Friday, December 9, 2016

S. Weinberg, “The Quantum theory of fields: Foundations” (1995), Eq. 9.2.15


In Weinberg's book The Quantum Theory of Fields, Volume 1 on p.388 (Chapter 9), the following identity is used (with f being any "reasonable" function):


f(+)+f()=lim



I don't understand the identity in a qualitative / heuristic way.



Answer



I can understand the identity qualitatively now:


If the function f(t) has a well defined value at t = \pm\infty, then for large values of |t| the function f is essentially constant with values f(-\infty) for t < 0 and f(+\infty) for t > 0.


For tiny \epsilon the Exponential factor e^{-\epsilon |t|} is essentially equal to 1 for |t| < 1 / \epsilon , and almost Zero for large |t| > 1 / \epsilon.


We then have approximately: \epsilon\int_{-\infty}^\infty \mathrm{d}t \, f(t) e^{-\epsilon |t|} \approx \epsilon\int_{1 / \epsilon}^0 \mathrm{d}t \, f(-\infty) + \epsilon\int_0^{1 / \epsilon} \mathrm{d}t \, f(+\infty) = f(-\infty) + f(+\infty)


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