Once quantum theory was fully formulated,
scientists tried to unify electromagnetism with quantum theory.
The basic phenomena of electromagnetism are fields.
This would eventually result in quantum field theory.
It could also be viewed as unification of quantum theory and special relativity.
Scientist knew that for each electromagnetic wave, there is a quantum particle, the photon.
They needed to describe how charged particles like electrons and protons, interact with photons.
The goal was a fully consistent theory of Quantum ElectroDynamics, or QED.
Once QED was understood, the goal was to extend quantum field theory,
to strong and weak nuclear forces.
This required two new principles.
The first principle defined what electromagnetism, and these nuclear interactions,
have in common.
It is called the gauge principle.
This leads to unification of all the three forces.
The second principle explains why, although unified, the three forces are different.
It is called spontaneous symmetry breaking.
These two principles together form the basis, of the standard model of particle physics.
Protons and neutrons have three quarks.
Other particles, called mesons have two quarks.
That is, a quark and a antiquark.
The interaction of protons and neutrons with other particles was very complicated.
It was hoped that the force between quarks might be simple.
The forces between molecules are complicated.
The forces between atoms can be understood more easily, with electromagnetism.
The laws governing the parts are simpler,
than the laws governing the whole.
This led to the discovery of the deep commonality that connects,
the strong and weak nuclear forces with electromagnetism.
All three are consequences of the simple but powerful gauge principle.
The gauge principle is best understood in terms of what scientists call as symmetry.
Symmetry is an operation that does not change, how something behaves,
relative to the outside world.
For example, if you rotate a ball, it does not change it.
It is still a sphere.
When scientists referred to symmetry, they could be referring to any kind of change,
we can make to an experiment, that does not alter the outcome.
Consider an experiment in which a beam of protons is accelerated, and aimed at a target.
A certain pattern is observed when the protons scatter of the nuclei in the target.
If we substitute neutrons for protons, the patter of scattering hardly changes.
This is due to the symmetry of forces between protons and neutrons,
and the nuclei in the target.
It tells us that certain nuclear forces can't tell the difference between a proton and a neutron.
There are some special situations in which the symmetries completely determine the forces.
This is true for a class of forces called gauge forces.
All the properties of a force can be determined by knowing the symmetries.
This idea is one of the most important discoveries of the 20th century.
It is called as the gauge principle.
There are two interesting features of this principle.
One is that, the forces it leads to are conveyed by particles called gauge bosons.
The second is that electromagnetic, the strong and the weak nuclear forces,
turn out to be forces of this kind.
The gauge boson that corresponds to the electromagnetic field, is called the photon.
Those that correspond to the strong force holding the quarks together,
are called gluons.
Those that correspond to the weak force, are called as weak bosons.
Other field theories could be constructed using the gauge principle.
This is done basing them on symmetries involving various kinds of elementary particles.
They are called as Yang-Mills theories.
The new forces would have an infinite range, like electromagnetism.
Scientists knew that the strong and weak nuclear forces, had a strong range.
They could not be described by gauge theory.
The question facing scientists, is how to unify forces, that manifest themselves differently,
as electromagnetism and the strong and the weak nuclear forces.
Even if some hidden unity was discovered,
they still have to understand why, and how it appears to be different.
Einstein had a wonderful way of solving this problem,
for special and general relativity.
He realised that the apparent differences, between the phenomena,
were not intrinsic to the phenomena,
but were due entirely to the necessity of describing the phenomena,
from the view point of the observer.
Electricity and magnetism,
motion and rest,
gravity and acceleration,
were all unified by Einstein in this way.
The difference that observers perceive, only reflect the view point of the observers.
Scientists came up with a new way of looking at unification.
The laws may have a symmetry that is not respected by all features of the world they apply to.
We can take the example of the social laws.
The law applies equally to all people.
This does not mean that all people are the same.
We might start out with equal opportunity, but as life goes on the initial symmetry goes away.
Scientists who speak of equality as a symmetry, would say that the symmetry at birth,
is broken by situations we encounter, and choices we make.
When we look at a nursery full of infants, we know that the symmetry will break,
but we cannot predict how and when it will be broken.
Scientists say that the symmetry is spontaneously broken.
The spontaneous symmetry breaking is the second great principle,
that underlies the standard model of physics.
Another example of symmetry is a group of freshers in college.
Much of the structure of the world, both social and physical,
break symmetries present in the space of possibilities.
The symmetric situation is unstable.
We trade the unstable freedom of potentiality, for the stable experience of reality.
This is true for physics also.
When a pencil is balanced at a point, it is symmetric, but it is unstable.
When the pencil falls, it will fall randomly in one direction or other.
Once fallen, it is stable, but it is no longer symmetric.
The laws describe only what possibly may happen.
The actual world governed by this laws,
involves the choice of one realisation of many possibilities.
The mechanism of spontaneous symmetry breaking can happen,
to the symmetries between the particles, in nature.
When it occurs for the symmetries that, by the gauge principle,
give rise to the forces of nature, it leads to differences in their properties.
The forces become distinguished.
Before the symmetry breaks, all four fundamental forces have an infinite range,
like electromagnetism.
Afterwards they have a finite range like the two nuclear forces.
This along with the gauge principle,
is one of the most important discoveries of the 20th century.
The idea of combining spontaneous symmetry breaking,
with gauge theories was discovered by scientists, including Higgs.
They showed that a particle exists, as a consequence of spontaneous symmetry breaking.
This is called the Higgs boson.
Scientists later discovered that the combination of the gauge principle,
and spontaneous symmetry breaking, can be used to construct a theory,
that unified the electromagnetic and weak nuclear forces.
The theory is called the Weinberg-Salam model of the electro weak force.
This implied, among other things, that there must be particles analogous to the photon,
which carries the electromagnetic force, to carry the weak nuclear force.
There are three of them, called the W plus, W minus, and Z.
Spontaneous symmetry breaking means the properties elementary particles,
depend in part on history and environment.
The symmetry may break in different ways, depending on conjunctions like density and temperature.
More generally the properties of elementary particles, depend not just on equations of the theory.
But on which solution to those two equations that applies to all universe.
This opens up the possibility that many- or even all- properties of elementary particles,
are contingent and depend on which solution of the laws, is chosen in our region of the universe,
or in our particular era.
They could be different, in different regions.
They could even change in time.
In spontaneous symmetry breaking, there is a physical quantity, whose value signals,
that the symmetry is broken and how.
This quantity is usually a field, called the Higgs field.
It manifests itself as the Higgs boson.
Recent experiments have confirmed the existence of this particle.
Quarks are associated with the three colours, red, blue and green.
The strong nuclear force binds the quarks.
The gauge principle was applied to the strong nuclear force,
and it was found that a gauge field was responsible for this force also.
The resulting theory is called quantum chromodynamics or QCD.
The Weinberg Salam model and QCD form the basis of the standard model.
The discovery that all the three forces,
are expressions of a single unifying principle, - the gauge principle,
- is the deepest accomplishment, of theoretical particle physics.