[Phys-L] velocity-dependent mass, or not

[John Denker <jsd@av8n.com>]

On 07/21/2014 12:05 PM, Tom Sandin wrote:
> For example, "refuting" the concept of relativistic mass by use the
> supposed quote from Einstein to a journalist.

That's a tricky example.

If you read the E=mc^2 paper, there is no doubt that Einstein
intended m to represent the plain old mass, i.e. the invariant
mass, aka the rest mass. The issue of velocity-dependent
"relativistic" mass does not arise here.

Later Einstein wrote one (and only one, AFAICT) paper that
discussed velocity-dependent mass, or rather masses.  There 
are lots of different velocity-dependent masses that crop 
up if you go down that road ... which I do not recommend.
Really not.

After 1908, it took Einstein a while to figure out what
Minkowski had done ... but then he wised up and hopped on
the spacetime bandwagon.  All of his later work, notably
general relativity, depends on the spacetime point of view, 
including proper length, proper time, invariant mass, et 
cetera.

See e.g. the kinetic energy expression in the middle of 
the chalkboard here:
  http://coraifeartaigh.files.wordpress.com/2014/03/einsteinjpgjpg-4a389e85f92a0547.jpg

===============

In any case, history is one thing, and physics is another.

Even if -- hypothetically -- Einstein had endorsed velocity-
dependent mass with his dying breath, it wouldn't change
the physics.  There are excellent physics reasons for
execrating relativistic mass, time dilation, and Lorentz
contraction, and instead focusing attention on invariant
mass, proper time, and proper length.
  http://www.av8n.com/physics/spacetime-welcome.htm

Many introductory-level physics textbooks are about 100 years 
out of date, but evidence suggests things are moving in the
right direction, however slowly:
  Gary Oas,
  “On the abuse and use of relativistic mass”
  http://arxiv.org/abs/physics/0504110