3 ms·

By no means is this my forte, but would this violate the conservation of mass?

by nkozyra 12y ago

cboursnell 12y ago
Nope. As energy equals mass times the speed of light squared you're just converting energy into matter. It's just the opposite of what the atomic bomb does.
asolove 12y ago
Conservation of mass is not applicable in the conditions under discussion, see Wikipedia's fun list of exceptions to conservation of mass: http://en.wikipedia.org/wiki/Conservation_of_mass#Exceptions_or_caveats_to_mass.2Fmatter_conservation http://en.wikipedia.org/wiki/Conservation_of_mass#Exceptions...
leephillips 12y ago
Mass by itself is only conserved in nonrelativistic classical physics. When you are in a relativistic regime the conserved quantity is a combination of momentum and energy. (You've probably seen E = mc^2, which shows that mass and energy can be transformed into each other.)
ktran03 12y ago
Wiki: The law of conservation of mass, or principle of mass conservation, states that for any system closed to all transfers of matter and energy (both of which have mass), the mass of the system must remain constant over time, as system mass cannot change quantity if it is not added or removed.
- oddthink 12y ago
  Wait, what? Photons have no mass, but they have energy. Ergo, that "both of which have mass" is flat-out wrong. Methinks wikipedia has problems.
  - gress 12y ago
    Photons have zero rest mass.
    - oddthink 12y ago
      Rest mass is mass. Photons have zero mass. "Relativistic mass" is more confusing than helpful and is generally not used in modern GR. Cite: ask any actual physicist. I hate to appeal to authority, but this is the view I was taught in grad school (PhD, UCB, 2001), and I have never spoken to anyone in physics or astrophysics who thought "relativistic mass" was helpful.
      - Steuard 12y ago
        I'm entirely on your side on this (PhD, UChicago, 2006), but there are still a bunch of physics teachers out there (mostly not doing research in related fields, is my rough impression) who will vigorously defend teaching the "relativistic mass" concept. They raise some decent points, but in my opinion none of it comes close to outweighing the arguments in favor of treating mass as an invariant property.
      - codeflo 12y ago
        If this is a recent terminology change, then people who use the old terminology are not factually "wrong", they just use an outdated definition. I do have a question though, if you or anyone else wants to clear this up for me. Which kinds of energy are no longer considered part of a system's "mass"? I gather that kinetic energy is not. What about various forms of binding energy?
        millstone 12y ago
        Only kinetic energy does not contribute to mass. In fact, it's even narrower: only the kinetic energy of the center of mass does not contribute. For example, heat energy is really just kinetic energy of the constituent particles, and heat definitely contributes to mass. The essential idea is that mass is Lorentz invariant, which means it does not change under changes of coordinate systems. So mass must be independent of velocity and orientation, which are just artifacts of how you measure anyways.
timnic 12y ago
Mass is not a conserved quantity. Energy for example is a conserved quantity. So like in a nuclear fission reaction we loose mass but this lost mass is then converted into energy. I have not read the article in detail, but I suppose they want to turn part of the photons energy into masses for the electron and the positron. Mass and energy are linked via E = mc^2.
- millstone 12y ago
  In fact, both energy and mass are conserved. If you accept e=mc^2 as "true," it's easy to show. If energy is conserved and mass is not, then what can E=mc^2 possibly mean? Is it a relation, like F=ma? Then if E is constant and of course c is constant, then m must be constant too. Or is it an exchange rate: you can convert this much mass into that much energy? But then energy is no longer conserved, if you can make it from mass. Last try. Maybe e=mc^2 means, within a given quantity of mass, there is necessarily a minimum energy? And this is true, but notice that the equation is phrased as an equality. Equalities go both ways: we can just as easily derive m=e/c^2, and now the "minimum energy" idea runs aground. In modern interpretations, energy and mass are both conserved, and e=mc^2 is understood to be the energy in the rest frame of the system. Individually photons are massless, but multiple photon systems can and usually do have mass. The price we pay for conceiving of mass as conserved and invariant is that it no longer adds nicely.

By no means is this my forte, but would this violate the conservation of mass?

by nkozyra 12y ago

By no means is this my forte, but would this violate the conservation of mass?

Nope. As energy equals mass times the speed of light squared you're just converting energy into matter. It's just the opposite of what the atomic bomb does.

asolove 12y ago

Conservation of mass is not applicable in the conditions under discussion, see Wikipedia's fun list of exceptions to conservation of mass: http://en.wikipedia.org/wiki/Conservation_of_mass#Exceptions_or_caveats_to_mass.2Fmatter_conservation http://en.wikipedia.org/wiki/Conservation_of_mass#Exceptions...

leephillips 12y ago

Mass by itself is only conserved in nonrelativistic classical physics. When you are in a relativistic regime the conserved quantity is a combination of momentum and energy. (You've probably seen E = mc^2, which shows that mass and energy can be transformed into each other.)

ktran03 12y ago

Wiki: The law of conservation of mass, or principle of mass conservation, states that for any system closed to all transfers of matter and energy (both of which have mass), the mass of the system must remain constant over time, as system mass cannot change quantity if it is not added or removed.

oddthink 12y ago
Wait, what? Photons have no mass, but they have energy. Ergo, that "both of which have mass" is flat-out wrong. Methinks wikipedia has problems.
- gress 12y ago
  Photons have zero rest mass.
  - oddthink 12y ago
    Rest mass is mass. Photons have zero mass. "Relativistic mass" is more confusing than helpful and is generally not used in modern GR. Cite: ask any actual physicist. I hate to appeal to authority, but this is the view I was taught in grad school (PhD, UCB, 2001), and I have never spoken to anyone in physics or astrophysics who thought "relativistic mass" was helpful.
    - Steuard 12y ago
      I'm entirely on your side on this (PhD, UChicago, 2006), but there are still a bunch of physics teachers out there (mostly not doing research in related fields, is my rough impression) who will vigorously defend teaching the "relativistic mass" concept. They raise some decent points, but in my opinion none of it comes close to outweighing the arguments in favor of treating mass as an invariant property.
    - codeflo 12y ago
      If this is a recent terminology change, then people who use the old terminology are not factually "wrong", they just use an outdated definition. I do have a question though, if you or anyone else wants to clear this up for me. Which kinds of energy are no longer considered part of a system's "mass"? I gather that kinetic energy is not. What about various forms of binding energy?
      - millstone 12y ago
        Only kinetic energy does not contribute to mass. In fact, it's even narrower: only the kinetic energy of the center of mass does not contribute. For example, heat energy is really just kinetic energy of the constituent particles, and heat definitely contributes to mass. The essential idea is that mass is Lorentz invariant, which means it does not change under changes of coordinate systems. So mass must be independent of velocity and orientation, which are just artifacts of how you measure anyways.

timnic 12y ago

Mass is not a conserved quantity. Energy for example is a conserved quantity. So like in a nuclear fission reaction we loose mass but this lost mass is then converted into energy. I have not read the article in detail, but I suppose they want to turn part of the photons energy into masses for the electron and the positron. Mass and energy are linked via E = mc^2.

millstone 12y ago
In fact, both energy and mass are conserved. If you accept e=mc^2 as "true," it's easy to show. If energy is conserved and mass is not, then what can E=mc^2 possibly mean? Is it a relation, like F=ma? Then if E is constant and of course c is constant, then m must be constant too. Or is it an exchange rate: you can convert this much mass into that much energy? But then energy is no longer conserved, if you can make it from mass. Last try. Maybe e=mc^2 means, within a given quantity of mass, there is necessarily a minimum energy? And this is true, but notice that the equation is phrased as an equality. Equalities go both ways: we can just as easily derive m=e/c^2, and now the "minimum energy" idea runs aground. In modern interpretations, energy and mass are both conserved, and e=mc^2 is understood to be the energy in the rest frame of the system. Individually photons are massless, but multiple photon systems can and usually do have mass. The price we pay for conceiving of mass as conserved and invariant is that it no longer adds nicely.