Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A couple of qestions on QM

  1. Jul 5, 2011 #1
    Hi there, I am on high school physics level and came here to have a few questions answered that keep boggling my mind and challenging my standards of logic... please keep explanations simple, since I don't have any extensive knowledge of QM and English is not native to me, thanks...

    _______________________

    1 - Quantum entanglement - is it even real:

    I got sick and tired of witnessing utter misconception about quantum entanglement, the web is full of people who are actually convinced entangled particle means manipulations to one particle continuously transfer to the other, so you can continuously jerk one particle and all effects will simultaneously be mirrored to the second particle...

    From what I read about entanglement - any manipulation, measurement or whatever to either of the particles destroys the entanglement, which brings me to the logical question, if we have a system of two particles and only one measurement allowed, how do we even confirm entanglement, I mean in order to account for a change we must measure the initial and final conditions of both particles in order to account for any changes which take place? Yet we have only one measurement, of one particle, and no way of knowing anything about the system prior to that measurement, since any measurement breaks the entanglement, common logic dictates it is theoretically and practically impossible to attain tangible evidence of it...

    I read that when two particles are created/emitted simultaneously, standard physics conservation of energy dictates they are created in opposite states, and that is what entangled means, but just because two identical particles are created simultaneously in opposition to one another, it doesn't mean they are entangled. I can set two 12 hour timers one at 12 and one at 6 o'clock and activate them simultaneously, then send them on the opposite sides of the globe concealed in boxes and claim they are entangled, and opening the box to verify that breaks the entanglement so both clocks assume opposite states, which were BTW their initial states... but that doesn't mean they are entangled, they have just been synced up...

    So how do we know anything about 2 entangled particles prior to their measurement, how do we verify in a tangible, empirical way they were indeed entangled, if one single measurement of a 4 bit system is all we have? Considering both were in opposition since the moment of their creation, and since they are the same and with the same lets say spin cycle rate, it is only logical for them to be in opposition in the moment of the supposed disentanglement even if there was no entanglement to begin with.
    How do we measure and confirm that? How do we measure it is instantaneous, how do we measure a change in the second particle if we know nothing about its prior state?

    _______________________

    2 - The Quantum Machine
    http://en.wikipedia.org/wiki/Quantum_machine
    (PDF thesis attached on bottom)

    I recently watched a TED presentation by one Aaron D. O'Connell, who spent 8 minutes trying to convince the crowd that quantum mechanics also apply to macroscopic objects, in a rather unscientific and deprived of any actual information way, he began with new age left brain/right brain stuff, went through a series of preposterous metaphors and finished with experiencing being at different locations at the same time, so I immediately turned extremely skeptical, and I was very surprised to learn his machine was credited "Breakthrough of the Year" by Science in 2010.

    To keep it short - he made a piezoelectric macro resonator, supercooled it to its ground state, and claims to have successfully measured it to be in a state of quantum superposition of quote "literally vibrated a little and a lot at the same time"

    When a piezoelectric element vibrates, it outputs current, and I kind of wonder what type of an output curve does "literally vibrated a little and a lot at the same time" output, plotted on a voltage/time graph?

    I'd suggest the type of curve it might be can very easily be the product of interference between 2 frequencies, giving a modulated third frequency as a sum, and keeping in mind he used a dielectric resonator, 2 materials would suggest 2 different resonance frequencies superimposed over each other, the sum of both creates a third frequency, that is more than capable of creating the illusion of "literally vibrated a little and a lot at the same time" - a state of quantum superposition applied to a macroscopic object - if so this experiment is anything but a breakthrough of the year, but somehow it doesn't seem like a plausible scenario, it is just too absurd to be true...

    So my question is, where is the actual evidence it was indeed a case of quantum superposition and not just a few nanoseconds worth of illusion/error/misinterpretation or whatever?


    1x0 in advance
     
  2. jcsd
  3. Jul 5, 2011 #2
    My quantum mechanics is admittedly very rusty. But as to your first question, I suggest looking into the Consistent Histories interpretation of quantum mechanics. In particular, check out the book "Consistent Quantum Theory" by Robert Griffiths. Dr. Griffiths was my grad instructor for QM, and he spoke on more than one occasion about interpretations of entanglement which avoid talking about superluminal influences. The discussion of EPR in his book is very careful.

    By the way, it's available online in its entirety for free:
    http://quantum.phys.cmu.edu/CQT/index.html

    You won't learn how to do quantum mechanics in his book -- he doesn't mention harmonic oscillators, the hydrogen atom, or other classic problems -- but you will get a good grounding in one of the leading approaches to interpreting its results. The kinds of questions you're asking (e.g. "So how do we know anything about 2 entangled particles prior to their measurement?") lead me to believe you'll find this approach satisfying.
     
  4. Jul 5, 2011 #3

    jambaugh

    User Avatar
    Science Advisor
    Gold Member

    I've got one up on you as I've had many more years of being sick and tired of the same thing. Yes that part is nonsense.

    Firstly understand entanglement as correlation. In quantum theory the two components of a composite system can be correlated far more completely than classical correlation of measured values.

    In particular entanglement is a property of the composite system which is complementary in the sense of Heisenberg's uncertainty principle to any measurements of the individual components (though the entanglement implies correlations between those measurements).

    In short entanglement means we know more about a composite system than we know together about each component system.

    The "entanglement" is a real effect but like interference you can only observe it by doing large numbers of experiments. However from the theory you can make a single measurement on the composite which will implies entanglement.

    There is another aspect of this. When we refer to parts of a composite system we typically are using some observable which we exclude from the quantum description, i.e. "the electron on the left vs. the electron on the right" (left and right position wise).

    We can subdivide a two electron system in multiple ways, as above using position, or using say z component of spin if we know the electrons spins are anti-correlated. In this way we can refer to "the left electron is in a superposition of z-spin up and z-spin down" or in a different division "the z-spin up electron is in a superposition of left position and right position". One begins to see how the confusion is in the use of classical object language to describe quantum systems.

    The point here is that whether a composite system is considered entangled depends on how one subdivides it into half systems. You can always choose a factorization in which no entanglement exists but the auxiliary observables you use to index the system may be rather weird and unintuitive. Hence entanglement is relative and more a property of how we describe the composite than it is a property of that composite itself. All the non-locality business just reflects the fact that most of the observables will be non-local in nature but that has nothing to do with violation of local causality (except to those trying to "interpret" the round peg of quantum theory within the square hole of a classical objective reality).
     
  5. Jul 5, 2011 #4

    DrChinese

    User Avatar
    Science Advisor
    Gold Member


    Welcome to PhysicsForums, afstgl! Great questions, especially for your level.

    Entanglement is seen in thousands of different experiments, and the technique varies with the individual setup. The usual method is to create entangled state, i.e. one which produces entangled state statistics. That means that an individual measurement will not be sufficient to show you have an entangled state, it takes a large series of measurements. This is consistent with your comment. In such a state, the outcome of one measurement (entangled Alice) is used to predict the outcome of another (entangled Bob) with certainty.

    You can create a state using sync'd up classical objects which will appear to match this. But guess what? You cannot do this with quantum objects UNLESS they are entangled. Unentangled quantum objects that are sync'd up will give PRODUCT state statistics instead of ENTANGLED state statistics.

    And just to clarify the difference: for polarization entangled photons, the correlation is cos^2(theta) where theta is the angle between. For unentangled photons with the same polarization (i.e. sync'd up), the correlation averages .25 + .5(cos^2 theta).
     
  6. Jul 5, 2011 #5

    DevilsAvocado

    User Avatar
    Gold Member

    Common logic also dictates; if you’re on high school physics level and are challenging the whole scientific community, and 75 years of research, with absolutely nothing but personal speculations – no one is going to take you seriously, and this thread will most probably be locked very soon.

    About The Quantum Machine; please read http://www.physics.ucsb.edu/~martinisgroup/theses/OConnell2010.pdf" [Broken].
     
    Last edited by a moderator: May 5, 2017
  7. Jul 5, 2011 #6

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

  8. Jul 6, 2011 #7
    Well, since I clearly stated I do not have knowledge in QM and as a product of that my mind is boggling, I cannot help but wonder how exactly did you figure that my post is challenging 75 years of scientific research? If that was a product of your logic, I'd say your sense of logic is not a very good one and should not be trusted blindly... Common logic also dictates if your response is arrogance ridden and does not contain anything useful, you are trolling... You put words in my mouth that I haven't spoken and neither can be extrapolated from the context, you are offensive, and it almost seems like your wishful thinking alters the context of my post, it almost seems you want to get my thread locked in order to feel better about yourself in some twisted way...

    No, I don't want the thread closed, not before it serves its purpose, not of challenging anything but of answering my questions, that is why I politely ask you to keep your zero contribution and conflict provocative responses out of it, the only viable bit of information you posted is one already implied in my very question, and furthermore I've seen enough of your responses in other threads to draw a pretty complete picture of your behavioral patters in this place, and the very fact those are exhibited by one, credited a "contributor" makes it that much worse... Thank you!

    _________________________________________

    To everyone else, who were helpful and constructive in their responses - I've read them carefully and I will do the same with the references you provided, but for the time being I still have one simple question I didn't felt got answered:
    What is this affecting expressed into? How does the second particle change as a result of disentangling, and how is the changed particle different compared to its prior, entangled state? I've seen so many different explanations of this that I simply want to know which one is the correct one.



    It might be a little off-topic, but as members of academia, I'd like you to elaborate on a matter that is less scientific and more ethical, that matter is the representation of QM in the mainstream media.

    Prior to registering and posting here, I've watched pretty much everything that was available on entanglement, and noticed a particular trend - explanations are extremely watered down in terms of facts and technical details, and substituted by not simply wrong, but also sugared nonsense, in the particular case of entanglement I've seen many people with scientific background to present entanglement as that continuous state, where you jerk one particle and everything mirrors to the other, I've seen the same people imply actual matter teleportation and a wide range of phenomenons that don't seem to have anything do with QM, instead being borrowed from the realm of science fiction.

    One of the most frequent oddities in this regard is the classical explanation of entanglement, where "you can send two particles to the opposite corners of the universe, and affecting one will immediately affect the other" - there are so many problems with it, no one really knows where the opposite corners of the universe are, but even if one did, it would be technically impossible to send two entangled particles there, even less possible to follow those particles there to apply measurement, and even less possible to verify any kind of a result across the entire universe, I mean each and every step of this scenario is impossible on its own, and combined together I struggle to find any term to describe this idea other than "utter and complete nonsense" - FTL interaction will be just as impressive from a mile away or even less, why does one have to involve that preposterous explanation instead - and yet this nonsense scenario gets reused, over and over again, which brings me to the following conclusion:

    It seems like the masses are not even supposed to understand entanglement for what it is and guide their curiosity accordingly, but rather NOT understand it, and instead be impressed and amazed by all the nonsense, which is substituting the actual details about entanglement. The mainstream scientific programs are not educating about QM, they simply plant utterly wrong misconceptions that barely have anything to do with reality, but WHY?

    Edit: Just to make a point, here is a video of overdecorated Kaku going way outside the realm of quantum teleportation, and claiming to have succeeded actual teleportation of actual atoms:
    http://www.youtube.com/watch?v=-FqLCLooayM&feature=related
     
    Last edited: Jul 6, 2011
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A couple of qestions on QM
  1. Running coupling (Replies: 1)

Loading...