One of the most astounding discoveries of the previous century was that biological activity at the micro level is literally grounded in the physical shape of biological molecules, particularly DNA, RNA and proteins. This discovery became possible only when X-ray crystallography had progressed to the point of allowing us to determine the extraordinarily complex detailed structure and foldings of these molecules.
The structure of these molecules is truly the secret of life, as Francis Crick and James Watson exclaimed when they discovered the double helix structure of DNA, helped by the work of Rosalind Franklin. This deservedly led to huge public excitement about how DNA molecules encode our genetic inheritance. However, it is the structure of other molecules – proteins and associated messenger molecules – that in fact makes things happen at the cellular level. DNA is important only because it codes for the proteins that do the real biological work. For example, haemoglobin in blood cells transports oxygen from the lungs to the rest of the body. Rhodopsin in the eye absorbs light and turns it into electrical signals. Kinesin and dynein are motor proteins that transport materials from one place to another in a cell. Enzymes speed up chemical reactions by such huge amounts that they essentially turn them on and off. Voltage-gated ion channels serve as biological versions of transistors, while ligand-gated ion channels allow messenger molecules ('ligands') such as neurotransmitters to convey information from one cell to another in the brain. And so it goes. And all this follows from the details of the complex shapes of these proteins.
This means that, to link physics and biology, we need to look at the theory that underlies molecular shape. And that theory is quantum chemistry, based in the fundamental equation of quantum physics: the Schrödinger equation. In quantum theory, the state of a system is described by what's known as its wave function, which determines the probabilities of different outcomes when events take place. The Schrödinger equation governs how the wave function changes with time. For example, it governs the process of quantum tunnelling, which in turn underlies important physical effects such as how the Sun generates energy via nuclear fusion, photosynthesis in plants, and flash memories you use to store data in computer USB flash drives.
I will take for granted the validity of the Schrödinger equation, which is one of the best-tested equations in physics. To link this to the functioning of life, we need to apply the Schrödinger equation to the wave function of the relevant molecules – in this case, proteins – so as to determine how their shape will change with time. So the actual question is: does the Schrödinger equation, together with the initial state of the wave function describing everything that existed in the early Universe, determine everything I think today because it determines the states of all the biomolecules in my body?
The confounding thing for free-will skeptics is that all outcomes don't depend only on the equations and the initial data. They also depend on constraints. An example is an apple under the influence of gravity, such as the one that Isaac Newton watched fall to the ground from a tree at Woolsthorpe Manor. That was its unconstrained motion.
Now suppose Newton had suspended the apple from a branch of the tree by a string attached to its stalk. It would thereby have been turned into a pendulum, because the string constrained its motion. Instead of dropping to the ground, it would have swung back and forth in a circular arc under the branch, with its state of motion determined uniquely by its initial position and velocity. Consequently, the motions of all the billions of atoms that make up the apple would then also be determined by the string. It would make each of them also move in a circular arc under the support. This is how constraints shape outcomes.
Now let's examine variation of the constraint over time. While the apple was swinging peacefully back and forth, imagine that Newton cut the string. The apple would then have fallen to the ground. The initial state (its speed in a circular arc when it started) no longer determined the outcome. It was the unexpected cutting of the string that determined what happened, because it removed the previous constraint. The moral of the story is that, when constraints vary, outcomes are not determined by initial conditions; they depend on the way that the constraints change with time.
From chaos to free will Page 65
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