I just picked up a book on QFT and have already found myself stuck on something that appears to be quite elementary. When disturbing the field, the potential gets an added term with a source function [itex]J[/itex],(adsbygoogle = window.adsbygoogle || []).push({});

[tex]J(x, t) \phi(x).[/tex]

I'm not quite getting what [itex]J[/itex] physically represents. It 'describes how the field is being disturbed', but how exactly? Also, why is the added term necessarily linear in [itex]\phi[/itex]?

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# [QFT] Source function

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