Human Universe (23 page)

This chapter is also about you. I suspect most of us have mused about the question ‘Why are we here?’ For some, the question and answer may be absolutely central to their lives. For others, myself included, it’s something I used to think about on a hillside desolate beside a punctured bicycle whilst wearing a secondhand overcoat I bought from Affleck’s Palace, but my existentialism faded with my hair.

Having said that, a little existentialism, like the Manchester rain, never did anyone any harm, so let’s place ourselves at the centre of things for a while and explore the immense contingency of our personal existence as a warm-up for the much deeper problem of the origin of the universe itself. It’s a pretty deep chapter this, so put on
Unknown Pleasures
, grab a bottle of cheap cider and let’s get going.

If, in a moment of solipsism, you decide to work out the odds of your own existence, you might come to the conclusion that you are astonishingly special. You began as a particular egg inside your mother, fertilised by a particular sperm from your father. There were 180 million sperm around that day, each with a different genetic code, only one of which became ‘you’ in combination with one of your mother’s million or so genetically unique eggs. So without going any further, you might feel lucky. If you chose to carry on, you might factor in the odds of your parents having sex on that particular day, because sperm are constantly manufactured. Then there are the odds of them meeting at all, and the odds of them being THEM. And whilst we’re picking up increasing armfuls of odds at the 1-in-a-100-million level, recall from Chapter 1 that there exists an unbroken line of your ancestors stretching back over 3.8 billion years to LUCA – the Last Universal Common Ancestor. If any one of those living things had died before it reproduced, you wouldn’t exist. That’s pretty lucky, but also completely devoid of any meaning at all. Yes, the odds of YOU existing are almost, but not quite, zero. But given the existence of the human race and a mechanism for procreation, someone has to be born. So whilst the probability of any given individual existing is tiny, it is inevitable that new babies will be born every day. Seen in this light, you are not special and your existence in the grand scheme of things is entirely understandable. Time for Joy Division and cider.

This demolition of your individual self-importance relied on the fact that a mechanism exists for the inevitable production of large numbers of human beings, given the important precondition that humans already exist. We’ve explored the road to human existence at length in the book already, and argued that complex multicellular life and intelligence at or beyond the level of humans may be rare in our universe. It is also clear that there are fundamental properties of the universe itself that are necessary for the existence of any form of life. The universe must live long enough and have the right properties for stars to form, and those stars must be capable of producing the chemical elements out of which living things are made, carbon being the most important. What do we mean by ‘properties’? We are back to the nature of the laws of physics once again, because they describe the behaviour of matter and forces at the most fundamental level. The laws restrict the possible physical structures that are allowed to appear in the universe, and stars, planets and human beings are all examples of such possible physical structures. Questions now naturally arise; more modest perhaps than our grand ‘Why are we here?’ puzzle, but more amenable to scientific enquiry. How do the laws of nature allow for human beings to exist, and by how much could those laws vary before life could no longer exist in the universe?

Let us begin in the spirit of taking small steps with a brief summary of the known fundamental laws of nature.

Attempting to describe the laws that govern the existence of everything from galaxies to human beings in a single paragraph of a book of a TV series might seem overly ambitious. It is at one level; otherwise everyone would complete physics, chemistry and biology degree courses in an afternoon. What we can do, however, is to outline the known fundamental laws in a concise and accurate way, so let us do that.

There are twelve known particles of matter, listed on
here
. They are arranged into three families, or generations. You are made out of particles in the first generation alone. Up quarks and down quarks bind together to make protons and neutrons, which in turn bind together to form your atomic nuclei. Your atoms are composed of electrons bound to those nuclei. Molecules, such as water and DNA, are built up out of collections of atoms bound together. That’s all there is to you; three fundamental particles arranged into patterns. Particles called gauge bosons carry the forces of nature. There are four known fundamental forces: the strong and weak nuclear forces, electromagnetism and gravity. Gravity is missing from the figure
here
, and we’ll get to that in a moment. The other three forces are represented in the fourth column. To see how this all works, let’s focus on the familiar electromagnetic force. Imagine an electron bound to the atomic nucleus of one of your atoms. How does that binding happen? The most fundamental description we have is that the electron can emit a photon, which you can think of as a particle of light. That photon can be absorbed by one of the quarks inside the nucleus, and this emission and absorption acts to assert a force between the electron and the quark. There is a vast number of ways in which the electrons and the quarks inside the nucleus can emit and absorb photons, and these all combine to keep the electron firmly glued to the nucleus. A similar picture can be applied to the quarks themselves. They also interact via the strong nuclear force by emitting and absorbing force-carrying particles called gluons. The strong nuclear force is the strongest known force (the clue is in the name) and binds the quarks together very tightly indeed. This is why the nucleus is significantly smaller and denser than the atom. Only quarks and gluons feel the strong nuclear force. Finally, there is the weak nuclear force. This is mediated by the exchange of the W and Z bosons. All known particles of matter feel the weak nuclear force but it is extremely weak relative to the other two, which is why its action is unfamiliar, but not unimportant. The Sun would not shine without the weak nuclear force, which allows protons to convert into neutrons, or more precisely up quarks into down quarks, which has the same result. This is the first step in the nuclear burning of hydrogen into helium, the source of the Sun’s energy. During the conversion of a proton into a neutron, an anti-electron neutrino is produced along with an electron. The neutrino is the remaining particle in the first generation we haven’t discussed yet. Because neutrinos only interact via the weak nuclear force, we are oblivious to them in everyday life. This is fortunate, because there are approximately sixty billion per square centimetre per second passing through your head from the nuclear reactions in the Sun. If the weak force were a little stronger, you’d get a hell of a headache. Actually, you wouldn’t because you wouldn’t exist, and this foreshadows the subject of the fine-tuning of the laws of nature we will undertake later in this chapter. The one remaining type of particle is the Higgs Boson, on its own in the fifth column. Empty space isn’t empty, but is jammed full of Higgs particles. All the known particles apart from the photon and the gluons, which are massless, interact with the Higgs particles, zigzagging through space and acquiring mass in the process. This is the counter-intuitive picture that was confirmed by the discovery of the Higgs Boson at CERN’s Large Hadron Collider in 2012.

Two further generations of matter particles have been discovered. They are identical to the first generation except that the particles are more massive because they interact with Higgs particles more strongly. The muon, for example, is a more massive version of the familiar electron. The reason for their existence is unknown.

This is all there is in terms of the description of the fundamental ingredients of the universe. There are almost certainly other particles out there somewhere – the dark matter that dominates over normal matter in the universe by a factor of 5 to 1 is probably in the form of a new type of particle which we may discover at the Large Hadron Collider or a future particle accelerator. The evidence for dark matter is very strong and comes from astronomical observations of galaxy rotation speeds, galaxy formation models and the cosmic microwave background radiation that we met in Chapter 1 and will meet again later in this chapter. But because we don’t know what form the dark matter takes, we are not able to incorporate it into our list.

The mathematical framework used to describe all the known particles and forces other than gravity is known as quantum field theory. It is a series of rules that allows the probability of any particular process occurring to be calculated. The whole thing can be described in one single equation, known as the Standard Model Lagrangian. Here it is:

It takes a lot of work to use this piece of mathematics to make predictions, but the predictions are spectacularly accurate and agree with every experimental measurement ever made in laboratories on Earth. This equation even predicted the existence of the Higgs particle; that’s how good it is. It probably looks like a set of squiggles unless you are a professional physicist, but in fact it isn’t too difficult to interpret, so let’s dig just a little deeper. The 12 matter particles are all hidden away in the symbol Ψ
_j
. The Standard Model is a quantum field theory because particles are represented by objects known as quantum fields. There is an electron field, an up quark field, a Higgs field and so on. The particles themselves can be thought of as localised vibrations in these fields, which span the whole of space. Fields will be important for us later, when we’ll want to think about a certain type of field that may have appeared in the very early universe, known as a scalar field. The Higgs field is an example of a scalar field. The mathematical terms between the two Ψ
_j
s on the second line describe the forces and how they cause the particles to interact. The forces are also represented by quantum fields. The term –
g
_s
T
_j
· G
_µ
for example, describes the gluon field that allows the quarks in the Ψ
_j
terms to bind together into protons and neutrons. The term
g
_s
is known as the strong coupling constant. It is a fundamental property of our universe that encodes the strength of the strong nuclear force. Each of the forces has one of these coupling constants. We will want to discuss these coupling constants later, because they define what our universe is like and what is allowed to exist within it. The last two lines deal with the Higgs Boson. The strength of the interaction between a matter particle and the Higgs field is contained in the
y
_j
terms, which are known as Yukawa couplings. These must be inserted to produce the observed masses of the particles of matter. That’s pretty much it.

Here ends our crash course on particle physics. The central point is that there exists a remarkably economical description of everything other than gravity, and it is contained within the Standard Model.

We considered the gravitational force in some detail in Chapter 1. It is described by Einstein’s Theory of General Relativity, which is what physicists call a classical theory. There are no force-carrying particles in Einstein’s theory; instead the force is described in terms of the curvature of spacetime by matter and energy and the response of particles to that curvature. A quantum theory of gravity, which we have already noted will be necessary to describe the first fleeting moments in the history of the universe, would involve the exchange of particles known as gravitons, but as yet nobody has worked out how to construct such a description. This is why Einstein’s theory remains the only fundamental non-quantum theory we have.

For completeness, let’s refresh our memory of Einstein’s Theory of General Relativity: