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Re: [Phys-l] Rest mass again?

What is the gravitational effect of a mass in motion?

Arnulfo Castellanos Moreno

-----Mensaje original-----
[] En nombre de Moses
Enviado el: Viernes, 22 de Octubre de 2010 09:04 a.m.
Para: Forum for Physics Educators
Asunto: Re: [Phys-l] Rest mass again?

Derek McKenzie wrote on Wed, October 20, 2010 6:42:13 PM

The term 'rest mass' seems to be quite misleading to me because although we
choose to view a lump of matter as 'at rest' in a particular frame, it >is
up of many moving constituents in that same frame...

Yes, so what? A "particular frame" is unambiguously defined as a frame in
the net momentum of a system is zero. It is a rest frame of a system.
What is misleading about it?
Similarly, the rest (invariant) mass is defined as the norm of a
4-momentum of
a system. Applying this to the rest frame of the system where the system's
3-momentum, by above definition, is zero, you get the rest mass. And since
does not matter what system you use for calculating an invariant
the invariant mass and the rest mass are synonyms. What is so misleading

>...and what one person calls 'rest mass' could equally well be called
sum of relativistic masses of its constituents.

Yes, so what? If we have one photon, we usually call it massless. I do
it myself, unfortunately, just to pay tribute to what is "customary". But if
insists that there is no mass other than the rest mass, then the term
for a photon becomes misleading, for a very simple reason. Add another
with the opposite momentum of the same magnitude. Now you have a system of
photons which (the system) has a rest frame and a non-zero rest mass. In my
view, it is more convenient to say that this rest mass comes from the
relativistic masses of constituents than to say that it is rest energy of
system divided by c^2. The first possibility avoids any necessity to explain
students (without invoking energy, which is a different property of an
object), why is each photon separately massless but the system of both is
The first possibility DOES NOT deny the second one. But those who discard
first one, should explain their students how come that energy/c^2 determines

mass in one special reference frame, but does not determine anything
in all other frames. This would single out that one frame as something
special, quite in the spirit of pre-Galilean physics, and totally against

Moses Fayngold,

Forum for Physics Educators

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