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Giteshwar
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nothing in the universe moves without force, so what does makes em waves move through space?
Orodruin said: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.
BobG said:Doesn't it also say that an object at rest will remain at rest?
BobG said:Besides, that applies to matter and the original post refers to energy.
BobG said:Doesn't it also say that an object at rest will remain at rest?
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.Orodruin said: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.
russ_watters said: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.]
Giteshwar said:nothing in the universe moves without force, so what does makes em waves move through space?
Giteshwar said:nothing in the universe moves without force, so what does makes em waves move through space?
rumborak said: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.
rumborak said:I don't see the medium, or lack thereof, an issue really. To my understanding, the EM field *is* the medium.
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.
EM_Guy said:You make it sound like the universe is filled with EM fields that are just waiting to be excited. i.e.
EM_Guy said: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.
morrobay said:Because in a mechanical wave the amplitude is not related to velocity.
morrobay said: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.
EM_Guy said: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.
If you imagine a transmission line, with lumps of shunt C and series L, when the first element is charged up it cannot discharge back towards the generator, because it is at the same voltage, but must discharge in the forward direction into the empty line. The same with the radiation field created by an accelerated charge.Giteshwar said:nothing in the universe moves without force, so what does makes em waves move through space?
If we consider any traveling wave, we find that there is a progressive phase delay in the direction of travel. This makes it go forward and makes it hard to come back. For a mechanical wave carried on a sequence of springs and masses, there is a small delay between each section caused by the inertia of the masses. This favours forward propagation. For a lumped transmission line, with shunt C and series L, the phase delay arises because inductors exhibit an inertia effect as it takes a finite time for them to build their magnetic fields. For an EM wave in a material, the delay possibly originates in part from electron inertia, arising from both the mass and the inductive action of the electrons i.e their need to build a magnetic field when the electric field causes them to move. As a matter of interest, Maxwell's Equations were devised using a mechanical analogue, and I think the aether was suggested and then disproved after his time.rumborak said:I think this site might be really useful in understanding how waves propagate:
https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html
The speed of the wave is totally independent of the amplitude or shape of the wave. When you change the "tension" on that applet, you change the speed of the wave.
Nugatory said: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.
Micheth said:If so, I guess if the object was the width of a single water molecule, the wave would just sit in the same place going up and down, due to no lateral displacement.
jtbell said:Are you assuming that the water molecules don't interact with each other? Consider surface tension, capillary action, etc.
This is a legitimate question but what I read are what politicians say to avoid a question.Giteshwar said:nothing in the universe moves without force, so what does makes em waves move through space?
Sorry, but this is simply just wrong. By definition, something moving at constant velocity has zero acceleration or the velocity would not be constant as the acceleration is the time derivative of the velocity.Neandethal00 said:This is a legitimate question but what I read are what politicians say to avoid a question.
Probably no one knows what really happens.
Something cannot have a constant velocity without an acceleration. Where and how does a photon gets an acceleration?
If the photon is emitted by an electron, either the electron achieves velocity c before emission or the photon undergoes an
acceleration.
Photons slow down when traveling through glass and then return to 'c' when it exits the glass meaning, probably, that an electromagnetic wave doesn't need or require more than its own nature to get up to its natural speed.Giteshwar said:nothing in the universe moves without force, so what does makes em waves move through space?
rumborak said:I think this site might be really useful in understanding how waves propagate:
https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html
The speed of the wave is totally independent of the amplitude or shape of the wave. When you change the "tension" on that applet, you change the speed of the wave.
Orodruin said:Sorry, but this is simply just wrong. By definition, something moving at constant velocity has zero acceleration
We know a lot about photons. One of the things we know is that they are a pure quantum field theoretical concept and you really should not expect them to behave as small classical balls. This also goes for electrons and other particles on small enough scales.Neandethal00 said:Boy, just a wrong word can make lots of heads spin. My sentence should have been
Something cannot "achieve" a constant velocity without an acceleration
Which is universally true. An acceleration precedes a constant velocity. If you say photons do not work that way, then we know little about photons,
or velocity of photons is not really its velocity. Can velocity be instantaneous without acceleration?