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Comment by TheOtherHobbes | Hacker News Reader

CPT symmetry isn't conserved, so that's not necessarily a given.

We don't know the mechanism which generates the gauge symmetry that makes charge discrete or generates conservation laws from symmetries, so it's risky to be too prescriptive about what is and isn't possible.

In the limit you can imagine a meta-technology which manipulates the mathematical structures that generate reality instead of just assuming the rules are fixed. That might or might not be possible, but the rules have to come from somewhere and there's no absolute guarantee they're hardcoded.

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So, what I meant to say was that using the computational gadget from physics, none of the branches will violate conservation of charge.

However, my post sucked and I didn't get around to emphasizing that. In quantum mechanics, it's possible for some state to have zero probability, and it's those probabilities that we're talking about everything being possible.

There's also the epistemic side where we know that there's a chance our theories are wrong in any given way. Those can't go to zero based on our model since the whole point is we're not restricting to a model. BTW, I really like your example as a good, clean limiting case.

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I'm sure people have come up with theories that violate conservation of charge, but they'd have to break U(1). I don't know what conservation CPT would break, if any; Noether's theorem shouldn't apply to it as a discrete symmetry.

Charge IS conserved!

The C in CPT is not only a change f charge but also a change of electron <---> positron, and quark <---> antiquark, and ...

The conservation or charge comes from the U(1) gauge symmetry. That is almost obvious if you ignore the mixing with the weak interaction, but after the mix you still have a U(1) gauge symmetry anyway.

I thought that in an expanding universe, mass isn't conserved. I don't see why charge would be conserved in that case.

Mass isn't close to being conserved. If an electron and positron annihilate, they become two massless photons, for example. You don't need to invoke an expanding universe.

In an expanding universe, energy isn't conserved. (Unless we do the trick we always do with energy and just define it to be conserved https://youtu.be/lCgKTpIzjRk?t=174 )

I don't know exactly why you think this applies to charge conservation. The U(1) symmetry is where charge conservation comes from and it isn't affected by an expanding universe.

The photons have energy, so through mass-energy equivalence, they aren't massless, I thought.

Sorry for the too late reply...

There is "rest mass" and "relativistic mass". The GP is talking about "rest mass" that is not conserved even in simple examples.

Most modern books try to eliminate "relativistic mass" because it's misleading, for example is good to calculate acceleration perpendicular to the movements (for example in a cyclotron) but it's not good to calculate acceleration in the direction of the movement.

So most modern books they call the same idea just "energy" and use that "energy" to calculate gravitation. "Energy" is conserved in special relativity and almost conserved in general relativity.

But the "relativistic mass" is useful in some problems and is widespread in popular culture and don't ask about the weird parts of the mass of the proton, so it's difficult to kill the idea.

When an electron flies beyond the cosmological event horizon, it disappears for good.

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