Criteria for choice between different physical principles

The violation of Bell's inequality is an example of a conflict not between different physical theories, but between different physical principles. The principles which are in conflict are the principles used in the proof of Bell's inequality.

Criteria for the decision between different physical theories have been in the focus of interest of scientific methodology. There is a large set of criteria which can be applied, if the experiment does not (yet) allow to decide the question: First, there is Popper's criterion of empirical content. Then, there are other criteria, like internal consistency, generality, simplicity, explanatory power, and beauty.

The situation is much less clear if we compare physical principles. Physical principles are properties of physical theories, not themself physical theories. Therefore, they do not make themself physical predictions. This is not a rigorous claim – the violation of Bell's inequality is, in some sense, a counterexample, because in this case a particular combination of principles allows to make a nontrivial physical prediction – namely Bell's inequalities – which can be falsified. But this is not really a counterexample, because the falsification of Bell's inequality is based on various physical theories as well, simply these theories are taken to be granted in our considerations.

So, even if we can falsify some combinations of physical principles, it is clearly not the typical property of a physical principle to produce, taken alone, falsifiable predictions. Thus, Popper's criterion of empirical content is inapplicable if we want to decide between physical principles.

Here we propose some metaprinciples or criteria which can be used to decide, in case of conflict, between physical principles:

We apply each of these metaprinciples to our particular conflict between realism and relativity. The result is quite one-sided, giving a strong argument in favour of realism: