Conservation of mass and energy

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Discussion Overview

The discussion revolves around the conservation of mass and energy, exploring whether matter, mass, rest mass, and energy are always conserved. Participants examine various scenarios and exceptions, including matter/anti-matter interactions and the implications of general relativity.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants propose that matter is not always conserved due to matter/anti-matter annihilation and creation.
  • Others argue that mass is not always conserved, referencing the same matter/anti-matter interactions.
  • One participant suggests that rest mass can change, using the example of heating a pile of potatoes.
  • There is a claim that energy is always conserved, but this requires considering all forms of energy and transformations, such as E=mc^2.
  • Another participant challenges the idea that energy is always conserved, suggesting that there are exceptions, particularly in the context of general relativity.
  • Some participants assert that the more general conservation law is the conservation of mass-energy, rather than just mass or energy alone.
  • A later reply clarifies that while mass has energy, energy does not necessarily have mass, citing light as an example.
  • One participant distinguishes between rest mass and invariant mass, stating that rest mass is not conserved due to particle creation/annihilation, while invariant mass is conserved, at least outside of general relativity considerations.
  • It is noted that invariant mass is independent of the observer frame, suggesting it should be conserved even in the context of general relativity.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the conservation of mass and energy, with no consensus reached on whether mass or energy is always conserved. The discussion remains unresolved with various interpretations and exceptions presented.

Contextual Notes

Participants highlight limitations in definitions of mass, rest mass, and invariant mass, as well as the implications of general relativity on conservation laws. There are unresolved mathematical steps and assumptions regarding the conditions under which these conservation laws apply.

noobphysicist
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Hmm, I
s matter always conserved?
Is mass always conserved?
Is rest-mass always conserved?
Is energy always conserved?

are there any exceptions?
 
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noobphysicist said:
Is matter always conserved?

No - matter/anti-matter annihilation and creation.

noobphysicist said:
Is mass always conserved?

No ... see above.

noobphysicist said:
Is rest-mass always conserved?

No ... you can change the rest mass of a pile of potatoes by heating them.

noobphysicist said:
Is energy always conserved?

Yes - but you need to take into account all forms of energy, and transformations such as E=mc^2.
With the possible exception of General Relativity ... but if GR does not apply then energy is conserved.
 
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UltrafastPED said:
No - matter/anti-matter annihilation and creation.



No ... see above.


I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?
 
noobphysicist said:
I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?

The more general conservation law here is the conservation of mass-energy, not just mass, not just energy.

Zz.
 
noobphysicist said:
I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?

No - mass has energy, but energy does not necessarily have mass.

For example, light has no mass, but does have energy.

Nor does kinetic energy contribute to the mass:
http://en.wikipedia.org/wiki/Energy–momentum_relation
 
The OP asked if mass is conserved and if rest mass is conserved. Mass is often taken to mean the same thing as rest mass, but in this context, you could also interpret mass to mean invariant mass. Rest-mass is not conserved because of particle creation/annihilation, but invariant mass is conserved (at least, not counting any general relativity weirdness). This is because a pair of photons traveling in opposite directions has a combined invariant mass.

In fact, invariant mass is independent of observer frame, so I think it should be conserved even in GR, since the problems of parallel transport do not apply.
 
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