# What pushes the EM waves in forward direction?

• Giteshwar
In summary: 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.In summary, photons always move at c for as long as they exist. Photons never experience acceleration. Photons never even experience time.
Giteshwar
nothing in the universe moves without force, so what does makes em waves move through space?

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.

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.

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.

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

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.

BobG said:
Besides, that applies to matter and the original post refers to energy.

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.

BobG said:
Doesn't it also say that an object at rest will remain at rest?
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.
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.]

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.]

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.

Giteshwar said:
nothing in the universe moves without force, so what does makes em waves move through space?

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.

Giteshwar said:
nothing in the universe moves without force, so what does makes em waves move through space?

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).

The changing electric field induces a magnetic field. The changing magnetic field induces an electric field.

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 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.

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.

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>

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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.

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.

EM_Guy said:
You make it sound like the universe is filled with EM fields that are just waiting to be excited. i.e.

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.

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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.

rumborak
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.

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.

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.
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.

morrobay said:
Because in a mechanical wave the amplitude is not related to velocity.

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.

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.
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.

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.

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.
Giteshwar said:
nothing in the universe moves without force, so what does makes em waves move through space?
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.
rumborak said:
I think this site might be really useful in understanding how waves propagate:

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.
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.

Greg Bernhardt and davenn
tech99, that is some keen insight. Basically, we have shunt capacitors (aka - permittivity) and series inductors (aka - permeability) all around us and throughout all of space. The changing electric field across a shunt capacitor causes the neighboring series inductor to build up its magnetic field. The changing magnetic field induces a back emf - an electric field in the circuit that is set to oppose that change. This electric field starts to charge up the next capacitor, etc., etc.

I always have a hard time remembering and thinking of voltage being dimensionally equivalent to rate of change of magnetic flux.

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.

Hi,
I'm thinking that if you, say, place an object in water, there are two things going on: 1) downward pressure which, because of the limited compressibility of the water, winds up springing back upward again, eventually rising above the water surface, and then falling downward again, in a cycle (of course until it loses energy by friction and entropy, returning to equilibrium). And then 2) Because the the object is also displacing the water laterally, it causes that up-down cycling to propagate in all directions in the plane of the water surface.

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.
Maybe that's not right but it seems so.

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.

Are you assuming that the water molecules don't interact with each other? Consider surface tension, capillary action, etc.

Personally I actually don't think water waves are that great for understanding how waves work. They have both a transversal and a longitudinal component, which makes them rather complex. A wave on a string is much better.

jtbell said:
Are you assuming that the water molecules don't interact with each other? Consider surface tension, capillary action, etc.

For the lateral component I am assuming they interact directly (transferring their momentum outward).

For that extreme thought experiment (width of a water molecule), yes, assuming negligible side interactions (hydrogen bonding etc.)

It doesn't have to move, other than described from a observer. We're in a observer dependent universe. I think it also depends on how you think of the universe, or for that sake 'all possible universes', as able to be described 'objectively' from some 'fifth dimension'. If you think of many worlds theory the assumption, as I see it, is that everything 'happens', although only one part observable by me existing in some specific chain of outcomes. I prefer another way to look at it where 'everything' has the probability of happening, but only can be defined by a observer, in the outcomes. That is in some way a static view of a universe (or universes), and possibly a 'whole description' of what it is, but it doesn't need to create 'parallel universes'. It would make this universe a symmetry break, using 'time'. Without 'time', no outcomes, and neither any 'propagation'. You could express it as time creates the infinity and inside we are presumed to exist in. It also takes care of a non-existent 'outside', well, in my eyes :)

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Giteshwar said:
nothing in the universe moves without force, so what does makes em waves move through space?
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.

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.
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.

Photons are for all intense and purposes born at velocity c. There is no contradiction here, only your mind trying to impose one.

sophiecentaur
Giteshwar said:
nothing in the universe moves without force, so what does makes em waves move through space?
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.

rumborak said:
I think this site might be really useful in understanding how waves propagate:

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.

At last someone has brought this thread to a sensible level again. We don't need the big guns to explain quite satisfactorily how waves propagate. Huygens (look him up) proposed a process which happened to apply to classical light but applies just as well to any wave. He envisaged anyone point on a broad wavefront (i.e. a plane wave traveling in just one direction) radiating energy in all directions (so-called secondary wavelets). These wavelets will add to (/ interfere with ) each other and it so happens that, in any other direction than forward, they cancel and they only enhance in that direction. The energy has only one way to go. If you take a simple wave propagating on a string and consider the phases and amplitudes along one section, starting at a few points along the string, you will find that the phases of the 'backward' secondary waves will be such as to produce a zero phasor sum backwards and add together in the forward direction.

Forget about em not needing a medium / photons / QM and all the rest. Just sort out the fundamentals of classical waves first.

Orodruin said:
Sorry, but this is simply just wrong. By definition, something moving at constant velocity has zero acceleration

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?

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?
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.

Neandethal,
Since you are getting hung up on acceleration, it is worth pointing out that energy is discretized. I assume you are familiar with the concept of kinetic energy and its relationship to velocity. And you are obviously familiar with the classical ideas regarding the relationship between acceleration and velocity. So, this may make your head spin - (and you shouldn't just accept this - you should study and be convinced yourself of the veracity of this claim). But one of the consequences of Max Planck's work is that energy of objects is discretized. Meaning that an object can only possesses an energy that is an integer multiple of hf. Meaning that energy is not continuous. So, for example, when you push your little sister on the swing, you may have thought that her velocity could be plotted as a continuous function. At her apex, her velocity is zero. And at some point she reaches a maximum velocity, before she beings decelerating. And so, if she goes from 0 to vf back to 0, you might have thought that she was experiencing every single value of velocity that there is between 0 and vf. But in fact, she did not. She only experienced integer multiples of hf. Now, h is really, really small, so you never noticed that your sister was "skipping" a whole bunch of values of velocity as she accelerated, but she did. When you start looking at really, really small particles, and/or really, really high frequencies, hf becomes significant. And then we have to deal with the reality of quantum physics - as strange as it may seem to us.

EM Guy, I don't think kinetic energy is quantized.

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