Physics Asked on December 8, 2020
The field action in flat spacetime is
$$ S = int d^4x, mathcal{L}(phi,partial_muphi).tag{1} $$
The variation in $S$ leads to
$$ delta S = int d^4x, delta mathcal{L}. tag{2} $$
Proceeding this way, one gets the Euler-Lagrange thing upon integration by parts
$$ delta mathcal{L} = frac{partialmathcal{L}}{partialphi}deltaphi + frac{partialmathcal{L}}{partial(partialphi)}delta(partialphi).tag{3} $$
But in many places, people also write
$$ delta S = int d^4x frac{delta S}{delta phi} deltaphi. tag{4} $$
I am confused by this way of writing. How is it consistent with the second equation? Also on the RHS, isn’t $$frac{delta S}{delta phi} deltaphi=delta S~?tag{5} $$ So what is the integral sign doing here?
According to Ortin, Gravity & Strings (Chapter 2), we define
$$dfrac{delta S}{delta phi} equiv dfrac{partial mathcal{L}}{partial phi} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi)}bigg)$$
Now, as you can easily see,
$$delta S = displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi} delta phi - dfrac{partial mathcal{L}}{partial(partial_muphi)} delta (partial_mu phi)bigg) $$
$$= displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi)}bigg)bigg) delta phi $$
$$=displaystyleint d^4x dfrac{delta S}{delta phi} delta phi$$
Edit From this answer, I realize that we don't really need to define an explicit $dfrac{delta S}{delta phi}$. Rather, we can derive it from the definition of $S$ as it should be the case. Based on the answer linked above, I derive here $dfrac{delta S}{delta phi}$ in the simplest manner apparent to me. The key is to realize that there is an implicit label associated with $phi$ which decides what $phi$ we are talking about - the label is the coordinates $x$.
$$S=displaystyleint d^4x mathcal{L}(phi(x), partial_mu phi(x)) $$ $$delta S = displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi (x)} delta phi(x) - dfrac{partial mathcal{L}}{partial(partial_muphi(x))} delta (partial_mu phi(x))bigg) $$
$$= displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi(x)} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi(x))}bigg)bigg) delta phi(x) $$
Now, since $x$ is already used in the integration and it runs over all of spacetime, we should use a different variable for coordinate label when we want to define $dfrac{delta S}{delta phi}$ which is the variation of action with respect to variation in the field at any point in spacetime. Let's use $y$ to denote coordinates of this point where we vary the field and notice the variation in action.
$$dfrac{delta S}{delta phi} = dfrac{delta S}{delta phi (y)}$$ $$= displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi(x)} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi(x))}bigg)bigg) dfrac{delta phi(x)}{delta phi(y)} $$ $$= displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi(x)} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi(x))}bigg)bigg) delta (x -y) $$ $$=dfrac{partial mathcal{L}}{partial phi(y)} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi(y))}bigg)$$
So, $$dfrac{delta S}{delta phi(y)} = dfrac{partial mathcal{L}}{partial phi} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi)}bigg)$$ And, thus, $$delta S = displaystyleint d^4x bigg(dfrac{partial mathcal{L}}{partial phi(x)} - partial_mu bigg(dfrac{partial mathcal{L}}{partial(partial_muphi(x))}bigg)bigg) delta phi(x) $$ $$=displaystyleint d^4x dfrac{delta S}{delta phi(x)} delta phi(x)$$
Or, concisely, ${delta S} = displaystyle int d^4x dfrac{delta S}{delta phi} delta phi$.
Answered by Dvij D.C. on December 8, 2020
I guess the confusion is caused by (implicitly) different variables and their associated derivatives. I'll stick to the action of a single classical scalar real field in the following, and try a heuristic explanation.
The different notions of variables, $delta$'s and $partial$'s (and later $mathscr{D}$'s!) can be confusing, but usually it becomes clear from the context what is meant.
Answered by Toffomat on December 8, 2020
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