A Derivation of QM limit of QFT in "QFT and the SM" by Schwartz

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The discussion focuses on the derivation of the quantum mechanical limit of quantum field theory (QFT) as presented in Schwartz's "QFT and the SM." It explains how the one-particle states are expressed using the field operator, leading to the formulation of the Schrödinger picture wavefunction. The key point of confusion is why the time derivative is applied to the field operator ##\phi## instead of the state vector ##\psi##. This is clarified by noting that the equation is framed in the Heisenberg picture, where observables, such as the field, evolve over time while the state vector remains static. Understanding this distinction is crucial for grasping the dynamics of QFT.
Hill
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How the time derivative moved into the braket?
In this derivation, a basis of one-particle states ##\langle x|=\langle \vec x,t|## is expressed with the field operator, $$\langle x|=\langle 0| \phi (\vec x, t)$$
"Then, a Schrodinger picture wavefunction is $$\psi (x)=\langle x| \psi \rangle$$
which satisfies $$i \partial _t \psi (x) = i \partial _t \langle 0| \phi (\vec x, t)|\psi \rangle = i \langle 0| \partial _t \phi (\vec x, t)| \psi \rangle$$

I need help to understand why the time derivative is applied to the field ##\phi## and not to the state vector ##\psi##.
 
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Hill said:
I need help to understand why the time derivative is applied to the field ##\phi## and not to the state vector ##\psi##.
Because the equation is written in the Heisenberg picture, where observables depend on time and state does not depend on time.
 
Demystifier said:
Because the equation is written in the Heisenberg picture, where observables depend on time and state does not depend on time.
Thank you.
 
For the quantum state ##|l,m\rangle= |2,0\rangle## the z-component of angular momentum is zero and ##|L^2|=6 \hbar^2##. According to uncertainty it is impossible to determine the values of ##L_x, L_y, L_z## simultaneously. However, we know that ##L_x## and ## L_y##, like ##L_z##, get the values ##(-2,-1,0,1,2) \hbar##. In other words, for the state ##|2,0\rangle## we have ##\vec{L}=(L_x, L_y,0)## with ##L_x## and ## L_y## one of the values ##(-2,-1,0,1,2) \hbar##. But none of these...

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