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The principle of least action seems to be one of the most fundamental of high-energy/fundamental interactions physics. But is there some other possibility ton construct a theory of interactions?

Or, is there some theories that explains the least action principle by some other, more fundamental, principle?

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The classical principle of least action can be explained through the Feynman path integral formulation of quantum mechanics. In that context, it can be understood as the system propagating along all possible paths, with destructive interference cancelling out all paths other than the one fulfilling the principle of least action.

For further reading I recommend "Quantum Field Theory of the Gifted Amateur" by Lancaster and Blundell, pages 16-17 and chapter 23, but surely there are good online resources out there as well.

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  • $\begingroup$ I remember the principle of least action being used in classical mechanics. Do you mean that the explanation by the path integral formulation, which seems plausible to me for microscopic objects, also extends to macroscopic objects? $\endgroup$ – Hartmut Braun Nov 27 '20 at 17:34
  • $\begingroup$ The connection hinges on the fact that classical mechanics is quantum mechanics, when the latter is considered in the limit of macroscopic objects (often formulated as $\hbar \to 0$). In that sense, everything which holds for a quantum mechanical object also holds for a classic object, but obviously not the other way around, since quantum mechanics is the more fundamental theory. $\endgroup$ – Codename 47 Nov 27 '20 at 17:46
  • $\begingroup$ I knew about the path integral formulation (well, vaguely) and the principle of least action (from classical mechanics) but it’s the first time I learned that they are so closely related. Great stuff! $\endgroup$ – Hartmut Braun Nov 27 '20 at 19:58

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