Talk:Mass–energy equivalence

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)

The paragraph was a mess. Rather than expand the introduction with discussion of closed/open systems, I simply corrected the sentence. Please review. Johnjbarton (talk) 14:44, 19 June 2024 (UTC)

De Pretto

The De Pretto stuff seems wildly over-represented in this article, given the shaky ground it is on (a totally different derivation in a totally different theoretical context by a total outsider published only in Italian). The idea that Einstein would have been aware of this is completely without evidence. Here is the purported evidence from the linked-to source:

It is very likely that Augusto De Pretto, another of Olinto's brothers, gave a copy of Olinto's article to his friend and co-worker, Beniamino Besso, who lived in Rome. Beniamino Besso sent it to his very dear nephew, Michele Angelo Besso, (Michele Angelo was guest at his uncle's house when an university student in Rome) who was a patent examiner in that same Swiss Patent Office in which Einstein at that time was working too. Michele Angelo Besso in my opinion could have given it to Einstein, who was, more than Besso's co-worker, Besso's close friend. Laurent Mousson, the head librarian at the Swiss Patent Office, has written about De Pretto's work that "we do not currently have this volume on our shelves". Nevertheless, he cautioned that the volume may have been available in 1905 and that it was later discarded. Anyway, there is no need that the whole volume in which De Pretto's essay was published should have been in the shelves of the Swiss Patent Office: it would have been enough for Besso to have a reprint of that paper, which surely Olinto's brother was happy to make circulate between friends. Interestingly enough, in his great article exposing the theory of relativity, written in 1905, Einstein acknowledged the help of only one person, that is to say Michele Angelo Besso.

I will say as a historian that this is weak. There's a lot of "likely" and "in my opinion" and "surely" here, without any actual evidence that _any_ of this transmission of knowledge took place. There's a lot of "jumps" here.

The current Wikipedia article asserts that De Pretto was separated by Einstein through "only" 3 degrees of separation, but the linked-to source says that the "links" were (with my numbering):

1. Olinto De Pretto <--> Augusto De Pretto brothers

2. Augusto De Pretto <--> Beniamino Besso co-workers and friends

3. Beniamino Besso <--> Michele Angelo Besso uncle and nephew

4. Michele Angelo Besso <--> Albert Einstein co-workers and friends.

Which is 4 links, not 3, if I'm counting correctly. Which is not all _that_ profound (I am 2-3 links from several US Presidents, but that does not imply actual proximity, it just means I know people who once knew cabinet-level appointees, and it doesn't imply any exchange of information along that pathway) — the only "real" interesting link here is between a brother of De Pretto and the uncle of Besso, which seems like a interesting coincidence. But there's a lot of "leaps" here. Not only would everybody in this chain have to think this paper had value, and read their way through it and its particular logic, but somehow out of all of that, the idea of E=mc^2 would have to be the one thing that Besso communicated to Einstein, and that Einstein... what? Recognized the value of just that one equation? And then re-derived it in a totally different way in a totally different physical context? This not how scientists work.

This is all just very weak. The fact that De Pretto came up with a similar derivation in another context is interesting and is one of _several_ such "near" derivations — a sign that Einstein's work was not coming out of "nowhere" but emerging out of the kinds of electromagnetic questions that a lot of people had in the early 20th century. To credit De Pretto as _influencing Einstein_ is an interpretation that is absolutely not supported by the evidence given. I do not think this should be part of an article on the history of mass-energy equivalence's history — it is totally misleading, and no actual historian of physics credits De Pretto with anything, as far I can tell. NuclearSecrets (talk) 18:51, 30 December 2024 (UTC)

Thanks. I agree and cut the section back. To go further we could use any credible secondary history to simply rewrite that section. I'll try to look into the Jammer book next time I'm at the library. Johnjbarton (talk) 19:20, 30 December 2024 (UTC)