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Re: [Phys-l] Rest mass again?

On 10/22/2010 01:53 PM, Dr. Arnulfo Castellanos Moreno wrote:

What is the gravitational effect of a mass in motion?

That's an innnteresting question. The answer is complicated.

Here's a hand-wavy sketch:

By way of analogy: In a static situation, electric charge is the
source-term for the electric field in accordance with Coulomb's

Similarly, in a static situation, mass is the source-term for
the gravitational field in accordance with Newton's law of
universal gravitation.

If you have a charge in steady motion (relative to some frame) things
are more complicated. There will necessarily be a magnetic field
(in that frame) in addition to the aforementioned electric field.

Similarly, if you have a mass in steady motion, there will be
post-Newtonian terms in the gravitational equations, terms
proportional to velocity (in the chosen frame).

If you have an accelerated charge, it will radiate EM waves

Similarly, if you have an accelerated mass, it will radiate
gravitational waves.


If you want the details on all this, you are going to need a book on
general relativity. It's not something that can be explained by email.


On 10/22/2010 04:35 PM, Jeffrey Schnick wrote:

It is my understanding that the sum of the rest masses of 2 up quarks
and a down quark is far less than the mass of a proton and the sum of
the rest masses of 2 down quarks and an up quark is far less than a
neutron so that most of the mass of a proton is from the motion of the
quarks and likewise for a neutron.

The motion is nowhere near enough to explain the observed mass.
It can't be, because the KE is necessarily less than the binding
well depth; otherwise the thing would fly apart.

If you want to know how much mass a quark has when it's by itself,
you are out of luck, because of quark confinement. There is no
such thing as a quark by itself. Consequently, the fundamental
definition of quark mass is what we call "model dependent" and
the choice of model is somewhat arbitrary. This is absolutely
not a nitpick; models have dramatically changed recently, so
opinions about quark masses have changed by orders of magnitude.
On the other hand, this doesn't change the physics; it just
rearranges and renames some of the terms in the equations.

In any case, you can't talk about "the" mass of the quark without
also talking about the mass in the associated gluon field. To get
an idea of how this works, see the excellent discussion of "electro-
magnetic mass" in the Feynman lectures (and replace electrodynamics
with chromodynamics).