Sunday, March 6, 2011

Who Ordered That? - Part 1

"Three quarks for Muster Mark!
    Sure he has not got much of a bark
                  And sure any he has it's all beside the mark."
        - James Joyce, Finnegans Wake 


By the start of the 1930's, physicists believed that all the constituents of the atom's inner structure had been discovered and all that remained was the task for formulating a theoretical framework to explain their behaviour. If we consider the constituents of matter; electron, proton and neutron, to be the building blocks of matter, then the forces of nature is the cement which binds them. We will see that there are four distinct fundamental forces in nature which govern all the interactions of matter, from the galactic, to the sub atomic scale.

This belief in these three elementary particles was rocked by several new theories. A slew of new, exotic particles were discovered, even some which weren't predicted. As particle accelerators grew in size, more and more particles began to emerge. This exciting, albeit confusing time in physics is best summed up by Isidor Isaac Rabi (1898-1988), who upon the observation of the muon, a hitherto unknown particle, exclaimed "Who ordered that?".

The Forces of Nature

The ebb and flow of the tide, a bright summers day, the myriad array of elements and the colour of this ink are all governed by four fundamental forces. The interaction and properties of these forces - gravity, electromagnetism, strong and weak nuclear force - is the most important idea in understanding the universe.
Gravity was the first force to be formulised into a theory. In 1687 Sir Isaac Newton (1643 -1727) published his Philosohhiæ Naturalis Principia Mathematica, within which he theorized his Universal Law of Gravitation. This theory helped explain a vast array of events, from the falling of an apple to the orbit of the planets. The force of gravity is what is known as an inverse square force, the strength of attraction between two interacting bodies is proportional to the inverse of the distance between them. 

The force of gravity acts over an infinite range and as far as we know is only attractive. In comparison to the other forces, gravity is very weak so its effects at the atomic level are negligible. However as it is only an attractive force its effects add up and this is particularly noticeable in acting over cosmological scales.

The electromagnetic force (EM force) is the next most obvious force as we can readily see it effects in everyday life. This force is both attractive and repulsive. It is magnitudes of strength more powerful than gravity and acts over an infinite range, so why doesn't it prevail over the galactic scale? It is its very nature of both attraction and repulsion which tends to cancel out its influence. It is the force which is responsible for binding atoms to form molecules and the magnetic shield which protects the Earth from the harmful effects of the solar wind. Prior to the 1800's the effect of electric and magnetic fields were observed but their mutual relationship seemed independent. In 1820, Danish physicist, Hans Christian Ørsted (1771-1851), observed that a changing electric field caused a change in motion of a magnetic compass needle. It would take several more years of intense research from many great scientists until in 1864 a brilliant Scottish physicist, James Clerk Maxwell (1831-1879), unified electricity and magnetism to what we now call electromagnetism. Maxwell's equations predicted many new phenomena and formed the basis for a major development in physics in the 20th century.

James Clerk Maxwell

The two remaining forces of nature are the strong and weak nuclear forces. Both of these forces act within the confines of the sub atomic world. The stability of the nucleus of the atom was a major snag of early atomic theory, how could positively charged protons be in such close proximity in the nucleus? With the discovery of the neutron many believe that this particle held the key to solving this dilemma. In 1935 Hideki Yukawa(1907-1981) theorized that some unseen force was present which only acts over extremely short ranges between protons and neutrons. This force was dubbed the strong nuclear force as it prevailed over the electromagnetic force at these short ranges. This strong force (and the addition of the EM force) is what drives the emission of alpha particles and gamma rays in radioactivity.

The beta decay of atoms raised many questions. For example: what governed the change of a neutron to a proton, electron and antineutrino? This decay could not be explained by the strong nuclear force or the EM force, so there had to be another unseen force which drove this reaction. This force would again be limited to extremely short ranges like the strong nuclear force, but must also be much weaker than the strong force. In 1934 Italian physicist Enrico Fermi (1901-1954) theorized a new force which explains the decay structure responsible for beta decay. At the time this theory was deemed "too remote from reality" for it to be published in the world renowned Nature journal (this was overturned in 1939). This new force was named the weak nuclear force.

These four fundamental forces were the foundations upon which the whole universe is based. As will be shown in later sections, all of these forces have an associated messenger particle. Now that we knew of these four distinct forces, was there some underlying relationship between them? This question would fuel an ever increasing ambition within the physics community to combine these fundamental forces under a unified law. 

Summary of the four fundamental forces


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