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Do I Genghis Khans' electrons?

  1. Feb 18, 2013 #1
    I am wondering if there is an exchange of electrons in the flow of electricity. If I shove a fork in an outlet (i'm not going to do this) will electrons enter my body? Am I "absorbing new electrons" ? If so, where did they come from. I know how power generation works, but did those electrons have a life prior to working for the city electric company?
     
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  3. Feb 18, 2013 #2

    Drakkith

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    Yes, electrical current is the flow of electric charges through a material. In standard power lines the charges are negatively charged electrons that existed in the line and generator. If you stick a fork in a socket and get a shock electrons are moving into and out of you. In other materials, such as inside batteries, current can also be ions, which have the opposite charge of an electron.

    Practically all electrons were created about 10 seconds after the big bang, after which the universe had cooled off enough for the creation of new leptons (a class of particle electrons belong to) to cease.
     
  4. Feb 18, 2013 #3
    Electrons are identical except for their position and spin. They are indistinguishable and, the way that the statistics works out, you cannot attach labels to the electrons and say this electron came from here and this other one came from there. Depending on your philosophy, you could maybe estimate the probability that some of your electrons came from Genghis Khan (probably about 1), but in reality, there is no way to tell so the question doesn't really have a true meaning.
     
  5. Feb 18, 2013 #4

    Drakkith

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    I take this to mean that if you have two electrons and you let them mingle amongst themselves for a bit and come back and measure them, you cannot tell which one was which since they are identical.
     
  6. Feb 18, 2013 #5
    I take it to mean that its not sensible to attribute any of them to Genghis Khan or not. They cannot be labeled as "his" or "not his" thus such a label fundamentally does not apply. There is no such thing as "which one was which", they simply are and they dont have an individual history or hysteresis.
     
  7. Feb 18, 2013 #6

    Drakkith

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    I don't think I agree. This sounds like you're saying that an electron that is launched from an electron gun cannot claim to have a history, so how could it have been launched from the electron gun? Furthermore, if you extrapolate this out to atoms and molecules I'm pretty sure we can track where they've been.

    It seems more likely that what is meant is that they are identical in all aspects, so once you shove a bunch in a bottle or something and stop observing them you cannot then claim to know which one did what in the past.
     
  8. Feb 18, 2013 #7
    The electron gun did something. This results in us observing an electron with a velocity. I dont think it makes sense, fundamentally, to say that was "the" electron that was in the gun. Maybe it makes intuitive/colloquial sense, but not physical sense, because there is no physical marker or label that tells us which one it is. We just say that it is "the" electron from the gun because we observe the velocity.

    I see it as akin to tracking which dollars are which in a bank account. It makes no sense to label them as such. You can transfer a dollar from one account to another, but there is no fundamental ability to distinguish which dollar was there before and which came from the transfer. You can only observe the dollars and the history of transactions, individual dollars have no history.

    I dont really know much about physics though, this is just the impressions I got from classes and real experts. So maybe you are right... I dont know.
     
  9. Feb 18, 2013 #8

    Hurkyl

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    There is a physical marker: it on the trajectory that was fired from the gun, whereas other electrons aren't.

    By symmetry it might make sense to say that electron could have been any electron in the universe, however we can still note that it is the same electron that is there is the same electron as the one that came out of the gun. (barring caveats such as tunneling or interactions with other electrons that might make this only mostly true)

    Much like in classical physics, due to symmetry, "position" has no meaning, but "position relative to other things" does.
     
  10. Feb 18, 2013 #9

    Drakkith

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    But we can single out an electron, trap it, move it around, and do all sorts of stuff with it without it ever interacting with another electron. I think it's safe to say it's the same electron that we initially trapped.
     
  11. Feb 18, 2013 #10
    Is that possible? I dont think so. I would say its "a" trapped electron, not "the" trapped electron.
     
  12. Feb 18, 2013 #11

    Drakkith

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    By "interact" I actually meant to say that it wouldn't ever get close to another electron like it would inside a material. Obviously whatever method we use to trap the electron is going to require interactions with other electrons at a distance.

    As for "A" or "The" electron, it makes little sense to say that the one and only electron you trapped somehow isn't the same one five minutes later or that the distinction somehow doesn't apply.
     
  13. Feb 18, 2013 #12
    Well, now Im thoroughly confused. lol
     
  14. Feb 18, 2013 #13
    You can follow an electron following a classical trajectory from an electron gun, but this is only a classical approximation. Particles only move in straight lines in the classical approximation. In quantum electrodynamics, you cannot know exactly what path an electron took from point A to point B, because the probability for an electron to go from point A (such as the front of the gun) to point B (some target) must take into account every possible path when calculating the probability amplitude. The most important contribution comes from the obvious straight line path from the gun to the target, but there are all sorts of weird paths (such as the electron bumping into "another" electron and the "other" electron moving to B, or the electron annihilates with a positron, forms some photons which create an electron and positron pair, of which the electron goes to B). The point is, since there is some small but nonzero contribution to the probability which comes from the electron switching places with an electron in you, you cannot say where the electron is originally from.

    The approximation can be very good if you trap an electron, because trapping the electron means that the probability amplitude of the electron swapping with another electron outside the trap is very small.

    But, if you trap two electrons, you can forget about labeling them 1 and 2. You could put one in a spin up state and one in a spin down state, and they will be "mostly" distinguishable for a while, but if they are given the opportunity to switch spin states then they are two units of the same thing. This isn't simply a matter of human ignorance. The quantum statistics actually state that they are indistinguishable even in principle. See, for example, http://en.wikipedia.org/wiki/Indistinguishability
     
    Last edited: Feb 18, 2013
  15. Feb 19, 2013 #14
    Makes sense to me. If you cannot create even an idealized experiment to test whether an electron was in Genghis Kahn then it is not a scientific question to ask. There is no quantum number that is such a label. And if you could, in some idealized setting, distinguish between having G.K.'s electron and not having it then that would be catastrophic for our statistical theories which do so well at providing band gap data, etc. Otherwise, any claim of being able to tell which electron is which is not science, its philosophy or personal belief projected onto scientific theories. Its not testable, no experiment can support it. Claims that go beyond the need to verify and predict observations are thrown out as superfluous via "Everything should be made as simple as possible, but no simpler."
     
  16. Feb 19, 2013 #15
    I'm going to have to go with Khashishi on this one.

    Consider the double slit experiment with electrons. You can fire electrons through the screen 1 at a time and still get an interfearance pattern which means the each electron went through both slits. Now how much sense would it make to fire them 2 at a time and ask which electron went though slit A and which one went through slit B? If a single electron can co-exist in 2 places at once, as demonstrated by the double slit experiment, I can't really say if a particular electron in my left eye-ball is not the same electron that some MIT student has in an electron trap somewhere.
     
  17. Feb 24, 2013 #16
    So on Genghis its a no? I reasoned that by laws of conservation there is only so much mass. Electrons have mass, so there is a finite number of electrons. If so, then they have to be shared. I know that this only applies in a closed system, so maybe I can't assume that (mostly) earths electrons stay on earth? But the question still remains, after dispute, about sharing electrons through contact. If you and I shake hands is there an electrical reaction? Is there a even molecular exchange, beside the BBQ sauce still on my hands from lunch? Do you walk away with a little bit of tempneff?
     
  18. Feb 24, 2013 #17
    Electrons are indistinguishable. Yes there is a finite number of electrons but they are *EXACTLY THE SAME* except for position, energy, angular momentum and spin. It is not possible to tell 2 electrons apart.

    You are thinking about this *classically*. You are thinking that electrons are hard balls that are either inside a system, or outside a system. But electrons are not classical. They are described by a wavefunction.

    It is hard to explain this without math so lets take an example. You have 2 electrons running into each other head on when launched from electron beams directed towards each other from beams A and B. They elastically scatter off each other and hit detectors C and D. Can you tell whether A hit C and B hit D, or A hit D and B hit C?

    Well, how can you tell? Electrons from A and B are identical, there is absolutely *NO WAY* to tell whether it was A that hit detector C or B that did. In classical physics you could paint a ball a certain color and optically track it, but in QM, electrons aren't even *things*, they're waves, so how can you paint a wave? And if you could track an electron optically, the light will knock it off its course anyways!
     
  19. Feb 24, 2013 #18
    But, something did hit D and C? Where do they get the electrons to 'shoot'? Have those electrons been transferred to the detector? Whether or not we know which is which, can we say there has been an exchange?

    I've only had classical physics so its hard to think of it any other way.
     
  20. Feb 24, 2013 #19

    A.T.

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    Believe me, you don't want to know.
     
  21. Feb 24, 2013 #20
    I would take this a step further.

    If there is a 50% chance that A hit C and B hit D, and a 50% chance that A hit D and B hit C, then those 2 options BOTH exist in superposition. They both happened.
    If these were macroscopic particles like marbles and you performed this experiment you could say that 1 event or the other happened, you just can't tell which one because the marbles are identical. With electrons you cannot say that. All you can say is that 2 electrons were fired and 2 electrons were detected. What happened in between is not just unknowable, it is a sum over all possible histories of the particals.
    This is just like the douple slit experiment where a single electron can travel through both slits.
     
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