Noether and the derivative of the Action

In summary: For time translation symmetry, it is energy (Hamiltonian) and for phase symmetry, it is electric charge (the current associated with this symmetry).In summary, the conversation discusses the Noether conservation of energy and momentum in relation to the action's units and derivatives. It is noted that if the Lagrangian is independent of time or a spatial coordinate, then the corresponding quantity is conserved. This is not a coincidence, as it is a result of Noether's theorem and the symmetry associated with the Lagrangian. The derivative of the action with respect to phase is electric charge, further illustrating this connection.
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nemuritai
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I know that the Action has units Energy·time or Momentum·position. A second fact is that the derivative of the action with respect to time is Energy and similar with momentum-position, consistent with a units ie. dimensions check.Is it a coincidence that both are Noether conserved quantities? For example will the derivative of the Action with respect to phase be electric charge?
 
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  • #2
Angular momentum is a conserved quantity, but action is not though the both has same dimension.
 
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:welcome:

nemuritai said:
I know that the Action has units Energy·time or Momentum·position. A second fact is that the derivative of the action with respect to time is Energy and similar with momentum-position, consistent with a units ie. dimensions check. Is it a coincidence that both are Noether conserved quantities?
If the Lagrangian is independent of time then energy is conserved. And, if it is indepenent of a spatial coordinate, then momentum in that direction is conserved. I don't see that as a coincidence.
nemuritai said:
For example will the derivative of the Action with respect to phase be electric charge?
I don't understand this question.
 
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It's of course not a coincidence, because if the Lagrangian doesn't depend on some coordinate ##q## (but only its generalized velocity ##\dot{q}##), then the canonical momentum is conserved for the solutions of the equation of motion, i.e., it is a conserved quantity:
$$p_q=\frac{\partial L}{\partial \dot{q}} \; \Rightarrow \; \dot{p}_q = \frac{\partial L}{\partial q}=0.$$
This canonical momentum is the generator of a symmetry (Noether's theorem).
 
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1. What is Noether's Theorem?

Noether's Theorem is a fundamental principle in physics that states that for every continuous symmetry in a physical system, there exists a corresponding conserved quantity. It was first developed by mathematician Emmy Noether in 1915.

2. What is the Action in physics?

The Action, also known as the Lagrangian, is a mathematical quantity that describes the dynamics of a physical system. It is defined as the difference between the kinetic and potential energies of the system.

3. How is the derivative of the Action related to Noether's Theorem?

The derivative of the Action is used to derive the equations of motion for a physical system. In the context of Noether's Theorem, it is used to determine the conserved quantities associated with symmetries in the system.

4. What is the significance of Noether's Theorem in physics?

Noether's Theorem is a powerful tool in theoretical physics, as it allows for the prediction of conserved quantities in physical systems. This has important implications in understanding the fundamental laws of nature and has been applied to various fields such as classical mechanics, quantum mechanics, and general relativity.

5. Are there any limitations to Noether's Theorem?

Yes, there are some limitations to Noether's Theorem. It only applies to continuous symmetries, and not all symmetries in a physical system can be described as continuous. Additionally, it does not apply to systems with dissipation or external forces. However, it remains a powerful and widely used tool in theoretical physics.

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