An electromagnetic plane wave has an electric field and a magnetic field. Each component contributes(adsbygoogle = window.adsbygoogle || []).push({}); equallyto the energy density. Mathematically it is very straight forward to show this is true.

The question is, "Fundamentally, why is this true?" Again, I'm not looking for a derivation. There are no issues with the math, just interpreting the physics from the math. With such a beautiful symmetry there must be a fundamental reason why.

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# Electromagnetic wave energy density

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