It is useful to mention antiparticles here. They shall be discussed
more extensively later, but here we wish to state explicitly that
while antiparticles may have some properties different from those
of the corresponding particles, they are still just “particles”. For
example, a particle and its antiparticle have exactly the same mass
and both fall downwards in the earth’s gravitational field. The
antiparticle of the electron is called a positron, and it has the same
mass as the electron, but the opposite electric charge. That’s all.
Do not see anything particularly mysterious in antimatter. It is
just a name given, one could equally well have spoken of mirror
particles. Also, it is a matter of convention which is called the
particle and which the antiparticle. One could equally well have
called the positron the particle and the electron the antiparticle.
That particles and antiparticles may react with each other quite
violently is true, but there are many other (violent) reactions that
do not particularly differ in principle from electron-positron
reactions. For example, at very high energies two protons colliding
with each other produces something quite similar to proton–
antiproton collisions.
The importance of the concept of antiparticles follows from a
law of nature: to each particle there corresponds an antiparticle
that has precisely the same mass, and whose other properties are
exactly defined with respect to those of the particle. For example,
the electric charge has the opposite sign. The law mentioned
allows for the possibility that the antiparticle corresponding to a
particle be the particle itself. In that special case the charge of the
particle must necessarily be zero. The photon is such a particle. It
is its own antiparticle.
There is a standard way to denote an antiparticle: by means of
a bar above the particle name or symbol. Thus one could write
electron and that would mean a positron. And also, to make the
point once again, positron means an electron.
Bound states also have their associated state. For example, a
proton contains three quarks, two u and one d quark, and an
antiproton simply contains the corresponding antiparticles: two
antiup quarks u and one antidown quark d. At CERN antihydrogen
has been created: one positron circling an antiproton.
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