4,200 words

Edited by Sally Davies

On a mountain road from Koya to Ryujin, Japan. 1998. Photo by Peter Marlow/Magnum

The French mathematician Pierre-Simon Laplace (1749-1827) believed that the Universe was a piece of machinery, and that physics determines everything. Napoleon, who had read up on Laplace’s work, confronted him about the conspicuous absence of a creator in his theory. ‘I had no need of that hypothesis,’ came the reply. Laplace might have said the same thing about free will, which his mechanistic universe rendered superfluous.

Since Laplace’s day, scientists, philosophers and even neuroscientists have followed his lead in denying the possibility of free will. This reflects a widespread belief among theoretical physicists that if you know the initial values of the variables that characterise a physical system, together with the equations that explain how these variables change over time, then you can calculate the state of the system at all later times. For example, if you know the positions and velocities of all the particles that make up a gas in a container, you can determine the positions and velocities of all those particles at all later times. This means that there should be no freedom for any deviation from this physically determined trajectory.

Consider, then, that everything we see around us – rocks and planets, frogs and trees, your body and brain – is made up of nothing but protons, electrons and neutrons put together in very complex ways. In the case of your body, they make many kinds of cells; in turn, these cells make tissues, such as muscle and skin; these tissues make systems, such as the heart, lungs and brain; and these systems make the body as a whole. It might seem that everything that’s happening at the higher, ‘emergent’ levels should be uniquely determined by the physics operating beneath them. This would mean that the thoughts you’re having at this very moment were predetermined at the start of the Universe, based on the values of the particle physics variables at that time.

Now you might be forgiven for doubting whether William Shakespeare’s sonnets, Winston Churchill’s speeches and the words in Stephen Hawking’s book A Brief History of Time (1988) really came into being in this way. And you would be right to doubt: there are many problems with the skeptics’ position.

At very small scales, quantum theory underlies what’s happening in the world. Heisenberg’s uncertainty principle introduces an unavoidable fuzziness and an irreducible uncertainty in quantum outcomes. You might know the value of one variable, such as a particle’s momentum, but that means you can’t accurately detect another, such as its position. This seems to fundamentally undermine the allegedly iron-clad link between initial data and physical results. However, this is controversial, so I’ll set it aside for now, as important as it is. Instead, I’ll focus on key aspects of causation that occur in the molecular biology of neurons in the brain.

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 functioning 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.[…]

Continue reading: https://aeon.co/essays/heres-why-so-many-physicists-are-wrong-about-free-will