[Ovah]

Kind of pedantic but I've never understood why it's E=mc^2 and not E=Δmc^2. In Einsteins original paper he derives the equation with a delta m: a change of mass corresponds to some amount of energy. To me that is different than to say that a whole mass corresponds to a some amount of energy. I've never seen a justification why the delta can be omitted and why the equation still would hold true.

[benchaney]

There isn't a delta because E=mc^2 isn't just describing a reaction, or a conversion. It is describing the fundamental equivalence between mass and energy. It is true even in situations where ΔE and Δm aren't defined.

[Ovah]

"It is describing the fundamental equivalence between mass and energy" I agree that this is widely agreed upon when referring to E=mc^2. It's almost a dogma by this point. But the derivation Einstein used to come up with his equation doesn't actually support said dogma. I'd really like to understand how this "fundamental equivalence" came about and the proof behind it, or if it's just dogma. E=mc^2 is a lot more profound than E=delta m * c^2. IIRC Einstein in the final sentence of his paper tries to generalize his result and jumps on the E=mc^2 train without backing it up.

[tim333]

I think you could say Einstein proposed the fundamental equivalence between mass and energy as a hypothesis and there has been much evidence since. Apparently it was based partly on a proposed symmetry between space and time. Einstein being Einstein probably had some deep reasoning behind it which was not necessarily all put down in that paper.

[mannerheim]

I would assume it has to do with relativistic mass, which used to be somewhat commonly used (m used for relativistic mass, m_0 for rest or invariant mass), but which is now disfavoured. For m as relativistic mass, E = mc^2 holds.

[snissn]

usually the momentum is really low so it's fine, you could read this wikipedia article about it, share questions if that doesn't clear things up

https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...

[Ovah]

I'll give it a read-through this weekend, thanks!

[colinmhayes]

The delta is implied. If you're being rigorous yea you'd include it. But you never see it used rigorously.

[Ovah]

Physics is usually pretty rigorous. In the context of science for a general audience, or printing pretty T-shirts, I get why it's omitted. I just find it funny that when it comes to E=mc^2 physics suddenly lack some rigour even in the college physics classes I've taken. It's probably OK to skip it I'm just curious as to why. I don't think I've ever seen the delta mentioned except in Einstein's original paper.