Questions tagged [action]
The action is the integral of the Lagrangian over time, or the integral of the Lagrangian Density over both time and space.
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Addition or disappearance of symmetries due to a particular field transformation [closed]
I have a question related to symmetries in field theory... I request someone to please help me with it:
Let us have an action $$S= \int d^4 x \mathcal{L}(A,B) $$ of fields $A,B$. Suppose we make a ...
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Transformation to linearise action over time
Let a body $m$ be in motion such that it follows the path of least action. We can define the total action of this body by computing $$S=\int_{t_0}^{t_f}T_m-V_m\,dt.$$ Furthermore, we can define action ...
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Intuition for Maupertuis' action and the principle of least action
I'm familiar with Lagrangian mechanics and Newtonian mechanics, though I'm currently building an intuition for the concept of "action" or "least action" (i.e what unites the two ...
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Why is integral of relativistic action $-\alpha \int_{a}^{b} \, \mathrm ds$ minimised with respect to $\mathrm ds$?
While reading some aspects concerning the conclusion that bodies follow geodesics of spacetime, I ran into relativistic action on p.24 in chapter 2 $\S8$ of the 2nd volume of Landau & Lifshitz:
$$...
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How does this analytic expression for the bounce action reduce to Coleman's in the thin-wall limit?
In the recent paper False Vacuum Decay Rate From Thin To Thick Walls
, it is shown that in 4 dimensions, the bounce action is given by : $$S = \frac{2\pi^2 m^2}{4\eta^2}\frac{1}{6\epsilon_{\alpha}^3}\...
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What should I vary the Tetradic Palatini Action with respect to in order to find its equation of motion?
I'm trying to calculate the EoM for the Tetradic Palatini Action. I've gained an understanding for tetrads/vierbeins, the spin connection, Omega tensor, and the action, but when it comes to applying ...
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Supercurrent from supersymmetric
In the book "Superstring Theory" by Green-Schwarz-Witten on page 189 (vulume 1) the author give the formula for the supercurrent (by Neother method) as
$$J_{\alpha}=\frac{1}{2}\rho^{\beta}\...
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Lorentz force law, Action of relativistic particle in e.m. field with interaction between field and particle
This question is about the derivation of the Lorentz force law and may be answered quickly. Nonetheless, I will give some context. In lecture we defined the overall Action of a particle + field (in ...
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Transversality condition for Euler-Lagrange Equations with 1 variable end point
I am able to follow the derivation of the Euler-Lagrange equations, for 1 variable end point, but cannot make the final step regarding the additive term.
Specifically, I arrive at the path minima ...
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Why don't we talk about time-dependent Maupertuis' principle? [closed]
I read some books and articles and found that all of the authors present Maupertuis' principle as follows:
Abbreviated action $S_{0}$ is defined as $$S_{0}:=\int_{q'}^{q''}pdq\tag{1}$$
where $q'$ and ...
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Why does relative velocity need to be very small? (Lagrangian for a free particle) [duplicate]
I'm currently studying Landau's mechanics book and am wondering about the "Lagrangian for a free particle" part.
If an inertial frame K is moving with an infinitesimal velocity $\vec{\...
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Intuition for $L = T - V$ in classical mechanics without Newton's 2nd law? [closed]
I'm trying to find some helpful intuition as to why the lagrangian in classical mechanics has the form $$L = T - V.$$ So far, all the explanations that I've found resort to motivating this definition ...
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Derivation of the Einstein-Infeld-Hoffmann (EIH) Lagrangian and the field action contribution
I'm struggling with understanding the field action contribution to the PN (Einstein-Infeld-Hoffmann) and PPN Lagrangians. In general, the action for a test particle is given by $ S=S_m +S_g $, with
$$ ...
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What does the Lagrangian $L$ being a functional mean in classical field theory? How do we evaluate action integral now if $L$ gives out real values?
In the book of Greiner while introducing classical field theory, it says the Lagrangian now would be functional of fields. But as a functional maps a function to a real value, how will we then ...
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Is action quantized to integer multiples of $h$ or $ħ$?
As I understand it, any observable/meaningful amount of Action (Energy * Time or Momentum * Distance) is always going to be an integer multiple of some value… $h$ or $ħ$, depending on the source.
I ...
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