Talk:Mass/Archive 1

"the attractive force all objects in the universe have for each other". The definition does not try to explain how these two seemingly unrelated phenomena can be related by this singular characteristic - "mass". It also does not even begin to explain what mass is -- how is arrises from an object. Omitting the fact that these basic questions about this basic phenomina is "sketchy" is to overstate our thin understanding. Addressing what we don't know is just as, if not more important than addressing what we do. Doing so stimulates the creative mind.


 * Under GR, the fact that inertial and gravitational mass are the same can be reduced to the "principle of the universality of free fall" - basically a symmetry argument. As for declaring our lack of knowledge: well, we have a mathematical description of inertial and gravity which works in all but the most extreme situations (black holes and the big bang). Don't you think that counts for something? Our understanding of mass is on roughly the same footing as our understanding of the other three forces. With any scientific theory, someone can find a fundamental principle or assumption and say "but you don't know why that is the case". Theories which don't have such assumptions or principles are non-scientific. -- Tim

When I wrote standards and special provisions for the Connecticut Department of Transportation's Bridge Design Unit, we tried to make them complete, brief and easy to understand: I think the following statement defining mass is what we might might have written if we had been asked to do so:

Mass can be defined in terms of either (a) "the mutual resistance of two particles, bodies or masses of material matter from simultaneously occupying and/ or passing throught the exact same place" and/or (b) "the mutual resistance of the penetration of a body resting on a planet's terra firma surface." Where the bodies physically exert mutual force on each other and are accelerated - their velocities forcibly changed - inversely to their weight, and/or massiveness.

One body cannot exert more (mutual) force on the other's nor can one exert greater impulse's than the other's. The impenetrability of matter, is the principle that relates these two seemingly unrelated phenomea, and makes this a scientific theory.

Respectfully submitted, Donald G. Shead 54 Chaplin St, Chaplin CT 06235 e-mail 

Relativistic mass VS rest mass
A convention should be adopted as to how wikipedia uses the word mass. It is modern convention for mass to mean *rest mass* - and I think this should be noted on this page. Discussion needed. Fresheneesz 06:48, 22 November 2005 (UTC)

Question about the origin of mass
The following is from the article on Standard_model: "Mass is really a coupling between a left handed fermion and a right handed fermion. For example, the mass of an electron is really a coupling between a left handed electron and a right handed electron, which is the antiparticle of a left handed positron. Also neutrinos show large mixings in their mass coupling, so it's not accurate to talk about neutrino masses in the flavor basis or to suggest a left handed electron neutrino and a right handed electron neutrino have the same mass as this table seems to suggest."

Does that make sense? If so, is there a way to expand on it in some way that would make sense to non-experts? JWSchmidt 21:52, 25 Mar 2004 (UTC)

Mass and Energy
This discussion started in WikiProject Science.

I quote:


 * Mass is now considered as one form of energy, i.e. mass can sometimes disappears into energy, and some energy can be converted to mass.

This is what I mean by a "totally wrong section on mass-energy equivalence". The article already points out the following:


 * Historically, the term "mass" was used for the quantity E/c². This was called the "relativistic mass", and m called the "rest mass". This terminology is now discouraged by physicists, because there is no need for two terms for the energy of a particle, and because it creates confusion when speaking of "massless" particles.

The issue is not one of whether amateurs are allowed to contribute to science articles; of course they are, but I should hope that they take the trouble to make sure the contributions are actually correct! -- CYD


 * I happen to believe that this statement is "perfectly good" and publishable, although it may not be perfectly phrased (remember, it's coming from an amateur, and amateurs are allowed to edit). The statement is substantiated by the Einstein article, for example, which says: matter and energy are simply different forms of the same substance, and A simple calculation using the mass of the uranium nuclei and the masses of the products of nuclear fission reveals that large amounts of energy are released upon fission.  Could you then propose another phrasing that is both correct and understandable by regular readers like me ? (or do you expect regular readers of the encyclopedia to understand the 'relativistic mass' jargon ? Sorry, I don't)  Pcarbonn 18:46, 1 Oct 2004 (UTC)


 * I've corrected the Einstein article. By the way, here is a 1948 quote from Einstein. -- CYD


 * It is not good to introduce the concept of the mass M = m /(1-v&sup2;/c&sup2;)1/2 of a body for which no clear definition can be given. It is better to introduce no other mass than `the rest mass' m.  Instead of introducing M, it is better to mention the expression for the momentum and energy of a body in motion. -- Albert Einstein


 * You may be right, but then, the equivalence between mass and energy is such a prevalent notion that it should be clearly discussed so that anybody can understand it. Let's work on it together (but not today, I have no time). Pcarbonn 06:07, 2 Oct 2004 (UTC)


 * It looks like you did not need me ! Congratulations for a job well done ! Pcarbonn 19:17, 4 Oct 2004 (UTC)

Demostration on the Moon

 * The demostration of equivalance of the time of falling object was carried out before the "Apollo 15 Moon walk". It was carried out a lots of times (it even was a classical laboratory experiment) in a vacum tube. I have not a precise date for the first experiment (I suppose it was in XVII or XVIII century since the vacum technique was enougth good at the time for this experiment). The demostration on the Moon is obviously a very spectacural one, but it is not hte first. Even more also the experiments in the vacum were very spectacular centuries ago, when people was not used to vacum technique. AnyFile 19:23, 26 Oct 2004 (UTC)

Unit of mass eV or eV/c2

 * In unit measure it should pointed out that the use in particle Physics to use electron volts came out from the equivalence between mass and energy (in the meanig this equivlence means). In rigid way the unit to be used should be eV/c2 (eV is an utnit of Energy, not Mass), but for shortness it is wrtitten eV only (giving for sure that who reads knows how the equivalence works) AnyFile 19:25, 26 Oct 2004 (UTC)
 * I agree. I have added a sentence with some explanation of this. -Lethe | Talk
 * In actual practice, I think most people use "eV". Saying that "eV/c^2" is more "correct" is nonsense; a unit is simply a ratio between one quantity and another fixed quantity, and as long as the fixed quantity is unambiguously defined you can do whatever you want (e.g. physicists measure time in units of distance all over the place, setting c=1). As the article already noted, using eV as a unit of mass relies on there being a precise equivalence between (rest) mass and (rest) energy.  &mdash;Steven G. Johnson 23:01, Dec 3, 2004 (UTC)
 * Using eV/c2 as a unit of mass relies on the equivalence of mass and rest energy. Using eV as a unit of mass relies on both the equivalence of mass and energy AND the choice of units in which c=1.  I know you agree with this, because you said the same exact thing in your comment.  This distinction should be made clear in the article.  I don't understand your reasoning, and am inclined to bring back my edits.  -Lethe | Talk
 * Not really. Using eV as a unit of mass merely relies on there being a well-defined mass associated with 1 eV, to which all other quantities are referenced.  Of course, you then have to use consistent units for force, etc., but that's no real obstacle.  (If you also use eV for energy, that implies c=1, but it's conceptually a separate question.  You could "just as easily" use eV for mass and Joules for energy.) &mdash;Steven G. Johnson 02:37, Dec 4, 2004 (UTC)
 * (Philosophical arguments aside, the real question is what people use in practice. In practice, I think the answer is "eV". &mdash;Steven G. Johnson)
 * OK, so I agree that if we decided to use units where we want to use eV for mass, then eV are good units for mass, regardless of what assumptions we make about the units for energy (and therefore value of c). However, I think this is specious reasoning.  No one thinks that eV is a unit for mass.  In fact, eV is defined to be the amount of energy that an electron blah blah blah...  The article text even mentions it thus: "it is common in particle physics to measure mass in terms of electron volts (eV), a unit of energy".  eV is a unit of energy, and we are allowed to use it as a unit of mass only when we use both mass-energy equivalence and the units where c=1.  I remain unconvinced by your argument.  I still feel that a reader who strolls into this page who isn't very familiar with these units and the usage of mass-energy equivalence might be confused by the usage here without explanation, someone who perhaps came from a textbook that uses eV/c2.  I do know of textbooks which use those units throughout, so you must admit that even if you don't use them, there are physicists who prefer eV/c2 for mass and eV for energy, at least for didactic purposes (and I do think that one of the main purposes of an encyclopedia should be didactic. So your approach to units is against at least some physicists.  I wonder if we can get a third opinion in here? -Lethe | Talk
 * (see comment below...resetting indenting &mdash;Steven G. Johnson)
 * I'd be interested to know what textbook uses eV/c2.
 * Sources provided below -Lethe | Talk 04:49, Dec 7, 2004 (UTC)
 * The fact is that many experimental particle physics papers use eV as a unit of mass (I just did a quick lit. search to confirm this). e.g. Phys. Rev. Lett. 83 (1), 41 (1999): "We thus exclude an effective Majorana neutrino mass greater than 0.2 eV...". Or Phys. Rev. Lett. 85 (17), 3568 (2000): "Super-Kamiokande can detect an electron neutrino mass as small as 1.8 eV, and the proposed OMNIS detector can detect mu and tau neutrino masses as small as 6 eV."
 * Not just experimental particle physics. Everyone who isn't teaching undergrads uses eV as a unit for mass.  The question which I am disputing here is whether all these physicists in the world are using eV as a unit of mass because they're scale is fucked up, or because in units where c=1, mass and energy have the same units.  My point all along is that it is the latter. -Lethe | Talk 04:49, Dec 7, 2004 (UTC)
 * As for the purpose of Wikipedia, the purpose of the aside about eV was to describe the kinds of units that are used in practice, and we aren't serving readers by saying eV/c2 when people actually use eV (which also happens to be a unit of energy). &mdash;Steven G. Johnson 02:32, Dec 6, 2004 (UTC)
 * Okay, I found an older PRL that uses eV/c2 (Phys. Rev. Lett. 46 (2), 80 (1981)). I concede that people also use eV/c2 in practice.
 * I am not arguing that people use eV/c2 in practice (maybe they once did, but these days, no one does, in my experience), what I am arguing is that everyone in the world uses units of mass for mass, and units of energy for energy, and never the twain shall meet, unless they happen to also be using units where c=1 (natural units). See the sources below. -Lethe | Talk 04:49, Dec 7, 2004 (UTC)
 * The question is, what will serve readers more &mdash; to learn that there is more than one unit of mass (obvious), or that it is perfectly possible and even convenient to use a unit of energy also as a unit of mass? &mdash;Steven G. Johnson 02:41, Dec 6, 2004 (UTC)


 * For whatever it's worth, I'm with Stevenj on this one. It's not as though the article doesn't explain how to convert this energy unit into kilograms. -- CYD
 * It's worth something. I did, afterall, ask for a third opinion, and so I'm glad you gave one (and sad that it didn't agree with mine).  Since I'm now in the minority here, if you and Steven aren't convinced after my last attempt, I'm going to give up and go home and cry. -Lethe | Talk 04:49, Dec 7, 2004 (UTC)

(This reminds me of a funny story, by the way. An experimental colleague of mind told me about an exciting result that one of the theorists at his university predicted...he proceeded to do the eperiment, and after taking great pains he was unable to observe the result.  He went back to the theorist and asked him what was wrong...the theorist went back over his work and came back the next day to apologize: "I know what happened, sorry - I accidentally left out a factor of c2."  So, the predicted effect was actually 16 orders of magnitude smaller. &mdash;Steven G. Johnson)

various books and their conventions for mass units
these first couple of undergraduate textbooks use MeV/c2 as their mass units. So we see that for didactic purposes (teaching undergrads), explicit mass units are prefered.


 * Williams (1991). Nuclear and Particle Physics. Clarendon Press, Oxford
 * Povh, Rith, Scholz, Zetsche (1999). Particles and Nuclei.  Springer Verlag.
 * Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons.

This last book has the most enlightening explanation of this choice of units, which I quote here:


 * "typical energies in particle physics are MeV, GeV or even TeV. Momenta are measured in MeV/c (or GeV/c, or whatever), and masses in MeV/c2.  Thus the proton weighs 938 MeV/c2=1.67x10-24"
 * "Actually, particle theorists are lazy (or clever, depending on your point of view - they seldom include the c&apos;s and the &#8463;&apos;s in their formulas. You're just supposed to fit them in for yourself at the end, to make the dimensions come out right.  As they say in the business, &#8220;set c=&#8463;=1&#8221;.  This amounts to working in units such that time is measured in centimeters and mass and energy in inverse centimeters."

Another undergrad textbook (Martin & Shaw (1997). Particle Physics.  John Wiley & Sons.) which does use MeV for mass says:


 * "In practice [...] Energies are measured in MeV, GeV etc., while momenta are MeV/c etc. and masses are MeV/c2 etc. This should be compared with natural units, where energy, momentum and mass all have the same dimension of energy, and are all measured in, for example, MeV."

So teachers who use MeV/c2 for mass and teachers who use MeV for mass both agree: we are allowed to use MeV only by virtue of units where c=1.


 * You're confusing two issues. If you use MeV for both energy and mass, then that does imply c=1 units (unless you alter the equations of motion).  And, indeed, that choice of both units is common.  However, it is two independent choices of units.  &mdash;Steven G. Johnson
 * Hmm... So you know, when you say it that way, I think I'm coming around to your point of view; I think you are right. So if you choose to measure mass and energy both in energy units (saying that they are both energy units means that [E]/[m] is a dimensionless number), this forces c to be a dimensionless number (the square root of [E]/[m]), but unless [E] and [m] are the same unit, then c will not be 1.  Right?   So we could choose [m]=eV and [E]=MeV, and then c would equal 1000. -Lethe | Talk 08:18, Dec 11, 2004 (UTC)

Now for some more advanced books:
 * Weinberg (1995). The Quantum Theory of Fields, Vol I.  Cambridge University Press.
 * Polchinski (1998). String Theory, Vol I.  Cambridge University Press.

The first one says about units only "Except in Chapter 1, we use units with &#8463; and the speed of light taken to be unity." and the second says even more succinctly "The constants &#8463; and c are set to 1."

So particle theorists agree: we are allowed to use MeV only by virtue os units where c=1.


 * Don't put words in their mouths. They say that they use c=1 units (and I agree that these are almost universal with theorists), but your books don't claim that these are required for eV units of mass.  I agree that the two choices are strongly correlated in practice, and indeed it is convenient to pair them, but that's a different issue.  (See above regarding what the undergrad. texts say.)  &mdash;Steven G. Johnson
 * OK, you're right, they never claimed that this choice of units was required. But you should at least concede that this convention (using eV/c^2 for mass in units where c is not 1, and using eV for mass in units where c=1, the former sometimes preferred for teaching, the latter for doing physics) is a sort of textbook standard, and so deviating from it, while not bad, deserves mention.  Actually, I guess you did already concede that point, more or less.  -Lethe | Talk 08:18, Dec 11, 2004 (UTC)

The upshot of all this as I see it is this: in practice, all sensible people use eV for mass, but not because they feel like using an arbitrary scale for mass, but rather for the reason that these people are all working in units where c=1. No source I have ever seen thinks that they can use eV for mass without setting c=1. The whole concept of natural units and using eV/c2 is already confusing for undergrads, it is my position that the idea of using eV for mass without explicitly stating that this may be done only by virtue of using units where c=1 will be even more confusing for those kinds of readers. -Lethe | Talk 04:49, Dec 7, 2004 (UTC)


 * I'm afraid that's false. In actual practice, as opposed to undergraduate textbooks that apparently oversimplify matters, people use whatever units are convenient for a given quantity, independent of what units they use for other quantities. As long as you combine them in appropriate ratios (which is what units really are: ratios), you are fine.  For example, in the 2000 Phys. Rev. Lett. article I quoted above, in a single equation they use time in seconds, distance in parsecs, mass in MeV, and mass in ktons.  It is an experimental paper, and c=1 "theorist units" appear nowhere.  (Distance is always in cm or parsecs and time is always in seconds or ms.) &mdash;Steven G. Johnson 05:48, Dec 7, 2004 (UTC)

This argument is going nowhere. The article seems to have changed in the meantime; does anyone have any objections or suggestions to Stevenj's new "units of mass" section? -- CYD
 * I enjoy a good disagreement.-Lethe | Talk 08:18, Dec 11, 2004 (UTC)

mass vs weight
I'm not sure what they do in commerce, but I'm pretty sure that the typical measure of body weight in humans via a bathroom scale, for example, is weight, not mass. If you take the same scale to the moon, and put the same person in it, the scale will read a smaller number. They are essentially springs measuring the force. They measure weight, not mass.

Also, why do you say "an object will exert more force" in a stronger gravitational field? This is true, but rather irrelevant. Weight is the force that your planet exerts on you. and that's what the scale measures.

Finally, what's with the clause "the quantity the weight we use in commerce"? I can't quite convince myself that this clause is grammatical. Can you parse it for me? Lethe | Talk 18:24, Feb 4, 2005 (UTC)


 * Have you ever turned back a paycheck because you do so little work, Lethe? How about you, CYD?  I'd bet that the jargon usage there doesn't cause either of you any difficulty.  Why, then, do you have such great problems understanding similar ambiguities in the word weight?


 * If you are pretty sure about the issue of human weight, then you obviously haven't put much thought into it at all.
 * What happens when you get serious about your weight, and go weigh yourself on one of those beam balances at the doctor's office or the gym? Isn't that a better indication of what you want to measure, than some substitute we put up with in our homes because it is cheap?


 * What happens in your little thought experiment about taking these scales to the moon? Those balances are mass-measuring devices, not force measuring devices. Those cheap bathroom scales (which didn't even exist until 1937) are no more accurate in measuring force than thay are in measuring mass on Earth.  If you hop on your mother's bathroom scale, and it reads 5 lb or even 5 kg less than your bathroom scale at home, you don't automatically credit it to success of your weight-loss diet, do you?


 * Of course, those pounds and kilograms, units of mass, are what people use when they weigh themselves, all around the world. Or stones, and while there are kilograms-force and pounds-force, I've never heard of any stone-force.
 * Even in the United States, many hospitals measure body weight in kilograms. Often, they can even measure this weight in either pounds or kilograms.  But hospitals like to be more accurate, not having variations between the scales in different departments, or if a patient is transferred to a different hospital.
 * So how do you think those hospitals calibrate their scales? They place a test weight (an object of known mass, exerting an unknown and irrelevant amount of force due to gravity) on the scale and compare its known mass to the scale's reading.  Nobody ever calibrates these scales differently for measuring pounds than they do for measuring kilograms, either.  In fact, it is often the same scale that can measure both&mdash;either by flipping the bar on those old balance beams, to get a different set of detents for the weights, or by a constant conversion factor (the same anywhere in the world) programmed into modern electronic-readout scales.
 * NASA doctors use the same terminology their counterparts with Earth-bound patients use, when they study "weight loss" of astronauts in space, etc.
 * Then, of course, there is our Body Mass Index; weight in kilograms, divided by the square of height in meters. Or, as the article puts it "(This is 703.07 times the weight in pounds, divided by the square of the height in inches.)"  Once again, note the constant conversion factor.  If the kilograms and pounds were measures of different things, we'd need a variable there.


 * Here are some of standards organizations on this point. American Society for Testing and Materials, Standard for Metric Practice, E 380-79, ASTM 1979:


 * 3.4.1.2 Considerable confusion exists in the use of the term weight as a quantity to mean either force or mass. In commercial and everyday use, the term weight nearly always means mass; thus, when one speaks of a person's weight, the quantity referred to is mass.


 * NIST Guide for the Use of the International System of Units (SI), section 8.3
 * Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb "to weigh" means "to determine the mass of" or "to have a mass of."
 * Examples:  the child's weight is 23 kg


 * I'll fix the typos in the language you didn't understand this time around. Gene Nygaard 22:53, 4 Feb 2005 (UTC)

My edition of Webster's Encyclopedic Dictionary says:

weight (weit) 1. n. the force acting on a body in a gravitational field, equal to the product of its mass and the acceleration of the body produced by the field. Strictly speaking, the value for the acceleration due to gravity depends upon position in the gravitational field and thus weight depends on where it is measured. However, since the value of the acceleration due to gravity is approximately equal (9.8m/sec&sup2;) everywhere on the surface of the earth, and exactly the same when measured at different times but in the same place, this factor is often neglected. The value of the mass (with mass units) is often used instead, to mean the force (weight) on an object of given mass measured at the surface of the earth.

-- CYD

Irrelevant references
I deleted the mass of references to "relativistic mass" because it's simply irrelevant to the article. The article should introduce relativistic mass, explain what it is, how it is used, and why many physicists don't like it, and move on. Listing a bunch of introductory textbooks and popular physics books that refer to relativistic mass does absolutely nothing for the reader. What needs to be said, is already said:


 * Some authors define a quantity known as the relativistic mass, which is basically the quantity E/c2. This makes the "equivalence" of "mass" and energy true by definition, though neither quantity is frame-independent! "Relativistic mass" was used in many early writings on relativity, and it is still used in books for laymen as well as introductory physics classes.

How is this "pretending that relativistic mass doesn't exist", as User:Gene Nygaard claimed? -- CYD


 * The general tone of this article was getting to be at the stage of a sham notion that relativistic mass is never used, and deleting the specific examples someone added is one step in getting back to that. Much of the quoted part above has had to be added back in after overzealous editing by the ones who would like to pretend that relativistic mass doesn't exist, or even that it is somehow "incorrect", and especially that it is never used any more.  In particular, if this "moving on" part, it is very misleading to simply say that:


 * "E = mc2 is not a "good" relativistic statement; it is true only in the rest frame of the object."
 * This is especially bad because of the use of the term "relativistic" as an adjective modifying statement. That sentence is only true if mass is used to mean invariant mass, not if mass means relativistic mass.  That's one reason why these comments had to be added at this point in the article.  Gene Nygaard 15:49, 3 May 2005 (UTC)


 * A significant part of the point is that, even if all, or almost all, working physicists use invariant mass today, no significant change has been made unless and until they make the effort of selling this idea to the rest of the world. The use of E = m c&sup; in both popular literature and in introductory textbooks, a use in the general sense and not in the sense which is only true "in the rest frame", is one example of the still very common use of relativistic mass.  The references CYD has been blanking, of course, include not only popular literature, but professional journals as well, showing that even the notion that no physicists use relativistic mass any more is a falsehood.  Gene Nygaard 16:03, 3 May 2005 (UTC)


 * Please indicate the statement in the article that says that relativistic mass is never used. I could throw in an equally large list of references to articles that specifically don't use relativistic mass; but, as I already mentioned, that would be belaboring the point.


 * Have you bothered actually reading the article? All the points you bring up are already in there -- it is clearly stated that the equation E = mc2 is correct in all frames provided you make m the relativistic mass, though it also points out that neither side is a frame invariant quantity. -- CYD

It is still mentioned in *Relativistic mass* that "it was realized that the invariant mass was the more useful quantity and people stopped referring to the relativistic mass altogether" This statement is either a result of intellectual blunder or intellectual dishonesty. -- AlphonsusW


 * Then edit that article; don't mess up this one. -- CYD

Misleading?
I do not understand this (end of the first section):
 * if one were to treat inertial mass mi, passive gravitational mass mp, and active gravitational mass ma distinctly, Newton's law of universal gravitation would take the form $$m_ia=\frac{Gm_pm_a}{r^2}$$.

Are there implicitely two bodies? What is r? It seems misleading to me and if you agree, I would like to remove it. --Philipum 13:41, 27 May 2005 (UTC)


 * Yes, there are two bodies. Newton's Law of Universal Gravitation states that the gravitational force between two bodies is inversely proportional to the square of the distance between them.  That explains the r2.  You haven't said what is misleading about this statement, only that it is unclear.  Why do you want it removed? -Lethe | Talk 17:39, May 27, 2005 (UTC)

I suppose I find it misleading because it gives the impression to be a bizarre way to define the different mass concepts of inertial mass, passive gravitational mass, and active gravitational mass. I also react against the asymmetry of the expression: it gives the impression that we have a mass that creates the field and a mass that feels it, but in reality both masses play both roles. Removing it would not make any harm since the laws of gravitation are explained in more details later in the article. --Philipum 06:35, 30 May 2005 (UTC)
 * But that's the whole point of the formula: there are three different places where what we call mass enters into the equations, therefore three different operational ways to measure mass. The fact that they should all be equal is not a priori obvious, is subject to experimental verification, and is the basis of several theoretical foundations.  It's hard to make an experimental verification if you do not recognize the formula to follow. -lethe talk 22:50, 5 December 2005 (UTC)

Since no real objections were made, I remove these two lines. --Philipum 29 June 2005 14:49 (UTC)


 * I clarified it.--Patrick 02:06, 6 December 2005 (UTC)
 * OK, that's fine. --Philipum 08:49, 6 December 2005 (UTC)


 * Its still not fine, noone defined what a is. What is it? None of these types of mass make sense to me, one of them doesn't even look like mass, its a force. Active gravitational mass looks like a force (later note: i'm sorry, not force, gravitational field - which is still not mass). Someone needs to define a at the very least. Fresheneesz 05:41, 9 December 2005 (UTC)

Gravity couples to energy not to (rest) mass
In this article 'mass' is used exclusivily as the invariant rest mass of a particle. According to general relativity gravity couples to the energy momentum tensor. So, for gravity, the total energy matters not the mass (as defined in the article).

Perhaps it should be mentioned that in modern physicis mass is a redundant concept. The invariant rest mass of a particle is just the rest energy. And inertia, gravity etc. are all linked to total energy. Count Iblis 29 June 2005 13:07 (UTC)


 * I agree completely with you. The concept of mass is useful only in classical mechanics - but this seems to be clear in the article, which is mainly about classical mechanics... but maybe the part on relativity needs to be rewritten. --Philipum 29 June 2005 14:56 (UTC)


 * I disagree, in physics nowadays, relativistic is almost exculsively used in the classroom, and never in real physics. The concept of mass is very useful in non-classical physics, because it is something which can be explicitely measured, and it is not dependant on speed. This page should *not* be "mainly about classical mechanics" and should expalin all aspects of mass. I still think that mass should be conventionally defined as "rest mass" on wikipedia. Fresheneesz 05:47, 9 December 2005 (UTC)


 * Yes, but you could just as well use rest energy instead of rest mass. Mass is a redundant concept in modern physics.Count Iblis 15:00, 9 December 2005 (UTC)


 * I have recently noticed that using rest mass in equations tends to make the equations more complicated than they should be. See Bohr model, Bohr radius, Rydberg constant, Hartree energy.
 * GoldenBoar 13:19, 4 January 2006 (UTC)

Mass in rest and relativist mass OK

 * With the formulation of the theory of the special relativity of Albert Einstein the concepts of relativist mass were introduced, m, that depends on the speed of a frame of reference in rectilinear movement uniform and mass in rest, m0 that talks about to a frame of reference taken in rest. These two states, for the inertial marks, that is to say, nonsubject to forces, are symmetrical due to the relative character of the movement. Of such form, that of its equivalence origin the equivalence between kinetic energy and mass, that was extended by heuristic route to all class of energy. For example, in a frame, the mass of a body, that in opposite directions absorbs the same amount and frequency of electromagnetic energy, increases its mass. The call modern relativity, that constitutes a revision of special relativity rejects the concepts of relativist mass and mass in rest that it replaces and it unifies by invariant mass, that is, of a mass whose magnitude is independent of all frame of reference. This concept is borned of Lorentz´s transformation. This revision is controversial, to see: (3 the energy has mass):
 * The law of the inertia of the energy and the speed of the gravity. October, 2004.

Concept?
Not having had much success before, I will ask again, in its own section:


 * 1) Why do we refer to "mass" as a "concept" and not a quantity?
 * 2) How can a "concept" have units?

Brian Jason Drake 08:28, 22 September 2009 (UTC)


 * I've gone ahead and rewritten the introduction with some operational definitions of mass. It's far from perfect but it's a start. Strad (talk) 23:25, 22 September 2009 (UTC)

Weight and amount section partly irrelevant
The discussion in the Mass section is partly irrelevant to this section. The fact that the two sides of a balance scale are subject to the same gravity force because they are so near is such a tiny detail that needs certainly not be mentioned in this section. So, all the part about gravity being almost equal on the Earth is scarcely relevant and would be better separated in a section immediately preceding Mass, called Mass.--Pot (talk) 09:02, 26 September 2009 (UTC)


 * I think it would be better to completely rewrite that section as something concerning "Measurement of mass". Much of the current speculation about atomic weights and the atomic mass unit is quite simply wrong, for example! On the other hand, I think it is fair to mention that masses in the everyday range of around one kilogram are usually measured by comparing weights in a constant gravitational field. Physchim62 (talk) 12:08, 26 September 2009 (UTC)


 * If we are speaking about the second paragraph of the Mass section I agree completely. But, for a start, it would be better to split it and put it into a new section that precedes the current one. Once this is done, it will be easy to rewrite it. As it stays now, the second paragraph has tone and content which are quite extraneous to the rest of the section. --Pot (talk) 12:14, 27 September 2009 (UTC)

Lead sentence is inconsistent within itself
"In physics, mass (from Ancient Greek: μᾶζα) commonly refers to any of four properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass."

It says "four properties", then lists only three. Facts707 (talk) 07:31, 17 January 2010 (UTC)


 * Fixed. Strad (talk) 15:53, 17 January 2010 (UTC)

Confusing and clumsy reference to mass/energy equivalence
"Special relativity provides a relationship between the mass of a body and its energy (E = mc2). As a consequence of this relationship, the total mass of a collection of particles may be greater or less than the sum of the masses of the individual particles."

The statement only makes sense with reference to sub-atomic particles, yet this important distinction has not been made; rendering the statement misleading to anyone not already in posession of a detailed understanding of the subject matter. A better way to introduce the subject, would be to simply say that: According to The Theory of Special Relativity mass and energy are equivalent, and related by the formula... This equivalence is apparent in fission and fusion reactions, whereby mass is converted to energy, by th process of splitting or fusing atoms. Airophile (talk) 06:52, 29 October 2009 (UTC) --99.156.167.179 (talk) 01:22, 4 November 2009 (UTC)drerennc99.156.167.179 (talk) 01:22, 4 November 2009 (UTC)


 * The problem is that mass is NOT "converted" to energy. This is popular misconception (one of the most pervasive in science) and it's just wrong. Those of us who know better have to deal endlessly with people who think they know what E = mc^2 means. It MEANS that mass and energy are the same thing, and neither one appears without the other. Both are separately conserved (for a given observer) over time. The only way either one increases or decreases is if you're looking at a sealed system and you let some escape. Then both mass and energy go out together, however, the total (including what escaped) remains the same. This article would be much simpler if it said "Mass is a conserved quantity (it can neither be created or destroyed), and special relativity did not change this understanding. However, relativity did add the fact that all types of energy have a mass, and this mass is added to systems when the energy is added, and the mass is subtracted when the energy leaves." S  B Harris 03:46, 27 January 2010 (UTC)

Blatant plagarism
In the section about equivalence of inertial and gravitational masses, the paragraph detailing galileo's experiments has at least two full sentences written by Stephen hawking in his book "a briefer history of time". More plagiarism may be present but I have not investigated beyond that point —Preceding unsigned comment added by Ahuramazda8 (talk • contribs) 20:58, 8 November 2010 (UTC)

The gram (g) is electronvolt is common in particle physics.
This is what Wikipedia tells in the first chapter. I can't understand neither am able to improve. Please discuss. --193.56.241.75 (talk) 12:21, 12 October 2011 (UTC)


 * Thanks for spotting that error. I've restored the missing information.  I hope it makes sense now.  I'm surprised that this (accidental?) deletion was not spotted and corrected earlier.    D b f i r s   08:12, 19 November 2011 (UTC)

Mass as length
Last lines in "Unit of mass" section include "...identified with its inverse Compton wavelength (1 cm−1 ≈ 3.52×10−41 kg)". I guess it should be just "1 cm" instead of 1 cm−1. Am I missing something? manya (talk) 11:25, 30 September 2011 (UTC)

you may have missed the word "inverse", as in, λ= 2π/m, in natural units. --dab (𒁳) 11:28, 13 December 2011 (UTC)

An idea if not inane
I'm not maven but to me, the current definition is not felicitous therefore I devise the following with the help of theories of gravity.

Since gravity or changing curvature of spacetime is the innate property of matter therefore

1- A mass of an object may be defined as "a quantitative might of its prime acceleration [gravitational]" - A prima facie might of its prime acceleration"

2- A mass of an object may be defined as "a qualitative appraise of its distortion of spacetime"68.147.52.247 (talk) 05:01, 11 January 2012 (UTC)Eclectic Eccentric Khattak No.1


 * Go look at mass in general relativity. The devil is in the details of how you "quantitate." S  B Harris 05:53, 11 January 2012 (UTC)

Mass in quantum physics?
I have a somewhat oddball question that doesn't appear to be covered by this section. Is the mass of a particle distributed according to its wave function? What happens to the mechanics of a particle when the wave function becomes distributed over a classical-sized region of space? (E.g. for a very weakly-interacting particle.) Does a gravitational field just further disperse the wave function?

Is this something that we have an answer for? If so, can it be covered in this section? Thank you. Regards, RJH (talk) 20:20, 14 May 2012 (UTC)


 * The answer is no, not exactly. Mass is concentrated around a fundamental particle in some small region, and the size of that region is not related to the wave function. You can think of the wave function as a probability (and phase) of the particle being at a particular place. The mass is concentrated at that place.


 * It might seem like a confusing and arbitrary distinction until you add a second particle. In a two-particle system, there is only one wave function, and it is a function of the two positions, i.e. 6 real number parameters $$\Psi(x_1, y_1, z_1, x_2, y_2, z_2)$$. So you would find the gravitational potential energy with a 6-dimensional integral averaging the probabilities of the two positions, and the integrand would have the particle masses concentrated at points (or small regions). -- Tim Starling (talk) 23:40, 14 May 2012 (UTC)
 * Thanks Tim. It's weird to think about, but then so is quantum mechanics. Regards, RJH (talk) 16:26, 15 May 2012 (UTC)

History of inertial mass
There is some (modern) history of gravitational mass with Kepler etc. Very much less so with inertial mass. If history is no part of this article, shouldn’t there be at least a reference to the pages about inertia, about impetus? — Preceding unsigned comment added by DominiqueM (talk • contribs) 14:57, 29 July 2012 (UTC)

Active, passive, and inertial masses
by definition of active and passive gravitational mass, the force on mass1 due to the gravitational field of mass0 is:
 * $$F_1 = \frac{Mass_0^{act} * Mass_1^{pass}}{r^2}$$

likewise the force on a second object of arbitrary mass2 due to the gravitational field of mass0 is:
 * $$F_2 = \frac{Mass_0^{act} * Mass_2^{pass}}{r^2}$$

By definition of inertial mass:
 * $$F = mass^{inertial} * acc$$

if mass1 and mass2 are the same distance r from mass0 then by the experimentally proven Weak equivalence principle they fall at the same rate (their accelerations are the same)
 * $$acc_1 = \frac{F_1}{mass_1^{inertial}} = acc_2 = \frac{F_2}{mass_2^{inertial}}$$

hence:
 * $$\frac{Mass_0^{act} * Mass_1^{pass}}{r^2 * mass_1^{inertial}} = \frac{Mass_0^{act} * Mass_2^{pass}}{r^2 * mass_2^{inertial}}$$

therefore:
 * $$\frac{Mass_1^{pass}}{mass_1^{inertial}} = \frac{Mass_2^{pass}}{mass_2^{inertial}}$$

in other words, passive gravitational mass must be proportional to inertial mass for all objects.

Further by Newtons third law of motion:
 * $$F_1 = \frac{Mass_0^{act} * Mass_1^{pass}}{r^2}$$ must be equal and opposite to
 * $$F_0 = \frac{Mass_1^{act} * Mass_0^{pass}}{r^2}$$

it follows that:
 * $$\frac{Mass_0^{act}}{Mass_0^{pass}} = \frac{Mass_1^{act}}{Mass_1^{pass}}$$

in other words, passive gravitational masses must be proportional to active gravitational mass for all objects. Lemmiwinks2 (talk) 00:35, 25 September 2009 (UTC)

The above Newtonian formulas are utterly irrelevant: there is no distinction between "active" and "passive" gravitational force in Newtonian gravity. The distinction is made solely by advanced theoreticians working with General Relativity. Only a post-graduate physics student would be qualified to understand the distinction. I don't know why it is even mentioned in Wikipedia, let alone in the intoductory paragraph.77Mike77 (talk) 15:46, 16 February 2013 (UTC)


 * To summarise, inertial, active gravitational and passive gravitational masses must all be proportional, assuming the following are all true:
 * the equivalence principle
 * Newton's second law of motion
 * Newton's third law of motion
 * Brian Jason Drake 06:46, 25 September 2009 (UTC)


 * I have created a list of known places where the above comment appears. Brian Jason Drake 11:41, 29 September 2009 (UTC)


 * Lemmiwinks2 deleted the section on their talk page containing that list. Revision 318510298 was the list revision containing that list. A cleaned-up version of the above comment now appears in the article "Equivalence principle". Brian Jason Drake 07:44, 28 October 2009 (UTC)

Re "faster" and "slower" as vocabulary re Time.
I basically reverted a recent edit about the rate of time passing on the Earth, compared to outer space. There is a common confusion of words about "fast" and "slow", re time. In a heavy gravitational field, time travels more slowly (e.g. at the event horizon of a black hole, time doesn't pass at all, and events are frozen). A clock at the earth's surface will tick more slowly than a similar clock in outer space. The confusion arises from thinking that this slowness "expands" the time, but when you think about it, the fact that the clock ticks more slowly means that less time is registered on the clock, compared to a similar clock in outer space.77Mike77 (talk) 00:50, 22 March 2013 (UTC)

Basic disagreement about the lead section of this article
Everyone please weigh in so we can reach a consensus and have a good (and stable) lead section.

This is (supposed to be) a current encyclopedic article about the concept of mass in physics. Steve Quinn has apparently written his own lead section to make it, at best, an article about how mass was understood in the nineteenth century. Of course it's useful to say how the contemporary concept of mass has special cases that correspond to the classical definition, but those special cases are no longer primary to the way mass is understood and the way the term mass is used by physicists today. You can't completely redefine mass away from its mainstream definition in physics, even if you found a couple of articles or essays related to classical aspects of mass or the calibration of the standard kilogram, or by quoting Newton, whose views are obviously not the most current in the scientific community. Perhaps some of this material could go somewhere in a section of the article, but certainly not as the entire lead.

Here are a few specific comments as well:


 * In physics, mass ... refers to the quantity of matter in an object.

To whom in modern physics are you attributing this statement? The term "matter" by itself isn't even defined unambiguously in physics today -- it has several meanings in different context. Precisely what definition of matter are you referring to? According to mass-energy equivalance, every type of energy has mass, not just "matter".

Even if you mean rest mass (which you didn't say) a system can have rest mass without having any matter in it -- for example if you had massless box containing nothing but light bouncing around inside it, this system has mass while it's at rest (which can in principle be measured inertially/gravitationally). (If that system as a whole is a body having a nonzero velocity in some frame of reference, then it will also have momentum and thus its total (relativistic) mass will be greater than its rest mass.)

Or what about the example of the positron, which the anti-particle of the electron, called "anti-matter" by some definitions, but it has exactly the same rest mass as an electron (both masses are positive, btw).


 * Also, the basic unit for mass is the kilogram (kg).

The kg is the unit we use by convention to measure and quantify mass; it is not basic or fundamental to the meaning or definition of mass itself. In fact, physicists often use a different system of units in which the units of mass are the same as those of energy, since they're equivalent anyway.


 * Using the kilogram helps simplify because it correlates with other basic units.

"Helps simplify" is not part of the basic definition or concept of mass, which is all that should be in the lead. Also, this article is about mass, not a particular measurement unit.


 * In other words, the basic unit of force, the newton, is the push or pull that changes the velocity of a 1 kilogram (mass) object by 1 meter per second, when the force acts upon the object (mass) for one second. Therefore, how much force needed describes the quantity of mass, or quantity of matter. This can be called an operational definition, which helps describe mass. Similarly, with an operational definition, Isaac Newton stated that mass is a measured quantity arising from a substance's conjoined "density and bulk".


 * On the other hand, inertial mass is a quantitative measure of an object's resistance to changes in uniform velocity, (acceleration).

"On the other hand" suggests that inertial mass is 2nd physical quantity or concept, separate from mass itself which was being discussed before this sentence.


 * In addition to this, gravitational mass is a quantitative measure that is proportional to the magnitude of the gravitational force which is
 * exerted by an object (active gravitational mass), or
 * experienced by an object (passive gravitational force) when interacting with a second object.

Now you've introduced gravitational mass as seemingly a third distinct kind of mass. As explained in the article itself, inertial and gravitational mass are equivalent, not separate. When one says that a system has a certain mass, it isn't necessary to specify whether you mean its inertial or gravitational mass, because these will not be different. There is only one modern concept of mass in physics, regardless of the fact that it is involved in multiple phenomena and can be measured in multiple ways (inertia, gravity, etc.).

Even if some of what I said above is mistaken, we should consider what level of technicality we want to get into immediately as the lead of the article.

DavRosen (talk) 08:04, 7 July 2013 (UTC)


 * I didn't notice any discussion here. Now I have. This is the modern definition of mass, I didn't invent it. I added refs, and the lead is built on the former lead. It got rid of the confusion, where the description of interial mass was being confused for the description of mass, just before I changed the lead. The basic unit for mass is the kilogram, check the literature. I'll come back and review the rest of your comments later. Steve Quinn (talk) 22:30, 9 July 2013 (UTC)


 * Hi Steve. "Quantity of matter" is just one traditional "definition" of mass, and it's fine to state it as such. Unfortunately, it isn't actually used to perform physical science or engineering (with the possible exception of people who study mass unit standards themselves) today because nothing follows from it: stating that definition doesn't help you measure mass, nor predict mass, nor create or destroy or convert [rest] mass, nor determine what effects or phenomena result from mass, nor how physical systems will be affected by mass, nor determine the relationship between mass and energy, force, etc. It also isn't useful because it defines mass in terms of something even more ambiguous: matter.  Just look at how many ways matter can be defined in the modern context (i.e. beyond just saying "atoms"), and you'll see that defining mass in terms of matter leaves you almost nowhere.  Many of the definitions of matter involve mass (or rest mass), so then the definitions become circlular: you can't define both matter and mass in terms of one another.
 * Fortunately, the other traditional definitions of mass are operational and much more useful: mass is a quantity that manifests in its well-known inertial and gravitational effects; using terms like "quantity of matter" have no effect on these phenomena. Inertial and gravitational mass aren't distinct physical quantities; they're just two (or 3) types of phenomena manifested by one physical quantity called mass.  This isn't controversial; it has been known since Galileo's experiments and culminating with Einstein's equivalence principle.
 * You are confusing a physical quantity with its units -- these are not the same thing. Mass is a valid physical concept under any (consistent) system of units.  You don't learn anything about the physics of mass by studying the properties of the standard kilogram  bar.  If you can't explain the concept of mass in a way that's independent of the kilogram itself, then you don't understand mass.  Of course the fact that we do, by convention, use the kilogram, is an important fact in itself that should be stated, but it doesn't define or lead to an understanding of the physical quantity called mass.  There is no reason to mention other units like the Newton in the lead section of this article.  The lead should just state what mass is (possibly mentioning multiple conceptions of this), in the simplest possible terms.  Further elucidations can go in particular sections of the article, where appropriate.
 * DavRosen (talk) 23:27, 9 July 2013 (UTC)
 * At the moment, essentially, I don't have a problem with the current lead. And "quantity of matter" can be inferred from the introduction of the NIST reference I provided, and the Scientific American article again, agrees, with this description. When discussing "rest mass" then kinetic and potential energy contribute to a particle's mass. If you want to put something about that in the lead, to show a distinction, that would be fine with me.
 * At the same time, I see describing matter in terms of the Newton demonstrates another characteristic of mass, because it is simple (rudimentary) and shows its relationship to a simple process. In other words, it shows that scientific endeavors that account for mass are not static. There is a dynamic involved. The "Newton" equation shows how mass can come in to play, and it seems to me that this would be both helpful and interesting to the reader. Maybe we could elaborate on this in a new first section.
 * Additionally, I don't appreciate having motivations misattributed to me. For example stating "Steve Quinn has apparently written his own lead section...", "You can't completely redefine mass away from its mainstream definition in physics..." and so on. I did neither of these things, and really such statements are irrelevant to creating a quality article. I find it is best to discuss the editing, as much as possible. In any case, I don't start attributing motivations to other editors - believe me - it doesn't go over well. Have you heard of "edit wars"? Well, that is one way they start; especially if two or three start hurling such things at each other. Then things get really out of hand, as you can imagine. Perhaps review WP:CIVIL.
 * Also, articles do have flexibility, within reason. It would be OK to include the Force (Newton) equation in the lead with all the basic units, because it is related to mass.
 * My intent for the lead was to distinguish terms, which I see you have done. I was not, and am not meshing anything together. Again, this was a misattribution of motivations, and perhaps also a misreading - since you have kept the same intact. In any case, it serves its purpose, does it not? Steve Quinn (talk) 02:20, 10 July 2013 (UTC)
 * I have a source here that directly supports what I am saying . I was trying to remember where it is when working on the introduction, but could not recall. This is the HyperPhysics site, which is hosted by Georgia State University's Department of Physics and Astronomy . I think it is well known to the Physics community on Wikipedia. Steve Quinn (talk) 02:55, 10 July 2013 (UTC)


 * Steve, you're right -- sorry I didn't do a better job of assuming good faith. DavRosen (talk) 05:18, 10 July 2013 (UTC)


 * To illustrate a problem with mass as a quantity of matter, mass is already a physical quantity before mentioning matter. Say the mass of a given body is 1 kg, i.e. the quantity of mass in or of the body is 1kg.  If mass is the quantity or amount of matter, then the quantity or amount of matter in or of the body must also be 1 kg.  So we have 1 kg of mass and also 1kg of matter.  How does this help define what mass is or means? As far as quantities go, it seems to make mass and matter simply act like synonyms, and we still don't know what either of them is or means.


 * Okay, that hyperphysics ref says "The mass of an object is a fundamental property of the object; a numerical measure of its inertia; a fundamental measure of the amount of matter in the object." So right there it gives two definitions, inertia and amount of matter. (It brings in gravitation later.) The site uses the inertia and gravitational aspects of mass quantitatively all over the place, but can you find anywhere that it actually uses the definition of mass as a measure of the amount of matter in any substantial way?  Also, I thought you objected to calling mass a property of something when you said "Mass is a physical quantity not necessarily a [physical property]]".


 * I'm not entirely satisfied with the current lead, but it's getting there. I only made what I felt were the least-controversial changes so as to avoid an edit war. It still gives the impression that there are actually three or four physical properties needing to be separately defined, although they end up being numerically equal.  I'd like it to be clearer that mass is a single property that can be *defined* by the two or three observable/measurable phenomena.  Just focusing on the first paragraph, the 1st para should stand on its own and explicitly answer "what is mass" at a simplified level, e.g.:


 * In physics, mass (from Greek μᾶζα "barley cake, lump [of dough]") is a property of a physical system or body, observable and measurable as a resistance to acceleration or in the strength of its mutual gravitational attraction with other bodies. In many common situations, mass can be thought of as representing the quantity of matter in an object.  It is often measured using a mass balance or scale.  The SI unit of mass is the kilogram (kg).


 * The reason for "in many common situations" is that systems with no matter at all can still have mass -- even light itself has mass (not rest mass, but it's never at rest anyway) which causes it to curve when it passes by large bodies like stars, and light can even orbit a black hole due to gravitational attraction, just like any body having mass can orbit another body.
 * DavRosen (talk) 05:18, 10 July 2013 (UTC)

In any common situation it is just WRONG to consider an object's mass to be the amount of "matter" in it. First of all, there is no good definition of "matter" (as pointed out above) so using it here is doubly confusing. Second of all, even for the energy entities that nobody considers "matter", they all add mass to closed systems in special relativity. A hot bar of iron has more mass than when it was cool. Here we observe the mass of the heat we added, but heat is not matter. The same would happen if we added light, mechanical work, or any type of energy at all. Lastly, there are not trivial proportions if we define "matter" as those elementary fermions (quarks and leptons) that have rest mass. In that case most of the mass associated with matter (98% of it) is NOT MATTER. Rather, it is kinetic energy and other massless particles associated with massless fields, like gluons. So in most ordinary circumstances with that defintiion of matter, an ordinary object's mass is 50 times the amount of matter in it, and the rest consists of the mass of trapped massless gluons, adding mass to closed systems of baryons, even though the gluons themselves are massless. Thus, I suggest that rather than have to explain this in the lede, let us avoid the word "matter" like the plague right here. S B Harris 22:53, 10 July 2013 (UTC)


 * Dave, in any case, I think your proposed lead is good and I think we should use that. If Sbharris's content can be incorporated as well, I think that would be great. --Steve Quinn (talk) 07:01, 11 July 2013 (UTC)


 * A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation . A photon is massless, has no electric charge, and is stable . The photon is currently understood to be strictly massless. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c. Its speed would be lower and depend on its frequency. . The speed of light is the speed at which all massless particles and associated fields (including electromagnetic radiation such as light) travel in vacuum..


 * It seems to me that light does not have mass. I think that saying it does have mass, is a misunderstanding of some sort.---Steve Quinn (talk) 07:01, 11 July 2013 (UTC)


 * One photon had no mass, but two photons (so long as not traveling in the same direction) do have a mass. An invariant mass. If trapped in a system, a system rest mass. This allows mass conservation during particle decay (like neutral pions decaying to photons). Matter converts to non matter! But the requirement of two photons to carry mass is the reason you never see a single massive particle decay to a single photon! S  B Harris 16:00, 11 July 2013 (UTC)


 * I think this is beyond the scope of this article and I am not going to get into nit picking. We have whole articles and the science of relativity that are based on the massless photon, and the fact that light travels at the speed of light, therefore light has no mass, otherwise it would travel at a speed less than "c".  Steve Quinn (talk) 18:04, 11 July 2013 (UTC)
 * OK, I changed the lead per this talk page discussion and per User:DavRosen and User:Sbharris. Thank you both for your contributions. Steve Quinn (talk) 18:21, 11 July 2013 (UTC)

Distinction between mass and weight
The distinction between mass and weight is a recent aspect as scientific concept.--86.120.44.145 (talk) 18:56, 15 July 2013 (UTC)

Lead section: continuing discussion
Discussion started in Talk:Mass but that section got unwieldy in length.

Steve, I agree that amount of "matter" is not fundamental to the modern understanding of mass, and the lead should reflect that. On the other hand, we shouldn't bury a notable viewpoint, just because we (as editors) don't agree with it or even can disprove it by some argument. Especially if that viewpoint may (whether intentionally or not) represent a [flawed] attempt to capture some valid aspects of the article's subject. For example, "amount of matter" (or "quantity of matter") may suggest intuitively that we can choose to talk about mass as "stuff" (whether material or not), beyond being "merely" a property of a system. When you transfer "stuff" from one system to another, you expect one system to now have less of it and the other system to have more of it, by the same amount unless some transformation or conversion of the "stuff" also occurs. E.g. "I transferred 1 kg of mass". You wouldn't say "I transferred 3 kg/m^3 of density" from one system to another -- there's no reason the second system's density would increase in general by the same amount by which the first system's density decreased. This may correspond to mass being an extensive quantity (or additive) while density is an intensive property. If a system comprised two identical (but isolated/non-interacting) subsystems, then, for any extensive property like mass (or volume, moles of hydrogen, electric charge, or of course any form of energy for example), the system would have twice the amount of it as either subsystem has. Amount or quantity may also allude to it being a ratio scale, meaning it has a non-arbitrary "zero" and that it is possible to say one system has "twice as much" or twice the magnitude compared to that of another system, as is the case with all the extensive and intensive examples above as well as absolute temperature(an intensive ratio scale), but unlike an interval or location scale such as degrees Celsius, or today's date (say Julian day), or electric potential, or my latitude or longitude, all of which have a zero value as merely a reference point which could just as well have been chosen differently.

As for the first sentence of lead starting with "mass can be described as an associated measure of the total quantity of energy in an object", this is true but it doesn't directly tell us very much about the nature of mass, until we find out something about the nature of that measure being alluded to, and put it all together. If this were an article about mass as merely a physical quantity, then we could simply define it as E/c^2 and refer the reader to Energy article for E, and to Mass-energy equivalence for the formula itself. Otherwise, this article should be about the concept of mass (especially the modern one but also eventually the historical ones), which can't be separated from inertia/gravitation, because these are the particular characteristic measures/manifestations/phenomena/consequences/effects/behaviors of energy that characterize the concept of mass. When we say light bends as it passes by massive bodies "because it has mass", we mean that it exhibits those inertial/gravitational phenomena that collectively characterize mass (relativistic mass in this case). When we say a body gets "heavier" as it accelerates towards the speed of light, we don't simply mean "it has more energy, of which, as usual mass is simply one measure", but that it has more inertia. That's why I (and now Steve) favor a first sentence involving something like:
 * Mass is a property of a physical system or body, observable and measurable as a resistance to acceleration or in the strength of its mutual gravitational attraction with other bodies.

This captures this nature of mass in the simplest possible terms, and it captures the aspect of it that held in the 19th century as well as today. It doesn't mention energy, but that could be in the second sentence, or maybe worked into that 1st sentence. DavRosen (talk) 19:07, 11 July 2013 (UTC)
 * Your idea to start with inertia and gravitation-source/sink as the definition, is a good one, and I approve. Feel free to move it around. Then (if you like) you can add that we also know that the "stuff" we're talking about as mass is ALSO always a property of all kinds of energy (the ability to do work = force X distance), as though it was the universe's bookkeeping agent to "remember" how much energy has been stored or deposited, as a never-changing-quanity in the universal "bank." Or perhaps the total does change as the universe expands, like some currency inflation (pun). Energy is a very squirelly thing, and it's not at all obvious how the different types of energy are like each other-- the frequency of a wave, the velocity of a particle, the storing of potential in a field. But they all leave behind this calling card of increased inertia and gravitation/space-time bending that we call "mass." To me the oddest thing is that this "mass" sometimes has no location, anymore than the energy associated with it does. Where (for example) is the mass of the kinetic energy of two particles A and B relative to each other? From the viewpoint of A, the extra kinetic mass-energy is all located at B, and from the view of B, it is all located at A. From an observer in the COM frame, it is divided between the two. So where IS it? Hard to say. It's stored in the structure of space-time somehow, as a RELATIONSHIP between the two particles, that has no specific THERE or SPOT. They could be light years away from each other and the universe still keeps track of the balance, which is somewhere associated with the two, but no place in particular, anywhere in the universe inside their light cone. The reminds me of nothing so much as quantum entanglement. Kinetic energy must be stored as entanglements, otherwise it makes no sense at all. The same with the energy of photons, which individually do not HAVE an energy, since it can be anything you like (pick your observer). Only when we get to photon pairs are we forced to stop that game, and now (again) behold, it's all in relationships. Long distance relationships if you must, but relationships it is. Grav waves and EM waves have points where the fields are zero, and energy/volume is zero. So where is the energy carried by the wave? Spread out over a volume; one part of space doing something different relative to ANOTHER. It's always like that. Nothing is pinned down. Finally, I'm sorry to be longwinded, but your idea that we use the horrid word "matter" as a stand-in for the "stuff" that we transfer from here to there as "mass" and that has "energy", I think it's a bad idea. The "stuff" we're talking about is mass-energy. It is mass (which always has energy), or energy (which always has mass). Only when pinned down as nice stopped particles with a rest mass is all this pretty and looks like "stuff." Otherwise, it's not stuff!! It looks like kinetic energy, which doesn't look like stuff, and has no location. It looks like EM radiation, which isn't any stuff you can hold in your hand ("How do you hold a moonbeam in your hand...?"). It is E or B fields, which aren't stuff, but force fields. Other force fields as well-- virtual pions, or weakons or gluons or whatever. This is not what anyone ever meant by matter, which originally was a word that differentiated things from the realm of pure energy. But pure energy has mass! Most of the mass of ordinary things is not particles with rest mass. Calling that stuff "matter" is cheating. If that's "matter" then the only thing that isn't "matter," is field-free vacuum. And perhaps vacuum with static gravitation fields in it. S  B Harris 00:39, 12 July 2013 (UTC)
 * Dave, I already altered the lead with your opening statements, and follow up sentences. Then I added Mr. Sbharris' contributions after that. So, I would think that the opening of this discussion is a step or two behind the editing of the article; except I see that Dave and Sbharris have already done some deft copy editing. Maybe I am in a time warp or time loop after discussions of photons, light, relativity, and energy. Anyway, the lead is really good, especially after the copy editing. I am really glad we have been discussing this. Steve Quinn (talk) 01:04, 12 July 2013 (UTC)
 * Sbharris, I appreciate the statement you made in the edit history: "Your mass is mostly gluons and glueballs, and who is to say if these are 'matter'??". I think this is a really good point, and one that I didn't think about before this. I think this would be a good point to add in the lead paragraph. We know it is gluons, "glueballs???", and quarks, but are we really referring to matter? Also, I see some really good points being made in the three paragraphs of your previous entry. Such concepts and possible conclusions are certainly mind blowing. Yes, where is the kinetic energy located? Also, why not use the following from your first above paragraph: "But they all leave behind this calling card of increased inertia and gravitation/space-time bending that we call "mass."" We can give a background explanation, as you did in the first paragraph. That is really clear to me what you are saying. Based on energy is conserved (in a closed system?) and mass is conserved, where the heck does it really go? Anyway, I say, let's use everything we can from these three paragraphs Steve Quinn (talk) 01:45, 12 July 2013 (UTC)

Wow it was a chore getting up to date on all the discussion about the lead here. Anyway, I think the move away from defining mass as a collection of "matter" or "stuff" was a good idea. I've made a couple modifications to the lead, but it seems to me that the main gist of the thing is intact. First, scales do *not* measure mass, they measure *weight*. Mass is never measured directly. Second, I've removed the link to physical system. The reference to physical body seems more than sufficient, and it helps cut jargon. Thirdly, I've made the bullet points on inertial and gravitational mass into a more comprehensive list of the ways in which mass is determined. I feel like the lead is significantly more straightforward this way, but of course this is not definitive.Forbes72 (talk) 22:46, 3 December 2013 (UTC)

Article is a disaster!
This article is such a mish-mash of re-writes, irrelevant technical details, and distantly related trivia, that it is worth preserving as a spectacular example of how badly things can go wrong. Nevertheless, I will (fairly soon) attempt to re-write the introductory paragraphs so that wikipedia readers can gain some useful and relevant information from the article. It should be trashed, except that it is an interesting freak show. This is no insult to the contributers, it's just that there is so much patchwork that the quilt has been lost. 77Mike77 (talk) 16:20, 16 February 2013 (UTC)


 * And it's still a mess. There's even two sections named "Newtonian gravitational mass". I just moved what I can only describe as an inline footnote into the mess of footnotes already here. This article needs work, pretty badly. I'm going to downgrade the quality from "B"to "C". Hopefully I can help fix this.Forbes72 (talk) 22:27, 3 December 2013 (UTC)


 * Everything seems ok when you consider it alone, but the overall structure is ridiculously scattershot. The history of the kilogram is laid out 4 times?!
 * "Since 1889, the kilogram has been defined by the international prototype kilogram.[note 2]The other two ad hoc units are the second and the kelvin. Historically, both the kilogram and the kelvin are derived from physical properties of water, while the second is derived from the length of the solar day on Earth."
 * "The gram was first introduced in 1795, with a definition based on the density of water (so that at the temperature of melting ice, one cubic centimeter of water would have a mass of one gram; while the meter at the time was defined as the 10,000,000th part of the distance from the Earth's equator to the North Pole). Since 1889, the kilogram has been defined as the mass of the international prototype kilogram, and as such is independent of the meter, or the properties of water."
 * "When the French invented the metric system in the late 18th century, they used an amount to define their mass unit. The kilogram was originally defined to be equal in mass to the amount of pure water contained in a one-liter container. This definition, however, was inadequate for the precision requirements of modern technology, and the metric kilogram was redefined in terms of a man-made platinum-iridium bar known as the international prototype kilogram."
 * Also, the original kilogram was defined to be equal in mass to a liter of pure water (the modern kilogram is defined by the man-made international prototype kilogram). Thus, the mass of the Earth in kilograms could theoretically be determined by ascertaining how many liters of pure water (or international prototype kilograms) would be required to produce gravitational fields similar to those of the Earth.
 * This is just silly. Forbes72 (talk) 03:04, 4 December 2013 (UTC)

Yes, it is a total disaster. Some improvements were made a while back to bring the general tone of it down from the clouds, to a level that an undergraduate science major might understand, but those were all reverted. The article, as it is, would be incomprehensible to a layperson, a high school student, a first year physics student, or just about anyone else. There is no point in wasting time trying to improve it, because it just gets reverted and further mangled. This article is an epic fail.77Mike77 (talk) 23:46, 4 December 2013 (UTC)

Question - Mass and curvature of spacetime
Ok comments coming from an novice.

We know mass curves space time right. We have learnt that from lensing of light around stars / galaxies etc.

Shouldn't mass be represented as a fundamental forumla for the curvature of space time then? Is this formula already out there?

I'm just thinking the curvature exists well outside the volume that is matter, i.e. by light curving around the galaxy, so the mass formula should decrease at some function of the distance.

E=MC^2 has always bothered me as a fundamental law, its seems to much of an oversimplication using a scalar value taken from matter. It seems like mass should be expanded to properly account for space time. In other words, energy / mass is what the universe is and trying to describe it in a fundamental formula that does not acknowledge the dimesion it is framed in seems odd.

Also a single curvature for say the sun is a massive simification isnt it? In reality wouldn't space time be more of a cumulative craterfield from each mass from a fundamental particle.

Are people getting me here?

Cheers

Clayton Fairs — Preceding unsigned comment added by 202.58.224.16 (talk) 06:53, 23 September 2011 (UTC)


 * I guess I think I know what you mean: you figure that the "E=MC^2" sprinkled with some hand-waving about "curvature" you were fed by the media is a "massive simplification". You are spot on, it is a massive simplification. The problem is, the full description is right there, only, it is fiendishly difficult to solve, so even sharper minds than either of ours are reduced to assuming massive simplifications, simply because if you don't simplify, you are not going to get any result. --dab (𒁳) 20:44, 13 December 2011 (UTC)

Thank you for the response and the link provided. Hopefully one day I'll have some time to go back to uni and study Physics in depth. Regards, Clayton — Preceding unsigned comment added by 202.58.224.16 (talk) 02:36, 6 December 2013 (UTC)

Kepler doesn't belong here
The article goes into some detail about Kepler's modeling of the planets' motions. But Kepler used geometric and proportional reasoning, and this was not tied to mass at all. From the article: "Johannes Kepler was the first to give an accurate description of the orbits of the planets, and by doing so; he was the first to describe gravitational mass." This doesn't follow at all. The motion of the planets is governed by their masses,(in the modern understanding) but Kepler believed that they were governed by the proportions of platonic solids and the music of the spheres. Galileo's experiments and Newton's laws are obviously relevant, but I think Kepler doesn't deserve a whole section here. Forbes72 (talk) 02:57, 6 December 2013 (UTC)

Lede discussion - continued.
My general impression of this article is that it is a mess. Its a mash-up of distinct meanings for the term "mass" and badly fails to spearate meanings from their repsective contexts of use. Worse, the most used meme is wrong...or rather obsolete. Velocity does not impart mass to an object or system. I am watching Leonard Susskind's video lectures The Theoretical Minimum, specifically Special Relativity and Electrodynamics (2012), Lecture 3. (http://theoreticalminimum.com/courses/special-relativity-and-classical-field-theory/2012/spring/lecture-1). At 1 hour 00 minutes 30 seconds he states that the concept used in most of THIS article, that mass is variable, is NOT used by any physicists he knows of, and is an anachronism, only being used in physics (undergraduate) textbooks. Since there seems to be general agreement here, in this case, that Wikipedia should go with the textbooks rather than with modern Scientific usage (ie with the teachers, rather than with the professionals), I'm probably sneezing into a strong wind, but I felt I should at least point out that the consensus here is not shared by at least one eminent Physicist, and moreover he states it is shared by virtually none. He specifically states that mass is what was previously called rest mass, that it IS invariant (since it is defined as the mass at rest) and that photons DO NOT HAVE any. He also states that the mass of a system of bound particles is less than the mass of the individual particles (due to the energy used in bonding, by implication by (some of) the potential energy of the system) and has NO contributions from velocity. At the least this viewpoint (the modern view) should be given prominent position. The different meanings (including the obsolete one) should be separated and then the theory should be developed for each separately. I suspect that Classical Physics, Special Relativity, General Relativity, the Standard Model (particles), and possibly even the Λ-CDM model have distinctly different definitions for the term. Keep in mind that the same term can be used for an "atomic" particle as well as for systems of particles in some (or all?) of these domains.

Here are two other things I think are wrong:

First: The lede states that mass is difficult to "directly" measure. I don't know what that means. I think mass is EASY to measure. I can do it gravitationally or inertially quite easily (depending on precision). The symmetry breaking (LOL) between weight and mass only occurs when the gravitational curvature is substantially different from "standard". (A recent paper in Science magazine described a technique to differentiate the difference in gravity equivalent to a couple of inches of altitude at sea level. Not easy.) It is just factually not true that mass is difficult to measure. This point of view seems to be confused about what measurement is, or perhaps the author thinks he has some clear idea of what constitutes a direct (compared to an indirect) measurement. Comparisions are direct (1-to-1 correspondance is the most fundamental mathematical operation). I can compare two masses EASILY.

Second: Quote:"From a fundamental physics perspective, mass is the number describing under which the representation of the little group of the Poincaré group a particle transforms." Unquote. This monstrosity from section "Weight vs. Mass". Need I say more?

Needs a rewrite. Needs clarity. And definitely needs more on Higgs Boson and the conceptual difference between say claiming that the mass of a proton (as a particle) is X and that the mass of a proton (as a system of particles and fields) is mostly "energy".173.189.72.14 (talk) 22:30, 12 February 2014 (UTC)

Re recent correction
"The kilogram is 1000 grams (g), which were first defined in 1795 as one cubic decimeter of water at the melting point of ice." This is still awkward, because it wasn't 1000 little grams that WERE defined as a cubic decimeter of water; rather, a kilogram WAS defined as that. Hard to correct...maybe this: "The kilogram is 1000 grams (g), and was first defined in 1795 as one cubic decimeter of water at the melting point of ice."77Mike77 (talk) 23:52, 11 March 2014 (UTC)

Negative mass
Could there be negative mass? http://www.triplenine.org/articles/Negative_Mass.htm --46.115.15.253 (talk) 15:51, 4 August 2014 (UTC)


 * I think mainstream science would have picked up this idea long ago if it had any legs. Besides which, the simulation was very obviously still evolving and, were it to continue, the particles would likely continue to migrate from the centre and re-combine into two clusters that coast away from each other at a constant rate. The simulation ended prematurely and did not resemble the filament and void structure of the known Universe, IMHO.
 * Or in simple Wikispeak, Jim's Negative Mass Page is not a reliable source.  nagual  design   16:54, 4 August 2014 (UTC)

Robert Hooke quote
I performed a small edit the other day that seemed to have polished away the last remnants of a quote from Robert Hooke (in the section about Newtonian mass). I was interested to see what the actual quote was/is, and whether anything had been lost. Here I've transcribed it verbatim (from p.27-28 of the reference) with typography preserved:

I realize it's quite lengthy, but I'd like some or all of it to be (re)integrated into the article. The current paragraph as I've left it simply tells the reader in a rather drab way the bare facts about gravitation that they probably already know, rather than use the opportunity to present them with some poetic, scientific, hiſtorical quote that they've probably never read. It's more interesting to tell the reader what Hooke actually said than what he meant, right?  nagual  design   04:38, 28 September 2014 (UTC)


 * Okay, I've reformatted the quote (blockquote → Quote box) so that it might be more suitable for inclusion in the article. Anyone care to comment..?  nagual  design   20:34, 5 October 2014 (UTC)

Quantum mechanics / Higgs boson
I've been confused for a long time by the fact that this article doesn't mention at all quantum mechanics. For instance, we have lots of QM articles that discuss the (hypothetical) role of the Higgs in giving mass to particles (which would imply it's responsible for everything that we perceive to have mass). However, the word Higgs doesn't appear anywhere in this article, nor as far as I can tell does anything related to quantum mechanics. How so? bogdanb (talk) 14:04, 19 January 2010 (UTC) MASS —Preceding unsigned comment added by 66.91.105.146 (talk) 01:56, 25 March 2010 (UTC)

I have tried to address it. I believe I got the gist of it right, but the presentation may still be improved. I think the presentation of the classical Euler–Lagrange equation, the Schrödinger equation and the Dirac equation side-by-side is useful towards understanding where this m is coming from (its pedigree is the term for kinetic energy in classical mechanics), but because the wave function in the Schrödiger equation really corresponds to p (the impulse) in the classical case, the m now appears in the denominator. So, I believe the presentation can still be improved in the interest of making this accessible to the moderately educated reader. Perhaps somebody can help me, or else I'll try to fiddle with it some more myself. --dab (𒁳) 20:07, 13 December 2011 (UTC)

Many very good aspects to this section. Fine to have the equations. I would like more said about how Higgs field is theorized to give mass to "particles", ie field excitations. User: HeJF — Preceding unsigned comment added by HeJF (talk • contribs) 12:21, 21 December 2014 (UTC)

Misleading headline
The headline sentence is currently "In physics, mass is a quantity which describes the capacity for a physical body to absorb and store energy." That seems to be somewhat misleading, surely that is more like a definition of thermal capacity? A free electron in space can't absorb energy but that doesn't mean it has zero mass for example.

Relativity and QM give different definitions, in QM via the Higgs mechanism and in relativity from energy and momentum. Can I offer two alternatives based on the relativistic understanding, first "Mass is a measure of the energy bound in an object." or better "Mass is a measure of the amount by which the energy of an object exceeds its momentum: m2 = (E/c2) - (p/c)2 where E is the total energy and p is the magnitude of the momentum."

The latter is a bit more technical but is the most accurate and is directly comparable with the energy-momentum relation on this existing Wikipedia page. It has the advantages that it works for all particles, it gives a value which is Lorentz invariant (i.e. it does not depend on speed so avoids the anachronistic concept of "relativistic mass") and it is also valid for system of particles, so for example it gives zero mass for a single photon but a non-zero value for a pair of photons (or more). The combined value is also invariant even though individual photon energies and momenta will vary with the frame.

It could also be noted that taking energy and the three momentum vector components as a 4-vector results in mass being defined as the magnitude of that vector which is then obviously invariant under a Lorentz Boost but that would be more suitable for the body of the article than a headline and could connect to this section. George Dishman (talk) 09:41, 7 March 2015 (UTC)

Orphaned references in Mass
I check pages listed in Category:Pages with incorrect ref formatting to try to fix reference errors. One of the things I do is look for content for orphaned references in wikilinked articles. I have found content for some of Mass's orphans, the problem is that I found more than one version. I can't determine which (if any) is correct for this article, so I am asking for a sentient editor to look it over and copy the correct ref content into this article.

Reference named "Sen": From Tachyon: A. Sen, "Rolling tachyon," JHEP 0204, 048 (2002). Cited 720 times as of 2/2012. From Tachyonic field:  

Reference named "Feinberg67": From Tachyon: See also Feinberg's later paper: Phys. Rev. D 17, 1651 (1978) From Tachyonic field:  

Reference named "Kutasov": From Tachyon:  <li>From Tachyonic field: </li> </ul>

I apologize if any of the above are effectively identical; I am just a simple computer program, so I can't determine whether minor differences are significant or not. AnomieBOT ⚡ 00:37, 23 June 2015 (UTC)

Decrease according to the double of the distance?
The article currently says: Hooke conjectures that gravitational forces might decrease according to the double of the distance between the two bodies.(click)  What does that mean? Is it equivalent to "inversely proportional to the square of the distance"? Ceinturion (talk) 16:06, 28 March 2016 (UTC)

Measuring mass
This article is sorely missing a section on measuring mass. The article Proposed redefinition of SI base units sort-of gets into this. This article could/should provide a section explaining *why* the measurement of mass is related to planck's constant, etc. 67.198.37.16 (talk) 17:25, 10 April 2016 (UTC)

Inertial mass
The article says


 * $$\frac{m_X}{m_Y}=-\frac{\boldsymbol{a_Y}}{\boldsymbol{a_X}}\!.$$

which is probably wrong (or at least an abuse of mathematical notation), as aY and aX are supposed to be elements in a vector space and vector division is not defined there. Suggest taking norms before performing division. Thanks. JacobRodrigues (talk) 13:34, 27 December 2013 (UTC)


 * The notation was certainly incorrect. Anyone familiar with the subject would know what was meant, but the mistake was lazy. Additionally, using X and Y to denote the objects instead of Cartesian coördinates seems confusing. I have changed it to objects 1 and 2 instead. Forbes72 (talk) 02:43, 21 January 2014 (UTC)


 * The above is Mach's definition of mass. The accelerations are not vectors but scalars pointing along the line separating the two masses. However, this definition is wrong for two reasons.  First, it is impossible to measure instantaneous acceleration; this was pointed out by Poincare as an "insurmountable problem" in Mach's definition.  Second, Mach's definition is wrong because it fails to take into account the energy needed to bring the two objects together; that energy (potential energy) alters the mass of the system as a whole by P.E./c^2. So, for example, the mass of a proton in deuterium is *less* than the mass of a proton in free space.  See for example:  "There Is No Really Good Definition of Mass" - Eugene Hecht. 67.198.37.16 (talk) 18:31, 10 April 2016 (UTC)

Recent edit - WP:OR?
A recent edit by Ggonzalm states "The difficulties in obtaining a single definition of mass are probably caused by the lack of a unified theory of physics. For this reason Einstein, Schrödinger and others pursued a geometric unified theory. It was clear to them that... " IMO, this either requires references or might be considered original research. WikiDMc (talk) 15:50, 8 March 2011 (UTC)


 * Its not OR, its the truth! Max Jammer has a book on this, Frank Wilczek has a recent paper:  http://arxiv.org/abs/1206.7114 67.198.37.16 (talk) 18:34, 10 April 2016 (UTC)

Wrong, wrong, wrong; ALL wrong!
 Mass  is the measure of  the amount matter of an object. Weight is a Force.  Inertia  is a property of that amount of matter and  Weight  is the  Gravitational Field Strength . You have these concepts mixed up. This article is all terrible. Ishango (talk) 03:20, 29 January 2016 (UTC)


 * Please put new talk page messages at the bottom of talk pages. Thanks.
 * Then WP:SOFIXIT or propose some changes here, but make sure everything is properly sourced. - DVdm (talk) 09:14, 29 January 2016 (UTC)


 * Ishango,


 * 1. The article is confused. Very confused. But not as bad as Mass versus weight which approaches crime against humanity status.


 * 2. I can not agree with your short statement. In fact, it is very hard to define mass, whichever way it is approached. Frank Wilczek recently attempted "Origins of Mass" but it has already been criticized on various grounds, eg treatment of Hadrons.


 * Note how the start of Wilczek's article dispenses with your statement as well as the sheer ignorance of the lede of this article (obtained from dictionary.com!). Wilczek starts with: "Newtonian mechanics posited mass as a primary quality of matter, incapable of further elucidation. We now see Newtonian mass as an emergent property. Most of the mass of standard matter, by far, arises dynamically, from back-reaction of the color gluon fields of quantum chromodynamics (QCD)."


 * What can be done by whoever has free time is to read the best classic on the topic: Max Jammer Concepts of Mass in Classical and Modern Physics. Cambridge (Mass): Harvard U.P. ISBN 0-486-29998-8 and get the basics right. It is amazing that this article does not use or refer to that. But hey, it is free. This article is a crime, but not a capital crime. I am not really dead (talk) 13:04, 21 February 2016 (UTC)


 * Here's one fairly simple one: "There Is No Really Good Definition of Mass" - Eugene Hecht which points out that Mach's definition of mass fails due to relativity. (Mach's definition is used in the third sentence of this article: "its resistance to acceleration or directional changes" -- viz the third sentence of this article is wrong.)   Its also cute in pointing out that one kilogram of ice becomes more than one kilogram when melted into water - gaining 4 nanograms due to thermal energy (i.e. 80kcal/gram specific heat of melting divided by c^2 gives 4 nanograms) -- thus, mass cannot be a "quantity of matter". 67.198.37.16 (talk) 18:17, 10 April 2016 (UTC)
 * Yes, but the cuteness there comes from the fact that there is really no definition of MATTER in science, not mass. S  B Harris 01:59, 11 April 2016 (UTC)

How to measure mass
What tool is used to measure the mass of a hydrogen molecule? What tool is used to measure the mass of the planet Mercury?

This article mentions a few tools that can be used to measure masses of a few milligrams to a few metric tons. However, my understanding is that balance scales, spring scales, the oscillating massmeter, Dumas method of molecular weight determination, etc. are all impractical for measuring the above objects. My understanding is that in practice, a series of different tools are used to measure the mass of much smaller and much larger objects -- similar to the way the distance ladder uses a series of different tools that each measure distances in some range.

What are those different tools for measuring the mass of an object?

Is there some other article that lists all of those tools (with links to the individual article on each tool)? Should those tools be listed in this mass article or perhaps the orders of magnitude (mass) article? --DavidCary (talk) 00:32, 29 April 2016 (UTC)

Article is too technical
I added a "too technical" tag to the article. This is supported by the following discussion sections (and probably others):


 * 1)
 * 2)
 * 3)

"Mass" is such a common and fundamental quantity that an article on it in a general encyclopedia should be accessible by everyone (or at least there should be a clear distinction between the common meanings and the advanced physics ones). Brian Jason Drake 08:19, 22 September 2009 (UTC)

Brian Jason Drake 08:19, 22 September 2009 (UTC)

Yes, this article is a mess of technicalities that are not stated correctly.77Mike77 (talk) 15:40, 16 February 2013 (UTC)
 * I really liked the intro. I found it geared nicely to the novice.  And I think an encyclopedia article can get as technical as it likes, as long as the intro is written for the novice and the subject is built from there.  Unfortunately, the most common error I've found in Wiki is intros written by  experts for experts, unfathomable to the novice. Pb8bije6a7b6a3w (talk) 01:53, 31 May 2016 (UTC)

The difference between "active" and "passive" gravitational mass IS "unfathomable to the novice", and anyone who thinks it's simple is either a post-doc or a deluded novice. The lede was quite good, though, up until those four bullet points, at which point a novice would become very lost. My suggestion is to end the lede with the paragraph before the one with the bullet points, and put the university-level technicalities in another section further down. — Preceding unsigned comment added by 77Mike77 (talk • contribs) 03:35, 31 May 2016 (UTC)

Active versus passive gravitational mass
Does this distinction need to be made in the initial definition? The average person isn't interested in arcane distinctions that are meaningless outside of General Relativity and advanced field theory. The average person deals with the common-sense world of Newtonian gravity, in which the distinction is meaningless. If two objects have the same mass, they interact gravitationally according to the formula, and it is impossible to say which is "active" and which us "passive". This seems to have been introduced into physics teaching several decades ago, for some reason. It is of no value to people who are not taking a PhD in physics.

Newton would have done it this way: take a known mass M, an unkown mass m, at distance r (between centres of mass), and measure the force pulling them together. Use the formula to calculate the unknown, m, which is THE gravitational mass. (You could make the larger mass, M, the unknown instead, and measure THE gravitational mass the same way). There is no meaning to the words "active" and "passive" in this Newtonian picture.

Later in the article, if, say, the behaviour of a lepton near the event horizon of a black hole is being discussed, the words "active" and "passive" could be introduced. It is of no value to someone buying a pound of butter.

This link gives an idea of where this distinction arose http://www.edition-open-access.de/sources/5/chapter_21.html

It is quite out of place in the basic layman's definition of mass that should begin a wikipedia article.

77Mike77 (talk) 21:53, 11 February 2013 (UTC)
 * I don't know why it is being said that this is an advanced topic. Consider the test proposed by Galileo to drop two masses from the tower of Pisa. Since the mass of the Earth dominates and the gravitational effect is therefore effectively the same for both test objects, what is tested is their passive gravitational mass. The Earth is the mass generating the 1g field at the surface hence that field is G times its active gravitational mass. That is quite simple even in the Newtonian view. George Dishman (talk) 19:33, 7 July 2014 (UTC)

No it isn't. That's absurd. Newtonian gravity acts between objects, and is the same force acting on each to pull them together. You are simply applying the word "active" to the larger object, and "passive" to the smaller, which is a meaningless exercise in the case of Newtonian physics.77Mike77 (talk) 03:45, 31 May 2016 (UTC)

Just to clarify
Just to clarify, the lead section of this article says that, "Mass....is the measure of an object's resistance to acceleration (a change in its state of motion) when a net force is applied." Is that not a property of an object, arising from its mass, called inertia? isn't mass the measure of the amount of mass in a substance? reply soon please.

P.S: i'm new to this editing stuff, so if this is a dumb question or something, cut me some slack.--RainbowsAndPonies (talk) 16:05, 19 January 2017 (UTC)


 * Don't be discouraged, but... mass is a (quantitative) physical property of an object (more generally, a system), as is energy, temperature, etc.  Inertia is a concept and a phenomenon that is (one of) the manifestations(s) of the physical property called mass. You might get (or find) a more complete answer to your question on Quora.com . DavRosen (talk) 13:43, 20 January 2017 (UTC)

== Undoing this good-faith new definition in 1st sentence: mass is a property of a physical body. It is the measure of the quantity of matter an object contains. ==

There was a edit that replaced 1st sentence with this good-faith attempt to "correct" it to:


 * In physics, mass is a property of a physical body. It is the measure of the quantity of matter an object contains.

There are several problems with it:
 * 1) It does not correctly summarize the body of the article; please read the article again
 * 2) That's an old conception of mass and not the one that is accepted today
 * 3) Simply copying a definition from a dictionary (a tertiary source) is not adequate for wikipedia which is based on secondary sources from experts in the field (physics in this case)
 * 4) even when it was used as a common definition of mass, it was not a usable definition in practice since it provides no way to know, even in principle, whether "its mass is 1.2 kg" is a correct statement about a particular object; it is a non-falsifiable definition
 * 5) It defines a fundamental concept mass in terms of the less fundamental and less uniquely-defined concept of matter (pls. read that article too)

I'm undoing it back to what it had been:


 * In physics, mass is a property of a physical body. It is the measure of an object's resistance to acceleration (a change in its state of motion) when a net force is applied.

DavRosen (talk) 13:34, 20 January 2017 (UTC)


 * Right. I'm new to this editing stuff, so guess I gotta learn.--RainbowsAndPonies (talk) 18:09, 20 January 2017 (UTC)

Survey of mass in some other language wikipedii from a couple of years ago, just FYI
Of course we aren't required to match the articles in other languages, and these certainly aren't cite-able references, but I just thought it was interesting to see what 10 other groups of editors, perhaps much like ourselves, used for a "definition" of mass. Of course some of these may have originated as translations from one of the other languages, but presumably most have been edited since then.

This was a couple of years ago. I used Google translate (automatically via Chrome browser), and then I edited results so they seemed to make sense to me in English (may not be an accurate translation) and just included as much as I thought was most relevant to how to define mass.

The slashes separate some possible alternative translations (according to me as much as google) for some seemingly-ambiguous words.

The mass is a property of matter and a basic physical parameter. It is given in the SI unit of kilogram. The symbol is usually m. Mass is an extensive quantity.
 * German

The gravity of a body is proportional to its mass. At the same time its mass determines the inertia with which it moves in reaction to forces. This dual role of mass belongs to the equivalence principle. Moreover, the mass of a body is equivalent to its energy in its rest frame, i.e. the two quantities differ only by the constant factor c2.

Mass is a property of matter, which reflects the degree of inertial effects of matter, or measures the gravitational effects of matter. The equivalence of inertial and gravitational force effects is postulated by the general theory of relativity, and experimentally verified with great precision[1].
 * Czech

Mass is similar to other characteristics of matter such as energy, electric charge, etc.

Mass - a physical quantity, which is one of the main characteristics of matter that determines its inertia, energy, and gravitational properties. Mass is usually denoted by Latin letter m.
 * Ukranian

Historical Review

...

In fact, Newton uses mass in only two ways: as a measure of inertia and as a source of gravity. Its interpretation as a measure of "quantity of matter" is nothing more than a visual illustration, and it was criticized in the 19th century as unphysical and meaningless.

Mass (Ancient μάζα, identification m) is the basic physics variable that describes, on the one hand, a body's inertia when a force acts on it, and, on the other hand, a body's ability to feel and cause gravitational forces.[1]
 * Finnish

The mass (from the Greek μᾶζα, "barley cake, lump (of dough)") is a physical quantity, that is a property of material bodies, which determines their dynamic behavior when subjected to the influence of outside forces.
 * Italian

In the course of the history of physics, particularly in classical physics, the mass was considered an intrinsic property of matter, represented by a scalar value (independent of direction), and which is preserved in time and space, while remaining constant within an isolated system. Furthermore, the term mass has been used to indicate two potentially distinct quantities: the interaction of matter with the gravitational field, and the relation linking the force applied to a body with the acceleration induced on it (see below paragraphs inertial mass and gravitational mass). However, the equivalence of the two masses was verified in numerous experiments (since the first ones which performed by Galileo Galilei)[1].

...

The intuitive pre-physical concepts of quantity of matter (not to be confused with the amount of substance, measured in moles ) are too vague for an operational definition

.... distinct from Newton's earlier theory (Newtonian dynamics) that introduces the mass in quantitative terms.

Mass (from the Greek μάζα) - a scalar physical quantity, one of the most important values ​​in physics. Initially (17th - 19th centuries), it characterized the amount of substance in the physical object, which, according to the ideas of the time, affected the ability of an object to resist applied force (inertia), and gravitational properties - weight.
 * Russian

In modern physics, the concept of amount of substance has a different meaning, and the mass is closely related to the concepts of energy and momentum (according to modern concepts - the mass is equivalent to the rest energy). Mass is manifested in nature in several ways:
 * Passive gravitational mass shows the force with which the body interacts with the external gravitational fields - in fact, this mass is a basis for measuring the mass by weighing in modern metrology.
 * Active gravitational mass shows how the gravitational field is created by this very body - the gravitational mass appear in the law of universal gravitation.
 * The inertial mass characterizes the inertia of the body and appears in one of the formulations of the second law of Newton. If an arbitrary force in the inertial frame of reference equally accelerates initially-motionless bodies, then these bodies are assigned the same inertial mass.

The gravitational and inertial masses are equal to each other (with high accuracy - about 10−13 - experimentally [1] [2], and in most physical theories - including certainly all experimentally-confirmed theories), ...

Mass is a physical quantity/property/indication of a material object. Mass exerts gravity, thereby affecting other surrounding lots and electromagnetic radiation (causing a change in the local space-time). Mass also means inertia, the property to exercise a movement resistance with respect to an actuation force, and determines the acceleration the force causes. These two properties of the material are sometimes called the weight mass and the inertial mass. Already Galileo is said to have demonstrated that they have the same value for a given body. A large mass under the influence of Earth's gravity falls to the ground as fast as a smaller mass. The larger mass both feels a greater attraction from the earth and causes a greater resistance to acceleration, and these differences will exactly offset each other. This is also called the equivalence between the inertial and weight mass.
 * Swedish

Mass is measured in kilograms, according to the SI system (mass units).

Mass - one of the fundamental physical quantities determining the inertia (inertial mass) and gravitational interaction (gravitational mass) of physical objects. Is a scalar. Commonly understood as a measure of the quantity of matter a physical object. In special relativity, associated with the amount of energy contained in a physical object. Most often marked with the letter m.
 * Polish

In physics, the mass is a property of a body measuring the amount of matter and energy. Unlike the weight, the mass always remains the same, wherever you are in the field. The invariant mass also does not depend on the reference system with which we look at the body. The mass plays a central role in classical mechanics and related areas. The reference frame in relation to the inertia is called inertial. The mass appears in many forms of relativistic mechanics.
 * Hungarian

Mass is a physical quantity/parameter/variable/scale that indicates a property of matter. That property, which can be described as the amount of matter, manifests itself in two ways. On the one hand, matter is 'heavy', which means it is subject to gravity, and on the other hand, matter is 'sluggish', meaning that it resists change in motion. In the first case, we speak of gravitational mass; in the second case, of inertial mass. The SI unit of mass is the kilogram. On earth, the mass of an object is usually determined by measuring its weight or by comparison with the weight of known masses.
 * Dutch

DavRosen (talk) 22:33, 20 January 2017 (UTC)

Units of Mass
It's easy to say that the units of mass are kilogram, slug or pound but how did physicists determine / calculate the mass of an object first in such units while formulating F = ma = mg?2001:56A:7399:1200:1C6E:259D:5630:25AE (talk) 04:45, 5 February 2017 (UTC)EEK
 * For questions about the subject you can go to the wp:Reference desk/Science. Here we can only discuss the article—see wp:Talk page guidelines. Good luck. - DVdm (talk) 15:47, 5 February 2017 (UTC)

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Weight vs. mass
This is explained well enough in mass versus weight, weightlessness and even weight. But it's hard to summarize in three paragraphs. Weight is the force of mechanical contact forces, and results in mechanical stresses, like the ground pushing up on your shoes (or an elevator floor, or rocket deck, or giant centrifuge pushing on you). Weight is NOT simply a "force of gravity" or caused by a force of gravity, since if only the force of gravity acts on you (as in orbit or any type of free fall), you are weightless and don't feel weight. To put it another way, you certainly feel the force of weight, but nobody has ever felt any naked unopposed force of gravity (the best you can feel is gravity gradients, and then only if large). So this has to be said correctly. What we feel are the forces that RESIST gravity, and that's all. I probably haven't done a great job, but what was in the article before, was just flatly wrong, and something had to be done. S B Harris 01:48, 17 July 2013 (UTC)


 * Wrong. Weight is the magnitude of the force of gravity. Thus when you are in free-fall you still have weight and are not weightless. The only place you'd be truly weightless is in intergalactic space. Or are we changing the definition of weight so that free-fall does mean weightlessness? If so, then what formula do we use for weight? The only formula I've EVER seen for weight is W = | Fg | = mg Lehasa (talk) 11:11, 6 June 2018 (UTC)


 * The literature seems to disagree with you:
 * {| class="wikitable" style="text-align: center"

! Google !! Scholar !! Books
 * "weight vector" force
 * 34.800
 * 3.680
 * }
 * For instance, have a look at how Resnick and Halliday talk about the "weight vector" here in their Fundamentals of Physics.
 * So, it depends. See https://books.google.com/books?id=OfmPlYZh4WQC&pg=PA34, where it is a vector, and https://books.google.com/books?id=OyBbDwAAQBAJ&pg=PA103 where it is a magnitude.
 * I have undone your second edit per wp:UNSOURCED (and wrong). - DVdm (talk) 11:47, 6 June 2018 (UTC)
 * So, it depends. See https://books.google.com/books?id=OfmPlYZh4WQC&pg=PA34, where it is a vector, and https://books.google.com/books?id=OyBbDwAAQBAJ&pg=PA103 where it is a magnitude.
 * I have undone your second edit per wp:UNSOURCED (and wrong). - DVdm (talk) 11:47, 6 June 2018 (UTC)


 * Thanks for investigating. You've shown that sometimes weight is a vector. okay. But there is still a huge mess because the references you cite define weight at the force of gravity on an object (and provide exactly the same equation that I posted above), but then the wikipedia article talks about an object in orbit being weightless even though the force of gravity doesn't change. It also (insanely!) claims that any force due to acceleration is weight. "The force known as "weight" is proportional to mass and acceleration in all situations where the mass is accelerated away from free fall." I'd love to see an actual equation from whoever wrote this confusing stuff. The textbooks don't seem to agree with this statement.   td;dr: this section in the article is still a mess. Lehasa (talk) 19:17, 7 June 2018 (UTC)