Double Slit Question

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If an electron moves through a double slit without being subjected to any measurement device it will act as a wave and cause
interference patterns on a screen. If the electron it measured before or after the slits it will act as a particle and no interference patterns
will be be seen on the screen. What if the screen where to act as both screen and measurement device (to measure spin for example).
Is this possible? Would the screen act as screen and measurement device at the same time "fooling" the electron ?
Can it be done? and what would be on the screen: interference patterns or no interference patterns?
 
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‘Fooling’ the electron not needed & it does not need to be detected at the slits.
The point is when they set up a detection field at just one of the two slits, sufficient to detect any electron going through that one slit. And then they eliminated the screen detections when the slit detector did detect an electron going though that slit. Therefore the remaining screen detections we know are for electrons going though the other slit where there is no detection at the slit. But still no interference pattern is produced for that undetected group at the screen, just the sure knowledge of which way seems to be enough to destroy pattern production – THAT IS THE PARADOX.
Only solved by non-local assumptions like QM.
 
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RandallB said:
just the sure knowledge of which way seems to be enough to destroy pattern production – THAT IS THE PARADOX.
Only solved by non-local assumptions like QM.
What do you mean by knowledge is enough? :confused:
 
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By placing the detector at slit A you will know when the electron passes through this slit. Since slit A is blocked, only those electrons that go through B will strike the detector at the back.

Since you've blocked the detector for electrons going through A, you know that the pattern produced on the back detector is from slit B - hence you have knowledge of which path the electron travelled, this knowledge is enough to destroy the interference pattern.
 
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ptabor said:
By placing the detector at slit A you will know when the electron passes through this slit. Since slit A is blocked, only those electrons that go through B will strike the detector at the back.

Since you've blocked the detector for electrons going through A, you know that the pattern produced on the back detector is from slit B - hence you have knowledge of which path the electron travelled, this knowledge is enough to destroy the interference pattern.
But to assert that it is our knowledge of the situation that causes the "destruction" of the interference pattern is rather unscientific IMHO, it is more like some anthropomorphic view of reality.

I think that "our knowledge" has absolutely nothing to do it.

But since you make the assertion, would you care to demonstrate or explpain that it is in fact our knowledge that causes the "destruction" of the interference pattern?
 

SF

Well, what exactly _is_ the measurement process?

The Wikipedia page on "[URL [Broken] cat[/url] sais:

Today, the mainstream view is that the thermodynamically irreversible triggering of the device acts as if it were an observation, in that the triggering apparatus generates decoherence that appears to "collapse" the wave function.
Apparently, the act of observation doesn't have anything to do with knowledge (or counsciousness) of the effect, but rather what matters is whether the particle produces an irreversible thermodinamic event or not.

Can this be generalised, to say that :
thermodinamically irreversible efect => no interference patterns
else => interference patterns
?

The current way of looking at things leaves lots of room for New Age theories who say that you can counsciously change/create your own reality simply by observing the world around you .. which is too "Matrix" for me to accept.
 
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selfAdjoint

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SF said:
Well, what exactly _is_ the measurement process?

The Wikipedia page on "[URL [Broken] cat[/url] sais:



Apparently, the act of observation doesn't have anything to do with knowledge (or counsciousness) of the effect, but rather what matters is whether the particle produces an irreversible thermodinamic event or not.

Can this be generalised, to say that :
thermodinamically irreversible efect => no interference patterns
else => interference patterns
?

The current way of looking at things leaves lots of room for New Age theories who say that you can counsciously change/create your own reality simply by observing the world around you .. which is too "Matrix" for me to accept.
Hear, hear. But then what is it that can be "collapsed" by an irreversible thermodynamic process? If we regard the state function as a kind of library or database of states possible after the next I.T.P. then where is it kept?
 
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SF

I don't think that's something anyone can answer, but then again, i'm not an expert, so I'm just quoting some guy:
The other consequence of the nature of the wavefunction is
even more striking. The reason it yields the probabilities for vari-
ous results that may arise when one makes a series of measure-
ments on equivalent systems is that the wavefunction is given by
the sum of the different states--each state implying a different
result of the measurement--each multiplied by a coefficient
related to the probability that the system will be in that particular
state when it's measured.
This may not sound so strange at first, but think about it for a
minute. The wavefunction can incorporate two mutually exclu-
sive configurations--say, spin up and spin down--at the same
time. Since the wavefunction governs the evolution of the quan-
tum-mechanical particle system, this means that the particle is
neither spinning up nor spinning down before the measurement,
but rather is, in some weird sense, doing both. When you make
the measurement, you find one or the other result (with the prob-
ability having been determined by the wavefunction). Moreover,
after the measurement, since the particle is now restricted to
existing in the spin state you measured, the nature of the wave-
function describing the particle will have changed. It will now
not involve a sum of both states, but only one state.

[..]

This sum of different quantum states which makes up the
wavefunction describing a system is called coherence. As long as
the different states in the sum all exist in the wavefunction, it
describes a "coherent superposition" of states. However, by the
act of measurement, you can reduce the wavefunction to a single
quantum state, destroying this coherence. As long as my electron
wavefunction is made up of a coherent sum of many different
quantum states, the single electron can behave as if it is many
electrons.
 
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shaun11 said:
What if the screen where to act as both screen and measurement device [..] Is this possible?
Back to the OP, I think the answer is no. QM implies that some measurements fundamentally are mutually exclusive (which is difficult to grasp classically).

Unfortunately, just like perpetual motion machines, it often takes less time to propose a method of "circumventing the known laws of physics" than what it does to understand why that particular method will fail.
 

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