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Dirac and Antimatter
In 1927 Paul Dirac was working on the
problem of combining the theory of the
very small (quantum mechanics) with
Einstein’s special theory of relativity.
He came up with this equation
describing the electron:
  



i

eA
(
x
)

m


  x 
 ( x)  0

 

You DON’T need to learn this
equation!!
Solutions to the Dirac equation
The equation explains very neatly the magnetic properties of the electron.
Electrons he visualised as spinning like tiny gyroscopes
but surprisingly they always have the same amount of “angular momentum”
Which we now call spin.
There is one very odd property of this equation: It has two solutions.
As a comparison the much simpler equation below has two solutions
y 4
y2
y  2
Solutions to the Dirac equation
Dirac predicted that there must be a particle like the electron but with
opposite properties (except mass).
He called the new particle he predicted THE POSITRON
Particle
Symbol
electron
e
positron
e
There are properties other than charge, of the positron which are
exactly opposite the properties of the electron some of which you will
discover later in the course. Just like the electron can be written ethe positron is sometimes written e+
The confirmation of the positron
• The positron was the
first particle of
antimatter predicted.
• Anderson discovered
the positron in cosmic
rays in 1932.
Antimatter.
• Every subatomic
particle has its
counterpart with
exactly opposite
properties.
p
p
n
n
e
e
Some examples of
particles and their
antiparticles
Proton and antiproton
Neutron and
antineutron
Neutrino and
antineutrino
When a matter particle meets its counterpart antiparticle. They are both
destroyed and produce gamma ray radiation.
Measuring Mass-the Electron Volt
------------------------------------------------------------
++++++++++++++++++++++++++++++++++++
An electron has a negative charge so it is repelled by a
negatively charged plate (electrode) and it is attracted to a
positively charged eletrode
It is accelerated and gains kinetic energy.
The amount of energy it gains if it is accelerated by a p.d. of
1volt is called
1electronvolt 1eV
This is an exceedingly small amount of energy
A potential
difference
of 1volt
between
the plates
The MeV
• Because mass is just
“compressed energy”, It is
possible to use the electron
volt units instead of kilograms
to quantify mass.
• But the electron volt is too
small so we use the MeV
which is equivalent to a million
electron volts
Particle
electron
Mass in
MeV
0.511
positron
0.511
proton
938.27
antiproton
938.27
neutron
938.27
antineutron
938.27