Quantum Electrodynamics is one of the most thorougly tested and universably applicable theories of modern physics. Almost everything we experience is mediated by charge/photon interactions, and QED is the theory which describes them.

You'd think it would give a valuable insight into what's "inside" an electron or a photon, but alas of this it says nothing.

In the 5th lecture in his "All of Physics" series, John WIlliamson shows us the extent and limits of QED. There's a surpise at the end too: with a minimal addition, perhaps there is some insight into what's inside the box after all...

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