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  #1  
Old 09-21-2001, 04:29 PM
Clarissa_xx Clarissa_xx is offline
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Ok, my head hurts. I had a bf once who was into this stuff but I guess I never really paid attention. There was a thread here with this link:

http://www.pipeline.com/~rhodesr/bot...ity/chap2.html

It seems they went through all sorts of creative methods to test this. My question is this. What if you recorded which slit the electrons went through but did not look at it and put it in a safe. You then analyze where the light hit and see if it a wave or particle pattern. If it is a wave pattern, which means nobody will analyze the data, then go unlock the safe and analyze the data. If it is a clumping/particle pattern, which means someone analyzed the data then go to the safe and destroy the data without analyzing it.

What would happen if someone did this???
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  #2  
Old 09-21-2001, 04:37 PM
erislover erislover is offline
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You'd have a Schrodinger's cat with electrons.

Truly, it doesn't matter whether the scientist observes the data or not, merely that quantum information has been measured; that is, that quantum data has been magnified to a macro perspective so we may measure it.
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  #3  
Old 09-21-2001, 04:41 PM
Clarissa_xx Clarissa_xx is offline
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But that's not what the link says. It says that if you record the data but then erase it before you analyze it will be a wave. So the measuring appears not to matter but whether it was analyzed.
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  #4  
Old 09-21-2001, 04:45 PM
Clarissa_xx Clarissa_xx is offline
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Schrodinger's cat ?? I'll have to look it up. I got this link in a thread here in general questions but can't find it now.
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  #5  
Old 09-21-2001, 04:46 PM
Great Dave Great Dave is offline
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Check the Sept issue of discover magazine for a real head bender of an explanation. Basically, observing it really doesn't do anything- why should phyisical properties change because of the presence or absence of sentient beings? Instead, there are other universes in which the photon goes thru the other slit. Or something like that. I only read it once.
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  #6  
Old 09-21-2001, 04:51 PM
Saltire Saltire is offline
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The story of Schroedinger's cat (an epic poem)

I try never to miss a chance to post a link to this column. In my opinion, it is the best of the master's work.
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  #7  
Old 09-21-2001, 05:01 PM
Ferrous Ferrous is offline
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Well, seriously, it depends what your definition of "is" is.
Electrons are not actually anything like objects in the macro world. They can only really be described using mathematics. However, for purposes of daily life, they can be thought of as behaving like a particle or a wave. Which one depends on what experiment you are doing or what the application is.
(Or something like that...IANAQP. It confuses me too.)
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  #8  
Old 09-21-2001, 10:38 PM
Ring Ring is offline
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I don't think this can be explained to someone who has no experience with quantum weirdness, but here is what I posted in another thread

"You have to resign yourself to the fact that no one knows. Quantum mechanics is outside the realm of your intuition, and probably always will be. As Richard Feynman said:

Quote:

quote:

"There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe that there ever was such a time. ... On the other hand, I think it is safe to say that no one understands quantum mechanics. ... Do not keep saying to yourself, if you can possibly avoid it, `But how can it be like that?', because you will get `down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."

R. P. Feynman (1967)
Von Neumann proved that if a quantum system is interacting with a measuring device, then the superposition of these systems remains a smoothly evolving wave(function). And that only when the superposition encounters a system (a conscious brain) which is not subject to the time symmetrical laws of QM does it collapse.

This collapsing of the wavefunction is one of biggest sticking points in QM, and it is also why so many theories have been proposed to get around this need for consciousness. Many worlds is one of these theories and it is mathematically correct, but so are all the others - so you buy your tickets and you take your chances."

If you have any questions about this probably incomprehensible post, let me know and I'll try to answer them. However, it's probably futile. But it's futile because of my inability to clearly explain it... not your ability to understand it.
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  #9  
Old 09-22-2001, 01:21 AM
DaveW DaveW is offline
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BigDaddyD wrote:
Quote:
Check the Sept issue of discover magazine for a real head bender of an explanation. Basically, observing it really doesn't do anything- why should phyisical properties change because of the presence or absence of sentient beings? Instead, there are other universes in which the photon goes thru the other slit. Or something like that. I only read it once.
It's a "headbender" because it's not really science. The extreme version of the Many Worlds Interpretation revealed in that article cannot be used to predict anything, nor can it be tested. Therefore, it's not science.

A person can believe in it all they want - the facts seem to be that not one aspect of that extreme view make a bit of difference to how the equations turn out if you assume that there's only one universe (this one).

Geez, Deutsch claims that he's more cautious now in the hopes that the other 'copies' of him in other universes will be more cautious, also, and not run down defenseless children on the highway? What a crock! He's hypothesizing nearly an infinite number of universes (since the quantum effects go back to the beginning of time), and so it would be almost a certainty that at least one of the copies of him has not only just now run down a child, but did so on purpose due to the "quantum" choices "his" parents made dozens of years ago. One of Deutsch's 'copies' was involved in the recent WTC destruction, and is still 'alive' to continue to terrorize the world! And this is equally as true for me, you, and everyone else on the planet. Under extreme MWI, there's an Earth out there in which George W. Bush is being hunted down as the prime suspect (perhaps the same universe in which Gandhi is Supreme Overlord, and failure to kiss his feet means an instant beheading). Under extreme MWI, the plots of Jumping Jack Flash, Dumb and Dumber, Friday the 13th, and the first 3/4s of Repo Man are likely to be occuring, in "real life," right now.

Enough ranting (sorry, but this has stuck in my craw since I read the article). The main problem I can see with Discover's coverage of this "secret" of physics is that whole photon interference thing (that, and the reporter seems to have been "wowed" by Deutsch, which made the reporting less than unbiased - see the "impeccable credentials" quote, where the reporter begs us to ignore the "Argument from Authority" logical fallacy).

The trick is that photons, under quantum 'rules', cannot be thought of as waves or particles, but must be thought of as waves and particles simultaneously. Doing otherwise forces you to ignore half the evidence we've got at some point or another.
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  #10  
Old 09-23-2001, 01:49 AM
Chronos Chronos is offline
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Quote:
But that's not what the link says.
The link is wrong, on that point at least. If anything interacts with a particle in a manner sufficient to tell if the particle is there, you lose the interference pattern. It doesn't matter if the data is recorded, or if a sentient being ever looks at the data. Turning your measurement devices off might or might not restore the pattern, but that depends on how your device works, and what it means to turn it off.
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  #11  
Old 09-23-2001, 01:04 PM
Ring Ring is offline
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Quote:
Chronos wrote:

The link is wrong, on that point at least. If anything interacts with a particle in a manner sufficient to tell if the particle is there, you lose the interference pattern.
Although I freely admit that you know far more than I (me?) on most physics topics I still must say that I don't believe the above statement has been proven to be true.

First, who or what is going to "tell if the particle is there"?

Second, your statement is contrary to the Copenhagen interpretation.

Third, although most physicists now ascribe to the macroscopic decoherence of the superposition I don't believe that anyone has proposed that this is also true at the microscopic or particle level.
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  #12  
Old 09-23-2001, 04:52 PM
jbird3000 jbird3000 is offline
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Quote:
Originally posted by Chronos
If anything interacts with a particle in a manner sufficient to tell if the particle is there, you lose the interference pattern.
Wouldn't an electron interact with all charged particles causing them to be attracted/repulsed by it, even if there were a great distance away? And couldn't you measure that?
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  #13  
Old 09-23-2001, 06:11 PM
erislover erislover is offline
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I admit I have had conceptual problems understanding what exactly constitutes "revealing quantum information" so that the waveform collapses, but I'm with Chronos-- I have never read (until this point) anything stating that humans actually had anything to do with it.
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  #14  
Old 09-23-2001, 07:59 PM
Ring Ring is offline
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Quote:
jbird3000 wrote

And couldn't you measure that?
Yes you could and this would constitute a collapsing of the wavefunction by a conscious observer.

Quote:
erislover wrote:

I have never read (until this point) anything stating that humans actually had anything to do with it.
From the physics faq

http://www.cs.ruu.nl/wais/html/na-di...ent-in-qm.html

Quote:
”This is the oldest of the interpretations. It is based on Bohr's notion of `complementarity'. Bohr felt that the classical and quantum mechanical models were two complementary ways of dealing with physics both of which were necessary. Bohr felt that an experimental observation collapsed or ruptured (his term) the wave function to make its future evolution consistent with what we observe experimentally. Bohr understood that there was no precise way to define the exact point at which collapse occurred. Any attempt to do so would yield a different theory rather than an interpretation of the existing theory. Nonetheless he felt it was connected to conscious observation as this was the ultimate criterion by which we know a specific observation has occurred.”
Underlining mine
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  #15  
Old 09-23-2001, 10:41 PM
Chronos Chronos is offline
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Well, if we're going to be discussing the different interpretations of quantum mechanics, we need to be over in Great Debates. Experimentally, though, there is no difference in results between a person telling me that he saw which slit the photon went through, and a computer telling me. If the computer doesn't collapse the wavefront until a person looks at the data, then we've got causality paradoces: Suppose that I set up a computer to quietly record which slit an electron passes through, and record that data on disk. Suppose, also, I plan to erase that data immediately on conclusion of the experiment. Clarissa_xx's link claims that in that situation, we'll get an interference pattern on the screen. Now, I do the experiment, and see the pattern on the screen. Upon seeing the interference pattern, I change my mind, and decide to look at the slit data anyway. Uh-oh.
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  #16  
Old 09-24-2001, 12:19 AM
jbird3000 jbird3000 is offline
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Quote:
Originally posted by Chronos
If anything interacts with a particle in a manner sufficient to tell if the particle is there, you lose the interference pattern.
Say we're a great distance apart in a void. You're doing the slit experiment and I'm just trying to figure the location of an electron. My electron is repulsed by your electron, even though it's real far away. I detect this movement, although I don't know where it's from. But I know there's another electron out there. Do you get the interference pattern?
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  #17  
Old 09-24-2001, 08:57 AM
Ring Ring is offline
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Quote:
Chronos wrote:

Now, I do the experiment, and see the pattern on the screen. Upon seeing the interference pattern, I change my mind, and decide to look at the slit data anyway. Uh-oh.

Once you’ve seen the interference pattern on the screen you’ve made an observation, and collapsed the wavefunction. Since you saw an interference pattern there could not have been a detection made at either slit. When you, at a later time, examine the computer data it will confirm this.
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  #18  
Old 09-24-2001, 09:24 AM
Clarissa_xx Clarissa_xx is offline
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Ring/Chronos -- are you saying that the pattern will always be a clumping/particle pattern unless something is wrong with the recorder? In other words, I record the data and see an interference pattern. I then rub my hands together evil-like and go to my safe and get the data and start analyzing but find out it is defective?

I absolutely hate reading something offical like and find out it is wrong!!! Do you have a better link Chronos??
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