ABSTRACT
This chapter shows the peculiar role played by paths in quantum physics. The well-known two-slit experiment has already been discussed as one of the starting points of the whole path-integral approach to quantum measurements. This experiment is a classic illustration of the peculiarities of quantum mechanics. The advantage of the language of paths as a tool for solving the two-slit paradox is more evident if one takes into consideration a possibility such as the delayed choice considered by Miller and Wheeler. This is one of the most important advantages of the path-integral approach: the same procedure of integral restriction gives both the probability distribution of the measurement outputs and the reduction of the state under the influence of this measurement. This is why the path-integral approach results in absolute quantum restrictions on measurability. No device providing the required information can have less influence than is described by restriction of the path integral.
