Drop a stone in the pond...a wave propagates radially from the source. The conservation of energy says the wave must decay proportionally to the radial distance. If I drop a steel I-beam in the pond, the same concept holds, only it can be considered a finite linear array of point sources. If the energy must still decay radially, then how do plane waves ever form?
I know we can generate them in labs and obviously express the math, but in nature, do plane waves really exist?
OK...so let's assume it was an infinitely long I-beam. Then I guess you could argue you get plane waves. But look at the assumptions: 1) infinite I-beams don't exist (nothing is infinite) and 2) you have to assume there are ZERO irregularities in the I-beam. These concepts are purely mathematical.
OK...OK....you're "zooming in" on the radial wave and it appears like a plane wave. But do you account for the fact that the wave must always be decaying in time (assume there is no external dissipation involved)?
Restating the main question...do plane waves really exist in nature? If they don't then why do they show up in theories? (Good example is plane waves hitting an open slit for diffraction studies...you started with something that is conceptual, not physical?)