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I Particle Exchange Forces

  1. Sep 10, 2016 #1
    So sorry to post what seems to be a well worn question. So the 4 forces of nature work by "particle exchange" ? Yes we understand the maths of "Fields" and we have the elegance of Maxwell and Newton/Einstein and these mathematical descriptions lead us to force exchange particles which can actually be predicted and detected. Whether these exchange particles are Particles or Waves (Photons, Virtual Photon, Long Wavelength Photons..................) my question is how do they create an attractive force ? Or is the physics of these realms now solely the domain of the mathematician ?
  2. jcsd
  3. Sep 10, 2016 #2
    Welcome to the physics forums.
    As far as I know it is not the force is being exchanged, the force is applied to both. The particles are exchanging information through other particles.
    Even if not, a force is a vector, so essentially you can have a photon emitted that tells the other particle what to do. You can fire a photon that says "come to me" and a photon that says "get away from me".

    No realms are bound to mathematicians, physicists, surprise surprise, have to deal with quantum mechanics not just mathematicians. But quantum mechanics is so heavy on maths it is often really hard to communicate with the layman. When a physicist and mathematician who is familiar with the physics behind it, see this math and it explains them very well, often better than words, what is going on in those realms.
    For example, the best explanations for the layman about the higgs boson is a party of particles where higgs is the most popular which gives mass to others, the higgs field is like a liquid that makes friction etc. You just can't explain it with words to make sense of it.
  4. Sep 10, 2016 #3
    Hi DB - many thanks for your answer and your welcome.

    I got to say, it's difficult to accept your answer above although I do like it because it implies we could create force fields if we could figure out how to generate these "message photons".

    I think we need some theory that explains how a wave or particle or (small) wave-particle or particle-wave can create an attractive or repulsive force. Maybe it exists within the realm of Quantum Chromodynamics but is unknown to nobodies like me ?

    Probably more likely that ElectroMagnetism, Magnetism and Strong/Weak Nuclear forces are some sort of variation (I wont say distortion) of the space/time fabric as Gravitation is - and, therefore, Exchange Particles are not necessary at all ? We can't detect Gravitons ? We can't detect the photons associated with magnetic fields (or can we ?) ? so maybe they don't actually exist ?

    But that's got to be wrong cos electrons and "electron clouds" do actually hold all readily manifest matter together ? So am I looking for a "Photon Cloud" that surrounds Magnets with some similarity to an electron cloud ? Need to get my head around that first I think !

    Thanks for your time

  5. Sep 10, 2016 #4
    You could get your head around some of the principles in mechanics, and move on to maxwell's equations. Unfortunately without some tough math it is hard to answer, at least for me, maybe someone else in the forum could answer that properly.
  6. Sep 10, 2016 #5


    Staff: Mentor

    I once asked the same question. Here is the answer I got on PF.

    That link will take you to the true answer. Be warned though, when I read that answer my brain melted.
  7. Sep 11, 2016 #6

    Simon Bridge

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    It is an ill wind that blows no minds - pleased to be of service :)
  8. Sep 11, 2016 #7
    Muchos appreciados Anorlunda and Simon Bridge - the article you identify certainly seems to address my question - the only place I have found it directly addressed.

    At gross risk of mind melt I am reading it.

    Once again, thanks a lot

  9. Sep 11, 2016 #8

    Simon Bridge

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    It may be useful to turn it into an "insights" article on PF, with better graphics ... let us know if it helps or where it doesn;t quite work.
  10. Sep 13, 2016 #9
    Hi Simon
    Had a few goes at that article - it can mess with your mind (as in making it feel inadequate) but it's the ONLY thing I've seen that ever tries to explain how particle exchange forces (and there are no other forces) work.

    I can't say I totally buy it because if it were "proven" or "hypothesised" then there would be a robust mathematical theory (like Relativity or like QM or QCD or QED) laying it out rigorously and I would have been referred to it by now ?

    As I understand it so far from the article it relies on a Feynman interaction and momentum exchange and to do with the properties of the exchange photons as to whether the force is attractive or repulsive ? If it is to some degree random then it's hard to see why these forces are always either repulsive or attractive ?

    I'm working on it though.

    What sort of wavelength are we looking at for these exchange photons ?
    Does a magnetic force have infinite range ? (like gravity) - I'm guessing the immediate answer is yes ?
    Maybe gravity and EM forces don't have infinite range after all and that explains the theoretical necessity for Dark Matter and Dark Energy under General Relativity as is ?

    Sorry to keep asking questions borne out of a depth of ignorance.

  11. Sep 13, 2016 #10

    Vanadium 50

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    And this is the problem. The story you have been told is sort of a cartoon picture. It's popular because it's "easy to understand", by which I mean "it's easy to understand things that are not true". It's a story that gives you a little flavor of the truth, and it can be told in five seconds instead of five years. Your choices are to either accept it as a cartoon picture and realize it's not exactly the truth, or to spend the years doing the work to really understand.

    To answer your question directly, these photons are not real, so they don't have a wavelength.
  12. Sep 14, 2016 #11

    Simon Bridge

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    See post #10 as well... this is where your enquireys start to need more of a physics lesson approach.
    Remember, if we could explain it all to you using common sense type approaches, then we would not need quantum mechanics at all and there would be no such theory The virtual particle model for force is something students seldom meet in detail before they graduate... you are asking post-grad level questions: there's a reason it take most people 3+ years to get to this stage.
    There is one - it is called "the standard model of particle physics" and it comes from Field Theory which includes wave mechanics and QED.

    They are not always repulsive or attracting ... at "random" is not the same as saying that just anything can happen: some things are more likely than others. What sort of photon is possible depends on the situation ... in this case, two charged particles with some momentum is one state, and two particles with other momenta and a photon is another state, and two charged particles with some other momentum is another state. The QM tells us about the probabilities of transitioning between the states. We like to limit inquirey to specific subsets of the set of all possible states depending on what we are interested in, but quite a lot of stuff is possible, most of which averages out.
    One thing to get out of is the idea that the virtual photons are somehow fired from the charge ... that's not the right picture.
    Like vanadium says, there is a lot of cartoony handwavey stuff in the explaining.

    Have I given you a link to Feynman's QED lecture series?
    ... watch all of them since he does a better job than most at describing this stuff. Be warned, itis not intended for a lay audience: the lectures were delivered to staff and grad students at the University of Auckland NZ. However, he is not expecting an audience conversant with the ideas or the rigorous maths.
    One of the lectures shows you how the photon statistics model works for reflection, I think it will help you here.

    ... I suppose I could find you a post-grad lecture series on virtual particles if you like............. that would count as directing you to the math laying it out rigorously.
    Probably nobody has yet because they are trying to pitch to their perception of your math background. Would you like the real maths?

    Depends on the momentum being transferred.
    But these are virtual photons - they exist primarily as a step in a calculation, a bit like the steps in calculating a long division.
    Or, for example, you may want to bake a small cake but only have a recipe for a huge one ... so you take the amounts of ingredients for the big one and, say, divide them in half 3 times (for a cake 1/8th the size). At each step the mathematics represents a certain amount of ingredients, but when you come to make the actual cake, you don't actually gather all the ingredients and physically half it etc. The amounts of ingredients in each step are virtual ingredients.
    If someone asked you what the virtual ingredients cost, you know, the ones that didn't make it into your cake, you'd look at them funny right? Same sort of thing with asking what the wavelength of a virtual photon is.

    However, QM is not like that. In QM, the steps in a calculation that look like virtual particles can be tested for in particle accelerators - they, or at least the real versions of them, have been detected. be clear on this - the virtual particles are never detected.

    There is a decent illustration of the principle in the lecture series above - the reflection part.
    There, an experiment is set up where there is a source and a detector and a mirror and we consider those photons, from the source, that reach the detector via the mirror.
    The classical law of reflection is ignored, and a calculation done involving the possibility that photons may reflect from any part of the mirror to derive the classical law of reflection as an average over all possible paths. All the non-classical paths are "virtual" in a sense.
    So each individual path used is just maths right? A photon wouldn't really follow the non-classical path right?
    This can be tested - and it turns out that it is possible to make the reflection brighter by removing most of the mirror... this only works if there is more to this idea than "it's just steps in a calculation". On the other hand, I cannot expect a reflection from putting a mirror just anywhere ... what this tells you is that the cartoon description we are handing you as a stop-gap is incomplete.

    The immediate answer is "sort of" ... the range of the electromagnetic interaction itself has no absolute maximum - however, there are lots if charged particles so the vast majority interact with the ones nearby. This is how most everyday objects do not have much of a charge even though they are composed of charged particles ...

    There are people who like to think that the electromagnetic interaction, by various means, explains "dark matter" etc. This is junk and can be safely ignored. This is something that has been thought of and discounted a long time ago.

    You don't even need to go to QM to get there ... the gravitational force is much weaker than the EM one, however, there is only one kind of gravitational "charge" while there are two kinds of EM charge. The two EM charges result in fields that can cancel each other out on large scales, while mass is only additive on large scales. On the scale of, say, a room, the gravitational interaction is dominant over EM for pretty much anything in the room. So much so that the Cavendish experiment works.
  13. Sep 14, 2016 #12
    Hi Vanadium 50

    Grateful for your input here.
    I do realise it's a bit like a kid can keep asking "why ?".

    My questions now would be:

    1) The Exchange Photons are "not real" - therefore they can't be detected ? You can't have that - if they are part of a mathematical theory then physics must seek to show their existence otherwise they are simply a mathematical artifice and I return to my original question of "What is the mechanism of "particle exchange force" ? ". There must be one (unless it's magic ?).
    2) Physics spends a lot of time and money experimentally seeking things predicted by mathematical theories (e.g. Laser Beams, EM Waves, Atom Bombs, allsorts of Elementary Particles recently the Higgs-Boson, Time Dilation, etc, etc). Presumably these non-existent photons have similarities with Gravitons (as they are particle exchange force intermediaries) which we are currently spending a lot of money trying to detect ?
    3) The particle exchange force intermediaries must carry Energy ? (E = hv). Therefore they must have a frequency/wavelength (or, QM wise, a band of frequencies) ? If they are EM (as Photons) then they must fit into the EM Spectrum somewhere ?

    The beauty of a theory is that once someone has done it the rest of us don't have to spend years understanding it. Maxwell's Laws are taught to undergrads. Special Relativity is taught to undergrads. General Relativity too ?

    If I asked (100 years ago) what is the mechanism for someone speaking in London to be received by someone in New York you would say EM Waves as predicted by Maxwell's Laws. You wouldn't tell me to spend the years totally understanding Maxwell's Laws ? The mechanism, and intermediary, is now totally understood and we have benefitted massively.

    There is (must be) a mechanism for "force at distance" as there are 3 (or 4 or 5 ?) such forces known.

    Thanks again Vanadium 50.

  14. Sep 14, 2016 #13
  15. Sep 14, 2016 #14

    Vanadium 50

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    Fine. Believe whatever you like. What do I know compared to you? I only have a PhD in physics, whereas you've read popularizations! I have better things to do than to argue.
  16. Sep 14, 2016 #15
    Sure thing Doc

    You wanna see my CV ?

  17. Sep 14, 2016 #16

    Vanadium 50

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    Like I said, I don't want to argue. Good luck to you.
  18. Sep 14, 2016 #17
    Me neither Doc

    Just after a better understanding
  19. Sep 15, 2016 #18

    Simon Bridge

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    These questions were addressed in post #11 ... one of the questions that arises in QM is what we mean when we say that something is true.
    It may be that I have not been direct or clear enough ... but I did also ask questions to help clarify the kind of answers you need ... you have yet to respond.
    If you will not follow advise, if you will not answer questions, then nobody can help you. Good luck.

    True as far as it goes. It looks like you have been asking for the understanding part though. If all you want to know is how to use the theory without understanding it then we'll have to take a different tack to the one we've been on.

    General relativity is not taught with any rigour below post-grad level and students have to specialize. Same with Field Theory.
    The broad strokes of einsteins relativity is covered at secondary school level - all that stuff about how time is another dimension of space and rubber sheet analogy for gravity... this is the carton version. Senior secondary students can get the fact of special relativity with time dilation and length contraction becuase they have the maths for that, but will not get the kind of depth you seem to be looking for with particle physics. The point of the theory is that someone else figures out the detailed background so that engineers etc can just use the results without understanding where they come from. They just need enough of a picture to be confident in the tools they are using. They can use it without understanding.

    The results of the standard model for particle physics, which is what you are asking about, is covered as the kind of cartoon you have met at the senior level in secondary schools (in NZ) along with a working knowledge of emergent facts like blackbody radiation and atomic transitions, maybe some of the basic exchange model for forces ... but no detail of how the underlying theory works. QM wave mechanics is seldom introduced before stage 2 at University and students are required to have a working knowledge of linear algebra. Matrix mechanics is at third year, with perturbation theory ... Field theory and the underpinnings of the standard model are covered in detail at post-grad level.

    You would have received something like Einstein's cat analogy - a cartoon version. If you has asked about the underlying theory as you have done for particle physics above then you would have got something less cartoony but still no maths. If you insisted on the underlying details for that, but wanted to avoid the maths then you would be met with frustration and told to go study a college course on the physics of radio or maybe do an apprentiship with a radio company. It would be axplained to you that the kind of answers you seek require a background you do not appear to have.

    EM theory needed to understand wireless transmissions is taught, in detail, at the second year college level, and it takes a while to get used to it.
    Nobody will expect to be able to provide an understanding of the underpinnings of classical electrodynamics in an internet forum... so we do not try.
    What we do here is attempt to steer people in their perosnal journey - you go read stuff, ask about what you don't understand. We try to answer on the level that you ask questions and/or direct to further reading if you are interested in more depth,

    There are 4 fundamental forces in physics.

    GR models gravity as a local effect of the local of space-time - leaving the remaining three to be modeled by particle physics.
    Particle physics is also not an action at a distance model ... all this particle exchange stuff is local.

    So far these are the best, simplest, models available - but work is ongoing to improve on that.

    Note: there is nothing that says that their "must be" any mechanism for the classical action at a distance stuff, just like we do not need a mechanism for local action ... it may be that this is just how the Universe works. It may sit uncomfortably with us, many people prefer explanations in terms of causes and effects in close proximity in space and time, but Nature does not care what you or I or anyone would feel comfortable with.

    Now: where are you going with this?
    What are you trying to achieve?
  20. Sep 16, 2016 #19
    Hey Simon

    Appreciate your answers here. Really do.

    You gave me a lot of stuff to look at - not necessarily easy to digest - but I am working on it (as I said).
    So if you can be patient I will get back to you.

    I think I've asked my questions with the appropriate clarity but I will re-state:

    I would like to know how "particle exchange" creates attractive (and repulsive) forces.
    Specifically relating to magnetism (like a magnet picking up a pin) - force at distance.
    We understand (mathematically) magnetic fields EM waves via Maxwell.
    However, I have no clear understanding of what actually happens to create the attractive and repulsive forces at a distance.
    I would like to understand that.

    Incidentally there seems to be a misunderstanding here that I am kind of "dumb" mathematically.
    Incidentally there seems to be a misunderstanding here that I am kind of "dumb" of physics.
    This may or may not be true but I never said I was just looking for "cartoon explanations" - this has been an assumption on the part of the people to whom I am truly grateful for a contribution.

    The best superficial explanation I've seen is:

    Which kind of illuminates "virtual particles" as something of a badly named artifice.
    This suitably answers my questions as to what is their wavelength etc.
    In the wake of that article, I think what I seek is the mathematical theory demonstrating that "disturbances" in the "photon field" ("EM Field") ["cartoonly" referred to as "virtual particles"] - surrounding a magnet give rise to attraction and repulsion.

    Prof Strassler says it exists so I need to chase that.

    I apologise for not acknowledging your gratefully received guidance sooner - I was working on it though.


  21. Sep 16, 2016 #20
    I referenced 3 (or 4 or 5).

    4 because I am a little confused by weak nuclear or is it weak EM
    5 because there was some talk of a discovery/hypothesis of a 5th force recently.

    Gravity has infinite range (so they say) but I take your point about "local".

    My original question related to magnetism (action at 4 or 5 inches) is that the domain of particle physics ?

    Surely there is no "local" ? Action at a distance is action at a distance whether the distance is very very very large or very very very small ?

  22. Sep 19, 2016 #21
    Hi Simon (again)

    As promised, and due in large part to your guidance and recommended reading etc, I have ordered some thoughts relating to my original question:

    1) Magnetism is due to the action (and interaction) of electrons (as part of the ferrous metal) and the associated ElectroMagnetic (Photon) Field

    That kind of answers my original question but I'll summarise a few general things I have learnt before returning to it:

    2) The concept of "Forces" (action at a distance) and "Fields" are linked. Probably understanding Fields is more important than Forces. Fields always have an intermediary (which we may consider as part wave/part particle as per quantum mechanics). If we consider the (at the moment 4 fundamental forces) they might be summarised as:

    - Gravity Force - Gravitational Field (Intermediary is "Gravitons" which we seek but have not yet discovered)
    - Weak Nuclear Force - Boson Field (Intermediaries are W & Z Bosons - discovered and mathematically understood)
    - Strong Nuclear Force - Gluon Field (Intermediaries are Gluons - discovered and mathematically understood)
    - ElectroMagnetic Force - Electromagnetic Field (Intermediaries are Photons - discovered and mathematically understood)

    to this I should add (as there are similarities):
    - Mass - Higgs Field (Mathematical necessity for currently accepted theories of QM which explain much of the Standard Model and, so, are well accepted) - (Intermediary is Higgs Boson, mathematically predicted and now discovered).
    The Higgs Field is everywhere like some sort of Aether ?

    3) If a "wave/particle" such as an electron is in an EM Field (which we might also call a Photon Field, as it is composed of Photon wave/particles) then it creates a "disturbance"of this field. If there are 2 electrons close (let's not get bogged down in a definition of close) by each other then these "disturbances" interact and can create an attractive or (if 2 electrons) repulsive force (electron and positron). The mechanism for this can be TOTALLY explained mathematically by quantum theories. These "disturbances" are often misleadingly referred to as "virtual particles". This quantum maths theory accounts in full for attraction or repulsion forces of nature as far as we understand.

    So returning to my question about magnetism:

    Aligned atoms or molecules have orbiting electrons (domain theory)
    These flowing currents give rise to a magnetic field (
    Magnetic field is electromagnetic and therefore a Photon field (not a field of virtual photons)
    If the field interacts with other electrons (of the right type - i.e. within another ferrous metal) then a disturbance is created and gives rise (by the same mechanism as above) to the appropriate attractive or repulsive force that we know as Magnetism.

    The only bit I'm worried about at the moment is whether the photons forming the magnetic field are of one particular type ?
    Do they have a fixed energy or frequency ?
    Whereabouts in the EM Spectrum do they fit ?

    I'm sure there'll be lots of errors in the above so I'm happy to be corrected.

    Thnxalot Simon for your help

  23. Sep 19, 2016 #22


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    That's right... and we have many many threads about this. You might try the Insights article: https://www.physicsforums.com/insights/misconceptions-virtual-particles/

    Why must there be a mechanism? Chances are that you didn't insist on a mechanism when you first encountered Newton's law pf gravitation.... That law just says that the gravitational force between two masses is given by ##F=Gm_1m_2/r^2## but says absolutely nothing about why this should be. We might say that it's the gravitational field at work, but that's no answer at all because there's still no explanation of why masses create gravitational fields and feel a force from them. Not having a mechanism didn't stop us from using Newtonian mechanics for centuries, to solve everything from planetary motion to dropped objects to the trajectories of artillery shells.

    The situation with quantum electrodynamics is different only in that the mathematics are much more complex.
  24. Sep 21, 2016 #23
    Hi Simon and Nugatory

    As promised (Simon) I've been trying to follow up the reasoning on this and I'll try and summarise the understanding I've ended up with:

    A magnet has a magnetic field (no great insight there)
    It exists because all the little magnets ("orbiting" electron clouds (sorry, cartoon depiction) are aligned and they radiate EM Waves (Quantised))
    We totally understand that field in terms of it's spatial variation (same as a gravitational field Nugatory, except we use Maxwell instead of Newton/Einstein, but this doesn't tell us what the field is made of or what the mechanism of the force manifesting itself is)
    The Magnetic Field is made up of Photons (NOT Virtual Photons), it is an EM Field and the quanta are photons
    If (say) 2 electrons are in an EM Field (i.e. a Photon Field) they create a "Disturbance" (This disturbance is what is misleadingly referred to as a "Virtual Particle")
    These Disturbances are wave motions (i.e. solutions to the wave equation but not generally constrained to act as quanta, I,e, travel at light speed or whatever)
    The Disturbances can then interact with each other and create the attractive or repulsive forces we see as "Magnetism" (The existence of this force may be shown mathematically using the quantum maths of solutions to the wave equations (Disturbances) in the field and their subsequent interaction - now this bit IS complicated and requires some convoluted (literally) maths
    In the common magnet case (force at a distance of ~6") the electrons that interact via the EM Field/Photon Field and thus create the force are in the Magnet (aligned Electron Clouds) and in the Pin (non-aligned Electron Clouds)

    As a general points I would add that I've also learnt that Fields and their carrier particles are as follows:

    Gravity Force - Gravitational Field (Carrier "particles" are Gravitons (yet to be detected))
    Electromagnetic Force - EM Field (Carrier particles are Photons - as with common magnetism)
    Weak Nuclear Force - Carrier Particles are W & Z Bosons
    Strong Nuclear Force - Carrier Particles are Gluons

    The creation of Force at Distance by all these Field/Particle interactions are similar - but they do not interfere with each other

    We ought to also mention:

    Mass - Higgs Field (Associate Particles are Higgs Bosons)
    Mathematically predicted and (pretty reliably) detected by study of Proton/Proton collisions.
    Interesting thing here is that the Higgs Field permeates all space (bit like the old fashioned Aether)

    I hope the above makes sense and I'm not too far off the mark
    Anyway, that's as far as I got.............so far......................

    Thanks for all your help - it's a lot clearer than it was

    Last edited: Sep 21, 2016
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