1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What pushes the EM waves in forward direction?

  1. Jun 3, 2015 #1
    nothing in the universe moves without force, so what does makes em waves move through space?
     
  2. jcsd
  3. Jun 3, 2015 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Your premise is false. Newton's first law will tell you that an object in motion will stay in motion if there are no forces acting upon it.
     
  4. Jun 3, 2015 #3

    BobG

    User Avatar
    Science Advisor
    Homework Helper

    Doesn't it also say that an object at rest will remain at rest?

    Besides, that applies to matter and the original post refers to energy.
     
  5. Jun 3, 2015 #4

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Indeed, but the OP is indicating that there needs to be a driving force. Of course there needs to be a source starting the wave, but once underway it will propagate without any kind of driving force.

    There is not such a big difference and the OP is based on the equivalence of the two. Technically the OP refers to electromagnetic fields, which abide by the conservation of energy and momentum just as other forms of matter.
     
  6. Jun 3, 2015 #5

    russ_watters

    User Avatar

    Staff: Mentor

    Just to clarify with respect to Bob's question, I don't really like the words "starting" and "once underway" here. They imply to me that before being underway, the EM wave is stationary. It isn't. Once created, the EM wave is at C. Since it is never at any other speed (and never stationary), Bob's question/objection is moot.
    [I'm sure you know that, I just didn't like the wording.]
     
  7. Jun 3, 2015 #6

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I am not sure that "once underway" implies that it was ever stationary (to me it doesn't say anything about what was before), but yes, EM waves are never stationary. The thing which is always there is the EM field.
     
  8. Jun 3, 2015 #7

    Nugatory

    User Avatar

    Staff: Mentor

    Let's try a different approach to this question. If I were to ask you what makes water waves move along the surface of the water, what would you say? Bear in mind that although the wave is moving sideways along the surface of the water, each individual drop of water is just moving up and down.
     
  9. Jun 3, 2015 #8
    It is an interesting question.

    Photons always move at c for as long as they exist. Photons never experience acceleration. Photons never even experience time.

    This is an interesting question though. There is energy in the photon. Or, if you prefer to stay classical, there is energy in the field. But a photon has zero mass. Maybe in a limiting sense, you can think of the emission of a photon (which requires energy) as an acceleration acting on a particle of zero mass, making it go from zero to c instantaneously?

    Or perhaps a related question can be asked, "What happens when you hit a photon?" When you, or any thing, hits a photon, the photon is hitting you (or the thing).
     
  10. Jun 3, 2015 #9
    The changing electric field induces a magnetic field. The changing magnetic field induces an electric field.
     
  11. Jun 3, 2015 #10
    I agree with the general sentiment that this shouldn't be analyzed so much in terms of momentum and inertia, but that it's a wave that propagates through "exciting" its neighbors in space.
     
  12. Jun 3, 2015 #11
    This seems problematic to me. Unlike ripples in a pond, EM waves need no medium to travel though. A brief EM pulse can propagate through the universe. The energy is propagating. Photons are traveling.
     
  13. Jun 3, 2015 #12
    I don't see the medium, or lack thereof, an issue really. To my understanding, the EM field *is* the medium, as it is an aspect of space.
    And the EM wave equation also suggests to look at it like a wave. Just like the water wave, it has a Laplace operator on the E (or B) field, which is the local derivative in space.
    The quantization of light into photons is a different matter, and does not detract from the wave nature of light in this aspect.

    <edited to be less ambiguous>
     
    Last edited: Jun 3, 2015
  14. Jun 3, 2015 #13
    I don't think there is a medium. You make it sound like the universe is filled with EM fields that are just waiting to be excited. i.e. You seem to be treating the EM field as the ether. There is no ether.

    Energy is traveling. Even if we stay purely classical, we have a pulse of EM radiation. This field doesn't "stay put" exciting its neighbor field, which in turn excites its neighbor field, etc. Rather, a changing electric field induces a magnetic field in its vicinity, and a changing magnetic field induces an electric field in its vicinity, and we have propagation of energy - through space - without a medium.
     
  15. Jun 3, 2015 #14

    I take it you have heard of Quantum Field Theory?

    http://en.m.wikipedia.org/wiki/Quantum_field_theory

    I think you misunderstood the conclusions of the Ether experiments back in the day. The idea about ether was that the EM waves were exciting something that was uniformly spread in space (kinda like air). The experiment results showed however that it is space *itself* that is the carrier of the EM field.
     
    Last edited: Jun 3, 2015
  16. Jun 3, 2015 #15

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The entire point of a field is that it has a value at each point in space-time, so yes, it is everywhere. It may have the value zero at some points or in some region, but the field itself is there. This does not imply an aether.
     
  17. Jun 3, 2015 #16
    I have heard of quantum field theory. But I understand none of it.

    My training is 100% classical. I have studied electromagnetic propagation purely in terms of Maxwell's equations (Faraday's law, Ampere's circuital law, Gauss' law, Gauss' law for magnetics). I know next to nothing about quantum physics.
     
  18. Jun 3, 2015 #17

    morrobay

    User Avatar
    Gold Member

    Could you show a conceptual explanation how the EM wave propagates at c from this equality: ω/k = Em /Bm = c ?
    Taken from kEmcos(kx-ωt) = ωBmcos(kx-ωt). That is how the amplitude of the magnetic and electric components of the EM wave are equated to c.
    I asked this question here:https://www.physicsforums.com/threa...m-wave-w-k-c-how-equated.813755/#post-5109183.
    The question is not how ω/k = Em/Bm= c simply drops out of above equality but a conceptual/mechanical explanation for EM propagation.
    Because in a mechanical wave the amplitude is not related to velocity.
     
  19. Jun 4, 2015 #18

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    It is not for an EM wave either. In an EM wave, the ratio of the amplitudes of the electric and magnetic field is c. This is a constraint on the amplitudes of the different fields (they are not two independent waves) rather than a dependence of the speed on the amplitude.
     
  20. Jun 4, 2015 #19
    I'm not sure that I can give you a satisfactory conceptual answer, other than to say that it comes down to Faraday's law, Ampere's circuital law, the permittivity of free space, and the permeability of free space. Unfortunately, for me, permittivity is nothing more than a constant relating the D-field to the E-field. Likewise, permeability is nothing more than a constant relating the B-field to the H-field. This betrays the fact that I don't have a firm grasp on the nature of permittivity and permeability. I can't really tell you the difference between E-fields and D-fields, except that they have different units.

    Permittivity is capacitance per unit length. And permeability is inductance per unit length.

    A charge establishes an electric flux density, that is, a D-field, independent of the permittivity of the space. The electric field intensity (E-field) depends on the permittivity of the space. The smaller the permittivity, the larger the E-field.

    A current (moving charge - note inertial frames of reference here) establishes a magnetic field intensity, that is, an H-field, independent of the permeability of the space. The magnetic flux density (B-field) depends on the permeability of the space. The smaller the permeability, the smaller the B-field.

    A time-changing magnetic flux density produces an electric field intensity, independent of the permittivity. The electric flux density depends on the permittivity.

    A time-changing electric flux density produces a magnetic field intensity, independent of the permeability. The magnetic flux density depends of the permeability.

    So, the time rate of change of the magnetic flux density determines the "spatial rate of change" of the electric field intensity. By spatial rate of change of the E-field, I mean the curl of the E-field.

    And the time rate of change of the electric flux density determines the "spatial rate of change" of the magnetic field intensity.

    So, starting with a time-varying magnetic flux density, the higher the time rate of change of this magnetic flux is, the greater the establishment of the E-field and the closer the establishment of the E-field. (Think curl here; I'm speaking of a spatial rate.) The greater the permittivity, the greater the D-field. The E-field and the D-field would also vary at the same frequency as the B-field. The higher the time rate of change of the D-field, the greater the establishment of the H-field and the closer the establishment of the H-field. (Think curl here). The greater the permeability, the greater the B-field. So, the greater the frequency of the oscillating fields, the shorter the wavelength. So, we have frequency and wavelength. But frequency (inverse of time period) and wavelength depend both on the inertial frame of reference. The speed of the wave is not determined by the frequency and the wavelength (both of which are subject to the inertial frame of the observer). Rather, the speed of the wave is determined by the permittivity and permeability of the medium.

    I hope this helps. I'm not sure that I gave you a good conceptual reason to demonstrate that the speed of EM waves in free space is c. But I hope that I at least have demonstrated why frequency and wavelength are inversely proportional to each other.
     
  21. Jun 4, 2015 #20
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What pushes the EM waves in forward direction?
Loading...