Question About The Role of Observation in Quantum Mechanics

[ray3400]

Yes, see nondemolition measurements. But this doesn't apply for the double slit.

The pattern changes since a detector at the slit has a serious impact on the microscopic stuff going through the slit:

The change in an observed object due to an observation is large if the means of observing it is comparable (or larger) in size and impact to the observed object. If you magnify the situation sufficiently strongly, it is qualitatively like (though not really like) observing a sand castle by a big wave from the shore.

Thanks, this makes a lot more sense. A lot of YouTube videos make it seem like there is some sort of magical thing that happens - "the particle is aware that it is being detected and therefore behaves differently". As though the particle has some sort of consciousnesses and the change isn't just a result of cause and effect relationship between the measurement device and the quantum particle.

 

[vanhees71]

Indeed, that's true, as my discussion in #18 showed. You don't need to put additional detector in front of one or both slits to gain which-way information. It's also sufficient to place your detector, i.e., the screen where the particles are registered close enough to the slits to be sure through which slit each particle came, but then you cannot observe the interference pattern as detailed in this posting too.

On the other hand, also the gain of which-information through detection at the slit generally destroy the interference pattern. How to explain this depends on the specific way of the detection.

One simple example is to use quarter-wave plates, oriented in a 90-degree angle relative to each other in the double-slit experiment with polarized photons. If you start with photons polarized at an angle of $0^{\circ}$ in the plane perpendicular to their momentum (which is parallel to the plane of the slits) and orient one quarter-wave plate in one slit at $45^{\circ}$ and the one in the other slit at $-45^{\circ}$ you mark the photons themselves such that you can gain which-way information at any distance from the slit, because the any photon coming through slit 1 is left-circular polarized and any photon coming through slit 2 is right-circular polarized and thus the parts of the corresponding electric-field-operators (note that for photons I cannot argue with wave functions as in #18, because photons do not have a position operator nor a well-defined "wave function") annihilate/create photon states that are perpendicular to each other and thus there's no interference term in the detection probability for these photons at a CCD cam placed no matter how far away from the slits and thus no (two-slit) interference pattern.

Now you gained which-way information with 100% certainty and have no interference pattern. If there were no quarter-wave plates you'd get an interference pattern but are left with no which-way information whatsoever. Of course the difference in the two settings are thus these quarter-wave plates and here of course the reason for the disappearance of the interference pattern is the placement of these quarter-wave plates in the slits and the interaction of the photons (i.e., the em. field) with them, leading to the change of the polarization from linearly polarized to either left- and right-circular polarization depending through which slit the photons came. This very setup has entangled the photon momentum with its polarization in such a way that the state of a photon behind the slits allows with 100% certainty to know through which slit it came.

You can in such a setup also choose to know only "somewhat" through which slit each photon came, i.e., you just orient the quarter-wave plates not with $90^{\circ}$ relative orientation to each other. Depending on which angles of the plates you choose, the probability for a photon with a certain polarization state behind the screen is larger to have come through one slit than the other (say 70% through slit 1 and 30% through slit 2). Then it's more likely that the photon came through slit 1 (70% probability) but you cannot be very certain about it because there's still a 30% chance of having come throuh slit 2. In this case you get in turn some interference pattern back, but not of as high a contrast as the one without the quarter-wave plates in the slits. You can also choose, not to get any which-way information with quarter-wave plates at the slits. Then you simply have to orient them in the same direction. Then there's 50% change for the photon to have come from either slit and that's the most uncertain state concerning which-way information the photons can be prepared in. Then you get the two-slit interference pattern with full contrast.

Indeed, as you see you don't need to really detect the photons to gain the which-way information, but an addition like the quarter-wave plates can lead to "imprinting" this information into the photons themselves, but that's so because of the interaction of the photons with the quarter-wave plates.

So there's some objective reason whether there's which-way information, no which-way information, or some uncertain information, which excludes and interference pattern, leads to an interference pattern of maximal possible contrast, or leads to an interference pattern with less contrast in these three cases, respectively.

However according to the laws of QT you cannot have both, i.e., certain which-way information and an interference pattern. These are mutually exclusive possibilities to prepare the photons.

What's also very clear with these examples is that there's no such thing as "wave-particle duality" or any other "weird things" attributed to QT as soon as you accept, as a fundamental law of nature that cannot be explained by any simpler laws, that there's some "irreducible randomness" in nature, which however is precisely described by modern quantum mechanics or, as here for photons, relativistic quantum field theory.

As @DrChinese said above, physics does not explain "why" nature behaves the way she does, she just accurately describes "how" nature behaves. Sometimes you can explain "why" some phenomenon is observed as it is by using theory to "explain" it by calculations according to the fundamental laws described by this theory, but there's no "deepter explanation" of the "why" of these fundamental laws.

Despite some criticism in this forum, I consider the introduction to (non-relativistic!) QM in the Feynman Lectures as among the best there are in the textbook literature. That's because Feynman is a role model for me in explaning QT as it is without any ado with strange philosophical ideas about some apparent paradoxes (there are none within the minimal statistical interpretation). He just sticks to the physics facts and leaves it at this: The probabilistic nature (i.e., Born's rule to intepret the wave function or more generally any quantum state) is just a fact of nature, deduced in about 120 years of experience with quantum theory (or 94 years if you count only the modern quantum theory, which is the one still up to date with today's knowledge).