Quantum frogs and jumping to conclusions

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  • #26
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Originally posted by FZ+
I may be wrong in this, but I believe that by classical theories, either all of the photons should go through or none of the light should go through. Since the photons coming out of the first filter are in terms of polarisation identical, they should all be changed in the same way by the second polariser. However, only some of them change, and we have a probability of changing the polarisation instead of a definite yes/no.
But this is practically tha same as the QM interpretation right ? The classical interpretation should be that of simple vectortransformation. Split the vector of each wave hitting the polarizer into two perpendicular components, one of which is aligned with the polarizer.

Your statement that all lightwaves should be changed the same way by the second polarizer is correct, they are. The classical answer lies in realizing that all waves pass through, each one with a lower amplitude than before. Think waves, not photons.
 
  • #28
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Originally posted by Chemicalsuperfreak
The problem is that Zoobyshoe and Chi Meson are wrong. They state probably the most common misunderstanding of any scientific principle.
Zoobyshoe is not wrong.
What I wrote was a simplified but
accurate paraphrase of the prob-
lems Heisenberg described one
would encounter when trying to
determine the probabiities of
both the location and momentum
of an electron in orbit via a
microscope. I did not claim this
to be an explanation of The
Heisenberg Uncertainty Principle.
Hesenberg said what I said he
said:
Heisenberg's Physics and Philosophy
Address:http://www.marxists.org/reference/subject/philosophy/works/ge/heisenb3.htm

My goal was to explain the origin
of the "Urban Legend" about quanta
"playing tricks" on the observer.
I believe Heisenbergs concerns
about how observation and measure-
ment may change what is observed
and measured are the primary
source of any such legend. If you
read the linked paper you will see
it is an issue he raises in more
than one situation. I am not aware
to what extent these concerns
informed his final formal state-
ment of The Uncertainty Principle.
 
  • #29
Chi Meson
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Originally posted by Chemicalsuperfreak
The problem is that Zoobyshoe and Chi Meson are wrong. They state probably the most common misunderstanding of any scientific principle.
Um, I disagree. I am aware of the extreme simplification of the condition, but what I said is quite correct.

ALso what I said does not go against what C'freak said, and what I said does not indicate a misunderstanding of the principle (In fact I was not even referring to HUP).

What I said was what I believe to be the most basic answer to the original question: It remains true that you can not observe anything (in classical or quantum terms)about a single particle without interacting with it, and interacting with a particle will change the condition of the particle.

Please let me know, specifically, what is wrong about this.

BTW, I just remembered what could be an exception to this: the analogy of the shoe: if you put a left shoe in one box and a right shoe in another box, but didn't know which was which, as soon as you looked in one box (say its the left shoe) then you immediately know that the other box contains the right shoe "without interacting with it." I think this is discussed in the book "in search of Schroedinger's cat"
 
  • #30
selfAdjoint
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There is nothing wrong with the statement that to observe a particle you must interact with it and thus change it. What is wrong is to state that this is what the uncertainty priciple is about. The U.P. as was said, does not concern your observation but basic existence. It says that where the photon frequency is perfectly known (for example), its location is truly undefined, not just unknown.

The problems of interacting field theory are real problems, but they are at a different level from uncertainty.
 
  • #31
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Numerous experiments (like the Aspect experiment) have disproven the so-called EPR paradox.
Wait, what? Are you saying that quantum entanglement has been disproven? No more quantum teleportation? Oh man my novel's completely ruined...
 
  • #32
jcsd
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Originally posted by CJames
Wait, what? Are you saying that quantum entanglement has been disproven? No more quantum teleportation? Oh man my novel's completely ruined...
No the Aspect experiment PROVES quantum entanglement.
 
  • #33
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Cool. Can I get a link?
 
  • #35
FZ+
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Originally posted by CJames
Wait, what? Are you saying that quantum entanglement has been disproven? No more quantum teleportation? Oh man my novel's completely ruined...
No... I mean the paradox is disproven - ie. Einstein et al's conclusion that QM cannot be a complete description of the world because it allow quantum entanglement etc. (Though some people still dispute the conclusiveness of the Aspect experiment.)
 
  • #36
selfAdjoint
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Exactly right. John Bell, who was a partisan of Bohm's hidden variable theory, devised his inequalities in the hopes of supporting that theory and atttacking quantum mechanics. See his book Speakable and Unspeakable in QM. But in fact quantum mechanics turned out to explain the effect and the Aspect and subsequent experiments demonstrated that the Bell correlations really happen. As you say, some quantum attackers haven't given up and are nit-picking the experiments.

QM says that what subsists between the entangled particles is a correlation between attributes, not a causal relationship. This knocks message sending on the head. I think the Bell-Aspect story is the greatest demonstration of the quantum nature of the world since the two slit-one electron experiment described in Feynmann's textbook.
 
  • #37
Chi Meson
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Originally posted by selfAdjoint
There is nothing wrong with the statement that to observe a particle you must interact with it and thus change it. What is wrong is to state that this is what the uncertainty priciple is about.
I totally agree. And that's not what I said. It's like a "Three's Company" episode: it's all just a big misunderstanding!

Getting beyond it: SelfAdjoint said:

"This knocks message sending on the head."

Which way? Does this business allow for using quantum weirdness to send information faster than light, or dies this prove it impossible. Or does it have nothing to do with this?

As I recall, single pieces of data can be determined instantaneously (such as that shoe analogy) but it was believed that you couldn't string such data into "information." (No hyper-c phone calls).
 
  • #38
selfAdjoint
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message sending FTL is impossible with the quantum understanding of the Bell-Aspect results. Suppose you have one of a pair of entangled particles. You want to change its state to send a bit to the other station, where the other particle will change its state and be recorded.

But you can't measure the state of your particle, because that will destroy the entanglement and with it your hopes of a link. So you would just have to trust that your particle is still in its entangled state. But somebody, or even some random event, could have "measured" the particle at the other end. That would have destroyed the entanglement too. So you're up the creek either way. You might have a link or not, but you'll never know for sure.

When all of this is developed in the math of the theory, it is known as Eberhardt's theorem. Quantum entanglement gives enhanced correlation, but it doesn't give cause, and you can't use it to send messages.
 
  • #39
Tom Mattson
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Originally posted by carla
What does quantum mean exactly?
It corresponds to the concept of discreteness, as opposed to continuity. For example, in classical mechanics the energy of a particle bound in a potential is a continuous function of the state variables. Upon closer inspection of microscopic systems however, we see that there this is not the case. Only certain allowed energy levels are occupied. For instance, in order for an atomic electron to be promoted from a lower energy state to the next higher energy state, it cannot do so via a continuous gaining of energy. Rather, it must absorb a quantum of energy.

What is a 'quantum leap'?
A quantum leap is a stroke of genius on par with the one(s) that led to quantum mechanics.

And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?
Thanks...
This is due to a popular misconception that "observation" is somehow related to "cognizance". When a physicist speaks of "observations" in QM, it means only that the system interacts with the detection apparatus. No mind or consciousness is implied or needed.
 
  • #40
Tom Mattson
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Originally posted by FZ+
A demonstration of the problem is easy to arrange - get 3 polarised filters. They, as you may recall, allow light through in only one alignment. If you put two of the filters aligned at right angles to each other, as predicted, no light goes through. The only light that gets through the first is cut out by the second filter. But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory.
No, classical electrodynamics accounts for it just fine. What is completely contrary to the classical theory is when you do a similar experiment using particles and Stern-Gerlach filters.
 

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