# If Mass Isn't Energy, Then What Is It? **URL:** \ **Category:** Factual Questions\ **Created:** [March 18, 2011, 10:16pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201 "2011-03-18T22:16:31Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 20, 2011, 3:49am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/41 "2011-03-20T03:49:36Z")
> [@Earthworm\_Jim](#): > > Is there a cite available for the “local mass defect” concept? So far nearly all hits on this phrase I’ve found via google are on message boards by a poster named Ring. Do you mean “local mass deficit” perhaps? Thank you! Both appear to be ok, but mass deficit, as you say, would probably be better. > **[Binding energy](https://en.wikipedia.org/wiki/Binding_energy#Mass_defect)** > > In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly used in condensed matter physics, atomic physics, and chemistry, whereas in nuclear physics the term separation energy is used. A bound system is typically at a lower energy level than its unbound constituents. According to relativity theory, a ΔE decrease > [@](#): > > **Mass defect** > > In simple words, the definition of mass defect can be stated as follows: > > Definition: The difference between the unbound system calculated mass and experimentally measured mass of nucleus is called mass defect. It is denoted by Δm. It can be calculated as follows… > > In nuclear reactions, the energy that must be radiated or otherwise removed as binding energy may be in the form of electromagnetic waves, such as gamma radiation, or as heat. Again, however, no **mass deficit** can in theory appear until this radiation has been emitted and is no longer part of the system. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 20, 2011, 3:19pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/42 "2011-03-20T15:19:42Z")
This thread is turning into a creepy clone of all the other threads on physics. It grows more technical all the time. Where is the non-technical, popular science explanation I asked for? Can nobody supply one? ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [March 20, 2011, 4:00pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/43 "2011-03-20T16:00:33Z")
> [@Exapno\_Mapcase](#): > > Where is the non-technical, popular science explanation I asked for? Can nobody supply one? Did you see my posts? I admit I didn’t spoon feed my answer, but I gave it. > [@](#): > > I have no problem with the phrase “mass is converted into energy” > [@](#): > > So too with “mass”. An excited carbon-12 nucleus has some amount of mass. If it de-excites by emitting a gamma ray, that gamma ray will do its gamma thing, and we’ll still be looking at a nucleus that is the bound state of six protons and six neutrons, just like before. It was a carbon nucleus before and it is still a carbon nucleus – a well-defined object under scrutiny. But, it’s mass is now different. It has lost mass. The nucleus has lost mass. Where did it go? Into energy. And the most direct answer: > [@](#): > > The thread started with the OP wondering why… > \*…the popular notion of mass being convertible to energy, as in a fusion reaction, is…incorrect. > \* > I am arguing that it is not incorrect. I am arguing that these are valid uses of the words involved, and issues arise only when one tries to prescribe particular definitions to these multi-use words. You are looking for a good pop-sci way of describing the ideas related to mass-energy interconversion, and I’m saying you already have it. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 20, 2011, 5:21pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/44 "2011-03-20T17:21:09Z")
> [@Pasta](#): > > You are looking for a good pop-sci way of describing the ideas related to mass-energy interconversion, and I’m saying you already have it. OK. But when Ring starts hitting me I’ll come crying to you. 🙂 ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [March 20, 2011, 5:54pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/45 "2011-03-20T17:54:39Z")
Now that the mass=energy question seems to be answered, the next question is: is time money? ---
**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\ **Post date:** [March 20, 2011, 6:04pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/46 "2011-03-20T18:04:24Z")
Maybe it’s answered to _your_ satisfaction, but I’m still trying to chew my way through the Wiki page on momentum four-vector. ☹ ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [March 20, 2011, 7:22pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/47 "2011-03-20T19:22:29Z")
> [@Washoe](#): > > Maybe it’s answered to _your_ satisfaction, but I’m still trying to chew my way through the Wiki page on momentum four-vector. ☹ Well, I wouldn’t want to give you the impression that I just became a world class physicist by reading 45 posts on the internet, but I think I’m close 🙂 (I wasn’t even going to read about momentum four-vectors because I got the impression from **Pasta’s** posts that it’s a complex situation and trying to just use 2 words to cover everything that is going on is part of the problem, but now that you mention it I’m going to burst my simplistic bubble of surface understanding and read that wiki page) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 20, 2011, 7:25pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/48 "2011-03-20T19:25:41Z")
I just can’t stand it. Pasta wrote: > [@](#): > > But, it’s mass is now different. It has lost mass. The nucleus has lost mass. Where did it go? Into energy. That does not in any way differ from what I have been saying. It’s just that I’m saying that the loss of mass isn’t the _source_ of the emitted energy it’s the _result_ of the loss of this energy. And I can’t believe you would disagree with this statement. You keep saying that the disagreement is a semantic issue, and that the definition you are using is a different one; well, what is that definition? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 20, 2011, 7:39pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/49 "2011-03-20T19:39:07Z")
> [@Washoe](#): > > Maybe it’s answered to _your_ satisfaction, but I’m still trying to chew my way through the Wiki page on momentum four-vector. ☹ Try this instead. I think you’ll find it a lot easier to understand. > **[Four-vectors in Relativity](http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/vec4.html)** ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [March 20, 2011, 8:03pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/50 "2011-03-20T20:03:42Z")
I would be better equipped to convey to you what I’m trying to say if I knew your answer to my question in Post 38. Could you take a look there? [QUOTE=Ring] It’s just that I’m saying that the loss of mass isn’t the _source_ of the emitted energy it’s the _result_ of the loss of this energy. [/QUOTE] In this particular context, I’m saying that it is fine when conveying the idea to a layperson (or not!) to say that the lost mass is the source of the energy. So, yeah, I guess I’m disagreeing with the statement you made. But not because I disagree on the physics. Just on the six-of-one-half-dozen-of-the-other choice of words. Regarding a particular definition for mass in this case, I am talking about the scalar sum of the masses of the experimentally persistent constituent particles (which may be composite). But, other contexts might be using other definitions. My point is simply that one can’t force “mass” to mean only “invariant mass of the whole system” and nothing else ever. Consider this passage: “It rained last Christmas eve, so mass was smaller than usual. The pastor’s sermon had a lot of energy, though.” This context is far removed from physics, but it is the _ad absurdum_ extension of the claim that mass and energy can only have one definition each. I claim that even within physics, there are multiple contexts and that the language has overloaded several words to mean multiple things. This extends (especially [but not solely] in lay descriptions) to saying that mass is “converted” to energy, or vice versa. > [@](#): > > You can know the name of a bird in all the languages of the world, but when you’re finished, you’ll know absolutely nothing whatever about the bird… So let’s look at the bird and see what it’s doing – that’s what counts.- _Richard Feynman_ I’m saying that we shouldn’t worry so much about what we call the bird. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 20, 2011, 9:30pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/51 "2011-03-20T21:30:29Z")
I agree. From now on we’ll just answer any physics question with; “it’s magic. BTW here’s what [Mati Meron,](http://cars9.uchicago.edu/chemmat/pages/news20101110undergrad.html) who is a senior scientist at the university of Chicago has to say about it. (This is from sci.physics) > [@](#): > > Just that there is energy associated with mass, that’s all. But > calling mass “a form of energy” is apt to be confusing, I would > refrain from it. > “There is no such thing as “conversion”. I probably wrote it a dozen > times already, over the last few years, but I’ll write it again: The > mass change is the _ **consequence** _ of energy release, not the cause.” > As you wrote above, mass is just the magnitude of the energy-momentum > vector. Moreover, both the space and time components of his vector > are conserved, so the magnitude is conserved as well. So, no, you > can’t convert mass to energy. But, you can convert energy from one > form the another and some forms are more useful than other. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [March 20, 2011, 10:06pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/52 "2011-03-20T22:06:15Z")
You again skipped over my question from Post 38. This Meron fellow aside, I can pull up plenty of counter examples. A quick Google search yields these examples right away… [http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/eneein.html](http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/eneein.html) [http://library.thinkquest.org/17940/texts/radioactivity/radioactivity.html](http://library.thinkquest.org/17940/texts/radioactivity/radioactivity.html) [http://www.lbl.gov/abc/Basic.html](http://www.lbl.gov/abc/Basic.html) [http://imagine.gsfc.nasa.gov/docs/ask\_astro/answers/970912b.html](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970912b.html) [http://www.newton.dep.anl.gov/askasci/phy05/phy05158.htm](http://www.newton.dep.anl.gov/askasci/phy05/phy05158.htm) [http://www.osti.gov/accomplishments/nuggets/einstein/speedoflight.html](http://www.osti.gov/accomplishments/nuggets/einstein/speedoflight.html) > [@Ring](#): > > From now on we’ll just answer any physics question with; “it’s magic. ? ---
**Author:** ![ChrisBooth12](https://avatars.discourse-cdn.com/v4/letter/c/e95f7d/32.png) [@ChrisBooth12](https://boards.straightdope.com/u/ChrisBooth12)\ **Post date:** [March 20, 2011, 10:16pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/53 "2011-03-20T22:16:20Z")
Matter is Energy isnt it? Isn’t that why light acts as both a partical and wave? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 20, 2011, 11:12pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/54 "2011-03-20T23:12:42Z")
> [@Pasta](#): > > You again skipped over my question from Post 38. > > This Meron fellow aside, I can pull up plenty of counter examples. A quick Google search yields these examples right away… > ? You didn’t really need to post those links as I have plenty of books that say the same thing, and I think we both know why they say what they say. On the other hand, I also think we both know, when you get down to what’s really going on, they are wrong. I used Meron because he posted it on the old sci.physics and none of the other physicists called him on it. And believe me they would have if he’d been wrong. I said “Magic” since it’s not necessary, in physics, for words to have a real meaning. BTW, I’m done. It appears I’m not going to convince you, and you’re not going to convince me. ---
**Author:** ![glowacks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/glowacks/32/5548_2.png) [@glowacks](https://boards.straightdope.com/u/glowacks)\ **Post date:** [March 20, 2011, 11:32pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/55 "2011-03-20T23:32:48Z")
This difficulty is because physicists have created a new definition of mass that includes what they want to think of as mass in order for physics to work right. With this definition, mass cannot be created or destroyed, but saying that is like the statement of my favorite theorem of differential geometry: “An oval has at least four vertices.” Anyone with typical knowledge of what an oval is and what a vertex is will tell you that said theorem is obviously false: an oval has no vertices. But I didn’t exactly tell you what “oval” and “vertex” mean in this context.\* Thus, the argument is semantics. Lay people think of mass as a number that tells you how much matter is there, but that’s oversimplifying it. Mass is what interacts via the fundamental force of gravity, and can take different forms. I can’t vouch for the accuracy of Wikipedia, but it basically agrees with what has been said, and states it in a fairly clear manner: > [@](#): > > Mass (so long as the type and definition of mass is agreed upon) is a conserved quantity over time. … However, relativity adds the fact that all types of energy have an associated mass, and this mass is added to systems when energy is added, and the associated mass is subtracted from systems when the energy leaves. > [@](#): > > Thus, all matter has the property of mass, but not all mass is associated with identifiable matter. That last part struck me as quite a surprise when I heard about it in the thread that started this whole discussion many months ago or whenever, and is the essence of what people are missing. Energy can/does have mass associated with it according to Einstein’s famous equation. Sometimes it will be easy to measure on a scale, other times not so much. When it changes from the former to the latter, you tend to say that mass was converted into energy. \*An oval is a basically any “smooth” simple closed curved and a vertex is point where that the curvature is at a local minimum or maximum. When stated that way the theorem can be seen as making sense at least, even if proving it is a bit tricky. ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [March 21, 2011, 2:42am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/56 "2011-03-21T02:42:34Z")
> [@glowacks](#): > > When it changes from the former to the latter, you tend to say that mass was converted into energy. So then wouldn’t it be accurate to say that _matter_ was converted to energy instead? Or is matter not a well-defined term in Physics? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [March 21, 2011, 2:04pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/57 "2011-03-21T14:04:31Z")
It seems to me to me mostly semantics. In special relatvity, mass is _usually_ representative of the rest energy of a system. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [March 23, 2011, 10:28pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/58 "2011-03-23T22:28:29Z")
> [@Ring](#): > > It appears I’m not going to convince you, and you’re not going to convince me. I thought this was true, but then in [this other thread](http://boards.straightdope.com/sdmb/showthread.php?t=601648), you indicated that you take John Wheeler’s terminology in _Spacetime Physics_ to be acceptable. So, while I’m no fan of appeals to authority, I thought I’d take another stab in this thread by using the same source. The _exact same book_ that you cite in the other thread provides numerous examples of “mass is created”, “mass is destroyed”, “mass is converted to energy” – exactly the usage you oppose as invalid here. Section headings… [INDENT]p. 233: “Photon used to create mass” p. 234: “Material particle used to create mass” p. 237: “Converting mass to usable energy: fission, fusion, annihilation” Passages: “For a final perspective on the evanescence of mass and the preservation of momenergy, turn from processes where mass is created to three processes where mass is destroyed: fission, fusion, and annihilation.” Problems: Sample problem 8-5, (a) – How much mass is converted to energy every second in Sun to supply the luminous energy that falls on Earth? (b) What _total_ mass is converted to energy every second in Sun to supply luminous energy? [/INDENT] If your own cite (used in defense of terminology in the other thread) doesn’t convince you in _this_ thread that mass, energy, and related verbs (_e.g._, “converted”) don’t have multiple useful and valid definitions, then I do agree that I cannot convince you. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 24, 2011, 12:10am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/59 "2011-03-24T00:10:00Z")
Hey, Ring convinced me… not to change. 🙂 ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 24, 2011, 12:49am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/60 "2011-03-24T00:49:33Z")
> [@Pasta](#): > > I thought this was true, but then in [URL=“[How were the concepts of Energy, Momentum and Mass developed? - Factual Questions - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=601648)”]… Curse you for making me look those up. If you would have actually read the articles you would have easily seen that he is talking about local effects, which is in effect, changing the definition of the system in the middle of the stream. I suggest you read, chapter 8 “Use and Abuse of the Concept of Mass” > [@](#): > > Mass of an isolated system, being the magnitude of its momenergy 4-vector [God I hate that made up word] can never change (as long as the system remains isolated.) My other post you referred to: > [@](#): > > John Archibald Wheeler on page 251 of Spacetime Physics says: > Quote: > In order to make this point clear, should we call invariant mass its “rest mass”? > > That is what we called it in the first addition of book. But a thoughtful student pointed out that the phrase rest “mass” is also subject to misunderstanding: What happens to the “rest mass” of a particle when the particle moves? In reality mass is mass is mass > It would seem that a reasonable person, of some intelligence, would see that one of the biggest names in relativity, John Wheeler, is clearly saying that there is no difference between rest mass and invariant mass. > > Mass is mass is mass, and it is defined as the magnitude of energy momentum four-vector. And therefore mass cannot be converted to energy, and is absolutely conserved. [Previous page](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201.md?page=2) [Next page](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201.md?page=4)