Re: Increasing Mass of Particles

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Mon, 23 Feb 1998, Lowell Herr wrote:

> 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?

Lowell,

The density of a moving *object* certainly increases since its volume
decreases (due to Lorentz contraction) and its mass either stays the same
or increases (depending only on how you want to define mass.)  Since the
density is nothing more than a matter of definition, however, no
significance whatsoever should be attached to this fact. 

If on the other hand, you really mean to talk about a *particle*--i.e., an
entity with no spatial extent--then the density is infinite in any event
and the question is moot.

John
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