Physics Asked on July 4, 2021
I have a very simple question, but strangely I cannot find any answer on the internet; maybe the answer is too simple that I don’t notice. I go straight to the point: if I define a Lagrangian from a Lagrangian density, and so from a definite integral in the coordinate space, how there can be an explicit coordinate dependence on the Lagrangian? In the picture I try to calculate the action along a fixed $s$-parametrized trajectory in the $n+1$-dimensional space in which a scalar field $phi$ is present ($y_i$ is the generical $i$-component of the $n+1$-dimensional vector $boldsymbol{y}=(boldsymbol{x},t)$ ).
begin{equation}
Sboldsymbol{=}intlimits_{tleft(s_1right)}^{tleft(s_2right)}!!!!mathrm dt,Lleft(boldsymbol{x}left(sright),tleft(sright)vphantom{tfrac{a}{b}}right)boldsymbol{doteq}!!intlimits_{tleft(s_1right)}^{tleft(s_2right)}!!!!mathrm dt!!intlimits_{mathcal M_n}!!mathrm d^n x,mathscr Lleft(phi,{partial_{y_{i}}phi},boldsymbol{x},tvphantom{tfrac{a}{b}}right)
tag{01}label{01}
end{equation}
I don’t know if it is a stupid question, but this indetermination on what actually is the nature of the quantities I’ve got in front of me, completely mess my conception up, about what I can and what I can’t do, such as when is the case to treat total and partial derivatives in the coordinate, of the lagrangian density; so thanks really in advance to anyone that will answer!
OP's equation is a bit unclear. In this answer we will for simplicity only consider the case where OP's $s$-parametrization is spacetime-filling, i.e. $s=n+1$. Also to be concrete, let $n=3$. Then we are back at standard field theory in 3+1D.
In that case the Lagrangian $$begin{align}L_V[q(cdot,t);v(cdot,t);t]~:=~&int_V! d^3x~ {cal L}left(q(vec{x},t);v(vec{x},t),frac{partial q(vec{x},t)}{partial vec{x}};vec{x},tright),cr v(vec{x},t)~:=~&frac{d q(vec{x},t)}{d t},end{align}tag{A}$$ can still depend explicitly on time $tin [t_i,t_f]$. It can obviously not depend on the integration variable $vec{x}$. However it can in principle depend on the spatial positions of the spatial boundary $partial V$ of the spatial volume $V$. Here $$underbrace{Vtimes [t_i,t_f]}_{text{spacetime}}quadstackrel{q}{longrightarrow}quad underbrace{M}_{text{target space}}.tag{B}$$
Answered by Qmechanic on July 4, 2021
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