Single slit diffraction experiment

[svnaras]

I'm a novice at this quantum business and was just trying to understand the single slit diffraction experiment and when an electron's position is getting measured in particular.

Given my understanding it looks like the electron's position gets measured twice. Once when it is just about to pass through the slit and the second time when it hits the screen.

As I see it from the source to the single slit the electron's waveform has a hugespread (including all possible coordinates). However, when it gets to the slit it can pass through only if its waveform is much smaller (of the width of the slit). So at that particular moment doesn't the electron's huge waveform collapse to either one that is within the slit's width or somewhere else (in which case it doesn't pass through the slit).

And because of the reduction in the bandwidth of the position waveform its conjugate momentum waveform acquires a huge spread and this makes it go in any direction after it passes through the slit giving rise to the diffraction pattern. Is this right? Please correct me if I'm wrong because this seems to be quite crucial to understanding what measurement is.

But frustratingly I've never seen anyone attribute the passing through of the slit as a measurement process. Only when it impinges on the phosphor screen do people say a measurement is performed (and sometimes only when a conscious being sees the phosphor screen).

As an analogy consider polarised light (along the horizontal direction) with a polarizer at angle 45`. When the photon gets near the polarizer a measurement is made of its polarization (ie. the wavefunction collapses into its components along 45' and its perpendicular). Now with some probability the photon passes through the polarizer.

I see the above situation similar to an electron passing through the slit. And I consider that a measurement is being made of either quantity. This way I don't see an issue of there being a need for an observer in order to collapse the function. It appears that anything that constrains the free form of an entity's wavefunction (that is a change of co-ordinate in co-ordinate space needs to be performed) constitutes a measurement. Is this correct?

 

[Phrak]

I'm a novice at this quantum business and was just trying to understand the single slit diffraction experiment and when an electron's position is getting measured in particular.

Given my understanding it looks like the electron's position gets measured twice. Once when it is just about to pass through the slit and the second time when it hits the screen.

As I see it from the source to the single slit the electron's waveform has a hugespread (including all possible coordinates). However, when it gets to the slit it can pass through only if its waveform is much smaller (of the width of the slit). So at that particular moment doesn't the electron's huge waveform collapse to either one that is within the slit's width or somewhere else (in which case it doesn't pass through the slit).

And because of the reduction in the bandwidth of the position waveform its conjugate momentum waveform acquires a huge spread and this makes it go in any direction after it passes through the slit giving rise to the diffraction pattern. Is this right? Please correct me if I'm wrong because this seems to be quite crucial to understanding what measurement is.

First, attributing an element of physical reality to reduction of the wave function, or 'collapse' is an interpretation of quantum mechanics.

But frustratingly I've never seen anyone attribute the passing through of the slit as a measurement process. Only when it impinges on the phosphor screen do people say a measurement is performed (and sometimes only when a conscious being sees the phosphor screen).

If, by some means, the reaction of the electron upon the screen were measured, this would constitute a measurement. Otherwise the electron field and the screen remain entangled.

 

[svnaras]

First, attributing an element of physical reality to reduction of the wave function, or 'collapse' is an interpretation of quantum mechanics.



If, by some means, the reaction of the electron upon the screen were measured, this would constitute a measurement. Otherwise the electron field and the screen remain entangled.

Maybe "collapse" was the wrong word to use. By collapse I mean the point at which a superposition in some basis changes to one of the eigen values of the basis. For eg: a photon with polarization +1 in the 0 degree basis changes to either +1 or -1 in the 45 degree basis when it passes through a 45 degree polarizer.

 

[zonde]

I'm a novice at this quantum business and was just trying to understand the single slit diffraction experiment and when an electron's position is getting measured in particular.

Given my understanding it looks like the electron's position gets measured twice. Once when it is just about to pass through the slit and the second time when it hits the screen.

As I see it from the source to the single slit the electron's waveform has a hugespread (including all possible coordinates). However, when it gets to the slit it can pass through only if its waveform is much smaller (of the width of the slit). So at that particular moment doesn't the electron's huge waveform collapse to either one that is within the slit's width or somewhere else (in which case it doesn't pass through the slit).

And because of the reduction in the bandwidth of the position waveform its conjugate momentum waveform acquires a huge spread and this makes it go in any direction after it passes through the slit giving rise to the diffraction pattern. Is this right? Please correct me if I'm wrong because this seems to be quite crucial to understanding what measurement is.

But frustratingly I've never seen anyone attribute the passing through of the slit as a measurement process. Only when it impinges on the phosphor screen do people say a measurement is performed (and sometimes only when a conscious being sees the phosphor screen).

If you take laser then without the slit it makes compact spot on the screen. When you add single slit this spot becomes stretched (blurry) in direction perpendicular to the slit.
I can't imagine other explanation than that trajectories of photons are deflected at the slit.

It's not hard to perform that experiment on your own using laser pointer.
Take two paper cards, put them on the edge of table so that they make small slit between them and then point laser pointer at the slit. On the floor you can put sheet of white paper as a screen.

 

[DrAupo1]

What effect does the thickness of the slit material,relative to the width and length of the slit itself, have on the resulting pattern? Is the width of the slit proportional to the wavelength? What about spacing between the slits?
What if one slit is rotated 90 degrees with respect to the other? What about two circular holes? Other shapes? Triangles,squares,etc?
It is easy to understand how 2 slits at right angles could produce interference,because the center of each slit could simply be interpreted as an infinite line of small circles,thru which only a particle could pass, yet the slit itself allows a wave to pass.(imagine a "+". now look at the very center of the "+".
Now slowly rotate the lines until they are parallel.Now it allows a wave and particle to pass.What am I missing here?


I realize that is a lot of questions, but I too am new to this realm.
Thanks for being patient with a novice.