[no subject]

[Leigh Palmer <palmer@SFU.CA>]

> Charge density zero?  Yes. Mass density zero?  No.  Particles and
> antiparticles have positive mass.

The mass density is indeed zero in a vacuum; that's the mass
density of virtual particles.

> A spherical (isotropic) distribution of *real* particles, as observed
> by one stationary with respect to the particles' center of mass, will
> be contracted (anisotropic) along the direction of relative motion, as
> observed by one moving with respect to the particles' center of mass.
> Why would it be different for a distribution of virtual particles?

Then you would claim that the mass density of a homogeneous
medium is anisotropic in the frame of someone moving with
respect to it. Please explain to me what observable quantity
is associated with an anisotropic mass density.

Leigh