A brief history of time 5

5.  Elementary particles and the forces of Nature

Chapter 5

 Elementary particles and the forces of nature

 

·         Aristotle believed that one could divide a piece of matter into smaller and smaller bits without any limit.

·         Democritus however, before him held that everything was made up of large numbers of various kinds of atoms (indivisible matter).

·         It was at the beginning of the 20^th century though that this argument was finally settled in favour of the atomists.

·         Soon, however the electron was discovered, a particle (negative charge) of matter that had mass less than the one thousandth of the lightest atom.

·         In 1911 the protons were discovered (from the Greek word meaning first) having positive charge, in the nucleus of the atom.

·         In 1932 the neutrons were discovered, another particle in the nucleus with no electrical charge.

  In 1969, experiments showed that when protons collided with other protons or electrons at high speeds other smaller particles were made, which were called quarks (a strange word taken from a quote from James Joyce)!

There are a number of different quarks, the sixth and last of them being discovered in 1995.

A proton or neutron is made up of 3 quarks.

We now know that neither the electrons nor the protons or the neutrons are indivisible.

Using the wave/particle duality, everything in the universe, including light and gravity, can be described in terms of particles. These particles have a property called spin. What the spin of a particle really tells us is what the particle looks like from different directions.

All the particles in the universe can be divided into two groups: particles of spin ½, which make up the matter in the universe and particles of spin 0, 1, and 2 which give rise to forces between the matter particles.

The matter particles obey Pauli’s exclusion principle, which says that two similar particles cannot exist in the same state: that is they cannot have both the same position and velocity, within the limits given by the uncertainty principle. If the world had been created without the exclusion principle, quarks could not form separate, well-defined protons and neutrons. Nor would these, together with electrons, form separate, well-defined atoms.

 In 1928 Dirac formulated a theory that was consistent with both quantum mechanics and the special theory of relativity. The first of its kind. It predicted that the electron should have a partner: an antielectron or positron, which was discovered in 1932 and confirmed Dirac’s theory.

We now know that every particle has an antiparticle, with which it can annihilate. Force-carrying particles,can be grouped into four categories according to the strength of the force that they carry and the particles with which they interact.

 

1. The first category is the gravitational force. This force is universal and it is the weakest of the four.

The gravitational force between the sun and the earth is ascribed to the exchange of gravitons between the particles that make up these two bodies. Although the exchanged particles are virtual, they make the earth orbit the sun!

 

2. The next category is the electromagnetic force, which interacts with electrically charged particles like electrons and quarks, but not with uncharged particles such as gravitons. It is much stronger than the gravitational force. A large body such as the sun or the earth contains nearly equal numbers of positive or negative charges, thus the attractive and repulsive forces between the individual particles nearly cancel each other out.

However, on the small scales of atoms and molecules electromagnetic forces dominate. The electromagnetic attraction between negatively charged electrons and positively charged protons in the nucleus, causes the electrons to orbit the nucleus of the atom, just as the gravitational attraction causes the earth to orbit the sun.

The electromagnetic attraction is pictured as being caused by the exchange of large numbers of virtual massless particles of spin 1, called photons.

 3. The third category is called the weak nuclear force, which is responsible for radioactivity and which acts on all matter particles of spin ½, but not on particles of spin 0, 1, or 2, such as photons and gravitons.

This force was not well understood until in 1967 it was unified with, electromagnetic force just as Maxwell unified electricity and magnetism 100 years earlier. It was suggested that in addition to the photon there were three other spin-1 particles, known collectively as massive vector bosons that carried the weak force.

 

4. The fourth category is the strong nuclear force, which holds the quarks together in the proton and neutron, and holds the protons and neutrons together in an atom.

 

Our very existence is the consequence of the production of protons, or more simply, of quarks (there are 3 quarks inside a proton) from an initial situation in which there were no more quarks than antiquarks, which is the most natural way to imagine the universe starting out.

 

Matter on the earth is made up mainly of protons and neutrons which in turn are made up of quarks.

 

·         Grand unified theories do not include the force of gravity. This does not matter too much, because gravity is such a weak force that its effects can usually be neglected when we are dealing with elementary particles or atoms. However it is gravity that determines the evolution of the universe as for a sufficiently large number of particles, gravitational forces can dominate over all other forces. The attractive force of gravity can even cause a star to collapse.