Is Feynman's QFT Unitarity Issue Resolved?

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The discussion centers on Richard Feynman's unitarity issue in quantum field theory (QFT) as presented in his Nobel Prize speech. Feynman noted that replacing the delta function with a function of width a2 led to calculations that did not preserve unitarity, meaning the sum of probabilities was not equal to one. However, it was concluded that modern renormalized perturbation theory has resolved this issue, ensuring that unitarity is preserved in contemporary QFT calculations.

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From Richard Feynman's Nobel Prize Speech.

It must be clearly understood that in all this work, I was representing the conventional electrodynamics with retarded interaction, and not my half-advanced and half-retarded theory corresponding to (1). I merely use (1) to guess at forms. And, one of the forms I guessed at corresponded to changing d [delta function] to a function f of width a2, so that I could calculate finite results for all of the problems. This brings me to the second thing that was missing when I published the paper, an unresolved difficulty. With d replaced by f the calculations would give results which were not "unitary", that is, for which the sum of the probabilities of all alternatives was not unity. The deviation from unity was very small, in practice, if a was very small. In the limit that I took a very tiny, it might not make any difference. And, so the process of the renormalization could be made, you could calculate everything in terms of the experimental mass and then take the limit and the apparent difficulty that the unitary is violated temporarily seems to disappear. I was unable to demonstrate that, as a matter of fact, it does.​

Does anyone know whether this difficulty has been resolved?
 
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PatrickPowers said:
Does anyone know whether this difficulty has been resolved?
Yes, a long time ago. Modern renormalised perturbation theory has been shown to
preserve unitarity.
 

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