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Thread: Uncertainty

  1. #1 Uncertainty 
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    Dear fellow physics lovers,

    There is something I'm curious about. This statement of Heisenbergs Uncertainty Principle is found in Modern Quantum Mechanics - Revised Edition by J.J. Sakuri, Addison-Wesley. See page 103
    ...a simultaneous precision measurement of position and velocity would necessarily violate the uncertainty principle.
    Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in this text. Not according to some other phrasing of the uncertainty principle.

    I see people tak about the uncertainty principle a lot in forums. What I'm curious about is what you think uncertainty is. Do you know how its defined?

    I ask so that I'm better prepared to help in the future. Thanks,


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    I'm not sure what you're question is. The Heisenberg Uncertainty Principle states that is impossible to know the velocity and position simaltenously of a particle, only one or the either. For example, electrons: you can know how fast one is moving around the nucleus but you don't know where it is. Conversely, you can know where the electron is but you no longer know how fast it is moving because this is a snapshot in time.


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    Quote Originally Posted by pmb View Post
    Dear fellow physics lovers,

    There is something I'm curious about. This statement of Heisenbergs Uncertainty Principle is found in Modern Quantum Mechanics - Revised Edition by J.J. Sakuri, Addison-Wesley. See page 103
    ...a simultaneous precision measurement of position and velocity would necessarily violate the uncertainty principle.
    Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in this text. Not according to some other phrasing of the uncertainty principle.

    I see people tak about the uncertainty principle a lot in forums. What I'm curious about is what you think uncertainty is. Do you know how its defined?

    I ask so that I'm better prepared to help in the future. Thanks,
    The quote is phrased as a statement of fact, not a definition.
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    Quote Originally Posted by mathman View Post
    The quote is phrased as a statement of fact, not a definition.
    Exactly. It is one of the consequences of the HUP. (Another important one is electrons not falling into the nucleus.)
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    Quote Originally Posted by mathman View Post
    The quote is phrased as a statement of fact, not a definition.
    That's true. But that's not whatI asked either.

    So far I've seen two responses, niether of which completely answered the questions I posed. I'll rephrase

    Question #1) Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in that text?

    Question #2) What do you think the definition of uncertainty is, i.e. do you know how its defined?
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    Quote Originally Posted by pmb View Post
    That's true. But that's not whatI asked either.
    You asked "Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in this text".

    Are we only allowed yes or no answers?
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    Quote Originally Posted by sumwun18 View Post
    I'm not sure what you're question is. The Heisenberg Uncertainty Principle states ....
    I wasn't asking for the definition. I was asking who believes that the statement given in that text is correct.


    Quote Originally Posted by sumwun18 View Post
    ...that is impossible to know the velocity and position simaltenously of a particle, only one or the either. For example, electrons: you can know how fast one is moving around the nucleus but you don't know where it is. Conversely, you can know where the electron is but you no longer know how fast it is moving because this is a snapshot in time.
    You haven't stated the definition if uncertainty yet. Were you aware of that? Uncertainty has a very precise mathematical definition.

    Here's another question; What determines the value of the uncertainty in an observable? How can it be changed?
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by pmb View Post
    That's true. But that's not whatI asked either.
    You asked "Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in this text".

    Are we only allowed yes or no answers?
    There are only two answers, yes and no. Either the phrasing is correct or it isn't. Which do you believe is true. You can always add there reason for your answer or why you don't wish to respond.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by mathman View Post
    The quote is phrased as a statement of fact, not a definition.
    That's true. But that's not whatI asked either.

    So far I've seen two responses, niether of which completely answered the questions I posed. I'll rephrase

    Question #1) Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in that text?

    Question #2) What do you think the definition of uncertainty is, i.e. do you know how its defined?
    Yes you did ask for the definition. and Yes this is the correct definition. And No I do not believe that a simultaneous measurement of velocity and position would negate Heisenberg.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by mathman View Post
    The quote is phrased as a statement of fact, not a definition.
    Exactly. It is one of the consequences of the HUP. (Another important one is electrons not falling into the nucleus.)
    I don't know where you got that idea from. Do you remember? Electrons don't fall ino the nucleus because the electrons exist in quantized states. According to QM there are only an infinite number of those states but none of them are the electrons being dashed into the nucleus. I don't see where uncertainty comes into it.
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    Quote Originally Posted by pmb View Post
    Either the phrasing is correct or it isn't. Which do you believe is true. You can always add there reason for your answer or why you don't wish to respond.
    which is exactly what two of us did. To which you responded that it wasn't what you asked for so I am confused ...
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    Quote Originally Posted by dmwyant View Post
    Yes you did ask for the definition. and Yes this is the correct definition.
    I didn't see the definition stated here.

    Maybe I should be more specific. What is the mathematical definition of uncertainty. How would you calculate it?
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    Quote Originally Posted by Strange View Post
    which is exactly what two of us did. To which you responded that it wasn't what you asked for so I am confused ...
    I asked trwo questions. You only answered the first one. I'm waiting for the second answer to be answered. I'll wait another two hours to see if someone answers it.
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    Quote Originally Posted by pmb View Post
    Maybe I should be more specific. What is the mathematical definition of uncertainty. How would you calculate it?
    (for position and momentum)

    Quote Originally Posted by pmb View Post
    I don't know where you got that idea from. Do you remember? Electrons don't fall ino the nucleus because the electrons exist in quantized states. According to QM there are only an infinite number of those states but none of them are the electrons being dashed into the nucleus. I don't see where uncertainty comes into it.
    Well, the informal description I had in mind was that if the electron was confined to the nucleus, the uncertainty in its momentum (and therefore kinetic energy) would be huge. My (limited) understanding (1) was that the quantized states are a result of the HUP (the "particle in a box" model).

    (1) It is many decades since I studied any of this and that was as in chemistry/electronics rather than physics classes so I wouldn't be surprised if I have been wrong all this time.
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    Quote Originally Posted by Strange View Post
    (for position and momentum)
    All that is is a relationshipp between two uncertainties. It is not the definition of uncertainty and it mot certainly can't tell you how to define it. At best you can make a statement about the relavent magnitude between two undertainties.

    I'll give you a hint: A synonym for uncertainty is standard deviation.
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    Quote Originally Posted by Strange View Post
    Well, the informal description I had in mind was that if the electron was confined to the nucleus, the uncertainty in its momentum (and therefore kinetic energy) would be huge. My (limited) understanding (1) was that the quantized states are a result of the HUP (the "particle in a box" model).
    No. You can calculate the energies and states of the hydrogen atom withouyt any knowledge of the uncertainty.
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    I should say this: When I studied QM as an undergrad my professor asked me what uncertainty was. I had no idea what he was talking about, even after studying it in my PHY-III course or in my chemistry courses. I was quite suprised at the answer.
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    Quote Originally Posted by pmb View Post
    All that is is a relationshipp between two uncertainties. It is not the definition of uncertainty and it mot certainly can't tell you how to define it. At best you can make a statement about the relavent magnitude between two undertainties.

    I'll give you a hint: A synonym for uncertainty is standard deviation.
    ooh! ooh! Sir! Sir! Me sir!


    But any more than that is totally over my head!

    So maybe you need to explain how ] and are each defined ...
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by pmb View Post
    All that is is a relationshipp between two uncertainties. It is not the definition of uncertainty and it mot certainly can't tell you how to define it. At best you can make a statement about the relavent magnitude between two undertainties.

    I'll give you a hint: A synonym for uncertainty is standard deviation.
    ooh! ooh! Sir! Sir! Me sir!


    But any more than that is totally over my head!

    So maybe you need to explain how ] and are each defined ...
    Sorry. I didn't read this closely enough. I answered it below.
    Last edited by pmb; June 11th, 2012 at 06:20 PM.
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    Here's the definition of an observable:

    Average of A:

    Uncertainty of A:

    There are two important things to note here.

    1) The uncertainty of an observable is dependant on the state the system is in.

    2) The uncertainty of a single measurement has no meaning.

    3) The uncertainty is independant of of the precision of individual measurements.

    Answer to questio#3. No. That is an invalid statement of the Heisenberg Uncertainty Princple. That statement in that text is about single measurements. The HUP is about the a standard deviation of observed measurements on systems which are identically prepared.

    Later on I'll try to figure out how to explain that notation so that those not familiar with Dirac notation can understand it.
    Last edited by pmb; June 11th, 2012 at 06:30 PM.
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    Quote Originally Posted by pmb View Post
    Here's the definition of an observable:

    Average of A:

    Uncertainty of A:

    There are two important things to note here.

    1) The uncertainty of an observable is dependant on the state the system is in.

    2) The uncertainty of a single measurement has no meaning.

    3) The uncertainty is independant of of the precision of individual measurements.

    Answer to questio#3. No. That is an invalid statement of the Heisenberg Uncertainty Princple. That statement in that text is about single measurements. The HUP is about the a standard deviation of observed measurements on systems which are identically prepared.

    Later on I'll try to figure out how to explain that notation so that those not familiar with Dirac notation can understand it.
    You're raising the same objection that Popper raised many decades ago (1930s?). Just as you have, he argued that standard deviations have no meaning in single-point experiments and thus the HUP must properly be regarded as a statement concerning ensembles of measurements.

    He wasn't able to come up with a valid falsifiable way to distinguish his interpretation from the Copenhagen interpretation, however, so the Popper interpretation of the HUP ironically remains unfalsifiable in the Popperian sense.
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    Quote Originally Posted by tk421 View Post
    You're raising...
    Woa!! I'm raising? Me???? Moi??? Why in the world would you ever claim that it's I who's saying this? I'm stating how the term is actually defined in the field of Quantum Mechanics universally. And I have no intention of discussing anything other than standard physics in this or any other discussion forum

    Pick up any decent QM text such as Liboff', and you can confirm this. Without this definition of the uncertainty principle the HUP wouldn't exist in its modern form. If you changed the meaning of uncertainty to mean something else then that too would be "unfalsifiable" as you say.

    The HUP does not pertain to single experiements. If you were to consider a system of the spin state of an electron and then calculate its uncertainty then you'd get a better idea of what I'm saying.

    And what you spoke of is not a theorem, it's an intepretation. Popper is the scientiic philosopher who came up with the idea of falsification. That only applies to physical hypotheses, not interpretations. The Copenhagen Interpretation isn't falsifiable either. But since its not a theorem it makes no difference.

    Now consider a system in which the z-component of an electrons spin is measured. There are only two measurements which can result from such a measurement. One is "up" and the other "down" (actually the eigenvalues are angularmomenta of which there are only two possible results. If the inittial state of the system is "up" then the uncertainty of s_z is zero. If the initial state is

    |psi> = (1/sqrt(2)){|+> + |->}

    then the uncertainty is non-zero and has a value that depends only on the initial state. It's fixed and can be measured. Uncertainty for such a system can have no other meaning.

    Now you can choose to define it otherwise and I'll be glad to listen. But you'll hae to be precise in your definition. I'll consider it if comes from a QM text or if you an justify your new definition.

    One last thing: I don't buy this falsification bit. Uncertainties can be measured and compared with observation and those observations are in accordance with the HUP.
    Last edited by pmb; June 13th, 2012 at 03:45 PM. Reason: Rquired clarification
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    Doesn't the uncertainty principle come right from fourier analysis?
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    Quote Originally Posted by TheObserver View Post
    Doesn't the uncertainty principle come right from fourier analysis?
    If I recall correctly, that's true when the number of eigenvalues is infiite and continuous set of values, not in finite discrete cases like the spin system I mentioned above where there is a finite number of eigenvalues/states.

    That's the beautiful thing about Dirac notation. It can be used to represent any kind of state.
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    I have to admit that I'm a little bumbed out. I was hoping to learn what people believe that "uncertainty" is in QM. I used to get the impression that people thought it meant the same thing as the error in precision of an instrument and that to decrease the amount of uncertainty that one got/used more precise instruments.

    Can you give me an idea of what you always believed uncertainty was? E.g. in your mind do you believe that the uncertainty of an observable was something that you could change? If so then how would you decease the uncertainty in an observable?

    Here's a question whose answer might help me: Consider rolling a single die and recording the number on the face of the die. Question: What do you believe the uncertainty is of the number N that appears on the die face.
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    Quote Originally Posted by pmb View Post
    I have to admit that I'm a little bumbed out. I was hoping to learn what people believe that "uncertainty" is in QM.
    I think part of the problem might be that you started off with a simple "plain English" description. When people responded to that, you immediately insisted that answers be given in a high level of mathematical detail and formalism that is probably over the heads of many readers who might have wanted to respond to the initial questions.

    It is beyond me and I am no longer very interested in where it is going...
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    I sort of echo Strange's view, but would expand on it. The way you asked the follow up questions strongly suggested an agenda. Frankly you sounded very much like a creationist trying to set up a trap for an evolutionist. I'm sure this wasn't your intention, but that was certainly how you came across to me. I chose not to answer because of that. Had I answered I would have said, in plain English, uncertainty relates to an absolute property of the universe that there is a limit, unrelated to practicalities of measurement, as to how precisely we may know the position and velocity of a particle.
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    Quote Originally Posted by Strange View Post
    I think part of the problem might be that you started off with a simple "plain English" description. When people responded to that, you immediately insisted ...
    You got the wrong impression. You seem to be irritated with me and all I was doing was asking what people think uncertainty is. And I didn't start off with a description. Those who responded , responded with a description. Regarding what you thought I "insisted" can only have come from this statement
    Quote Originally Posted by pmb
    You haven't stated the definition if uncertainty yet. Were you aware of that? Uncertainty has a very precise mathematical definition.
    That's far from insisting. It was more of a hint than anything else. So there was never me doing any "insisting" here.

    Quote Originally Posted by pmb
    that answers be given in a high level of mathematical detail and formalism that is probably over the heads of many readers who might have wanted to respond to the initial questions.
    And if that's the case then that's what I wanted to know. If the people here who don't know math don't know how its defined then it could go a long way to explain what it is to them. I guess I should have know that people won't openly admit that they don't know something.

    Quote Originally Posted by John Galt View Post
    I sort of echo Strange's view, but would expand on it. The way you asked the follow up questions strongly suggested an agenda.
    Yes. I know. I realized that might be the case when I was writing my responses. It was very difficult posing the questions without giving that impression. Then again people are always free to ask me what my intent was. I'm new here so people can't read me yet but that will come with time.

    Quote Originally Posted by John Galt View Post
    Frankly you sounded very much like a creationist trying to set up a trap for an evolutionist. I'm sure this wasn't your intention, but that was certainly how you came across to me. I chose not to answer because of that. Had I answered I would have said, in plain English, uncertainty relates to an absolute property of the universe that there is a limit, unrelated to practicalities of measurement, as to how precisely we may know the position and velocity of a particle.
    Thanks. That was very helpful .Creationsist? Yipes!!!

    My intention? The topic of uncertainty quite often arises in QM forums. Since I plan n working on my web page I was trying to find out exactly what people are assuming about it when they talk about uncertainty. That's a very very difficult task and I'm bound to say something people will misunderstand. For example: After I asked what they thought it was I responded with this comment
    Quote Originally Posted by pmb
    Uncertainty has a very precise mathematical definition.
    At least one person assumed from that that I was insisting on a mathematical definition.

    What you said goes back to the quote I posted. Given what you said here there is no way that I'm aware of to determine the correctness of the statement of the HUP that I quoted. I will say that I'm quite confused aboiut this anger I got. I was just trying to learn what people think uncertainty means. It's not like I wanted to judge people. I hate when people do that. But I guess I have my answer. Thanks.
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    I see uncertainty as a basic property of the universe that physically translates to some properties not being determined until measured directly, like the electron clouds around a nucleus or pair-produced particles on the border of event horizons existing both inside and outside the event horizons until "measured". Also that this "measurement" has nothing to do with a consciousness doing the measuring, but only that another particle interacts with it, collapsing the wave function. Particles are "fuzzy". I have no understanding of the math either. I have never liked the parallel universe interpretation, but only for aesthetic reasons. As my signature suggests though, I am wholly open to correction.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by KALSTER View Post
    I see uncertainty as a basic property of the universe that physically translates to some properties not being determined until measured directly, like the electron clouds around a nucleus or pair-produced particles on the border of event horizons existing both inside and outside the event horizons until "measured". Also that this "measurement" has nothing to do with a consciousness doing the measuring, but only that another particle interacts with it, collapsing the wave function. Particles are "fuzzy". I have no understanding of the math either. I have never liked the parallel universe interpretation, but only for aesthetic reasons. As my signature suggests though, I am wholly open to correction.
    Here is one way to picture it: Consider the following setup. We have a device which produces a beam of electrons, all of which are in the spin-up state. Everytime an electron is emitted from the box we have an electron in the exact same state as the previous one. We now measure the x-component of spin and will get either or . From now on we'll call these spin-up and spin-down, respectively. We repeat this over and over and over again, for perhaps 10,000 times. Then we take the data we collected i.e. the number of up vs down results and then determine what is called the standard deviation of the data. That's what uncertainy is. Notice that each measurement yielded an exact result. Icould have measured the y-component of spin just after the first measurement and gotten exact values there too. But the uncertainty of each component is still non-zero.

    I worked this out for a classical die. You can think of it as each die through being the numerical result of a quantum measurement where each of the seven values are as likely to come up as any other value. The entire scenario is worked out here
    Probability

    Notice that each measurement yields an exact result yet the uncertainty is non-zero.
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    Quote Originally Posted by John Galt View Post
    I sort of echo Strange's view, but would expand on it. ....
    I've decided not to ask a question of this nature again, or at least get some advice from you as how I might find the information I'm seeking.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by Strange View Post
    I think part of the problem might be that you started off with a simple "plain English" description. When people responded to that, you immediately insisted ...
    You got the wrong impression. You seem to be irritated with me and all I was doing was asking what people think uncertainty is.
    Not really (baffled/bemused, maybe). On the other hand, you sounded irritated with the initial responses you got (a bit schoolteacher-ish). But I am sure that this is just a shortcoming of the medium.

    I appreciate your goals of trying to get a better understanding of your audience. That is very important.
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    Couldn't you have said this.

    I'm looking to put together a blog discussing Heisenberg's Uncertainty Principle and the some of its ramifications. The intent of the piece will be to educate people who have an interest in it, but limited knowledge of it. So that I can pitch what I say at the right level I need a better handle on what people think uncertainty is. I'd appreciate it if you would take a couple of minutes to write what uncertainty means to you in the context of physics.

    I can see no obvious downside to this approach and it is the one most likely to generate the response you were looking for. What would have been wrong with that? From where I'm sitting you made the whole thing unecessarily complex and secretive.
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    Quote Originally Posted by Strange View Post
    On the other hand, you sounded irritated with the initial responses you got (a bit schoolteacher-ish). But I am sure that this is just a shortcoming of the medium.
    I try never to come off that way. I fo try to be as logical as I can and I'm sure that comes of stuffy sometimes. It's a learning process,
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    Quote Originally Posted by John Galt View Post
    Couldn't you have said this.

    I'm looking to put together a blog discussing Heisenberg's Uncertainty Principle and the some of its ramifications. The intent of the piece will be to educate people who have an interest in it, but limited knowledge of it. So that I can pitch what I say at the right level I need a better handle on what people think uncertainty is. I'd appreciate it if you would take a couple of minutes to write what uncertainty means to you in the context of physics.
    I like it!

    I hope you don't mind but I created a new thread in another forum I frequent. I think that using your phrasing and going to a place where the subject hasn't been discussed yet then I just might get what I'm looking for.

    Looking back to my own education I believe that I thought uncertainty was somehow related to experimental error. Does that sound familiar to any of you?
    Last edited by pmb; June 13th, 2012 at 12:34 PM.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by tk421 View Post
    You're raising...
    Woa!! I'm raising? Me???? Moi??? Why in the world would you ever claim that it's I who's saying this? I'm stating how the term is actually defined in the field of Quantum Mechanics universally. And I have no intention of discussing anything other than standard physics in this or any other discussion forum
    You need to calm down. You asked a question (or so it seemed), and I endeavored to answer. In response, you seem to be pushing a particular idea (without explicitly telling us what it is), and then displaying anger when you don't get the answers you wanted. I find that rude, at minimum.


    And what you spoke of is not a theorem, it's an intepretation.
    You seem to be putting words into my mouth, while ignoring the words that (virtually) actually did come out of my mouth. Look again at what I wrote. Nowhere did I write "theorem." I referred only to interpretations. So you seem to be violently agreeing with me, but yet not. I am puzzled.

    Popper is the scientiic philosopher who came up with the idea of falsification.
    I acknowledge this fact in my post above.

    That only applies to physical hypotheses, not interpretations. The Copenhagen Interpretation isn't falsifiable either. But since its not a theorem it makes no difference.
    I have actually read Popper. He makes no such limitation, as far as I am aware. He is talking not so much about theorems as about what constitutes a scientific theory. Interpretations reside in a somewhat gray area. As they are unfalsifiable, one may freely choose from among unfalsifiable interpretations that otherwise lead to identical experimental outcomes.


    One last thing: I don't buy this falsification bit. Uncertainties can be measured and compared with observation and those observations are in accordance with the HUP.
    I'm not asking you to "buy" anything. As in the difference between sample and population standard deviations, one can still properly speak of the uncertainty of a single measurement. One can calculate it. You seem to be objecting to some notion of "reality" that forbids one definition. I see no a priori reason to follow your prescription.

    Or am I missing your point?
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    Quote Originally Posted by tk421 View Post
    Or am I missing your point?
    Yes. You're being way too defensive for my taste. Sorry but I don't find it wise to discuss physics with someone who appears to be so defensive.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by tk421 View Post
    Or am I missing your point?
    Probably, but I stopped reading after you claimed that I needed to "calm down". That told me that you were reading things into my posts that aren't there. Frankly I found it rude.
    Then you suffer from a lack of self-awareness. Your post consists of boldface, multiple question marks, and argumentative combativeness. It is you, sir, who exhibit rudeness, especially to those attempting to do you the favor of answering your question.
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    I believe we all know now who finds who offensive. Can we get back to the (now clear) topic?

    Pmb, there have been a few instances now where you have found yourself at odds with respected members. I find it distracting. I think it has more to do with people needing to get used to your way of doing things than anything else though and not due to a serious error of conduct or intent on your part. I want to ask you to be as clear as you can from now on to help avoid having to go through a slugging match every time you post something. I believe your intentions are pure and I welcome your contributions.

    To the rest: let's give pmb the benefit of the doubt and let's try to stear around any perceived instances of rudeness. I don't think that is his intent. 'Cmon, let's make this work
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    Strong Moderator Suggestion: Lets get back to the subject and drop ALL the personal stuff.

    Edit: Sorry KALSTER, you slipped your post in ahead of me, but I shall leave my warning intact anyway
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    Quote Originally Posted by KALSTER View Post
    Pmb, there have been a few instances now where you have found yourself at odds with respected members. I find it distracting. I think it has more to do with people needing to get used to your way of doing things than anything else though ..
    Yes. That is precisely the case.
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    Quote Originally Posted by pmb View Post
    Looking back to my own education I believe that I thought uncertainty was somehow related to experimental error. Does that sound familiar to any of you?
    As far as I can recall the very strong point made at the oustset, when it was first introduced, that this was not related to experimental error or limitations of measuring devices. This was so thoroughly communicated that one could not think uncertainty without thinking inherent property of matter. I wonder if you are trying to solve a problem that existed more for yourself than for most others?
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    Quote Originally Posted by pmb View Post
    Looking back to my own education I believe that I thought uncertainty was somehow related to experimental error. Does that sound familiar to any of you?
    I guess that when I first came across the idea in popular science sources, it was presented in the "conceptual photon bouncing off an electron" model (as even Heisenberg first described it). I'm pretty sure that very soon after that I was made aware that it was a more fundamental description of the nature of these things.
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by John Galt View Post
    I wonder if you are trying to solve a problem that existed more for yourself than for most others?
    I understand all of that. I wasn't talking about uncertainty as I now know its meaning. I was talking about my understanding/ipression of uncertainty before I learned QM. To be precise, I learned its precise meaning in the graduate level QM, not in the undergraduate QM course I took.

    In that comment I was merely asking whether others had the same impression of what uncertainty is as I used to.
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    Quote Originally Posted by Strange View Post
    I guess that when I first came across the idea in popular science sources, it was presented in the "conceptual photon bouncing off an electron" model (as even Heisenberg first described it). I'm pretty sure that very soon after that I was made aware that it was a more fundamental description of the nature of these things.
    Now that you mention it I too recall that thought experiment. I believe that was the source of my impression.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by John Galt View Post
    I wonder if you are trying to solve a problem that existed more for yourself than for most others?
    I understand all of that. I wasn't talking about uncertainty as I now know its meaning. I was talking about my understanding/ipression of uncertainty before I learned QM. To be precise, I learned its precise meaning in the graduate level QM, not in the undergraduate QM course I took.

    In that comment I was merely asking whether others had the same impression of what uncertainty is as I used to.
    I understood that. That is why I used past tense - a problem that existed , rather than a problem that exists. I was suggesting that your experience may have been an uncommon one.
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    Quote Originally Posted by John Galt View Post
    I understood that. That is why I used past tense - a problem that existed , rather than a problem that exists. I was suggesting that your experience may have been an uncommon one.
    Ah! Okay. I have another thread going in another forum. I figured I'd start from scratch there, using what I learned here. We'll see what happens.

    Note: When I posted exactly what you wrote, some people thought that I was asking them to teach me what uncertainty is. I was afraid that was going to happen.
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    [QUOTE=pmb;330816]
    Quote Originally Posted by John Galt View Post
    Note: When I posted exactly what you wrote, some people thought that I was asking them to teach me what uncertainty is. I was afraid that was going to happen.
    If you had anticipated this you should have amended the post. The following addition might have worked.

    Please note that I have, through my university education a sound grasp of the subject. I need a sense of how it is perceived in general.
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    OK, I have been AWOL for a while.

    I confess I am totally mystified by this thread. Let me try to summarize - not chronologically but perhaps logically:

    1. The OPer claims to have a full and detailed understanding of the HUP. We have no reason to doubt this (but see point 6 below), but refuses to reveal this understanding

    2. The OPer invites us to give our own understanding of the HUP so that he can "help" others. Who among us asked for such help?

    3. Under prompting from John Galt, he admitted it was to improve his blog

    4. This seems to suggest that the "misconceptions" (in his view) of we mere mortals will be adressed in his blog, where he "reveals the truth"

    5. Nonetheless,the result of this canvassising of supposed misconceptions drew some very negative comments from the OPer. Moreover in this case this "canvassing" is not a miilion miles from "market research", which itself not a million miles from spamming

    6. The OPer claimed that the HUP has something to do with standard deviation. It has not - this is a result from large sample theory, whereas the HUP applies equally well to single measurements made on a single system acted on by a pair of non-commmuting operators (recall in QM a "measurement" is an eigenvalue of an operator on a complex Hilbert space of state vectors)
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    Quote Originally Posted by John Galt View Post
    If you had anticipated this you should have amended the post. The following addition might have worked.

    Please note that I have, through my university education a sound grasp of the subject. I need a sense of how it is perceived in general.
    I didn't know a better way to explain it. I thought that me saying that I had a sound grasp of it would make me come off as arrogant or stuffy. I don't want to have that kind of reputation. I want people to feel free and comfortable talking to me so I have to be veryu careful with stuff like that. Some people seem to hate it when you mention your education so I try to leave that out unless asked directly.

    It didn't matter that much since my understanding isn't the topic of conversation. I'll answer those who ask me directly (and politely). The purpose was to be able to understand what people have in mind when I'm discussing the HUP and uncertainty itself. I have this belief that people confuse error in measurement with uncertainty. I wanted to see if that impression was true. From the responses I believe that its close. Now I have a better understanding of how to interact with people who want a better idea of uncertainty and the HUP.

    It doesn't really matter anyway. I decided to leave that forum. I can't believe how hard it is finding a nice discussion forum where people aren't so judgemental. People are more concerned with peoples motives than what they're talking about. Sometimes that's justified. I had an experience where the persons motives were anything but acceptable. This forum seems to be good that way though. The moderators seem quite kind. This is my new home now.
    Last edited by pmb; June 14th, 2012 at 12:49 PM.
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    Quote Originally Posted by Guitarist View Post
    1. The OPer claims to have a full and detailed understanding of the HUP. We have no reason to doubt this (but see point 6 below), but refuses to reveal this understanding
    Since you choose to be harsh in your responses (your words) I choose not to respond to any of your posts after this one. If someone wants to know the answers to one of your questions then they can ask me. Besides, there are too many errors in your post and I don't have the strength to correct them all so I'll just correct the first one

    Fact - I never refused to reveal my understanding of uncertainty. Frankly, nobody asked me. And it would have been counter productive to the purpose of this thread to state the definition right off. People don't care what I think in this thread (I don't think). I care what people think.

    Contrary to your false claim that I refuse to reveal my understanding I direct all who really cares about this to reread post #20 where I gave the precise mathematical definition of the uncertainty in a variable. I also gave a worked out example in a link I provided in post #30 which also has the definition in it. The link is to -- Probability

    Now I'll provide a quote from a graduate level QM text on the definition of uncertainty. In fact I'm going to quote the text that UMass Lowell uses in their graduate QM course, i.e. Principles of Quantum Mechanics - Second Edition, R. Shankar, page 128
    The Uncertainty

    In any situatin descibed probalistically, another useful quantity to specify besides the mean is the standard deviation. which measures the average fluctation around the mean. It is defined as



    and often called the root-mean-squared deviation. In quantum mechanics, it is reffered to as the uncertainty in . ...
    I could quote any QM text one wishes but they're all identical. At least I can see the error in your belief on what means.

    I recommend that you pick up a better QM text, say one that is used in a graduate level course. You can learn all about uncertaintly and how to derive the HUP using that definition.
    Last edited by pmb; July 14th, 2012 at 06:28 PM.
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    pmb, just FYI, you do come across as extremely arrogant, which is one reason people react so badly to your posts.

    {Back to lurk mode}
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    Quote Originally Posted by MeteorWayne View Post
    pmb, just FYI,...
    Please follow your own advice and leave the personal stuff out of the thread. It's unwelcome. If you have something personal to tell me then that's what PM is for. And even then I'm not intersted in the false impressions you folks seem insisted on making.

    I'm really getting tired of people confusing confidence with arrogance. I'm really not interested. If I need to leave to avoid these personal attacks (and yes. I consider it an ad hominem attack) the let me know.
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    What I'm curious about is what you think uncertainty is. Do you know how its defined?
    Let's say you have a rope tie to a tree and an excited 12 year old jiggling the free end of the rope up and down at a regular interval. We have a series of waveforms on the rope.


    He keeps jiggling the rope and refuses to stop. I can look at the rope at determine the frequency of the jiggling with ease, but what is the position of the waveform? I can't really tell exactly.

    I tell the 12 year old to stop and just send one wave down the rope. Now I can tell its exact position. But what is its frequency? I can't really tell again because I only have one.

    This at least was the explanation given to me by a grad student. I need to delve deeper.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by MeteorWayne View Post
    pmb, just FYI,...
    Please follow your own advice and leave the personal stuff out of the thread. It's unwelcome. If you have something personal to tell me then that's what PM is for. And even then I'm not intersted in the false impressions you folks seem insisted on making.

    I'm really getting tired of people confusing confidence with arrogance. I'm really not interested. If I need to leave to avoid these personal attacks (and yes. I consider it an ad hominem attack) the let me know.
    I was just trying to help you understand why you get the reactions you do. I won't waste my time anymore.
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    Won't dare to argue too much with you guys' advanced knowledge of the topics. But as someone with only undergrad physics courses, I thought the uncertainty principle as as guitarist and tk421 wrote -applying to a single measurement of a single system and not in a statistical sense ; also that is a feature of reality and nature. Basically the more narrowly you know the ranges of its' position then the greater the possible ranges of its' momentum and vice versa.
    Last edited by ballyhoo; June 15th, 2012 at 07:29 AM.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by MeteorWayne View Post
    pmb, just FYI,...
    Please follow your own advice and leave the personal stuff out of the thread. It's unwelcome. If you have something personal to tell me then that's what PM is for. And even then I'm not intersted in the false impressions you folks seem insisted on making.

    I'm really getting tired of people confusing confidence with arrogance. I'm really not interested. If I need to leave to avoid these personal attacks (and yes. I consider it an ad hominem attack) the let me know.
    pmb, this ends here.
    1. Your initial posts and the motivation for them were obscure. You agreed this was true.
    2. Several members, myself included, 'bit our tongues' to remain positive and helpful.
    3. You raised, or certainly expanded, the issue of how your posts are perceived.
    4. Several members tried to give you insight into how you came across.
    5. Now you are complaining about those attempts to help.

    Moderator Mode: I have a suggestion, but you might be better to think of it as a demand. Stop being so frigging sensitive, get off your high horse and just be a useful, contributing member of the forum. You are very welcome here, but your petty, self-centred attitude is not.
    Last edited by John Galt; June 15th, 2012 at 09:19 AM. Reason: corrected 'of' to 'off'.
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    Quote Originally Posted by pmb View Post
    I recommend that you pick up a better QM text
    Righto, will do. I must say I was labouring under the (obviously mistaken) impression that P.A.M Dirac was reasonably reliable. Clearly I am wrong
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    Quote Originally Posted by MeteorWayne View Post
    I was just trying to help you understand why you get the reactions you do. I won't waste my time anymore.
    Thanks. I'm just trying to let you know that nobody needs to be told something more than once. Someone already told me their impressions. E.g. I see people here trying to tell me what my emotional state is and its waaaay out of whack.I and oly I can tel what my emotional state is. People can easily misread things such as puncition and bolding/italics etc. What I meant as being surprise and a bit irritation at a presumption was misinterpreted as me flying off the hande. See? People first make and assumption and then act on it, never ever asking me what the tuth is, i.e. what I meant at certain usage of punctuation.

    I keep telling people that their pereceptions are wrong but they refuse to consider it. There's nothing I can do about that. It's just the way I write.

    Let's drop the personal stuff from herein, shall we? Thanks!
    Last edited by pmb; June 17th, 2012 at 10:41 AM.
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    Quote Originally Posted by Guitarist View Post
    Quote Originally Posted by pmb View Post
    I recommend that you pick up a better QM text
    Righto, will do. I must say I was labouring under the (obviously mistaken) impression that P.A.M Dirac was reasonably reliable. Clearly I am wrong
    Nobody is perfect. However I'd find it hard to believe that Dirac was wrong on this point. Can you relay to me what he ays on this or do I need to hit the library and see for myself?
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    Im in better spirits today so I'll respond to Guitarists inquiry

    Quote Originally Posted by Guitarist View Post
    1. The OPer claims to have a full and detailed understanding of the HUP. We have no reason to doubt this (but see point 6 below), but refuses to reveal this understanding
    Nobody asked me but I think you simply missed it as I mentioned in my first response to you.

    Quote Originally Posted by Guitarist View Post
    2. The OPer invites us to give our own understanding of the HUP so that he can "help" others. Who among us asked for such help?
    What I posted in the opening thread is

    Quote Originally Posted by pmb
    I ask so that I'm better prepared to help in the future.
    You assumed that someone needed to ask for help before I needed to prepare for it. That’s not the way I work. I try to get a good grip on misconceptions in particular areas of physics so that I can (1) help when people asks about it in the future, either in this forum or on the other forums I frequent (2) prepare for a possible FAQ for my website that I might create in the future and (3) prepare for a teaching job I might take at our local highschool.

    The current question is only a very small portion of what I hope to learn. Mostly I’m curious about what people in general think it is so that I can be prepared to respond readily. E.g. posting all the math is difficult for me and in situations where theres a lot of math I create a web page on my web site and then resuse it as time goes on and the subject arises again.

    Quote Originally Posted by Guitarist View Post
    3. Under prompting from John Galt, he admitted it was to improve his blog
    Actually I don’t even know what a blog is. And it was John who phrased it that way, not I. And for the most part he’s wrong. It’S not my sole purpose, only a possible use of the knowledge I gain. From the way you say it you seem to think that it’s a bad thing. I don’t know why that’d be considered a bad thing.


    Quote Originally Posted by Guitarist View Post
    4. This seems to suggest that the "misconceptions" (in his view) of we mere mortals will be adressed in his blog, where he "reveals the truth"
    A mnisconception. What “seems” to you to suggest is wrong.

    Quote Originally Posted by Guitarist View Post
    6. The OPer claimed that the HUP has something to do with standard deviation.
    Already addressed in a previous post.
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    Quote Originally Posted by pmb View Post
    Dear fellow physics lovers,

    There is something I'm curious about. This statement of Heisenbergs Uncertainty Principle is found in Modern Quantum Mechanics - Revised Edition by J.J. Sakuri, Addison-Wesley. See page 103
    ...a simultaneous precision measurement of position and velocity would necessarily violate the uncertainty principle.
    Do you believe that this version of the Heisenbergs Uncertainty Principle is true as it is written in this text. Not according to some other phrasing of the uncertainty principle.

    I see people tak about the uncertainty principle a lot in forums. What I'm curious about is what you think uncertainty is. Do you know how its defined?

    I ask so that I'm better prepared to help in the future. Thanks,
    I understud that the uncertainty principle is because to detect the position and velocity of a particle, it is stated that impacts on it something (photon, wave, ...) that when the impact on the particle, you can measure only one of two things, but varies the other.

    If we were able to detect a particle without impact anything on it that did change one of two things (position or velocity), then the principle would have no validity?
    "If everyone is thinking alike, then somebody isn't thinking". George S. Patton
    "Science does not know its debt to imagination". Ralph Waldo Emerson

    "Why settle with the known models and patterns (but not underlying laws) of Our Universe , if we might understand them better if we could puzzle out them from outside its limits?"
    (The common sense)
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    Quote Originally Posted by dapifo View Post
    I understud that the uncertainty principle is because to detect the position and velocity of a particle, it is stated that impacts on it something (photon, wave, ...) that when the impact on the particle, you can measure only one of two things, but varies the other.

    If we were able to detect a particle without impact anything on it that did change one of two things (position or velocity), then the principle would have no validity?
    Absolutely not. It is about the fundamental nature of reality (or at least what is possible to observe of reality). It is not about errors introduced by the mechanism of measurement. If you could make measurements with out disturbing the particle you would still find that the more accurately you measured one, the greater the uncertainty in the other.
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    Quote Originally Posted by Strange View Post
    Absolutely not. It is about the fundamental nature of reality (or at least what is possible to observe of reality). It is not about errors introduced by the mechanism of measurement. If you could make measurements with out disturbing the particle you would still find that the more accurately you measured one, the greater the uncertainty in the other.
    Here is how it works. Suppose the system is in a particular state , a state which we can consistently reproduce. We measure say, the momentum and record its value. When we measure its value there is no limit to the accuracy we can achieve in its measurement. We then prepare the system to its original state and repeat the experiment. We measure the momentum again and record its value. We keep doing this over and over and over again. Afer several tens ouf thousands of measurements we take the momentum measurements and calculate the standard deviation. That value is given the symbol . Now we do the exact same thing except this time, instead of measuring momentm we measure the position of the particle. We measure the position (with arbitrarilysmall precision), record it, prepare the system in the exact same state and repeat. We do this a few tens of thousands too. We then calculate the standard deviation of the position measurements and give it the symbol . If we do this with any state of the system what we find is the following



    Notice that even though we measured the position and momentum of the particle with arbitrarily small precision we still got finite uncertainties in the momentum and position. The standard deviation in p is also called the uncertainty in p etc.
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