Assuming the distance to move and the acceptance rate are appropriate, the Metropolis–Hastings algorithm is a faster way of finding the best solution to a problem than deterministically evaluating the likelihood at each point on a grid laid across the likelihood surface and mapping the entire mountain range. The algorithm is also only one of very many that computers use to solve all sorts of problems in statistics and artificial intelligence. Computers are often described as mimicking the human brain, and perhaps we too use randomness to make decisions on what to do and how to behave. The only known source of randomness we could use is quantum randomness at the level of the fundamental particles, atoms and molecules, but perhaps science will one day uncover other sources of randomness in the universe.
Evolutionary biologists use a similar analogy when thinking about how evolution can produce more competitive individuals, lineages and species. Instead of being parameters, latitude and longitude are now values of phenotypic traits, and altitude is replaced with how good individuals with particular trait values are at surviving and reproducing. Random mutation means some individuals are less good at surviving and reproducing than their parents because they have less fit phenotypic trait values. However, every now and then a mutation generates phenotypic trait values that increase survival and reproductive rates. Evolution slowly climbs towards the survival and reproductive peak, but in doing so produces individuals that struggle to survive or are unable to reproduce.
Randomness is potentially pervasive through biology, yet research into its role is still in its infancy. Quantum biology and quantum consciousness are very new fields of scientific endeavour. If I were starting out in science today, I think I would seriously consider immersing myself in one of these fields. I reached this conclusion, because in researching this book, I have formed the opinion that until physicists are able to produce data to demonstrate that quantum mechanics is attributable to some underlying deterministic process, then I will take the quantum strangeness at face value, and conclude that the universe is stochastic. If I was to rerun my replicate universe experiment, each outcome would be different. Perhaps I would find the physical universe is deterministic, and our solar system will exist in each universe rerun, while life is stochastic. Maybe sometimes when we visit Earth in each universe, humans will exist, while at other times perhaps a species of malevolent intelligent lizard is the dominant species on our planet. Or perhaps I am just trying to find a way to bring the physicists who argue for a deterministic universe and the biologists who are convinced the universe is random together. Regardless, this discussion does not answer the question of why we exist.
I started my personal journey to try to understand why I existed when I lay on my deathbed. Writing this book does not, I hope, signal my imminent demise, as there is still much more I want to understand. Nonetheless, I have some idea of why it is we exist.
One downside of the universe being stochastic, and the Earth being the only planet where we have been able to study life, is that it is impossible to confidently assign probabilities to events that we only have evidence of happening once here on Earth. In particular, Earth is the only planet on which life is known to exist, so assigning an accurate probability of life evolving on other planets, even if they are similar to Earth, is currently impossible. We need more data before we can state if life arises frequently or not.
Despite this challenge, scientists have made remarkable progress in working out what had to happen for us to exist, why it happened, and even how it happened, but there are a few key events in our history we can still only speculate about.
The first key event we cannot assign a probability to is the birth of our universe. We have never observed another universe, and we have never observed anything beyond our universe. We don’t even know where the edge of our universe might be, or indeed if it even has one. We cannot look beyond our universe in an attempt to observe others coming into existence (or not). We consequently do not know whether universes are common or if there is only one, and if they are common, whether ours is typical or unusual. Put another way, we do not know why there is something rather than nothing.
What we do know with reasonable confidence is that when our universe formed, it consisted of a singularity of extremely hot energy. The four fundamental forces quickly emerged as the singularity expanded and cooled, and the fundamental particles from which all matter is made came into existence. Much like the formation of the universe, the four forces emerged only once, so we do not know whether it was inevitable or fortuitous that gravity, electromagnetism and the weak and strong nuclear forces emerged with the strengths that they have. Perhaps a different set of forces would emerge if we were to conduct the rerun experiment, and there may be fewer, or more, than the four we observe in our universe. Another unknown is consequently why we have the forces we do.
What we do know is that life as we know it requires all four forces, and computer simulations reveal that even small differences in each of their strengths could result in universes without atoms, molecules, stars or planets. Ultimately the reason we exist is that the fundamental forces took the values they did, and this allowed protons, neutrons, atoms, stars, molecules, galaxies and planets to emerge. We are here because the fundamental forces are just right. Given they have these values, science has a very good, but not yet complete, understanding of why these things formed and why they behave as they do.
Some scientists and philosophers have marvelled that our universe has forces of the right strength for life to exist. Some even interpret this coincidence as evidence that our universe could not have arisen by chance. However, if our universe did not have forces that were just right for life, then life could not evolve and no one could observe it. We can only observe the universe because it is just right for life to evolve. There may be many other universes that weren’t right for life, and they have never been observed. The fact we can observe our universe because things were just right for life to evolve is called the anthropic principle. We exist because the early universe developed in a way that would become favourable for life (at least in one location). We don’t know why this happened, but we should not be surprised, because if our universe hadn’t developed like this we could not exist or wonder about our existence. I think there are more exciting questions to ponder than the anthropic principle, which I find a circular argument.
Unifying the standard model of electromagnetism and the strong and weak nuclear forces with Einstein’s theory of the way gravity works would provide a more complete understanding of our universe, and is an exciting research area in theoretical physics. Theories such as string theory and quantum loop gravity provide approaches to unify the four forces, but testing them, and showing which theory is right, requires so much energy it is currently beyond our technical expertise. Nonetheless, Einstein’s theory of general relativity and the standard model provide good explanations for why matter behaves as it does. In principle, large particle accelerators might one day be able to re-create conditions a little more similar to those found in the early universe than we can achieve today, and experiments in these machines might shed light on why and how forces emerge with particular properties, but currently our technology is not sufficiently advanced to build such a facility.
If I was to create universes, each with our gravity, electromagnetism and strong and weak nuclear forces, where quantum behaviour is truly random, the universes would look similar, but not identical, to ours. In any universe with identical forces to ours, physical and chemical processes will be the same as those uncovered by scientists in our reality. The first generation of stars would consist of hydrogen and helium, and their deaths would create heavier elements in the ratios we observe. The molecules we are familiar with would also form. Water, carbon dioxide, nitrogen, methane, gold, uranium and other elements and molecules would be as common as they are in our universe. The fundamental forces determine the dynamics of all elements and molecules, so as long as these forces emerge as they are in our universe, and quantum behaviour is random, the processes we observe in our universe will be identical to those in our experimental reruns, yet each universe would be unique. We could exist, but we wouldn’t necessarily do so.