Re: a relativity question

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

Hugh Haskell wrote:

> I agree with the concept of "invariant mass" WRT imputed mass changes
> which depend on the vantage point of the observer, but haven't we
> already given some status to the
> variable-mass-under-the-influence-of-external-fields camp when we
> talk about, and explain to our students, how the "mass defect"
> (terrible term) comes about in nuclei? I don't understand how the
> measurable difference in mass that occurs when one combines a proton
> and neutron into a deuteron, under the influence of the strong
> interaction, differs in concept from the unmeasurable difference
> (because it is many orders of magnitude too small to be detectable)
> in mass that occurs when an object is moved from a high elevation to
> a low one under the influence of the gravitational interaction.

I'm not sure I understand your point, but you seem to be suggesting
that there is a conflict of some sort between the idea of "invariant
mass" (or, for me, just "mass") and the idea that the mass of a
system should depend on how it is put together.  There isn't.  The
mass of the Earth depends on its structure just like that of an atom,
that of a nucleus, and that of any other complex system that has
interacting parts.  The mass of a system is "invariant" wrt changes
of reference frame, not wrt changes of structure.

Am I speaking to your point or have I missed it?

> Granted that in the case of the deuteron, we are not part of the
> system whose total energy is changing and that in the case of the
> gravitational change, we are, but how does that change outlook we
> take to mass being dependent upon an object's location within field
> that it responds to? Perhaps we can apportion the mass change among
> the two objects involved, which in the case of the earth and a box,
> would make the box's mass change even smaller than it already is by
> about 20 orders of magnitude, but two gravitationally interacting
> objects (a double star system, for example) ought to show, if we
> could determine it, the same sort of "mass defect."

Right.  For instance, the Earth has a gravitational mass defect on
the order of of GM^2/Rc^2 ~ 10^16 kg.  That is to say, if we took the
Earth apart and separated all of its bits of mass (against their
mutual gravitational attraction) by large distances, those bits of
mass would add up to somewhere around 10,000 trillion kg more mass
than the mass of the Earth.

--
John Mallinckrodt        mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.