Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Warp wrote:
>>>   So when the electron hits the sensitive film *after* it has passed the
>>> slits, it goes back to the past and changes it so that it goes through
>>> only one of the slits after all?
> 
>> No. It either interferes with itself or not. You're assuming the only 
>> way it can interfere with itself is to go through both slits. There's no 
>> evidence that's the case, and much evidence that it isn't.
> 
>   So why does it interfere with itself when there are two slits but not
> when there is only one? If it was just one regular physical macroscopical
> particle it wouldn't matter how many slits there are: If it goes through
> one of them, it just goes through one of them, that's it. It doesn't even
> "know" that there are other slits.
>   However, when there are two slits, the electron passes through and starts
> interfering with itself, as if it has passed through both and changed
> direction in different ways.
> 
>   How else can this be explained? How does the electron "know" that there's
> another slit so that it "knows" to start interfering with itself, other than
> actually going through the other slit as well?
'It' knows that the second slit is there because an electron has an 
infinite size. Part of the problem is that you use 'it', implying that 
it is an identifiable object and it has a subconscious connotation of 
something finite.
> 
>   (I believe this has something to do with wave-particle duality: In the
> double-slit experiment the wave nature of the electron shows up: The wave
> goes through both slits and starts interfering with itself.)
Both 'wave' and 'particle' are concepts from classical physics that 
don't apply here.
In the double slit experiment you have a source of electrons, a double 
slit and a screen. You know that you had an electron at the source and 
you can compute the likelihood of the position on the screen that will 
light up, enabling you to estimate the pattern if you use a large number 
of electrons. Between the source and the screen, the electron passes 
through ever point in the universe and you are not even sure that the 
same electron hit the screen as the one you started with. Simply because 
'same' is not defined here.
A basic rule of quantum mechanics: don't try to visualize what happens. 
You can either visualize it or compute it, not both at the same time. ;)

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