Powstini

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Hijacked by non-experts with possible ulterior motivations

This is a warning to casual reader of this page looking for scientific understanding of mass and its conservation. The page has been hijacked by people who prefer to mystify the discussion than give the simple statements. The revision history shows a variety of vandalism claims and reverse edits, while this talk page shows that one of the abiding contributors Steve adopts an unusual definition of "mass" thereby making the discussion cumbersome and confusing.

I am not bothering to edit the main article because it is likely be vandalised by these authors. Sorry, I am not Being Bold the way Wikipedia exhorts us to be.

The message is very simple :

1. In chemistry and physiology, being phenomena at low energies, mass can be taken to be conserved within the observational approximation needed.
2. In Special Relativity the only useful definition of mass is the rest mass. This makes mass or inertia a property intrinsic to the particle, independent of the frame of observation.
3. Rest mass is not conserved in interactions as particle types can change. That is, the sum of rest masses of initial reactants is not necessarily the same as that of the final products.

The classic example of the last point is electron and positron with total rest mass twice that of the electron can combine to produce two photons, the rest mass in the final state being zero. This mismatching of rest masses is still consistent with the law of conservation of energy.

That's all there is to it folks, you can read the continuously updated article at your own risk and amusement. Powstini (talk) 12:52, 23 February 2014 (UTC)

On the contrary, in special relativity, the "only useful" definition of mass is NOT the narrow "rest mass." The only useful definition of mass is invariant mass which (indeed) is the same as rest mass for single massive particles, but has no definition for systems of particles except invariant mass (the system rest mass when bound, the system mass in the COM frame when unbound). Yet most objects are systems of particles, not simple particles! So what does relativity do in discussing the mass of compound objects? When you weigh (or find the mass of, by balance beam) a system of particles (any ordinary object) you are measuring the invariant mass of the object, not the sum of rest masses of the particles that make it up. So you end up counting the mass of the photons inside a hollow object, and the kinetic energy of the particles moving in the object, too ("thermal energy"). Yet none of these has a rest mass, as individual photons have no rest frame, and kinetic energy disappears in any given particle's rest frame.

In the system of two photons, the invariant mass of the two photons (as a system) is the same as the invariant mass of the electron and positron that produce them, so invariant mass is conserved. If you put the electron and positron in a can, and let them annihilate while the can sits on a scale, as the photons bounce around inside the can, the number on the scale does not change. The photons also add inertia, gravitation, and so on (none which changes with the annihilation). That is because invariant mass is conserved, so long as you keep the system closed. But if you like open systems, nothing is conserved, so what's the point in discussing that?

As for your remarks about my "unusual" definition of mass in relativity, it is the same as adopted by Taylor and Wheeler in Spacetime Physics, a well regarded mathematical introductory text on SR for physicists.

I am genuinely curious as to how it is, that as a physicist, you have not come across the concept of invariant mass. And why you think "rest mass" applies to ordinary objects, which are always compound, and always contain energy that contributes to the mass of the object but that cannot be measured using any simple definition of "rest mass" (which really applies only to single elementary particles). Enlighten me. I can take it. SBHarris 21:41, 20 March 2014 (UTC)

Response to SBHarris

Appreciate your enthusiasm and also agree that invariant mass is a valid and useful concept in its own right. However Invariant mass is not a concept we use for bound states. We then simply call it the _rest mass_ of the bound state.

Invariant mass is used in scattering experiments primarily to keep track of the input energy at which the experiment is being carried out. The magnitude of invariant mass can be engineered to the experimentalist's desire. It is confusing to bring this into the discussion regarding conservation of "mass" as known in chemistry and non-relativistic physics. On a page like wikipedia please file the invariant mass concept separately and distinctly -- it does not actually "weigh up" anything. Aside from being irrelevant to this page, a lengthy article with numerous caveats confuses and discourages a first time reader. I hope you will restructure the article suitably. Powstini (talk) 06:15, 22 March 2014 (UTC)

Okay, now I'm confused. My purpose on the conservation of mass page was simply to point out that conservation of mass holds in relativity also. That simple. Mass in relativity (invariant mass) is as much conserved as energy and momentum, and in some ways even more so, as invariant mass is Lorentz invariant, and energy and momentum are not (this doesn't affect conservation, but it does affect the quantity that each inertial observer SEES conserved). You then started this argument (is it an argument-- where do we disagree?) with a comment on the talk page attacking this idea, and saying that the page had been "hijacked" (your word) and that "rest mass" is NOT conserved in interactions. You gave the example of electron-positron annihilation. But rest mass (what you flatly call "mass") IS conserved for that system, if it is bound (i.e., positronium in a can). And even if unbound (free electron and positron decay to free photons) invariant mass is conserved in any system where energy and momentum are conserved. Invariant mass is generally conserved in the same way energy and momentum are. So, I fail to see what kind of "fair" system exists where mass is NOT conserved (over time, in a single reference frame, which is what this phrase "conserved" means).

Now, before you go and give me a case where the sum of rest masses of particles are not conserved in an interaction, I'm well aware of that. But that's not a single reference frame, but many frames where you go around and look at each moving particle at rest. That's not permitted! If you do that, energy and momentum are not conserved, either! So it proves nothing. Second, I'm well aware that mass is not conserved in systems where (over time) you allow some mass to cross your system boundary (system is not closed). But again, energy and momentum are not conserved in that case. You've broken the rules that go along with any conservation law.

I think I know what you mean when you say the invariant mass can be engineered to the experimentalist's desire (by boundary drawing-- though not by frame-shifting in the case of invariant mass), but I don't see the point. Energy and momentum of a system can be engineered to be anything you like, in the same way (and by frame-shifting, too-- but let's pretend you can't do that). That doesn't mean energy and momentum of unbound systems are not conserved. You can pick any system you like, but once you have, over time and interaction, and with it closed after that, from t1 to t2, its total energy stays the same through any interaction. And also its momentum. And also its mass (either relativistic or invariant). That's handy.

So again I ask: give me a counter example of an interaction in special relativity where energy and momentum ARE conserved, but mass (either invariant mass or relativistic mass) are NOT conserved. The relativistic mass is conserved trivially, since it's just system E/c^2. But you'll find invariant mass is just as hardy. So is "rest mass of a single bound system" if you like to call that the "true" mass. SBHarris 03:07, 23 March 2014 (UTC)

Don't strain too much, as this is an impossible task. If E and p are conserved in any system, then obviously the quantity E² - p² is conserved. And that's just m², the square of invariant mass. Which you like as bound system rest mass. So that quantity must be conserved, end of proof. Every type of legitimate mass in relativity is conserved whenever energy and momentum are.

The problem in pedagogy is years of playing with toy systems where energy is allowed to leak out (like Einstein's 1905 thought problem where a body gives off two photons in opposite directions, which are then ignored). Also a severe problem is introduced in conflating mass with matter, a scientifically doubtful term that is NOT well-defined and often is not conserved. Example: most of your mass is gluons and quark kinetic energy-- are gluons matter or energy?? So are YOU mostly matter or non-matter? You can see the matter article for the problems, here. But matter as defined as common fermions and non-matter as defined in common bosons like photons, gives us non-conservation of matter! As in your electron-positron annihilation. But that just confuses things badly with a poorly-defined term, and by the time we get through deciding "matter" isn't conserved, we've often decided mass isn't either, as we think sloppily mass and matter are the same thing.

I've tried to clean all this up, but obviously if people like you come to these pages with these issues, I haven't hit the main points hard enough, early enough. So your input, as a newbie, is sought. SBHarris 00:31, 25 March 2014 (UTC)

Inertial mass

Dear SBHarris

To draw your attention once again : Invariant Mass does not actually "weigh up" anything. The reader who arrives on this page simply wants to understand the fate of the usual "inertia" type of mass. The definition of Invariant Mass has the word mass stuck in it only to remind experts of the analogy to rest mass. Please don't clutter up an informative article with advanced technical information. Once more with feeling : Keep the article simple.

Please note that covariance is an elegant tool. But all that is covariant is not physical and some of the most important physical quantities are not covariant.

Powstini (talk)

If you don't like the exotic term invariant mass, and think it's too technical, that's one thing. But you're the one who pointed out that invariant mass is the rest mass of systems and objects in SR, and therefore IS the ordinary "inertial mass" of baseballs and chemistry. Homely ordinary inertial mass, where present, is conserved wherever energy and momentum are. Give me a counterexample or admit your error like a man. Then feel free to make suggestions on terminology here after we agree in basic physics fact. You can't measure the inertia of unbound systems (obviously a quantity undefined or unmeasurable in the first place is hard to conserve), but if you trap photons they add inertia as E/c^2. Einstein's 1905 thought experiment works just as well if the photons are bouncing around inside a can, then let out, as if they are newly created and emitted. To imagine otherwise would be to hold that an empty can that created new thermal photons in its cavity, would suddenly lose mass, inertia, and weight because the trapped photons don't contribute. But of course that's nonsense. SBHarris 17:30, 26 March 2014 (UTC)

It is not a matter of taste, i.e, of my liking something or not. It is a problem of a non-expert not understanding the manner in which physics literature uses words.

The word invariant is being used in two different contexts. The general sense and then more specifically, "invariant mass" to identify a useful concept in scattering experiments. The use of the word mass in the term Invariant Mass related to scattering experiments is purely suggestive. There is no mass there in the sense of inertia.

We do _not_ call the mass of the proton (or baseball) its invariant mass. It is called its rest mass.

But finally to clarify ( at the risk of perpetuating the confusion at your end), both concepts, in the _general_ sense, are invariant quantities under SR transformations.

Please clean up the page by leaving out Invariant Mass from the discussion of Conservation of Mass. If at all mentioned in passing, provide it as an external link.

Powstini (talk) 15:57, 27 March 2014 (UTC)

How do I find the page on which I commented

If you want top find what pages you've edited then click the "Contributions" link in the upper left hand corner (those are your contributions.) If you want to see past versions of a page go to that page (can be either article or talk page) and click the "view history" link next to the edit link, it will show a list of all the versions that have ever been saved. (Both of these will, however, not show deleted versions.) Let me know if you have further questions or if this doesn't answer your question. RJFJR (talk) 13:39, 25 March 2014 (UTC)

I left the answer to the question to posted on my talk page here, on your talk page. I'll place a note to that on my talk page as well. RJFJR (talk) 14:04, 26 March 2014 (UTC)