Last Friday I was at the Waikato Science Teachers' one day conference in Cambridge. There was a wide range of different material talked about, which made for an interesting day. One of the questions which was tackled (led by Simon Taylor) was 'When is an experiment valid?' Or, what is 'validity' all about when it comes to science. Some thoughts from the audience were 'When it does what it is supposed to', 'When it agrees with theory', 'When it is controlled', 'When it is repeatable', 'When it measures what it is supposed to'.
All of these I think are reasonable responses, depending upon the situation. f you are illustrating a physical principle to a class then you certainly want your experiment to behave - we all know that physics experiments are too often characterized by the fact that they don't work - especially when they have a big audience.
But an experiment can never 'not work'. It does what it does. The fact that it didn't do what you wanted or expected could be for a variety of reasons - bad experimental design, poor control, statistical variation, or maybe because of some 'new' phenomenon. After all, where would science be if every experiment agreed with current theory? Major strides forward in science have usually been triggered through experiments that didn't do what the experimenters were expecting.
I think a good definition of 'valid' would be 'that the experiment measures what you think it is measuring.' That, from memory, was basically Simon's point. If we achieve that, then it doesn't matter whether we find a new phenomenon, validate an existing theory, or just make a mundane measurement of electric current in the lab. We've done some good experimental physics.
Sorry - short entry this one - baby wants attention.