text: In the causal theory of waves, permittivity is a complex quantity. The imaginary part corresponds to a phase shift of the polarization  relative to  and leads to the attenuation of electromagnetic waves passing through the medium.  By definition, the linear relative permittivity of vacuum is equal to 1, that is , although there are theoretical nonlinear quantum effects in vacuum that become non-negligible at high field strengths.
  The following table gives some typical values.
  The relative low frequency permittivity of ice is ~96 at −10.8 °C, falling to 3.15 at high frequency, which is independent of temperature.  It remains in the range 3.12–3.19 for frequencies between about 1 MHz and the far infrared region.