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Why is local gauge invariance needed in qft? I read that is allows interactions whereas global gauge invariance does not but was given no reason.
Why it is needed we don't know :)Why is local gauge invariance needed in qft? I read that is allows interactions whereas global gauge invariance does not but was given no reason.
By itself gauge invariance does not guarantee renormalizability, right? I mean that there is no problem in writing down gauge invariant terms of arbitrarily high dimension that are therefore non-renormalizable.You can look at local gauge invariance in a number of ways (try Wiki for some detail), but I like to think of it as a postulate that happens to provide a class of field theories that satisfy experiment (the standard model).
One of the reasons its nice, is that it guarentees renormalizability of the ensuing quantum theory. It also allows you to vastly constrain the number of possible models you can write down into a small finite subclass (each of which has been studied to death and turn out to be important).