(TheScientificTruth) writes:

I understand that most of our mass (3599/3600)comes from neutrons and
protons. I also understand that nucleons are made from up and down
quarks. I even understand that only a small percentage of the mass of
the nucleons come from the quarks, while the rest comes from the force
field that holds them together. But I have several questions:

1. Does this imply that most of our mass (85%) is made of a force
field and not matter?

The fraction is considerably more than 85%, and it mostly consists of
quark kinetic energy, not "force fields." The `u' and `d' quarks have quite
low masses compared to a proton mass; our best estimates are that both of
them probably mass less than 10 MeV, or less than 1% of the nucleon mass!
(Note that our "best" is still not very good in this case --- the `u'
and `d' masses are not know to even a single significant digit!
http://pdg.lbl.gov/2003/q123.ps, http://pdg.lbl.gov/2003/q123.pdf)
If you work out the compton wavelength corresponding to these masses,
you'll find that it is on the order of several tens of fermi, which is
_MUCH_ larger than a proton's radius, which is only on the order of a fermi;
likewise (and essentially equivalently), if you work out the uncertainty
in the momentum of a quark localized inside a proton, you'll find that
it is over an order of magnitude larger than m_q*c. Both these preceding
observations imply that the `u' and `d' quarks are "rattling around" inside
a nucleon at ultra-relativistic velocities --- i.e., their kinetic energies
are more than an order of magnitude larger than their rest masses.


2. Is it gluons that make up this force field and thus the energy of
the gluons that accounts for the 85% of our mass?

No. See my response to your preceding question.


Please explain this strong nuclear force field in detail.

A _detailed_ description would require a textbook-length post covering
the entire theory of QCD --- and frankly I have better things to do
with my time than write Yet Another Textbook On QCD! :-T


How do sea quarks and mesons (which I think are exchanged by the nucleons
to hold them together) play into this?

Nucleons do not "exchange" sea quarks. To leading order, they exchange
a gluon, then do a one-for-one swap of a "valence" quark, and finally
exchange another gluon to balance the energy, momentum, and "color" books;
this process is to leading order equivalent to the exchange of a pion.


3. My final question has more to do with biophysics (and nothing to do
with the above), but I think it is appropriate to ask it here. During
an exothermic chemical reaction, a very tiny amount of rest mass is
converted to energy (supposedly). Where does this rest mass come from
that turns into energy?

From the binding energy of the electrons in the molecules. The electrons
in the reaction products are more tightly bound (i.e., in a state of lower
total energy) than the electrons in the reactants; the difference in binding
energy between the reactants and the product is released as "reaction heat."


I mean, the rest mass of the individual particles has to be the same
right?

Yes --- however, in relativistic physics, rest mass is not "additive,"
i.e., the rest mass of a composite object is =NOT= necessarily the sum
of the rest masses of its constituents!


So is it the rest mass of the atoms or molecules that is decreasing?

Yes. The sum of the rest mass of the reaction products is microscopically less
than the sum of the rest masses of the reactants. (Since typical atoms have
rest masses on the order of `A' GeV, where `A' is the "molecular weight,"
whereas typical chemical energies are on the order of an eV or so, the
change in rest mass during a chemical reaction is typically on the order
of a part per billion or so --- which is very small, but theoretically
measurable.)


-- Gordon D. Pusch

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