with idea that 98% of the mass of matter is not matter (but is rather something we now define somewhat arbirarily as "energy" due to it having no rest mass), but several of us (including me) thought this was crazy. This gluon stuff DOES have rest mass in systems, like.... nucleons. Some of the nucleon mass is also quark kinetic energy (I have no idea how much). What a pain.

The truth is that nobody really has a good definition of "matter." So a unified presentation can hardly be expected. Perhaps the best way to look at this, is in the last paragraph of the article, not the intro, in which it is pointed out that the word is now almost meaningless, except when used as part of a qualified phrase like "dark matter" or "orginary matter" or whatever. Astronomers tend to use the word his way.

Astronomers do tend to lump the photons in with the "ordinary matter" mostly because it doesn't matter if you do or don't (as you point out). Things like "weakons" (W/Z) are not seen in ordinary matter except as virtual fields, and these don't add much mass.

Less than 1% of the mass of free H is released when it binds into Fe-56 and other elements (where the fraction is less). It's hard to say what this 1% consists of. It's sort of a mix of nuclear force "field" that is destroyed and half as much "mass" of new electric field that is "made" as the positive charges are shoved together in fusion. These fields both have mass since they are energy (one positive one negative, adding to a net negative) but the particles that make up the fields are virtual particles (if they are particles at all) because these fields are static fields. Virtual photons for the increased E field (giving it more mass) and virtual pi's and rho's for the nuclear field (which loses even more mass in fusion than is gained in E field; the reverse for fission). Again, all this is weighable in ordinary matter (iron weighs only 99% of 56 hydrogen atoms), but is this mass "matter"? I dunno.

It's not quite true that "Atomic matter is not "usually" composed of electrons, protons, and neutrons - it is ALWAYS composed of them!" At least if you don't mind defining "atomic matter" as unstable atomic matter (and why shouldn't you-- do we not define radioactive atoms as matter, too?). There are exotic atoms in which chemical matter binds some short-lived lepton or even hadron and what you have before the thing decays, is still matter. Hence the qualification in what you read, although the article on exotic atoms should have been referenced.

Astronomers usually consign gravitational energy to be negative rather than positive, and various experiments suggests that our universe is "flat" and thus very near the "critical ratio" of having just enough negative gravitational energy to balance out the TOTAL positive mass-energy of [ordinary+dark] matter plus dark energy. This assumption is part of what is behind the idea of dark energy, which really only must be 3 times as much as all the rest, which isn't all that much (considering that the ratio of gravity to mass-energy could in theory be anything). Dark energy would have positive mass, but would not be matter.[4] So the mass-energy of the entire universe including gravity adds up to exactly zero, giving rise to the cute idea that everything is just a quantum fluctuation, of the sort that happen from time to time. The Big Bang was a really big one.SBHarris 23:40, 3 June 2013 (UTC)

Clyde Greene (talk) 20:10, 8 July 2013 (UTC)== Normal Image: a 3d array ==

Normal Image: a 3d array, references as spheres defined by the charge / mass density . . the nucleus, the object projected and the electron shells being the image formed by the projection?

(1) Within the nucleous provides reaction sites thats extends as a region of frequencies [ie wavelengthes] give form or exist as nuclear particles. (2) Reflected Image: an array of electron configurations; references reaction sites [molecular form] formations of chemical bonds, etc. Note: Principle chemical analysis of matter defined by [radiation\absorption] over range of frequencies identify elemental orbitals while other wavelengths give bonds identifying electron configurations forming compounds, etc.

'Ah, . . early conceptions' of the atom! Yes, we agree that is matterrather losely defined concept just as element is a rather losely defined term. Classical concept of matter (form) mass and motion (energy) being represented as atoms. These particles of matter are then composed of subparticles; Having a spatical arrangement (reference) to charge. The (neutral) mass, neutron => positively charged particles (proton) and a negative particles (electron). Elements refer to the number and configuration of matter; each element has unique number of charged particles. These charged particles form specific confiurations within individual atoms of elements, but only those elements which form noble elements exist as indiviual (stable) atoms. The other elements/compounds form molecules (multiple nucli) attached or bonded via the outer negatively charged particles (electrons).

gas or smoke resembling gas[edit]

Shall we use smoke to resemble gas because it looks nicely with the other images with a black background when we have a good image of one of the few colored gases that can be seen well? Darsie42 (talk) 16:51, 14 November 2013 (UTC)

Lead sentence: MOS compliance[edit]

Starting a discussion thread following an apparent disagreement with Headbomb ...

At this time the article starts out as follows:

Matter is a poorly defined term in science (see definitions below). The term has often been used in reference to a substance (often a particle) that has rest mass. Matter is also used loosely as a general term for the substance that makes up all observable physical objects.

This violates MOS on two levels. First, the lead sentence does not provide a definition; rather it provides a disclaimer. This is a violation of WP:Lead. In general the first paragraph as a whole only vaguely defines the topic. Second, and more importantly, though, the lead violates WP:NAD. Whether or not the term matter has more than one definition is mostly beside the point (it is only really significant to the extent that one should make sure it is an appropriate title for the article). The lead should be defining the topic of the article, not the words in the title of the article. The fact that the words may have other definitions is a different issue (it may deserve a passing mention for clarity at the end of the article's lead but certainly should not be at the beginning of the lead).

The article is currently covering a broad range of overlapping sub-topics. It may perhaps be worthwhile to break these into more than one article (I don't have a strong opinion about that at this time). Nevertheless, whatever is the final result the articles need to following WP:Lead and WP:NAD.

The revised lead version I had suggested was

Matter is any substance that composes physical objects in the universe.[1][2][3] The two most basic properties of matter are the mass that it possesses and the volume that it occupies. It is contrasted with energy, which acts on matter.

Whether or not this version is used, the MOS issues need to be fixed.

-- MC— Preceding unsigned comment added by (talk)


  1. ^ "matter". Random House, Inc. November 29, 2013. 
    "matter". Encyclopædia Britannica Online. November 29, 2013. 
  2. ^ R. Penrose (1991). "The mass of the classical vacuum". In S. Saunders, H.R. Brown. The Philosophy of Vacuum. Oxford University Press. p. 21. ISBN 0-19-824449-5. 
  3. ^ "Matter (physics)". McGraw-Hill's Access Science: Encyclopedia of Science and Technology Online. Retrieved 2009-05-24. 
The idea of defining matter as the stuff that makes up any Penrose "physical object" ("physical body" in your piped link MOS:PIPE) is droll, since if you go to the article on physical bodies it says they are identifiable collections of matter, all stuck together. How are we helped by such circularity? SBHarris 04:38, 27 August 2014 (UTC)
The distinction you make between matter and energy is incorrect. Kinetic energy, for example, does not "act on matter", rather it is an attribute of a physical object. In a more fundamental sense, mass and energy are the same, so if matter has mass it must also have energy. Gandalf61 (talk) 04:32, 1 December 2013 (UTC)


Been reading you with great interest, from old discussion to this. My own view lends to using the Pauli exclusion principle for defining matter. Doesn't matter if there are other 'forces' involved, 'virtual' or not'. Just as a electron becomes a 'electron cloud' in the modern definition, instead of singular particle uniquely defined to a momentum and position at each instant. But whatever ordinary matter is it takes a unique space to us humans, touch-ably so. So Pauli's definition suits me just fine, as for using 'energy' to define matter? Anyone that can give me a kg pure 'energy' here? Don't think so :)

Also, you might rethink "The reason for this is that in this definition, electromagnetic radiation (such as light) as well as the energy of electromagnetic fields contributes to the mass of systems, and therefore appears to add matter to them. For example, light radiation (or thermal radiation) trapped inside a box would contribute to the mass of the box, as would any kind of energy inside the box, including the kinetic energy of particles held by the box."

It (heat for example) adds mass to a material, but not 'matter' per se. Don't see them as the exact same myself? And would not formulate it as it now got 'touchable matter' added. Heat something and it will have the same constituents as when unheated, but will add vibrational behavior of those constituents inside that materials confinement (their kinetic energy). that 'energy' is temporarily held inside the material, adding a mass, until the materials constituents kinetic energy 'cools down'. All as I see it then.

As far as I know there is no experiment existing in where we by forcing a lot of EM 'energy' in a confined state can create a stable particle, as making a stick. What we have is a theory, or theory's, discussing regimes in where it should be possible, but unless proven as a experimental fact I would avoid defining 'energy' as equivalent particles (of rest mass). Furthermore this is a discussion about touchable matter right? Then let's stay with that, and leave 'pure energy' to theory. (talk) 14:49, 26 August 2014 (UTC) Yoron.

A lot of EM energy makes "matter" all the time. If you shoot photons of more than 10 MeV or so through matter, you get a lot of pair production of electrons and positrons from the gammas. Those are matter particles and they are made directly out of gamma EM radiation. Matter (all kinds of particles) is also routinely made from kinetic energy in accelerator experiments. Protons that have never existed before can be made in an accelerator, and they become "touchable" hydrogen as soon as they pick up an electron. Does it make a difference to you that "touchable matter" can be made from various pure energies? And be converted back to them?

MC, it seems to me that this article is in no danger of violating WP:NAD until it focuses on the word "matter" more than the concepts behind it. It is in no danger of doing that. Wikipedia contains very many articles on topics which have a great many concepts subsumed into a single word, like religion,philosophy and alternative medicine, and none of them are in danger of violating WP:NAD either. But a lot of varying definitions are certainly given. SBHarris 04:35, 27 August 2014 (UTC)

Pentaquarks "not generally accepted"[edit]

This sentence is rather dated now that we have a peer-reviewed observation of two pentaquark resonances to high significance. Here is the journal paper: -- (talk) 18:25, 12 August 2015 (UTC)


What is flam? It is a matter or energy? Ankit468 (talk) 22:37, 25 March 2016 (UTC)

Good question, but here we must discuss the article, not aspects of the subject, or of other subjects—see wp:talk page guidelines. For general questions, you can go to the wp:Reference desk/science. Good luck. - DVdm (talk) 22:56, 25 March 2016 (UTC)