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In principle, then, quantum electrodynamies is the theory of all chemistry,
and of lie, ïf life is ultimately reduced to chemistry and therefore Just to physics
because chemistry is already reduced (the part of physics which is involved in
chemistry being already known). Purthermore, the same quantum electrodynam-
1cs, this pgreat thing, predicts a lot of new things. In the first place, it tells the
properties of very high-energy photons, gamma rays, etc. It predicted another
very remarkable thing: besides the electron, there should be another particle of
the same mass, but of opposite charge, called a poszron, and these two, coming
together, could annihilate each other with the emission of light or gamma rays.
(After all, light and gamma rays are all the same, they are just different points
on a frequency scale.) The generalization of this, that for each particle there is an
antiparticle, turns out to be true. In the case of electrons, the antiparticle has an-
other name——it is called a positron, but for most other particles, ¡t 1s called anti-so-
and-so, like antiproton or antineutron. In quantum electrodynamies, ÉuUo mumnbers
are put in and most of the other numbers in the world are supposed to come out.
'The two numbers that are put in are called the mass of the electron and the charge
of the electron. Actually, that is not quite true, for we have a whole set of numbers
for chemistry which tells how heavy the nuclei are. Thhat leads us to the next part.
2-4 Nuclei and particles
What are the nuclei made of, and how are they held together? It ¡is found
that the nuclei are held together by enormous forces. When these are released,
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the energy released is tremendous compared with chemical energy, in the same
ratio as the atomic bomb explosion is to a NT explosion, because, of course,
the atomie bomb has to do with changes inside the nucleus, while the explosion
of TNT has to do with the changes of the electrons on the outside of the atoms.
'The question is, what are the forces which hold the protons and neutrons together
in the nucleus? Just as the electrical interaction can be connected to a particle,
a photon, Yukawa suggested that the forces between neutrons and protons also
have a field of some kind, and that when this fñeld jiggles it behaves like a
particle. Thus there could be some other particles in the world besides protons
and neutrons, and he was able to deduce the properties of these particles from
the already known characteristics of nuclear forces. For example, he predicted
they should have a mass of two or three hundred times that of an electron; and lo
and behold, in cosmic rays there was discovered a particle of the right massl But
1t later turned out to be the wrong particle. It was called a -meson, or muon.
However, a little while later, in 1947 or 1948, another particle was found, the
7-meson, or pion, which satisied Yukawa/s criterion. Besides the proton and
the neutron, then, in order to get nuclear forces we must add the pion. Now,
you say, “Oh greatl, with this theory we make quantum nucleodynamics using
the pions just like Yukawa wanted to do, and see if it works, and everything will
be explained” Bad luck. It turns out that the calculations that are involved in
this theory are so dificult that no one has ever been able to figure out what the
consequences of the theory are, or to check it against experiment, and this has
been going on now for aÌlmost twenty yearsl
So we are stuck with a theory, and we do not know whether it is right or
wrong, but we do know that it is a i2 wrong, or at least incomplete. While we
have been dawdling around theoretically, trying to calculate the consequences of
this theory, the experimentalists have been discovering some things. For example,
they had already discovered this -meson or muon, and we do not yet know where
it fts. Also, in cosmic rays, a large number of other “extra” particles were found.
Tt turns out that today we have approximately thirty particles, and it is very
difficult to understand the relationships of all these particles, and what nature
wants them for, or what the connections are from one to another. We do not
today understand these various particles as different aspects of the same thing,
and the fact that we have so many unconnected particles is a representation of
the fact that we have so much unconnected information without a good theory.
After the great successes of quantum electrodynamics, there is a certain amount
of knowledge of nuclear physics which is rough knowledge, sort of half experience
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and half theory, assuming a type of force between protons and neutrons andÌ seeing
what will happen, but not really understanding where the force comes from. Aside
from that, we have made very little progress. We have collected an enormous
number of chemical elements. In the chemical case, there suddenly appeared a
relationship among these elements which was unexpected, and which is embodied
in the periodic table of Mendeleev. For example, sodium and potassium are
about the same in their chemical properties and are found in the same colunn
in the Mendeleev chart. We have been seeking a Mendeleev-type chart for the
new particles. One such chart of the new particles was made independently by
Gell-Mamn in the U.S.A. and Nishijima in Japan. The basis of their classification
1s a new number, like the electric charge, which can be assigned to each particle,
called its “strangeness,” Š. 'This number is conserved, like the electric charge, in
reactions which take place by nuclear Íorces.
In Table 2-2 are listed all the particles. We cannot discuss them mụch at this
siage, but the table will at least show you how much we do not know. Ứnderneath
cach particle is mass is given in a certain unit, called the MeV. One MeV is
equal to 1.783 x 10~?” gram. The reason this unit was chosen is historical, and
we shall not go into it now. More massive particles are put higher up on the
chart; we see that a neutron and a proton have almost the same mass. In vertical
columns we have put the particles with the same electrical charge, all neutral
objects in one column, all positively charged ones to the right of this one, and all
negatively charged objects to the left.
Particles are shown with a solid line and “resonances” with a dashed one.
Several particles have been omitted from the table. 'hese include the important
zero-mass, zero-charge particles, the photon and the graviton, which do not fall
into the baryon-meson-lepton classiication scheme, and also some oŸ the newer
resonances (KŠ, ó, ). The antiparticles of the mesons are listed in the table, but
the antiparticles of the leptons and baryons would have to be listed in another
table which would look exactly like this one reflected on the zero-charge columm.
Although all of the particles except the electron, neutrino, photon, graviton, and
proton are unstable, decay products have been shown only for the resonances.
Strangeness assignments are not applicable for leptons, since they do not interact
strongly with nuclel.
AII particles which are together with the neutrons and protons are called
baruons, and the following ones exist: There is a “lambda,” with a mass of
1115 MeV, and three others, called sigmas, minus, neutral, and plus, with several
masses almost the same. 'Phere are groups or multiplets with almost the same
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Table 2-2
Elementary Particles