What is Noether's theorem: Definition and 96 Discussions

Noether's theorem or Noether's first theorem states that every differentiable symmetry of the action of a physical system with conservative forces has a corresponding conservation law. The theorem was proven by mathematician Emmy Noether in 1915 and published in 1918, after a special case was proven by E. Cosserat and F. Cosserat in 1909. The action of a physical system is the integral over time of a Lagrangian function, from which the system's behavior can be determined by the principle of least action. This theorem only applies to continuous and smooth symmetries over physical space.
Noether's theorem is used in theoretical physics and the calculus of variations. A generalization of the formulations on constants of motion in Lagrangian and Hamiltonian mechanics (developed in 1788 and 1833, respectively), it does not apply to systems that cannot be modeled with a Lagrangian alone (e.g., systems with a Rayleigh dissipation function). In particular, dissipative systems with continuous symmetries need not have a corresponding conservation law.

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  1. T

    I Noether's Theorem in the Presence of a Charged Operator

    I am trying to understand the following idea that I found from some notes: Generally, a system with U(1) symmetry will have a conserved current: ##\partial_{\mu}j^{\mu} = 0##. The notes then state that in the presence of a local operator ##\mathcal{O}(x)## with charge ##q\in \mathbb{Z}## under...
  2. G

    I Taylor expansion about lagrangian in noether

    I was studying a derivation of noether's theorem mathematically and something struck my eyes. Suppose you have ##L(q, \dot q, t)## and you transform it and get ##L' = L(\sigma(q, a), \frac{d}{dt}\sigma(q,a), t)##. ##\sigma## is a transformation function for ##q## Let's represent ##L'## by...
  3. H

    I Noether's second theorem: two questions

    A technical subject, well above my level it seems (I'm still learning about quantum physics and special relativity), but one about which I absolutely must get some clear ideas as soon as possible. From what I 'understand', Noether's second theorem applies to infinite-dimensional symmetry...
  4. PhysicsRock

    I Why isn't the Lagrangian invariant under ##\theta## rotations?

    I just calculated the Lagrangian of a particle of mass ##m## in a radially symmetric potential ##V(r)##. I thought it would be a good idea to switch to spherical coordinates for that matter. What I get is $$ L = \frac{1}{2} m \left( \dot{r}^2 + r^2 \dot{\theta}^2 + r^2 \dot{\varphi}^2...
  5. T

    B What's so great about Noether's theorem?

    I read about Noether's theorem that says how for every symmetry there is a conserved quantity. Seems kind of obvious. Does anyone understand it well enough that they can explain precisely why that notion is profound?
  6. N

    A Noether and the derivative of the Action

    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...
  7. K

    Find the Conserved Quantity of a Lagrangian Using Noether's Theorem

    So Noether's Theorem states that for any invarience that there is an associated conserved quantity being: $$ \frac {\partial L}{\partial \dot{Q}} \frac {\partial Q}{\partial s}$$ Let $$ X \to sx $$ $$\frac {\partial Q}{\partial s} = \frac {\partial X}{\partial s} = \frac {\partial...
  8. Ebi Rogha

    I Vacuum energy and Energy conservation

    Also, I have heard from physicists that vacuum energy fluctuation (creation and destruction of virtual particles) violates energy conservation. The reason, they justify, is based on uncertainty principle (energy-time form of uncertainty principle), energy can exist and disappear for a very short...
  9. T

    A Noether's theorem for finite Hamiltonian systems

    The Noether's theorem for finite Hamiltonian systems says that: My question is: If I know a symmetry how can I write the first integral?
  10. F

    Lagrangian for the electromagnetic field coupled to a scalar field

    It is the first time that I am faced with a complex field, I would not want to be wrong about how to solve this type of problem. Usually to solve the equations of motion I apply the Euler Lagrange equations. $$\partial_\mu\frac{\partial L}{\partial \phi/_\mu}-\frac{\partial L}{\partial \phi}=0$$...
  11. JD_PM

    Using Noether's theorem to get a constant of motion

    Noether's theorem tells us that an invariance of the Lagrangian yields a constant of motion. In this problem, that constant is: $$Q_v = p^a \Big( \frac{\partial q_a^{\lambda}}{\partial \lambda}\Big)_{\lambda = 0} + p^b \Big( \frac{\partial q_b^{\lambda}}{\partial \lambda}\Big)_{\lambda = 0}=...
  12. M

    Nother current of given symmetry

    This is my first time dealing with scaling symmetry, so I'm sorry if the following is fundamental wrong. My approach was the same as if I was trying to show the same for translation or Lorentz symmetry. We have $$\delta\phi(x)= \phi'(x')-\phi(x)=...
  13. peguerosdc

    Noether's theorem with non-finite transformations

    Hi! I am given the lagrangian: ## L = \dot q_1 \dot q_2 - \omega q_1 q_2 ## (Which corresponds to a 2D harmonic oscillator) And I am given two transformations and I am asked to say if there is a constant of motion associated to each transformation and to find it (if that's the case). I am...
  14. Van Ladmon

    I Conflict of domain and endpoints in Noether's theorem

    In the derivation of energy conservation, there is the transformation ##q(t)\rightarrow q'(t)=q(t+\epsilon)##, whose end points are kind of fuzzy. The original path ##q(t)## is only defined from ##t_1## to ##t_2##. If this transformation rule is imposed, ##q'(t_2-\epsilon)=q(t_2)## to...
  15. D

    Trick for conserved currents in classical field theory

    First I found the equations of motion for both fields: $$\partial_\mu \partial^\mu \psi = -\frac{\partial V(\psi^* \psi)}{\psi^*}$$ The eq. of motion with the other field is simply found by ##\psi \rightarrow \psi^*## and ##\psi^* \rightarrow \psi## due to the symmetry between the two fields...
  16. Abhishek11235

    A Physical meaning of Noether's theorem

    This is the image of theorem from V.I Arnold's Mathematical method of mechanics. I understood the example given in text. But I want to know what is physical meaning of example? Can anybody help?
  17. D

    I Noether's theorem for discrete symmetry

    I am wondering if it existes some discret version of the Noether symmetry for potential with discrete symmetry (like $C_n$ ). The purpose is to describe the possible evolution of the phase space over the time without having to solve equations numerically (since even if the potential may have...
  18. Q

    Understand Logic of Wald & Zoupas' Expression on Conserved Quantities

    Wald and Zoupas discussed the general definition of ``conserved quantities" in a diffeomorphism invariant theory in this work. In Section IV, they gave one expression (33) in the linked article. I cannot really understand the logic of this expression. Would you please help me with this?
  19. H

    A Derivation of the Noether current - Lorentz Transformation

    We make an infinitesimal Lorentz transformation of the Lagrangian and require it to be invariant. We then arrive at the following expression. $$\epsilon^{\mu\nu}j_{\mu\nu} = P_{\mu}\epsilon^{\mu\nu}X_{\nu}$$ which can be written as $$\epsilon^{\mu\nu}j_{\mu\nu} =...
  20. H

    I Noether's Theorem: Exploring Infinitesimal Transformations

    We can look at infinitesimal transformations in the fields that leaves the Lagrangian invariant, because that implies that the equations of motions are invariant under this transformations. But what really matters is the those transformations that leaves the action invariant. So we can always...
  21. gibberingmouther

    I Emmy Noether's Theorem: Learning STEM for Beginners

    I bought "Emmy Noether's Wonderful Theorem" by Dwight E. Neuenschwander. After flipping through it, I realized a lot of the math is over my head. For example, multivariate calculus and differential equations. Has anyone else bought this book or really studied how to apply her theorem? I want...
  22. N

    On differentiation and indices in field theory

    I'm self-studying field theory and trying to solidify my understanding of index manipulations. So I've been told that there is a general rule: " If the index is lowered on the 'denominator' then it's a raised index". My question is whether this is just a rule or something that can make sense...
  23. Toby_phys

    Using Noether's Theorem to get conserved quantities

    Homework Statement N point particles of mass mα, α = 1,...,N move in their mutual gravitational field. Write down the Lagrangian for this system. Use Noether’s theorem to derive six constants of motion for the system, none of which is the energy Homework Equations Noethers Theorem: If a...
  24. J

    A Time independence of a Noether charge in QFT?

    In classical field theories, I believe I understood how to derive a Noether charge that corresponds to a symmetry of action. And there is no problem in understanding its time independence. But in quantum field theory, it looks like the two different approaches, 1) Canonical quantization...
  25. J

    Conserved quantity for a particle in a homogeneous and static magnetic field

    The equation of motion for a charged particle with mass ##m## and charge ##q## in a static magnetic field is: ##\frac{d}{dt}[m{\dot{\vec{r}}}]=q\ \dot{\vec{r}}\times \vec{B}## From this, we can see that ##\frac{d}{dt}[m\dot{\vec{r}}-q \vec{r}\times \vec{B}]=0## and so the following quantity is...
  26. parshyaa

    I What does Noether's theorem actually say?

    I don't know much about classical physics(such as lagrangian function), but as i was reading conservation of energy, i came to this theorem and it tells that if a system is symmetrical in certain transformations(such as translation, rotation etc) then it will have a corresponding law of...
  27. binbagsss

    Solving Geodesic Equations with Euler-Lagrange and Noether's Theorem

    Homework Statement Homework Equations There are 5 equations we can use. We have the fact that Lagrangian is a constant for an affinely parameterised geodesic- 0 in this case for a light ray : ##L=0## And then the Euler-Lagrange equation for each of the 4 variables. The Attempt at a Solution...
  28. Ken Gallock

    I Lorentz transformation and its Noether current

    Hi. I'd like to ask about the calculation of Noether current. On page16 of David Tong's lecture note(http://www.damtp.cam.ac.uk/user/tong/qft.html), there is a topic about Noether current and Lorentz transformation. I want to derive ##\delta \mathcal{L}##, but during my calculation, I...
  29. C

    I Technicality with Noether's theorem

    If we consider a transformation of a field ##\Phi \rightarrow \Phi + \alpha \frac{\partial \Phi}{\partial \alpha}## which is not a symmetry of a lagrangian then one can show that the Noether current is not conserved but that instead ##\partial_{\mu}J^{\mu} = \frac{\partial L}{\partial \alpha}##...
  30. S

    I Noether's Theorem: Confused About Last Step

    Hello! I looked over a proof of Noether theorem and I am a bit confused about the last step. So they got that ##\delta q(t) p(t)## is constant (I just took the one dimensional case here) where ##\delta q## is a variation of the q coordinate and p is the momentum conjugate of q. I am not sure I...
  31. F

    I Does Noether theorem explain the constant speed of light?

    I learned in Analytical Mechanics: "Emmy Noether's theorem shows that every conserved quantity is due to a symmetry". The examples I learned where conservation of energy as symmetry in time and conservation of momentum as symmetries in space. Now I wonder, do universal constants are also due to...
  32. Q

    A Does an infinitesimal generator of acceleration exist?

    I am trying to determine what types of field theories have a Lagrangian that is symmetric under an Infinitesimal acceleration coordinate transformations. Does an infinitesimal generator of acceleration exist? How could I go about constructing this matrix?
  33. CassiopeiaA

    A What Are Practical Applications of Noether's Theorem for Beginners?

    Hi I need a little help in my homework. It is not a direct problem to be solved. Rather I am supposed to find an application of Noether's theorem. All the article or papers I have found are very difficult for me to understand. In fact, I still don't understand any application of Noether's...
  34. It's me

    Using Noether's Theorem find a continuity equation for KG

    Homework Statement Consider the Klein-Gordon equation ##(\partial_\mu \partial^{\mu}+m^2)\varphi(x)=0##. Using Noether's theorem, find a continuity equation of the form ##\partial_\mu j^{\mu}=0##. Homework Equations ##(\partial_\mu \partial^{\mu}+m^2)\varphi(x)=0## The Attempt at a Solution...
  35. ibkev

    I Background required for Noether's Theorem?

    Can someone please explain the progression of topics I would need to study in order to tackle Noether's Theorem? I keep hearing how important it is and am setting a self-study goal for myself to eventually understand it with enough rigour that I can appreciate it's beauty. I have a feeling I...
  36. JulienB

    A Noether's Theorem: Julien's Calculation Mistake?

    Hi everybody! I'm currently studying Noether's theorem, but I'm a bit stuck around a stupid line of calculation for the variation of the symmetry. The script of my teacher says (roughly translated from German, equations left as he wrote them): "V.2. Noether Theorem How does the action change...
  37. J

    A Noether's Theorem to Multi-parameter Transformations

    When you have single parameter transformations like this in Noether's Theorem \begin{array}{l} {\rm{ }}t' = t + \varepsilon \tau + ...{\rm{ }}\\ {\rm{ }}{q^\mu }^\prime = {q^\mu } + \varepsilon {\psi ^\mu } + ... \end{array} The applicable form of the Rund-Trautman Identity is {\rm{...
  38. Z

    What Values of \( n \) Make the Given Lagrangian a Total Derivative?

    Homework Statement We have the Lagrangian $$L=\frac{1}{2}\dot q^2-\lambda q^n$$ Determine the values for n so that the Lagrangian transform into a total derivative $$\delta q = \epsilon (t\dot q - \frac{q}{2})$$ Homework Equations The theorem says that if the variation of action $$\delta S =...
  39. C

    In an infinite quantum well, why Δn=0?

    I've been reading up a bit on semiconductor quantum wells, and came across a selection rule for an infinite quantum well that says that "Δn = n' - n = 0", where n' is the quantum well index of an excited electron state in the conduction band, and n is the index of the valence band state where...
  40. G

    Newtonian formulation/proof of Noether's theorem

    Hi. I've only ever seen Noether's theorem formulated ond proven in the framework of Lagrangian mechanics. Is it possible to do the same in Newtonian mechanics, essentially only using F=dp/dt ? The "symmetries" in the usual formulation of the theorem are symmetries of the action with respect to...
  41. T

    Help Me Understand This Author's Point: Noether's Theorem

    I don't understand how the author get to these point. Please help me as i have been spending so much time trying to figure this out but to no avail. Thanks for your help Source: http://phys.columbia.edu/~nicolis/NewFiles/Noether_theorem.pdf
  42. A

    Noether's currents under dilatations (scaling transformations)

    Hello, 1. Homework Statement Suppose we have the following Lagrangian density, in ## 3 + 1## dimensions: $$L = \frac{1}{2}\partial_{\mu}\phi \partial^{\mu}\phi - g \phi^4$$ Under the dilatation (scaling transformation): ##x \rightarrow \lambda x^{\mu}, \phi (x) \rightarrow \lambda^{-1}...
  43. L

    Noether's theorem -- Time inversion

    Noether's theorem said that because of homogeneity in time the law of conservation of energy exists. I am bit of confused and I am not sure is also time inversion some consequence of this. For example in the case of free fall we have symmetry ## t \rightarrow -t##. I am sometimes confused of...
  44. davidbenari

    Relation of Noether's theorem and group theory

    I'm doing a small research project on group theory and its applications. The topic I wanted to investigate was Noether's theorem. I've only seen the easy proofs regarding translational symmetry, time symmetry and rotational symmetry (I'll post a link to illustrate what I mean by "the easy...
  45. P

    What is the relationship between dynamical symmetry and Noether's theorem?

    Hi, I am learning classical mechanics right now, Particularly Noether's theorem. What I understood was that those kinds of transformations under which the the Hamiltonian framework remains unchanged, were the key to finding constants of motion. But here are my Questions: 1. What is...
  46. sweet springs

    Noether's Theorem and the real world

    Hi. Noether's theorem comes from the symmetries of the world. In the real world the distribution of galaxies and materials are inhomogeneous. Noether's theorem does not stand for the real world, so conxervations of energy, momentum, angular momentum do not stand exactly. Is it OK...
  47. N

    Is Weak Isospin Conservation Provable Using Noether's Theorem?

    Is it possible to prove that weak isospin associated with SU(2) is conserved using Noether's theorem?
  48. C

    Noether's theorem and constructing conserved quantities

    Homework Statement A particle of mass m and charge e moving in a constant magnetic field B which points in the z-direction has Lagrangian ##L = (1/2) m( \dot{x}^2 + \dot{y}^2 + \dot{z}^2 ) + (eB/2c)(x\dot{y} − y\dot{x}). ## Show that the system is invariant under spatial displacement (in any...