Conservation of mass and energy

[noobphysicist]

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

are there any exceptions?

 

[UltrafastPED]

Is matter always conserved?

No - matter/anti-matter annihilation and creation.

Is mass always conserved?

No ... see above.

Is rest-mass always conserved?

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

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.

 

[Bandersnatch]

Is energy always conserved?

That should also be a "no".
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

 

[noobphysicist]

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?

 

[ZapperZ]

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.

 

[UltrafastPED]

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

 

[Khashishi]

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.