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I have recently read that different interpretation of QM define measurement differently, but that some realists (examples?) refuse to define measurement at all..why is this?

I'm taking a class on the philosophy of physics and currently studying the measurement problem. I'm trying to understand the significance of why defining what a measurement 'is' is so important to this problem. I made a statement "Measurement plays a fundamental role, but is defined differently with respect to alternative interpretations of the quantum theory", but my lecturer pointed out that some realists do not define measurement at all, but I don't understand why?

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    Where did you read that? Can you maybe give references and a context? – iphigenie May 3 '14 at 10:22
  • It's from my lecturer, I'm taking a class on the philosophy of physics and currently studying the measurement problem. I'm trying to understand the significance of why defining what a measurement 'is' is so important to this problem, I made a statement "Measurement plays a fundamental role, but is defined differently with respect to alternative interpretations of the quantum theory" but she pointed out that some realists do not define measurement at all, but I don't understand why? (This is my first philosophy class) – sarahusher May 3 '14 at 10:26
  • Did your lecturer name any of these realists that do not define measurement? – Mozibur Ullah May 7 '14 at 10:28
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I expect you've come across the following:

Heisenberg argument for his uncertainty principle took into account the act of measurement in 1927. Specifically, measurement causes a disturbance that it quantifies in terms of the energy carried by the measurement itself - and then one takes a limit. Leo Szilard's solution to Maxwell's demon also revolved around measurement. The demon needs to measure the speed of molecules and the act of acquiring this information requires energy.

Prior to this, of course, was Poincare's & Einstein's theorizing that the measurement of time had to be rethought in 1905.

These examples, hopefully, give at least some heuristic indication as to why measurement became a closely studied topic.

Realism, broadly construed, takes the objective existence of the world out there as a given. Newtonian Physics took this picture for granted - the clockwork universe - the objective character of time & space. In this picture one does not need to measure the world out there to know that it is real and has an objective character - and is independent of observers. This is of course a metaphysical supposition.

Presumably it was the impact of the discoveries outlined above that made the scientific community rethink this supposition, and to tighten up what it is that was known properly. They turned to epistemology where one considers how one can be sure that the knowledge one has is in fact valid.

It was the Vienna and Berlin school of logical positivism that developed out of this that became the main steam of Anglo-American philosophy - today referred to as Analytic philosophy - though one sees here that it had continental roots. This philosophy, because it considered only measurement to be valid, and thus true, dismissed that which could not be measured as being ontologically not there at all. They argued, if there are no possible conditions under which a particular phenomena can be measured, then what right have we have to say that it exists. For example, Since one cannot see isolated quarks, they would argue that quarks are not ontologically real. Whereas, a realist physicist, would say they're obviously real - they're always confined.

Hence, despite the emphasis on measurement, logical positivism is seen as an anti-realist school.

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It is difficult to answer your question when you have not given the exact context. The term "realism" is sometimes used in the context of quantum mechanics to mean that measurable quantities always have single values. But you could also use realism in the sense of thinking that there is a world that exists apart from our measurements of it. The many worlds interpretation is not realist in the sense of requiring all observables to be single valued but it is realist in the sense of thinking that there is a world independent of our measurements of it. I'm going to address the possibility that what the lecturer was saying was intended to apply to the many worlds interpretation.

Some advocates of the many worlds interpretation say that it is not necessary to define measurement because measurements do not play a fundamental role in physics. Rather, some interactions copy information between systems so that it is present in more than one system after the interaction where it was present in only one system before. If somebody happens to take note of the result of such an interaction then they may describe it as a measurement but that is not a fundamental part of the laws of physics and so measurement is not part of such laws and realists shouldn't have it as a distinct category.

I do subscribe to the many worlds interpretation but not to that theory of measurement because I think it is uncritical. Some interactions are more suited to be measurements than others and this is a result of their physical properties, so the theory of measurement should be considered part of the laws of physics.

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A realist who is also a physicalist cannot consider measurement special because under those premises a measurement is just another interaction between two physical systems (the observer, and the observed system). Therefore there should be no need to have a generic definition of measurement because the actual physics should not depend on whether your interaction is a measurement or not. Whatever the laws of motion and interaction are should automatically include those interactions we call measurement.

If the measurement process is not a fundamental building block of your theory, you don't need to have a general criterion for what is a measurement and what isn't; it suffices if you can decide that the measurements you actually do are suitable for the observer to learn about the observed system. If you make a general definition, you might needlessly exclude some yet-unconsidered methods of measurement which also provide you information about the system, but don't fit your measurement definition.

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