ABSTRACT

Experiments in fundamental physics tend to be very costly and complicated. Normally the same particle, or the same atom, or whatever it is that we are studying, is passed through a sequence of measuring systems, each one of which will try and extract some information about what is going on. e chances are that each measuring device will have an e ect on the state of the system. e set of amplitudes describing what might happen at one measuring device will depend on the outcome of the previous measurement. If we are going to be able to deal with this, we need some way of tracing amplitudes through a sequence of situations. We need to know how to combine and convert amplitudes.