Peskin and Schroeder, equation 2.51

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SUMMARY

The discussion centers on the justification of the approximation D(x-y) ≈ e^{-i m t} as t approaches infinity, derived from the integral expression ∫ dE √(E² - m²) e^{-i E t}. The user explores discretization techniques and attempts to manipulate the expression G: (E² - m²)^{3/2} e^{-i E t} into a differential equation involving D(x-y). The method of stationary phase is highlighted as a necessary approach for resolving the issues presented in the derivation.

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  • Understanding of quantum field theory concepts, specifically propagators.
  • Familiarity with the method of stationary phase approximation.
  • Knowledge of complex analysis, particularly in evaluating integrals.
  • Proficiency in differential equations and their applications in physics.
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  • Study the method of stationary phase approximation in detail.
  • Review quantum field theory textbooks focusing on propagators and their derivations.
  • Learn about discretization techniques in quantum mechanics.
  • Explore differential equations related to quantum mechanics and their solutions.
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This discussion is beneficial for theoretical physicists, graduate students in quantum field theory, and anyone engaged in advanced mathematical physics, particularly those working with propagators and integral approximations.

Chris.X
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Hi

I am struggling to justify

D(x-y) \approx e^{-i m t} as t \rightarrow \infty

from

\int dE \sqrt{E^2-m^2} e^{-i E t}.

I thought I might get some insight from discretizing, as

e^{-i m t} \sum_{n=0}^{\infty} \epsilon \sqrt{ n \epsilon ( 2 m + n \epsilon ) } e^{-i n \epsilon t}

but I don't understand how to approximate or take the limit of the sum.

I also tried to work backwards from

G : (E^2-m^2)^{3/2} e^{-i E t}

and replacing extra E's with d/dt's and ended up with a differential equation
for the result,

G = i [ t (d^2/dt^2 + m^2) + 3 d/dt ] D(x-y)

but I am having trouble putting the pieces together to solve it.

Assistance would be greatly appreciated.
 
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