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Question that’s killing me this evening…

  1. Jun 18, 2005 #1
    -We have one electron that bumps on one positron, they are particles of equal charge but different sign, between them we have attraction, culminating with annihilation.

    -We have one electron that bumps on one proton, they are particles of equal charge but different sign, between them we have attraction, culminating with creation of hydrogen atom.

    C’mon how, what electron “sees” in both cases is the same electric field of different charge and feels the same attraction force proscribed by Coulomb law.
    What X field and X force makes electron to act different in this two cases, what?
    OK proton has larger mass than electron, but it’s certainly not gravity, spin of both proton and electron/positron is s=1/2 so no difference there.

    OK intuitively I would say that electron will not annihilate with proton because of that mass difference, but what field/force makes that electron to spread through space we call 1s orbital, why doesn’t that happens when electron meets positron ? What’s that difference that I’m missing here ?
    Last edited: Jun 18, 2005
  2. jcsd
  3. Jun 18, 2005 #2
    You may be right that the effect of gravity can be ignored for all that practical reason. But, have you considered the fact that the mass of proton is 3000 times greater than the positiron? So the two cases you're looking at isn't really showing you the kind of symmetry. For example, when studying hydrogen atom, you will have to 'assume' the proton is fixed at the center. For the case of positron, this may well not be assumed.
  4. Jun 18, 2005 #3
    You really need to take a particle physics class, but here is the short story:

    The positron is the anti-particle of the electron. They have all the opposite quantum numbers except spin. They are exactly the same except for these quantum numbers being opposite. Same type of material, etc.

    The proton is a baryon, while the electron and positron are leptons. Conservation of baryon number and lepton number does not allow the reaction:
    Note: Below I will use the following: [itex] e^- [/itex] is the electron; [itex] e^+ [/itex] is the positron; [itex] p [/itex] is the proton.

    [tex] p+e^- \rightarrow \gamma 's [/tex]

    since on the left hand side we have one lepton and one baryon and on the right hand side we have none of either.

    The reaction:

    [tex] e^+ + e^- \rightarrow \gamma 's [/tex]

    is allowed since lepton number is conserved. There is one lepton and one antilepton on the left- giving a net lepton number of zero. And the right hand side has no leptons.

    So you can think about it as nature obeying certain conservation laws which tell you what kind of reactions to expect. This is similar to conservation of energy or angular momentum, etc.

    Hope this helps.
  5. Jun 18, 2005 #4


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    The mass of the proton is approximately 1837 times the mass of a positron...How about you eat some physics,too...?

  6. Jun 19, 2005 #5
    I think its a really good question and I dont think that mass alone explains this, its not a law of nature that things with different masses cant interact in otherways than that of electron and proton.

    EDIT: added word mass
    Last edited: Jun 19, 2005
  7. Jun 19, 2005 #6
    Here is a start of explaination:In both cases, we have an intermidiate state: the H atom and the positronium atom.
    Therefore, the question should be why positronium is not stable while the H atom seems to be stable.

    Norman has given a piece of answer: the interaction.

    Electromagnetic interaction allows the transition of e+ + e- --> photons (relatvistic effect: requires the dirac equation). The energy-momentum-spin conservation and the lepton numbers are conserved in this reaction including solely the electromagnetic interaction. => ground state of positronium is not stable.

    Now why H atom is stable (without an external help): once again: the electromagnetic interaction does not allow the reaction: p+e- --> new particle(s) (problem of conservation of either the energy-momentum-spin or/and the lepton and baryon numbers by the em interaction).

    In addition, when we introduce the electroweak interaction, we still have the same problem: p+e- --> n + nu is not possible in the H atom without external help (mainly the problem of energy conservation ) => H is stable because the energy of the neutron is greater than the energy of the proton plus the ground energy Eo of the H atom.

  8. Jun 19, 2005 #7
    Actually when the electron meets the positron, it does form a kind of 1s orbital -- the electron-positron pair become a positronium atom, which has some similarities to the hydrogen atom. Unlike hydrogen, however, positronium is unstable and rapidly decays into photons.
  9. Jun 19, 2005 #8
    Thank you all.
    I feel much easier now, knowing about positronium creation. That fits like a puzzle in my imagination or understanding of these phenomena. Norman was talking about conservation of baryon and lepton number, I’m sorry because my knowledge doesn’t involve this topics (-quantum p. layman), but as I remember Schrödinger’s equation doesn’t deal or include this terms, numbers and limitations arising from those laws, and still it theoretically predicts existence of 1s orbital at hidrogen.

    Now knowing all this you’ve sad, it seems logical to me that electron or positron goes into spiral from 1s orbital to annihilation positron/electron in positronium. And it seems to me that 1s orbital isn’t that stable as I thought so.

    Even now knowing about restrictions Norman presented to me, I still don’t know anything about real carrier/executor of that restriction (field, force or...) that presents and executes this phenomena.
    Last edited: Jun 19, 2005
  10. Jun 19, 2005 #9
    "You really need to take a particle physics class, but here is the short story:"

    I’d love to but I’m something like a med student ;)
  11. Jun 19, 2005 #10
    From what I've learned so far of particle physics, NOBODY really understands why lepton number, baryon number, etc. are conserved. Someone will probably point to Noether's theorem and say that conservation of these quantum numbers is due to certain gauge symmetry relations. But that just substitutes one abstract, phenomenological concept (gauge symmetry) for another (conserved quantum numbers). It still leaves unanswered the question "what is an electron?".

    Be warned, this kind of question is not well received in the physics community, and the usual reply is "shut up and calculate".
  12. Jun 19, 2005 #11
    Simple. Anything which has the properties and only those properties of what we define an electron as.
  13. Jun 19, 2005 #12
    we cant define an electron! This question has no answer in physics,, its shut up and calculate as previously mentioned.
  14. Jun 19, 2005 #13
    Spin 1/2, lepton number -1, charge -1, mass of 0.5MeV...
  15. Jun 20, 2005 #14
    You speak as if the electron were an immutable, permanent thing, as in the original meaning of the term "atom". But this is not what is observed.

    Take muon decay for example -- at one instant you have a muon, something that does not meet the common definition of electron-ness, and in the next instant you have an electron (and some neutrinos) that has taken its place. The electron and muon may not be such fundamental objects if they can change back and forth into each other. Perhaps they are different states of the same object, whatever that is.
  16. Jun 20, 2005 #15
    The definition content is permanent. The physical object may change.

  17. Jun 20, 2005 #16
    This is my opinion aswell, its the electron that defines the definition not the other way arround.

    True! its not FALSE that electrons have arround 1837 times the mass of electrons! Its rest mass not gravitational mass we meassure and as we all know mass gives inertia.

    Miguel what are you trying to tell us ? This isnt physics its philosophy with pieces of inaccurate physics facts.
  18. Jun 20, 2005 #17
    This is blatantly false. Electrons and protons don't attract? Please... This is just an attempt to package and sell his (what I can only assume based upon this posting) crank in print form. It is so frustrating that people who seek legitimate advice and help on understanding some concept have to deal with this [expletive removed]. This post is no better than the posting of a conversation bot.
  19. Jun 20, 2005 #18


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    When you encounter posts like this, do not "feed the trolls". Use the "Report Bad Post" button to alert the Mentors.

  20. Jun 21, 2005 #19
    That's not quite true. If there is a box containing all identical properties and the box contains many particles, all of which have all the same poperties all of whose properties are identical to each properties then in the box then you can say that all of he partiess are of the same. If two properties share all the properties of an electron then that particle is an electron.

  21. Jun 22, 2005 #20
    yes, but he electron has an exisistance without the definition of it! There might be things we are missing if we say "an electron is defined as this.." perhaps we dont know it all yet. Say someone discover some new quantum number that describes the electron what then? Should we change the definition or say that its no longer an electron?
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