Observation in Young's double slit experiment

Question

I'm new to YDSE and recently I found out that there was also an experiment done when humans were able to separate photons (What I mean is we were able to shoot photons onto the double slit) so it was observed that it showed same pattern as light does, but weirdness started when we kept a detector at slit entrance and tried to know exactly from which slit did the photon passed through but when we did so the wave interference wasn't shown instead it was like shooting particles onto a screen.

Scientists came to a conclusion (well I don't know if they did or not but I just heard it on YouTube channels) that when there is an observer, photons behave like particles. When there is no detector, it showed a wave interference. But since we are looking at it, aren't we also an observer? We do have consciousness. So why don't we, as an observer, cause the photons to behave like particles here?

I may be wrong about this whole concept so I welcome any correction and happy to know what actually is YDSE if I'm wrong.

Answer 1

No humans required! The apparatus setup can be varied to yield either an interference pattern or not without ever learning which slit the photons go through. It is enough that it is possible to learn that information.

See this experiment (which unfortunately is fairly advanced) which explains and demonstrates this.

Young’s Double Slit Experiment with Single Photons and Quantum Eraser https://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/single_photon_paper.pdf

Answer 2

You can dim a laser beam until you find that light is only detected in chunks if you have a very sensitive detector. Those chunks are called photons.

You can arrange a double or single slit experiment with a very dim laser beam so that there is a very high probability that there is less than one photon at a time going through the experiment. When a photon goes through one slit you will see a particular pattern of light and dark stripes. If you open up a second slit you will see a different pattern of light and dark stripes: this is called quantum interference. Some places that were formerly lit will be dark and vice versa. If you consider a place that was lit with one slit and dark with two slits then something must be coming through the second slit to stop a photon from arriving there. That thing is blocked by opaque materials, deflected by lenses and mirrors as a photon would be. You will only ever see a photon pass through one slit if you measure it but the other invisible photon has to be there or you can't explain the results of experiments. If you put detectors in front of just one slit you will see a single slit pattern so the invisible photon is interacting with something that blocks photons as a detector would but the detector you see doesn't go off so there must be an invisible detector too. Quantum theory claims that each object exists in multiple versions that are approximately sorted into layers each of which looks like the universe as described by classical physics. This is often called the many worlds interpretation of quantum theory, but it is the only known explanation of single photon interference and many other experiments and it is a consequence of the equations of motion of quantum theory unless they are modified. See "The Fabric of Reality" by David Deutsch Chapter 2 for a popular explanation.

A measurement or observation is just an interaction with a system that copies information out of that system. This kind of interaction tends to suppress quantum interference through an effect called decoherence:

https://arxiv.org/abs/quant-ph/0306072

https://arxiv.org/abs/1111.2189

If you're just looking at an experiment while photons are going through it that doesn't change anything because you're not actually interacting with the photons so you're not causing decoherence and the interference remains intact.

Answer 3

Photons always have both particle and wave properties just like we have arms and legs. When we are observed walking we see the leg properties, when we are waving we see the arm properties. For the photon the slits can show the wave properties.

Even mass particles like electrons and small molecules will show wave properties thru slits ... that is because like the photons the interaction between the a screen/detector and the particle is governed/ruled by the EM field.

If you want to understand light you can always say that light is a very small (local in science) and individual wave in the EM field. All photons are ruled by the EM field.

Answer 4

Let me put it like this: According to the postulate of quantum mechanics, "The act of making an observation inherently disturbs the quantum system being observed".

Imagine you are observing the motion of a moving car. How do you even see the car? It is because the light particles (which have momentum) are hitting the car, gets reflected and come straight to your eyes. Will the motion of the car change if you don't observe it? NO, because the photons have very small momentum to change the motion of that car.

But imagine if the car gets replaced by an electron. Now, if you try to observe it, you have to do something. This act of observation disturbs the system and the photons now have enough momentum to change the motion of an electron. Hence the system gets disturbs and yields different result.

In the double-slit experiment, the system gets disturb when we observe it and changes the result.