Re: What Hath Einstein Writ?

[Leigh Palmer <palmer@SFU.CA>]

> I had thought of mass as a constant in equations like
> F=ma, where mass represents a resistance to
> acceleration.  If moving objects have a higher
> resistance to acceleration, it seems that they have a
> higher mass.  I think the iceberg story makes sense.
> An object at a higher temperature has more mass than
> the same object at a lower temperature.

That's the way I would have preferred it, but there
are two problems. I like the second law definition too
(with the exception that I write it as F = dp/dt), but
there is no test of this law on macroscopic objects.
The rest mass convention has problems when one applies
it to macroscopic objects. What is the rest mass of an
iceberg before and after melting? Presumably this is a
question which can be answered *in principle* given
whatever definition one wishes to use, but it cannot
be answered in practice. I consider that a serious
problem, at least with the empirical approach to
science that I take with my students. It is also clear
to me that when one takes the rest masses of the
constituent particles which make up the iceberg (or
even of a nucleus in it) then the rest mass is not a
conserved quantity. The relativistic mass is conserved
in some sense.

I do not find it difficult to live with what I consider
to be the "new" convention, but I have adopted the term
"proper mass", removing the implicit inertial meaning
by placing the observer in the rest frame of the mass
in question. We do have the option to tell our students
when the textbook is wrong or misleading, or when it is
out of step with the rest of the world. In doing so we
help prepare them for life.

Leigh