World tube

In physics, a world tube is the path of an object which occupies a nonzero region of space (nonzero volume) at every moment in time, as it travels through 4-dimensional spacetime. That is, as it propagates in spacetime, a world tube traces out a three-dimensional volume for every moment in time. The world tube is analogous to the one-dimensional world line in that it describes the time evolution of an object in space, with the difference that a world line represents the path of a point particle (of nonzero volume), whereas a world tube occupies finite space at all moments in time.