The discussion clarifies the roles of the S, U, and T channels in particle interactions, emphasizing that all channels contribute to a process if they are possible. The physical significance of these channels is equivalent to that of the S channel, and it is impossible to definitively attribute an interaction to a specific channel unless all other channels are rendered impossible. Additionally, the conversation highlights the relevance of loop diagrams in understanding these interactions.
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And not even then. There are also loop diagrams.mfb said:They all contribute if (!) they can happen for a given process. Their "physical significance" is the same as for the s channel. You cannot point to an observed process and say "it happened via this channel!" unless all but one channel are impossible.