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I've heard it said that the vacuum permittivity is the lowest possible permittivity for any substance, that no substance can have $ε_r<1$. Is this true? If so, why is this different from permeability, where common materials like copper and graphite have $μ_r<1$? Is there some connection to the nonexistence of magnetic monopoles, which is the only other asymmetry I'm aware of between electrical and magnetic phenomena?

Edit: So it seems it is possible to have $ε_r<1$ for sufficiently high frequencies. I'm not considering high-frequency effects here; I'm asking specifically and only about electric permittivity at DC.

I would be interested to know what causes it to be frequency-dependent, but that's for another question.

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    $\begingroup$ Similar: physics.stackexchange.com/q/411248 $\endgroup$
    – jinawee
    Commented Apr 6, 2019 at 17:07
  • $\begingroup$ @jinawee That actually does answer my "can it happen" question. I wasn't considering optical frequencies as the context of this was thinking about capacitor dielectrics, which certainly won't be experiencing optical frequency signals as part of normal operation. $\endgroup$
    – Hearth
    Commented Apr 6, 2019 at 17:13
  • $\begingroup$ Check out researchgate.net/post/… $\endgroup$
    – jim
    Commented Apr 6, 2019 at 18:03
  • $\begingroup$ It appears to me that all the materials with $\epsilon$r less than one are conductive. I am pretty sure there are no practical dielectric materials with $\epsilon$r less than 1. Is your interest only in dielectric materials, or does it also extend to conductive materials? Are you wanting to build a capacitor or something? Or is this just a quest for knowledge? $\endgroup$
    – mkeith
    Commented Apr 6, 2019 at 19:14

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Plasma can have permittivity less than 1 (even negative:-)) at low frequency (http://farside.ph.utexas.edu/teaching/em/lectures/node100.html)

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  • $\begingroup$ What does low-frequency mean in this context? Is it a specific range or is it all the way down to DC? $\endgroup$
    – Hearth
    Commented Apr 6, 2019 at 21:55

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