4
H. Schopper
Pauli, thought that this symmetry could not be violated. Such a believe indeed goes
back to Emanuel Kant 2 who claimed that certain ‘a priori’ concepts have to be valid
so that we would be able to explore nature. Since it seemed obvious that nature does
not know whether we observe it directly or through a mirror a violation of mirror
symmetry seemed unacceptable. This phenomenon is still not understood, although
the fact that also C conservation is completely violated and the combined symmetry
PC seemed to hold has reduced somewhat the original surprise. The whole situation
has become more complicated by the detection that PC is also violated, although
very little. A deep understanding of the violation of these ‘classical’ symmetries is
still missing. So far experiments show that the combined symmetry PCT still holds
as is required by a very general theorem.
In field theories another class of more abstract symmetries has become
important—the gauge symmetries. As is well known from Maxwell’s equations the
electrodynamic fields are fully determined by the condition that gauge symmetry
holds, which means that the electric and magnetic fields are independent against
gauge transformations of their potentials. It was discovered that analogous gauge
symmetries determine the fields of the strong and weak interactions in which case
the (spontaneous) breaking of the symmetries plays a crucial role.
In summary, we have abandoned the description of nature in terms of hard
indestructible spheres in favour of abstract ideas—the symmetries and there breaking. From a philosophical point of view one might, in an over-simplistic way,
characterize the development as moving away from Democritus to Plato.
Finally, it should be mentioned that in particle physics progress was only possible
by an intimate cooperation between theory and experiments. The field has become
so complex that by chance discoveries are extremely rare. The guidance by theory
is necessary to be able to put reasonable questions to nature. This does not exclude
great surprises since many theoretical predictions turned out to be wrong. Indeed
most progress could be made by verifying or disproving theories.
Although the Standard Model of Particle Physics SM (with some extensions, e.g.
allowing for masses of neutrinos) has achieved a certain maturity by being able to
reproduce all experimental results obtained so far, it leaves open many fundamental
questions. One particular problem one has gotten accustomed to, concerns P and C
violations which are put into the SM ‘by hand’. And as has been mentioned above
the SM leaves open many other questions which indicate that it cannot be a final
theory.
In 2008 I wrote the concluding paragraph of this introduction as “Many
arguments indicate that a breakthrough in the understanding of the microcosm will
happen when the results of LHC at CERN will become available. LHC will start
operation in 2008, but it will probably take several years before the experiments
will have sufficient data and one will be able to analyse the complicated events
before a major change of our picture will occur, although surprises are not excluded.
Hence it seems to be an appropriate time to review the present situation of our
H. Schopper
Pauli, thought that this symmetry could not be violated. Such a believe indeed goes
back to Emanuel Kant 2 who claimed that certain ‘a priori’ concepts have to be valid
so that we would be able to explore nature. Since it seemed obvious that nature does
not know whether we observe it directly or through a mirror a violation of mirror
symmetry seemed unacceptable. This phenomenon is still not understood, although
the fact that also C conservation is completely violated and the combined symmetry
PC seemed to hold has reduced somewhat the original surprise. The whole situation
has become more complicated by the detection that PC is also violated, although
very little. A deep understanding of the violation of these ‘classical’ symmetries is
still missing. So far experiments show that the combined symmetry PCT still holds
as is required by a very general theorem.
In field theories another class of more abstract symmetries has become
important—the gauge symmetries. As is well known from Maxwell’s equations the
electrodynamic fields are fully determined by the condition that gauge symmetry
holds, which means that the electric and magnetic fields are independent against
gauge transformations of their potentials. It was discovered that analogous gauge
symmetries determine the fields of the strong and weak interactions in which case
the (spontaneous) breaking of the symmetries plays a crucial role.
In summary, we have abandoned the description of nature in terms of hard
indestructible spheres in favour of abstract ideas—the symmetries and there breaking. From a philosophical point of view one might, in an over-simplistic way,
characterize the development as moving away from Democritus to Plato.
Finally, it should be mentioned that in particle physics progress was only possible
by an intimate cooperation between theory and experiments. The field has become
so complex that by chance discoveries are extremely rare. The guidance by theory
is necessary to be able to put reasonable questions to nature. This does not exclude
great surprises since many theoretical predictions turned out to be wrong. Indeed
most progress could be made by verifying or disproving theories.
Although the Standard Model of Particle Physics SM (with some extensions, e.g.
allowing for masses of neutrinos) has achieved a certain maturity by being able to
reproduce all experimental results obtained so far, it leaves open many fundamental
questions. One particular problem one has gotten accustomed to, concerns P and C
violations which are put into the SM ‘by hand’. And as has been mentioned above
the SM leaves open many other questions which indicate that it cannot be a final
theory.
In 2008 I wrote the concluding paragraph of this introduction as “Many
arguments indicate that a breakthrough in the understanding of the microcosm will
happen when the results of LHC at CERN will become available. LHC will start
operation in 2008, but it will probably take several years before the experiments
will have sufficient data and one will be able to analyse the complicated events
before a major change of our picture will occur, although surprises are not excluded.
Hence it seems to be an appropriate time to review the present situation of our