What is action: Definition and 5 Discussions

ACTION (Australian Capital Territory Internal Omnibus Network) is a bus operator in Canberra, Australia, and is owned by the Government of the Australian Capital Territory.

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  1. LarryS

    I Is Relativistic Action for a beam of light = zero?

    Under the RELATIVISTIC definition of Action, is the Action for a beam of light always zero? Thanks in advance.
  2. ergospherical

    I Ricci tensor from this action

    Here is an action for a theory which couples gravity to a field in this way:$$S = \int d^4 x \ \sqrt{-g} e^{\Phi} (R + g^{ab} \Phi_{;a} \Phi_{;b})$$I determine\begin{align*} \frac{\partial L}{\partial \phi} &= \sqrt{-g} e^{\Phi} (R + g^{ab} \Phi_{;a} \Phi_{;b}) \\ \nabla_a \frac{\partial...
  3. D

    P&S Exercise 3.4 Majorana Fermions Derivative of ##\chi##

    I am stuck at the final part where one is supposed to show that the derivative of the second term of the action gives the mass term in the Majorana equation. For $$\chi^T\sigma^2\chi = -(\chi^\dagger\sigma^2\chi^*)^*$$ we get $$\frac{\delta}{\delta\chi^\dagger}(\chi^\dagger\sigma^2\chi^*)^*$$...
  4. P

    I How does the quantum of action cause indeterminism?

    How and/or why does the existence of the quantum of action (Planck's constant) cause indeterminism?
  5. ergospherical

    A Extra (boundary?) term in Brans Dicke field equations

    Here is the action: ##S = \frac{1}{16\pi} \int d^4 x \sqrt{-g} (R\phi - \frac{\omega}{\phi} g^{ab} \phi_{,a} \phi_{,b} + 16\pi L_m)## the ordinary matter is included via ##L_m##. Zeroing the variation ##\delta/\delta g^{\mu \nu}## in the usual way gives ##\frac{\delta}{\delta g^{\mu \nu}}[R\phi...
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