Counterfactual thinking in physics

Counterfactual thinking plays a key role in research in physics and, I believe, in research in all natural sciences. In this contribution I will describe a few examples of counterfactual thinking, how it is used, the power of this method of inquiry, and the types of results that can be achieved. A brief account of the way physicists carry out research will be given and three main types of questions will be identified. Two of them will be used to illustrate the value of counterfactual thinking, one example regarding astronomy, and the other example dealing with electromagnetic forces. The latter might be quite tough for non-physicists. The last, most extended, part of this paper gives an analysis of counterfactual situations: what would the world look like if the constants of nature had different values. This discussion leads to the conclusion that slight changes of these numbers leads to uninhabitable worlds.