# 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:** 14\ **Page:** 4
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [March 24, 2011, 1:07am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/61 "2011-03-24T01:07:42Z")
> [@Ring](#): > > 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” I did actually read all of those sections. And if you read carefully, you’ll notice he uses “mass”, etc., in these other ways in this very section (“Use and Abuse of the Concept of Mass”), even while writing about clarifying these exact issues. (You might not have noticed this because (if I may WAG) you would naturally read right through these usages, as they are the cleanest, clearest way of presenting the ideas.) Yes, he is talking about local effects when he uses “mass”, etc., in these ways, but that’s exactly my point – there are times, contexts, constructions, …, when it is helpful to do this. Even Wheeler does this, and I do not fault him for it anymore than I would fault **Exapno Mapcase**. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [March 24, 2011, 1:11am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/62 "2011-03-24T01:11:46Z")
> [@Ring](#): > > momenergy… [God I hate that made up word] Hear, hear. “Momenergy” is just plain silly. But, it does go to show that language and terminology is malleable, so long as context is clear. Wheeler (and Taylor) got their required pedagogical mileage out of it. ---
**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, 1:29am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/63 "2011-03-24T01:29:58Z")
> [@Pasta](#): > > Hear, hear. “Momenergy” is just plain silly. But, it does go to show that language and terminology is malleable, so long as context is clear. Wheeler (and Taylor) got their required pedagogical mileage out of it. Finally, I can end this damn thread. I forget the page number but somewhere he says: > [@](#): > > Pasta is wrong and Ring is right ---
**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, 1:57am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/64 "2011-03-24T01:57:07Z")
> [@Ring](#): > > Finally, I can end this damn thread. I forget the page number but somewhere he says: > > > [@](#): > > > > Pasta is wrong and Ring is right Yeah, but you have to flip an infinite number of pages to get to it. ---
**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, 3:00am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/65 "2011-03-24T03:00:13Z")
> [@Exapno\_Mapcase](#): > > Yeah, but you have to flip an infinite number of pages to get to it. Fine with me. Anything to end this God forsaken thread. BTW if you decide to go with Pasta’s, “anything can mean anything philosophy,”🙂 at least put a modifier in front of the word, i.e _local_ mass defect or deficit. ---
**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, 3:05am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/66 "2011-03-24T03:05:32Z")
Shit that should have been e.g. not i.e. Normally it probably wouldn’t matter, but with exapno, Mr. Language himself, I’ve got to watch it. ---
**Author:** ![RadicalPi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/radicalpi/32/9806_2.png) [@RadicalPi](https://boards.straightdope.com/u/RadicalPi)\ **Post date:** [March 24, 2011, 4:25am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/67 "2011-03-24T04:25:19Z")
> [@Ring](#): > > **Pasta** you know more physics than I ever will, but surely you’re not saying that a system of two photons, that have a center of momentum frame, has no mass? Are you trying to say that two photons, each of which weighs absolutely nothing, weigh more than nothing when they are in some sort of system? If so, what sort of system? And if the mass isn’t in the photons, where the hell is it? ---
**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 24, 2011, 11:13am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/68 "2011-03-24T11:13:09Z")
> [@RadicalPi](#): > > Are you trying to say that two photons, each of which weighs absolutely nothing, weigh more than nothing when they are in some sort of system? If so, what sort of system? And if the mass isn’t in the photons, where the hell is it? When their velocities are not parallel there will always be a frame which minimizes the total energy of the system this will be the rest frame of the system and this energy is the rest energy of the system. ---
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [March 24, 2011, 12:34pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/69 "2011-03-24T12:34:54Z")
> [@Exapno\_Mapcase](#): > > OK. But when Ring starts hitting me I’ll come crying to you. 🙂 As someone lurking in the thread up to this point, I just want to say how fascinating this “god-forsaken” thread has been in showing how the role of communication and intent play in conveying meaning. Certain disciplines require fixed, tight definitions - per **Ring**. Communication - especially to non-expert audiences - allows for broader, looser definitions - as long as the intended meaning appears to be exchanged. That is was \*\*Exapno \*\*was looking for and \*\*Pasta \*\*provided (thanks **Pasta**!). It is when folks try to cross streams - e.g., trying to communicate to non-experts while requiring expert-strict definitions - that things start to go wonky… Mass. mass masses massively - or something like that…😉 ---
**Author:** ![MuleSkinner](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@MuleSkinner](https://boards.straightdope.com/u/MuleSkinner)\ **Post date:** [March 24, 2011, 11:19pm UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/70 "2011-03-24T23:19:03Z")
> [@The\_Tao\_s\_Revenge](#): > > I think you mean e = mc² where c is the speed of light. If you assume the speed of light to be 1, which I believe is an important idea in physics maybe, then you get: > > E = M1² > which simplifies to > > E = M1 because 1² is 1 > > which further simplifies to > E = M because 1 times anything is that anything. Well, one thing I learned on this board is that the equals sign is never used loosely in mathematics. So, isn’t it correct to say that according to Einstein, the OP is right? Mass is energy? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 25, 2011, 1:42am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/71 "2011-03-25T01:42:33Z")
> [@MuleSkinner](#): > > Well, one thing I learned on this board is that the equals sign is never used loosely in mathematics. So, isn’t it correct to say that according to Einstein, the OP is right? Mass is energy? There are two types of equations: equalities and identities. Equalities means something equals something, and identities mean something is something. E[sup]2[/sup] = m[sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup] is an equality. ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [March 25, 2011, 2:28am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/72 "2011-03-25T02:28:21Z")
> [@MuleSkinner](#): > > Well, one thing I learned on this board is that the equals sign is never used loosely in mathematics. [QUOTE=Ring] There are two types of equations: equalities and identities. [/QUOTE] The symbol ≡ is sometimes used to express identity, but it’s not exactly handy on our keyboards. ---
**Author:** ![MuleSkinner](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@MuleSkinner](https://boards.straightdope.com/u/MuleSkinner)\ **Post date:** [March 25, 2011, 3:04am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/73 "2011-03-25T03:04:38Z")
> [@Ring](#): > > There are two types of equations: equalities and identities. Equalities means something equals something, and identities mean something is something. > > E[sup]2[/sup] = m[sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup] is an equality. In that case, isn’t it correct to say that according to Einstein, the OP is wrong? 😃 ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [March 25, 2011, 3:54am UTC](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201/74 "2011-03-25T03:54:30Z")
> [@Ring](#): > > There are two types of equations: equalities and identities. Equalities means something equals something, and identities mean something is something. > > E[sup]2[/sup] = m[sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup] is an equality. In that equation, what does p mean, and for E and M, please fill in the blank “the energy of \_\_\_\_\_\_” and “the mass of \_\_\_\_\_\_\_” [Previous page](https://boards.straightdope.com/t/if-mass-isnt-energy-then-what-is-it/575201.md?page=3)