# The observer detail of double slit experiment

I have been searching internet for a while and I couldn't find any detailed information about the observer type in double slit experiment of electrons.

For example is the observer a camera or something like that?

The more detail of how they observed the electron the better. I hope to find my answer here.

• This could be a CCD, CMOS or other electronic camera as well as a plate with photo emulsion. In special cases it could be a photon multiplier too. Dec 2 '15 at 12:03
• Possible duplicate of What is an observer in quantum mechanics? Dec 2 '15 at 16:29
• physics.stackexchange.com/questions/203784/… there is the answer you are looking for. May 10 '16 at 8:51

Maybe your question is about the instrumentation used, in which case I cannot help much, but if it is about the concept of observing something in such experiments, then here's a short/rough discussion.

The observer is a black-box type of description of a more involved physical concept behind, namely that of interactions and measurements.

I guess you already know that we cannot visually see say an electron as that would imply probing the electron with the visible part of the light spectrum and in this range the wavelengths are too large compared to the system size to be probed.

But more generally, when we observe something experimentally, some form of interaction takes place, for example a scattering between the electron and a photon coming from a detector. In doing so we inevitably change the state of our system, and this is hidden behind that experimental act of observation.

To give you more intuition, take a particle under a microscope. The precision of position measurements of the particle is limited by the resolution of our microscope which itself is limited by the wavelength $\lambda$ of the light used. In order to improve this, $\lambda$ has to be made as small as possible, which results in using more energetic photons. The scattering that results between the photon and our system will jolt the system, and cause its momentum to change. All this process can be labeled as observing the position of a particle.

If you are interested in the instrumentation part, start by reading up about CCD's.

A neat explanation by Mark Eichenlaub can also be found on this Quora post, here's a small quote from it:

Let's give one example. In a real double-slit experiment, the electrons are bounced off layers of atoms in a crystal, not slits. Mostly, the electrons won't interact with the crystal except to bounce off. The state of the crystal is unchanged. However, sometimes an electron will interact with the crystal when it bounces, flipping the spin of different electron in the crystal as it does so. In that case, if we go to the crystal and see where the spin has been flipped, it tells us where the electron bounced off. That means the spin-flipping constitutes an observation of the electron, and electrons that flip a spin in the crystal will not show interference. As long as there is any system in the universe we can examine that keeps a record of where the electron bounced, we destroy interference because the states don't cancel any more.

So the detector is a charged couple devise which transfers the electron hits (the charges generated by the electron hitting the plate) and is recorded as an image. There are photos of single electron hits at the end of the article.

When talking about how the double-slit-experiment can be manipulated to result in an interference pattern or a lack thereof, most (popular) scientific publications use the term "observer" as the cause of the collapse of the interference pattern.

I too have long wondered what is meant by "observer" since none of the publications I've read ever bothered to define it.

So, here's my take on it.

Now, it is my understanding that an "observer" is any system that tries to interact with the wavicle(*) in question (in this case an electron) in order to extract from it some property (eg.: position, speed, direction or which slit it passed through).
Such an observer typically will consist of three components:

1. a medium; something that interacts with the wavicle. This can be an electric field, a magnetic field or other wavicles (eg: light, electrons), whatever, even a piece of wire
2. a generator; something that generates the detecting medium (eg: a magnet, a lightbulb)
3. a detector; something that detects a change in the medium (a CCD, a magentometer, a machine that goes "ping"). This change is then quantified and interpreted to mean something like "its moving south".

As long as nothing tries to interact with the wavicle (according Feynman) it will be on every possible path between the source and the target. Some paths however have a higher probability than others. And the path that it took is only revealed at the moment it is observed by the target. The interference pattern is there to show that not all paths are equally probable.
When you add an "observer" you effectively destroy the wavicle's interference pattern with your medium defining its location and direction and losing its wave-characteristics. Its particle-properties then determine where it will hit the target.

Note that for the wave-function to collapse there doesn't need to be a detector. The interaction with the medium is sufficient.

(*) Until it is observed (by the "observer" or the target CCD or photoemulsion plate) the electron has both properties of waves as well as of particles. So I'm using the contraction wavicle here to indicate its duality.