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

I Infinite range of E-M field vs finite age of particles

  1. Oct 9, 2018 #1

    nomadreid

    User Avatar
    Gold Member

    It is said (hopefully no need to give references for such a common statement) that the electromagnetic field of a given charged particle is infinite in range (albeit converging to zero as the distance goes to infinity). However, given that charged particles apparently did not exist at the beginning of the expansion of the universe, and that the electric field expands at the speed of light modulo being carried along by the expansion, that the field would be finite? (I am assuming the universe having started out as already very large, possibly infinite.)
     
  2. jcsd
  3. Oct 9, 2018 #2

    phinds

    User Avatar
    Gold Member

    I believe that the "infinite extent" of charged particles' electromagnetic fields as well as that of the gravitational field of massive bodies are Classical Physics statements (and therefore assume a static space-time)
     
  4. Oct 9, 2018 #3

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    The field is actually the fundamental object in modern physics, not the particles. So the field existed before the particles.
    As a technical note, particles are modeled as excitations of their underlying fields in modern quantum theory (specifically quantum field theory). So photons are excitations of the EM field, electrons are excitations of an electron field, etc.
     
  5. Oct 9, 2018 #4

    phinds

    User Avatar
    Gold Member

    While I agree w/ what you say, I still think that the "infinite extent" is a Classical Physics statement (and in practical terms is just a mathematical convenience anyway)
     
  6. Oct 9, 2018 #5

    nomadreid

    User Avatar
    Gold Member

    From what I understand (which is too little), the field of which a particle is the excitation of is distinguished from the field which is the object of the effect (attraction/repulsion) of one charged and/or massive particle on other (test) particles in space. The former is infinite, but I am referring to the latter.

    And hence invalid, so my idea about a finite field is not off the mark?
     
  7. Oct 9, 2018 #6

    phinds

    User Avatar
    Gold Member

    If it is "invalid" I think it is only "invalid" in the same sense that Newtonian Gravity is invalid. They both work just fine except in extreme cases.
     
  8. Oct 9, 2018 #7

    nomadreid

    User Avatar
    Gold Member

    In other words, an E-M field emanating from an electron is "infinite for all practical purposes", i.e., very big (albeit finite, if one wishes to get picky)?
     
  9. Oct 9, 2018 #8

    phinds

    User Avatar
    Gold Member

    Yeah, I think that's it. As I said, I've always thought of the "infinite range" as just a mathematical fiction for all practical purposes, but then I'm an engineer, not a physicist.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted