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Usual Schrodinger lagrangian is $ i(\psi^{*}\partial_{t}\psi ) + \frac{1}{2m} \psi^{*}(\nabla^2)\psi $. It gives correct equation of motion, with conjugate momentum for $\psi^{*}$ vanishing. This lagrangian density is not real but differs from real Lagrangian density $ \frac{i}{2}(\psi^{*}\partial_{t}\psi -\psi \partial_{t}\psi^{*} ) + \frac{1}{2m} \psi^{*}(\nabla^2)\psi $ by a total derivative. My trouble is that these two lagrangian densities lead to different conjugate momenta and hence when setting equal time commutation relations, I am getting different results (a factor of 2 is causing the problem). I can rescale the fields but then my hamiltonian also changes. Then applying Heisenberg equation of motion, I don't get the operator schrodinger equation.

Is it possible to work with real lagrangian density and somehow get the correct commutation relations? I would have expected two lagrangians differing by total derivative terms to give identical commutation relations (since canonical transformations preserve them). But perhaps I am making some very simple error. Unless all conjugate momenta are equivalent for two lagrangians differing by total derivatives, how does one choose the correct one?

I guess same thing happens for other first order systems like Dirac lagrangian also.

I am learning QFT for the first time and so I would appreciate any help!

thanks a lot

Ankit

*Thanks a million to Qmechanic for very detailed answer. It makes perfect sense.

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I don't have time for a detailed reply to your question, but it may help to have a look at the end of Sec. 7.2 in Weinberg's textbook (vol. 1). He discusses the effect of adding a total time derivative to the Lagrangian and shows that while modifying the canonical momentum, it does not affect the commutation relations of the theory. –  Tomáš Brauner Sep 30 '11 at 10:17
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1 Answer 1

Here we will for simplicity only consider the Schrödinger system. We will assume that

$$\phi~=~(\phi^1+i\phi^2)/\sqrt{2}$$

is a bosonic complex field, and that

$$\phi^*~=~(\phi^1-i\phi^2)/\sqrt{2}$$

is the complex conjugate, where $\phi^a$ are the two real component fields, $a=1,2$. [Note the change in notation $\psi\longrightarrow\phi$ as compared with the OP's question(v1).]

1) The Lagrangian density

$${\cal L}~:=~ i\phi^{*}\dot{\phi} + \frac{1}{2m} \phi^* \nabla^2\phi$$

for the Schrödinger field $\phi$ is already on the Hamiltonian form

$${\cal L}~=~ \pi\dot{\phi} - {\cal H}. $$

Simply define complex momentum

$$\pi~:=~i \phi^{*}, $$

and Hamiltonian density

$${\cal H}~:=~-\frac{1}{2m} \phi^* \nabla^2\phi.$$

More generally, this identification is a simple example of the Faddeev-Jackiw method.

2) Recall that the classical equations of motion do not change by adding a $4$-divergence $d_{\mu}\Lambda^{\mu}$ to the Lagrangian density

$${\cal L} ~~\longrightarrow~~ {\cal L}'~:=~{\cal L} + d_{\mu}\Lambda^{\mu}.$$

[We use the symbol $d_{\mu}$ (rather than $\partial_{\mu}$) to stress the fact that the derivative $d_{\mu}$ is a total derivative, which involves both implicit differentiation through the field variables $\phi^a(x)$, and explicit differentiation wrt. $x^{\mu}$.] Therefore, we can (via spatial integration by part) choose an equivalent Hamiltonian density

$${\cal H} ~~\longrightarrow~~ {\cal H}'~:=~\frac{1}{2m}|\nabla\phi|^2 ~=~\frac{1}{4m}(\nabla\phi^1)^2 +\frac{1}{4m}(\nabla\phi^2)^2,$$

and we can (via temporal integrations by part) choose an equivalent kinetic term

$$i\phi^*\dot{\phi}~=~ \pi\dot{\phi} ~~\longrightarrow~~$$ $$\frac{1}{2}(\pi\dot{\phi}-\phi\dot{\pi}) ~=~ \frac{i}{2}(\phi^*\dot{\phi}-\phi\dot{\phi}^*)~=~\frac{1}{2}(\phi^2\dot{\phi}^1-\phi^1\dot{\phi}^2)~~\longrightarrow~~\phi^2\dot{\phi}^1. $$

The last expression shows (in accordance with the Faddeev-Jackiw method) that

The second component $\phi^2$ is the momenta for the first component $\phi^1$. $\qquad$(1)

3) Alternatively, we can perform a Dirac-Bergmann analysis$^1$ directly. Consider for instance the Lagrangian density

$${\cal L}'~=~ (\alpha+\frac{1}{2})\phi^2\dot{\phi}^1+(\alpha-\frac{1}{2})\phi^1\dot{\phi}^2 - {\cal H}', $$

where $\alpha$ is an arbitrary real number. [The term $d(\phi^1\phi^2)/ dt$, which is multiplied by $\alpha$ in ${\cal L}'$, is a total time derivative.] Let us check that the quantization procedure does not depend on this parameter $\alpha$. We introduce canonical Poisson brackets

$$ \{\phi^a({\bf x},t),\phi^b({\bf y},t)\}_{PB} ~=~0, $$ $$ \{\phi^a({\bf x},t),\pi_b({\bf y},t)\}_{PB} ~=~\delta^a_b ~ \delta^3 ({\bf x}-{\bf y}), $$ $$ \{\pi_a({\bf x},t),\pi_b({\bf y},t)\}_{PB} ~=~0, $$

in the standard way. The canonical momenta $\pi_a$ are defined as

$$ \pi_1~:=~\frac{\partial {\cal L}'}{\partial \dot{\phi}_1}~=~(\alpha+\frac{1}{2})\phi^2,$$ $$ \pi_2~:=~\frac{\partial {\cal L}'}{\partial \dot{\phi}_2}~=~(\alpha-\frac{1}{2})\phi^1.$$

These two definitions produce two primary constraints

$$\chi_1~:=~\pi_1-(\alpha+\frac{1}{2})\phi^2~\approx~0,$$ $$\chi_2~:=~\pi_2-(\alpha-\frac{1}{2})\phi^1~\approx~0,$$

where the $\approx$ sign means equal modulo constraints. The two constraints are of second-class, because

$$ \{\chi_2({\bf x},t),\chi_1({\bf y},t)\}_{PB}~=~\delta^3 ({\bf x}-{\bf y})~\neq~0. \qquad(2) $$

Thus the Poisson bracket should be replaced by the Dirac bracket. [There are no secondary constraints, because

$$ \dot{\chi}_a({\bf x},t) ~=~\{\chi_a({\bf x},t), H'(t)\}_{DB} ~=~ 0, \qquad H'(t)~:=~ \int d^3y \ {\cal H}'({\bf y},t), $$

are automatically satisfied.] The Dirac bracket between the two $\phi^a$'s is

$$\{\phi^1({\bf x},t),\phi^2({\bf y},t)\}_{DB}~=~\delta^3 ({\bf x}-{\bf y}), \qquad(3)$$

leading to the same conclusion (1) as the Faddeev-Jackiw method. Note that the eqs. (2) and (3) are independent of the parameter $\alpha$.

4) In all cases, the canonical equal-time commutator relations for the corresponding operators become

$$ [\hat{\phi}^1({\bf x},t), \hat{\phi}^2({\bf y},t)] ~=~ i\delta^3 ({\bf x}-{\bf y}), $$ $$ [\hat{\phi}({\bf x},t), \hat{\phi}^*({\bf y},t)] ~=~ \delta^3 ({\bf x}-{\bf y}), $$ $$ [\hat{\phi}({\bf x},t), \hat{\pi}({\bf y},t)] ~=~ i\delta^3 ({\bf x}-{\bf y}). $$

--

$^1$: See, e.g., M. Henneaux and C. Teitelboim, Quantization of Gauge Systems, 1992.

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Thanks a lot to Qmechanic for very detailed answer. I really appreciate the help. –  user5468 Oct 8 '11 at 17:39
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