Waching particles go through the double slit?

Question

Sorry in case this is a duplicate, I haven't studied physics or maths, and I can't find answers anywhere. The double-slit experiments are frequently explained online like the one here: https://www.youtube.com/watch?v=A9tKncAdlHQ

The explanation says:

1. Shoot particle through two-slit: get interference (wave behavior).
2. Shoot particle one by one through two-slit: still get interference (wave behavior).
3. Shoot particle one by one through two-slit AND watch them: no interference (particle behavior).
4. Shoot particle one by one through two-slit AND PRETEND to watch them but switch off the camera: get interference (wave behavior).

My main question is (ref. 3, 4):

How can you WATCH which slit the particle goes through, surely you need photons to bounce off them in order to detect them – that would be directly interfering with the trajectory of the particle in the experiment. Surely you need a perfect vacuum and darkness to run this experiment. So any camera would be blind. Does this mean the video is wrong in talking about experiments 3, 4 (7mins in)?

Wikipedia says "An experiment performed in 1987 produced results that demonstrated that information could be obtained regarding which path a particle had taken without destroying the interference altogether." What does it mean by "altogether"? So did it work or not?

Secondly, just checking if I understand correctly:

Is the result of experiment $3$ what the various interpretations try to explain? (Copenhagen, Penrose, Von Neumann-Wigner, Everett)

Finally:

Can someone point to me the REAL test examples of experiments 3 and 4?

Answer 1

You cannot watch photons. They are either created or annihilated in interaction. Watching them will annihilate them. However, you can do the experiment with electrons. The results are shown on the wiki page you linked for the case where you "switch off the camera", i.e. you don't actually watch them. It is seen that you don't actually get wave behaviour for the electrons, they always arrive on the screen at a point, as expected for a particle. The wave behaviour applies only to the probability for where the particle will arrive. But probability is not a real physical thing. It only exists as a mathematical estimate of likelihood.

Answer 2

That's not actually such an easy observation. In general you'll be looking for quantum eraser experiments. One attempt, the delayed-choice quantum eraser, does not "watch" so much as determine after the fact which photon goes through which slit. (I'm looking specifically at Kim's experiment in the second link.) That is done with down-converters that split one photon into two entangled photons, one of which goes to a detector that can tell you which slit the original photon came from. The path information provided by the so-called idler photons was not measured until 8 ns AFTER the signal photons were detected, making it "delayed". And yet, if the path information is known the interference pattern disappears, if the path information is not known then the interference pattern is present. It's a little complicated, you'll have to study the diagram and the description carefully. But it is an heroic effort to observe "which slit?"

Answer 3

As mentioned in the answers, this experimental setup with photons simply does not work. Since I did not watch the video in detail, I hope that they explained that the explanations are about electrons and not photons.

In the arrangement with photons they use two measuring instruments, the observation screen and a camera. What they are not saying is how the camera is exposed. There must be a light that illuminates the electron. Some photons from the light source hit the electron and some of these expose the camera.

But how do the photons that hit the electron interact with the electron? Ask yourself, does the scattering process change the trajectory of the electron? The answer is yes, and the conclusion is that this is the reason for the destruction of the fringes on the screen. BTW the electron setup includes a vacuum camber, otherwise the air stops the electrons.

Long story short. The camera has no influence on the experiment. The light source required for this is the disturbing source.

Answer 4

Regarding watching photons, Photons are not watched. They may be observed. The observation is not conducted by another photon. Usually photons are observed by exciting an electron, and absorbed during the excitation. In the double slit experiment, the photons should be observed in less destructive method. As a concrete example for the trajectory observation, consider polarizers on each of the slits and on a CCD sensor. where the CCD is located in a plane that enables to record the interference in absence of polarizers. If all polarizers are parallel (that is, their polarization axes are parallel), they have no effect, other than decreasing the photons that reach the CCD by factor of two. If one of slit polarizers is rotated by 90 degrees, the trajectory may be inferred with certainty but the interface disappears. (actually, in this case, the polarizer and CCD better be replaced with Polarizing beam splitter and two CCDs, to get both slits scattering pattern). Different polarizers angle allow trade off between knowledge about the trajectory and the emergence of the interference pattern. Where the emergence is correlated to the contrast. As an extreme example consider perpendicular slit polarizers and 45deg CCD polarizer, the knowledge on trajectory is washed away and the interference pattern is completely restored.