# Waching particles go through the double slit?

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?

• see this related answer of mine physics.stackexchange.com/questions/285142/… – anna v Apr 24 '20 at 17:53
• thanks anna, but do you know any experiments actually watching which slit the photon goes through? – Tian Apr 24 '20 at 17:58
• @Tian You are correct. The only way to watch a photon is to absorb it. As soon as you do that, the photon stops traveling and never reaches the detection screen. There can be no interference if you eliminate the interfering photons. – Bill Alsept Apr 24 '20 at 18:20
• @BillAlsept Thank you! I mean not just photons but any particle in a two-slit, sorry I amended my terms to clarify. Does that mean that what Jim Al-Khalili says in the video (7mins) is wrong? Was the which-way experiment ever conducted? What if you fired bigger atoms, so that when you detect them by bouncing photons off them, the effect is negligible? I want to know what happens when you look at a slit, does the interference pattern really disappear? – Tian Apr 24 '20 at 21:41
• This, for me, explains it phys.org/news/… . It is with electrons. Detecting an electron changes the quantum mechanical wavefunction , from a wave coherent in both slits to one slit unchanged and the other becoming a point source wave starting at the slit, incoherent with the other wave.. Waves always probability waves. – anna v Apr 25 '20 at 3:41

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?"

• Thanks! It is so complicated, but it shows that which-way experiments really do change how the particles behave...right? Am I correct that the various interpretations (Copenhagen, de Broglie-Bohm, Von Neumann-Wigner, Everett) are efforts to explain how particles fired one-by-one in two-slit still manage to create interference? – Tian Apr 25 '20 at 12:18

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.

• Thank you! Which part are you referring to on the wiki page? Wiki says: "they sent single electrons onto nanofabricated slits...and, by collecting the transmitted electrons with a single-electron detector, they could show the build-up of a double-slit interference pattern." So electrons do behave as wave-particles. But my question is about watching which slit the particle goes through. I amended the question to particles instead of photons. Has that actually been done, and how? Because you must bounce off a photon to detect the particle, then you affect its trajectory. – Tian Apr 24 '20 at 21:40
• Yes. If you detect the particles as they pass through the slit, you affect the trajectory and this destroys the interference pattern. – Charles Francis Apr 24 '20 at 21:45

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.

• If light is able to influence an electron on its flight, how do the edges of the slit experiments do that? What is claimed is ultimately the self-interaction of the photons at the edges. The fringes also appear behind a single edge and in single electron experiments Is it really true that the electric and magnetic field from the surface electrons of the edge does not influence the impact of electron or photon? If the result of the interference experiments can be explained in this way, the time that has been spent on this question for 100 years could be better used for mankind. – HolgerFiedler Apr 25 '20 at 9:10
• Moreover, the design of slits by manipulating their electric and magnetic fields could perhaps improve photolithography. – HolgerFiedler Apr 25 '20 at 9:10
• The EM field from the edges does influence the photons and electrons as they pass by. Polarizing them and or diffracting them. – Bill Alsept Apr 25 '20 at 19:40
• @BillAlsept The task to be examined is how the edge does this. With or without some periodicity in the deflection as a function of the distance of the particles from the edge? – HolgerFiedler Apr 25 '20 at 21:22
• Proximity to the edge definitely plays a part. – Bill Alsept Apr 25 '20 at 22:00

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.

• My answer was taught from Goren Gordon. Elitzur–Vaidman bomb tester in Wikipedia provides more coherent explanation of a non destructive test that assume no prior knowledge in quantum mechanics. – עומר כורך Apr 24 '20 at 22:42