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

In summary, "Derivation of QM limit of QFT" in "QFT and the SM" by Schwartz outlines the transition from quantum field theory (QFT) to quantum mechanics (QM) by examining how QFT reduces to QM in appropriate limits. It focuses on the conditions under which field operators behave like quantum mechanical observables, emphasizing the role of particle states and the significance of the vacuum state. The derivation illustrates the mathematical framework that connects the two theories, highlighting the implications for understanding fundamental particles and their interactions within the Standard Model.
  • #1
Hill
717
567
TL;DR Summary
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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  • #2
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.
 
  • #3
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.
 

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

What is the main goal of deriving the QM limit of QFT in Schwartz's "QFT and the SM"?

The main goal is to demonstrate how quantum mechanics (QM) emerges as a low-energy, non-relativistic limit of quantum field theory (QFT). This derivation helps to bridge the gap between the more fundamental QFT, which describes particles and fields at high energies, and QM, which is an effective theory at lower energies.

How does Schwartz approach the derivation of the QM limit from QFT?

Schwartz approaches the derivation by starting with the QFT formalism and systematically taking the non-relativistic limit. This involves focusing on the low-energy excitations and using techniques such as the path integral formalism, effective field theory, and integrating out high-energy modes to show how the Schrödinger equation and other QM phenomena emerge from the underlying QFT description.

What role do effective field theories play in the derivation?

Effective field theories play a crucial role in the derivation by allowing the separation of high-energy and low-energy phenomena. They provide a framework where the high-energy degrees of freedom are integrated out, leading to a simpler theory that describes the low-energy behavior accurately. This simplification is key to showing how QM can be seen as an effective description of the low-energy limit of QFT.

Why is it important to understand the QM limit of QFT?

Understanding the QM limit of QFT is important because it provides a deeper insight into the relationship between two fundamental theories of physics. It helps to clarify how the principles and equations of QM arise from a more comprehensive theory and ensures consistency between the descriptions of physical phenomena across different energy scales. This understanding is also crucial for developing new theories that unify or extend current models.

What are some key concepts or techniques used in Schwartz's derivation?

Some key concepts and techniques used in Schwartz's derivation include the path integral formulation, the concept of integrating out high-energy modes, the use of effective field theories, and the application of non-relativistic approximations. These methods collectively allow for a systematic reduction of QFT to QM by focusing on the relevant degrees of freedom and energy scales.

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