To understand how our enormous universe behaves it is necessary to examine the behaviour of the tiny particles from which you, I and all matter are made, along with how these particles interact with one another. These particles, as best as we can measure them, have a diameter of about a thousand trillionth of a millimetre. Scientists refer to these particles as fundamental because they cannot break them down into smaller parts. These particles are the building blocks from which all things are made.
The behaviour of large objects like rocks, cars, aeroplanes, asteroids, the moon and other planets in our solar system is familiar to us. These are known as macroscopic objects because they are large compared to fundamental particles. Everything we experience in our daily lives, from other people, through books, to a glass of water, is macroscopic. Macroscopic objects move through time and space just like us. They do not exist as a blur, nor do they disappear from one location to mysteriously appear in another, as some fundamental particles, such as electrons in an atom, can do. In contrast, on the spatial scale at which fundamental particles interact, the microscopic scale, particles behave very differently. They are hard to pin down to a location, and this makes the behaviour of these particles appear deeply peculiar. For some of this chapter and the next, when I describe the behaviour of fundamental particles it helps to suspend disbelief a little. Reality as we know it is not reality at the scale of fundamental particles.
Up until the first half of the twentieth century, most scientists assumed the universe was constant in size, was unchanging, and had always existed and always would. But as telescopes increased in power, it became apparent that the universe was not constant but was expanding, and this suggested it must have had a beginning, and at that beginning our universe must have been very small. It has grown and changed a lot since its birth as the tiny dot of intense energy that it must have been.
Our universe started out tiny but is now vast. In every direction we look from Earth we see universe. We cannot see the universe’s edge, but there is still a lot to look at. We can visualize what we can see by considering a hypothetical space voyage. Imagine being an astronaut on a spacecraft that is 400 kilometres from the surface of our planet – that’s the orbit of the International Space Station. You look out of the window, and you can see our beautiful home world. Let us now start a journey away from Earth. As our planet recedes into the distance, the next object we encounter is the moon, which, like the space station, orbits the Earth but further away. We then start to encounter other planets in our solar system. We see a total of eight planets orbiting the sun – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune – along with a dwarf planet called Pluto, and millions of smaller rocks. As we leave the solar system, the light from the sun dims the further we travel, until other stars shine brighter than our own. We will eventually start to pass some of these other stars, each with its own set of orbiting planets and rocks. In due course, the stars start to become less frequent, and we realize that our sun, now lost in a vast cloud of other stars, is just one of at least one 100 billion stars that are spinning around a central point. All these stars are in our galaxy, the Milky Way.
There are about a trillion galaxies in the part of the universe we can see. When we look at galaxies beyond the Milky Way through telescopes, we discover something interesting: most of them are moving away from us, and the further away a galaxy is, the faster it tends to be moving. If we were to reverse time, something that cosmologists have done in computer simulations of the universe, we discover that all the matter and energy that is now spread across the vast tracts of the cosmos must once have been compressed into a single point called a singularity. The singularity would have been smaller than the smallest of fundamental particles and would have been extremely dense and hot, and from this point emerged the universe in which sentient life evolved at least once, here on Earth. Physicists have shown that key to the way our universe behaves, including why it is expanding, are four fundamental forces that are central to our story. These forces determine how particles, atoms and larger objects including planets, stars and galaxies interact with one another. Because these forces are so fundamental to our existence, before considering how they work it is helpful to introduce them. They are the stars of the book, as without them you and I would not be here, and nor would the atoms from which we are made.
All objects in the universe are made up of atoms. Each atom consists of a central blob called a nucleus and electrons that whizz around the nucleus at various distances from it. Atomic nuclei are made from smaller particles called protons and neutrons. Each atom of an element has the same number of protons and electrons, and, except for one type of hydrogen that has no neutrons, each atom also contains one or more neutrons, with the number depending upon the element. Hydrogen atoms always have one proton and one electron. However, in nature, hydrogen nuclei can have either zero, one or two neutrons. These different types of hydrogen are called protium, which has no neutrons, deuterium, which has one, and tritium, which has two. These three types of hydrogen atom are known as isotopes of hydrogen. Neutrons and protons are built from quarks. The quarks are held together by a force called the strong nuclear force. This force also holds together the protons and neutrons to make atomic nuclei. The strong force prevents neutrons and protons from drifting apart, while also preventing quarks from separating from one another. If the strong nuclear force did not exist, neither would atomic nuclei, nor life.
The next force to introduce is called the weak nuclear force. It also operates within atomic nuclei, and it is the reason that isotopes of some elements are radioactive. Most atomic nuclei are stable and one of their neutrons doesn’t suddenly morph into a proton, or vice versa, creating an atom of a different element. However, some isotopes of some elements have an unstable configuration of neutrons and protons, and this can cause a proton to become a neutron, or a neutron to become a proton. When this happens, a type of radiation is emitted. Not all radioactive elements decay like this, but ones that do, like tritium (the isotope of hydrogen with two neutrons), which decays into helium, do so because of the weak nuclear force. You might think that the weak nuclear force has little to do with daily life, but without it the nuclear fusion that powers our sun could not happen. Nuclear fusion is caused by the weak nuclear force, and in the sun it is responsible for creating helium from hydrogen isotopes, and this generates a lot of energy in the form of heat and light. Without the weak nuclear force, our universe would be dark. No weak force, no life.
Protons and neutrons are often depicted in diagrams as tiny balls that stick together to form atomic nuclei, and that is a reasonable analogy, even if it is not quite accurate. A blur, or fuzz, of electrons whizzes around the nuclei in a type of orbit that chemists call an orbital. The electromagnetic force keeps the electrons in place. If electromagnetism did not exist, electrons would drift away from atomic nuclei and atoms could not exist.
Electromagnetism does more than this. Atoms come in different types, determined by the number of protons, neutrons and electrons. The number of protons and electrons in an atom is equal, and this number determines the element. For example, lithium has three protons and three electrons, while carbon has six of each. When a chemical reaction occurs, atoms share or exchange electrons with one another thanks to the electromagnetic force. When they do this, they form molecules. Water is an example of a common compound made of molecules. Each water molecule consists of two atoms of hydrogen and one atom of oxygen bound together by the way they share electrons via the electromagnetic force. Carbon dioxide, methane and rust are examples of other molecules. Molecules occur in an extraordinarily diverse array of forms. but each is made of atoms, and each atom is made of protons, neutrons and electrons. No electromagnetic force and no atoms or molecules.