Speed of water and relation to light

[Serge58]

Fill a bowl with water, lay a piece of fabrics over the bowl, one end touching the water and the other end laying down on the table. Due to the capillarity effect the water will follow the fabrics and after a while it will be all wet. From this little experiment we can deduce that water travels, at a speed that depends of the type of fabrics, and has a maximum speed in each type of fabrics. We can even extrapolate that the speed of water in nylon can be something like 1 cm/minute. Silly isn't it? But its apparent speed is dictated by the medium that carries it.

That brings my questions related to light.
1. Could it be possible that light has an infinite speed but it is slowed down by the medium where it travels? Isn't there a vacuum permeability in the vacuum of space that would limit its speed?
2. (my favorite option) Is it possible that light doesn't move at all? Like the water in the previous analogy? That it just dissipates and follow the fabrics of space, even empty ones, as water in nylon? That would supply a different explanation of why light slow down in water and speed up when it get out.

 

[harrylin]

Hi Serge, welcome to physicsforums!

The generally accepted model for light propagation in relativity is a wave model. However there are variations in the interpretations, as there are problems with details; in practice what people do is know how to calculate based on rules from wave mechanics. In that context it's easier to point out how your suggestions conflict or agree with currently used models:

- the speed of light in vacuum is a local constant determined by the vacuum and when light travels through water, it is effectively delayed due to interactions with the water molecules. See:
https://www.physicsforums.com/showthread.php?t=613481
https://www.physicsforums.com/showthread.php?p=3772953

- concepts as "vacuum" and "fabric of space" can be used with relativity theory.

However, this forum is not meant for discussing personal possible new theories (see Rules at top of the page). As you can see in the linked threads, refraction is very well explained by current scattering theory.

 

[mfb]

1. Could it be possible that light has an infinite speed but it is slowed down by the medium where it travels? Isn't there a vacuum permeability in the vacuum of space that would limit its speed?

It has a fixed, constant speed in vacuum, for all observers. This is not possible if the vacuum would be some medium and light would travel relative to that medium.

2. (my favorite option) Is it possible that light doesn't move at all?

It can travel from A to B, that is usually called "movement".

Like the water in the previous analogy?

Water moves in your (wrong) analogy.

 

[harrylin]

It has a fixed, constant speed in vacuum, for all observers. This is not possible if the vacuum would be some medium and light would travel relative to that medium.[..].

In fact, it has been known since 1904/1905 that this is possible and some of the early relativists related to such a wave model*. The problems with specific wave models do not depend on relativity. To mention a basic problem: As far as I know, a spherical transverse light wave emitted from a light source is an impossibility.

* see for example:
https://www.physicsforums.com/showthread.php?t=631954
https://www.physicsforums.com/showthread.php?t=590601
https://www.physicsforums.com/showthread.php?t=561128
(and many more)

 

[Dale]

To mention a basic problem: As far as I know, a spherical transverse light wave emitted from a light source is an impossibility.

Why is that a problem? That is just another way of saying there is no monopolar EM radiation. Why is it a problem that there is no monopolar EM radiation?

 

[mfb]

In fact, it has been known since 1904/1905 that this is possible and some of the early relativists related to such a wave model*. The problems with specific wave models do not depend on relativity. To mention a basic problem: As far as I know, a spherical transverse light wave emitted from a light source is an impossibility.

You can construct some co-moving ethers and whatever, but then you give up the concept of a preferred reference frame, and get the same predictions as SR (maybe just with a more complicated framework). Otherwise, you get predictions different from SR, which are inconsistent with experiments.

 

[harrylin]

You can[..] give up the concept of a preferred reference frame, and get the same predictions as SR [..]

That's right; I simply corrected an erroneous claim. However, we were next both inaccurate. When I wrote that "the problems with specific wave models do not depend on relativity", I was only thinking of detailed mechanisms and not about what you call, "some co-moving ethers and whatever". While a Lorentz ether gives the same predictions, it is held that pluralistic "co-moving ethers" can't work as a model for SR (the prime argument was stellar aberration).

 

[harrylin]

Why is that a problem? That is just another way of saying there is no monopolar EM radiation. Why is it a problem that there is no monopolar EM radiation?

I think that you mean something else. I merely referred to an idealized classical spherical uniform light source that supposedly emits transverse spherical waves around it. Did you ever try to draw that in 3D perspective? I did and if I'm not mistaken, then that is a geometric impossibility. However, if I am mistaken, then I'll be delighted to find out how it can be done!

 

[Dale]

It is impossible, I just don't know why that is a problem according to you.

 

[Serge58]

I wish to reformulate my question. Why is there a maximum speed of light in vacuum? is it because it won't go faster or because it can't?

 

[mfb]

Light does not have a will, or "tries" to do something.
It has a specific speed (that is an observation), and you can derive this based on the Maxwell equations, for example.
The laws of physics are the same in all reference frames (another observation), this makes sure that the speed is the same for all frequencies and all directions.

 

[harrylin]

I wish to reformulate my question. Why is there a maximum speed of light in vacuum? is it because it won't go faster or because it can't?

According to Maxwell's theory on which SR was based, it's a property of vacuum, so that light can only propagate at the speed of light - similar to sound which propagates at the speed of sound.
See: http://en.wikipedia.org/wiki/Vacuum_permittivity

 

[harrylin]

It is impossible, I just don't know why that is a problem according to you.

Impossible means that such a basic classical model is plain wrong - simply due to self contradiction, without any need to introduce QM. I'm not aware of a more severe form of "problematic".

 

[Dale]

Impossible means that such a basic classical model is plain wrong - simply due to self contradiction, without any need to introduce QM. I'm not aware of a more severe form of "problematic".

It is only a self contradiction if the model predicts it and it is impossible, but Maxwells equations do not predict a spherical transverse wave. The lowest order of radiation allowed by Maxwells equations is dipole.

Since it is not predicted the fact that it is impossible is not a problem.

 

[Serge58]

According to Maxwell's theory on which SR was based, it's a property of vacuum, so that light can only propagate at the speed of light - similar to sound which propagates at the speed of sound.
See: http://en.wikipedia.org/wiki/Vacuum_permittivity

So i can suppose that if light doesn't travel faster, it is not because it won't, but because it can't. It is possible to assume that light would go faster if it wasn't of the permittivity of vacuum?

 

[Dale]

So i can suppose that if light doesn't travel faster, it is not because it won't, but because it can't.

Can you think of an experiment which could distinguish between "it won't" and "it can't"? If not, then the concept is philosophy, not science, and doesn't belong here. If so, then please describe the experiment and we can analyze what SR would predict.

 

[harrylin]

It is only a self contradiction if the model predicts it and it is impossible, but Maxwells equations do not predict a spherical transverse wave. The lowest order of radiation allowed by Maxwells equations is dipole. [..]

Good point! Assuming that that's correct, I'm afraid that this is not sufficiently recognized. A quick internet search produces on the one hand claims in courses such as
"light consists of [..] fields that travel through space as transverse waves", e.g.
http://ubpheno.physics.buffalo.edu/~dow/lectures/phy102/ch24_print.pdf ,
but also claims like "Spherical EM wave" and "The E.M. wave has a spherical wave front", e.g. both on slide 7 in http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and not to forget the typical SR computation based on "consider a pulse of light that [..] propagates as a spherical wave" such as in
http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and of course "let a spherical wave be emitted" in
http://www.fourmilab.ch/etexts/einstein/specrel/www/

 

[harrylin]

[..] It is possible to assume that light would go faster if it wasn't of the permittivity of vacuum?

If vacuum permittivity and permeability are reduced, then also c is increased. In fact, as measured with a reference system on Earth, the speed of light is slightly reduced near the sun. That was predicted by GR and confirmed by experiments.
I found nice and clear explanations plus animations here:
- http://www.astro.ucla.edu/~wright/deflection-delay.html

 

[Austin0]

Good point! Assuming that that's correct, I'm afraid that this is not sufficiently recognized. A quick internet search produces on the one hand claims in courses such as
"light consists of [..] fields that travel through space as transverse waves", e.g.
http://ubpheno.physics.buffalo.edu/~dow/lectures/phy102/ch24_print.pdf ,
but also claims like "Spherical EM wave" and "The E.M. wave has a spherical wave front", e.g. both on slide 7 in http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and not to forget the typical SR computation based on "consider a pulse of light that [..] propagates as a spherical wave" such as in
http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and of course "let a spherical wave be emitted" in
http://www.fourmilab.ch/etexts/einstein/specrel/www/

But aren't they actually talking about aggregate collections of photons isotropically emitted from a symmetric source. Not a single wave in any real sense.

 

[harrylin]

But aren't they actually talking about aggregate collections of photons isotropically emitted from a symmetric source. Not a single wave in any real sense.

It looks to me that they are not talking about photons but about spherical waves just as they state; for sure Maxwell wasn't thinking of photons and neither was Einstein implying photons in his SR paper.

 

[Q-reeus]

Good point! Assuming that that's correct, I'm afraid that this is not sufficiently recognized. A quick internet search produces on the one hand claims in courses such as
"light consists of [..] fields that travel through space as transverse waves", e.g.
http://ubpheno.physics.buffalo.edu/~dow/lectures/phy102/ch24_print.pdf ,
but also claims like "Spherical EM wave" and "The E.M. wave has a spherical wave front", e.g. both on slide 7 in http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and not to forget the typical SR computation based on "consider a pulse of light that [..] propagates as a spherical wave" such as in
http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and of course "let a spherical wave be emitted" in
http://www.fourmilab.ch/etexts/einstein/specrel/www/

Harald - don't you think it's about time to grab a decent textbook or use any of the many good online resource, and actually learn the basics of EM theory? You argue a lot about validity of Maxwell's eqn's in relation to SR, without having any real understanding of ME's. To answer your conundrum about 'spherical radiation' which is actually dipolar or other multipole form: Spherical simply means that the constant-phase wavefront from say a dipole radiator is spherical at large r from such a source, not that the amplitude of that wave is spherically symmetric - an impossibility. It certainly is possible for a glowing sphere such as a star to output spherically symmetric radiation at large r from such a source. But the resolution is quite simple. Radiation from a star, or even a frosted light bulb, is the result of a huge number of chaotically phased and oriented individual radiators (colliding ions, atomic transition radiation). They do not cancel each other out owing to that random phase and orientation feature characteristic of thermal emitters.

 

[harrylin]

Harald - [..] You argue a lot about validity of Maxwell's eqn's in relation to SR [..]

Hi Q-reeus, I did not argue at all "about validity of Maxwell's eqn's in relation to SR" - quite the contrary, I mentioned that SR is based on it!
I mentioned in relation to the OP that we don't really understand what light exactly is but that for sure certain wave models can't be correct, such as spherical transverse waves which appear to be an impossibility. Next it was Dalespam who brought up Maxwell's equations by saying that those do not allow spherical transverse waves and I found that valuable input.

use any of the many good online resource [..]

Regretfully online university courses are supposed to be good online resources. Now, you seem to merely disagree with us about words and not about content:

Spherical simply means that the constant-phase wavefront from say a dipole radiator is spherical at large r from such a source, not that the amplitude of that wave is spherically symmetric - an impossibility.[..]

In my book a wavefront designates a constant phase, and I find two poles where this is impossible, so that the amplitude at those poles must be zero. Zero amplitude still implies no wavefront and no phase at those points. Are you really arguing about words?

 

[Q-reeus]

Hi Q-reeus, I did not argue at all "about validity of Maxwell's eqn's in relation to SR" - quite the contrary, I mentioned that SR is based on it!
I mentioned in relation to the OP that we don't really understand what light exactly is but that for sure certain wave models can't be correct, such as spherical transverse waves which IMHO are an impossibility. Next it was Dalespam who brought up Maxwell's equations by saying that those do not allow spherical transverse waves and I found that valuable input.

Now, you seem to merely disagree with us about words and not about content:

Sorry, I should have been a little more specific. Didn't mean you were arguing with or against compatibility of ME's and SR. Rather that you were arguing for their compatibility, but then getting into issues - e.g. of impossibility of spherically symmetric radiation from a single oscillator as somehow a problem for classical EM/ME's.

In my book a wavefront designates a constant phase, and I find two poles where this is impossible, so that the amplitude at those poles must be zero. Zero amplitude still implies no wavefront and no phase at those points. Are you really arguing about words?

Well it gets down to agreed definitions I suppose. It is simply an agreed and universally adopted convention that 'spherical wave-front' in respect of radiation refers to just that - the phase. Mathematically, although the amplitude drops to zero at the poles of a dipole oscillator far-field, we still have that the phase is defined. Otherwise we have some kind of axial 'singularity crisis' is implied. Anyway, keep happy I say.

 

[harrylin]

Sorry, I should have been a little more specific. Didn't mean you were arguing with or against compatibility of ME's and SR. Rather that you were arguing for their compatibility, but then getting into issues - e.g. of impossibility of spherically symmetric radiation from a single oscillator as somehow a problem for classical EM/ME's.

I don't think that I mixed up Maxwell's equations with descriptions of wavefronts in some of the literature; however it was Dalespam who stressed their incompatibility.

Well it gets down to agreed definitions I suppose. It is simply an agreed and universally adopted convention that 'spherical wave-front' in respect of radiation refers to just that - the phase. Mathematically, although the amplitude drops to zero at the poles of a dipole oscillator far-field, we still have that the phase is defined. Otherwise we have some kind of axial 'singularity crisis' is implied. Anyway, keep happy I say.

That was exactly my point, which means that this is probably not about definitions. Thus I repeat:
Did you ever try to draw a [edit: transverse] spherical wavefront (thus with defined phase everywhere) in 3D perspective? I did and if I'm not mistaken, then that is a geometric impossibility. However, if I am mistaken, then I'll be delighted to find out how it can be done!

 

[Q-reeus]

I don't think that I mixed up Maxwell's equations with descriptions of wavefronts in some of the literature; however it was Dalespam who stressed their incompatibility.

That was exactly my point, which means that this is probably not about definitions. Thus I repeat:
Did you ever try to draw a spherical wavefront (thus with defined phase everywhere) in 3D perspective? I did and if I'm not mistaken, then that is a geometric impossibility. However, if I am mistaken, then I'll be delighted to find out how it can be done!

Odds are you will have visited and studied the Wiki site here In which case you have noted that even when θ and thus the sin(θ) part goes to zero, phase is a well defined function of radius for a dipole oscillator far-field. It all reminds me of a website I came across where the individual there was hinging his/her entire case on that the field line of a bar-magnet that points exactly north or south 'never ends' - thus soundly 'refuting' the usually accepted position that lines of B always form closed loops. But I'm quietly confident you wouldn't subscribe to such an argument.

 

[harrylin]

Odds are you will have visited and studied the Wiki site here In which case you have noted that even when θ and thus the sin(θ) part goes to zero, phase is a well defined function of radius for a dipole oscillator far-field. [..]

A dipole oscillator far field is definitely not a transverse spherical wavefront.
But I will not continue this distraction from the OP's questions. [EDIT: OK then, one last time!]

 

[Q-reeus]

A dipole oscillator far field is definitely not a transverse spherical wavefront.

Well it's always transverse in direction wrt a radius vector from source point (assumed a 'point source' at large r). And it's always a field having constant phase at a given r. So I'm sort of lost as to the point of actual dispute here Harald. Enlighten please.

 

[harrylin]

Well it's always transverse in direction wrt a radius vector from source point (assumed a 'point source' at large r). And it's always a field having constant phase at a given r. So I'm sort of lost as to the point of actual dispute here Harald. Enlighten please.

A last remark on this: you apparently agreed that a spherical wavefront means a wavefront with the same phase everywhere on the spherical surface. Using the picture of that Wikipedia dipole article, you can see the transverse phase of the radiation going to the right; in particular the E-vector is indeed transverse to it and pointing "up" in the picture.
Now try to indicate the same for the radiation straight out of the top pole, thus the radiation that is going "up" in the picture and the electric field perpendicular to it. You will see that the transverse direction is indeed necessarily undefined; it has no transverse direction.

BTW you provided a useful link as it not only serves to explain it with a picture but it also links on to an article that explains this same point, although ignoring the phase aspect; there is no wavefront at the poles, it only lacks clear explanation why this is necessarily so:
http://en.wikipedia.org/wiki/Isotropic_radiator#Antenna_Theory

And with that, this little excursion away from the topic is for me closed.

 

[Q-reeus]

A last remark on this: you apparently agreed that a spherical wavefront means a wavefront with the same phase everywhere on the spherical surface. Using the picture of that Wikipedia dipole article, you can see the transverse phase of the radiation going to the right; in particular the E-vector is indeed transverse to it and pointing "up" in the picture. Now try to indicate the same for the radiation straight out of the top pole, thus the radiation that is going "up" in the picture and the electric field perpendicular to it.

Right - and the point is? Let me guess. Field strength, both absolute, and component along polar r direction in particular, goes to zero for θ = 0 (axial direction). Well that has been covered. We are talking about the very far field where only 1/r radiation component counts. At any point even infinitesimally displaced from the polar axis, there is a finite rms field strength, and the phase is as given there - strictly a uniform function of radius r only, entirely independent of θ. It's fine to say field has disappeared at strictly θ = 0, and in a sense phase has no meaning if E, B is zero there. But mathematically phase is always well defined, even in that limit as |E, B| goes to zero. And closer in, when near fields are appreciable, there is meaningful use of phase as fields are non-zero when θ = 0. In this near-field case, phase of net field, in any direction one may choose, is definitely not a spherical wavefront.

BTW you provided a useful link as it not only serves to explain it with a picture but it also links on to an article that explains this same point, although ignoring the phase aspect; there can't be a wavefront at the poles
http://en.wikipedia.org/wiki/Isotropic_radiator#Antenna_Theory
And with that, this little excursion away from the topic is for me closed.

Caught that just in time - glad it (hopefully) all adds up in the end.

 

[Serge58]

If vacuum permittivity and permeability are reduced, then also c is increased. In fact, as measured with a reference system on Earth, the speed of light is slightly reduced near the sun. That was predicted by GR and confirmed by experiments.
I found nice and clear explanations plus animations here:
- http://www.astro.ucla.edu/~wright/deflection-delay.html

Nice article but I don't get the relation with light being stuck at a certain speed due to permittivity and permeability.

 

[Serge58]

Is it possible that light doesn't move at all? Like the water in the previous analogy? That it just dissipates and follow the fabrics of space, even empty ones, as water in nylon?

Can someone help me with this one? The way I see that is that water is not pushed.. (and you might say it moves, I don't know) it looks like it is pulled, sucked, and it is spreading, diffusing, filling the gaps though the fabrics. It doesn't look like there is a force pushing it per se.

What about light? Is there a force pushing it? Or it is just "filling the gaps", being pulled by the vacuum of space?

 

[harrylin]

Nice article but I don't get the relation with light being stuck at a certain speed due to permittivity and permeability.

Those two properties (magnetic and electric) of the vacuum regulate the propagation speed through it, which was already explained in the article that I referred to before. I thus referred to that next article to demonstrate that the neighbourhood or far distance of heavy bodies affects those vacuum or "space" properties, with the result that light propagates slower or faster near or far away from the sun, as measured from Earth.

The way I see that is that water is not pushed.. [..] it looks like it is pulled,[..], filling the gaps though the fabrics. [..]
What about light? Is there a force pushing it? Or it is just "filling the gaps", being pulled by the vacuum of space?

Light motion is more like sound than like water; it's not a coincidence that the same symbol "c" is used for both. The little side discussion related to how extremely well Maxwell's theory predicts wave motion of electromagnetic waves (light is also an electromagnetic wave). It doesn't predict a "filling the gaps" motion, and that would give wrong predictions. With such a theory we would perhaps not even have radio!

So, to obtain even a faint idea of light motion, one should understand ordinary waves. Do you understand how sound propagates? Would you say that sound propagates somewhat like water filling gaps, or somewhat like water waves? Is there a force pushing or pulling sound so that it gets to your ear?

- http://en.wikipedia.org/wiki/Speed_of_sound

 

[Serge58]

So, to obtain even a faint idea of light motion, one should understand ordinary waves. Do you understand how sound propagates? Would you say that sound propagates somewhat like water filling gaps, or somewhat like water waves? Is there a force pushing or pulling sound so that it gets to your ear?

- http://en.wikipedia.org/wiki/Speed_of_sound

Thanks for your time and replies by the way. It is much appreciated.

Yes i understand the basics behind water waves, sound waves and even electromagnetic waves. However, even if it is obvious that a force need to be applied to create a sound or a water wave, it doesn't seems that obvious for electromagnetic waves.

It is assumed that absolute light has an infinite speed?

 

[harrylin]

Thanks for your time and replies by the way. It is much appreciated.

Yes i understand the basics behind water waves, sound waves and even electromagnetic waves. However, even if it is obvious that a force need to be applied to create a sound or a water wave, it doesn't seems that obvious for electromagnetic waves.

It is assumed that absolute light has an infinite speed?

A common feature is that no external force is required to keep them moving.
Further, I have no idea what "absolute light" could mean. For example, what would be "absolute sound"?

 

[Serge58]

A "absolute sound"?

I meant light that isn't limited by the permittivity of the vacuum.

 

[harrylin]

I meant light that isn't limited by the permittivity of the vacuum.

Once more: please explain what you would mean with a similar "absolute sound", which is not "limited" by the properties of air.

 

[Serge58]

Once more: please explain what you would mean with a similar "absolute sound", which is not "limited" by the properties of air.

I've got it. :-) No air, no water = no waves. No fabric of space = no wave. So it can't fill the gaps.

Now what about the photon? Is it created when a wave touches matter? or does he travel as well?

 

[harrylin]

[..]
Now what about the photon? Is it created when a wave touches matter? or does he travel as well?

There you touch a dispute of QM, that only apparently was settled. Please ask questions about photons in the Quantum physics forum.

 

[Dale]

and not to forget the typical SR computation based on "consider a pulse of light that [..] propagates as a spherical wave" such as in
http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
and of course "let a spherical wave be emitted" in
http://www.fourmilab.ch/etexts/einstein/specrel/www/

These are probably OK, as long as they haven't specified coherent light. E.g. you can have a flash bulb which emits an incoherent spherical wave. There is no problem with such wavefronts.

 

[harrylin]

These are probably OK, as long as they haven't specified coherent light. E.g. you can have a flash bulb which emits an incoherent spherical wave. There is no problem with such wavefronts.

To me, an "incoherent wave" is a kind of self-contradiction (or, with some word play, an "incoherent" expression). "Incoherence of one wave" is like clapping with one hand.

 

[Dale]

To me, an "incoherent wave" is a kind of self-contradiction (or, with some word play, an "incoherent" expression). "Incoherence of one wave" is like clapping with one hand.

The geometric impossibility that you mentioned in post 8 is only an impossibility for a coherent wave. I'm sorry that you don't like the term "incoherent wave", but it is a common and well-defined term.

 

[harrylin]

The geometric impossibility that you mentioned in post 8 is only an impossibility for a coherent wave. I'm sorry that you don't like the term "incoherent wave", but it is a common and well-defined term.

Well, I'm certainly more interested in essence than in expressions! Even when allowing for varying transverse direction I still don't see how to obtain a wave that has a transverse component (that is: non-zero so that it exists there) at all points of the sphere. Please show/sketch an example.

 

[Dale]

Well, I'm certainly more interested in essence than in expressions! Even when allowing for varying transverse direction I still don't see how to obtain a wave that has a transverse component (that is: non-zero so that it exists there) at all points of the sphere. Please show/sketch an example.

I am not certain, but I think that two orthogonal dipoles of different frequencies will do it.

 

[Q-reeus]

Well, I'm certainly more interested in essence than in expressions! Even when allowing for varying transverse direction I still don't see how to obtain a wave that has a transverse component (that is: non-zero so that it exists there) at all points of the sphere. Please show/sketch an example.

Was not this all settled back between #21-29? In particular #21 should have sufficed. Apparently not. Why not just accept that as a matter of common definition 'spherical radiation' is not synonymous with 'spherical wave'. Perfectly ok for a spherical intensity distribution of EM radiation to be comprised of an incoherent superposition of waves - e.g sunlight. Whereas no individual component wave can have such spherically uniform intensity. A distinguishing difference between character of EM radiation ('light') vs acoustic radiation ('sound') is that for the latter monopole radiation = spherical radiation = spherical sound wave is quite ok. That's because of the longitudinal nature of pressure waves. As your sketching has discovered, that cannot apply for a transverse wave. Definitions can and are at times bent to suit - one could for instance talk about intensity waves from a blinking light-bulb putting out essentially spherically uniform intensity incoherent radiation. But stick to standard usage, and distinction between radiation per se and wave should finally clear up this ongoing problem. And if it's really still needed, here's a resource that just may finally settle it for you: http://www.scribd.com/doc/27753743/Coherence-Incoherence-And-Light-Scattering

[Note that the single bit labelled 'Spherical Waves' in that article is NOT saying there is an isotropic intensity solution to Maxwell's equations - expression there is concerned with field decay as a function of r - angular dependence is simply omitted. We covered angular dependence back in earlier posts in reference to dipole oscillator fields.]

 

[Serge58]

There you touch a dispute of QM, that only apparently was settled. Please ask questions about photons in the Quantum physics forum.

Thanks. I will check the quantum forum for my queries on photons. I'll have a lot of reading to do. :-)

In the mean time, since the speed of sound in air varies with its density, it is the same for light in vacuum? Are there any variations of densities in the fabric of space? Perhaps caused by gravitational fields? or by any other phenomena?

 

[DrGreg]

In the mean time, since the speed of sound in air varies with its density, it is the same for light in vacuum? Are there any variations of densities in the fabric of space? Perhaps caused by gravitational fields? or by any other phenomena?

The concept of the "fabric" of spacetime can't be taken too literally. We model gravity not by "density" but by "curvature".

If, on a flat piece of paper, you draw a graph of distance-v-time in the absence of gravity, freely moving objects are represented by straight lines. To include gravity you have to draw the graph on a curved surface instead, and then an object falling freely under gravity is represented by "as straight a line as possible" on the curved surface.

Measuring velocity in space is equivalent measuring an angle between two lines on spacetime graph. If you try to map a curved surface on a flat piece of paper, things look distorted and the angles on the paper may not be the same as the true angles on the surface. This is what causes apparent changes to the speed of light -- local observers always measure the same value c, but a distant observer may be using the "wrong map" and get a different value.

 

[harrylin]

I am not certain, but I think that two orthogonal dipoles of different frequencies will do it.

Well, then neither of us is certain about this one! I came to the conclusion that a perfectly spherical transverse excitation (notably for the purpose of detection anywhere simultaneously at distance R) is not possible for a simple geometrical reason: as it uses one of the three spatial dimensions, it looks to me that there always has to be a direction in which it is "in the way" so to say. But if you think that you can get around that with two dipoles of different frequency, and want to show how, then it may be worth a special topic.

Was not this all settled back between #21-29? In particular #21 should have sufficed.

According to me, this was all settled at posts #14 and 17:

"#14 Maxwells equations do not predict a spherical transverse wave. The lowest order of radiation allowed by Maxwells equations is dipole."
#17 "Good point! [..] this is not sufficiently recognized."

[..] Why not just accept that as a matter of common definition 'spherical radiation' is not synonymous with 'spherical wave'. Perfectly ok for a spherical intensity distribution of EM radiation to be comprised of an incoherent superposition of waves - e.g sunlight.

I agree with that: "radiation" is for me sufficiently vague to permit a non-perfect (or "rough") intensity surface IMHO. However, the latest suggestion of Dalespam was not a simple issue of words (I hope!); and so a new little excursion away from the topic was started!

[..] As your sketching has discovered, that cannot apply for a transverse wave. [..] And if it's really still needed, here's a resource that just may finally settle it for you: http://www.scribd.com/doc/27753743/Coherence-Incoherence-And-Light-Scattering

[Note that the single bit labelled 'Spherical Waves' in that article is NOT saying there is an isotropic intensity solution to Maxwell's equations - expression there is concerned with field decay as a function of r [..]

Regretfully that article IS saying what Dalespam convincingly stated to be wrong; as it is just another example to my post #17, the author should correct the following, if it was MEANT or not:

"A spherical wave has spherical wave-fronts [..] A spherical wave is also a solution to Maxwell's equations"

 

[harrylin]

[..] In the mean time, since the speed of sound in air varies with its density, it is the same for light in vacuum? Are there any variations of densities in the fabric of space? Perhaps caused by gravitational fields? or by any other phenomena?

Nobody knows what "space" or "vacuum" is, except that it's not mere nothingness. The only thing we know is the equations that describe its properties (it's those space-time equations that have "curvature" etc.). And concerning gravitational fields, I linked you in post #18 to a clear example of how nearby matter affects those properties in nearby areas.
Einstein phrased it in 1920 as follows: "the metrical qualities of the continuum of space-time [..] are partly conditioned by the matter existing outside of the territory under consideration."

 

[Q-reeus]

According to me, this was all settled at posts #14 and 17:
"#14 Maxwells equations do not predict a spherical transverse wave. The lowest order of radiation allowed by Maxwells equations is dipole."
#17 "Good point! [..] this is not sufficiently recognized."

Fair enough then. So it all get's down imo to this issue of hang-ups over definitions was getting at last post.

Regretfully that article IS saying what Dalespam convincingly stated to be wrong; as it is just another example to my post #17, the author should correct the following, if it was MEANT or not:

"A spherical wave has spherical wave-fronts [..] A spherical wave is also a solution to Maxwell's equations"

Reason for bracketed edit was in anticipation that part would be a focus of yours! I agree it was worded not the best and even the accompanying diagram was suggestive of spherically uniform intensity. It all has to be taken though in context of what preceded that part and interpreted accordingly. A sensible reconciliation is that that piece was intended to illustrate the time-averaged scattered field of a 'point scatterer' receiving, over time at least, an incoherent flux of incident radiation. On that statistical time-averaged basis it can be considered to be a spherically symmetric source of radiation without any conflict. This assumes the scatterer itself has an effectively spherical symmetry at least on a time-averaged basis. Which should apply to say an ion at some cubical lattice sight. As we discussed previously, the term 'spherical' wrt e.g. dipole oscillator wave is in respect of the wavefront phase, not amplitude. [Which also implies everywhere-radial propagation vector.] The earlier referenced Wiki article on dipole oscillator gives the full expression inclusive of field angular amplitude which was absent in above cited article. And there are in general higher-order multipole moments to consider. Did the rest of that article help at all? Hope so.

 

[harrylin]

So it all get's down imo to this issue of hang-ups over definitions was getting at last post.[..] As we discussed previously, the term 'spherical' wrt e.g. dipole oscillator wave is in respect of the wavefront phase, not amplitude. [..]

Both side discussions concerned the phase on a spherical surface as given by Maxwell's equations. If Dalespam decides to start a thread on his last idea, we can discuss it there.