Re: [Phys-l] velocity-dependent mass (or not)

[John Denker <jsd@av8n.com>]

On 06/23/2009 11:03 AM, David Craig wrote:

> > I'm astonished anyone is still talking about this, let alone teaching  
> > it.   (Having seen how egregious are the treatments of relativity in  
> > many of the standard "modern physics" textbooks, I do in fact know  
> > better than to be surprised.   But I am anyway.)
> >
> > I can't recall meeting ANY professional relativist who employs the  
> > concept of a velocity-dependent mass, with the very occasional  
> > exception of mention as a throwaway heuristic.  (I have seen one or  
> > two of the very old guard use it in public talks, though.)
> >
> > The bottom line is that the concept IS dead among those who actually  
> > do relativity for a living.

Quite so.

Dead for many, many decades.




On 06/27/2009 07:11 PM, moshfarlan@yahoo.com wrote:
>  .... As an extreme
> example, annihilation of positronium produces two photons, which,
> according to the "true religion", can only be named as massless
> particles. Then where does the rest mass of the system of these
> masless particles come from?  


It doesn't "come from" anywhere.  You shouldn't assume it needs to
"come from" anywhere.  Asking where it comes from has no physical 
significance, because mass is not conserved.  There is no reason 
why it should be.

By way of analogy, when I am standing I have no lap, but when I
sit down I have one.  Where did the lap come from?  That would 
be a perfectly reasonable question if laps were conserved.   But 
in fact the question has no physical significance, because laps 
are not conserved.  There is no reason why they should be.

  Here mass means the invariant mass, which is equal to the rest
  energy divided by c^2.  Energy is conserved.  Rest energy (by
  itself) is not conserved.  There is no reason why it should be.

  Keeping track of where things "come from" makes sense in the 
  context of conservative flow.  For details on this, see
    http://www.av8n.com/physics/conservative-flow.htm

I don't have a problem with non-conserved mass.  If you have a
problem with it, then, well, that's /your/ problem, and Henny
Youngman has the only solution.  There is plenty of evidence 
(including the "extreme" example cited above) to tell us that 
mass is not in fact conserved.

Mass is Lorentz invariant.  That means it is invariant with respect
to Lorentz transformations.  That does *not* mean it is invariant
with respect to all imaginable transformations (such as annihilation
reactions).

I apologize to the list members who think I am belaboring the obvious.

Also:  Let's keep in mind that velocity-dependent mass is not the only
dead idea that is haunting us.  In the same category are the Fitzgerald-
Lorentz contraction and time dilatation, i.e. rulers that cannot be
trusted and clocks that cannot be trusted.  All three of these stinking 
corpses need to be buried once and for all.

Relativity is not weird or paradoxical.  It is just the geometry and
trigonometry of spacetime.  The smart way to handle this has been known
since 1908, i.e. for 101 of the 104 years that relativity has existed.

Maybe in a few more decades, textbook writers will get a clue.