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:
- Shoot particle through two-slit: get interference (wave behavior).
- Shoot particle one by one through two-slit: still get interference (wave behavior).
- Shoot particle one by one through two-slit AND watch them: no interference (particle behavior).
- 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)
Can someone point to me the REAL test examples of experiments 3 and 4?