Thursday, March 10, 2011

Who Ordered That? - Part 5

The Electro-Weak Interaction

Each of the four fundamental forces in nature were well established by the 1960’s. How were they linked or how they functioned was a matter of great debate and theory. It was assumed that some form of messenger particle was responsible for passing information to and from interacting particles. The electromagnetic force was described by a theory called quantum electrodynamics (QED). This theory stated that the interaction between charged particle was mediated by the photon. This allowed the electron and proton to sense each other's presence. In a similar vein, a theory for the weak force was developed, which had at its heart the massive W and Z gauge bosons. These particles mediated the weak force between all particles. Due to these particles' relatively large mass, this force was extremely short ranged.

The success of the weak force theory led physicists to the conclusion that the EM force and the Weak force were somehow linked at a more fundamental level. These two forces were two sides of the same coin and so developed the electroweak theory. This theory parallels the work of Maxwell and his unification of electricity and magnetism years earlier. From this new theory, predictions for the mass and charge of the W and Z particles were made. Conformation of these predictions came in the early 1980’s at CERN.

The Theory of Quantum Chromodynamics

The theory of the strong force had been theorized to account for the interactions between nucleons. This force allowed for the nucleus to remain stable even though it should fly apart as the protons repelled one other. With the discovery of the quark, was there now an interaction between them which bound them internally within the hadron. Was this the same force which bound the nucleus? From this theory a messenger particle dubbed the gluon was predicted. From this prediction the gluon was much like the photon, it was massless and carried the information from one particle to another. For interactions with quarks the gluon had to have certain properties which it must carry. One such property which was newly developed was the idea of colour.

This is not colour in the sense of what we see in everyday life, i.e. the manifestation of certain wavelengths of EM radiation. This colour property comes in three flavours: red, yellow and blue. This theory formed the basis of what we now call quantum chromodynamics (QCD). According to this theory quarks can only combine in combinations that are colour neutral. There are several possible ways this can occur:
  •  Combination of red-yellow-blue is white so therefore is colour neutral
  • Combination of anti-red, anti-yellow and anti-blue is colour neutral
  • Combination of any colour and its anti-partner is also colour neutral
To achieve balance with this system we must have what is called gauge symmetry. This symmetry is central to understanding QCD. A gluon carries with it a colour and an anti-colour (not necessarily of corresponding pairs). When a quark changes colour it emits a gluon, this gluon in turn interacts with another quark which immediately changes colour in the right manner so as to maintain colour neutrality within the hadron. This allows the quark to change colour continually which does not affect the colour neutrality of hadrons.

This new theory could account for the force that binds not only protons and neutrons together but also the force that binds quarks together to form hadrons. This is an oversimplified view of QCD, the theory is one of the great achievements of 20th century physics but at its core is a very mathematical model which it difficult to adequately describe in a few paragraphs.

The Birth of the Standard Model and its Limits

It you were to ask any scientist what are the greatest accomplishments of science, the most likely response would include the like of Darwin’s Theory of Evolution, the discovery of DNA, penicillin, the internet and the Standard Model. The standard model is the crown jewels of modern physics. If offers a common ground between the leptons, quarks, QCD and QED.  Within this model all members of the particle zoo could be accounted for. The interaction between these particles could be predicted with accuracy. This theory helped to link the two major fields in physics: QED and QCD. Within the confines of the standard model we have a common framework for three of the fundamental forces.

The standard model predicted many of the masses and properties of the myriad of sub atomic particles. These were subsequently discovered at facilities such as CERN throughout the 1980’s and 1990’s.

The standard model helps answer the question as to why, if these fundamental forces are so intrinsically linked, they are they so different? Why does the photon have no mass yet the W and Z boson are massive? To answer these questions the development of a theory dating back to the mid 1960’s is required. Peter Higgs (b.1929) proposed that there was an all-encompassing field that permeates all space which gave particles their apparent mass. Since the photon does not interact with this field it has no mass. So what we perceive as mass is the interaction between particles and this Higgs field.

The standard model has helped us predict that there are only three generations of quark and leptons. Have we reached the end of the rainbow and found our pot or gold? The very simple answer is no. The standard model offers us a very accurate model for the majority of what we see but it has limits in application.

The standard model does not include gravity nor can it explain the phenomena of dark matter or dark energy. We also cannot use the standard model to return back to the initial starting point of the universe. The standard model breaks down as we approach time zero. From observation, neutrinos would appear to have mass, the standard model has no explanation as to why this is so. 


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