Talk:Mass–energy equivalence

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Former good article nominee Mass–energy equivalence was a Natural sciences good articles nominee, but did not meet the good article criteria at the time. There are suggestions below for improving the article. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.
March 16, 2009 Good article nominee Not listed
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I think the fonts used for variables do not look good in this article. I think this would be much better:

It expresses the law of equivalence of energy and mass using the formula

where is the energy of a physical system, is the mass of the system, and ...

That way all formulae would be renderd using the same fonts. Any objections to that? --Physikerwelt (talk) 17:30, 11 October 2016 (UTC)

Mass-energy equivalence should be matter-energy equivalence[edit]

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Although terminology like "mass-energy equivalence" and "interconversion of mass and energy" is common, it is nevertheless incorrect.

Mass is an extensive property of matter. It is also an extensive property of energy. If you take a small isolated system containing matter, and weigh it, and if the matter inside the isolated system then gets converted to energy, and you weigh that isolated system again, there will be no change in its weight. If the matter within the isolated system had a mass of N grams, the mass of the resulting energy is also N grams.

So mass cannot be converted into energy. Really, it's matter that can be converted into energy. The mass of the resulting energy is equal to the mass of the matter that we began with.

Even physics researchers sometimes use sloppy language, figuring that everybody understands that it's not mass, but matter, that can be converted into energy. But they misjudge their audience, and the result is confusion everywhere. The result is an entire Wikipedia article that uses confusing language.

Rahul (talk) 01:08, 2 March 2017 (UTC)

The literature and most part of the web seem to disagree with you:
Google Scholar Books Web
"Mass-energy equivalence" 2,220 7,170 142,000
"Matter-energy equivalence" 65 186 27,900
- DVdm (talk) 07:55, 2 March 2017 (UTC)
I'm skeptical of a search that just counts the number of Google hits. This type of search can be used to prove a lot of wrong things. Also, please note that I'm not denying that "mass-energy equivalence" is used. Rather, it's used by researchers who expect their audience to understand that it means something other than its literal meaning. The average Wikipedia reader would misunderstand this phrase. Rahul (talk)
@Dhesi: Exactly how would you "weigh" energy? Weight is not the same as mass, let alone energy.--Jasper Deng (talk) 09:04, 2 March 2017 (UTC)
You can weigh energy, but please be cautious when phrasing such a question -- you're implying by your question that I mentioned weighing energy, and I did not. If this were a conventional discussion forum, you would have quoted me as saying "weigh that isolated system". It's hard to weigh energy alone -- it usually is found in conjunction with matter, so you would weigh both of them together. Typically the isolated system in this experiment would be an insulated box that you would weigh. Inside the box would be some combination of matter and energy, and the weight you get will be the weight of both. Also note that it's hard to convert matter into energy in a box that you can weigh, so this is obviously a hypothetical experiment that would be hard to actually do. We're essentially talking about exploding some sort of nuclear device inside an insulated box and weighing the box before and after. Rahul (talk) 21:17, 2 March 2017 (UTC)
I think then it's important to distinguish between the rest mass and the "relativistic" mass defined in this article. You are correct that the latter is conserved in an isolated system, but as soon as kinetic energy is gained by objects in the system (as observed from our reference frame), their relativistic mass is no longer the same as the rest mass. Now what do we need to do to measure the rest mass? We'd have to bring the objects in the system to rest with respect to our reference frame, i.e. robbing them of any kinetic energy they might have, which isn't allowed since we assume the system to be isolated. In order to gain that kinetic energy, their rest mass had to decrease (where else would you get the energy?). In other words, rest mass would not be a measure of the inertia of the system as-is, but is what we usually call "mass" in common parlance. You are, however, completely correct that the "relativistic" mass is invariant in a closed system relative to a given frame of reference.--Jasper Deng (talk) 22:32, 2 March 2017 (UTC)
By all means, let's be clear whether we're referring to rest mass or what you call "relativistic" mass (which I just call "mass" in my insulated box analogy). My point still remains that mass never appears and it never disappears, so mass does not convert into anything. The relativistic mass of a particle increases as the particle accelerates, but that increased mass does not appear out of nowhere -- something else must lose the same amount of mass somewhere, so again, mass does not interconvert. Rahul (talk) 04:28, 3 March 2017 (UTC)
@Dhesi: Well the dominant viewpoint is that "mass" without further qualification refers to rest mass.--Jasper Deng (talk) 05:19, 3 March 2017 (UTC)
I have no quarrel with "mass" being used to mean "rest mass" in the Article, just so long as this is consistently done throughout the article, and the definition is made clear in the beginning. In my daily life, however, I use "mass" to mean "whatever you measure when you weigh something", and that (in the case of my insulated box example) is what you have described as the relativistic mass. Consider for a second a hot frying pan. Its relativistic mass is a few picograms higher than its rest mass. If the average person thinks about it, he will instinctively consider the relativistic mass the mass of the frying pan, because that's what he would measure if he could weigh it (accurately enough) on his kitchen scale. (Ignoring air currents etc.) The rest mass cannot be measured in his kitchen, because he would have to cool the frying pan down to absolute zero first. Oversimplifying a bit, we can say that the rest mass of the frying pan is a hypothetical quantity, while the relativistic mass is real (as opposed to hypothetical) because it's actually measurable at a temperature that can be achieved in our kitchen. If we're going to use terms like "mass" in a manner that is not intuitive to the average person, then we should be very, very clear up front that we are doing so in any text intended for a lay audience. Rahul (talk) 07:17, 3 March 2017 (UTC)
I have a different perspective. I believe most folks interpret mass as something not dependent on an object's velocity, which is the classical view. The concept of inertia is less intuitive to those who have not taken physics, especially in relativity, so my opinion is that pedagogically, the physical definition of mass as a measure of inertia (which, to be clear, is the one I consider most fundamental) takes a backseat. The article explains that the equation is only valid when m is taken to be the rest mass, and is a special case of the more fundamental and general identity . In fact, the latter identity is also only valid when m is taken to be the rest mass.--Jasper Deng (talk) 08:05, 3 March 2017 (UTC)
Rahul, energy and mass are both merely quantitative *properties* of an object. The object is ultimately *composed* of quantum particles, some of which may be considered matter and some of which may not, e.g. photons can be considered "radiation" particles instead of matter. You can't "convert" matter to a property of matter! You can't convert any particle to its own property! You can convert some particles (matter or not) to some other particles (matter or not), but in both cases they have the same energy and the same mass both before and after the conversion. The problem is that you are considering something like light to *be* (a type of) energy -- in fact light *has* a property called total/relativistic energy (and an equivalent/proportionate one called its total/relativistic mass), just like matter does, but light is not a form of energy (nor of mass) and matter is also not a form of energy (nor of mass). You can't "weigh energy" because energy isn't an object that can be weighed. Energy and mass are both quantities that are merely the *result* of a measurement made on an actual system/object. You can weigh a system composed of particles (protons, photons, etc.), or measure its mass or equivalently its energy. Energy and mass are in fact the *same* physical property, but for historical reasons we use these terms according to different manifestations of this property. If we are measuring the work or heat that can be performed by a system, we usually call it energy, and if we are measuring the inertia or the strength of interaction with a gravitational field, then we call it mass, but they are always proportionate if measured in different units, or identical if measured in the same units. DavRosen (talk) 00:51, 3 March 2017 (UTC)
What an interesting discussion! Sadly, Wikipedia is not a good place for this, as we're shoehorning a discussion into wiki software. Anyway, when the average person reads a Wikipedia article, they are not going to easily make sense of quantum particle-based mathematical models containing abstract energy or mass terms. They will, however, easily relate to the example I gave, which could be restated as: "If you take a perfectly insulated box containing matter, and weigh it, and if the matter inside the box then gets converted to energy, and you weigh that box again, there will be no change in its weight." So why don't we try to make this Article use everyday analogies to make physics understandable to the lay person? The interconversion of matter and energy is understandable with my box analogy. The alleged interconversion of mass and energy by contrast has no real-life analogy. Rahul (talk) 04:28, 3 March 2017 (UTC)
His point is that you can't convert matter to energy and vice versa, since the latter is a property of the former, whereas mass and energy are in fact manifestations of the same thing.--Jasper Deng (talk) 05:18, 3 March 2017 (UTC)
I invite you to consider the journal reference below (sorry for the crude formatting, I'm not a Wikipedia expert and anyway this is the Talk page so I won't invest any time with wiki markup below). Rahul (talk) 07:47, 3 March 2017 (UTC)


Energy has Mass: A common misunderstanding is re-examined.

Hermann Bondi and C B Spurgin.

Physics Bulletin, Volume 38, Number 2, 1987.


The incorrect notion that mass can be converted to energy probably owes its origin to simplified popular accounts of nuclear fission processes, where emphasis is laid on the fact that the particulate fission products of uranium have a total rest mass somewhat less than that of the uranium atom and initiating neutron, while a very considerable amount of energy seems to have appeared from nowhere (as kinetic energy of products, energy of photons etc). But this energy has mass equal to the mass that seems to have disappeared. The energy has not come from nowhere; it was formerly present as potential energy of the arrangement of protons and neutrons prior to the fission — potential energy which has been diminished by the rearrangement into more stable fission products. It is the loss of this potential energy which gives rise to the apparent reduction in mass which is observed if one ignores the mass of the energy released. Potential energy has diminished and kinetic energy has increased. Mass of potential energy has diminished and mass of kinetic energy has increased. Energy has been conserved and mass has been conserved, each separately.


The best way to appreciate Einstein's conclusion is to realise that energy has mass. The best way to express it is to say that the mass of energy E is m, given by m = E/c².

Students should be taught that:

(i) energy has mass;

(ii) energy is always conserved;

(iii) mass is always conserved.

They should be warned against believing erroneous statements that mass and energy are interconvertible, and they should be urged to avoid such terminology as 'the equivalence of mass and energy'.



@Dhesi: Let me ask you an elementary question. What is mass a property of? What is energy a property of? I consider the notion that "energy has mass" to be absurd, because it is matter that has mass.
If you really want, I could ask professors at my university (which is one very well-known for physics, having two elements named for it), but I doubt they'd disagree.--Jasper Deng (talk) 07:56, 3 March 2017 (UTC)
If you want to invest some time, and if your campus has easy access to online reference materials, perhaps you could do a citation search, and look for any journal articles that cite the one by Hermann Bondi and C B Spurgin (above), and see if there are other articles that agree or disagree with their assertions. Such citations will be very useful in improving the Article. But by all means if your professors have any insights supported by reliable published references those would be helpful. Re the notion that "energy has mass" being absurd, simply consider my insulated box again. Suppose it contains matter and antimatter particles that annihilate one another after you have weighed the box. The box now contains (almost) pure energy. Does the box weigh the same? If it does, you're weighing energy (after subtracting the weight of the box itself). If you can weigh something, it must have mass. Energy has mass. Try to find the entire paper by Hermann Bondi and C B Spurgin (cited above). — Preceding unsigned comment added by Dhesi (talkcontribs) 08:51, 3 March 2017 (UTC)
It is not true that the stuff left in the box "is energy" or "is almost pure energy". What's left in the box are photons, which *have* have a property called energy! The original matter and antimatter already had that same property -- there is no difference in this respect. It is no more true that photons "are" energy than that the original matter & antimatter "are" energy. Energy (or its equivalent, mass) is equally a property of both! DavRosen (talk) 08:58, 3 March 2017 (UTC)
@Dhesi: As an example of how absurd that statement is, can you even tell me the "momentum of the energy"? Conservation of momentum is, after all, the original motivation for the notion of relativistic mass. You are attempting to frame a property of matter as a property of a property of matter.
You cannot define energy without matter, anymore than you can define momentum without matter. When you annihilate, you don't have the matter disappear. True, the result is often zero-mass particles, but they're particles nonetheless. The energy hence evolved is then an attribute of those particles.--Jasper Deng (talk) 09:28, 3 March 2017 (UTC)

OK peeps. This talk page is for discussing improvements to the article. Not a general discussion as to the meaning of "mass", "matter", "Energy" and whatever, see WP:NOTAFORUM. The article is supported by reliable sources and we do not do our own original research here. You could discuss on Wikipedia:Reference desk/Science if you are struggling to find references and welcome to continue here if there are changes that you feel are needed to the article that are supported by sources. Best wishes Polyamorph (talk) 10:00, 3 March 2017 (UTC)

@Polyamorph: But this discussion is related to whether we should accept Dhesi (talk · contribs)'s suggestion. He provided a reference, after all. --Jasper Deng (talk) 10:02, 3 March 2017 (UTC)
Then simple answer is no. We should use the term that is supported by the reliable sources which are cited in this article. Polyamorph (talk) 10:12, 3 March 2017 (UTC)
I agree. This is not the place to discuss interpretations of a paritcular —likely wp:UNDUE— source. Discussions of this kind tend to never end. - DVdm (talk) 10:33, 3 March 2017 (UTC)
This is not a discussion involving original research, but rather, that the terms used may mislead a lay reader. As another example, consider: "[I]f a body is stationary, it still has some internal or intrinsic energy, called its rest energy, corresponding to its rest mass." This would be clear to a physicist. A lay person would assume that the mass of book resting on a table in his living room is its rest mass. But the physicist knows, and a lay person does not, that this assertion either assumes absolute zero or assumes that the thermal energy is negligible for the calculation. Throughout the article implicit assumptions are made designed for physicist readers, not for lay persons. it's this type of misunderstanding that Bondi and Spurgin are addressing in the cited reference when they address what students should be taught (which you incorrectly suggest is wp:UNDUE). It's true that the Wikimedia software is poorly designed for discussions, but that's a reflection on the software, not on the discussion. — Preceding unsigned comment added by Dhesi (talkcontribs) 20:02, 3 March 2017 (UTC)
@Dhesi: You started this section saying "matter-energy equivalence" when that has been clearly established to be absurd, as is your later "energy has mass". Your intuition argument is weak at best, because like I said, people don't think of mass as something dependent on velocity. Additionally, thermal energy of an everyday object is rather neglible compared to the rest energy: a heated clothes iron is basically not any harder to lift up than one cooled by liquid nitrogen. And I highly suggest you actually read WP:UNDUE because it does not say what you think it says. Your source is a small minority viewpoint.--Jasper Deng (talk) 20:14, 3 March 2017 (UTC)

I'm closing this discussion. If you want to talk more on this please take to Wikipedia:Reference desk/Science. Thanks. Polyamorph (talk) 20:39, 3 March 2017 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Universe design[edit]

Mass and energy works alternative due to force. Force generate by formula 0=-0/-0 to create universe. 0= null sign. Its also involved in dark matter. Its nothing but some parts of null inside in something. Prashant Nanda 14:52, 14 March 2017 (UTC) — Preceding unsigned comment added by Twisindia (talkcontribs)

'Equivalence' is wrong term.......[edit]

e = mc^2 does not assert equivalence. How could mass and energy possibly be the same thing when there's blatantly a c-squared involved. They are interchangeable not equivalent. CMJAWHM3 (talk) 14:18, 18 April 2017 (UTC)CMJAWHM

But the literature seems to have no problem with the term equivalence:
Google Scholar Books
"Mass-energy equivalence" 2,760 7,210
- DVdm (talk) 16:11, 18 April 2017 (UTC)