216 The Poetry of Physics and The Physics of Poetry
(the ratio of the proton mass to the electron mass) needed for the creation
of an electron-positron pair. The observation of the antiproton did not
take place immediately following Dirac’s prediction because it was
impossible to create photons with the necessary energy, 2mpc^2 , where mp
is the mass of the proton.
The discovery of the antiproton had to await the development of
particle accelerators, which could yield such high energies. Such a
machine was built in the early fifties at the University of California at
Berkeley. Shortly after the completion of this machine the long awaited
antiprotons appeared exactly as Dirac’s theory had predicted. These
particles have the same mass and spin as the proton but opposite charge.
antiprotons and protons annihilate each other to produce photons in the
same way as electrons and positrons. The reaction of the proton and
antiproton is more complicated, however, because of the nuclear force
but the essential features of Dirac’s theory still persist. Shortly after the
discovery of the antiproton the antineutron was also detected. This
particle has the same mass and spin as the neutron. Its charge is also
equal to zero since the opposite of zero is still zero, i.e. (–0 = 0). Neutron
antineutron pairs are also created and annihilated just as expected by
Dirac’s theory.
One of the interesting features of the antiparticles is that their
interactions among themselves are exactly identical to the interactions of
the particles among themselves. The antiproton antiproton interaction is
the same as the proton-proton interaction. A positron and an antiproton
interact with each other just like the proton and an electron and hence,
form an antihydrogen atom, which behaves exactly like the hydrogen
atom. The antihydrogen atom has been observed and found to emit
the same spectral lines as the hydrogen atom. There is no way of
distinguishing matter from antimatter. Given enough positrons,
antiprotons and antineutrons, one could build a world identical to our
own. In fact, it is purely convention to call the material of which we
are made matter and those particles, which annihilate our particles
antimatter. For all we know, we are made of antimatter and the other
particles are matter. There is no way of determining whether the other
galaxies or clusters of galaxies in our universe are made of matter or
antimatter. There are, in fact, cosmological models, which predict that
half the galaxies in the universe are made of antimatter and the other half
of matter.