What counts as an observer of quantum particles?

[Groovy]

I have heard so many times that subatomic particles are in no specific state until they are observed and about the double slit experiment. I have no doubt this is correct,but I don't understand 1- If it hasn't been observed yet, how would anyone know? And 2- What counts as an observer? A scientist who knows what they are looking for? A fly in the room that sees the experiment? A passing photon? Is it literally observation through sight apart from the other senses? How has this been proven if presumably any experiment would itself be an obervation?

[mathematic]

An "observer" is anything which interacts with the particle.

[Sensei]

Observation = measurement.

Photon emitted by system, is absorbed by your eye, or by digital, or analog camera, and is gone from system under observation. You, or device, "eat" photons and change state.

In the case of double-slit, it could be polarization filter.

 

It's working this way that if we have unpolarized photons beam, and they are emitted toward polarization filter, 50% of them will pass through, 50% of them will be reflected.
But if beam is already polarized, and right polarization filter angle is used, 100% of photons will pass through polarization filter, and 0% being reflected.
Or after rotating polarization filter +-90 degrees, 0% of photons will pass through polarization filter, and 100% being reflected.
So by putting polarization filter, you made measurement of particles-photons.
You learned their polarization.

But beams that are passed through, or reflected, are no longer unpolarized, nor they are linearly polarized.

So you can put yet another polarization filter behind them, and see what happens. It's quite interesting. Do it below water with blue laser, to see beams.

Act of observation (measurement) changed state in which are particles under observation.

Edited February 28, 2016 by Sensei

[Enthalpy]

1- If it hasn't been observed yet, how would anyone know?

If I understand you query properly...

 

Many observations need the particle to have had all the possible states prior to detection. Only this explains the observation.

You cite the double slit: a photon must pass through both slits so you get an interference pattern on the screen.

 

Pure state... this is possible to a reasonable extent. But a state pure for one quantity is a mix for an other one. If the energy is certain then it takes time to measure: the instant of the measure is uncertain. So a state can be "pure" but not for all quantities at a time - only for the "compatible" ones.

 

"Observation" is often a process where one of the particle's attributes gets more accurately defined, for instance its position. But better doesn't mean certain: every measure has some incertainty. So it isn't "known" with perfect precision nor certainty.

 

My general suggestion would be: do not concentrate on the double-slit experiment, because it easily suggests false interpretations. You might invest time for instance in pictures of molecular orbitals by the atomic force microscope, as this one refutes many false interpretations.

http://education.mrsec.wisc.edu/SlideShow/slides/scanning/pentacene.html

http://www.nature.com/nchem/journal/v3/n4/fig_tab/nchem.1008_F3.html

for instance, the observed electron has not decided to be at one point just because the microscope observed it. The observed and observing electrons interact permanently over all their possible positions.