Today’s vexing aesthetic physics of handwavium problem:
INASMUCH as the energy levels and resulting orbitals of muon-proton atoms are completely different from those of electron-proton atoms –
WELL, obviously, or what would be the point in making muon metals in the first place –
AND INASMUCH as this makes muon-photon interactions differ remarkably from electron-photon interactions, thus changing radically the emission spectrum and other optical properties from their electronic equivalent –
WHAT do the blasted things look like?
(It is on those mornings when I find myself contemplating this before my first cup of coffee, inasmuch as said metals are a vitally important and visible component of a hypothetical fusion torch drive, that I have some sympathy for the technobabble approach to doing things. Somehow, I doubt the Star Trek writers ever had to deal with this sort of thing…)
Maybe Quora or one of the Stackoverflow sites can help?
Radiating in starkly coruscating ultraviolet? (Seriously, I have zero clue. But it might be that the high energy levels involved mean that they don’t absorb much visible-or-lower wavelenghts, per https://www.fourmilab.ch/documents/golden_glow/ .)
Apparently yes, according to the physicists and near-physicists I’ve got to weigh in on this one.
(Well, ultraviolet, anyway. Can’t speak as to the starkly —
Oh, wait, I can. Space opera, therefore definite stark coruscations incoming.)
Just don’t look at it. It will drive you mad. It will sear out your retinas and cause massive seizures. Some things aren’t meant to be looked upon! Actually the question is who is looking at it? I would be rather surprised if the Eldrae are trichromatic like humans or even tetrachromatic like some animals. Some could even be dodecachromatic like mantis shrimp just for shear aesthetics. So I say handwave the handwave and create unique color names in the Eldrae tongue for what it looks like and just say humans would see a shiny black substance.
Ah, but in this case I need to know for cover art purposes, too, so the human eyeball response is relevant.
(Of course, it’ll appear rather more spectacular to the eldrae, whose canonical chromatic range lets them see ivén and serís (low and high UV, respectively) with the Mark I Eyeball, radiating in the UV as it will.
On the other hand, a running fusion torch would be radiating enough UV, pretty much, to give anyone close a fatal sun-tan and never mind the actual ionizing radiation, so it’s not like there are that many onlookers…)
I would assume the appearance depends on the muon-element equivalents of atomic number and phase state. The exact muon-element (or alloy) chosen for a task will depend on the properties desired. So, I expect you can just pick a look that is reasonable for the task at hand, or at least doesn’t actively conflict….if it did, they wouldn’t use it or would put a shell on it.
Probably it just looks silvery, if by “metal” you mean the following: a substance consisting of positively charged nuclei surrounded by negatively-charged particles, at least some of which are free to wander throughout the material, and thus possess sufficient degrees of freedom to couple with photons of arbitrary wavelength. Alternatively, it might be graphene black if it absorbs rather than reemits photons.
As to whether there might be muonic analogs to how copper and gold look reddish-yellow due to details of their electron shells, that’s way beyond my pay grade.