Q:
Hi,As a non-physicist, I am unsurprisingly struggling to acquire a simple layman�s understanding of Entanglement. Over the years I�ve searched high and low for a layman-friendly explanation but to no avail. Here�s where I am�The simplest experiment to show how entangled particles differ from non-entangled particles uses entangled photons of light that shoot off in different directions. In each direction there is a polarised filter behind which is a detector that clicks each time a photon is detected (that has passed through the filter).Prior to 1982 all such experiments (called Bell tests) used single-channel polarisers. This simpler set-up is the one I think I have the best chance to get my head around.My understanding is we can represent a detected photon with a �1� and not-detected with a �0�. I imagine that to perform such an experiment would require the ability to be able to fire off entangled photon pairs one-at-a-time (otherwise how could you ever get a 0,0 result?) Am I correct so far?Apparently the results are correlated in a way that cannot be explained by saying that each of the two entangled photons carry along with them a (hidden) variable with the same (randomly selected) angle (from vertical, say).When one photon hits the lens, something called a �wave function� is said to collapse. Apparently, this is the first point where QM says something different occurs, compared to where the particles carry variables. The first photon to hit a polariser will instantly become aligned with the lens, and this alignment is communicated to its partner photon, which also changes (even though it has not yet hit its lens).The entangled particles were in �superposition�, effectively existing in all possible angles at the same time. When the first entangled particle hit its polarisor, it was forced to choose the angle that aligns with the polarisor. This in-turn forces its partner photon to choose the same alignment.If the two polarisors have the same alignment, then 100% of the ones that get through one of them also get through the other. Only 50% of photons get through either individual polarisor, but the ones that do and the ones that don�t all seem to match up.My attention now turns to those that fail to get through. Following the same logic, their wave functions must collapse and they must be forced to have an alignment that is perpendicular to that of the first polarisor to be hit. It must be almost impossible to position both polarisors exactly the same distance from the source, and so one will inevitably be the 1st to be encountered. This nearest one will dictacte the �forced� polarisation.With this in mind, I thought about a widely-used scenario. The nearest polarisor is vertical, which I�ll call 0 degrees, and the further-away polarisor is at an angle of 45 degrees. By my logic described above, the second polarisor should encounter all untangled photons (as all their wave functions will have collapsed) and 50% are vertically aligned and 50% are horizontally aligned.Now as far as I�m aware, this means that the furthest-away polarisor should let through 85% of the photons (because it is at an angle of 45 degrees to both sets of them).So in my simplified view of the world, this is the obvious test for this spooky action at a distance. To pass the test, these detector alignments should produce a significant difference in the number of photons that get through the polarisors. Is this what really happens, (can you control the intensity of one direction by changing the polarisor alignment in the other) and if not where has my reasoning gone wrong?Mnay thanks.
- Karma Peny (age 25)
England
A:
Most of what you write is a correct picture of entanglement, although some of it presumes certain collapse-type interpretations of quantum mechanics. the problem come's right at the end. That 85% would be the transmission rate for spin-1/2 particles at 45° from the previous polarization. For spin-1 photons it's just 50%. So no signal is transmitted faster than light.
The language about a collapse one place triggering events elsewhere is probably inappropriate. Bell experiments have been done with moving detectors, each of which detects first in its own frame. The results show the standard violations.
One important thing to realize is that the Bell violations of local realism follow directly from the experimental results, with no mention whatsoever of quantum mechanics, wave functions, or anything like that. So even if someday the framework of quantum mechanics is changed, there;s no reason to suspect that the new framework will be less weird. Given that, almost by definition, we first come up with less weird ideas, quantum mechanics may be the least weird way of thinking about the results.
Mike W.
(published on 12/22/2017)