Talk:Mass–energy equivalence

Practical examples referencing

I'm concerned that § Practical examples could leave skeptical readers still-skeptical. The idea that "mass becomes energy" (or vice versa) is not intuitive, and the descriptive text, while seemingly accurate, is fairly technical. Readers who are new to all this and say "stop snowing me with math, I wanna see evidence" might reasonably look to this section. But the section is only working the math: plugging known starting values into preceding equations and saying "look, here's what the calculator says" and that seems to be all the references there say either. We don't have earth-mass data, measured product-mass of the A-bombs over Japan, etc. So it takes for granted that even the general ideas, and further, the unit-conversions and constant-factors, are actually true. That is, it's all a risk of "WP:INUNIVERSE" for a topic that is proposed as completely non-fiction. I think this article needs to highlight actual examples of the experimental validation of measured mass before/after (demonstrating that it actually changes) and ideally measured energy gained/released (demonstrating that the specific formula is correct). DMacks (talk) 16:43, 18 June 2022 (UTC)

I agree. One example would be that the plutonium in the Fat Man bomb weighed 5 pounds, but released the same energy as 15,000 tonnes of TNT. Unfortunately I'm not good enough on the physics to write it in, and we should really be quoting in Wikipedia. I guess books like Dancing Wu Li Masters are places to look for these examples. For Evolution, Richard Dawkins is a great explainer. So many of these science articles lack a simple, understandable explanation. Billyshiverstick (talk) 05:50, 16 August 2022 (UTC)

This is a bad article, too much talk, and NOT the most important issue: to show clear and clean Bold textin the initial paragraphs how Einstein deduced that Energy is mass x c2, not to speak of all the people from Newton to Poincare (who basically discovered all of Einstein's before him, as Broglie did with most of Bohr, it seems though French do not have the P.R.E.SS than 'you' know... who expressed the same ideas, ignored here, (which comes latter among many other partial explanations) it is a shame that no physicists have not written properly such a key article of their discipline — Preceding unsigned comment added by 79.151.76.210 (talk) 16:54, 10 April 2023 (UTC)

The statement about an isothermal open system is problematic

I just added a "Clarification needed" tag to this portion of the introduction:

"The equivalence principle implies that when energy is lost in chemical reactions, nuclear reactions, and other energy transformations, the system will also lose a corresponding amount of mass."

This is not usually how I see this concept taught, because it is only true if the particles and molecules in the system are brought back to the same kinetic energy, i.e. the same thermal energy. Basically, it is considering an open system and what might qualify as an isothermal process, although I'm not sure if it's technically isothermal.

I usually see this taught by using a closed system, and rather than saying "energy and mass both exit the system", you say that "within the closed system, the rest mass that went away now remains in the system as kinetic energy and/or heat, thermal energy, which is merely the kinetic energy of particles and molecules".

I've seen examples with both chemical reactions and nuclear reactions, where the masses of the products are compared to the masses of the reactants, and this is shown to be equal to the energy released by the reaction. If you imagine that the reaction took place in thermal contact with some "outside environment", then you start to have problems because reaching the same temperature will not necessarily cause the correct amount of thermal energy to leave the system. Did the volume of the system expand or contract? What is going on with entropy? Now the specifics of the chemical or nuclear reaction become relevant to what temperature it will be. It seems preferable to use a closed system, and notice that the temperature has increased and possibly the matter has expanded (perhaps it even exploded), and now the energy is in the molecules as a mixture of heat and/or kinetic energy of blast particles moving away from the center. A very small "system" might be one uranium atom that undergoes spontaneous fission and results in particles spreading out, it has no well-defined temperature but the net momentum starts and remains zero, and the lost mass is conserved as kinetic energy in the fragments flying outwards. Fluoborate (talk) 21:58, 14 April 2024 (UTC)