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Electric field , photons, charge

  1. Jul 8, 2013 #1
    Now another question ,
    We have two types of charges , positive and negative ones.
    The field between them (electric field) is said to be also positive and negative , but the particle that mediates the electric and electromagnetic field is photon which has no charge.(chargeless)

    Now the individual particles like protons and electrons have charge themselves but the corresponding photon made field doesn't.
    The thing I don't understand is is photon is only one of a kind without a charge then how does it know when two protons need to be repelled because they are of the same charge and when a negative charged particles needs to be pulled towards a positively charged one as they tend to attract.But in both case this interaction is mediated by the field which consists of photons and photons are only one kind so how do they decide how to act in each of these two situations?
     
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  3. Jul 8, 2013 #2

    sophiecentaur

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    No. Field is a vector, with a magnitude and a direction. It is the size direction of the force on a unit charge. It is also describable as the gradient of the Potential. That gradient is negative (repulsion) for like charges and positive for unlike charges (attraction).
    PS you need to be careful when you use the word "mediated"? The virtual photons involved in this 'mediation' are not the same as what you see when light enters your eye. Sort out the classical picture first and then move on to the clever stuff later.
     
  4. Jul 9, 2013 #3
    Yes I do know that the photons for static electrical fields are not the ones which hit my eyes every now and then when I get blinded by the light , but the questions was rather not about the particles themselves but the field , as a field has no charge it's just a field , yet the charged particles which interact with the field have polarities.

    Ok I'm not sure I quite understood what you said , if I imagine a picture where an equal number of + and - particles of equal strength of their charge are put together in a given size container then the like ones repel and the unlike ones would attract and they can do that because they find their way to the other particle because of the field , yet these interactions take two ways but the field is only one.So this was the part I had problems with.
    If you could elaborate a little on the part that deals with what's so specific about the field that it can do this sorting out between charges of particles which interact with it.
     
  5. Jul 9, 2013 #4

    BruceW

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    https://en.wikipedia.org/wiki/Static_forces_and_virtual-particle_exchange
    It seems like they are saying that the virtual photon is related to the energy change due to some disturbance. And this energy change is negative for an attractive force and positive for repulsive. I think you can say that the virtual photon has 'negative energy' when the interaction needs to be attractive, and has 'positive energy' when the interaction needs to be repulsive. The virtual photon is off mass shell, i.e. I think that it does not have to keep to the usual rule of positive energy because it is a virtual particle. Also, important to remember is that this idea of virtual particles... they are not like real particles. And I think some people don't even like using the word virtual particle. From the website, it looks like the interaction just spontaneously 'happens' between the two real particles. So it is not necessary to think of the photon as carrying information between the two... And in fact, it cannot carry actual information, since it is moving faster than light. So I think this idea of virtual particle is to be taken with a pinch of salt.

    disclaimer: I have no knowledge of quantum electrodynamics, I am just making a guess from the little bits that I have read. hopefully soon I will properly learn the subject.
     
  6. Jul 9, 2013 #5

    sophiecentaur

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    Those Feynman diagrams have a lot to answer for. People interpret them so literally. They see that squiggly line and come to quite unjustified conclusions about it represents. That squiggle is just a SYMBOL and not a picture. When two particles are deflected in their paths, you can think classically of two fields pushing against each of them. The field's amplitude varies in time. That is an EM wave. Ergo you can say that photons come into it. But that's just qualitative arm waving but photons don't go anywhere and does anyone actually model a photon as a wave? Dodgy ground, I think and miles away from this level of discussion.
     
  7. Jul 9, 2013 #6

    BruceW

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    haha, not a big fan of the term 'virtual particle' then? yeah, I can see where you are coming from on that one.
     
  8. Jul 9, 2013 #7

    sophiecentaur

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    Feynman seemed to have a bit of thing about photons having to be particles. Far be it from me to criticise him but I think that was a bit of a human streak and not as rational as most of what he said. After all, it's only a name and he used to get quite shirty about it, istr.
     
  9. Jul 9, 2013 #8

    BruceW

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    hmm. I guess propagator is a better word than virtual particle, since it doesn't suggest any notion of any kind of 'thing' moving around at super-luminal speeds. In some ways, the virtual particles are like particles, but in other ways not. Maybe Fenyman was just eager to have some kind of analogy for these propagators, rather than just leave them as a mathematical concept.
     
  10. Jul 9, 2013 #9

    sophiecentaur

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    Yes. But I have heard him say that they have to be particles because blah blah (memory lets me down) and it's not just waves involved. Well, the last bit has to be right.
    He had one enormous problem, of course, and that was that he was very very clever, had an ego the size of a house and I really don't think he could empathise with people with even slightly less intellect than his. He assumed people got what he got. And we didn't always / ever.
     
  11. Jul 9, 2013 #10

    BruceW

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    hehe, really? he did those famous lectures, so he must have had at least some capacity for empathy. But then those lectures have been judged as being not so successful. I think because he taught slightly unorthodox ways to understand the physics, that they were bound to be not so helpful to the 'Joe Bloggs undergraduate student'.
     
  12. Jul 9, 2013 #11

    sophiecentaur

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    He was very charismatic and people left his lectures with that 'understanding' feeling. I bet some of it leaked away soon after - when they tried to explain what they'd learned. Their tutors would have needed to perform lots of 'repair work' before they could actually pass their exams. I must say, I would have liked to have seen his lectures in person - I was the right age but wrong nationality (and not bright enough to get on those courses I think).
    The same thing happened to me, though, after a lecture by Herman Bondi in 1966. He made perfect sense and everyone enjoyed the guest lecture. Interestingly, his Cosmology ideas have since been dumped in favour of the BB.
     
  13. Jul 9, 2013 #12

    BruceW

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    cool. Well, even though Bondi's cosmology ideas were dumped, I'm guessing that's just because of the empirical evidence that came after those ideas. That's just how science goes, right? We think up several ideas, and the ones that survive are those that best match with observation.

    I don't think I have ever had a lecture where I got that 'understanding' feeling then some of it leak away... Probably because I have only been to lectures given by people in my own university. I have definitely been to lectures where I thought I understood it, but then tried to use the concepts in some problems and realized I actually don't know what it is about. But that's a slightly different thing I think, hehe.
     
  14. Jul 9, 2013 #13

    sophiecentaur

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    When I was a lad, the CMBR evidence was only just coming through and 'Steady state' is a very satisfying model to go for. Bit of a bummer for Bondi and Hoyle, though.
     
  15. Jul 9, 2013 #14

    BruceW

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    yeah. steady state would have been nice. Even nicer is this 'chaotic inflation' that I have read a bit about. It does not require a big bang either. And it pretty much erases any problem of 'why the fundamental constants', because there are many regions which cool down out of the quantum foam. And in each region, there will be a different set of fundamental constants, therefore we can use anthropic principle to explain why our region got these fundamental constants that happen to be well suited for life. But I think chaotic inflation is not so well-established as the usual 'standard model' of cosmology. Pretty interesting though.
    Andrei Linde "Particle physics and inflationary cosmology" http://arxiv.org/abs/hep-th/0503203
     
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