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Probability Clouds

  1. Oct 27, 2004 #1
    If say, in a particular atom, we have an electron cloud, and according to quantum theory, the particular electron would be in all places of the cloud at one time (right?). So, does this mean that they are actually physical electrons? If they are, won't this mean that an atom would possess a hell lot of negative charges?
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  3. Oct 27, 2004 #2


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    No it doesn't mean the electron "is" everywhere in the cloud at one time. It means that the electron has a probability* to be at each point of the cloud, a higher probability at some points than at others. You don't know where the electron is, and if you measure its position, the probability of findind it as at a point is given by the probability function (eigenvalue of the wave function).

    *Actually a complex amplitude whose squared modulus is the probability.
  4. Oct 27, 2004 #3

    James R

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    The theory doesn't say the electron is in all places at once. It gives the probability that the electron is found in any given place at a given time. We're still dealing with only one electron. If you make a measurement of its position, you'll only ever find it in one place.
  5. Oct 27, 2004 #4
    ahh, then how come in the double-slit experiment, the probability cloud of the electron actually spreads out in such a way that the electron goes through two slits at one go?
  6. Oct 27, 2004 #5
    No, it's the probability cloud the one who goes through bot slits, if you put a detector in both slits only one will fire.... 50%/50%
  7. Oct 27, 2004 #6


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    Yes, but misogynisticfeminist is still correct in asking that if you do not detect which path an electron takes, it will behave as if it went through BOTH slits and interfere with itself! You will detect such interference pattern with more and more electrons passing thru, even if you shoot them one electron at a time. I find this to be a very intelligent response on her part to the answers that she was given.

    Thus, if you look at at her original question about the elctron cloud, in an s-wave orbital for example, is there really just one well-defined electron that is moving all around the nucleus exhibiting a "spherical path", or is there really a smearing of the electron simultaneously as described by the wavefunction?

    Note that I think she was told that the former is the "right" way to look at it. I disagree. Other than the fact that an electron making such a path in a central force would be a VERY strange solution, having only ONE electron zipping around like that would result in a definite value of the angular momentum! Yet, s orbital has none!

    Secondly, the existence of an electron everywhere simultaneously is implied in the standard QM interpretation. In fact, we see the effect of this, and I have already cited several examples of the Schrodinger Cat-type states and observation - bonding-antibonding energy gap in H2 molecule and supercurrents in SQUIDs measurement. In both cases, orthorgonal properties of the object in question (electron in H2 and a supercurrent in SQUID) exist simultaneously! The electron occupies BOTH H location at the same time, causing its wavefunction to interfere and separates out the bonding band from the antibonding band. The supercurrent DOES move in both directions across the SQUID junction at the same time, again generating a gap between the two symmetries!

    This is a very fundamental aspect of QM, and yet, it has got to be one of the most difficult and strangest aspect to fully comprehend.

  8. Oct 28, 2004 #7
    But think of an ns-orbital with n large. With fast lasers one might be able to probe the orbit (if you could prepare such a state), and it would not be spherical. Classically you would have an atomic dipole oscillating like mad, and the state would probably be very short-lived and decay quickly to the (n-1)p state. But it would be a nice weird thing to look at. I suppose there would be a kind of perihelium precession of the orbit, so that the cloud would be spherical on average.
  9. Oct 28, 2004 #8
    hmmm thanks zapperz,.......and one thing, I am not a "she"...

    : )
  10. Oct 28, 2004 #9


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    Well then, get a sex-change operation, damn it!


  11. Oct 28, 2004 #10


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    Er... "one might"?!

    Are you inferring or are you basing this on an actual experimental results?

    We are also not talking about "large n", because we know that anything beyond a helium atom, there are simply no exact solution to the whole atom!

    The example of s-orbital is MERELY to illustrate that it DOES require that one visualize that an electron is smeared out all over the atom, and not, as has been suggested, an electron making a strange trajectory around the nucleus in which its average postion appears to look like a sphere. If you do not like the s-orbital, then use whatever orbital you like and the example will still be valid.

    BTW, in case people missed this, last week's PRL has a very elegant paper in the imaging of the interference between an s-wave and a partial d-wave from cold Rb atoms. [N.R. Thomas et al. PRL v.93, p.173201 (2004)].

  12. Oct 29, 2004 #11


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    Ahh yes, I remember my experiance with this, I thought exactly the same thing, and was really mad, because I was being told there were 7 electrons in whatever, not jillions.

    btw, the double-slit experiment is with photons.
  13. Oct 29, 2004 #12


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    No, the double slit experiment can be done with photons, electrons, protons, neutrons, cooper pairs, even LARGE buckyballs such as C60 and C70! The set-up may not be identical, but as long as you have a coherent interference of two possible paths that anything can take, you have a "double-slit". Each one of these have been observed experimentally.

  14. Oct 29, 2004 #13
    Zz. is entirely right on this one...
    indeed the double-slit experiment can be seen as a proof why waves can be used to describe particles. Not only photons can be expressed in terms of waves, all particles can...

  15. Nov 2, 2004 #14
    Isn't it so that, if particle beams of electrons or photons that are of diameter small enough that all of the particles clear the walls of the slits/holes, the detected pattern will be only a pair of elliptically shaped dots; doesn't that really mean that the wavelike appearance exhibited, when an isotropic source is used, is actually due to Fresnel diffraction around the edge of such openings rather than some alleged dual state of the particle. Cheers, Jim
  16. Nov 2, 2004 #15

    Hans de Vries

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    I hate to be negative on somebody’s experiment but,

    Zeilinger's experiment with buckyballs (720, 840 nucleons per ball !!)
    does not meet the requirements for diffraction and is rather straight
    forwardly falsifiable.

    It's in fact so that the setup will produce "positive results" for any
    arbitrary sized object. That is, if you scale up the size of the equipment
    enough then you can throw real living (Schroedinger?) cats through it
    and still get an "inference" result!

    The experiment blows "red hot" 900 degrees C buckyballs from an
    oven through a grating with a spacing of 1000 nm (six orders larger
    and wider than the diffraction wavelength of ~ 3 pm)

    There are two (equal) gratings in a row. The pattern that is formed
    after that equals the size and spacing of the grating, just like a
    shadow pattern. This pattern is then taken to be the "interference"
    pattern. That's why it works for any arbitrary sized objects. A third
    grating measures the result.

    If this sounds to strange to be true then have a look here for a
    detailed discussion and a list of some 8 on-line references to
    publications on the experiment:

    Buckyball inference

    They claim the shadow pattern of the grating should wash out
    at a certain distance and that all what’s left then should be a real
    inference pattern. The predictions they make for the shadow
    pattern however don't match with simple simulations.

    Taking the shadow pattern for an interference pattern leads to an
    extremely high expectation ratio as well, more than 50% of the
    buckyballs (or cats) is assumed to go through two or more slits at
    the same time!

    Excuse me for using the word cats here but I'm actually very serious
    in this. After all they are planning to throw real live viruses through
    the gratings.

    Regards, Hans
  17. Nov 3, 2004 #16


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    I have read both of Zeilinger's papers, and unless someone else would write a comment or a rebuttal IN PRINT (and not on an internet public forums) to those two papers, I will accept them as is till there are more reproducibile verifications. If you think that they have misinterpret their results, then it is imperative that YOU submit a rebuttal. Don't just whine on here, it does no one any good.

    Secondly, if you are equating this with a Schrodinger Cat-type experiment (by your repeated usage of "cats"), then let me remind you that in the SQUIDs experiment done at Delft and at Stony Brook, the Schrodinger Cat-type effects are observed in a system of particles of the order of 10^10 particles, way larger than even the molecular interference scale! These observations are reproducible, are VERY clear, and the definiteness of the results are astounding.

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