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Measurement disturbs the system.

  1. Oct 23, 2011 #1
    "Measurement disturbs the system."

    Okay so I explained the uncertainty principle to my non-scientist friend, and he came up with a basic thought experiment.

    I know there is something wrong with it, so I read paragraph 8 from here: http://www.oberlin.edu/physics/dstyer/TeachQM/misconnzz.pdf

    However, can someone clarify what the underlying idea of the uncertainty principle, if it is not the fact that "measurement disturbs the system".

    "In more detail, this misconception holds that each particle really does have denite values for both position and momentum, but these denite values cannot be determined because measurement of, say, a particle's position alters the value of its momentum."

    Is it true that a particle in QM has definite position and/or momentum at a particular time T?
    If so, what IS wrong with the above quotation?

    I might post the thought experiment if I don't fully understand replies and still need help understanding why the thought experiment doesn't work IRL.

  2. jcsd
  3. Oct 23, 2011 #2


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    Re: "Measurement disturbs the system."

    No, that is not true (thus the quotation is perfectly ok). It makes no sense to speak about particle position and momentum at the same time (with respect to precision - of course it makes sense to say that your car stays still while parked on your driveway). Assumtion that the particle has both those values defined (not known, just defined) leads to paradoxes and conclusions contradicting the observations.

    Do it!
  4. Oct 23, 2011 #3
    Re: "Measurement disturbs the system."


    Can you go into more detail?
  5. Oct 23, 2011 #4
    Re: "Measurement disturbs the system."

    Wait, I thought electrons (for example) DO have a definite position and momentum at a particular time T.

    Let's concentrate on just one of these for the moment. I thought that position simply cannot be determined. So for example you have your cloud of probability for the position of a particle because you are forbidden to determine precisely where it is without measuring it, but at each moment it DOES have a position, it isn't just a cloud of probability. Is this still wrong?? Now I think about it this doesn't make much sense, but I am confused / unsure about things.

    Now I don't know how electrons and photons interact, but let's say they "stick" to electrons and when you look at electrons, the photons are "released" from the electrons. I don't know how else they detect electrons...

    [PLAIN]http://img685.imageshack.us/img685/5434/diagram1k.jpg [Broken]

    In the diagram above, we pretend we are in a dark room and the only thing is an electron and two photons and no external interference. The electron starts with a definite momentum and velocity (it comes from the bottom left and moves up and right). There are two photons "sticking" to the electron. Now our "eye" - which is not really our eye, but represents our measuring devices - can measure the mass-energy of the incident photon, as well as it's momentum and direction.

    Now the "electron and remaining photon" is a free particle not under the influence of gravity (assumed), and so travels in a straight line at a constant speed (Newton's 1st Law).

    Soon after, we look at the electron again and the second photon falls into our eye. We take the same measurements as before and we then can determine precisely where the electron was when we first measured it, and precisely where it is now.

    Edit: I just realized we cannot measure the time taken for the first photon to reach our eye because that would mean we would know the position of the electron to begin with. So maybe we need to do this with 3 photons for it to work.
    Last edited by a moderator: May 5, 2017
  6. Oct 23, 2011 #5


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    Re: "Measurement disturbs the system."

    Sure. The most common explanation is double slit experiment, which cannot be simply explained under assumption that paths of particles have any meaning.
    Of course, you may go into weirdness of Many Words or Bohmian pilot waves, but it is up to your methaphysical taste if those constructs are more appealing than resignation from definite position when it is not measured.
  7. Oct 23, 2011 #6


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    Re: "Measurement disturbs the system."

    @Jewbinson - you may measure positions of your electron multiple times, every time getting some value (within some assumed accuracy). But those points won't form a straight line. More precisely you measure position - more curved the "trajectory" will be. So if you just measured the position, you may not predict next position - thus it makes no sense to say what is a direction of electron path. Direction is simply related to momentum - it is a line parallel to momentum vector.
  8. Oct 23, 2011 #7
    Re: "Measurement disturbs the system."

    If we have 3 photons instead of 2 we can solve the system and we will know the position and momentum of the electron at each of the 3 different times
  9. Oct 23, 2011 #8


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    Re: "Measurement disturbs the system."

    You could know positions at 3 different times. You may repeat that 1000 times. But those 3 (or 1000) points do not form a straight, uniform line. Knowing 999 first points, you may not predict where will the 1000th one be.
  10. Oct 23, 2011 #9
    Re: "Measurement disturbs the system."

    I don't want to advocate any specific interpretation, but I do like to be exact, so would you agree if I were to rephrase your quote
  11. Oct 23, 2011 #10
    Re: "Measurement disturbs the system."

    So in outer space with no external force a free particle does not move in a straight line? Because that is an assumption of the experiment which I mentioned.
  12. Oct 23, 2011 #11
    Re: "Measurement disturbs the system."

    I think questions like this are hard (impossible?) to answer, cause it uses concepts that simply are not part of the quantum formalism (but are part of some interpretations; as such answers exist, but are dependent on the interpretation)

    Frustrating, I know.
  13. Oct 23, 2011 #12
    Re: "Measurement disturbs the system."

    So what is the quantum mechanical equation for an electron moving through empty space - is THIS is SE for a free particle? Because if so, things are starting to click in my brain...
  14. Oct 23, 2011 #13
    Re: "Measurement disturbs the system."

    Can you rephrase? I don't understand.

    If you're asking if it's given by the Schrödinger equation: then the answer is yes.
  15. Oct 23, 2011 #14
    Re: "Measurement disturbs the system."

    typo...i meant "is this the SE for a free particle?"

    Okay, so basically Quantum Mechanics describes the cloud of probability of different particles and the consequences. Other things like whether or not a particle has definite position and other observables is open to interpretation. Correct?
  16. Oct 23, 2011 #15
    Re: "Measurement disturbs the system."

    Indeed, for me it goes even further: I'd say even the meaning given to a number on a position measuring apparatus is open to interpretation. After all, what does "the position of a particle" mean if you don't have a particle?

    For as far as I can see, the only use the word "particle" seems to have in the quantum formalism is to determine on how many variables your wave function depends...
  17. Oct 23, 2011 #16
    Re: "Measurement disturbs the system."

    Well the point is that we are dealing with a system based on "clouds", or as most people would say, "probabilities". The point is that the main object in our system is not a dot, but a cloud itself, or not necessarily a cloud, but something that can be interpreted as a cloud...
  18. Oct 24, 2011 #17
    Re: "Measurement disturbs the system."

    Does the particle travel between two certain points (i.e. the position found at time1, and the position found at time2), or is more like the particle was found there at time1, then it jumps to the new position at time2 without going inbetween?
  19. Oct 24, 2011 #18


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    Re: "Measurement disturbs the system."

    The assumption about mere existence of "path" leads to paradoxes. Take double slit experiment performed with single electrons (photons, whatever else) at a time. If you insist that electrons have defined paths, you could divide (just in set theory sense, you don't need to tag individual events to which class they belong) events into two classes: those going through left slit and those going through right one. The pattern created by all events should be a sum of patterns made by L and R classes. We know what pattern is created if only one slit is open. Experimental pattern is different than sum of patterns with one slit open. So - if you want to say that "electron passed left slit" you must accept that behaviour of the "electron travelling through left slit" is magically affected at a distance by opening or closing the right slit.
    As above - it makes no sense to use the term "path" regarding the particle. The particle may be observed at some point (rather some area), but between observations "path" makes little sense.
    You may also create lots of experiments in which the straight line between source and detector is blocked, but particles are still observed.
    If any I would rather prefer the second view. Personally, I like most Zeilinger's approach: we use the word "particle" only regarding the very moment of detection or emission. In between we should speak about evolution of the wavefunction, but not about particle.
  20. Oct 24, 2011 #19

    Ken G

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    Re: "Measurement disturbs the system."

    Yet this is just what Bohmian mechanics asserts. Remember, we can generalize Arthur C. Clarke's "any sufficiently advanced technology is indistinguishable from magic" to "any physical effect we don't understand is indistinguishable from magic." I believe Bohm showed that quantum mechanics does admit the interpretation that a particle does always have a definite position and momentum, but it requires adding another layer to the theory, it requires a "pilot wave" to shepherd the various possible trajectories into acting as though they were indeterminate about those quantities if one can't see what the pilot wave is doing. Of course, a more standard interpretation of the situation is that if you can get rid of the pilot wave by invoking indeterminacy, and the pilot wave has no measurable consequences, then this is what you should do, and that's what you are doing. But there is no observation that tells us "the particle does not have a definite but unknown position and momentum at any given time."
    For the same reason, perhaps we should just say "there is no empirical evidence that it makes any sense to use the term 'path', and little but a kind of classical prejudice motivates its use in quantum mechanics. Of course, some view classical prejudices as a good idea, it really depends on personal preference."
    Yes, the notion of purely classical trajectories is definitely out the window. But pilot-wave guided trajectories are still possible, we just don't know and might not get to know.
  21. Oct 24, 2011 #20


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    Re: "Measurement disturbs the system."

    Of course, we may insist for "existence" of something which is fundamentally impossible to observe. It is a matter of taste, and metaphysical and religious formation.
    I (like Carl Sagan) keep an invisible dragon in my garrage.

    Bohmian mechanics preserves classic-like image of the particle of well defined parameters at the terrible expense of introduction of non-measurable 'real' entities, acting non-locally and backward in time, while still having problems to interprete some phenomena. It is a matter of taste if the 'reality' of particle between interactions is worth of such price. My Occamian nature definitely dislikes Bohm...

    Fully agreed!
    My positivistic nature tells me that in science we should not speak about existence of anything which is not backed by empirical evidence...
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