Worksheet on Time Travel

From George Ellis)
The Myth of a Purely Rational Life, pp14

World 3a: The world of physical possibilities, delineating possible physical behaviour

(it is a description of all possible motions and physical histories of objects). Thus it
describes what can actually occur in a way compatible with the nature of matter and
its interactions; only some of these configurations are realised through the historical
evolutionary process in the expanding universe. We do not know if laws of behaviour
of matter as understood by physics are prescriptive or descriptive, but we do know
that they rigorously describe the constraints on what is possible (you cannot move in a
way that violates energy conservation; you cannot create machines that violate
causality restrictions; you cannot avoid the second law of thermodynamics; and so
on). This world delineates all physically possible actions (different ways particles,
planets, footballs, automobiles, aircraft can move, for example); from these
possibilities, what actually happens is determined by initial conditions in the universe,
in the case of interactions between inanimate objects, and by the conscious choices
made, when living beings exercise volition.