Regular readers of Reflections of the University Lowbrow Astronomers know that the University of Michigan Physics Department holds a series of multimedia presentations aimed at the general public called “Saturday Morning Physics.” They have become very popular. This fall these presentations covered gamma ray bursts, particle physics and galaxies. In this article I will give an overview of the particle physics presentations given by Dr. Ken Bloom.
Other branches of physics study collections of matter such as rocks or planets, but particle physics studies the smallest parts of matter. Even so, particle physics can contribute to the study of larger objects. For example, the problem of dark matter (see ”The Dark Matter Mystery” by Lorna Simmons, Reflections, December 1999) might be solved by better understanding of particle physics. Also particle physics is necessary to understand how stars emit energy, how stars evolve over time and how cosmic ray particles travel through the galaxy.
Dr. Bloom began with a history of the development of particle physics. The ancient Greeks believed that all matter was composed of four elements, earth, water, air and fire. This idea persisted until 1808 when Thomas Dalton demonstrated that matter was made of atoms. Chemists were able to identify approximately 90 different types of atoms called elements. In time, these elements were organized into a pattern we now call the periodic table. Initially there was no explanation for why the periodic table worked; this would have to wait until the internal structure of the atom was determined.
Clues to this structure emerged in 1897, when Thomson discovered the electron. It is possible to detect charged particles, such as the electron, by creating a magnetic field. Charged particles passing through the magnetic field travel along curved paths. The exact shape of the path tells us the ratio of the electric charge to the mass. This led to a model of the hydrogen atom: negatively charged electrons distributed within a sea of positive charge similar to the distribution of plums in plum pudding.
The plum pudding model was shown to be incorrect by Thomas Rutherford. Rutherford sent alpha particles (a particle observed near radioactive materials) through a thin gold foil. Alpha particles appeared on the other side. A few were deflected at large angles, but most were not deflected at all. This was not consistent with the plum pudding model, but was consistent with another model. Electrons are in “orbit” around another particle called the nucleus. Sending small probes like alpha particles have become the standard technique for studying particle physics. The electron has about 1/1800 the mass of a hydrogen nucleus. By measuring the mass of different atoms, it is possible to guess that the nucleus is made of two particles, the proton and the neutron. Different kinds of atoms have different numbers of protons and of neutrons. The hydrogen atom consists of one proton; the alpha particle consists of two protons and two neutrons and so on. This leaves one question. Protons repel each other, so why don’t atoms fall apart? We will answer this later.
Soon other particles were discovered. Cosmic rays are particles that enter the earth’s atmosphere from space. Dr. Bloom demonstrated a device called a scintillation detector, whenever a particle passes through this device, a flash of light appears. If we measure the charge/mass ratios of cosmic rays, we find that some are protons, but we find new particles with names like muon, kaon, pion, omega, and so on. The particles with the highest energies detected so far have been from cosmic rays.
In the late 1920s, quantum mechanics predicted that there must be a particle that has the opposite charge to the electron, which we now call a positron. The positron was discovered in 1931. Almost all particles have an antiparticle with equal mass and opposite charge. All particle reactions can occur with antiparticles. Also when a particle and an antiparticle collide, they release energy that is carried off by two photons.
Understanding how particles interact has been a major preoccupation of particle physics. After studying many objects, physicists have concluded all interactions operate within two constraints, the conservation of energy and the conservation of momentum. If you measure the energy of a closed system before and after a particle interaction, you get the same value. This is also true of momentum. These laws have been used to understand subatomic particles. For example, early observations showed the neutron decaying into two particles, a proton and an electron. However this does not comply with the conservation of momentum. Therefore either the conservation laws are wrong or there is an additional unseen particle. Physicists soon were able to observe such a particle, called a neutrino. It has very little mass, perhaps no mass, and zero electric charge. Neutrinos rarely interact with matter, and can pass through many light years of lead. A similar reasoning process has been used to predict the existence of several new particles.
We now have a zoo of different particles, why are they so many? To answer this, we first categorize the particles into groups. The leptons consist of six particles and six antiparticles in three families, the electron family, the muon family, and the tau family. Note that there are three different types of neutrinos and they are all leptons. The bosons consist of the photon and a few related particles. The remaining particles are referred to as hadrons and are the most unmanageable group; a way organizing hadrons into patterns, reminiscent of the periodic table, was developed. This was called the eight-fold way and made valid predictions for hadron properties. This worked, except there was an empty space where there should have been a particle. Sure enough, a particle called omega-minus, was discovered which fit into this space.
At first there was no explanation for why the eight-fold way works. An experiment similar to the Rutherford experiment was devised were protons were bombarded with high-energy electrons. Most electrons were not deflected but a few were deflected by large angles (reminiscent of the Rutherford experiment). Protons are apparently mostly empty space and are composed of objects less than 1/1000 its size. These particles have the name “quarks.” The quarks are organized into families, each of which has two members: Family 1 with the up and down quarks. These two quarks plus with the electron are all that is needed to make most things we normally encounter. The other quarks only occur in exotic matter that is rarely seen under normal conditions. Family 2 has the strange and charmed quarks. Family 3 contains the bottom and top quarks. All these quarks have fractional electric charge (either +2/3 or -1/3). The top quark is the heaviest and this fact meant it was only recently that particle physicists were able to prove its existence. Current theory suggests it is impossible to observe a free quark. Any attempt to free a quark would release just enough energy to create a quark and anti-quark pair. One member of this pair would bind to the quarks that remain and the other member would bind to the other fragment. Instead of creating a free quark, all you have accomplished is splitting a hadron into two hadrons. All eight-fold way particles, all hadrons, are believed to be composed of quarks. Hadrons are divided into two groups, baryons which are composed of three quarks and mesons which are composed of a quark and the corresponding anti-quark.
Whenever two particles interact, we assume a “force” acts between them. Before the twentieth century, there were three known forces, namely gravity, electricity and magnetism. When physicists started experimenting, they discovered electricity and magnetism were related. Dr. Bloom gave several demonstrations, which showed that a changing electric field generates a magnetic field, a changing magnetic field generates an electric field and an electric field exerts a force on a changing magnetic field. There is a single electromagnetic force (EM force) which explains both electric and magnetic phenomenon. Physicists have since added two new forces, the strong force and the weak force.
Each of these forces has different characteristics. A particle interaction occurs through the exchange of particles and the particles exchanged depend on which force is involved. In any given situation, one of these forces tends to dominate. Gravity is the most important player on the scale of solar systems, globular clusters, galaxies and so on. It can operate over large distances but is much weaker than the other three forces. It operates by the exchange of particles which have never been observed called gravitons. The weak and strong forces don’t have the needed range and EM fields tend to cancel out over large distances and thus cannot compete with gravity. EM is the most important player within the human scale. Gravity is a player, but not as important as EM. EM holds most objects together more strongly than gravity. EM interactions involve the exchange of photons. Within an atomic nucleus, the strong force dominates. It is stronger than any of the other forces, without the strong force the protons within a nucleus would fly apart. When neither the strong or electromagnetic forces apply, the weak force becomes important. Weak interactions always involve neutrinos, the only particle that responds to neither the strong nor EM force. Weak interactions involve the exchange of particles known as vector bosons. Weak interactions occur in some forms of radioactive decay and contribute to energy production in some stars.
When physicists attempted to describe the strong interaction, they found some problems. It is possible to map particle interactions using special diagrams called Feynman diagrams. A single diagram describes up to four different particle interactions with the same mathematical formula. These descriptions frequently involved infinite quantities since quantum mechanics allows so-called virtual particles to be created and destroyed anywhere and accounting for virtual particles is not easy. Dr. Bloom showed examples of Feynman diagrams for the EM, strong and weak interactions. He also showed a Feynman diagram representing the experiment that proved the existence of the top quark.
As much progress as physicists have made, there are still unanswered questions that keep physicists busy. Among these questions: Why do quarks and leptons have different properties? Why do the different particles have the masses they do? There does not appear to be any pattern to particle masses. Why are there three families of quarks, three families of leptons and not four or five? Are quarks made up of anything smaller?