What is an observation or measurement?

[least_action]

What is an observation or measurement?

If it is not known exactly I would like to at least have some good appreciation of what the fuzzyness of the boundary is.

 

[Dmitry67]

There is no clear definition of what measurement is (and can't be, why some combinations of atoms, called 'measurement devices', should behave differently from the others?); this is a main reason why Copenhagen interpretation, which was mainstream before the discovery of the Decoherence is abandoned now (but it is still #1 in popular articles, so you can always find something about 'wavefunction collapse' without a definition why some combination of atoms caused it)

 

[arkajad]

You will get many different answers. Mine is simple: when something has been irreversibly recorded on a macroscopic readable device - that means some measurement/observation has taken place. It does not matter whether this fact will be discovered in 1000 years form now or never.

Of course one would have to explain "macroscopic readable device" and "irreversibly" - but usually we have no doubts about what these terms mean.

 

[skippy1729]

There is no clear definition of what measurement is (and can't be, why some combinations of atoms, called 'measurement devices', should behave differently from the others?); this is a main reason why Copenhagen interpretation, which was mainstream before the discovery of the Decoherence is abandoned now (but it is still #1 in popular articles, so you can always find something about 'wavefunction collapse' without a definition why some combination of atoms caused it)

Aren't you referring to what should be called the VonNeumann-Copenhagen interpretation? In the original Copenhagen interpretation, a measurement merely causes the physicist to update his wave function to reflect the new information he has gained as a result of his experiment. While not widespread, I believe it is still a viable viewpoint held by many people.

 

[Dmitry67]

Yes, there are 2 different flavors of Copenhagen Interpretation, in one wave function is "just knowledge" and is not objective. You're right, that viewpoint is held by some people, even that interpretation also has its own sort of "magic" (behavior of elementary entities is described using more complicated things like 'consciousness' or 'knowledge') That sort of interpretation also raises questions about “did collapse ever occur before the first living entity was born” or “if wave function is knowledge, and knowledge is subjective, why objective reality obeys the probability density described by wavefunction?

I do hope that page is turned.

 

[Fra]

Just to stand up for the view that is different from Dmitrys.

if wave function is knowledge, and knowledge is subjective, why objective reality obeys the probability density described by wavefunction?

Because the set of all subjective views in these cases has evolved relations by interacting, that fills the purpose of "objectivity" with respect to the set of subjective views. Each subject in this set will agree on certain things - these are the objective inter-subjective laws.

Subjectivity does not equal arbitrariness. The set of possible subjective views are still constrained, just like the set of observers in SR or GR.

About the OT; a measurement is thus a subjective event as well. But this does not mean that it's meaningless. Measurement theory is about measurements, and all measurements unavoidable are subjective at least in the sense that they required a contex; a measurement device or and observer.

The main confusion is I think howto understand this apparent conflict between subjectivity in measurement and the objectivity we required form science.

Either you try to understand how objectivity and scientific knowledge emerges, in a construction (such as measurement theory) which seems to be fundamentally subjective?

or you try to understand how you can understand emergence of subjectivity from an overall assumed objectivity.

Which way you choose is somehow significative for your level of structural realism.

/Fredrik