How to define measurement in quantum mechanics

spaghetti3451
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Homework Statement



Define measurement in the Copenhagen interpretation of quantum mechanics.

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The Attempt at a Solution



In the Copenhagen interpretation of quantum mechanics, measurement changes the state of a wavefunction, that is, it collapses the wavefunction into one its orthonormal basis vectors (eigenstates). We cannot assume that the system possesses properties if not measured...

I found the second sentence in a book of mine. Problem is I don't know what it means :-)

What do you think it means?
 
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I thought about it for a bit, read a bit on wikipedia and was going to summarize what I read, but i'll just give you the link.

http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics

I just think about it in the practical sense. A measurement is something you do in a lab with rulers and stopwatches that gives you a real number. For example we can measure the position of a photon at a particular time in a double slit experiment by putting a photomultiplier next to each slit, and it will make a loud noise when a photon goes through one of the slits, so we know the photons position (within a particular uncertainty) at the time when the photomultiplier made the noise.
 

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