# E=mc2 ?? **URL:** \ **Category:** Factual Questions\ **Created:** [January 4, 2003, 10:35pm UTC](https://boards.straightdope.com/t/e-mc2/146671 "2003-01-04T22:35:04Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![sum](https://avatars.discourse-cdn.com/v4/letter/s/6de8d8/32.png) [@sum](https://boards.straightdope.com/u/sum)\ **Post date:** [January 4, 2003, 10:35pm UTC](https://boards.straightdope.com/t/e-mc2/146671/1 "2003-01-04T22:35:04Z")
Energy can not be created nor destroyed…Einstein said that energy can become matter and matter can become energy…but the sum total of energy + matter is always equal. Right? Could some one explain what else this equation means but like in simple terms? ---
**Author:** ![Mastema](https://avatars.discourse-cdn.com/v4/letter/m/94ad74/32.png) [@Mastema](https://boards.straightdope.com/u/Mastema)\ **Post date:** [January 4, 2003, 10:46pm UTC](https://boards.straightdope.com/t/e-mc2/146671/2 "2003-01-04T22:46:04Z")
It means that a small amount of matter can be converted into a large amount of energy (Nuclear fission/fusion). Or that a large amount of energy can be used to create a small amount of matter. See this [Cite](http://leiwen.tripod.com/create_matter.htm) ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [January 4, 2003, 10:46pm UTC](https://boards.straightdope.com/t/e-mc2/146671/3 "2003-01-04T22:46:56Z")
What unit is “c” in? Miles? ---
**Author:** ![Manduck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manduck/32/256_2.png) [@Manduck](https://boards.straightdope.com/u/Manduck)\ **Post date:** [January 4, 2003, 10:51pm UTC](https://boards.straightdope.com/t/e-mc2/146671/4 "2003-01-04T22:51:00Z")
> [@](#): > > \*Originally posted by Speaker for the Dead \* > \*\*What unit is “c” in? Miles? \*\* Doesn’t matter (no pun intended:cool: ). If c is in miles per hour and m is in kilograms, then E will be in kilogram-miles per hour. The units used on one side of the equation will determine the units to use on the other side. ---
**Author:** ![Manduck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manduck/32/256_2.png) [@Manduck](https://boards.straightdope.com/u/Manduck)\ **Post date:** [January 4, 2003, 10:51pm UTC](https://boards.straightdope.com/t/e-mc2/146671/5 "2003-01-04T22:51:37Z")
> [@](#): > > \*Originally posted by Speaker for the Dead \* > \*\*What unit is “c” in? Miles? \*\* Doesn’t matter (no pun intended:cool: ). If c is in miles per hour and m is in kilograms, then E will be in kilogram-miles squared per hour squared. The units used on one side of the equation will determine the units to use on the other side. ---
**Author:** ![Manduck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manduck/32/256_2.png) [@Manduck](https://boards.straightdope.com/u/Manduck)\ **Post date:** [January 4, 2003, 10:54pm UTC](https://boards.straightdope.com/t/e-mc2/146671/6 "2003-01-04T22:54:04Z")
Darn, wasn’t quick enuff to stop my first, stupider, post ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [January 4, 2003, 10:57pm UTC](https://boards.straightdope.com/t/e-mc2/146671/7 "2003-01-04T22:57:47Z")
Er, I’ve had this explained to me before, but what is an “hour squared”? Also… what is a kilogram-mile? ---
**Author:** ![sum](https://avatars.discourse-cdn.com/v4/letter/s/6de8d8/32.png) [@sum](https://boards.straightdope.com/u/sum)\ **Post date:** [January 4, 2003, 11:03pm UTC](https://boards.straightdope.com/t/e-mc2/146671/8 "2003-01-04T23:03:53Z")
> [@](#): > > \*Originally posted by Mastema \* > \*\*It means that a small amount of matter can be converted into a large amount of energy (Nuclear fission/fusion). Or that a large amount of energy can be used to create a small amount of matter. > See this [Cite](http://leiwen.tripod.com/create_matter.htm) \*\* Thats what I dont understand… If it takes a small amount of matter to make alot of energy then how can they always be equal in the universe? Or did I get that part wrong? Im so totally confused… I’ll check that site out…thax. ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [January 4, 2003, 11:04pm UTC](https://boards.straightdope.com/t/e-mc2/146671/9 "2003-01-04T23:04:57Z")
Umm… well, an hour sqaured is an hour times an hour. And a kilogram mile is a kilogram times a mile. _shrugs_ Nothing too it, really. But also not very intuitively useful units. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 4, 2003, 11:19pm UTC](https://boards.straightdope.com/t/e-mc2/146671/10 "2003-01-04T23:19:04Z")
> [@](#): > > \*Originally posted by sum \* > \*\*Thats what I dont understand… If it takes a small amount of matter to make alot of energy then how can they always be equal in the universe? Or did I get that part wrong? > > Im so totally confused… > \*\* I think you’re confused. E=MC[sup]2[/sup] just means matter and energy can be converted back and forth into each other. It seems like you are assuming all the matter in the Universe is equal to all the energy in the Universe. That doesn’t make sense because matter and energy are equivalent…matter=energy=matter=energy…just different forms of the same thing (like liquid water versus ice water…both still water). ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [January 4, 2003, 11:26pm UTC](https://boards.straightdope.com/t/e-mc2/146671/11 "2003-01-04T23:26:41Z")
Staff Report [In E=mc[sup]2[/sup], what units of measurement was Einstein using?](http://www.straightdope.com/mailbag/memc2.html) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 4, 2003, 11:26pm UTC](https://boards.straightdope.com/t/e-mc2/146671/12 "2003-01-04T23:26:55Z")
A sample of many. > [@](#): > > \*Originally posted by Ring \* > \*\*Fusion/fision isn’t about converting mass to energy its about converting potential energy to kinetic and electromagnetic energy. In other words it’s about changing one type of energy into another. The remnant of the nucleus has less internal energy and as a _result_ of this decrease in internal energy it has less mass, not vice versa. > > And, as usual, I must add my normal caveat – the mass of the entire system does not change \*\* > [@](#): > > \*Originally posted by Ring \* > \*\*This has been discussed in great detail many times on this board but here are the basics. The relativistic equation that relates mass and energy is: > > m[sup]2[/sup] = E[sup]2[/sup] - p[sup]2[/sup] > > m = mass > E = energy > p = momentum > (c = 1) > > Since the system had zero momentum before the explosion it must have zero momentum after the explosion and therefore: > > m[sup]2[/sup] = E[sup]2[/sup] - 0[sup]2[/sup] or m = E or if you want to include c as a conversion constant then m = E/c[sup]2[/sup] and the system mass does not change. Mass cannot be converted to energy. > > A vault that contains nuclear weapon will weigh the same both before and after the explosion. \*\* ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [January 4, 2003, 11:32pm UTC](https://boards.straightdope.com/t/e-mc2/146671/13 "2003-01-04T23:32:57Z")
**Whack** is correct. When you are studying Einstein’s equations, it’s helpful to think of matter and energy as two aspects of the same thing, commonly called mass-energy. The amount of mass-energy in the Universe is a constant, no matter what percentage of it exhibits which set of properties at the moment. E=mc[sup]2[/sup] is like all physical equations: dimensionless. The dimensions used on one side determine the dimensions derived for the other side. Distance per time squared means distance per time _per time_. It’s a rate of acceleration. For example, g is 9.8 meters per second squared. That means every second, something falling on Earth (and exhibiting the acceleration defined by g) gets 9.8 meters per second _faster_. 0 s == 0 m/s 1 s == 9.8 m/s 2 s == 19.6 m/s 3 s == 29.4 m/s etc. For more about this stuff, look for the section on derivatives in an introductory calculus book. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 4, 2003, 11:45pm UTC](https://boards.straightdope.com/t/e-mc2/146671/14 "2003-01-04T23:45:34Z")
One more. > [@](#): > > \*Originally posted by Ring \* > \*\*Well, actually as you say mass and energy are properties of a system, and not things in their own right, but the mass and energy of _any_ closed system cannot change. The reason they can’t change is because momentum must be conserved. > > m[sup]2[/sup] = E[sup]2[/sup] - p[sup]2[/sup] > > So if the momentum can’t change then neither can the mass. > > > [@](#): > > > > _posted by someone else_ – If you take the system of a electron/positron pair, if those 2 particles collide and annihilate, the mass of the system drops to zero, as the energy in that system is no longer contained". > > No, the mass for the _system_ stays the same for the same reason as above. It’s true that the particles annihilate, but the property of the _system_ that we call mass stays the same. \*\* Oh, and one more thing. Mass and energy are not the same thing. In special relativity there’s a thing called the energy momentum four vector which in some respects is as basic as you can get. In any case, energy is the time component of the four vector, momentum is the space component and mass is the magnitude of the four vector. This is all summed up in the equation: E[sup]2[/sup] = m[sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup] E = energy m = mass p = momentum c does not equal 1 ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 5, 2003, 12:12am UTC](https://boards.straightdope.com/t/e-mc2/146671/15 "2003-01-05T00:12:08Z")
> [@](#): > > \*Originally posted by Ring \* > \*\*Oh, and one more thing. Mass and energy are not the same thing. \*\* Despite your further explanation you lost me. Just when you think you get a handle on something leave it to a physicist to screw it up for you 😉 . I’m really hoping we’re getting caught on semantics here. Stop me if this analogy is inappropriate: Liquid water and Frozen water are not the same thing but they are different forms of the same thing…water. VERY loosely speaking I thought energy and matter worked the same way. From your own quote of yourself if you explode a nuclear device in a vault and convert mass to energy (or vice versa) from outside the vault all measurements will be the same. Less matter, more energy…doesn’t matter. Just as if I melted my ice to a liquid then turned the liquid into a gas…I still have the same amount of water and have merely changed its aspect. What have I missed that doesn’t require 8 years post grad work to understand? ---
**Author:** ![sleeping](https://avatars.discourse-cdn.com/v4/letter/s/839c29/32.png) [@sleeping](https://boards.straightdope.com/u/sleeping)\ **Post date:** [January 5, 2003, 12:13am UTC](https://boards.straightdope.com/t/e-mc2/146671/16 "2003-01-05T00:13:56Z")
From **Ring** : > [@](#): > > m2 = E2 - p2 Whoa! How did you write the exponent? I was answering a math question and had to use carrots. ---
**Author:** ![sleeping](https://avatars.discourse-cdn.com/v4/letter/s/839c29/32.png) [@sleeping](https://boards.straightdope.com/u/sleeping)\ **Post date:** [January 5, 2003, 12:16am UTC](https://boards.straightdope.com/t/e-mc2/146671/17 "2003-01-05T00:16:08Z")
Now I see from my post that it doesn’t work for me even when I quote! ---
**Author:** ![Mastema](https://avatars.discourse-cdn.com/v4/letter/m/94ad74/32.png) [@Mastema](https://boards.straightdope.com/u/Mastema)\ **Post date:** [January 5, 2003, 1:09am UTC](https://boards.straightdope.com/t/e-mc2/146671/18 "2003-01-05T01:09:41Z")
To get really strange, the new quantum theories hypothesize that at the sub-atomic level energy and matter are made up of the same stuff. For instance, in string theory, one of the more popular quantum explanations, the very smallest unit of anything is the string (hence the name) which can be visualized as a rubber-band with a certain number of whole waves in it. The number of waves, or the frequency of the band determines whether you get photon, graviton, proton boson, muon etc . . . This is of course a **VERY** simplified explanation. For more in depth info in terms even I could understand I would recommend: [ul] [li]The Dancing Wu Li Masters[/li][li]In Search of Schroedingers Cat[/li][/ul] ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [January 5, 2003, 1:36am UTC](https://boards.straightdope.com/t/e-mc2/146671/19 "2003-01-05T01:36:07Z")
> [@](#): > > \*Originally posted by sleeping \* > \*\*From **Ring** : > > Whoa! How did you write the exponent? I was answering a math question and had to use carrots. \*\* They’re called carets. ^ is a caret. Note spelling. Try the [sup] tag. Type x[**sup]2[**/sup] to get the desired effect. The [sub] tag does the right thing, too. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 5, 2003, 2:01am UTC](https://boards.straightdope.com/t/e-mc2/146671/20 "2003-01-05T02:01:39Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > \*\*VERY loosely speaking I thought energy and matter worked the same way. From your own quote of yourself if you explode a nuclear device in a vault and convert mass to energy (or vice versa) from outside the vault all measurements will be the same. Less matter, more energy…doesn’t matter. Just as if I melted my ice to a liquid then turned the liquid into a gas…I still have the same amount of water and have merely changed its aspect. > > What have I missed that doesn’t require 8 years post grad work to understand? \*\* I think a lot of people get hung up on this because they think mass and energy are things. But they’re not things – they’re _properties_ of a _system_. This is not to say they’re not intimately related, they are. If you study the equation: m[sup]2[/sup] = E[sup]2[/sup] - p[sup]2[/sup] You can see that if the momentum is zero the mass of the system equals the energy of the system, or to put it another way – mass is equal to the energy of the system that cannot be transformed away. If you wanted to, you could abandon the concept of mass altogether and just talk about the magnitude of the energy momentum four-vector, but the concept of mass is so ingrained that I doubt this will ever happen. [Next page](https://boards.straightdope.com/t/e-mc2/146671.md?page=2)