Talk:Mass

Template:WP1.0

This definition needs work

1. The introductory sentence structure should be simple. The single idea of this sentence should be a clear definition of the most common meaning of the word 'mass'.

Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object".

This sentence is too complex: i) It starts by defining mass as a 'concept'- non-existent except in the mind; ii) it then disguises this definition by cloaking it as 'an intuitive idea'- not rational or intellectual; iii) finally it declares that it is a quantitive measure of 'matter' - so not intuitive, now it is a unit of measurement. It appears to be saying that, in physics, 'mass' is a fundamental concept underpinning the whole science of physics and alludes to an intuitive estimate of the quantity of matter (undefined) that exists in an object. This is not helpful and should be re-examined with a view to changing it for a definition that is declarative, clear and unambiguous.

Mass is mentioned in the definition of 'matter'.

Anything which occupies space and has mass is known as matter.

This sentence contributes additional confusion to the definition of mass by creating a circular reference to the definition of 'matter'. Taken together, these sentences try to explain to the reader that 'mass' is a unit of quantative measure of the amount of 'matter' in an object, and the 'matter' is only matter because it occupies space and has mass (presumably as a measure of itself).

This approach only confuses the reader by piling one misconception upon another. One would imagine that 'mass', being a fundamental concept of physics, should have an elementary definition that is undisputed.

Is 'mass' a 'thing'?; does it actually exist as a substance that can be called 'matter'? Or is it a quantative measure of the amount of 'matter' expressed in units of (what)? Does it refer to the amount of interaction between matter and force? Is it a real or an imaginary object? Is it a real or imaginary action or reaction? Is it a measure of existence or a measure of behaviour?

Mass is mentioned in the definition of 'Energy' - presumably another fundamental concept in the science of physics.

Several different forms of energy, such as kinetic, potential, thermal, electromagnetic, chemical, nuclear, and mass have been defined to explain all known natural phenomena.

Note the specific reference to mass as a 'form of energy'. This would make energy a form of quantative measurement of the amount of matter in an object (if one were to combine the two definitions).

These problems need to be resolved. A simple, common and clear definition of mass (as used and understood in general) should introduce the concept. This meaning should then contribute the foundation to all further extrapolations of its meaning in physics. All manipulations and connotations applied subsequently should point back to the original meaning and thus reinforce the fundamental nature of this concept in the mind of the reader.

If 'mass' (as a fundamental concept in search of a meaning) has undergone or is undergoing a change in definition; this needs to be clarified.

"Mass", in common usage, refers to how 'heavy' or massive an object is. It appears to be independent of the density or chemical properties of the matter that constitutes its existence. One can have a massive feather that is still soft but weighs a ton and would impale a body should it be thrown with considerable force. Conversely, a pebble-sized ball of lead would appear to be innocuous until it is fired from the mussle of a rifle. Now, its density and velocity are significant in determining its behaviour - what is the significance of its mass?

GPC (talk) 04:19, 15 February 2008 (UTC)

First things first, if a theory takes mass to be a fundamental concept, then it is not going to be able to provide a definition of it. Consider, Euclidean Geometry leaves the concept of point undefined, Set Theory leaves the notion of set undefined; Physics is a language used to discuss the physical world, thus it is going to be the case that something is bound to be a basic technical term; meaning that it will be explainable only by way of analogy and other such imprecise conversation. Matter and Energy are also fundamental, indeed, so any definition of them is going to end up relying upon the others. Also, if you want a "...simple, common and clear definition of mass", then realize that you would need a theory explaining mass to get it, and that this would be in terms of simpler fundamentals; in other words, if accurate, then it would be exceedingly far from common and simple, probably would entail a theory of quantum gravity and explain the mass problem. Yet, it would still have undefined fundamental terms, thus it would simply shift the impossible burden you wish fulfiled.

Also, Mass is a form of energy, or vice versa; consider E = mc^2, the equivalence of these concepts is what makes the formula important. Finally, responding to your final comment, mass is significant since it is how the density would be determined; its like asking how the time interval and distance is significant since the velocity determines the behaviour...Phoenix1177 (talk) 03:59, 4 July 2008 (UTC)

error?

In classical mechanics, massless objects are an ill-defined concept, since applying any force to one would produce, via Newton's second law, an infinite acceleration.

I thought that dividing by zero was undefined, rather than infinity? --The Missing Piece 16:01, 2 February 2007 (UTC)

• Yes probably, but if one were using limits and said as the denominator -> 0 (assuming the numerator is not 0 and not imaginary), the overall expression -> infinity. Though of course it never reaches infinity, since 0 is an undefined asymptote on the graph... My 2c. Of course there are branches of math that deal mathematically with "infinity." But, frankly, they're beyond me. ;o] Mgmirkin 23:15, 22 May 2007 (UTC)

It is common to talk about massless strings, for example, as part of a system whose other components have non-zero mass. Assuming the string to have zero mass greatly simplifies the analysis. The same applies to pulleys, as in the Atwood machine. Hope this helps. Perhaps this should be inserted in the page somewhere. Timb66 12:31, 29 May 2007 (UTC)

Another error?

The first line of the article appears to be in error, slightly. "Mass is the property of a physical object that quantifies the amount of matter and energy it is equivalent to."

My understanding was that according to Einstein's relation e=m(c^2) or m=e/(C^2) mass and energy are interchangeable but NOT matter. This seems to be a common physics error...

matter=/=mass, matter=/=energy

IE mass or energy is a PROPERTY of the matter, but NOT interchangeable with it. Or am I wrong?

Can we please clarify this? I've added a -Fact- tag until this issue is resolved. Please cite reputable sources (who know what they're talking about) on this one. Mgmirkin 22:59, 22 May 2007 (UTC)

From later in the article: "One may distinguish conceptually between three types of mass or properties called mass:

-Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.

-Passive gravitational mass is a measure of the strength of an object's interaction with a gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass.

-Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass."

None of these points refers to "matter" or the actual "stuff" something is made of, the only refer to the "strength": of an object's resistance to acceleration, of an object's interactions with a gravitational field, of the gravitational field due to a particular object.

Appears to have nothing to do with the actual composition of the thing itself. It's simply a "property of the thing" that describes how it interacts with other things. One cannot exchange matter of energy (at least not according to what I've read), only mass for energy. Or rather mass is perhaps a form of internal energy that can be converted into a form of external energy? However, no creation or destruction of the MATTER take place.

Again, correct me if I'm wrong here. Please! Mgmirkin 23:10, 22 May 2007 (UTC)

I agree that the lead sentence could use some improvements, but I'm not sure your critique hits the right point. My understanding of what the sentence tries to say (at least before somebody inserted "and energy" in a good-faith quest to be relativistically correct) is:

Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object". ...

Here "matter" is not meant as a technical term, but as a reference to the everyday intuition that an object may be made up of more or less "stuff", and "mass" is the formal physical concept that is "best" at capturing that notion within the modern physical world-view. (Of course, "best" is a subjective judgement, and there is no need to invite controversy by explicitly claiming bestness in the article). For this reason I don't think matter should be linked – the target article is mostly about how it is difficult to give that word a fundamental physical definition, at least under the tacit assumption that it is to mean something different from mass. Energy should not be mentioned at all here; it is certainly needed to make concept of mass precise in the precense of relativity, but it is not a part of the intuitive notion that the concept of mass refines. The lead sentence should be useful to a reader who doesn't need or want to understand the finer points of physics, and insisting that such a reader should consider mass-energy equivalence does him a disservice without helping more technically inclined readers (who might as well be told of the energy connection later in the lead paragraph). –Henning Makholm 15:27, 26 May 2007 (UTC)

I agree with Henning Makholm on the lead sentence and will be WP:bold in editing accordingly. Timb66 12:25, 29 May 2007 (UTC)

Except that in using the word "matter" we've only made things circular, and worse. Yes, I know most dictonaries define mass losely as how much matter a thing contains. But some also define matter as how much mass a thing contains. Matter is truly impossible to define (see the wiki on it for why), and so bringing it in doesn't help. Mass is easier to define: although there are several alternative definitions in physics, at least all of them are consistant. But the only really "intuitive" definition of mass which is close to true for "ordinary objects" is how much inertia they have, or how heavy they are in a gravitational field. A semi-good definition for "ordinary objects" is "the sum of the masses of the atoms that make it up", each atom having a characteristic mass. That only leaves out small effects of binding and thermal energy and so on. And it doesn't apply to things not made of atoms (neutron stars, white dwarves, black holes), and all the other things that contribute (slightly) to the inertia of compound objects. And it only delays the problem, as there's no good intuitive reason why atoms have the mass they have (it's approximately the sum of the hadron and lepton masses, but the binding energies are now starting to get significant (a few tenths of a percent) and when you get to truly fundamental particles (quarks), the atom isn't even close to being the sum of the masses of the quarks and leptons which make it up. So you're really screwed, finally, with no "intuitive" way to describe mass from the true ground floor up. So let's not even try. We don't know what "matter" is, but we DO know more or less what "mass" is-- it is that which warps space-time. But it's a real property of things (provided we choose the definition of invariant mass), and not a thing-in-itself. SBHarris 02:09, 11 October 2007 (UTC)

Your objection seems to assume that the lead sentence should be a definition of "mass". You are right that "how much matter there is" is horribly inadequate as a definition, but a definition is not what the lead sentence should be. As you point out, it is somewhat tricky to find a definition that stands up to scientific scrutiny, and the result is likely to require advanced concepts. Therefore, a general encyclopedia should not start out by subjecting the unprepared reader by a definition! Instead of a definition then, we start by presenting a fuzzy idea of what this is all about, and try to make it more precise later in the article. We explicitly call it out as being fuzzy (with scare quotes, "roughly", "intuitive") in order to prevent anybody from thinking that it tries to be a definition, which it is not. People were thinking usefully about mass before the concepts that we would now consider essential for a proper definition of mass were even invented, and our article should remain accessible to readers who need only, say, the imperfect understanding of mass that prevailed in the 1800s.

(By the way, "that which warps space-time" would not work as a definition either; the entire stress-energy-momentum tensor warps spacetime, but "mass" is a scalar and can only account for part of the tensor, no matter how we twist the definition). –Henning Makholm 11:45, 27 October 2007 (UTC)

Mass: Electromagnetic inertia, quantum polarization, elementary particles as tiny black holes, Higgs bosons, gravity of the whole universe causing local inertia, Lorentz invariant property, etc.

The above title pretty well sums up B.K. Ridley's take on the possible sources of "inertial mass" (i.e. mass in general). He discusses these in his Time, Space and Things 3rd Edition, Canto-- Cambridge University Press, (1976, 1984, 1994), ISBN 0 521 48486 3 paperback, asks the question: "What is inertial mass" (Chapter 8 Mass, pp. 133ff).

He starts out with a discussion the to the electron in motion and points out that the moving electron creates a magnetic field and this is in part responsible for its increased mass:

"Working out the magnetic-field energy of a moving electron, we come to the conclusion that the fraction of inertial mass which can be confidenlty taken to be of electromagetic origin is 1/137 . . .

"At least some part of inertia is of electromagnetic origin. What about the rest?" (p. 136)

He then discusses the possibility that the moving electron is "retarded" by spontaneous generation of a sea of quantum vacuum-fluctations:

"Bubbling around the electron is a seething mass of virtual electron-positron pairs" (p. 138)

But all this jiggling (positive energy) and the electron's spin (negative energy) cancel out (p. 138). So where does he look for the rest of the inertia?

"There are three choices, the strong interaction, the weak interaction, and gravitation . . . The idea that inertia is something to do with the drag of a field is too good to let go. . . it is natural to see the field acting as a thing to be lugged about when the particle moves."(p. 138-139)

Ridley introduces the Higgs boson as the "mass field", ". . . a quantum field which inhabits the whoe of space just as the electromagnetic field does. Mass comes about through the body interacting with this field." (p. 139)

But Ridley moves on quickly to closer look at gravity:

"But the intimiate connection between mass and gravity is a strong indication that the most basic source of inertial mass is ultimately to be found in the most evident of all interatctions, gravitation.

"It all stems from what Einstein has called the principle of equivalence." (p. 139)

"The equivalence of pure gravitation and an appropriate acceleration leads to bizarre predictions." (p. 141)

He explores both extremes of size. First he shrinks the gravitational potential until it collapses: "Such entities are called black holes" (p. 142). He concludes that "The gravitational radius for the electron is about 10^-57 m" and he can't quite bring himself to believe that electrons are tiny black holes of pure gravitation:

". . . our imaginations whould have to be fortified to embrace the description of the electron as a negatively charged black hole."

So he goes to the other extreme -- the entire universe -- and introduces the notion that the gravitation of the entire universe is responsible for inertial mass:

"But no one has yet produced a fully satisfactory theory of inertial mass based upon the influence of matter in the rest of the Universe, although Sciama has offered a simple model."

he describes Sciama's model invoving gravitational-inertial waves emitted by a moving particle:

"What a remarkable idea, that when you accelerate into a run, your msucles are fighting the influence of galaxies scarcely visibe even with the most powerful telescopes!" (p. 149)

He ends with:

"In these fundamental problems matter in the largest and smallest scales is involved, and one cannot avoid the feeling that there exists a strong skein in the substratum of nature, which connects the size of the Universe to the size of elementary particles. One day this skein will be unravelled." (p. 149)

The above probably sums up about all that is truly "known" (in the form of hypotheses) about what mass really is, or derives from. However, there are some subtle and important points to be found in a second book by Marc Lange, (2002) An Introduction to the Philosophy of Physics: Locality, Fields, Energy, and Mass, Blackwell Publishing, Oxford UK. He points out that mass and energy are not really the same thing at all:

"This "equivalence" does not mean that energy really is' mass. It means only that when a certain amount of mass Δm disappears, the equivaent amount of energy (the amount for which it is exchanged, the amount that takes its place) is (Δm)c^2. This does not show that mass is the same thing as energy or even that the mass gets turned into energy -- only that when one disappears, the other replaces it." (p. 226)

He points out that in fact mass is "real" and energy is not, because mass is "Lorentz-invariant" (the same measured in one inertial frame to another inertial frame -- here's an example (I hope I've got it right). I am in a rocket going close to the speed of light relative to you on earth. I weigh my pocket calculator -- 0.1 kilogram. because I am moving really fast relative to you, you would say that it has an apparent mass (equivalent mass of) about 10x this, for a total "mass" of 0.1+10*.01. But almost all of this extra is "energy of motion" as opposed to the intrinsic mass of 0.1 kg, which remains "invariant". If my calculator were to crash into the fixed frame (i.e. earth) it would give off one hell of a bang (its kinetic energy 1/2mv^2) but the remanents afterwards would still be 0.1 kilogram of dust):

"In fact, howevr, it is mass rather than energy that is Lorentz-invariant.

"The reality of fields, and hence spatiotemporal locality, will turn on properly interpreting mass in relativity theory." (p. 228)

After quite a discussion, and taking exception with statements from common physics texts, he concludes that

"Mass is a real property whereas energy is not. Mass is no more a form of energy than it is a form of momentum. Energy and mass are not two sides of the same coin in the manner of E and B (as we found in chapter 7 [E is the electric field, B is the magnetic field]); a body's combination of energy and momentum in a given frame reflects its mass and that frame.

"Energy, like mass, is a conserved quantity. The explosion [of an atomic bomb] redistributed energy within the system. In so doing, it turned some energy associated with masses of the system's constituents (mc^2) into other forms of energy, such as heat. . . . But this does not mean that the system's total mass diminished." (p. 240)

Likewise, for fields: ". . . the field has mass, a real (Lorentz-invariant) property, [so] the field is real." He is presupposing his previous assertion that "energy and momentum are frame-dependent, and hence unreal."(p. 247).

"In sum: When we think of the deuterium nucleus as a system of bodies rather than as a single body, we must ascribe masses not ony to the proton and neutron, but also to the field. The masses of the proton and neutron in the deuterium nucleus are not their masses in isolation (which reflect not just the "bare" particles, but also their fieds). A body's energy is mc^2 plus its kinetic energy, without a potential energy term, because that potential energy is ascribed to the field." p. 246

"A field is not made of energy any more than it is made of momentum. Since matter is not some sort of stuff, a field is not made of matter either. But if matter is defined as anything with mass, then a field is matter. Since the electromagnetic field has mass, the field is ontologically on a par with bodies . . ." (p. 247)

Although this book is much harder than Ridley's they both seem to associate "fields" (in particular gravity) and "mass" in some yet-to-be explained manner. He delves into "invariance" in some depth. He concludes that the speed of light c is invariant, mass is invariant as noted above, and so is what he calls "the spatiotemporal interval between two events":

"If Δs is the distance between two events relative to a certain inertial frame and Δt is the span of time separating them in that frame, then I = (cΔt)^2 - (Δs)^2." (p. 219)

This stuff is hard, and I'm not sure that anyone but a specialist can do it justice. But if we can figure out how to assimilate it and get it into the article the article would reflect more accurately what is known about mass at a "deep" level. wvbaileyWvbailey 17:30, 7 June 2007 (UTC)

Wikilinking matter

Can we wikilink matter as matter? --69.150.163.1 21:42, 11 September 2007 (UTC)

I'd prefer not to -- see my comments on the matter in the "Another error?" section above. –Henning Makholm 00:21, 12 September 2007 (UTC)

No! Matter is impossible to define, and bringing it in, even as a word, doesn't help at all. See the matter wiki for why.SBHarris 02:14, 11 October 2007 (UTC)

False and/or misleading weight paragraph

I had to edit the following text because it was false and misleading on several fronts:

“ In informal everyday usage, mass is more commonly, although incorrectly, referred to as weight, but in physics and engineering, weight of the object strictly means the value and the direction of the force acting on an object placed in gravitational field. ”

1. There is nothing "incorrectly" about it
2. There is nothing "informal" about it
3. The improper bold emphasis on the first weight is related to the first two points
4. The linking of the second "weight" rather than the first one was deceptive, probably inadvertently so
5. The scope of the second clause is actually narrower,
   1. It applies to the branch of physics called mechanics
   2. The "strictly" is also not exactly correct in actual usage, though I have left it for now while considering whether rephrasing is needed

Gene Nygaard 14:17, 23 October 2007 (UTC)

That weight is not used incorrectly

1. We are measuring exactly what we want to measure.
2. We are calling it what we've called it for over a thousand years.
3. Fortunately, nobody ever gave your physics teacher any say-so as to what "net weight 15 oz (425 g)" means on a can of beans. Nor as to the meaning of weight for a 401.23 ounce bar of platinum.

Gene Nygaard 14:17, 23 October 2007 (UTC)

There is nothing informal about that usage of weight

• Nobody ever talks about net weight with weight having anything other than mass. This isn't "physics and engineering" terminology.
• Nobody ever talks about troy weight of a bar of platinum with weight having anything other than mass.
• Nobody ever talks about carat weight of a diamond with weight having anything other than mass.
• Nobody ever talks about molecular weight of NaCl with weight having anything other than mass.
• Nobody ever talks about body weight of a rodent with weight having anything other than mass.
• In the medical sciences, the particular body weight discussed at human weight means mass; it is only in planetaria, physics textbooks, and a number of web sites where the word weight is used to mean force due to gravity in this context, often without ever explaining where you are going to put your scale to weigh yourself as you float on the cloudtops of Saturn, nor how you are going to float there where the atmosphere is about three-fourths that on the surface of the Earth, nor what effect it would have on the scale's if you were indeed floating.
  • All around the world, including many hospitals in the United States, we measure this weight in kilograms, and they are indeed the proper units for this purpose.
  • There is no stone-force, never has been. Unlike the kilogram-force and the pound force, no force unit has ever been spun off from this unit of mass. Nonetheless, our Wikipedia article is at stone (weight).
  • Nobody ever talks about curb weight of an automobile with weight having anything other than mass, especially not those who call it kerb weight.
• And on and on and on. Is it okay if I stop now? I will, anyway.

Not only is the word weight used in these contexts, but in some cases it is actually legally required to be used. Now that's certainly not "informal". For example, most food sold in the Unites States that is not sold by volume must have a label which must contain either the word "weight" or the specific abbreviation "wt". [21 CFR 101.105] Gene Nygaard 14:17, 23 October 2007 (UTC)

Weight is not the same as mass, weight is mass pulled down by gravity (measured in Newtons, N), the mass of an orange is about 100g on earth, on th moon, it is 100g. If you were to weigh it, on earth, an orange weighs 980 N (100 • 9.8 = 980 N), while pn the moon, an orange would weigh 160 N (100 • 1.6 = 160 N). Weight is measured by scale, using the mass of object and gravitational force of the earth, then reversing the calculaton, (using division), to come up with a approximitive awnser.Mass is measured with a balance, (single, double, triple beam), that gets an exact awnser. Androo123 (talk) 01:45, 1 February 2008 (UTC)

Mis-linking related to misunderstanding of scope of Wikipedia weight article

Usually, not much can be made of the fact that it isn't the first use of a word which is linked, but one farther down the page. In most cases, all that needs to be done is move it up to the top of the page.

In this case, however, my guess so far (will correct this if I find out otherwise) is that that wording was added all at once, and linked that way by the original contributor.

That is misleading and related to the other problems mentioned above. It was likely intentionally done that way, and intentionally meant to distinguish the usage characterized as "informal" and "incorrect" from the true, God-given meaning of this word. But I don't think it was intentionally meant to deceive.

Rather, my guess is that the originally contributor failed to understand the scope of the linked article, failed to realize that our article at weight deals with the first meaning mentioned, as well as the second one:

• In fact, the vast majority of the links to that article come from references to weight where its meaning is equivalent to the mass covered in this article. My informed guess is that less than 10% of the incoming links deal with weight meaning "force due to gravity", based in large part on my looking into it in some detail a long time ago and reporting about that at Talk:weight.

• The article at weight is within the scope of WikiProject Business and Economics as well as within the scope of WikiProject Physics, as you can see in the boxes at the top of the talk page.

Gene Nygaard 14:17, 23 October 2007 (UTC)

Reply

The paragraph stood for a long time with the following content

In informal everyday usage, mass is more commonly referred to as weight, but in physics and engineering, weight strictly means the size of the gravitational pull on the object; that is, how heavy it is, measured in units of force. In everyday situations, the mass of an object is proportional to its weight, which usually makes it unproblematic to use the same word for both. Distinguishing them becomes important for measurements with a precision better than a few percent, due to slight differences in the strength of the Earth's gravitational field at different places, and is essential when one considers places far from the surface of the Earth, such as in space or on other planets.

I created most of that wording, studiously avoiding any implication it is "incorrect" to use the word "weight" for mass. In this form the first use of "weight" is bolded, because it presents a synonym for this article's title. The second use of "weight" is linked because its target article clearly presents the "weight-as-force" meaning (which is a fact about that article's content even though most of the links possibly ought to point here instead). The point of the paragraph was exactly to prevent overzealous editors from inserting "MASS IS TOTALLY DIFERENT FROM WEIGHT YOU DIMWITS" statements, while still acknowledging that one sometimes need to distinguish between the two meanings. This worked well until an anonymous editor inserted "incorrectly" a few days ago, while also completely garbling the description of the weight-as-force meaning and removing the subsequent balancing discussion.

I would prefer reverting to the version I quoted above, but edited to take into account some of your objections that still apply to it:

In many contexts, mass is more commonly referred to as weight, but in mechanics the word weight strictly means the size of the gravitational pull on the object; that is, how heavy it is, measured in units of force. In everyday situations, the mass of an object is proportional to its weight, which usually makes it unproblematic to use the same word for both. Distinguishing them becomes important for measurements with a precision better than a few percent, due to slight differences in the strength of the Earth's gravitational field at different places, and is essential when one considers places far from the surface of the Earth, such as in space or on other planets.

However, I do not agree that the Weight article as it currently stands is an appropriate link target for weight-when-synonymous-with-mass. –Henning Makholm 14:59, 23 October 2007 (UTC)

Distinguishing them (them meaning, of course, mass vs. force, the things we ought to be distinguishing) is every bit as important for that 401.23 troy ounce bar of platinum. The "unproblematic" language glosses over that. We are measuring what we want to measure, and we are calling it "weight" just as we have for ages. It does mean mass; the fact that some who have been mistrained in their physics clases or whatever into believing that is this context it also means force should not be encouraged in that misbelief. Gene Nygaard 15:08, 23 October 2007 (UTC)

What it really boils down to is a poor choice of a word to use, when the physicists were shopping around for a term to use in their jargon, for the force due to gravity. Gene Nygaard 15:14, 23 October 2007 (UTC)

It also would not be appropriate for the majority of the links to weight to link here as some sort of Easter egg surprise, either. Many readers might just say, "Oh, another bad link" and say to hell with it. Gene Nygaard 15:35, 23 October 2007 (UTC)

Disruptive edits

Makholm: I (User: Greg L) placed the following post on Talk:Kilogram regarding Gene Nygard’s flagrant disregard for the facts. Please e-mail me or contact me on my talk page as to what we are going to have to do about this. Greg L (my talk) Here’s the post:

Gene. I’ve tried to be patient but your continued edits trying to change accepted facts in physics are becoming disruptive. Encyclopedia Britannica very simply defines “weight” as “[the] gravitational force of attraction on an object, caused by the presence of a massive second object, such as the Earth or Moon.” Wikipedia’s Weight article defines weight as follows: In the physical sciences, weight is a measurement of the gravitational force acting on an object. World Book (print edition) says this under Weight: Weight is the gravitational force put forth on an object by the planet on which the object is located. Further, the Kilogram article adheres perfectly to Encyclopedia Britannica’s discussion of the distinction between “weight” and “mass”. The article also gives proper and fair treatment to the fact that the term “weight” in common vernacular can occasionally mean “mass.”

With regard to Encyclopedia Britannica’s article on weight you’ve written here that we shouldn’t “stoop” to their level and you’ve also written that Wikipedia’s own article on weight, which is linked to in several places in this article can’t be trusted. Other editors besides me have made edits that counter yours. I’ve leaned over backwards and several parenthetical instances of “(force due to gravity)” have been placed after various instances of “weight” to help the reader understand the point. These parenthetical explanations go far beyond other encyclopedic treatments; my print edition of World Book doesn’t even mention that “weight" may sometimes mean “mass” in common vernacular. In light of these realities, which were carefully explained to you above, you’re placement of a “factual dispute” tag on this article just because you aren't getting your way borders on vandalism. I see from your edits and arguments in places like Talk:Mass and in various areas of the Weight article, that you’ve take up the cause there, trying to redefine the commonly accepted usage of the term to “force due to gravity.” Your continued edits to change reality are without foundation and are disruptive. You’ve been warned. Greg L (my talk) 16:28, 23 October 2007 (UTC)

I agree with the comments by Greg L. Timb66 16:33, 23 October 2007 (UTC)

Thank you for taking the time to post your comment Tim. The fact that you are a professor of astrophysics at the School of Physics, University of Sydney lends great credibility. Greg L (my talk) 16:59, 23 October 2007 (UTC)

Gee. Now, instead of addressing the issues that have been raised, we get appeals to authority (not by Timb66 but by Greg L). For starters now, Greg, maybe you could explain how, even if that is true, it gives him or her any special expertise as to what either "WT" (weight) or "kg" (kilograms) (or for that matter, LB (pounds)) mean on my bag of sugar:

• NET WT 10 LB 4.54 kg

Who exactly gave Timb66 some say-so in that regard? Gene Nygaard 17:46, 23 October 2007 (UTC)

Gene, Your arguments are circuitous beyond reason and too much of my time has been spent dealing with you on this. This issue you raised above is clearly explained here in the Kilogram article, where it says

In everyday use, given that all masses on Earth have weight and this relationship is usually highly proportional, “weight” often serves to describe both properties, its meaning being dependent upon context. For example, the “net weight” of retail products, which may be given in pounds (U.S.) and kilograms, refers to mass (see also Pound: Use in commerce).

We’ve gone through all of this and the Kilogram article is clear. It expands on the exceptions to an extent that goes well beyond that in other encyclopedias, some of which don't even give lip service to the exceptions and adhere strictly to the meaning of “weight” as understood in the physical sciences. Your arguments that three encyclopedias treat the subject of “weight” incorrectly and everything should be revised to conform with your views on the matter are without foundation. Greg L (my talk) 18:07, 23 October 2007 (UTC)

Greg, your World Book also claims that pounds are not units of mass, does it not? Mine tells me that "The inch-pound system measures the weight of various materials. . . . The metric system measures mass (amount of material something contains)."

Drat, are you going to start the WP:AfD on pound (mass), or are you going to make me do it? Gene Nygaard 18:16, 23 October 2007 (UTC)

Double drat! Forgot about the AfD on newton and dyne as well. Gene Nygaard 18:23, 23 October 2007 (UTC)

Gene, your facetious (3. lacking serious intent; concerned with something nonessential, amusing, or frivolous) comments are entirely beside the point. In any proper article dealing 1) with the physical sciences, and 2) with “mass”, “weight”, and the difference between them, “weight” is “gravitational force”. Period. Encyclopedia Britannica very simply defines “weight” as “[the] gravitational force of attraction on an object, caused by the presence of a massive second object, such as the Earth or Moon.” Wikipedia’s Weight article defines weight as follows: In the physical sciences, weight is a measurement of the gravitational force acting on an object. World Book (print edition) says this under Weight: Weight is the gravitational force put forth on an object by the planet on which the object is located. All three of these encyclopedias are correct and you are wrong. Your raising of the issue of “net weight”—something brought up in the Kilogram article and which you had to have known about—amounts to nothing more than disingenuous attempt to seize a virtue of that article and try to turn it around to your own advantage in an effort to divert from the real issue. I’d very much like to have simply addressed logical, scientific points with you but that seems impossible since you refuse to accept the reality of a proper encyclopedic treatment of the subject and instead resort to nonsense. Stop harassing other editors over this issue because your position is without proper foundation. Greg L (my talk) 19:28, 23 October 2007 (UTC)

A new attempt at clarification

(Outdent.) Um, this looks like the middle of a long ongoing argument, and it is difficult for a newcomer to see what the basic disagreement is about. My eyes start to glaze over when I see long paragraphs with more emphasized text (of several kinds) than plain text. Instead, let me try to argue from first principles how I think we ought to handle the case. For the purpose of focusing the discussion, I would be interested in hearing which steps, if any, we cannot all agree on:

(1) According to science, the mass of a body ("M", measured in kilograms) and the size of the force exerted on the body by gravity ("F", measured in newtons) are separate, distinct concepts. Both are valid and useful.

(2) Under various common assumptions that it would be tedious to repeat at each step in the discussion, M and F are, to a good first approximation, proportional to each other. Thus:

(2a) Pre-SI units of force were often derived from units of mass (pound-force, kilopond) and in all but the most formal speaking identified with the underlying mass unit (effectively non-dimensionalizing g to 1).

(2b) Practical devices intended to measure M actually react to F and convert to M-values either by a fixed calibration or by comparing it to the F of reference objects with known M.

(3) Even before M and F was recognized as being distinct, people were aware that some intrinsic (hence somewhat like M) "how-much-ness" of things could be quantified by measuring their "heavy-ness" (hence somewhat like F). "Weight" referred to this assumed single concept.

(4) In informal everyday language the word "weight" continues to denote an idea of how-much-ness that does not distinguish between M or F (though it is clearly distinct from, say, volume).

(4a) This is not problematic or incorrect within the domain of informal everyday language.

(5) In some contexts the word "weight" refers specifically to M. (Gene Nygaard makes a good case that commerce an example of this).

(5a) This is not problematic or incorrect within the scope of those contexts.

(5b) A typical feature of such contexts is that F is not considered a relevant quantity to speak about at all (except as a way to measure M).

(6) In some contexts the word "weight" refers specifically to F.

(6a) This is not problematic or incorrect within the scope of those contexts.

(6b) Such contexts include all cases where M and F are both potentially relevant quantities and a way to specify which one one speaks about is needed.

(7) As an encyclopedia that strives towards a neutral point of view, Wikipedia should not imply that one of the senses (4), (5), (6) is an objectively (i.e. when no particular context is given) more correct meaning of "weight".

(8) The concept M has a name "mass" which is unambiguous in all context; therefore our encyclopedic treatment of M should be in the mass article.

(8a) It is encyclopedically relevant in this article to present "weight" as a word that sometimes denotes M, and give a short capsule summary of why this is not always the case, with a link to a fuller discussion.

(8b) The fuller discussion is probably best placed in the weight article. Weight (disambiguation) is not an appropriate place for discussion, per the Manual of Style (disambiguation pages).

(9) The concept F has no other short name than "weight", so our encyclopedic treatment of it should go in the weight article, even though "weight" when stripped of context is ambiguous.

(10) Wikilinks from the word "weight" in other articles where the intended meaning is M should be (pipe-)disambiguated to the article about M, namely mass. The presence of an unlinked boldface weight in the lead section lets the reader quickly locate the word he clicked on, and find an explanation of why the link was piped.

Okay, I can see from the above the Gene Nygaard disagrees with (10). But surely that disagreement must arise from a more fundamental disagreement earlier on. I'm not certain what it is, though... –Henning Makholm 02:29, 27 October 2007 (UTC)

I'll address some of your points. It would be clearer if you italicized your variables, and even then use m rather than M for mass (yes, M is sometimes used, but usually only when there are two masses and they are distinguished by using uppercase for one and lowercase for the other (G is proportional to mM/r). I'll use m here.

1. No. Unless you are talking about the science of linguistics, the meaning of a word is not "according to" science, and even in linguistics, it wouldn't be the proper terminology. Scientists may well use a number of words with specialized meanings within the jargon of a narrow brance of the broad thing called "science", but that is a different story.

Otherwise, the rest of 1) following the introductory clause is okay by me.

2. Okay for talk page purposes only, and only when the context clearly is limited to Earth either explicitly or because the nature of the discussion precludes otherwise

4. It isn't "informal".

7. That summary of your points 4, 5, 6 is pretty reasonable.

8. No. Many people are more familiar with the "mass" used in a body-building context. Or with the some bigness, volume, field-of-view idea associated with the term "massive" in everyday usage. I just recently saw someone else change an astronomy-related article which talked about something about "massive objects" to make the intended meaning less ambiguous by saying "objects that have mass. In that particular case, using mass insteat was an improvement, but mass is an ambiguous word, too. It is less problematic than weight primarily because its other meanings are usually not associated with measurements involving specific numbers. But there are still times when the meaning needs to be made clear.

9. First, the concept represented by F is much broader than weight in anybody's book. Many of the disputes in various articles arise because people insist on using the word weight in connection with something that applies to force from other causes as well (and were we to adopt your argument, "force" is shorter than "weight", but see my third point here). Second, the fact that mass is indeed the quantity that we normally call weight in many contexts is quite important to this article, something that will help readers bridge the gap between what they already know. Third, we don't choose our terminology so that we always use the shortest word or phrase possible.

10. There's a reason mass isn't used in those articles, and that's probably a good reason not to send it here. Your proposal sounds to be like a devious first step in trying to remove the most common usage of the word weight from its own article. If you want to spin off a new article at Weight (mechanics) and move the handful of incoming links to weight which should go there to their appropriate new article, I'll listen to your arguments, though on initial impression I wouldn't be likely to favor that either. Gene Nygaard 13:34, 27 October 2007 (UTC)

(1) I fail to understand your objection. The point speaks not about words at all; it merely introduces two concepts and introduces the (deliberately non-standard) abbreviations M and F to refer to the concepts in the following discussion. What has that got to do with linguistics?

(2) This is a talk page. As I said: We do not need to repeat those assumptions in each comment; we all know what they are.

(4) The point makes a claim about informal language. It makes no claim at all about language that is not informal. Are you claiming that the point I make about informal language is an untrue statement about informal language?

(7) Good. It did look like you were suggesting that F should be considered a second-class or fringe meaning of "weight", but I accept your statement to the contrary.

(8) When "mass" is used in a body-building context, does it mean anything different from M? Similarly for the astronomy example; "objects that have mass" would seem to refer to M, too.

(9) First: It surprises me that you think that F is broader than "weight". Can you give an example of a sense of F that does not fall under a possible meaning of "weight"? Recall that the concept I denote by F is not just force in general but specifically the size of the gravitational force on a body. Second: Yes, that is why I support telling in mass that "weight" often means this, too, as described in (8a). Third: We chose our article titles as the common term for the subject we write about, and there is no other common names for the concept F. (Various longer descriptive phrases that one might think up are not common terms for the concept, and hence not good article titles).

(10) Please do not assume bad faith. I note that your theory about my supposed hidden motives is directly contradicted by (8b), and decline to argue further about what I want or do not want (upon which subject I do consider myself the supreme authority). –Henning Makholm 14:25, 27 October 2007 (UTC)

(1) I am objecting to "According to science". Science doesn't determine the meaning of ambiguous words. What you are taling about is probably better characterized as "in common usage in the physical sciences" or something along those lines. Science can be used to analyze what a gold trader means when he talks about the troy weight of a 400 oz gold bar, and the combination of common sense and the physics in that case requires that weight means what you are calling mass. In other words, what I'm objecting to the notion that just because there are two concepts that are distinguishable in science, it is not "according to science" that particular wording is often used in the jargon specific to that branch of science to describe those two concepts. The meaning of your statement is much clearer and less objectionable, if you simply drop that introductory clause.

(2) How we "handle the case" includes what we put in the article as well. I was just pointing out that what you said may help us in focusing the discussion here, but it isn't necessarily phrasing that is polished enough or relevant enough for inclusion in the article.

(4) Under that interpretation, my objection would of course be that the phrasing is deceptive and misleading, and likely to be interpreted differently from what you intended. Its "truth" in that overliteral (and problematic even in a strictly literal sense) doesn't matter; most people are going to see this as a claim that the usage described by you is always "informal usage". And you've repeated that phrase again in 4a, hiding the fact that, using the same reasoning you have used, it is every bit as much true that "This is not problematic or incorrect within the domain of formal language", yet I doubt you'd agree to that wording.

(8) Yes, it does mean something different. Exactly what, I'd be hard pressed to explain.

(9)I don't understand you at all there. We use F = ma in connection with the force exerted by a rocket engine, let's say it is 175 kilonewtons for our example. That force F = 175 kN is not ever called "weight" in anybody's book. But it still accelerates a mass (the very thing we should be talking about in this article about mass with that meaning). It is most often called "thrust". We use F for the tension on a bicycle spoke or on a piano wire. Neither of those forces is ever called "weight" in anybody's book. I understand your point about you having specifically limited yourself to a particular kind of force for which you are using he symbol F; the problem is that this is the vary same symbol used in general with a broader meaning. Others participating in the discussion, or if that were put in the text of the article, others reading the article, aren't likely to interpret it the same way (even if they see your specific definition of how you are using it, they're likely to gloss over it like I did). Something like that may be acceptable a formula set off from the text and followed by a "where x means ..." explanation, but not in this context.

(10)Maybe I went overboard there, with respect to the motives of "you" as an individual, and I have no reason to believe that this was your personal intention. When I talked about the "first step," I wasn't thinking that you would be someone likely to take the "next step". But someone would. Because of that, I cannot accept what you said as an agreed-upon principle, primarily on the basis that somebody, sometime is virtually certain to interpret it exactly that way. So I apologize for wording it as directed specifically at your intentions, and hope this helps clarify what I meant. Gene Nygaard 15:46, 27 October 2007 (UTC)

(1) Again, the text you are objecting to does not attempt to determine the meaning of any word, neither does it claim that science is determining the meaning of word. It simply presents concepts, explicitly without speaking about which words apply to them. Your objection still makes no sense to me.

(2) What I wrote is an attempt to focus the discussion here on the talk page. None of it is intended to be moved to article-space with its current wording.

(4) Indeed I do not agree that any word in contemporary formal language can mean a concept that does not distinguish between F and M. I am trying to discover which facts we can agree on. I would be counterproductive to extend it to something that I know we cannot agree on at this point. I do not understand how you think this is "deceptive and misleading" - who would it deceive?

(9) It seems that you are objecting to my use of abbreviations rather than to what I am actually saying. Very well - I invite you to suggest a pair of neutral shorthands that we can use (limited to this discussion on the talk page) to speak about the two concepts, without presupposing any conclusions about which one better matches the word "weight". Then we can start over from the beginning. There is no hope of reaching a consensus until we can agree to use a common language in which to express a consensus among ourselves.

(10). Apology accepted. However, I think your argument here just begs the question – it assumes that there is a predetermined truth about which concepts must be treated under which article titles, within what is supposed to be a reasoned discussion about which article titles should be treated under which article titles. –Henning Makholm 20:18, 28 October 2007 (UTC)

(9) Henning is using F to mean force due to gravity. It would be better to use W instead of F. Then it would be obvious that the examples given by Gene (thrust, etc.) are not relevant. Timb66 16:13, 27 October 2007 (UTC)

W was my initial thought, but I feared that Gene would object to the "force-of-gravity" meaning getting first choice of the initial letter of "weight". If everybody can live with calling it W, I'll happily change it. Or we could call it Q or Z. Just so we can move on without geting bogged down in meta-discussions about which terms to use during the discussion. (I would prefer not to italicize the letter, however. It is not intended as an algebraic variable, but an "editorial" abbreviation for a bit of cumbersome prose. Still, I apparently have to emphasize, only to be used in the talk page discussion while we agree about how to express things in the article). –Henning Makholm 20:18, 28 October 2007 (UTC)

I agree, except for one factor. It's probably going to be used in a context where F with the general meaning is more appropriate than W in the more specific meaning. Gene Nygaard 17:26, 27 October 2007 (UTC)

I don't understand this comment. Gene seems to be raising objections for the sake of it. This is not a difficult concept, why has it become so difficult? Please can we move on? Timb66 20:22, 27 October 2007 (UTC)

Ok... not a scientist... Net weight - bag of sugar... the weight of the sugar contained in the bag ("net" meaning the weight of the product without the product's container). Don. —Preceding unsigned comment added by Fishgolf (talk • contribs) 02:15, 8 February 2009 (UTC)

“Mass versus weight” article

The subject of “mass vs. weight” is currently being discussed at Talk:Kilogram: Location for “Mass versus weight”. A consensus has been so far achieved that the section Kilogram#Mass versus weight should be moved out of Kilogram. Consideration is being given as to where to move it to. Options are to move it to Mass, or to Weight, or to give it its own article, Mass versus weight that all other articles can link to. If you would like to express an opinion on this matter, please click here. Greg L (my talk) 22:23, 8 November 2007 (UTC)

Nice to see you are admitting that I was correct in labeling that section as disputed at the kilogram article. Since you are now claiming that dispute is resolved, have you removed the inappropriate content from Kilogram? -- Gene Nygaard (talk) 17:37, 16 November 2007 (UTC)

Machine translation?

To me the paragraph "The object of inertial mass lower than any significantly high number tend to have low responsiveness to a change of resistance to accelerated motion. We therefore conclude that objects of a high inertial mass tend to increase resistance force less rapidly while experiencing acceleration, so accelerate more rapidly." appears to have been translated into English from a foreign language. Do other people read it that way? Can anybody actually understand it (especially the first sentence). P Koil (talk) 15:53, 4 January 2008 (UTC) 15:55, 4 January 2008

I, too, fail to extract sense from this. The second sentence appears to claim that bodies with high mass accelerate faster (than bodies with lower mass?). Of course, this is false if we are to suppose everything else (including the acting force) being equal, and I cannot imagine any reasonable alternative boundary condition that would make it true.

The stilted style does not sound machine-translated to me, more like a crank with a non-standard perception of mechanics who tries to hide his basic incompetence behind random learned-sounding phrasing (such as "significantly", "tend to", or "more rapidly" instead of "faster").

This is the edit where the paragraph first appeared. I have removed it. –Henning Makholm 02:12, 6 January 2008 (UTC)

Not true.

> To date, no deviation from universality, and thus from Galilean equivalence, has ever been found, at least to the accuracy 1/10**12. More precise experimental efforts are still being carried out. <

This part of the article is not true. I am aware of experiments conducted in deep mineshafts with copper and steel balls, which showed quite significant (0.1-0.4%) difference based on material quality. These experiments, however, are DISPUTED (which is not the same as "has never been found"). This issue should be clarified! 82.131.210.162 (talk) 18:07, 9 January 2008 (UTC)

• And it takes brass balls to do fringe physics of this type, too. SBHarris 01:56, 1 February 2008 (UTC)

Citation please? --Slashme (talk) 05:45, 10 January 2008 (UTC)

Physics or chemistry?

Someone changed the opening paragraph a couple of days ago, saying mass is a concept of chemistry. Seems to be no discussing here on this talk page to categorize mass as a chemistry concept, so I changed back to physics. Also, the article's categorization doesn't deal with chemistry. Although mass is very important in chemistry, in my opinion it should be viewed as a physical property. Mårten Berglund (talk) 00:38, 20 March 2008 (UTC)

I agree. The current definition reads far too closely to the definition of amount of substance, which is definitely different from mass. Physchim62 (talk) 16:30, 14 December 2008 (UTC)

Higgs Boson or Mass Field

"the equality of inertial and active gravitational mass [...] remains as puzzling as ever".

Open Source (Vacuum Gravity and Inertial Forces) Project: http://www.wiki1.net/groups/pmwiki.php?n=BigCrash.Inertia

If you that gravity around massive objects is a curvature of space accelerating matter toward the massive object, and you accept vacuum energy, then very dense vacuum energy might be expected to generate the same curvature of space accelerating matter toward the dense energy (accelerate elemental particles outward in all directions, felt as resistance to change in motion, see proposed mechanics and math at BigCrash.org > Inertia) --Jtankers (talk) 12:51, 25 April 2008 (UTC)

... Because (astronomical) vacuum gravity force would only act on the smallest particles of matter, possibly quarks, ... seems to indicate that the force may be repulsive with respect to particles/fluctuations that are energy only and only travel at the speed of light. (A fluctuation is strongly pushed by vacuum energy... such a force might require light speed of massless particles. This push force on energy may also push strongly on vibrating strings of energy, possibly folding them into ... every possible bit of empty space. The pull force on matter particles should have the effect of giving the matter particle its size, pulling the quark to the radius that a quark is measured to be, and resisting change in motion, causing inertial forces. --Jtankers (talk) 12:51, 25 April 2008 (UTC)

Quarks aren't measured to have ANY radius. No "elementary particle" (quark, lepton, vector boson) has yet been found to have a radius or a "size." SBHarris 19:00, 16 October 2008 (UTC)

(Au sujet de la masse) Origin of Mass

The last link to http://unification.selfip.org:2008/ is innapropriate to wikipedia and should be removed. The concept of particle as "a magnetic spherical loop" developped in this "theory" would be regarded as bogus by the vast majority of physicists. If required I can loose my time translating parts of this for you, but I believe it is not worth. —Preceding unsigned comment added by Humanino (talk • contribs) 15:55, 18 August 2008 (UTC)

As nobody seem to comment, I will now remove this link. Humanino (talk) 19:06, 2 September 2008 (UTC)

Question

Why doesn't your mass increase by 100 g after you eat a 100g sandwich? —Preceding unsigned comment added by 71.17.75.192 (talk) 02:26, 15 December 2008 (UTC)

You're not a closed system, and you lose carbon when you breathe in oxygen and exhale CO2. Some of that sandwich is carbon. If you put a man in a bubble or sealed spaceship and he eats a 100 g sandwich, mass of the system never changes. SBHarris 00:50, 5 February 2009 (UTC)

Origin of Mass -- The Abstract

More than 99% of the mass of the visible universe is made up of protons and neutrons. Both particles are much heavier than their quark and gluon constituents, and the Standard Model of particle physics should explain this difference. We present a full ab initio calculation of the masses of protons, neutrons, and other light hadrons, using lattice quantum chromodynamics. Pion masses down to 190 mega–electron volts are used to extrapolate to the physical point, with lattice sizes of approximately four times the inverse pion mass. Three lattice spacings are used for a continuum extrapolation. Our results completely agree with experimental observations and represent a quantitative confirmation of this aspect of the Standard Model with fully controlled uncertainties.

Science 322, 1224 (2008); S. Dürr, et al. Ab Initio Determination of Light Hadron Masses

--151.200.247.100 (talk) 07:47, 3 January 2009 (UTC)

What happened to the relativity sections?

Article now reads:

Now, suppose that the mass of the body in question is a constant. This assumption, known as the conservation of mass, rests on the ideas that (i) mass is a measure of the amount of matter contained in a body, and (ii) matter can never be created or destroyed, only split up or recombined. These are very reasonable assumptions for everyday objects, though, as we will see, mass can indeed be created or destroyed when we take special relativity into account.

This is simply wrong! Both relativistic mass and invariant mass are conserved in closed systems over time. "Matter" might be created or destroyed (depending on how you define it), but mass is not. If energy is conserved, mass must be also. SBHarris 00:54, 5 February 2009 (UTC)

It's speaking of rest mass, not invariant/relativistic mass.Headbomb {^ταλκ_{κοντριβς} – WP Physics} 03:09, 5 February 2009 (UTC)

What is the rest mass of a system? The only answer that make sense is the system invariant mass, since nothing else can be measured (or defined). So rest mass (by any rigorous definition of "rest mass") is conserved in isolated systems. It is not created, nor destroyed.

Now, please don't tell me that the "rest mass" of a 2-photon system is undefined, and because it's undefined, it's not conserved. That only means we shouldn't use the term "rest mass" when we talk about systems (even though we must talk about the mass of systems, ergo we must use another definition of mass). For single particles, rest mass is obviously conserved. For it not to be, the particle would need to break up into a system, and then the preceding comment applies. SBHarris 20:17, 6 February 2009 (UTC)

you can't define mass using force, and defing force using mass..

title.. --89.139.9.247 (talk) 14:03, 24 February 2009 (UTC)

Some ridiculous admin reverted this page to its earlier form.

Please desist from such vandalism, Quantumobserver. Is this the way all Admins treat real people?