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I am currently reading the book ''The Outer Limits of Reason'' and encountered a description about which I am very confused. I am afraid to say, this may be due to the fact that I am not a native English speaker.

On pp.226, it says:

Researchers are not in total agreement as to why we do not see things in superposition. All that is known is that when we examine the results of a quantum experiment, or to use the right lingo, when the system is measured, we no longer see a superposition. We say the system collapses from a superposition of many positions to one particular position. The measurement problem asks why this collapse occurs and is one of the major discussion points in the philosophy of quantum mechanics.

and on pp.204:

Before we leave the double-slit experiment, let us rephrase the experiment in a slightly different way. The photon leaves the light source, and then depending on whether the barrier has one or two slits open, the photon will have a position or a superposition.

and on pp.218:

Wigner takes this as proof that the only thing in the world that can collapse a superposition to a position is human consciousness.

I have some information about superposition and the Schrödinger's equation and that the word superposition itself means that the act of placing upon; the state of being placed upon.

The thing that makes me confused is the usage of "superposition" and "position" in this sentence because I don't know if there is a definition of the "position" besides the common usage of it. Is it just means a "state" and we can easily exchange them with one another and it is the style that the writer picked for, I don't know, better understanding? The kind of way that popular science writers use or are there some other relation and meaning?

I know that This is not completely a "physics" question and if you think it is inappropriate for this forum, tell me to remove it.

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    $\begingroup$ Superposition in this context means being in multiple positions at once. When a particle goes through a double slit, it's in a superposition because it "goes through both slits" at the same time. This is being contrasted with "position," where the particle is only going through the single slit. Wigner is referring to the fact that mathematically, we make the correct predictions when we assume the particle goes through both slits (and interferes with itself after passing through the slits). However, when we measure the particle on the other side, we only ever see it at one position at once. $\endgroup$
    – Zonova
    Commented Feb 21, 2020 at 3:34
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    $\begingroup$ The word position in physics means pretty much the same as in english. The superposition term can mean the photon or particle can have many possible states or future possible positions before it is observed, at observation the position becomes known. Sometimes the concepts of interference and superposition are interchanged. It is not proven that a photon goes thru both slits or that it interferes with itself, in fact there are good theories like Feynman's that state the particle or photon takes the shortest path that is an integer multiple of its wavelength, probability also plays a role. $\endgroup$ Commented Feb 21, 2020 at 4:39
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    $\begingroup$ theories like Feynman's that state the particle or photon takes the shortest path that is an integer multiple of its wavelength I don’t think Feynman had any such theory. His path integral approach says that they take all possible paths, each with a probability amplitude that depends on the value of the action for that path. $\endgroup$
    – G. Smith
    Commented Feb 21, 2020 at 6:53

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Before we leave the double-slit experiment, let us rephrase the experiment in a slightly different way. The photon leaves the light source, and then depending on whether the barrier has one or two slits open, the photon will have a position or a superposition.

Strictly speaking this is wrong. There is a superposition in both single and double slit. Let me explain why. According to quantum mechanics, we can not predict the position of a particle before measuring it. How do we measure the position? If it’s a photon then a photographic screen can measure the position. Before the photon reaches the screen, we do not know from which path it came. Thus before measuring the position, there were infinitely many paths for the photon have been in. If we want to do any prediction about where the photon will strike the photographic plate it turns out that we get the correct answer only if we assume that the photon could come there from all possible paths. Still the prediction is not a deterministic one. We can only talk about how probable it is for a photon to strike any particular part of the plate.

enter image description here

This state where the photon can take all possible path is the superposition and it always only hits the plate at exactly any one spot. This spot will vary each time. This spot is the position of the photon.

The reason why we take straight line paths is because they are the most probable paths. So if we work with this simplification, then we can see straightforwardly why the double slit readily gives a superposition state.

enter image description here

There are two (simplified) paths that can give rise to a spot at that location. So before the photon striking that position of the plate, it was in a superposition of the two paths.

P.S. If you want more details about this I highly recommend you to read Feynman’s QED. It is highly readable and Feynman’s ability to explain is unmatched.

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Superposition means in physics the following: taking a function of a mathematical model and adding it to another mathematical function of the same model . This gives a new function which is the superposition of the two.

When these functions are sine and cosine functions, this results in a new function that can show interference effects, i.e. that sine and cosines are involved in the superposed functions.

This can happen both in classical physics and in quantum mechanics. The difference is that in quantum mechanics the sinusoidal functions added do not represent energy or momentum transferred in space and time, as are the function used for sound waves , water waves, even classical light waves.

In quantum mechanics the sinusoidal wave functions $Ψ$ describe the probability of finding a particle at (x,y,z,t), by the value of $Ψ^*Ψ$ (the complex conjugate squared). Adding two wave functions is their superposition $Ψ_1 + Ψ_2 =Ψ_3$ . In this case the $Ψ^*Ψ$ of the new superposed $Ψ_3$ will show in the probability distributions interference effects.

For a view on the double slit experiment see this answer of mine.

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"Position" and "superposition" are only distantly related in Physics English. "Position" refers to the location of an object. "Superposition" refers to overlapping in a general sense. "Superimpose" is related to "superposition": when two objects are superimposed, they usually overlap at the same location. But if a single particle has several distinct states that are at different locations, the overall state of the particle is obtained by simply adding the two states, yielding a wavefunction that has several peaks.

We say that the overall state is a "superposition" of the several states. The several states are "superimposed" in the sense that they are both associated with the same particle, even though they are not associated with the same physical location.

IMHO, the reason anyone would think that "wavefunction collapse" requires human consciousness is that they don't understand the fact that we humans are measurement devices subject to the same physical laws as the rest of the universe. A camera - or any other measurement device- itself ends up in a superposition of states when it measures the state of a quantum particle. But in each of its component states, the camera sees the particle in only one state. It never takes a "double exposure" picture.

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  • $\begingroup$ Those who like the consciousness point of view may regard your camera as a Wigner's friend and include it in the quantum system, still excluding themselves. $\endgroup$
    – Ruslan
    Commented Feb 25, 2020 at 16:14

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