Sorry for reopening a closed thread. But I have exactly the same doubt as this guy: https://www.physicsforums.com/showthread.php?t=346730(adsbygoogle = window.adsbygoogle || []).push({});

And the answer doesn't actually answer his question. I do get delta(p+p'), but they just help me in getting [itex]a_{p}a_{-p}[/itex] and [itex]a_{p}^{\dagger}a_{-p}^{\dagger}[/itex] Which I don't know how to get rid off, and shouldn't be in the final answer.

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# Total momentum operator for free scalar field

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