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gohu

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- TL;DR Summary
- Why does the same formula cos2(θ) apply in different situations, one where photons are sequentially modified by polarizers, and the other where entangled photons pass through separate polarizers simultaneously without prior modification?

I'm trying to make sens of the dirac's three polarizer experiment (Moderator's note: link removed) and the epr experiment and bell's inequalities, and i have a loooots of questions, but here i will focus on one first. (I have read some of the long and very interesting threads on the subject that are already on this forum, but I missed some I guess because i cannot understand this).

in the three polarizer experiment, photons pass through a first 0° polarizer, THEN a second one at 45° angle for example. The number of photons that has pass through both filters is, I believe, calculated with the formula cos²(45 - 0). And it's important to notice that the photons are modified after the first filter, their polarization align with 0°.

but in the epr experiment, a pair of entangled photons pass SIMULTANOUSLY (i'm not yelling :p i just emphasize) in two polarizer distant in space, one at 0° angle and the other at 45° angle (to be consistent with my previous example), and the amount of photons that pass through the two filter is also, I believe, calculated with the formula cos²(45 - 0). And this time, the photons are not modified by the filter. Well i mean they are of course, but the photons that pass through the 45° polarizer were not previously modified by the 0° polarizer. Or were they ? if they were, i will have other questions then !

so, what's going on ? Why do we use the same formula ?

I found a beginning of an answer in this post by jesseM :

It explains how they differ, but not why they look the same.

in the three polarizer experiment, photons pass through a first 0° polarizer, THEN a second one at 45° angle for example. The number of photons that has pass through both filters is, I believe, calculated with the formula cos²(45 - 0). And it's important to notice that the photons are modified after the first filter, their polarization align with 0°.

but in the epr experiment, a pair of entangled photons pass SIMULTANOUSLY (i'm not yelling :p i just emphasize) in two polarizer distant in space, one at 0° angle and the other at 45° angle (to be consistent with my previous example), and the amount of photons that pass through the two filter is also, I believe, calculated with the formula cos²(45 - 0). And this time, the photons are not modified by the filter. Well i mean they are of course, but the photons that pass through the 45° polarizer were not previously modified by the 0° polarizer. Or were they ? if they were, i will have other questions then !

so, what's going on ? Why do we use the same formula ?

I found a beginning of an answer in this post by jesseM :

Perhaps by "same law" you just mean that the classical Malus' law for polarized light and the law for entangled particles both involve a cos^2? The problem is that although the equation can be written in a similar form for both laws, the physical meaning of the symbols is completely different, so from aphysicalperspective they cannot be called the "same". If you write cos^2(a-b) in the classical context a would be the polarization angle of the light, b would be the angle of a single polarizer, and cos^2 would be giving the reduction in intensity of the light as it passes through the polarizer; but if you write cos^2(a-b) in the quantum context, a and b would both be polarizer angles, there would be no term for the polarization of the light, and cos^2 would be giving theprobabilitythat both photons give the same binary result (both passing through their polarizers, or neither).

It explains how they differ, but not why they look the same.

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