Speed of water and relation to light

[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. :tongue2:

 

[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. :tongue2:

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 :uhh:

[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 :uhh:

[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.

 

[Serge58]

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."

Very nice quote from Einstein. I will start a new topic since this discussion is too far way from the original subject. Thanks