In fact, burning fuel is only one of many things a spaceship has to do to accomplish its real purpose, which is to transport its payload from one point in space to another.

that is physically special. Within one universe it looks irregular; across universes it has a regular structure, like a crystal in the multiverse.

Facts cannot be understood just by being summarized in a formula, any more than by being listed on paper or committed to memory. They can be understood only by being explained. Fortunately, our best theories embody deep explanations as well as accurate predictions.

I shall assume that the ‘laws of physics’ include an as yet unknown rule determining the initial state or other supplementary data necessary to give, in principle, a complete description of the multiverse (otherwise these data would be a set of intrinsically inexplicable facts). In that case, an environment is physically possible if and only if it

So when a cosmic-ray particle strikes a DNA strand and causes a mutation, some of its counterparts in other universes are missing their copies of the DNA strand altogether, while others are striking it at different positions, and hence causing different mutations. Thus a single cosmic-ray strike on a single DNA molecule will in general cause a

High-level phenomena about which there are comprehensible facts that are not simply deducible from lower-level theories are called emergent phenomena.

Quantum theory is, as I have said, one such theory. But the other three main strands of explanation through which we seek to understand the fabric of reality are all ‘high level’ from the point of view of quantum physics. They are the theory of evolution (primarily the evolution of living organisms), epistemology (the theory of knowledge) and the

There are no measurable continuous quantities in physics.

For even in purely practical applications, the explanatory power of a theory is paramount and its predictive power only supplementary.