Electromagnetic wave questions

[alan.b]

Different substances do emit, reflect, and/or scatter different kinds of em waves besides visible light.

Thank you, that is all I wanted to say. In conclusion, "radio antenna" can indeed glow with visible light, but only if it was made from the right material.

 

[GreenLantern]

I think all these people that are so CLEARLY confused on light/photons etc. really need to check out DeBroglie's work.

Long story short guys, along the EM spectrum (and well with particles as well but let's not get into that) all EM waves behave like particles (photons/quanta) as well as waves. AT THE SAME TIME.

The difference lies in how much of particle and how much of wave each one characteristics each type of "light" exhibits.

Back to the question of Radio waves and the double slit experiment and what i just said above,

EM waves of longer wavelength (and correspondingly lower frequency) like Radio waves behaving MORE LIKE A WAVE than particle. Higher Frequency and shorter wavelength EM waves behave more like particles.

Someone asked the question earlier something about if a radio wave or similar could eject electrons when striking metal. The answer is NO. It is like this, Imagine long wavelength EM waves being like a foam hammer and short wavelength EM waves being like a metal hammer. Now, if i threw a whole crap ton of those foam hammers at you, it wouldn't really harm you. But if i threw ONE regular hammer at you, you'd have some issues. Hence the reason why some will and some will not eject electrons.

The higher energy EM waves (i.e. Gamma rays etc..) are better understood as particles because they behave more like particles (just think about it. CRAZY short wavelength and crazyy high frequency of oscillation propagating at speed C. Sounds like it would behave as a particle more than a wave. RADIO wave definitely do not behave as particles anywhere near as much because, if they did, how are they exactly going to travel over mountains and around all the other crap in our world so that we can get stations on our radios?)

Summary, Radio waves would most likely perform in the double slit even better than visible light does. Gamma rays most like would suckkkk in the double slit in producing the interference pattern viewed when done with visible light. but all have some properties of both waves and particles.

 

[GreenLantern]

Thank you, that is all I wanted to say. In conclusion, "radio antenna" can indeed glow with visible light, but only if it was made from the right material.

yeah man for sure. and think about it, you again only need find the right material, and if you could make the radio antenna produce gamma rays, DEATH RAY MAN :)

Emitting an EM wave with a frequency of something on the order of 10^21 Hz from a radio transmitter (which are usually linearly polarized emitting waves that, in a horizontal plane around the antenna, are polarized in the vertical direction) could be pretty cool LOL

 

[GreenLantern]

Now, take the red flashlight and shine it through different materials and see what happens - depending on the substance itself we will see this beam of red light, maybe it will be pinkish or yellowish rather than red and maybe beside these visible photons there will be other ranges of em waves scattered as well, right?

I don't think so man, Now don't quote me, and we kinda need to clarify what you mean by red flashlight. if you mean like a monochromatic (almost ;) ) light emitting source then I'm pretty sure the beam stays the same color through different materials. What changes in other materials is the WAVELENGTH not the frequency of the EM wave.


Now if your red flashlight was NOT a basically monochromatic source, then shining it through other materials may disperse the light some (like white light in a diamond or through a prism) making a little rainbow (granted the proper angles are there) of different "red lights". Because the speed of light (and since n=c/v the index of refraction of light in a specific material) through a medium is directly dependent on the wavelength of the light. (dispersion- the dependence of wave speed and index of refraction on wavelength)

(c= speed of light in vacuum. $$\lambda_{0}$$ = wavelength in vacuum. V=speed of light in material. $$\lambda$$ = wavelength in material. n= index of refraction)

n=c/v = $$\lambda_{0}$$ƒ / $$\lambda$$ƒ = $$\lambda_{0}$$ / $$\lambda$$

therefore $$\lambda$$ = $$\lambda_{0}$$ /n
independent of ƒ. The frequency ƒ of the wave does not change when passing from one material to another. That is, the number of wave cycles arriving per unit time is equal to the number of wave cycles that leave per the same unit of time. Essentially saying that the material boundary surface cannot create or destroy waves.

$$\lambda$$ IS different in general in different materials. When a wave goes from one material into another with larger index of refraction, the wave speed decreases. The wave length in the second material is then shorter than it was in the first and Vise versa. The waves get "squeezed" if the wave speed decreases and "stretched" (wave length gets longer) if the wave speed increases.

 

[Bob S]

EM waves of longer wavelength (and correspondingly lower frequency) like Radio waves behaving MORE LIKE A WAVE than particle. Higher Frequency and shorter wavelength EM waves behave more like particles.

1420-MHz microwaves (longer wavelength than the 2450 MHz power in microwave ovens) can flip the hyperfine structure dipole in hydrogen, like in intersteller gas. When a hydrogen hyperfine dipole radiates, it radiates a single 1420 MHz photon ("particle"). So this is a case of a radio wave behaving like a "particle".

Bob S

 

[GreenLantern]

i never said they only behave like waves ;)

 

[GreenLantern]

OH i just ran into something in my textbook incredibly relevant for this WHOLE discussion:

From University Physics 12th ed. by young and freedman:

"Electrons in the red and white broadcast antenna oscillate vertically producing vertically polarized electromagnetic waves that propagate away from the antenna in the horizontal direction."

"The situation is different for visible light. Light from ordinary sources, such as incandescent light bulbs and fluorescent light fixtures, is not polarized. The "antennas" that radiate light waves are the molecules that make up the sources. The waves emitted by anyone molecule may be linearly polarized, like those from a radio antenna. But any actual light source contains a tremendous number of molecules with random orientations, so the emitted light is a random mixture of waves linearly polarized in all possible transverse directions."

"No matter how this [incandescent] light bulb is oriented, the random motion of electrons in the filament produces unpolarized light waves"

So i guess if you could get that vertical radio antenna of yours to have it's electrons oscillate vertically as they do but at a visible light frequency then, which makes crazy sense, the light emitted would be polarized vertically. Interesting huh?! The electric field oscillation from these oscillating electrons would only be in the vertical direction producing POLARIZED LIGHT from this antenna.

So, would you be able to see it?? Sure I'd assume so. Just like how you can see light through your spiffy polarized sun glasses.

Would it GLOW?? well, i'd guess not.

I think it would be emitting light but i wouldn't define it as glowing. It would emit polarized light so i guess it would be like the difference between seeing unpolarized light (normal light from a bulb) without polarized sunglasses on, and saying its "glowing" verses looking at light through polarized sunglasses (no glare) and saying its not glowing. But, the same polarizer on that radio antenna most likely cannot effectively (or at all?) polarize EM waves with much shorter wavelengths than radio waves (like light has).

Hows that for an answer to your questions OP??

 

[alan.b]

EM waves of longer wavelength (and correspondingly lower frequency) like Radio waves behaving MORE LIKE A WAVE than particle. Higher Frequency and shorter wavelength EM waves behave more like particles.

Thanks for all the input. I call that "digital and localized" vs "analog and continuous".

Originally I wanted to talk about very simplified example of transmitting certain movie (collection of images) with certain resolution and frame rate in a given time. I wanted to look at this situation in terms of transmitted bytes of information and the minimum numbers of em waves sent (energy used) to accomplish this task with radio waves and light waves.

What is interesting here is that our picture is quantified by the number of pixels and bits of information need to be sent and received, so there seem to be a paradox in relation between the number of bytes transmitted, number of photons emitted and the number of photons and bytes received. The paradox is somewhere around the point where these em waves go from "more like wave" to "more like particle". Hmm?

I don't think so man, Now don't quote me, and we kinda need to clarify what you mean by red flashlight. if you mean like a monochromatic (almost ;) ) light emitting source then I'm pretty sure the beam stays the same color through different materials. What changes in other materials is the WAVELENGTH not the frequency of the EM wave.

Is it not the wavelength what defines the color? Is it not the wavelength just one side of the same coin where amplitude is another and where this coin is called energy or frequency, i.e. "color"? -- I mean some ordinary light with narrow beam and with some red paint over its glass to make the light beam cast a red circle on a white wall. Blue car looks green under yellow light, right? So, red beam passing through blue dust should make it look pinkish?

I think we better just talk about electron beams or simple electric currents, since we are talking about "radio antennas", but in any case the question is whether those "secondary" em waves, due to interaction with the medium, are the ONLY ones. I think we all agree now there should be certain materials and voltages that would produce wide spectra of electromagnetic radiation, including light and radio waves, and thanks for clarifying that.

Would it GLOW?? well, i'd guess not.

Is there anything that emits visible light and does not "glow"? Maybe it would not be as bright as a light bulb, but certainly the intensity would depend on the amount of em waves produced, which ought to have some relation with the amount of information transmitted, the distance, position and even the number or receivers.

So i guess if you could get that vertical radio antenna of yours to have it's electrons oscillate vertically as they do but at a visible light frequency then, which makes crazy sense, the light emitted would be polarized vertically. Interesting huh?! The electric field oscillation from these oscillating electrons would only be in the vertical direction producing POLARIZED LIGHT from this antenna.

You mean the light would encode the information the same way radio waves do?

Very interesting indeed, thanks for all that info.

 

[Dale]

I'm referring to electromagnetic radiation that comes as a byproduct of beam interaction with the propagation medium, I'm not talking about the energy of the beam itself.

Take a red flashlight and shine the beam through vacuum, looking at the beam from the side we should see nothing since there is no any scattering, right?

Now, take the red flashlight and shine it through different materials and see what happens - depending on the substance itself we will see this beam of red light, maybe it will be pinkish or yellowish rather than red and maybe beside these visible photons there will be other ranges of em waves scattered as well, right?

Sure. And the same is true of radio waves. That is exactly how the cop's radar gun works.

 

[Born2bwire]

OH i just ran into something in my textbook incredibly relevant for this WHOLE discussion:

From University Physics 12th ed. by young and freedman:

"Electrons in the red and white broadcast antenna oscillate vertically producing vertically polarized electromagnetic waves that propagate away from the antenna in the horizontal direction."

"The situation is different for visible light. Light from ordinary sources, such as incandescent light bulbs and fluorescent light fixtures, is not polarized. The "antennas" that radiate light waves are the molecules that make up the sources. The waves emitted by anyone molecule may be linearly polarized, like those from a radio antenna. But any actual light source contains a tremendous number of molecules with random orientations, so the emitted light is a random mixture of waves linearly polarized in all possible transverse directions."

"No matter how this [incandescent] light bulb is oriented, the random motion of electrons in the filament produces unpolarized light waves"

So i guess if you could get that vertical radio antenna of yours to have it's electrons oscillate vertically as they do but at a visible light frequency then, which makes crazy sense, the light emitted would be polarized vertically. Interesting huh?! The electric field oscillation from these oscillating electrons would only be in the vertical direction producing POLARIZED LIGHT from this antenna.

So, would you be able to see it?? Sure I'd assume so. Just like how you can see light through your spiffy polarized sun glasses.

Would it GLOW?? well, i'd guess not.

I think it would be emitting light but i wouldn't define it as glowing. It would emit polarized light so i guess it would be like the difference between seeing unpolarized light (normal light from a bulb) without polarized sunglasses on, and saying its "glowing" verses looking at light through polarized sunglasses (no glare) and saying its not glowing. But, the same polarizer on that radio antenna most likely cannot effectively (or at all?) polarize EM waves with much shorter wavelengths than radio waves (like light has).

Hows that for an answer to your questions OP??

This was already shown as a viable way of producing high frequency light via charge acceleration when we mentioned the synchrotron radiation. In fact, because the synchrotron accelerates the charges within a single plane, then the emitted radiation is polarized.

The difference with an incandescent bulb is that a tungsten bulb creates light by acting as a black body radiator. In this mechanism, the polarization of the light that is emitted is random giving a net effect of unpolarized light. But there are mechanisms, like the synchrotron, where we can generate polarized light by controlling the aspects of the light production. Of course, we can always just use a polarizer which is essentially the same for RF or visible light. A simple polarizer that would work across the spectrum is a grating. The only difference is that the grating's dimensions are dependent upon the wavelength of the light we wish to polarize. So the grating for RF is very coarse compared to the grating that you can use for visible light. Another simple mechanism is reflecting the light off of a planar surface. This is why polarized sunglasses are so effective, the light that reflects up into our eyes that creates the glare is polarized by the reflection. So, bouncing light off of a table or a lake will polarize it too.

As for mentioning deBroglie, ehhhh... I only like talking about him on the level of basic high school physics. His ideas of matter wave were the precursor to wave mechanics but I think it is usually more beneficial to use the far more accurate quantum mechanics if we are going to bring a discussion above the classical level. Otherwise, deBroglie is still a quasi-quantum theory and usually leaves more questions or misconceptions (what does the wavelength represent, is the particle oscillating in physical space, etc.). Not that wave mechanics are easier to understand but I feel that they can prevent some misconceptions.

Thanks for all the input. I call that "digital and localized" vs "analog and continuous".

Originally I wanted to talk about very simplified example of transmitting certain movie (collection of images) with certain resolution and frame rate in a given time. I wanted to look at this situation in terms of transmitted bytes of information and the minimum numbers of em waves sent (energy used) to accomplish this task with radio waves and light waves.

What is interesting here is that our picture is quantified by the number of pixels and bits of information need to be sent and received, so there seem to be a paradox in relation between the number of bytes transmitted, number of photons emitted and the number of photons and bytes received. The paradox is somewhere around the point where these em waves go from "more like wave" to "more like particle". Hmm?



Is it not the wavelength what defines the color? Is it not the wavelength just one side of the same coin where amplitude is another and where this coin is called energy or frequency, i.e. "color"? -- I mean some ordinary light with narrow beam and with some red paint over its glass to make the light beam cast a red circle on a white wall. Blue car looks green under yellow light, right? So, red beam passing through blue dust should make it look pinkish?

I think we better just talk about electron beams or simple electric currents, since we are talking about "radio antennas", but in any case the question is whether those "secondary" em waves, due to interaction with the medium, are the ONLY ones. I think we all agree now there should be certain materials and voltages that would produce wide spectra of electromagnetic radiation, including light and radio waves, and thanks for clarifying that.




Is there anything that emits visible light and does not "glow"? Maybe it would not be as bright as a light bulb, but certainly the intensity would depend on the amount of em waves produced, which ought to have some relation with the amount of information transmitted, the distance, position and even the number or receivers.



You mean the light would encode the information the same way radio waves do?

Very interesting indeed, thanks for all that info.

You really need to follow ZapperZ's advice and take the time to learn the theory before you continue with your speculations. There are problems with your undestanding over a broad spectrum of topics listed here and we cannot hope to clear up your misconceptions to any large degree. More importantly, if you took the time to learn how we encode information into light then most of these questions would be answered. There is no paradox here, we do not encode information by using photons nor are they related due to the mechanisms that we use. At best, we can only reliably count photons and measure their basic properties (spin, frequency). The real observables here are the electric and magnetic fields which we can easily manipulate en masse.

 

[alan.b]

You really need to follow ZapperZ's advice and take the time to learn the theory before you continue with your speculations. There are problems with your undestanding over a broad spectrum of topics listed here and we cannot hope to clear up your misconceptions to any large degree.

Please, explain:
- What statement you consider speculation?
- What are my problems and misconceptions?

More importantly, if you took the time to learn how we encode information into light then most of these questions would be answered.

I like to learn, that's why I took time and came here to ask about it, do you mind? I know how information can be encoded, there are many ways just like you explained previously, some methods are analog and some are digital. What I do not know is how it is done in practice and to what point can it be simplified, either theoretically or practically. Particle wave duality is paradox in itself, especially when you boil it to the point of individual quanta and specific discrete bits of information they carry, though I agree that in reality there is no paradox.

There is no paradox here, we do not encode information by using photons nor are they related due to the mechanisms that we use. At best, we can only reliably count photons and measure their basic properties (spin, frequency). The real observables here are the electric and magnetic fields which we can easily manipulate en masse.

I was referring to 'photons' as electromagnetic quanta of light and radio waves alike, having them being the same thing. Now, what you say seem against everything we were talking about here before, surely we do use "photons" (em waves), i.e. their properties, to encode information, like you previously said, so please explain your new opinion...

What do you mean by: - "we do not encode information by using photons nor are they related due to the mechanisms that we use."

What do you mean by: -"At best, we can only reliably count photons and measure their basic properties (spin, frequency)."


When did you change your mind about signal encoding via amplitude and/or frequency modulation and by encoding bits in the phase space?

 

[Dale]

what you say seem against everything we were talking about here before, surely we do use "photons" (em waves), i.e. their properties, to encode information, like you previously said, so please explain your new opinion...

What do you mean by: - "we do not encode information by using photons nor are they related due to the mechanisms that we use."

What do you mean by: -"At best, we can only reliably count photons and measure their basic properties (spin, frequency)."

When did you change your mind about signal encoding via amplitude and/or frequency modulation and by encoding bits in the phase space?

It is a subtle point, but Born2bWire is correct. This is probably going to sound like nonsense, but unfortunately I am not expert enough in the subject myself to simplify it well.

In quantum mechanics there is a specific kind of state, called a http://en.wikipedia.org/wiki/Fock_state" where there are a definite number of photons. Other perfectly valid quantum mechanical states are not Fock states and have some quantum uncertainty associated with the number of photons.

In particular, the quantum state known as a http://en.wikipedia.org/wiki/Coherent_state" does not have a definite number of photons. A coherent state is a state that is most analogous to "classical" EM waves and is the state that is most widely used for communication purposes. One interesting feature is that coherent states are eigenstates of the annihilation operator, meaning that detecting a photon in a coherent state does not change the state.

So, for most of our communication systems Born2bWire is correct, the information is encoded in the phase of a coherent state, and not in any particular property of individual photons.

 

[alan.b]

Maybe I missed your point, but you either must be talking about some practical and technical difficulties, or suggesting there is some additional or different physics involved with em waves of higher energies like visible light.


If he is talking about practical approach, he might be right, but then I'd like to know why and where exactly (at what frequency range) do we "loose" this ability to manipulate em waves in the same way we manipulate radio waves.


The whole point of this thread is how radio waves appear to be "more like waves" and how visible light appears to be "more like particles", but the conclusion was - NO, they are ONE AND THE SAME THING, only different energy state. Now, all of a sudden Born2bwire is talking about visible light radiation as something we can ONLY COUNT, as if he is talking about marbles, hence justifying and re-opening the question - is there some (unexplained) difference between radio and light waves?


But the most puzzling thing for me is that he refers to "counting photons" as something we can do "AT BEST", as if this counting and frequency readout is not the very best resolution we can hope for. What is better, what can possibly be more precise than reading the properties of individual quanta of em radiation?


If radio and light wave "photons" are the one the same thing then I expect:

a) if we can count light photons, we should be able to count radio photons, can we?
b) if we can modulate radio waves, we should be able to modulate light waves, can we?



Additionally, what happens with radio waves when they strike antenna? They produce electric current ok, but do they disappear or they pass through? Do they lose energy, what happens to information when these radio waves scatter, interfere, diffract, refract and reflect? If they lose or change energy, then that would be a serious problem for analog signal encoding, and even for digital encoding I would expect much more noise in the signal, but these radio waves sure seem indestructible in the way they manage to preserve information despite all the obstructions they pass through, and around.

And, when radio waves produce electric current in an antenna, just what part of its electric or magnetic component is responsible for this, I mean since em radiation (photons) is supposed to have zero electric charge? The only way I see photons (em waves) can be electrically neutral and yet still have electromagnetic properties is if it were composed of both positive and negative charges, like it's depicted in this Wikipedia article:

http://en.wikipedia.org/wiki/Electromagnetic_radiation

 

[Dale]

Maybe I missed your point, but you either must be talking about some practical and technical difficulties, or suggesting there is some additional or different physics involved with em waves of higher energies like visible light.

If he is talking about practical approach, he might be right, but then I'd like to know why and where exactly (at what frequency range) do we "loose" this ability to manipulate em waves in the same way we manipulate radio waves.

The whole point of this thread is how radio waves appear to be "more like waves" and how visible light appears to be "more like particles", but the conclusion was - NO, they are ONE AND THE SAME THING, only different energy state.

That is correct. Visible light and radio are different ranges of the same thing (EM radiation).

Now, all of a sudden Born2bwire is talking about visible light radiation as something we can ONLY COUNT, as if he is talking about marbles, hence justifying and re-opening the question - is there some (unexplained) difference between radio and light waves?

There is no difference (other than frequency). Both visible light and radio waves may be in a Fock state where the number of photons is definite. Both visible light and radio waves may be in a coherent state where the number of photons is not-definite. Neither visible light nor radio waves may simultaneously be in a Fock state and a coherent state. Both visible light and radio waves used in most modern communications equipment use coherent states where the number of photons is not definite.

But the most puzzling thing for me is that he refers to "counting photons" as something we can do "AT BEST", as if this counting and frequency readout is not the very best resolution we can hope for. What is better, what can possibly be more precise than reading the properties of individual quanta of em radiation?

Born2bwire can correct me if I misunderstand his intentions, but I believe his "at best" refers to the fact that you cannot "count photons" in a typical coherent state because due to quantum uncertainty there simply is not a definite number of them.

If radio and light wave "photons" are the one the same thing then I expect:

a) if we can count light photons, we should be able to count radio photons, can we?

For both, yes, but only if the system is in a Fock state (which is not usual)

b) if we can modulate radio waves, we should be able to modulate light waves, can we?

Certainly, that is how fiber optic communications work.

 

[Born2bwire]

You are asking questions that cover a huge range of knowledge and topics and you seem to have a cursory understanding of them at best. You need a better understanding of the basics of wave physics, classical electromagnetics, quantum physics, quantum electrodynamics, digital signal processing, analog signal processing, and optics to name just a few of the topics your questions have been hitting upon.

You keep asking how low frequency are more wave like and higher frequency are more "particle" like. This is simple basic wave physics. Unlike what Greenlantern suggest by looking into deBroglie, you do not need to bring in quantum physics to see this. Wave physics are most prominent when the feature size and observation scales are on the order of the wavelength. This is when diffraction, refraction, and interference effects are most prominent. As we go up the scale, by increasing the feature size and increasing the length scale compared to the wavelength, the waves behave like particles. One reason for this is that the phase dependence of the wave now varies rapidly over the distance of our problem. Before, we may be observing the scattering of a wave over an area where the wave may go through 5 full cycles in its phase. 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.

Even if you took classical electromagnetics and looked at it in the optical frequencies, it would behave like a "particle" because the wave properties would be practically non-existent. In high frequencies we thus often use simple ray optic physics as an approximation with techniques like Geometric Optics or Ray-Tracing Optics.

I never reversed myself on how information is encoded into a signal. You should take the time to research signal processing and how this is done to understand why this has nothing to do with photons as I keep stating. As DaleSpam stated previously, we cannot feasibly measure the properties of photons (let alone deterministically assign such properties to a group of photons we wish to send out). Instead, we manipulate the electric and magnetic fields and their phases, frequency and/or amplitudes. With quantum optics, we do talk about coherent states or squeezed states in terms of photons, but these are rather coarse way of dealing with photons. I hesitate to spend any time on this because this is WAY beyond the level which we should be on for this discussion. The purpose of these states are that the noise properties are better than the classical limit. This way we can improve the noise properties of our communications system. However, these states are not defined by the number of photons and such. They are a very complicated combination of number states which means that the number of photons that you will observe for identical states will vary. I don't think we should discuss this too much further but I was mainly implying that at best we can have a CCD or some kind of detector that simply counts photons. We do not have a way of dealing with assigning and detecting the properties of photons in large numbers. But this is immaterial because any reasonably large amount of photons give rise to electric and magnetic fields that behave very much like (if indeed exactly the same within error) to classical fields.

You should take the time to read more about wave physics, signal processing in terms of encoding the information, and basic classical electromagnetics. A lot of your misconceptions would be cleared up by a very basic understanding of these areas. Quantum mechanics isn't really needed to understand most of the basic stuff but unfortunately some of the weirder or more technical explanations require it and it usually just gunks up the conversation.

Born2bwire can correct me if I misunderstand his intentions, but I believe his "at best" refers to the fact that you cannot "count photons" in a typical coherent state because due to quantum uncertainty there simply is not a definite number of them.

Yes, yes. If we are talking about quantum optics where we are dealing with optics on the level of photons, the way we send signals will cause us to measure the number of photons for identical signals to vary. Only the distribution of the number of photons that would be measured will be the same (but of course you need to measure such an identical signal again and again and again to determine the photon number distribution). Instead, we are interested in other properties of this signal, like its phase.

 

[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.