- #141
- 24,488
- 15,024
Well, I think the first step to understand these things is to look at what's done in the lab. You have a very concrete setup consisting of a laser and certain types of birefringent crystals which through nonlinear optics enables you to create entangled photon pairs. That's the preparation procedure. This is also well understood by effective QED descriptions, i.e., using some constitutive parameters to describe the down-conversion process. It's all based on phenomenological experience and then brought into an efficient formalism to understand "how it works".
Then you have other equipment to measure polarization. In the most simple case you just use some polarization foil like "polaroid" in a certain orientation letting photons with some linear-polarization state through and the ones in the perpendicular polarization are absorbed. These filters you use on both sites where the photons are registered (or not registered). Then you can established in a series of measurements that the single-photon polarization is completely indetermined. Taking accurate measurement protocols to ensure that you can check the correlations of each of the entangled photon pairs you find a 100% correlation between polarization measurements in the same direction. The only thing that has to do with a human experimenter is that he decides what he wants to measure, and there's no subjective element in this, if that's what's behind your question.
The very idea that this is an interesting measurement is a prediction of the theory, but it's finally defined if you can set up such a concrete experiment to measure it. There's no more you can expect from natural science. What in addition do you expect? Why are such questions never asked about classical mechanics or electrodynamics? You never ask why Newton's postulates describe the Newtonian world accurately (and indeed that's true, i.e., within the now known limits of applicability Newtonian mechanics is a very good description of the corresponding phenomena observed in nature)? But why don't you ask? Is it, because the classical-physics description has no irreducible probability element in it? Isn't it a as pretty weird idea to think that everything is strictly deterministic, compared to our daily experience of pretty random events?
Then you have other equipment to measure polarization. In the most simple case you just use some polarization foil like "polaroid" in a certain orientation letting photons with some linear-polarization state through and the ones in the perpendicular polarization are absorbed. These filters you use on both sites where the photons are registered (or not registered). Then you can established in a series of measurements that the single-photon polarization is completely indetermined. Taking accurate measurement protocols to ensure that you can check the correlations of each of the entangled photon pairs you find a 100% correlation between polarization measurements in the same direction. The only thing that has to do with a human experimenter is that he decides what he wants to measure, and there's no subjective element in this, if that's what's behind your question.
The very idea that this is an interesting measurement is a prediction of the theory, but it's finally defined if you can set up such a concrete experiment to measure it. There's no more you can expect from natural science. What in addition do you expect? Why are such questions never asked about classical mechanics or electrodynamics? You never ask why Newton's postulates describe the Newtonian world accurately (and indeed that's true, i.e., within the now known limits of applicability Newtonian mechanics is a very good description of the corresponding phenomena observed in nature)? But why don't you ask? Is it, because the classical-physics description has no irreducible probability element in it? Isn't it a as pretty weird idea to think that everything is strictly deterministic, compared to our daily experience of pretty random events?