Q&A: The Law of Conservation of Momentum
The Law of Conservation of Momentum
Question
I heard the following argument against free choice:
The law of conservation of momentum states that the total momentum in a closed system is constant.
The universe is a closed system.
Therefore there is no free choice. Because if we had free choice to move our bodies as we wished, that would contradict the claim that the total momentum remains constant.
Is that correct?
Answer
This is a particular case of a more general argument: there is no choice, because if there were choice, it would follow that according to the laws of nature the world should do X, but the choice is to do Y. So what happened contradicts the laws of nature. QED. A similar argument is also raised against divine involvement in the world.
These arguments, of course, beg the question. Indeed, when a person chooses, the laws of nature are broken, and when God is involved, the laws of nature are violated. Anyone who believes in free choice or in divine involvement must assume that there are holes in natural/physical determinism. More simply: the electron that begins the process of translating my choice into action starts moving without a physical cause. This is of course against Newton’s laws (but not against the principle of causality, because there is a non-physical cause here. By contrast, our will has no cause at all).
All this was explained in my book The Science of Freedom.
On second thought, your question specifically is actually not difficult in my opinion. Every movement we make changes my momentum, but it will preserve the overall momentum of the world. Suppose I decide to move to the right; something will move to the left and balance it out somewhat (the Earth will slow down a bit). After all, determinists also have to assume this. If my brain decided that I would move to the right, then the decision in the brain is the cause of the movement, but it does not supply the momentum for that process. There still has to be something that balances the total momentum.