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QED is fully relativistic. Historically it derived from second quantization of the Dirac equation. Recall that anti-matter came out of the Dirac equation for the electron. So did spin. In the Schroedinger/Heisenberg version of quantum mechanics neither of those things really exist. Spin was just thrown in ad hoc because it was known to be there. But you can't produce it from first principles without relativistic treatment. Anti-matter kinda surprised even Dirac. But he stuck to his guns. You can't really do pair-production, negative energy, or virtual particles in QED without a relativistic basis.

Nobody likes to remember "second quantization" anymore; it's like the Klingons in the original Star Trek. Kind of embarrassing to High Energy guys these days. I was never a High Energy Physicist. I just found it interesting. Naturally my adviser regarded courses beyond the candidacy exam not related to my field as a waste of time.

Gold. Now you are really testing my memory. But ... I do not think so. I seem to recall that if you actually do a wild-ass guess (that works surprisingly well, kind of like that neutron/proton stability model that comes from viewing the nucleus as a particle-in-a-box) you find that the inner electrons in all elements from the Lanthanides and beyond have enough energy that they have to be treated relativistically.

Gawd that is a long time ago... anyway, no, color in pure metals is a result of band theory. I believe for most metals, there are so many energy levels so close at the top of the conduction band that they radiate essentially continuously (thus appear silver, as most metals do.) Copper and Gold have some weirdness in the Fermi-levels near the top of their conduction bands that cut-off blue light.

I'll see if I can dig up a web page in a while; must help daughter with math.

Oh geez. You are 100% correct! I had forgotten that the Dirac equation leads to a completely different aufbau than the Schrodinger/Heisenberg one. In addition to the Lanthanide contraction caused by relativistic effects (at least I remembered that part correctly) in Au there is an additional contraction of the 6s orbital so the 5d -> 6s transition (into the conduction band) is so small it absorbs in the blue, rather than UV and higher as is the case of the bound electron orbitals in most metals. Have a go: http://voh.chem.ucla.edu/vohtar/fall02/classes/172/pdf/172rpint.pdf.

A more lay accessible treatment is here: http://www.fourmilab.ch/documents/golden_glow/. Which has some errors (the transition is 4d->5s in Ag, not Au) but omits the complexity of band theory altogether; and seems "correct enough."

Now, that still doesn't explain Cu, and I sure don't remember any more. No time tonight, though.

Thanks. That was fun.

QED is fully relativistic. Historically it derived from second quantization of the Dirac equation.

Feh. That's what I get for relying on Feynmans QED: The Strange Theory of Light and Matter (ISBN-10: 0691024170 | ISBN-13: 978-0691024172, $7.77 at Amazon.com) instead of having taken a real course in it.

As far as the Gold, I'll take your word for it: it was an offhand comment during a discussion section after a paper at a long-gone Theoretical Chemistry conference, but it was odd enough that it lodged in the esophagus of my mind.

Incidentally, speaking of the Dirac equation, doesn't it leave room for magnetic monopoles? John Van Vleck would be deeply saddened...

Cheers!