172
R. Barrett and P. P. Delsanto
If the unification of the weak and electromagnetic interactions into the
electroweak interaction represented the first step towards a Theory of Everything, it remains unfortunately the only step. So far, no unification has been
achieved with the strong interaction. The weakest of the forces, gravity, also
remains very much on the outer. As we have seen in Chap. 7, the theory
of relativity interprets gravity as arising from a distortion of the geometry of
space-time, and a quantum theory of gravity, analogous to QED, has not yet
been found.
9.4 Sorting the Particle Zoo
Now that we have gained an insight into the role played by the four forces in
the subatomic domain, it is time to return to the Particle Zoo, with a view to
sorting out the exhibits that industrious experimentalists were rapidly discovering and attempting to classify. The advent of powerful new accelerators
in the 20th century provided physicists with access to particles with energies that had hitherto been unattainable. So many “fundamental” particles
turned up in experiments that it was soon realised that not all could be truly
fundamental; many of them were probably constructed from other smaller
units.
This suspicion was strengthened when the proton was examined more
closely. We saw in Chap. 8 how electrons behave like tiny magnets when
placed in a magnetic field. The strength of this behaviour is determined by
the electron’s magnetic moment, and its prediction to astonishing accuracy
is one of the triumphs of QED. However, when, in the nineteen-thirties,
measurements were made of the magnetic moment of the proton, the theoretical and experimental results were in mutual disagreement by hundreds of
percent. This was soon recognised as evidence that the proton has internal
structure of its own. The proton is not a fundamental particle, and like Alice
in Nanoland, we must go to an even smaller domain, to search for the most
basic constituents of nature.
The reader can probably guess one path physicists might take to further
progress: follow the example of Rutherford in the atomic domain, and smash
two particles together to see what eventuates. A high energy analogue of the
Rutherford experiment described in Chap. 8 was carried out at the Stanford
Linear Accelerator (SLAC) in California in 1970. Protons were bombarded
with very high energy electrons 5 . The distribution of the electrons after the
5 The energy was equal to that produced by accelerating the electrons through an electrical potential
difference of 4.5 to 20.5 billion volts.
R. Barrett and P. P. Delsanto
If the unification of the weak and electromagnetic interactions into the
electroweak interaction represented the first step towards a Theory of Everything, it remains unfortunately the only step. So far, no unification has been
achieved with the strong interaction. The weakest of the forces, gravity, also
remains very much on the outer. As we have seen in Chap. 7, the theory
of relativity interprets gravity as arising from a distortion of the geometry of
space-time, and a quantum theory of gravity, analogous to QED, has not yet
been found.
9.4 Sorting the Particle Zoo
Now that we have gained an insight into the role played by the four forces in
the subatomic domain, it is time to return to the Particle Zoo, with a view to
sorting out the exhibits that industrious experimentalists were rapidly discovering and attempting to classify. The advent of powerful new accelerators
in the 20th century provided physicists with access to particles with energies that had hitherto been unattainable. So many “fundamental” particles
turned up in experiments that it was soon realised that not all could be truly
fundamental; many of them were probably constructed from other smaller
units.
This suspicion was strengthened when the proton was examined more
closely. We saw in Chap. 8 how electrons behave like tiny magnets when
placed in a magnetic field. The strength of this behaviour is determined by
the electron’s magnetic moment, and its prediction to astonishing accuracy
is one of the triumphs of QED. However, when, in the nineteen-thirties,
measurements were made of the magnetic moment of the proton, the theoretical and experimental results were in mutual disagreement by hundreds of
percent. This was soon recognised as evidence that the proton has internal
structure of its own. The proton is not a fundamental particle, and like Alice
in Nanoland, we must go to an even smaller domain, to search for the most
basic constituents of nature.
The reader can probably guess one path physicists might take to further
progress: follow the example of Rutherford in the atomic domain, and smash
two particles together to see what eventuates. A high energy analogue of the
Rutherford experiment described in Chap. 8 was carried out at the Stanford
Linear Accelerator (SLAC) in California in 1970. Protons were bombarded
with very high energy electrons 5 . The distribution of the electrons after the
5 The energy was equal to that produced by accelerating the electrons through an electrical potential
difference of 4.5 to 20.5 billion volts.