I've been told recently that the vacuum permittivity, given a sufficiently strong electric field, is not a constant, as it can cause positron-electron pair to split out of the vacuum. 1) is this true? 2) if so, where do such pairs as the positron-electron come from in a vacuum? I did try to research this myself, but I wasn't able to find it. If there's a name for the phenomena, it would be helpful in searching.