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Re: [Phys-l] Mass and Energy



Hi Hugh-
Sorry, but the physics of gquarks and gluons is considerably more complicated than that. I don't want to get into great detail, ut the notion "kinetic energy of a quark or gluon inside a proton" may not have much meaning. The trouble is that neither a quark nor gluon has any existence outside of a proton (or other hadron) so the notion of mass is hard to pin down. (For ordinary particles we make our mass determinations - physically and conceptually - in terms of the isolated particle). There is no such thing as an isolated quark or gluon.
The "glue" that holds a quark inside a proton is not very stretcheable. Think of the quark as moving at times to the boundary of the proton and stretching the "glue". When this happens, the energy is in the stretch. So a model of what is going on is a harmonic oscillator that is continuosly transferring its energy back and forth between kinetic and potential.
Regards,
Jack



On Thu, 25 May 2006, Hugh Haskell wrote:

At 13:32 +0300 5/25/06, Savinainen Antti wrote:

Hugh wrote:

"The difference, originally quaintly called the "mass defect," is,
of course, the "binding energy" that serves to hold the nucleus
together. ... So where does the "binding energy" that holds these
particles together come from?"

I'm a bit confused here. I have thought that the binding energy
is the energy that a system *gives away* when bonds are formed;
the bonding could be chemical, nuclear or whatever. The mass of
the system decreases because part of the initial rest energy
('the mass defect') is transported to the surroundings or
manifests itself as kinetic energy of the products. Isn't this
the whole idea behind extracting nuclear power?

So I wouldn't like to talk about the binding energy as if it
were present in the system to keep it together. Should I correct
my view?

You are correct. I was sloppy in my speaking. When two objects are
bound together, their total energy is less than when they are
separate. But that clearly implies that the mass of "free objects" is
greater than their combined mass when they are joined together with
that excess mass being converted to energy which is emitted during
the formation. In the case of the quarks that make up the proton or
neutron, their combined mass is considerably less than the their
combined mass. What I was trying (not very well) to say, was where
does this mass come from? We know that in "ordinary particles" that
energy is given up by converting some of the mass of the
constituents. But in the case of protons and neutrons, the energy
that "creates" the mass that the protons and neutrons show us has to
come from somewhere, and it seems that it must be the kinetic energy
of the quarks and gluons that are "contained" within the composite
particle.

I hope that'a a little clearer than my original wording.

Hugh


--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley