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I don't see anything that appears to be an application that depends on the pi-meson like some of the other particles.

Originally Yukawa Hideki predicted the pi-meson while lecturing at Osaka Imperial University in 1935, this is somewhat ironic considering in 1939 the Manhattan project started.

General Groves must have known about Yukawa Hideki's work on the atomic nucleus, which was right up the ally of research of atomic power. I am wondering if he was concerned about his "enemy" having this knowledge.

It could be the case that the meson has no practical use in the development of the chain reaction and construction of the Atomic Bomb and perhaps has no apparent practical application thus the motivation for the question.

Qmechanic
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    Depends on your definition of "practical", doesn't it. See, for instance, https://physics.stackexchange.com/q/14999/ – dmckee --- ex-moderator kitten Aug 05 '18 at 06:28
  • @dmckee ...yes you are correct. Depends on one's point of view of "practical". I learned from the answers and do not necessarily regret asking but the way I worded the question shows ignorance on my part. There is no clear black and white answer to "practical". As for example Einstien's Field equations are usefull on all our GPS devices. What is only theoretical today may be of vital practical importance in the future. I should know this of all people and deeply apologize for phrasing the question in such an ignorant manner. –  Jul 30 '20 at 16:47

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The long-range forces holding together nuclei are transmitted by pions which can be described with Yukawa's potential. This is very much what he described. See https://en.wikipedia.org/wiki/Nuclear_force

So in that sense, while not fundamental, pions do find an application in any nuclear reactor or atomic bomb.

Now Yukawa was mistaken in that he assumed that the exchanged particle would be the particle found in the cosmic radiation (which we now call muon). But that error is forgivable because he fell into an elaborate trap laid by nature: on the one hand, it is an accident of nature (the pion being the Goldstone bosons of unbroken isospin symmetry) that the pion's mass is so low and thus so close to the muon mass. On the other hand, the source of the muons of the cosmic radiation actually are nuclear reactions: solar and galactical protons hit the atmosphere and undergo nuclear reactions. These reactions produce pions, some of which proceed towards the lower atmosphere. But what Yukawa couldn't know is that the pions decay before reaching the surface. The particle they decay into is the muon. Not knowing about the subsequent decay, one would therefore assume that the cosmic ray particle actually participates in nuclear reactions.

Additionally, pions serve as source of polarized muons which are useful tools for study. They are one of the main tools of study at the fixed-target experiments in CERN's North Area.

tobi_s
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Although Yukava has proposed his potential reasoning in terms of $\pi$-meson, the meson itself has little relevance to using nuclear energy (whether peaceful or military) or understanding nucleus - its value is mainly theoretical. Indeed, the nuclear force is a residual force - a limiting case of the forces acting between the quarks composing neutrons and protons. For the nuclear models relevant to practical applications, the description in terms of a phenomenological hardcore potential is quite sufficient.

Roger V.
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