What is a law of nature?

Laws of nature are impossible to break, and nearly as difficult to define. Just what kind of necessity do they possess?

In the original Star Trek, with the Starship Enterprise hurtling rapidly downward into the outer atmosphere of a star, Captain James T Kirk orders Lt Commander Montgomery Scott to restart the engines immediately and get the ship to safety. Scotty replies that he can’t do it. It’s not that he refuses to obey the Captain’s order or that he doesn’t happen to know how to restart the engines so quickly. It’s that he knows that doing so is impossible. ‘I can’t change the laws of physics,’ he explains.

We all understand Scotty’s point (although the Enterprise does somehow manage to escape). He cannot break the laws of nature. Nothing can. The natural laws limit what can happen. They are stronger than the laws of any country because it is impossible to violate them. If it is a law of nature that, for example, no object can be accelerated from rest to beyond the speed of light, then it is not merely that such accelerations never occur. They cannot occur.

There are many things that never actually happen but could have happened in that their occurrence would violate no law of nature. For instance, to borrow an example from the philosopher Hans Reichenbach (1891-1953), perhaps in the entire history of the Universe there never was nor ever will be a gold cube larger than one mile on each side. Such a large gold cube is not impossible. It just turns out never to exist. It’s like a sequence of moves that is permitted by the rules of chess but never takes place in the entire history of chess-playing. By contrast, if it is a law of nature that energy is never created or destroyed, then it is impossible for the total energy in the Universe to change. The laws of nature govern the world like the rules of chess determine what is permitted and what is forbidden during a game of chess, in an analogy drawn by the biologist T H Huxley (1825-95).