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usually on physical theories one would have Lagrangians or Hamiltonians with multiple fields; say, a vector $A_{\mu}$ and a scalar $\phi$ and one would postulate ad hoc a coupling between the fields which will be used to derive some physical states and then, retroactively support the coupling from a match with the physics

Usually one reads about 'minimal couplings' and for scalars with vectors on might see a factor like $\frac{ \partial{\phi}}{ \partial{ x_{\mu}} } A_{\mu}$

But my question goes on in what general or abstract sense such factors represent a minimal coupling? minimum of what? if is the total degree of powers of the fields? do we have some variational principle for lagrangian couplings that these terms can be called a stable point in such variations?

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They're the most renormalizable (or least non-renormalizable) couplings. If the term in the Lagrangian has a coefficient of dimension ${\rm length}^\Delta$, we want to minimize $\Delta$.

Equivalently, we want to minimize the mass dimension of the operator. That means reducing a combination of the powers of the fields, as well as the number of derivatives.

For example, $(\partial_\mu \phi) A^\mu$ is the minimal coupling between $\phi$ and $A_\mu$ (among the Lorentz-invariant terms) - this bilinear term is usually banned by gauge invariance or conservation laws (symmetries).

The dimension of the operator is 3 - one from $\phi$, one mass from $\partial_\mu$, one mass from $A_\mu$. Every factor of $A_\mu$, every new derivative, or every new copy of $\phi$ would increase $\Delta $ by one away from the minimality.

The points above are kind of modern - they are related to the Renormalization Group of the 1970s. The minimal couplings are those that are most important at long distances (by power laws) and least problematic (divergent) at short distances. During Einstein's times, the justification of the minimality would be understood just heuristically, based on feelings of beauty etc. That's no longer needed.

Of course, one could consider non-polynomial or even non-local interactions for which the discussion above would break down - but such terms would almost certainly not be called "minimal", anyway.

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  • $\begingroup$ Lubosh's answer shows a very bad attitude to this subject established nowadays in the theoretical physics. He listed speculations but omitted agreement with experiments. You know, interaction means inter action, exchange. What can be simpler than writing experimentally observed exchanges between experimentally observed entities? Nothing. But who follows this pattern? Too few. $\endgroup$
    – Vladimir Kalitvianski
    Commented Mar 28, 2011 at 20:46
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    $\begingroup$ I forgot to add a pedagogic comment for extremely slow readers: this question and the corresponding answer has nothing whatsoever to do with experiments. It's about the explanation of a word used while constructing theories. With minimal couplings, one may construct both experimentally valid and experimentally invalid theories. However, if a theory has the right field content, the minimal coupling (among those preserving the desired symmetries) is extremely unlikely to have a zero coefficient, and as long as it is nonzero, it dominates the long-distance physics. $\endgroup$
    – Luboš Motl
    Commented Mar 29, 2011 at 5:25
  • $\begingroup$ > Lubosh wrote: "With minimal couplings, one may construct both experimentally valid and experimentally invalid theories." I disagree. Because, apart from this minimal coupling (like $j\cdot A$), one has to introduce the corresponding counter-term - to repair this nice coupling. Agreement with experiment is attained after reparations. $\endgroup$
    – Vladimir Kalitvianski
    Commented Mar 29, 2011 at 9:39
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    $\begingroup$ That's OK, Vladimir, because Germany has already paid all the reparations. $\endgroup$
    – Luboš Motl
    Commented Mar 30, 2011 at 9:27
  • $\begingroup$ It's OK with you because you are not a victim but an idle observer. $\endgroup$
    – Vladimir Kalitvianski
    Commented Mar 30, 2011 at 21:11

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