0
$\begingroup$

This may be a duplicate, but how are (ideally) constant E- and B-/H-fields transmitted thru space? In this situation I like think of EM radiation as a note from a particle informing the universe of a change, but as the fields are constant, there aren't any notes/photons to send. For example, how will a bar magnet affect a cosmic ray passing by earth, and by what mechanism? Is this the province of virtual photons, or something else?

$\endgroup$
  • $\begingroup$ A field isn't 'transmitted', a field is defined on all of space and time. In particular, a constant E field does not change with time. A uniform E field is the same over all space. $\endgroup$ – Alfred Centauri Apr 15 '14 at 2:34
  • $\begingroup$ @AlfredCentauri But the field does not permeate the universe instantly. A constant E-field is what I intended, it will change predictably according to the inverse-square relation. How could a uniform E-field exist in space, as it has different start and end points as well as a definite orientation? $\endgroup$ – user40753 Apr 15 '14 at 2:58
  • $\begingroup$ you must sharpen your thinking about fields and the terms you are using. Frankly, I don't have a clue as to what your final sentence is supposed to mean. $\endgroup$ – Alfred Centauri Apr 15 '14 at 3:10
  • $\begingroup$ @AlfredCentauri a field may be defined over all of space and time, but time is the independent variable that determines the extent of the field, as in the case of a current flow, where the resultant magnetic field radiates outward at c. this is the field transmission the mechanism of which I am asking about. as to the E-field, by start and end points I'm talking about + and - charges, where the orientation/direction of the field is between them. the E-field is caused by imbalance of charge, and you cannot have a uniform imbalance of something, thus there can't be a "uniform" E-field. $\endgroup$ – user40753 Apr 15 '14 at 3:53
2
$\begingroup$

One has to define in what framework one is talking of electric and magnetic fields.

In the classical framework the field is defined, for simplicity lets take a point charge, as proportional to 1/r^2 and exists up to infinity. Thus classically there is no transmission for a static charge, it just is. When a charge is moving, i.e. changing its (x,y,z,t) dependence radiation is emitted, if there exists acceleration, as electromagnetic waves. The effect of the classical motion of these charges and consequent motion of field lines can be seen in this simulation . A video of constant motion ( non radiating) is here . It is a subject of interest for plasma physics. Another way of looking at static fields is that they are built up of electromagnetic fields of wavelength approaching infinity.

This brings us to the quantum framework, which as far as we have discovered with our experiments, is the fundamental underlying level of physics from which all classical theories emerge.

For example, how will a bar magnet affect a cosmic ray passing by earth, and by what mechanism? Is this the province of virtual photons, or something else?

Photons exist in the quantum mechanical domain and the classical electromagnetic field emerges from a large ensemble of photons. In this framework all information is transmitted by particles . So virtual photons will be transmitted between the bar of magnet and the cosmic ray with the velocity of light , though the effect will be infinitesimal. Virtual means that the exchange happens between known input and output particles and the exchanged photon is not on mass shell. Your example can be written up as such an interaction and the probability of the bar of magnet to change to path of the cosmic ray can be computed but it is not worth the trouble because it will be very very small ( 1/r^2 may be disguised in the solutions of the quantum mechanical problem but it is still there).

$\endgroup$
  • $\begingroup$ I used that example because I wanted the forces to be so small that there would be no other [obvious] way for the two to interact. Is there the possibility that they might not interact at all? Can we say that whenever a particle accelerates it emits one real photon, but is always emitting an effective infinity of virtual photons, which we could call "actionable potential"? How does distance constrain interaction? Is there a separation where forces become nonexistent? I ask that because I'm not clear on whether h allows doing less work over more time (vs. "borrowing"). $\endgroup$ – user40753 Apr 15 '14 at 4:43
  • 1
    $\begingroup$ For a static field these virtual photons will have a wavelength approaching infinity and they will collectively build up the electric field, and here the classical formalism is much more practical. The quantum mechanical tells us that the effect will propagate with the velocity of light. QM the probability of interaction is what can be predicted and measured and that will be infinitesimal, due to 1/r and h bar. The heisenberg uncertainty principle constrains tiny dimensions. see my answer here : physics.stackexchange.com/questions/107397/… $\endgroup$ – anna v Apr 15 '14 at 5:03
  • 1
    $\begingroup$ an accelerating charge has a computable probability of radiating photons as it is accelerated, and it is easier to compute the spectrum classically once we know how to go from one framework to the other hyperphysics.phy-astr.gsu.edu/hbase/particles/synchrotron.html . One off happens seldom in specific interactions of known particles. $\endgroup$ – anna v Apr 15 '14 at 5:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy