## What is the philosophy of physics?

Physics and its philosophy

Physics is the search for a mathematical model which describes the phenomena around us. New physics usually starts with observing something that cannot be explained, after which a hypothesis is formulated which explains the observations. The physicist then tries to come up with experiments that show that the hypothesis is true or false, or should be modified.

What is the philosophy of physics?

There are many difficulties with the picture of physics that I just described. What counts as an observation? How can we ever justify a general hypothesis on the basis of a finite number of observations? What is the nature of mathematics? These are questions about the philosophy of physics. On my blog I’m going to address these and similar questions, but here I’d like to discuss just one example of a topic in the philosophy of physics – determinism.

Determinism

The idea of determinism is that if we know all that there is to know about a physical system at one point in time – the position and velocity of all particles in the system – and we know the laws that tell us how the system changes, then we can calculate what is going to happen. As an example, think of a coin toss. We usually say that the probability of heads and tails are ½ because we have no reason to think otherwise.

But a strict believer in determinism might say something else. If we know exactly the situation when the coin was tossed, the position and velocity of all particles at that moment, we can use the laws of physics to calculate the outcome of the coin toss. The probability of an outcome is then either one or zero – it either happens, or it doesn’t. A physical model is deterministic if complete knowledge of the initial situation allows us to predict the outcomes with certainty, while a model that yields uncertainty about the outcomes is called indeterministic.

Sub-quantum theory

It is often stated that, since there is uncertainty in quantum theory, it must be the case that our reality is indeterministic. That conclusion is not justified, since the uncertainty in the quantum theory means only that quantum theory is indeterministic, and not the reality that it describes. It could be that there is some underlying theory that describes a deterministic reality, of which quantum theory is only an approximation. We do not have such a theory yet, but a minority of physicists believe that we will find a deterministic ‘sub-quantum theory’ in the future (e.g. the Cellular Automata version of quantum theory of Gerard ‘t Hooft).

Why should we care?

Why should we care about the philosophy of physics? If all we want is a better mathematical model, so that we can construct better cars and faster rockets, then why don’t we stick to physics itself? Why do we have to drag in philosophy?

Physics is the attempt to reshape our mathematical model so that it yields the best predictions, but questions about the model (or about different possible models) are philosophical questions. For example, when is a model a good model? When it is as accurate as possible, or when it is as broadly applicable as possible? Therefore, if it has to be decided where to invest money for scientific research, philosophical questions are important.

For me personally, there is a far more important reason: I’d like to understand reality as best as I can. Nothing can be known with absolute certainty, but the second best thing is that the philosophy of physics makes it possible for us to explore the limits of our own knowledge. Questions like “what is the nature of time and space?” and “what happened before the Big Bang?” are clearly connected to physics, but even questions involving our free will and the meaning of life are, in the end, questions in the philosophy of physics. Physics and philosophy are two sides of the same coin – both are applied logic.

[Thanks to Carlo Rovelli for proof-reading the text] 1. Nesar Ali Titumir says: