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Quantum Physics
Posted: 22 March 2007 06:39 AM   [ Ignore ]
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The following excerpt was taken from here

[quote:1a9432e703]

[u:1a9432e703]EPR paradox[/u:1a9432e703]

In quantum mechanics, the EPR paradox is a thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities and the values that can be accounted for by a physical theory. "EPR" stands for Einstein, Podolsky, and Rosen, who introduced the thought experiment in a 1935 paper to argue that quantum mechanics is not a complete physical theory. [/quote:1a9432e703]


My question is, at what point exactly was the deterministic, causal model for reality first threatened?  And was it threatened by an actual empirical observation?  Who made the observation? What did they observe?  And how?

And my second question is in regards to this:

[quote:1a9432e703]
The EPR paradox is sometimes referred to as the EPRB paradox for [b:1a9432e703]David Bohm[/b:1a9432e703], who converted the original thought experiment into something closer to being experimentally testable.

David Bohm’s first book, Quantum Theory published in 1951, was well-received by Einstein, among others. However, Bohm became dissatisfied with the orthodox approach to quantum theory, which he had written about in that book, and began to develop his own approach (Bohm interpretation) █ he devised a non-local hidden variable deterministic theory whose predictions agree perfectly with the nondeterministic quantum theory. [/quote:1a9432e703]

Ok, so why isn’t Bohm’s theory taken seriously?

The rest of my questions are more mundane and reflect my inexperience in this subject (but a person has to start somewhere)

[quote:1a9432e703]
During the last century, quantum theory has proved to be a successful theory, which describes the physical reality of the mesoscopic and microscopic world. Up to now, no method is known which contradicts [b:1a9432e703]the predictions made by quantum theory. [/b:1a9432e703] [/quote:1a9432e703]

Can anyone tell me what these predictions were and how they were made?

[quote:1a9432e703]
Quantum mechanics was developed with the aim to describe atoms and to explain the observed spectral lines in a measurement apparatus. [/quote:1a9432e703]

What are spectral lines?  And why do they exist in [i:1a9432e703]a measurement apparatus?[/i:1a9432e703]

[quote:1a9432e703]
During the development of quantum mechanics the fact that quantum theory allows for an accurate description of reality is obvious from many physical experiments, and has probably never been seriously disputed.
On the other hand, for the interpretation of quantum mechanics, things could not be more different. Since the theory of quantum mechanics has been formulated, the following question arises:

How can we interpret the mathematical formulation of quantum mechanics?

This question leads to a discussion, in which people with different philosophical backgrounds give different answers. [b:1a9432e703]Quantum theory and quantum mechanics do not account for single measurement outcomes in a deterministic way. [/b:1a9432e703]  [/quote:1a9432e703]

What is a measurement outcome?  When I open my fridge and look inside, the contents in the fridge are as they are displayed to me.  Is that an example of a measurement outcome?

[quote:1a9432e703]
One accepted interpretation of quantum mechanics is the Copenhagen interpretation. The Copenhagen manifest argued that a measurement causes [b:1a9432e703]an instantaneous collapse of the wave function which describes the quantum system[/b:1a9432e703] [/quote:1a9432e703]

Ok, so I’ve heard this term ‘wave function’ kicking around quite a bit.  Can anyone give me an explanation of it that makes more sense then the one given here?

[quote:1a9432e703]The system after the collapse is random - pure chaos. [/quote:1a9432e703]

Why? Have they observed this collapse with the naked eye?

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Posted: 22 March 2007 07:01 AM   [ Ignore ]   [ # 1 ]
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Those are good questions, Cory and I’d like to know more also.  Very interesting topic.

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Posted: 22 March 2007 07:08 AM   [ Ignore ]   [ # 2 ]
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Re: Quantum Physics

Cory, you’re asking all the right questions here. I am not an expert on QM, and giving a detailed lecture on its history would take more work than I am prepared to give it at this point.

However, I would simply say that QM was developed as scientists began to be able to investigate within the atom. The pre-QM idea of an atom was that it was like a small solar-system with electrons spinning around a stable nucleus at the center. But experiments to verify this led to the awareness that things were not so simple.

For places to start learning about it, I’d recommend books by people like Richard Feynman, his QED (about quantum electrodynamics, a theory for which he won the Nobel), as well as his Six Easy Pieces. It has a wonderful intro to QM in one of the chapters.

There are a multitude of other good popular-level books on QM out there, but do be careful to avoid the woo-woo authors, like Gary Zukav and the like. QM is not eastern mysticism.

[quote author=“CoryDuchesne”]Ok, so why isn’t Bohm’s theory taken seriously?

I’m not sure. I rather like Bohm’s theory myself, but for other reasons: because it allows for a “realist” interpretation of the wave-function collapse. But I don’t believe that Bohm’s interpretation gets round the fundamental issue of randomness. That is, I believe that Bohm’s theory also asserts that causal forces are fundamentally statistical in character.

[quote author=“CoryDuchesne”]Can anyone tell me what these predictions were and how they were made?

Well ... these would have been predictions dealing with subatomic particles. QM phenomena really are only apparent at the atomic or subatomic level. All of chemistry (= all chemical interactions) are at their basis peformed by quantum-mechanical forces.

So, they would have been predictions about the behavior of subatomic particles.

[quote author=“CoryDuchesne”]What are spectral lines?  And why do they exist in a measurement apparatus?

See HERE . The “measurement apparatus” for seeing spectral lines is a spectrograph.

[quote author=“CoryDuchesne”]What is a measurement outcome?  When I open my fridge and look inside, the contents in the fridge as they are displayed to me.  Is that an example of a measurement outcome?

Well, not sure precisely what this author meant by the phrase, but generally a “measurement outcome” is the result of a particular experiment. So you run your experiment (which we will assume is intended to measure some quantity X), and see what results. X would be your “measurement outcome”.

All examples of QM measurements are at the subatomic level. These weird effects do not occur in daily life, e.g., in your refrigerator. So QM is not required to send people to the moon, for example. If you want to “measure” the rate of fall of a ball down an inclined plane, you don’t need QM. So QM would be unnecessary for that “measurement outcome”.


[quote author=“CoryDuchesne”]

One accepted interpretation of quantum mechanics is the Copenhagen interpretation. The Copenhagen manifest argued that a measurement causes an instantaneous collapse of the wave function which describes the quantum system

Ok, so I’ve heard this term ‘waving function’ kicking around quite a bit.  Can anyone give me an explanation of it that makes more sense then the one given here?

Subatomic particles show weird behaviors. The only way to explain these behaviors is basically to say that a subatomic particle either is a sort of “wave function” (i.e. not a stable particle at all), or it is guided by a “wave function” (if you are Bohm).

The famous “ double slit experiment ” should give you some idea of what is going on here experimentally. I would also strongly suggest you look at some of the “external links” at the bottom of the Wikipedia page.

The wave function is said to “collapse” when it is observed at a particular location rather than another: where as before the observation that wave function was spread out over a large area, when it is observed the wave “collapses” into a particular part of its area. Where precisely it collapses is entirely random in character, following a certain function.

[quote author=“CoryDuchesne”]Have they observed this collapse with the naked eye?

They have observed thousands of experiments validating this theory.

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Posted: 22 March 2007 10:05 AM   [ Ignore ]   [ # 3 ]
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Re: Quantum Physics

[quote author=“CoryDuchesne”]


My question is, at what point exactly was the deterministic, causal model for reality first threatened?  And was it threatened by an actual empirical observation?  Who made the observation? What did they observe?  And how?

And my second question is in regards to this:


The EPR paradox is sometimes referred to as the EPRB paradox for David Bohm, who converted the original thought experiment into something closer to being experimentally testable.


Ok, so why isn’t Bohm’s theory taken seriously?

Interesting topic.
Maybe this will help: Heisenbergs uncertainty principle. One simply cannot measure, with accuracy, the position and velocity of a particle simultaneously. The more accurately you measure one- the more you simultaneously alter the other.

For example certain wavelengths of light are shown on the electron to measure its position but this then alters its velocity and the course it will take is random and unpredictable everytime. It pretty much put the nails in Laplace’s theory of hard scientific determinism.

http://en.wikipedia.org/wiki/Heisenberg_Uncertainty_Principle

As for Bohm, I’m not really familiar with him.

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Posted: 22 March 2007 02:49 PM   [ Ignore ]   [ # 4 ]
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Doug, do any of these books start out with the assumption that the reader knows nothing about Quantum Physic or do they all start out as though a person knows something about the subject?  I ask because I really and truly am ignorant about the subject.  I know what an atom and alike is, but if you start out too deep you go over my head very quickly.

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Posted: 22 March 2007 03:11 PM   [ Ignore ]   [ # 5 ]
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Posted: 22 March 2007 04:01 PM   [ Ignore ]   [ # 6 ]
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Thanks.  That was interesting.  I wouldn’t mind reading more stuff like this.

To be honest, about the only science I have a grasp on is Psychology because that the field I got my first degree in years ago. I can discuss neuro-psychology with the best of Psychiatrists even and I only have a bachelors degree in Psychology.  Now just because I can discuss it, doesn’t mean I’m not learning the process.  I have enough of the basics as a starting point to learn more about it all, get a grasp of what is being said, and that is about it, if that makes any sense.  Thus if your discussing humans and use Psychology in the process, a light bulb will go on in my head, in which I generate more questions using words that maybe over the general populations’ head based on what I know and understand.

It’s the same with anyone else who knows a certain amount of that field of science and I only wish I had that much understanding of other sciences, sometimes, because it can be very interesting.

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“Sometimes in order to see the light, you have to risk the dark.” ~ Iris Hineman (Lois Smith) The Minority Report

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Posted: 22 March 2007 04:22 PM   [ Ignore ]   [ # 7 ]
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[quote author=“Doug”] Subatomic particles show weird behaviors.
The famous “double slit experiment” should give you some idea of what is going on here experimentally.

Thanks for the replies Doug. 

Hey, I’d like to hear your comments on this youtube video which gives an intro to Quantum

(Mriana, this seems like a good place for us beginners to start)

  Quantum Physics on youtube.

The 2:45 minute mark in the video is where things become incoherent. 

And at the very end of the video, I’m pretty sure we see a clip from the movie what the bleep do we know.  :shock:

What bothers me about Quantum weirdness is how badly some people delight and celebrate its incoherence.  It’s almost like some people are glad that it doesnt make sense - and usually we see these types recklessly take what they want from quantum weirdness, using it to fuel their superstitious propensities.

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Posted: 22 March 2007 04:41 PM   [ Ignore ]   [ # 8 ]
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Posted: 22 March 2007 06:09 PM   [ Ignore ]   [ # 9 ]
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[quote author=“CoryDuchesne”][quote author=“Doug”] (Mriana, this seems like a good place for us beginners to start)

I do believe you are right.  :D

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Posted: 23 March 2007 01:17 AM   [ Ignore ]   [ # 10 ]
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[quote author=“Mriana”]Doug, do any of these books start out with the assumption that the reader knows nothing about Quantum Physic or do they all start out as though a person knows something about the subject?  I ask because I really and truly am ignorant about the subject.  I know what an atom and alike is, but if you start out too deep you go over my head very quickly.

IIRC, Feynman’s Six Easy Pieces are pretty elementary—not necessarily easy to understand, but very clear. QED is tougher. Once again, no equations or anything, but it’s a bit complex. OTOH nobody is better or more renowned than Feynman at explaining tough concepts in everyday language, without dummying anything down.

There may be some more elementary books on QM, but I don’t know for sure which. Maybe a dictionary entry would be a good place to start? I’d suggest going into a bookstore and flipping through a few books on the subject. If you aren’t sure, write the name down and we can take a look at it here and see if it’s likely to be good.

Although I haven’t seen the video clip, I’d agree with George about What the Bleep Do We Know ... it has a very poor reputation when it comes to science, so I’d be wary of taking any clip from that movie as necessarily accurate.

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Posted: 23 March 2007 02:04 AM   [ Ignore ]   [ # 11 ]
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[quote author=“dougsmith”]
Although I haven’t seen the video clip, I’d agree with George about What the Bleep Do We Know ... it has a very poor reputation when it comes to science, so I’d be wary of taking any clip from that movie as necessarily accurate.

Although what the bleep do we know is indeed horrible, I think that the clip on youtube gives a pretty good animation of the famous “double slit experiment”.

Doug, you should check out the clip.  I’d like to hear your, or any one elses critique of how the information was presented in the clip.

I thought it was fine. Well, up until the 2:45 mark at least.

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Posted: 23 March 2007 02:22 AM   [ Ignore ]   [ # 12 ]
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OK, I’ve seen it. With the caveat that I’m not a physicist nor play one on TV, this sounds to me an excellent quick overview of the situation. The issue about the “observer” is a complex one. Most physicists, like Feynman, basically throw up their hands and say they ‘make no hypotheses about the nature of the case’ like Newton said about gravity. That is, all they are doing is telling how the experiments came out. Using QM theory they can predict with an extremely high level of accuracy all these phenomena, and for a real experimental physicist, that’s enough.

Bohm’s theory (IIRC) is that particles are guided along so-called “pilot waves” that are associated with them. The interference patterns are in the pilot waves, and the particles follow those pilot waves along, in what I believe is a statistical sense. (The waves are waves of probability). The observer is in that case interacting with the pilot wave, hence skewing the resulting pattern of electrons.

There is also a “ many worlds ” interpretation of QM, in which the wave function isn’t said to “collapse” into one of a large variety of possible values, but rather that the particle actually exists in all its values, each one in a different branching universe.

... I told you QM was weird!

:wink:

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Posted: 23 March 2007 05:10 AM   [ Ignore ]   [ # 13 ]
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[quote author=“dougsmith”][quote author=“Mriana”]Doug, do any of these books start out with the assumption that the reader knows nothing about Quantum Physic or do they all start out as though a person knows something about the subject?  I ask because I really and truly am ignorant about the subject.  I know what an atom and alike is, but if you start out too deep you go over my head very quickly.

IIRC, Feynman’s Six Easy Pieces are pretty elementary—not necessarily easy to understand, but very clear. QED is tougher. Once again, no equations or anything, but it’s a bit complex. OTOH nobody is better or more renowned than Feynman at explaining tough concepts in everyday language, without dummying anything down.

Thanks Doug.  smile  I’ll be sure to read at least one of them when I can.

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Posted: 23 March 2007 05:58 AM   [ Ignore ]   [ # 14 ]
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[quote author=“Mriana”]Doug, do any of these books start out with the assumption that the reader knows nothing about Quantum Physic or do they all start out as though a person knows something about the subject?  I ask because I really and truly am ignorant about the subject.  I know what an atom and alike is, but if you start out too deep you go over my head very quickly.

Brian Greene has two books out explaining QM and String theory.  The first one is the Elegant Universe and the second one is The Fabric of the Cosmos: Space Time and the Texture of Reality.  Both are non-mathamatical mainly written for the lay person.

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Posted: 23 March 2007 06:06 AM   [ Ignore ]   [ # 15 ]
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Thanks for that, I haven’t read Greene’s books, but he does have a reputation as a good popularizer.

Do recall that string theory is a whole different kettle of fish from QM ... QM has an enormous amount of empirical evidence in its favor; string theory, as of yet, has none.

So when you’re reading about string theory, you may be reading about the way the world really is put together on an extremely small scale, or you may simply be reading about a very ingenious mathematical fiction.

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