Paul Martin wrote:
In article ,
Dave Farrance wrote:

This is not directed at any one person. Dave's message was merely the
most appropriate to hang my reply from.

Interesting discussion. Here's some food for thought:

If a sealed container does *not* allow particles to pass its walls but
*does* allow loss of energy via exchange of thermal radiation with its
outer environment, then a chemical reaction within that container could
increase the temperature within that container, and as it returns to its
original temperature it would lose energy and hence mass due to the
relativistic effect of slowing down the vibration (speed) of particles
within that container.

Thermal energy is based on motion, so is subject to special relativity.
Once the temperature returns to its initial value, the mass is the same
as initially.

On the other hand, consider potential energy. A mass at "rest" at
height does not mass any more or less when it's at "rest" having
produced work in descending. (We're neglecting the rotation of the
earth and other similar effects here, and assuming that there is no
measurable difference in the gravity gradient between the two points,
otherwise general relativity kicks in.)

What about a battery? It's a matter of definitions as to what you
consider an ideal battery to be. You *could* argue that an *ideal* battery
is sealed to prevent whole atoms from passing through its walls but is
allowed to exchange energy with its environment via more than one method.

Electrons go out; the same number of electrons come in, having done
work in the circuit. Net effect: no difference in mass.

But work is done accelerating them to get them to move.. if you like.

The mobile electrons thereby acquire extra mass.


Depending on the chemical reaction and the internal resistance of the
cell, there may be heat involved. That's a secondary issue. If you were
measuring the mass when experimenting, you'd allow the cell to return
to its initial temperature before taking the measurement.

Incidentally, good luck on measuring a 50kg car battery down to the
nearest nanogram.

well, exactly. Local variations in gravity like the Moon going by, would
be more measurable.

Nevertheless it happens. Its just that for all practical purposes it is
so slight as to be utterly irrelevant: hence all these claims that it
doesn't happen. Relativity says it MUST happen, maths shows that you
wont be able to measure it when it does.

The whole thrust of Relativity is that energy IS mass. If you take it
from a system, that system loses mass.