It is also possible to reexpress the nonrelativistic time evolution in terms of propagators going toward the past, since the Schrödinger equation is time-reversible. The past propagator is the same as the future propagator except for the obvious difference that it vanishes in the future, and in the Gaussian t is replaced by − t. In this case, the interpretation is that these are the quantities to convolve the final wavefunction so as to get the initial wavefunction: