I am currently reading Cohen-Tannoudji's Quantum Mechanics. In the book on page 23 there is a comment that states;(adsbygoogle = window.adsbygoogle || []).push({});

"A plane wave of type Ae^i(kx-wt) , whose modulus is constant throughout all space [cf. |ψ|^2=|A|^2], is not square integrable. Therefore, rigorously,it cannot represent a physical state of the particle (in the same way as, in optics, a plane monochromatic wave is not physically realizable). On the other hand, a superposition of plane waves can be square integrable."

My understanding of what the author is trying to get across is that a wave such as

A*cos(kx-wt) where A=1 integrating from -∞ to ∞ is not finite, fine. but I DON'T understand the optics comment that "a plane monochromatic wave is not physically realizable". I have not taken very much optics or electromagnetic theory, so I have not come across this phenomena. Can someone please explain this to me, or foward me to some reference that will describe trying to physically realize a monochromatic plane wave? thanks

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# Plane monochromatic wave is not physically realizable?

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