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Q1. What is the source for electrons used in double split experiment?

Q2. Electrons must come from some atom. When inside the atom, the electron is a probability wave function. If the electron is struck from the atom using photons or any other methos, it would be an interaction with the electron. So wouldn't it's wave function already have collapse at the start of the experiment itself?

Q3. If this electron's wave function has already collapsed before hitting the slits then it is a particle. It should then approach slit as a particle. Am I missing something basic?

Q4. If above assumptions are valid, does this particle electron change back to wave mid flight towards the slits? And produce interference?

Q5. Is the source slits and detector setup rules out any interaction of electron with the air molecules? Or the setup is inside a vacuum?

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  • $\begingroup$ The wave function evolves over time in accordance with the Schrodinger equation. If it stopped evolving after being "struck from the atom" then of course it would never interact with the screen. $\endgroup$
    – WillO
    Jul 30, 2016 at 3:05
  • $\begingroup$ Thank you for the information. This is new for me. The wave function evolves over time. Haven't read it yet. May be I just got started in QM. Looks interesting. $\endgroup$
    – Vivek
    Jul 30, 2016 at 3:23
  • $\begingroup$ Couple of misunderstandings here. Electrons are quanta, i.e. they are single measurements on a system. Wave functions don't describe probabilities but the state of ensembles of quanta, so to say that "the electron is a probability wave function" is meaningless, as it equates a single measurement with the description of all possible measurements. Collapse of the wave function is a fairly meaningless concept that one shouldn't use any longer. Electrons are not particles, as that is a term that is only useful on classical mechanics, even though the misnomer is still frequently found. $\endgroup$
    – CuriousOne
    Jul 30, 2016 at 3:30

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Q1. What is the source for electrons used in double split experiment?

A bit of search on the net is not wasteful. Following the links it is seen that the experiment is in vacuum as an electron microscope is used. In electron microscopes :

the whole of the inside of an electron microscope is under high vacuum in order to enable the electron beam to travel in straight lines.

The beam of electrons ins prepared so that it is in a plane wave pattern.

Q2. Electrons must come from some atom. When inside the atom, the electron is a probability wave function. If the electron is struck from the atom using photons or any other methos, it would be an interaction with the electron. So wouldn't it's wave function already have collapse at the start of the experiment itself?

Each measurement and each setup has a quantum mechanical solution of the appropriate quantum mechanical equation, and the appropriate boundary conditions. There is always a wavefunction which models an electron , but the quantum mechancical model is probabilistic. The mathematical function when complex conjugate squared gives the probability of finding the electron at (x,y,z,t). In the double slit experiment at the (x,y) of the screen of detection.

The wave function is the solution of the problem : plane wave electron impinging on boundary two slits

Q3. If this electron's wave function has already collapsed before hitting the slits then it is a particle. It should then approach slit as a particle. Am I missing something basic?

There is no magic in the wavefunction.It is always there and nature solves the differential equations for the given boundary conditions. The electron, as a quantum mechanical entity, in the small dimensions of the slits can only be described by the probability distribution which is the solution of the boundary conditions problem.

Q4. If above assumptions are valid, does this particle electron change back to wave mid flight towards the slits? And produce interference?

It is not a matter of change, it is a matter of boundary conditions and the square of the wavefunction constrained by them.

Q5. Is the source slits and detector setup rules out any interaction of electron with the air molecules? Or the setup is inside a vacuum?

The experiment has to be in vacuum since the boundary conditions change when other particles are on the way, and the problem becomes different.

To summarize:

electron ds

Single electron double slit

The macroscopic "particle" nature of the electron is seen in the dots on the (x,y) of the measurement screen surface. The macroscopic "wave" nature of an electron is shown in the probability distribution as measured in the accumulated screen shots , nature's solution of the boundary value problem.

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    $\begingroup$ @CuriousOne you must really be living in a parallel world then. In my universe, a macroscopic classical particle has an (x,y,z,t) with measurement errors by definition of a particle. As in the classical ideal gas. Planets are classically composed of innumerable such particles. $\endgroup$
    – anna v
    Jul 30, 2016 at 5:52
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    $\begingroup$ @CuriousOne the world people live in is classical. The envelopes of understanding observations is classical. If I see a smudge on the glas of the car I "know" it was a butterfly or bee. The definition of particle is classical, the assignment of the word is cognitive, semantic, whatever. But to confuse a newbie with esoteric definitions carrying a book behind them for understanding, is not constructive, nor does it motivate for further studies. $\endgroup$
    – anna v
    Jul 30, 2016 at 6:19
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    $\begingroup$ @CuriousOne I also don't understand what world you live in. You don't believe in photons and now you don't believe electrons can make these impact marks either? I too think it would be confusing for a newbie or some of these other physicist you talk about. $\endgroup$
    – Bill Alsept
    Jul 30, 2016 at 7:48
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    $\begingroup$ @BillAlsept It is not worth the effort as CuriousOne is a platonist. He believes that the very successful model of quantum field theory is THE REALITY, not just another beautiful mathematical model fitting data specific data . It seems also that he( she?) does not believe in relying on a specific classical frames for definitions, everything has to go to quantum fields. $\endgroup$
    – anna v
    Jul 30, 2016 at 8:15
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    $\begingroup$ You still cannot explain why it takes A field as big as the universe to account for one single photon. Your field and light wave are Great tools but ridiculous when it comes to reality. It cannot be explained in reasonable words how that is possible. Why make it more complicated than it needs to be. $\endgroup$
    – Bill Alsept
    Jul 30, 2016 at 8:43
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A1. Usually people use thermal electron source for the double slit experiment, as rightly pointed out by @annav the experiment is performed in vacuum. It need not be a electron microscope only a low energy electron gun is sufficient.

A5. The experiment can not be performed in the air because the travel range of such low energy electrons in air is very small (less than micron).

A low energy electron is an absolute necessity of this experiment. As you might know that for particle wave

$$\lambda=\frac{h}{mv}$$

and as the velocity/energy of the electron increases its wavelength decreases.

The diffraction pattern arises due to the interference between the portions of the probability wave of one electron passing through both the slits. As long as the slit separation is smaller than the electron wavelength the interference can occur because the probability wave of one electron can pass through both the slits but in other case it can (practically) pass from one slit only.

Now I think it does not matter if the electrons are coherent with each other before the slit because anyways the portions of the wave of one electron interfere with each other.

It may be noted here that the electrons must have a very small energy spread too because if the energy spread is large, then the peaks of diffraction pattern for one energy matches with the through of the interference pattern of other energy and the fringes vanish as we go away from center.

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  • $\begingroup$ Thank you hsinghal for the answer. The slit separation infomation was also helpful for me. I wonder what is the result if slits are separated beyond electron wavelength? The electrons would bounce off from middle of slit or go through one of the slits? How accurate is the alignment of apparatus is with the slit? $\endgroup$
    – Vivek
    Jul 30, 2016 at 19:54
  • $\begingroup$ I think the electrons will pass from slits and you will see the super position of the electrons from two slits but no interference pattern. Just the addition of the electron flux distribution from individual slits. It should look like Gaussian distribution. $\endgroup$
    – hsinghal
    Jul 30, 2016 at 19:57

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