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

Question about wave function collapse

  1. Jun 17, 2011 #1
    I’ve been fascinated with the different QM interpretations since I discovered them… but I want to sort of restrict my imagination to more solid ideas.

    I know about the Double-Slit experiment and that there are many misconceptions about the correct definition of an ‘observer’ and what role the brain plays in all of it but I have a question that I think will clear up the issue for me.

    I’m looking out of a window at some trees with birds in it and a sidewalk with people walking by. What I want to know is this:

    Does opening/shutting my eyes have any affect on the wave functions of the objects on the other side of the window? Does the act of ‘looking’ with my eyes collapse wave functions differently than if I were to shut my eyes? Or does QM determine the outcome of events strictly by my body’s mass and position of the mass relative to objects around me in spacetime?

    I’ll admit that I have trouble grasping the implications of QM. I think that’s the first step toward bettering ones understanding of it... so any help would be appreciated.
     
  2. jcsd
  3. Jun 17, 2011 #2
    When photons reach the inner cones of your eyes, they are detected at a definite position. So the uncertainty in position must be zero. If the uncertainty is zero than the wave function must be a dirac delta function. It looks collapsed. A better explanation is that the function is in a position eigenstate.

    Close your eyes and your not measuring anything.

    The wave function is like a list of probabilities before measurement, once a measurement has been made you now have certainty, the list of probabilities collapses into a single value.
     
  4. Jun 17, 2011 #3

    there are various ways to collapse a wave function, essentially there has to be some "select kind of interaction" with the quantum/photon.

    an interaction where we determine the position/location of the photon....when we pin-down/force the location .....this collapses the wave function...
     
    Last edited: Jun 17, 2011
  5. Jun 22, 2011 #4
    Thanks for the replies Lost Conjugate and San K. I'd appreciate some feedback from others!
     
  6. Jun 22, 2011 #5

    These have very clever answers, to equally clever questions....


    ''Does opening/shutting my eyes have any affect on the wave functions of the objects on the other side of the window?''

    Not in any great deal. If you mean local objects, outside, then the wave function does not spread out like it does for a single quantum. There is some question however that our observations on a cosmological scale could have effects in that it is shaping the universes past, but to understand that latter part, you need to know about absorber theory, the transactional interpretation and top-bottom models of quantum mechanics.

    ''Does the act of ‘looking’ with my eyes collapse wave functions differently than if I were to shut my eyes?''

    Wave functions on our scale are so small, they are negligable. If your eyes are closed, the wave functions which would normally smear a quantum object are also negligible.


    ''Or does QM determine the outcome of events strictly by my body’s mass and position of the mass relative to objects around me in spacetime?''

    Not sure what is meant, but your existence means nothing for the evolution of systems. Quantum mechanics however can be deterministic and can solve many current problems with QFT. The collapse of a wave function is just two parts of one solution. You have incoming waves and outgoing waves, usually named echo and offer waves. They form a collapse on the square of the wave function [tex]\int |\psi \psi*|= \int |\psi|^2[/tex]. This means that something ''a weak measurement'' has occurred as a coupling on the system. This happens all the time in the absence of human observers. Tiny particles are constantly having eigenstates being defined by the observation of other particles on them. This is how decoherence works.
     
  7. Jun 23, 2011 #6
    Thanks for the insight Goldstone. I don't completely understand your response... it sounds like i need to take physical chem. haha i seem to lack essential vocabulary.

    "Tiny particles are constantly having eigenstates being defined by the observation of other particles on them. This is how decoherence works."

    Do i have this right: So a particle's next state in QM is determined by whether a neighboring particle is 'observing' it which limits the possible states? Is it similar to a domino effect in that each particle imposes limits on a neighboring particle, and so on until it there is just one possible state?

    BBC recently released an excellent 2-part documentary called 'Everything' & 'Nothing' and was presented by Jim Al-Khalil. 'Nothing' was incredibly interesting. It examined the idea of empty space and eventually collapsed on the idea that there is no such thing as empty space, due to quantum fluctuations in a vaccuum where particles pop into existence by borrow energy from the vaccuum and immediately give it back, vanishing. The documentary is partially responsible for my curiosity on the subject.

    Forgive my lack of understanding/vocabulary I will eventually take P-Chem and cover the material but i cannot help but seek a novice understanding in the mean time.
     
  8. Jun 23, 2011 #7

    Yes, exactly. The act observation limits how many possible realities can exist.
     
  9. Jun 23, 2011 #8
    "Yes, exactly. The act of observation limits how many possible realities can exist."

    So to reiterate and expand: observing one particle real close causes surrounding particles to start snapping into their most probable states based upon the position/velocity of the observed particle... but this is only true at the quantum scale right?

    I'm now more interested as to why this doesn't happen on larger scales like humans are more familiar with. Is it because our eyes do not have the ability to see a particle close enough for it to be declared 'observed' or in a definite location?

    Once again my definition of 'observation' needs to be refined. The difference between the human eye and a QM lab instrument is not clear to me aside from the scales involved. Very interested in this question.
     
  10. Jun 23, 2011 #9

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Welcome to PhysicsForums!

    Your skin does the same thing as your eyes, and the same as the lab instrument. When a photon is absorbed, the appropriate "reality" takes shape. Assuming the photon came from an electron dropping from one orbital to another: that electron is now in a state which leaves it no different than any other electron in a similar orbital. So that means there isn't much impact on the electron itself, and so there aren't really going to be much influence as a result of how the photon was observed. So there is no need to start blinking your eyes just yet!
     
  11. Jun 23, 2011 #10
    ''So to reiterate and expand: observing one particle real close causes surrounding particles to start snapping into their most probable states based upon the position/velocity of the observed particle... but this is only true at the quantum scale right?''

    Yes, this is true at the quantum level. Always true at it's level.

    ''I'm now more interested as to why this doesn't happen on larger scales like humans are more familiar with. Is it because our eyes do not have the ability to see a particle close enough for it to be declared 'observed' or in a definite location? ''

    We cannot view the fundamental world... at which happens there is beyond human observation (usually in every day life). Entire objects like chairs and tables for instance, can have a wave function, but it's wavelength is extremely small - it is effectively unobservable. Though this small wave function does exist.

    ''Once again my definition of 'observation' needs to be refined. The difference between the human eye and a QM lab instrument is not clear to me aside from the scales involved. Very interested in this question.''

    Quantum mechanics right now states that the collapse is a measurement of a weak catagory. This is just a fancy way to say that one system has been coupled to another system through some interaction. This interaction is of course ''observation'' - the eye of the scientist in the lab may define the angular momentum for instance, of an electron, and collapse it's wave function for a short period of time, but this is no different to two electrons defining each other when they cancel out at close proximity to each other. Observation is just when a system obtains some real attributes, like energy and spin. These are what are called observables.
     
  12. Jun 23, 2011 #11
    And since the wave equation is a map of probabilities, this simply means that the object is built up of so many particles that statistically the probability is nearly removed and its position, momentum, energy, etc are all continuous.

    Taking a measurement of the entire object is like taking a measurement of each particle and finding the average, which would be the expectation value of your measurement, the classical result.

    This is why you do not see QM effects with Macroscopic objects, it is possible, just vastly improbable like flipping a coin 10^40 times and having it land on tails every time.
     
  13. Jun 23, 2011 #12
    “Welcome to PhysicsForums!”

    Thanks I like it here. Constructive responses from the informed are really appreciated. I have lots of questions.

    “When a photon is absorbed, the appropriate "reality" takes shape. Assuming the photon came from an electron dropping from one orbital to another: that electron is now in a state which leaves it no different than any other electron in a similar orbital. So that means there isn't much impact on the electron itself, and so there isn’t really going to be much influence as a result of how the photon was observed.”

    Is the focus of QM centered on the emission/absorption of photons? I didn’t realize it was so fundamental.

    “Entire objects like chairs and tables for instance, can have a wave function, but it's wavelength is extremely small - it is effectively unobservable. Though this small wave function does exist.”

    By having a ‘small’ wavelength does that just mean that the table or chair is well defined and ordered with little chance it will suddenly appear on the other side of the room, as opposed to much smaller particles whose possible positions are more random? i.e. the bigger the object, the less random the subsequent states will be?

    “Quantum mechanics right now states that the collapse is a measurement of a weak category. This is just a fancy way to say that one system has been coupled to another system through some interaction. This interaction is of course ''observation'' - the eye of the scientist in the lab may define the angular momentum for instance, of an electron, and collapse it's wave function for a short period of time, but this is no different to two electrons defining each other when they cancel out at close proximity to each other.”

    This explanation leads me to ask what I think are highly philosophical/untestable/unanswerable questions. I don’t expect real responses to them because of their open-ended/ridiculous nature and I certainly don’t want to dilute the thread but here they are:

    At the moment of the big bang the first two (sub-atomic?) particles of the universe would have been coupled with each other by interaction… causing a collapse… but collapsing into what? What about the first particle? Without a second particle observing the first how could what we see today exist without interaction and collapse? I’m done for now. I have a Quantum headache.
     
  14. Jun 23, 2011 #13
    Hey lovenugget! Gonna see if I can shed some light...

    I believe what you're referring to here is something called "Baryogenesis". Our knowledge of particle physics predicts that every particles like electrons, protons and neutrons should have an antiparticle that exists that will cancel out with it when in close contact, and usually produce some photons in the interaction. We believe that in the very beginning of the universe, the Big Bang, there should have been equal parts particle and antiparticle that would go through annihilation and disappear, leaving photons. However, we observe only "regular" particles and very few antiparticles (so named "anti" simply because they are not common, there's no other distinction between the two besides that fact that they are of opposite charge and something we call baryon number.) So far, Physicists have very little even in the way of theories as to why the Big Bang produced such uneven levels of particles to antiparticles, so this problem is at the forefront of theoretical physics right now. I just had a professor who was especially focused on this topic!

    So, to answer your question, why didn't particles just annihilate with each other at the Big Bang--we don't know! Yet... ;)
     
  15. Jun 23, 2011 #14
    Well, we have a good idea why particles and antiparticles did not annihilate each other during the big bang. It's called spontaneous symmetry breaking, just involves the idea of fields where information is either kept in symmetry or that there has been a difference in the end result.

    Originally in our search for a gauge invariant theory, we had equations which did not permit symmetries which we sought for:

    [tex]\partial \phi' = [\partial \phi + i \phi \frac{\partial \theta}{\partial x}]e^{i \theta}[/tex]

    [tex]\partial \phi'* = [\partial \phi* + i \phi* \frac{\partial \theta}{\partial x}]e^{i \theta}[/tex]

    multiplying the two we get

    [tex]= \partial \phi \partial \phi* + i (\phi \partial \phi* - \phi* \partial \phi) \frac{\partial \theta}{\partial x}+ \phi* \phi (\frac{\partial \theta}{\partial x})^2[/tex]

    which has no symmetry whatsoever! plus it is horrid to look at. Nice to note though that if the potential term was added into the equation (well actually taken away, but you get my drift :) it would not effect its symmetry). Why do people always forget to plug in the potential [tex]V(\phi* \phi)[/tex], it has some of the most interesting dynamics!

    Nevertheless, this was not good, so we had to add an extra four-field to our system, namely the four-vector potential [tex]A_{\mu}[/tex]. Added with our covariant derivative, which we have seen so far has the form:

    [tex]D_{\mu}\phi = \partial_{\mu}\phi + iA_{\mu}\phi[/tex]

    [tex]D_{\mu}\phi* = \partial_{\mu}\phi* - iA_{\mu}\phi*[/tex]

    simply has an addition to our field which has an appearance of [tex]\partial \rightarrow \partial - iA[/tex].

    This allowed us to have a nice symmetry in the making - interesting though how we had to mould the equations a few times, a bit of nip and tuck if you will. This math however works only for a gauge particle, however, symmetries can break for instance, and give this particle a mass. There was some work done on the Kaon particle and there where tiny differences found in the life expectancy compared to it's antiparticle. During big bang, there was almost likely some kind of symmetry breaking, making normal matter predominant.
     
  16. Jun 23, 2011 #15
    I didn't know any of this! And I'm years away from being able to understand some of that math, though I was able to follow and see the antisymmetry. What does[itex]\theta[/itex] represent in those equations?

    So, to see if I understand the conceptual part...Due to small asymmetries in values between particles and antiparticles in the early universe (like mass, for example?) there existed an equally small asymmetry in the lifespan of the particles versus the corresponding antiparticles which could have allowed such a number of normal particles to exists without accompanying anti particles? (after that lifespan had passed). These particles all annihilate to create photons, correct? Would this also explain the huge numbers of photons in comparison to baryonic matter?
     
  17. Jun 23, 2011 #16
    [tex]\theta[/tex] is part of the exponentiation of the field, and it is a phase.

    But yes, particles in the early universe may have been produced with antisymmetries which made a bundle of particles over antiparticles. What is there .... about [tex]10^{80}[/tex] particles in the universe? An example will maybe be that only 1% of this was antiparticles... just an example.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Question about wave function collapse
Loading...