What is a measurement? (layperson question)

[Galteeth]

I know this is a hotly debated subject here, but what is the general scientific consensus (or most popular definiton) on what constitutes a measurement?

Secondly, are there theories where the measurement problem becomes an illusion because of anthropomorphic bias? (To put in a crude metaphor, where the observation you choose to make isn't really a choice, it just seems that way, since you could conceive of a different outcome?)

 

[Dmitry67]

There is no consensus.
Otherwise a list of Interpretations would be much shorter.

 

[zonde]

There is definition of measurement in wikipedia.
But in QM it seems that problem is more in consensus about what physical quantity these measurements represent.

 

[JustSam]

See https://www.physicsforums.com/showpost.php?p=2331535&postcount=17" for John Bell's writing about problems with quantum mechanical measurement.

 

[alexepascual]

I know this is a hotly debated subject here, but what is the general scientific consensus (or most popular definiton) on what constitutes a measurement?

Secondly, are there theories where the measurement problem becomes an illusion because of anthropomorphic bias? (To put in a crude metaphor, where the observation you choose to make isn't really a choice, it just seems that way, since you could conceive of a different outcome?)

Take a look at "Interpretations of quantum mechanics" in Wikipedia.
I think there is also an entry on "The measurement problem"

 

[alexepascual]

If you are a "layperson" you may also find helpful to read some popularizations (books) on the subject. Some times they contain errors and biased statements, but they can give you a better picture of what is involved without forcing you to know the math.
But knowing a little linear algebra and calculus always helps, as (specially linear algebra) also provides you with some picture of what is going on.

 

[Galteeth]

See https://www.physicsforums.com/showpost.php?p=2331535&postcount=17" for John Bell's writing about problems with quantum mechanical measurement.

Right. I had actually read that post before. I was wondering if there has been any progress on this issue since, and apparently the answer I'm getting here is "not really."

 

[JustSam]

Right. I had actually read that post before. I was wondering if there has been any progress on this issue since, and apparently the answer I'm getting here is "not really."

I'm sure if I get something wrong, smart people will come and correct me, so here goes!

I think the main progress is that more people appreciate Bell's complaint. In the 60's, Bell was a voice in the wilderness, with most everyone else completely satisfied with the understanding of quantum mechanical measurement. In the 90's, the inconsistencies finally impinged on the sensibilities of enough physicists that there is now a general recognition of what is now called the "measurement problem".

There has been a corresponding refinement in the Copenhagen Interpretation, called the Consistent Histories approach, that tries to explain measurement as a mathematical procedure to obtain the probabilistic answers for a given experiment, rather than as the physical collapse of the wave-function. The Consistent Histories approach goes a long way towards providing a consistent set of rules for applying quantum mechanics that works for quantum erasure, interaction-free measurement, and so on. However, Consistent Histories still leaves some fundamental philosophical questions unanswered.

A second point of progress is the study of http://en.wikipedia.org/wiki/Decoherence" , which shows how the evolution over time of the wave-function in the presence of an environment can mirror the collapse formerly thought to have been caused by measurement. For example, in the double slit experiment, when the two slits are observed to see which slit the the particle goes through, the interference pattern is destroyed. But now, instead of explaining this by saying the observation was a measurement that caused a collapse, we can now say that the interaction with the observing photon caused the particle to get out of phase with the path through the other slit, thereby gradually decreasing the interference pattern.