# How exactly does energy "become" mass?

by Quantum_Grid
Tags: energy, mass
 P: 57 In Einstein's equation E=mc2 or more sepcifically the "inverse", m=E/c2, how exactly does mass "become" energy? I [vaguely] understand how energy may be released from mass, but how would mass come about from just energy? Maybe this question is unanswerable, or maybe I'm not asking it correctly. If its unanswerable in a few paragraphs, any suggestion on books that may be relevant to my question would help too.
 P: 15,325 When two particles fuse, such as in the heart of stars, their combined mass is greater than their separate mass. More later.
 P: 57 Thanks. My question was in relation to the theory on the origin of mass, or more specifically F. Wilzcek's description of the compromise between gluon field energy and and quark localization energy; and their resulting residual energy. How does this energy translate into mass?
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P: 4,739

## How exactly does energy "become" mass?

Mass is one form of energy, there is no such thing as "pure" energy.

 P: 57 Yeah, sorry, I was hoping to get a more specific reply than I had over there. I guess I'll just try the library. Thanks for your help, though! --edit: I'll search around the forums here for one too...
 Sci Advisor HW Helper P: 4,739 so what is wrong with the answers given by us? (same guys in these forums, so no need for double posting)
P: 57
 Quote by malawi_glenn so what is wrong with the answers given by us? (same guys in these forums, so no need for double posting)
Your right, won't happen again, sir! I didn't see your post describing mass as a form of energy, that does help clarify it a little.

I am starting physics as a major this semester in school, so I guess it will all come to me in time!

P: 224
 Quote by Quantum_Grid In Einstein's equation E=mc2 or more sepcifically the "inverse", m=E/c2, how exactly does mass "become" energy? I [vaguely] understand how energy may be released from mass, but how would mass come about from just energy?
Perhaps you should try to look into the mass velocity relation. If a mass has higher velocity then the mass equivalent of the extra energy is added to the mass.
P: 3,912
 Quote by malawi_glenn Mass is one form of energy, there is no such thing as "pure" energy. you have already asked this question in quantum forum.
Given that relativistic mass is not recognised in modern text books and only the invariant rest mass of a particle is considered as mass, is a photon not "pure" energy?
P: 114
 When two particles fuse, such as in the heart of stars, their combined mass is greater than their separate mass. More later.
You mean their separate masses are greater than their combined mass. The difference in mass is released as energy.
P: 15,325
 Quote by Denton You mean their separate masses are greater than their combined mass. The difference in mass is released as energy.
Not always, no. Depends on what the reaction is. Certainly, we prefer fusing hydrogen into helium because that releases energy, which is useful to us.

But pick the particles right and it works the other way. Fuse barium and krypton to get uranium, you will actually gain mass.

Stolen from wiki for brevity:
"The fusion of two nuclei with lower mass than iron generally releases energy while the fusion of nuclei heavier than iron absorbs energy"
P: 4,513
 Quote by kev Given that relativistic mass is not recognised in modern text books and only the invariant rest mass of a particle is considered as mass, is a photon not "pure" energy?
The OP question is about the stuff that depresses a scale; about that stuff that resists accelleration.
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 Quote by kev is a photon not "pure" energy?
why should it be? What is the definition of "pure" energy? I am a master student, but still never heard of it.
P: 15,325
 Quote by malawi_glenn why should it be? What is the definition of "pure" energy? I am a master student, but still never heard of it.
Aren't we describing electromagnetic radiation here?
 Sci Advisor PF Gold P: 1,767 Mass is bound energy they are (in GR and SR) exactly the same thing with different units. If for example you could measure accurately enough, the energy stored in a compressed or stretched spring would give it slightly more weight and inertial mass than that of a relaxed spring. But also as has already been mentioned... The net difference in energies between the components of an atom (separate electrons, protons and neutrons) and their bound atomic form all add up if you 1.) equate mass and energy with unit conversion: $E=mc^2$ and if you 2.) take into account the binding energies. Note that you must account for those binding energies = masses as they are what gives say a block of carbon its weight and inertia as compared to that of the constituent protons neutrons and electrons.
 P: 5,634 Nobody knows "exactly"...heck nobody knows "exactly" what mass or energy is..where it comes from,etc. But we can describe a lot of their characteristics. A completely different way of looking at an answer to your question: strings! In string theory mass is one type of energy vibration, energy is simply another form of vibrating energy. And different partciles (masses) have different vibrational modes... Regarding classical mass and energy conversion: we are really poor at converting either to the other...fission/fusion for example only releases a tiny portion of the available energy... is it maybe 1% or so mass that is converted in bombs?? Likely less in power reactors... "We know a lot, we understand little."
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 Quote by DaveC426913 Aren't we describing electromagnetic radiation here?
nope the general question was "how exactly does mass "become" energy?"

One guy claimed that photons, electromagnetic waves, are "pure" energy. But where is such definition stated and motivated?

To me, photons are one form of energy, mass is one form of energy. Energy can not be created or destroyed, only converted into different forms.

Look at the electromagnetic field, and the energy equations from SR:

$$E = \hbar \omega$$ photons

$$E = c^2 m_0$$ rest-energy for massive particles

Now how does one see that photons are "pure" energy? The field is described by an angular frequency (omega). Working in units where c = 1, mass has same units as energy. Working in units where hbar = 1, omega has same units as energy.

Why do we have to work in SI units? It is just that we are used to it and take it for granted.
P: 57
Thanks a lot for all your responses guys, this is fascinating stuff! I can't wait to start studying this in college this year.

 Quote by Naty1 Nobody knows "exactly"...heck nobody knows "exactly" what mass or energy is..where it comes from,etc. But we can describe a lot of their characteristics. A completely different way of looking at an answer to your question: strings! In string theory mass is one type of energy vibration, energy is simply another form of vibrating energy. And different partciles (masses) have different vibrational modes... Regarding classical mass and energy conversion: we are really poor at converting either to the other...fission/fusion for example only releases a tiny portion of the available energy... is it maybe 1% or so mass that is converted in bombs?? Likely less in power reactors... "We know a lot, we understand little."
Interesting, I think "we don't know", inspires me to seek out the real answers more than anything else. I'm a little wary of string theory, but I won't get off topic with that.

Thanks again for all the interesting replys!

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