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enceladus_
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I am doing some reading on Quantum Mechanics, and I do not understand this concept. Here is an excerpt from "The Universe and the Atom" by Don Lichtenberg. Sorry for the bad formatting.
I don't understand why we can't answer which slit the particle when through. What else could it have gone though? This is probably a terribly literal way of viewing the event, but the whole concept is pretty fuzzy to me.
I also don't understand the significance of entanglement. If two observers agree on the same axis when total spin is 0, this seems incredibly obvious. If the axises are then changed to be perpendicular of each other, the probability of prediction is 50%. Again, this seems very obvious, and insignificant.
Thanks in advance to those who give wisdom.
13.4 Interference in quantum mechanics
If we shine light through a thin slit in an opaque object and place a
screen behind the slit, then the light will form a pattern on the screen.
Because light is a wave, it will bend as it goes through the slit (we call
this phenomenon “diffraction”), and the image on the screen will be
wider than the slit itself. We can also shine light through a double
slit, and again we get a pattern on the screen. However, the pattern
is not simply the sum of the patterns caused by the individual slits.
Because light is a wave, the light going through the two slits exhibits
interference. In particular, there are regions on the screen that are illuminated
if either slit is closed, but the regions are dark when both
slits are open. the reason is that the wave exhibits destructive interference
in those regions.
Let us now make the light source so dim that only one photon
goes through the slits at a time and makes a tiny spot on the screen
as if it is a particle. Suppose the spots on the screen are recorded
as more and more photons impinge on it. The pattern of recordings
is the same as the diffraction pattern from two slits. Those who say
that each photon must go through only one slit get the wrong answer,
because they predict, in contrast to experiment, that the pattern is the
sum of the patterns from one slit at a time.
The only way we can explain the phenomenon is to say that even
a single photon acts as a wave when it goes through the slits, somehow
“being aware” of both slits as it passes through, but the photon
acts as a particle when it makes a spot on the screen.
If we do the same experiment with a beam of electrons going through two slits and hitting a screen that scintillates when an electron
strikes, we get the same answer as with a beam of photons. The
conclusion is that an electron acts as a wave when it goes through
the slits and it acts like a particle when it hits the screen.
The question, “Which slit did the photon or electron go through?”
cannot be answered. According to quantum mechanics, the question
does not make sense unless an experiment is performed to measure
which slit the photon or electron goes through. It is a difficult measurement
to perform in practice, but it can be analyzed in principle.
Such an experiment is called a “thought experiment.” The result of
the analysis is that if a measurement determines which slit the photon
or electron goes through, the interference phenomenon is destroyed,
and the pattern on the screen is the result of the sum of the patterns
with a single slit open at a time. In Figure 13.1 we illustrate the
diffraction pattern after a wave (of light or electrons) passes through
a single slit. We also illustrate the interference pattern after the wave
passes through two slits.
If a two-slit interference pattern is observed, one cannot say that
the photons or electrons “really” go through either one slit or the
other. They act as waves that pass through both slits at the same
time. Our intuition does not allow us to “understand” how photons
and electrons can act both as waves and particles. There is no inherent
contradiction in the theory of quantum mechanics, as far as
we know, and furthermore, the calculations of quantum mechanics
lead to predictions that agree with experimental measurements. But
the human mind does not seem to be able to comprehend what is
“really” going on.
I don't understand why we can't answer which slit the particle when through. What else could it have gone though? This is probably a terribly literal way of viewing the event, but the whole concept is pretty fuzzy to me.
I also don't understand the significance of entanglement. If two observers agree on the same axis when total spin is 0, this seems incredibly obvious. If the axises are then changed to be perpendicular of each other, the probability of prediction is 50%. Again, this seems very obvious, and insignificant.
Thanks in advance to those who give wisdom.