1. The Everett approach and quantum branching ↑
According to the Everett, or many worlds, approach to quantum mechanics, parallel worlds are not created every time we make a decision because we only branch when quantum interactions have macroscopic effects.
There are, however, a number of processes that do cause us to branch:
Any quantum experiment can have important macroscopic affects if the experimenter places bets on the outcome, and so we can create worlds of our own choosing in this way. We could, for example, decide to learn to play the guitar if an atom is measured with an 'up' spin state, and piano if it is 'down', by doing this we could ensure that we somehow get to do both. This sort of branching would not occur, however, unless we really intend to follow through with each action. Otherwise, we will be constrained by free will.
Many forms of technology rely on quantum tunnelling, including transistors, microchips, lasers, digital cameras, and USB drives.
Biology also offers many examples of quantum interactions that have macroscopic effects.
2. Quantum mechanics and biology ↑
Austrian physicist Erwin Schrödinger was the first to suggest that the genetic code can be regarded as a quantum code, with fluctuations causing mutations, in 1944. We now know that mutations can be caused by proton tunnelling, a quantum event[2a].
British-Irish geneticist Johnjoe McFadden and British physicist Jim Al-Khalili provided further evidence that the genetic code can be regarded as a quantum code in 1999 and, in 2001, Indian physicist Apoorva Patel showed that the polymerase enzyme, involved in replicating DNA, picks nucleotides in accordance with the Born rule.
In 2007, Gregory Engel and colleagues at the University of California, Berkeley, showed that photosynthesis, the process that provides fuel for almost all life on Earth, utilises quantum interactions in order to be more energy efficient.
That same year, biochemist Ismael Tejero and colleagues at the Autonomous University of Barcelona showed that quantum tunnelling is utilised by antioxidants called catechins, which are found in tea, wine, and some fruits, vegetables, and chocolate. Catechins work by neutralising free radicals, ions that can damage cells and react in the bloodstream, and would not be able to transfer an electron to the ion without utilising quantum effects.
Jennifer Brookes and colleagues at University College London showed that our sense of smell is dependent upon random quantum interactions in 2007. This theory was first suggested by biophysicist Luca Turin, while working at University College London in 1996.
The classical view suggested that different smells are triggered when molecules, called odorants, enter receptors in our nose. It was not known why different things smell the way they do, because molecules with similar shapes do not necessarily smell like one another. Turin suggested that different smells are related to different frequencies of vibrations that are caused by quantum tunnelling.
2.1 The anthropic principle ↑
Quantum interactions could even explain how life developed in the first place. British physicist Paul Davies claimed that it's very unlikely that life arose from classical theory. He stated:
"simple calculation shows that it would take much longer than the age of the universe, even if all the matter in the universe consisted of pre-biotic soup, for even a single protein to form by chance. So the classical chance hypothesis seems unsatisfactory"[2b].
If the Everett approach is correct, then it can provide a new interpretation of the anthropic principle. The anthropic principle was devised to explain why the universe appears to be perfectly designed for our existence, in a way that seems highly improbable.
The weak anthropic principle, first suggested by American physicist Robert Dicke in 1961, states that this is a selection bias, as we couldn't observe any world that is incapable of containing life.
The strong anthropic principle, first suggested by Australian physicist Brandon Carter in 1974, implies that life is somehow needed for the universe to exist.
British writer Douglas Adams illustrated the problem with the strong anthropic principle as follows:
"Imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!' This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on to the notion that everything's going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise".
Proponents of the weak anthropic principle argue that the universe would not be hospitable to life if any of a number of universal constants were different. These include, the ratio between the strength of the force of gravity and the nuclear forces, the proportionality constant between energy and mass, and the ratio between the rate at which dark energy is accelerating the expansion of spacetime and the rate at which the universe would otherwise collapse due to gravity.
If the force of gravity were much stronger than the strong and weak nuclear forces, then almost everything would be smaller because objects would get heavier more quickly. Stars would not need to be as massive in order to radiate, and everything else would shrink in turn. If life forms did exist, then they too would have to be tiny. Even creatures the size of insects would have to have thick necks to hold their heads up. If gravity were stronger still, then the universe would be a tangle of black holes, as everything would fall in on itself.
If the amount of energy produced in nuclear reactions were smaller, then stars would not be able to produce the elements that we need to live. If it were larger, then gravity would be important at smaller sizes and, again, everything would be too small for life to evolve.
The evolution of life was also reliant upon quantum fluctuations that occurred during the inflationary epoch of the early universe. If there were no fluctuations, then everything would be equally dispersed, and so mass and energy would drift apart before it could become gravitationally bound. If the fluctuations in symmetry were too vast, however, then everything would be distributed too closely together, creating another universe full of black holes.
Another important factor for any universe that contains life is the number of dimensions that we experience. If we lived in a world with only two spatial dimensions, then any holes in our body, such as the tubes that carry our food, oxygen, and blood, would cut us in half. If we were to experience four spatial dimensions however, then the force of gravity would be proportional to the cube of an object's mass, rather than the square. This would mean that stars would be too heavy to last very long, and life would not have time to evolve.
The weak anthropic principle states that the universe must be the way it is because we wouldn't be able to observe it if it were any other way. Yet this still doesn't explain why it happened to be suitable for life. It's like saying to someone who survived almost certain death that they survived because they wouldn't be here if they hadn't. This still doesn't explain what happened.
Everett's interpretation provides a natural extension of the weak anthropic principle by suggesting that there are an infinite amount of worlds and so, however improbable our existence is, we know that life must exist somewhere. This is analogous to the explanation that a person survived almost certain death because one in every million people always do, and a million people had already faced certain death that day. An unusual event occurred because it had a non-zero probability and a vast number of trials were run.