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

Electron's vanishing act

  1. Jul 18, 2011 #1
    When I was in School, my science teachers over-simplified everything to the point of being totally wrong. I guess, that's how they themselves were taught by their teachers.. or it could be that they have no idea they are wrong... or it could be that I didn't really understand them well (for a long time, I believed that electrons moved around the nucleus in a single plane, based on textbook projections).

    The kids these days learn a lot from the Internet and even correct their teachers. I wasn't surprised when I lost a bet to a High school kid challenging my rusty knowledge on the electron but it sure hurt my ego :D Trying to refresh the science that I learned at school, I have used the Internet (Wikipedia in particular) to a large extent to gain a better understanding. Unfortunately, for me though, the picture is not always complete.

    What I was taught at school: Electrons orbit around nucleus in discrete orbitals
    What I now know: Electrons don't orbit but disappear from one point in orbital space to reappear in another point in orbital space.

    What I want to know is:
    1. Does the electron instantly reappear at another spot in its orbital after it vanishes? or does the vanishing and reappearing happen simultaneously? Is there a time delay - (like, if time is quantized and the electron once vanished can reappear only after a certain quanta of time has elapsed)?
    2. Does the electron do the same Houdini vanishing act when it gets excited from a low energy shell to a higher energy shell and vice versa?
    3. When the electron vanishes and reappears, how does it maintain its state, say for example, it's spin?
  2. jcsd
  3. Jul 18, 2011 #2


    User Avatar
    Science Advisor

    In atoms, there is no such thing as electron position, only (quantum) energy levels.
  4. Jul 18, 2011 #3


    User Avatar

    Both views are equally wrong... You just can't view atom as a microscopic planetary system with electrons making possibly some tricks.

    Try mental excercise: imagine that electron do not exist at all except those rare moments you want to check where actually it is located (you may do it e.g. scattering X-rays on the atom). And imagine it just appears at the very moment of such measurement. Between measurements it makes no sense to speak about its position nor its other properties. All you know about atom is that if you examine it with X-rays, there is such-and-such probability to find electron in such-and-such place, and other probability to find it in another place.
  5. Jul 18, 2011 #4

    Ok, If I understand it correctly:
    a) We are limited in a way we cannot make truly continuous observation of the electron

    b) Observer would neither see electron appearing or disappearing. When observer 'sees' an electron it already exists at its position.

    Guess, it's like locating a rat in dark room using a strobe light. When you see it in the light flash, you can guess the locations the rat may move next ... but between the light flashes you would have no idea if it's still in the room :D
  6. Jul 18, 2011 #5


    User Avatar
    Science Advisor

    This is true for an atom with one electron, or approximately true for an atom with many electrons. In the latter case, the state of a single electron cannot be exactly described independently of the other electrons, since electrons interact. You can include the averaged effect of the other electrons, and get a relatively accurate description that way, but it's not exact. In a bit the same way that you can't describe the Earth's orbit around the sun without taking the locations and orbits of all the other planets into account (not that electrons move anything like planets, but the principle here is the same - it's a many-body system).

    They don't, really. If an electron is occupying an orbital, that's a complete description of the state of the electron, which (per quantum mechanics) only gives you probabilities of where you may find it. An orbital gives you a probability distribution over all of space.

    If you 'detect' the location of the electron, then it's no longer in a bound orbital state. Depending on how accurately you detected that location, you imparted more or less energy/momentum to the electron. (Uncertainty principle) If you were to detect an electron's location within an atom to within an Ångström or so, then you'd impart so much energy to the electron, it'd no longer be bound to the atom. The quantum-mechanical view here is that the idea of an electron having any precise location within an atom just doesn't have meaning, either in theory or in practice.

    What that 'disappearing' thing is illustrating is rather the fact that the motion of the electron in the atom (because of this uncertainty) doesn't follow any kind of classical trajectory. An electron can go from point A to point B without passing intermediate points in space. And you can tell that even without 'measuring' it, because the orbital itself can have regions (nodes) where the probability of locating the electron (were you to do so) would be exactly zero.
  7. Jul 18, 2011 #6
    What you learned in school was not wrong. There is nothing that says the motion of an orbiting object must be continuous in order to be an orbit.

    There are other problems that interfere with the concept of an orbit however, such as no angular momentum in the lowest energy state.
    Last edited: Jul 18, 2011
  8. Jul 18, 2011 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Thinktank, as you can see, this is not an easy question to answer. We can describe HOW an electron behaves, but not what it actually is. (Although that phrase is semi-meaningless in science)

    We know that we cannot predict where an electron or any other particle will be past a certain accuracy. We know that in many waves it behaves just like a wave would, yet at the same time it also behaves like a particle. What happens between our observations is unknown.
  9. Jul 18, 2011 #8
    oh yes there is...that statement is incorrect....quantum theory forbids it!!!

    Discontinuous energy jumps via quanta means CONTINUOUS orbits of classical descriptions are incorrect. Continuous orbits are fundamentally incompatible with the discontinuous jumps of quantum theory.

    See here for visualizations of atomic orbitals (electron clouds):

  10. Jul 18, 2011 #9
    I agree with alxm's description....post 5.......but maybe in "unscientific" terms one could crudly describe the absorption or emission of an energy quanta via your statement....I don't know what "disappear" means in terms of quantum theory....sounds more like a classical concept..if even that!!!........much better to say that jumps from one energy orbital state to another are discontinuous.
  11. Jul 19, 2011 #10
    its even stranger than that.
    for a finite time the electron is in both orbitals.
    thats called superposition.
  12. Jul 19, 2011 #11


    User Avatar

    a) That is not that we are limited. This fundamental impossibility of making continous measurement is a very foundation of Quantum Mechanics. It is so fundamental, that even thinking about electron position in between of measurement makes no sense. Mere assumption that electron has some position (even unknow to you) lead to paradoxes or contradiction with experiment (like in case of electron interfering on double-slit exp)

    b) I would be cautious with use of words like 'already exist' or even 'electron'... It was said about photons, not electrons, but the sense is the same: Anton Zeilinger - one of the top-class modern physicists working on quantum optics, when asked 'what the photon really is?' answered: "Photons are clicks in photon detectors". Thomas Jennewein explains this view: "nothing real is traveling from the source to the detector."
    My personal way to rephrase your statement would rather be: "I call 'electron' the mere fact of seeing it".

    David N. Mermin's comment on A.Zeilinger: `Would Anton agree that electrons are clicks in electron counters? Are fullerenes clicks in fullerene counters? Is Anton a click in an Anton counter?'
    Last edited: Jul 19, 2011
  13. Jul 19, 2011 #12
    This is not accurate: The Schrodinger wave equation, for example, describes the evolution of an electron...bound or unbound.

    You are getting closer, but trying to accurately describe electron behavior with only CLASSICAL concepts is impossible. Macroscopic thinking and descriptions usually don't apply to microscope phenomena.

    Thinktank says : Ok, If I understand it correctly:

    sort of....but WE are not the ones limited...

    instead how about saying:

    "...we cannot make truly continuous observations of the electron because its evolution (description) is not continuous". An example would be DISCONTINUOUS orbital jumps via emission or absorption of an energy quanta....like a photon. Or you can say: An electron cloud does not have continuous (classical) positions. "Superposition" as posted above is another "non classical" state.

    That's getting closer. But if your'e implying it exists the moment before observation at it's measured position, no: The act of measuring forces the electron to "materialize"...to "appear"....and disrupts it.

    The act of observation/measurement forces the electron wave (described by the Schrodinger wave equation) (the electron "cloud") to compress to some small position interval, where the various plane waves forming the Schrodinger wave exhibit constructive interference within that interval and destructive interference outside the interval.....so the act of measuring "disrupts" the pre existing electron wave behavior....
  14. Jul 19, 2011 #13
    I don't believe you're accurate in saying the electron's evolution is not continuous. The presence of d(wavefunction)/dt in the Schroedinger equation is evidence enough of the mathematical continuity of the electron's wavefunction. Measurement of orbital state, in the form of stimulated absorption/emission of radiation, does affect the system, but not in a discontinuous way: the interaction potential between the electric dipole and the external oscillating electromagnetic field is sinusoidal, which is continuous. So I don't see where any discontinuity can arise.
  15. Jul 19, 2011 #14
    "the interaction potential between the electric dipole and the external oscillating electromagnetic field is sinusoidal, which is continuous"

    That's a classical, approximate, interaction....

    Can you take the time derivative of a step function?? (answer: yes)

    So how do you explain the emission or absorption of an energy QUANTA?? Or superposition??
    Last edited: Jul 19, 2011
  16. Jul 19, 2011 #15
    You are implying there is an inconsistency between continuous evolution of the wavefunction, and discrete energy states? I don't believe there is one so to me this question is incomplete.

    Energy quanta arise from finite stationary orbital states (corresponding to finite energy levels). Finite number of states does not mean discontinuous wavefunctions, precisely because it can exhibit a superposition in moving from one to another. During this superposition its energy eigenvalue is undetermined because it's changing - in a sense it can be "half lower energy and half higher energy" without actually having an energy in between.
  17. Jul 19, 2011 #16
    I see what you mean.....I think I agree with second part of your post #15...so maybe we, or I, am into semantics ....maybe an "expurt" will clarify.......
  18. Jul 19, 2011 #17
    You are speaking of discrete energy levels or discrete changes between orbits. I was only referring to a single orbit, as in a single energy level and discontinuous motion of a particle with boundary conditions that arise in an orbit as was mentioned in the OP.

    What he now knows does not disprove what he was taught.

  19. Jul 19, 2011 #18
    Wow! interesting discussion. Thank you guys :smile:

    Guess, it all depends on what observer's definition of 'real' is. Energy may be an abstract form but the effects of expending energy are observable and real. Therefore, I would say energy is real and Photons being a form of energy transfer, is also real.

    If it's not real, consider this:

    1. The things we call real are mere effects on our senses. We could very well be in Matrix type universe, where, as long as our brain is told it is seeing something, feeling something, smelling something....we believe that something exists in real.

    2. My TV and remote control are separated in space. If the capacity of the batteries in my TV remote to do work decreased a bit when I pressed a button and consequently(after ruling out other causes) my TV changed channel, why is it different to say something real traveled the space to make this happen?

    Correct me if I am wrong, but Schrodinger's wave equation is all about probability. Describing something with probabilities -- unless it is 0 or 1 -- is mere opinion. It would be fair to say, the electron's state between observations is indeed unknown but predictable (may be better than your weatherman predicts the weather).
  20. Jul 19, 2011 #19


    User Avatar

    I see I've managed to pull this discussion towards pure metaphysics ;)

    I've quoted A.Z. (who is actually close to my personal metaphysical taste) and Mermin commenting him ironically (but I respect Mermin view and I agree with his thoughts even more often, than with Zeilinger's), just to picture how metaphysical are such disputes.

    You are perfectly right: it all depends on our definition of 'reaityl' is. This question remains open for at least 2000 years. And it remains an open question, as it is not about any experimentally verifyable property of the word, but rather lexical: about our definition of the word: 'reality'. Unfortunately millions of people tend to assign their intuitive meanings to that word, and then go into quarrels, rather that finding common meaning of words or just abandon usage of ambiguous words in disputes, where strict meaning is important.

    Yep, Schrödinger's equations is about probability. But there is a difference between an outcome of double-slit experiment and the outcome of thunderstorm prediction for tomorrow. Outcome of double-slit experiment is fundamentally unknown. It is realized (making real, the value is assigned to the variable) at the very act of measurement. But The weather tomorrow is not quite certain just because our prediction techniques and input data they require are not precise enough.
    Anyway - I just saw the lightning, and the forecast predicted a thunderstorm to be starting at 2 AM (I live in Central Europe - it is 23:19 - it came 3 hrs too early)
    Last edited: Jul 19, 2011
  21. Jul 20, 2011 #20


    User Avatar
    Science Advisor

    I would say that an 'orbit' is by definition a classical trajectory, specifically a closed trajectory.

    There's a reason electron states in an atom are referred to as 'orbitals' and not 'orbits'.
  22. Jul 20, 2011 #21
    exactly..."orbit" is a classical, continuous, crude concept analogous to planet orbits...
    "orbital" is the wave picture, the electron cloud interpretation. And is the one illustrated in my Wikiepdia link, post 8.
  23. Jul 20, 2011 #22


    User Avatar
    Staff Emeritus
    Science Advisor

    Science is my definition of reality. I can't speak for anyone else though.

    Nonsense. The weather tomorrow is as exactly as unknown as the exact position and momentum of each and every particle that makes up the thunderstorm and everything that interacts with it from now until tomorrow. Fortunately we are not trying to determine the position and momentum of every particle to an absolute, but to a much less accurate measurement that we can use to tell whether it will rain or not tomorrow. The biggest reason we cannot predict the weather is the sheer number of variables that we have to compute along with the number of said variables that are simply unknown.
  24. Jul 20, 2011 #23
    no...all I tried to say is that discrete quanta energy exchanges are DISCONTINUOUS events, incompatible with continuous classical trajectory orbit descriptions.
  25. Jul 21, 2011 #24
    It IS all about probability...but scientists would say it is NOT "opinion" it is all that can be known because that's the nature of quanta.

    I don't believe that's an accurate description, but we may be into word semantics. "unknown" is never a description of the Schrodinger description of quantum states I have seen...

    Here is how Albert Messiah explains it in QUANTUM MECHANICS:


    For another explanation see Wikipedia here:

    and note one example:

    See if you think all the above means "unknown".

    PS: thinktank..it's obvious from your comments subsequent to your original post that you have read what has been posted in reply...and learned...some have a hard time doing that....seems like you came a long way in this discussion....with QM sometimes it a matter of "The more I know the less I understand." or as Richard Feymann says "Nobody understands quantum mechanics."

    If you are interested look at the derivation of the Schrodinger equation here:

    That it did NOT come from first principles, but rather empirical [experimental] observations leading to "hypothesis" and "postulates", means to me we don't know WHY all this fits as it does, but we can explain observations and make predictions with the quantum theory.

    So don't be too hard on your HS teacher!!
    Last edited: Jul 21, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook