Electromagnetic wave questions

[Born2bwire]

Let me put this out there in hopes of further clarification. In classical electromagnetics, the electromagnetic wave's primitives, that is the basic components of the wave, are the electric and magnetic field. These fields have three basic properties, amplitude, frequency and phase. By manipulating these three properties we can encode information into the wave.

In quantum electrodynamics, the primitives are actually the vector and scalar potentials. These potentials are represented as quantum fields and the excitation of the energy level of these fields are photons. The observables of the vector and scalar potential fields are the electric and magnetic fields. So we can observe the electric and magnetic fields, but there are underlying fields that can give rise to consequences even if there are no electric and magnetic fields observed (Aharonov-Bohm Effect). With quantum electrodynamics, we still talk about the electric and magnetic fields and we now include photons as a possible measurement of the electromagnetic wave. However, there isn't much we can do with photons at the moment (in large numbers, for low amounts of money, etc.). They have some properties like spin which we can measure but we cannot really create a photon with a desired set of properties at will. Usually we just have a system that generates a crap load of photons and we filter out the photons that have the desired properties or something loosely to that effect. This is a hot topic with quantum computing, being able to copy the properties of photons, of maintaining coherence, etc. This means that for us, the photon is not a viable means of long distance communication. Probably the best measurement we can do reliably now is photon counting and frequency detection, ie the CCD on your digital camera. But there is not much we can garner from such limited information as DaleSpam and I have tried to show by briefly talking about coherent states in quantum optics. That is, even with photons in the picture with quantum optics we are not really using them directly to aid in our goals. Thus, photons are not really directly connected with the encoding of information even with systems designed around quantum physics.

 

[alan.b]

... But at the high frequency limit, we may have a length scale where the wave will go over 5,000 cycles. The phase is almost random because if we observe at A, the phase maybe 10 degrees but at A+1 cm, it may be -190 degrees because the phase changes so rapidly over a small distance. This causes most of the wave effects to cancel out or, more accurately, not be noticeable.

Why would phase spontaneously change in mid-air?

What does frequency have to do with the "phase change"?

... Instead, we manipulate the electric and magnetic fields and their phases, frequency and/or amplitudes.

Can we then manipulate these same electric and magnetic fields, their phases, frequency and/or amplitudes when emitting em waves in the visible light range, or not?

What is the amplitude of, say - red light? If two beams of red light emit em waves with the same wavelength but different amplitudes, would they appear same or different in color?

 

[Born2bwire]

Why would phase spontaneously change in mid-air?

What does frequency have to do with the "phase change"?




Can we then manipulate these same electric and magnetic fields, their phases, frequency and/or amplitudes when emitting em waves in the visible light range, or not?

What is the amplitude of, say - red light? If two beams of red light emit em waves with the same wavelength but different amplitudes, would they appear same or different in color?

At this point, I will again refer you to learn the basics of wave physics as all of these issues deal are covered by a very basic understanding of waves.

But as for the second question. We could, but it is my offhand response to say that the only reliable method is amplitude modulation. I believe that because of the relatively short coherence lengths of visible light sources that it would be difficult to use a phase or frequency modulation scheme for long distance communication. That is because the signal will drift in its phase/frequency after a given distance enough so that the modulation is lost. I have asked some of my colleagues for their views on the subject. Amplitude modulation is easy.

 

[alan.b]

At this point, I will again refer you to learn the basics of wave physics as all of these issues deal are covered by a very basic understanding of waves.

I have asked some of my colleagues, they say the phase should not spontaneously change in mid-air and that frequency has nothing to do with the possibility of some random changes of the phase.

They said you should take the time to read more about wave physics, signal processing in terms of encoding the information and basic classical electromagnetics, and that a lot of your misconceptions would be cleared up by a very basic understanding of these areas. Wait, no.. that's what you said to me. Ugh, can you at least point out some of those misconceptions so I actually know what is it you hallucinated I might be wrong about?

 

[Dale]

Why would phase spontaneously change in mid-air?

What does frequency have to do with the "phase change"?

The equation for a plane wave is:
A e^{i(k \cdot x-\omega t)}
Where A is the amplitude, k is the inverse of the wavelength (aka the wavenumber) and omega is the frequency. The phase of this number obviously changes by 2 \pi every time x changes by 2 \pi /k or t changes by 2 \pi / \omega

If two beams of red light emit em waves with the same wavelength but different amplitudes, would they appear same or different in color?

Same color.

I have asked some of my colleagues, they say the phase should not spontaneously change in mid-air

Your colleagues are also wrong, see the above. They are welcome to join the conversation.

 

[alan.b]

The equation for a plane wave is:
A e^{i(k \cdot x-\omega t)}
Where A is the amplitude, k is the inverse of the wavelength (aka the wavenumber) and omega is the frequency. The phase of this number obviously changes by 2 \pi every time x changes by 2 \pi /k or t changes by 2 \pi / \omega

I was thinking of different "phase". The one you create by emitting the waves with certain delay, so this time delay I was thinking it would be a "phase shift", which I believe is how it is understood or "accomplished" in regards to 'signal encoding'.


I can not understand the original information in the context of the meaning of "phase" you are talking about. If b2b was referring to those properties he would have said 'frequency' and 'amplitude' or 'wavelength', not 'phase', I think. But, none of those properties should really change randomly, all those properties must persist, otherwise the information will get corrupted, out of the sync or out of the band.

Your colleagues are also wrong, see the above. They are welcome to join the conversation.

I made that up... I don't have any friends, you are my only friend, and GreenLantern-Man!

 

[Dale]

I was thinking of different "phase". The one you create by emitting the waves with certain delay, so this time delay I was thinking it would be a "phase shift"

Yes, that is also known as phase. In my experience it is usually called "initial phase" to distinguish it from the usual meaning of "phase" which varies over time and space.

none of those properties should really change randomly, all those properties must persist, otherwise the information will get corrupted, out of the sync or out of the band.

Yes, that can certainly happen in a communication system, as any cell-phone user knows. It is all the same in principle for optical and radio, but the practical considerations obviously vary greatly.