I want to check out my response to a student question related to the
increase in mass of a particle moving at a high speed. The student asked
if the density of the particle changes or if there is an actual change in
the volume of the particle. I told him that I thought there was no
evidence for a change in volume and that I was unsure about density.
Rather, the increased mass was tied up in the energy of the particle.
Would a few of you care to expand on this puzzle and I will send comments
on to this curious student?