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    How can one event affect another instantly over a distance

    I was under the impression that the collapse postulate is only considered an approximation, since it does not define precisely what constitutes "measurement". So if all nonlocality in QM flows from the collapse postulate, then the nonlocality may not be physically real, just an artifact of the...
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    Entanglement, what is it?

    FYI, the singular of this word is "die" and the plural is "dice". I know no other English word that follows this rule ... what a crazy language!
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    What is light?

    My opinion is that you have hit upon one aspect of the so-called "macro-objectification problem", which is still an active area of debate and research in physics. Macroscopic objects have a definite position, thus we can easily speak of forces and the E and B fields of the Maxwell equations...
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    Under what conditions will quantum effects become important for gravity?

    Alledged WIMPs would exhibit gravitational effects in their quantum behavior much more than ordinary matter because they are unaffected by electromagnetism and nuclear forces. For example, if there is any condensed phase of WIMPs we can observe somewhere in the universe, its spectrum (in...
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    Under what conditions will quantum effects become important for gravity?

    The WIMPs (weakly interacting massive particles) that have been proposed to explain dark matter might exhibit quantum effects in a gravitational potential well.
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    Trouble understanding how charge density is the amplitude squared

    If you are talking about an ionized hydrogen molecule (H2+), the electron's charge density is mostly between the two protons. You could think of the system as three point charges along a line: (H+)===(e-)===(H+) From each proton's perspective, the electron is closer than the other...
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    Path Integral that is valid for a Particle

    Alternative theories/interpretations needn't predict ALL physical results identically to orthodox QM, only those QM predictions that have been verified by experiment.
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    One Electron at a Time?

    No, no, that's not what I meant. By "artifact of the detector" I don't mean a spurious effect, so maybe "artifact" is the wrong word. I mean it's the presence of the detector that causes the discretization of the beam. The double-slit experiment is always portrayed as if the clicks...
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    One Electron at a Time?

    Even if the detector clicks account for 100% of the electron current, there is still a question as to what's happening between clicks. The phrase "only one electron in the apparatus at a time" makes it sound like the beam emitted from the source is composed of very brief, discrete pulses, but...
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    One Electron at a Time?

    That's exactly my question. Is it "detecting one electron at a time" or "one electron-detection event at a time". I'm thinking one electron isn't plucked out of the many-particle system made up of source+vacuum gap until the moment of detection.
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    One Electron at a Time?

    What's clicking? Is it the same screen that displays the interference pattern?
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    One Electron at a Time?

    Presumably this calibration step would involve some kind of position measurement parallel to the beam direction. Such measurement would be necessary to establish the "distance between electrons" to which Tonomura et al. refer. But then the Born rule implies that the calibration doesn't just...
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    One Electron at a Time?

    In the electron double-slit experiment performed by Tonomura et al. (http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000057000002000117000001&idtype=cvips&gifs=yes [Broken]) it is claimed that only one electron can be in the apparatus at a time, because the electron...
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    Question on observer created reality

    I am no expert in Bohmian mechanics, so maybe you are right. I read some criticism of Bohm's account of wavefunction collapse, but it was just informal commentary on the 'net, not a serious analysis. On the other hand, I would point out where Bohm invokes some more "deus ex machina" as you...
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    Question on observer created reality

    I was under the impression that Bohmian mechanics does not explain well the collapse of wavefunctions, i.e. the sudden discontinuity in Bohm's "quantum potential" after measurement.
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    Question on observer created reality

    Does that mean, therefore, that if wavefunction collapse is "real", there must be some nonlinearity in the time evolution operator?
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    Why Pauli's Exclusion Principle?

    Even in QFT, I thought that the commutation rules for fermion and boson fields were taken as axioms. The spin statistics theorem just explains why fermions are spin n+1/2 and bosons are spin n, not why fermions and bosons exist. Please correct me if I am mistaken, I've only just started...
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    How is Momentum Measured?

    I am trying to imagine how one would display interference effects analogous to the double-slit experiment, except that the roles of position and momentum are reversed. With the usual double-slit setup, the beam components passing through the two slits interfere, resulting in some spots on the...
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    How is Momentum Measured?

    What kind of experimental apparatus provides the purest measurement of a low-energy particle's momentum? By "pure" momentum measurement, I mean allowing for the maximum achievable uncertainty in position, and therefore the most accurate possible reading of momentum as per the uncertainty principle.
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    Experimental Tests of Projection Postulate

    Thanks much, Patrick. Very clear explanation.
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    Experimental Tests of Projection Postulate

    I am just a grad student so my understanding of QM is still naive, but I am still not getting why the Born rule needs to be a fundamental postulate. Can't it just be viewed as an heuristic, an approximation to the hopelessly complicated unitary dynamics of a macroscopic system?
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    Experimental Tests of Projection Postulate

    Is it fair to say, then, that the macroscopic pointer states still formally interfere with each other, but the intereference effects are so minute that the pointer's behavior cannot be distinguished from classical behavior?
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    Experimental Tests of Projection Postulate

    What kind of interaction hamiltonians would prevent interference? Are they non-hermitian?
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    Experimental Tests of Projection Postulate

    Is it generally accepted, then, that the projection postulate is not formally true, but is an approximation of the measuring device's unitary evolution, in the limit as the number of particles goes to infinity?
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    Experimental Tests of Projection Postulate

    Even in a Stern-Gerlach apparatus, I would think there is no actual "measurement" until the electron actually hits a detector. Until that time it would be in a superposition of possible pathways through the magnets, wouldn't it?
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    Experimental Tests of Projection Postulate

    Have there been any experiments designed to explicitly test the projection postulate? I mean that part of it that says the measured particle is left in an eigenstate of the measured operator. The usual devices for measuring particles (photomultipliers, phosphor screens, etc.) don't really...
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    Electrical charge and Mass

    With such a short lifetime, the Z0 must be confined within a very small radius (~9x10^-17 m, if I'm calculating right) since its velocity is less than c in any reference frame. How does this length scale compare with quark confinement? In other words, is the lifetime of Z0 so short that...
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    What is meant by molecular temperature ?

    Yes, that is what I mean, except that the temperature concept is even applicable to molecules much smaller than DNA. Here is an except from one of the fullerene beam papers: They appear to mean "temperature" as the excitation of individual molecules. This is C70 they are talking about...
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    What is meant by molecular temperature ?

    But a molecule is itself an assembly of smaller bodies (nuclei and electrons). Consider that, for example, a large protein molecule may contain many thousands of constituent atoms.
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    What is meant by molecular temperature ?

    In my opinion, the term "molecular temperature" does make intuitive sense. A single molecule can have excited electronic states, vibrational states and rotational states. The molecule may also lie in an external potential well, in which it is bouncing around. All of these excitations...
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