Experiment is the sole judge of scientific 'truth'.
In these postmodern times this might seem to be an old-fashioned view, but it is, I believe, correct. Not everyone would agree. By valid, I mean that the experimental result has been argued for in the correct way, by use of epistemological strategies such as those discussed below. It might be useful here to distinguish between the theory of the apparatus and the theory of the phenomenon. Ackermann is talking primarily about the later.