Particle Physics: A Very Short Introduction

Book Preface

Particle Physics – A Very Short Introduction

A general introduction to particles, matter, and the universe at large.

Matter

The ancient Greeks believed that everything is made from a few basic elements. The idea was basically correct; it was the details that were wrong. Their ‘earth, air, fire, and water’ are made of what today we know as the chemical elements. Pure water is made from two: hydrogen and oxygen. Air is largely made from nitrogen and oxygen with a dash of carbon and argon. The Earth’s crust contains most of the 90 naturally occurring elements, primarily oxygen, silicon, and iron, mixed with carbon, phosphorus and many others that you may never have heard of, such as ruthenium, holmium, and rhodium.

The abundance of the elements varies widely, and as a rough rule, the ones that you think of first are among the most common, while the ones that you have never heard of are the rarest. Thus oxygen is the winner: with each breath you inhale a million billion billion atoms of it; so do the other 5 billion humans on the planet, plus innumerable animals, and there are plenty more oxygen atoms around doing other things. As you exhale these atoms are emitted, entrapped with carbon to make molecules of carbon dioxide, the fuel for trees and plants. The numbers are vast and the names of oxygen and carbon are in everyone’s lexicon. Contrast this with astatine or francium. Even if you have heard of them, you are unlikely to have come into contact with any, as it is estimated that there is less than an ounce of astatine in the Earth’s crust, and as for francium it has even been claimed that at any instant there are at most 20 atoms of it around.

An atom is the smallest piece of an element that can exist and still be recognized as that element. Nearly all of these elements, such as the oxygen that you breathe and the carbon in your skin, were made in stars about 5 billion years ago, at around the time that the Earth was first forming. Hydrogen and helium are even older, most hydrogen having been made soon after the Big Bang, later to provide the fuel of the stars within which the other elements would be created.

Think again of that breath of oxygen and its million billion billion atoms within your lungs. That gives some idea of how small each atom is. Another way is to look at the dot at the end of this sentence. Its ink contains some 100 billion atoms of carbon. To see one of these with the naked eye, you would need to magnify the dot to be 100 metres across.

A hundred years ago atoms were thought to be small impenetrable objects, like miniature versions of billiard balls perhaps. Today we know that each atom has a rich labyrinth of inner structure. At its centre is a dense, compact nucleus, which accounts for all but a trifle of the atom’s mass and carries positive electrical charge. In the outer regions of the atom there are tiny lightweight particles known as electrons. An electron has negative electric charge, and it is the mutual attraction of opposite charges that keeps these negatively charged electrons gyrating around the central positively charged nucleus.

Look at the full stop once more. Earlier I said that to see an atom with the naked eye would require enlargement of the dot to 100 metres. While huge, this is still imaginable. But to see the atomic nucleus you would need that dot to be enlarged to 10,000 kilometres: as big as the Earth from pole to pole.

Between the compact central nucleus and the remote whirling electrons, atoms are mostly empty space. That is what many books assert, and it is true as concerns the particles that make up an atom, but that is only half the story. That space is filled with electric and magnetic force fields, so powerful that they would stop you in an instant if you tried to enter the atom. It is these forces that give solidity to matter, even while its atoms are supposedly ‘empty’. As you read this, you are suspended an atom’s breadth above the atoms in your chair due to these forces.

Powerful though these electric and magnetic forces are, they are trifling compared to yet stronger forces at work within the atomic nucleus. Disrupt the effects of these strong forces and you can release nuclear power; disrupt the electric and magnetic forces and you get the more ambient effects of chemistry and the biochemistry of life. These day to day familiar effects are due to the electrons in the outer reaches of atoms, far from the nucleus. Such electrons in neighbouring atoms may swap places, thereby helping to link the atoms together, making a molecule. It is the wanderings of these electrons that lead to chemistry, biology, and life. This book is not about those subjects, which deal with the collective behaviour of many atoms. By contrast, we want to journey into the atom and understand what is there.

Inside the atom

An electron appears to be truly fundamental; if it has any inner structure of its own, we have yet to discover it. The central nucleus, however, is built from further particles, known as protons and neutrons.

A proton is positively charged; the protons provide the total positive charge of the nucleus. The more protons there are in the nucleus, the greater is its charge, and, in turn, the more electrons can be held like satellites around it, to make an atom in which the positive and negative charges counter-balance, leaving the atom overall neutral. Thus it is that although intense electrical forces are at work deep within the atoms of our body, we are not much aware of them, nor are we ourselves electrically charged. The atom of the simplest element, hydrogen, consists of a single proton and a single electron. The number of protons in the nucleus is what differentiates one element from another. A cluster of 6 protons forms the nucleus of the carbon atom, iron has 26, and uranium 92.

Opposite charges attract, but like charges repel. So it is a wonder that protons, which are mutually repelling one another by this electrical force, manage to stay together in the confines of the nucleus. The reason is that when two protons touch, they grip one another tightly by what is known as the strong force. This attractive force is much more powerful than the electrical repulsion, and so it is that the nuclei of our atoms do not spontaneously explode. However, you cannot put too many protons in close quarters; eventually the electrical disruption is too much. This is one reason why there is a heaviest naturally occurring element, uranium, with 92 protons in each nucleus. Pack more protons than this together and the nucleus cannot survive. Beyond uranium are highly radioactive elements such as plutonium whose instability is infamous.

Atomic nuclei of all elements beyond hydrogen contain protons and also neutrons. The neutron is in effect an electrically neutral version of the proton. It has the same size and, to within a fraction of a percentage, the same mass as a proton. Neutrons grip one another with the same strength that protons do. Having no electrical charge, they feel no electrical disruption, unlike protons. As a result, neutrons add to the mass of a nucleus, and to the overall strong attractive force, and thereby help to stabilize the nucleus.
When neutrons are in this environment, such as when part of the nucleus of an iron atom, they may survive unchanged for billions of years. However, away from such a compact clustering, an isolated neutron is unstable. There is a feeble force at work, known as the weak force, one of whose effects is to destroy the neutron, converting it into a proton. This can even happen when too many neutrons are packed with protons in a nucleus. The effect of such a conversion here is to change the nucleus of one element into another. This transmutation of the elements is the seed of radioactivity and nuclear power.

Magnify a neutron or proton a thousand times and you will discern that they too have a rich internal structure. Like a swarm of bees, which seen from afar appears as a dark spot whereas a close-up view shows the cloud buzzing with energy, so it is with the neutron or proton. On a low-powered image they appear like simple spots, but when viewed with a high-resolution microscope, they are found to be clusters of smaller particles called quarks.

Let’s take up the analogy of the full stop one last time. We had to enlarge it to 100 metres to see an atom; to the diameter of the planet to see the nucleus. To reveal the quarks we would need to expand the dot out to the Moon, and then keep on going another 20 times further. In summary, the fundamental structure of the atom is beyond real imagination.

We have at last reached the fundamental particles of matter as we currently know them. The electrons and the quarks are like the letters of Nature’s alphabet, the basic pieces from which all can be constructed. If there is something more basic, like the dot and dash of Morse code, we do not know for certain what it is. There is speculation that if you could magnify an electron or a quark another billion billion times, you would discover the underlying Morse code to be like strings, which are vibrating in a universe that is revealed to have more dimensions than the three space and one time of which we are normally aware.
Whether this is the answer or not is for the future. I want to tell you something of how we came to know of the electron and the quarks, who they are, how they behave, and what questions confront us.

Forces

If the electrons and quarks are like the letters, then there are also analogues of the grammar: the rules that glue the letters into words, sentences, and literature. For the universe, this glue is what we call the fundamental forces. There are four of them, of which gravity is the most familiar; gravity is the force that rules for bulk matter. Matter is held together by the electromagnetic force; it is this that holds electrons in atoms and links atoms to one another to make molecules and larger structures. Within and around the nucleus we find the other two forces: the strong and weak. The strong force glues the quarks into the small spheres that we call protons or neutrons; in turn these are held closely packed in the atomic nucleus. The weak force changes one variety of particle into another, such as in certain forms of radioactivity. It can change a proton into a neutron, or vice versa, leading to transmutation of the elements. In so doing it also liberates particles known as neutrinos. These are lightweight flighty neutral particles that respond only to the weak and gravitational forces. Millions of them are passing through you right now; they come from natural radioactivity in the rocks beneath your feet, but the majority have come from the Sun, having been produced in its central nuclear furnace, and even from the Big Bang itself.

For matter on Earth, and most of what we can see in the cosmos, this is the total cast of characters that you will need to meet. To make everything hereabouts requires the ingredients of electron and neutrino, and two varieties of quark, known as up and down, which seed the neutrons and protons of atomic nuclei. The four fundamental forces then act on these basic particles in selective ways, building up matter in bulk, and eventually you, me, the world about us, and most of the visible universe.

As a picture is said to be worth a thousand words, I summarize the story so far in the figures showing the inner structure of an atom and the forces of Nature.

1. Inside the atom. Atoms consist of electrons remotely encircling a massive central nucleus. A nucleus consists of protons and neutrons. Protons are positively charged; neutrons have no charge. Protons and neutrons in turn are made of yet smaller particles called quarks. To our best experiments, electrons and quarks appear to be basic particles with no deeper constituents.