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Electromagnetic wave questions

  1. Mar 3, 2010 #1
    [I have rephrased my questions below in reply #7. Basically, could there be 2 types of Electromagnetic Energy?]

    Here is what I meant:

    Here's the thing. I understand the 'textbook' explanations for these questions already so I'm not really searching for a recital of that information. I have reasons to challenge the status quo here and am hypothesizing that the properties of EM emitted from radio towers and microwave ovens has slightly different properties than EM emitted from the sun or heated body.

    So I guess my real question is, how can I test these hypotheses or find real empirical (not theoretical - I don't care what Einstein says) information to refute them?

    I hypothesize that if the frequency emitted from a radio tower was increased up to the point that it sits within the visible light spectrum, that the radio tower would not appear to 'glow'. The EM emitted from this tower would not be visible by the eye. It would not be equivalent to EM of the same frequency emitted from a light bulb.

    I hypothesize that EM emitted from a radio tower would never be capable of a Doppler Red Shift. If the radio tower were receding very quickly away from the observer, the frequency of the EM would not red-shift.


    Here was my original set of questions:




    I am trying to better understand electromagnetic energy and how radio waves emitted from an antenna or microwaves are the same as light from a light bulb or from an oven element.

    Here are some things that seem to be different about them:

    1) Do radio waves become ‘visible’ to the eye at visible light frequencies?

    2) Can heat waves be picked up on a radio?

    3) Do radio/magnetism/static-charge have a detectable ‘quantum’ (particle) form? For real, not just in theory? (ie, do they carry momentum that can be detected like with photons?)

    4) Are certain frequencies of radio waves blocked by glass like certain frequencies of light waves are?

    5) Does increasing the frequency of a microwave increase its energy; I think probably not since this increase in energy output would require an increase on the input side as well, but changing the frequency doesn’t likely require any more or less energy.

    Thanks to anyone who can help out!!! :)
     
    Last edited: Mar 4, 2010
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  3. Mar 3, 2010 #2

    Pengwuino

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    1) Yes, but then they are no longer radio waves
    2) It depends what you mean by "heat". If you mean radiative heat (such as infrared from humans), not really since a radio seeks to pick up electromagnetic radiation in the radio spectrum
    3) Electromagnetic waves are photons if you're thinking about that.
    4) Interesting question! Quite a coincidence since today I was recalling an episode of mythbusters that, although not as part of the myth, showed infrared radiation is blocked by whatever glass they were using. It was pretty neat.
    5) Higher frequencies electromagnetic radiation has higher energy. I'm not sure what you mean by "input side" and "output side" however.
     
  4. Mar 3, 2010 #3

    Matterwave

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    You should realize that these names we have for "radio waves" or "microwaves" or "visible light" just define a certain stretch of the EM spectrum.

    1) So, the question "will radio waves be visible at visible frequencies" is a contradiction. Radio waves are waves NOT at visible frequencies, otherwise, you'd call those waves "visible light". Kind of like how dollars, quarters, nickles, all define a certain amount of currency. Your question 1 is kind of like asking "Would nickles be worth 25 cents if they were quarters?"

    2) There are no such thing as "heat waves". Perhaps you are referring to infrared rays?

    3) Radio waves have quantum, also photons. All EM radiation have quantum of photon. Magnetism and static charges are not EM waves like radio and light. They are fields. They are also quantized into virtual photons...which are the force carriers; however, this is very different than the photons of EM waves.

    4) I'm not too familiar with radio wave transmission through glass...

    5) A photon of microwave radiation with a higher frequency will have higher energy; however, the energy delivered by a microwave is also dependent on how many photons are there (also known as the "Intensity" or "Flux" in various contexts).
     
  5. Mar 4, 2010 #4
    Thanks Pengwuino. In regards to your responses:
    1) I understand that increasing the frequency would 'classify' them as no longer being radio waves however I am referring to a situation where we still emit them from an antenna - but with a much higher frequency (created by a circuit board). In other words, would a radio tower start to glow visibly if you increased the frequency.

    2) I understand that a radio 'seeks' to pick up frequencies from the radio spectrum, but again, if we focused the crystals to pick up the appropriate frequency - that of radiative heat, would the radio pick up on it? If the only difference between radio and thermal radiation is frequency then it ought to work.

    3) I understand that radio waves are 'said' to be photons but do these photons in this case carry momentum? (ie, can they for instance collide with a metal plate causing electrons to be released if at a high enough frequency - again assuming a light-frequency but emitted from a radio tower). Or, do radio waves carry 'spin' and have quantum entanglement like photons from the sun?

    4) I expect that infrared radiation would be blocked by glass - since that is light from a lightbulb and glass is known to block various frequencies. I am asking about waves from a radio tower - do they get blocked by glass. and if those frequencies were increased to the frequencies of infrared light, would they be blocked by the glass in the same way?

    5) I understand that higher frequency means higher energy. This is the rule for light waves (from the sun, etc). But what about waves from a microwave oven? According to this theory, increasing the frequency of the waves sent to your food ought to increase the rate it is heated if the energy becomes higher (assuming the frequency remains within the absorbable range). But, from an electronics point of view, it does not take more energy to increase the frequency - you just shorten or lengthen the time between peaks of the wave you are sending. Therefore, if it requires no more energy to produce the higher frequency then according to the law of conservation of energy, this higher frequency cannot carry more energy - but that then conflicts with the statement that "higher frequencies carry higher energy." It seems that frequencies emitted by microwaves or radio towers don't vary in energy in the same what that light from the sun or a lightbulb do.

    Thanks.
     
  6. Mar 4, 2010 #5
  7. Mar 4, 2010 #6

    Born2bwire

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    Light is light, whether it be infrared, radio frequency, x-ray or visible light. The same physics that occurs over one spectrum of light applies to all spectrum. So any given spectrum of light will behave the same with a Young's double slit experiment provided you adapt the dimensions of the experiment accordingly.

    The absorption and transmission spectrum of objects is frequency dependent. Some objects will be transparent over a certain spectrum but opaque over others. Glass obviously is mostly transparent for radio waves and infrared light. You can feel the infrared from a light bulb or from sunlight passing through a window, both of which require transmission through some kind of glass. In addition, you have surely listened to your radio beside a closed window to see that glass is generally transparent to radio waves.

    Water is a good absorber of infrared I believe though a poor absorber of visible light.

    The energy of a photon is frequency dependent, the energy of a wave is independent of frequency and is dependent upon the amplitude/intensity of the wave. This differs because a photon is from quantum theory while we generally describe light using the classical wave theory. Suffice to say though, if you have two waves of the same incident power but different frequencies, then we know that the wave of higher frequency has a lower rate of photons being absorbed by the detector than the wave with the lower frequency. This would satisfy the energy relationship between the quantum and classical viewpoints.
     
  8. Mar 4, 2010 #7
    Here's the thing. I understand the 'textbook' explanations for these questions already so I'm not really searching for a recital of that information. I have reasons to challenge the status quo here and am hypothesizing that the properties of EM emitted from radio towers and microwave ovens has slightly different properties than EM emitted from the sun or heated body.

    So I guess my real question is, how can I test these hypotheses or find real empirical (not theoretical - I don't care what Einstein says) information to refute them?

    I hypothesize that if the frequency emitted from a radio tower was increased up to the point that it sits within the visible light spectrum, that the radio tower would not appear to 'glow'. The EM emitted from this tower would not be visible by the eye. It would not be equivalent to EM of the same frequency emitted from a light bulb.

    I hypothesize that EM emitted from a radio tower would never be capable of a Doppler Red Shift. If the radio tower were receding very quickly away from the observer, the frequency of the EM would not red-shift.

    Thanks.
     
  9. Mar 4, 2010 #8
    "I hypothesize that EM emitted from a radio tower would never be capable of a Doppler Red Shift. If the radio tower were receding very quickly away from the observer, the frequency of the EM would not red-shift..."

    You can hypothesize all you want....experimentalists have already proven such shifts do occur....and for gravitational potential differences as well....perhaps you could read about doppler shifting of all sorts of electromagnetic waves in cosmological science....

    What substantiation do you have for your hypothesis?? that's what you should test for...but experimental physics is usually not so easy as it appears....

    here's one example:

    http://en.wikipedia.org/wiki/Crookes_tube#Doppler_shift

    Oh, I just thought of an abvious proven example for you: doppler radar:

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

    If you think it doesn't work, just race down any highway at 90 mph and see what happens...
     
  10. Mar 4, 2010 #9

    jtbell

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    I don't know if anyone has actually done the double slit experiment with radio waves, but I expect that it would be possible in principle.

    Radio waves definitely undergo diffraction when passing an obstacle. In particular, knife-edge diffraction around the top of a hill or mountain follows the same basic equations as with light (differing only because of the wavelength), and is a well-known factor in radio and TV signal propagation in mountainous areas.

    Diffraction around nearby buildings, trees, etc. can have a significant effect on radio/TV reception. And signals reflected from the ground or other large surfaces can interfere with the main signal and produce maxima and minima in the net signal:

    http://www.hdtvprimer.com/ANTENNAS/siting.html

    The diagrams of diffraction around a tree modeled as a sphere remind me of the old joke about the "spherical cow of uniform density." :biggrin:
     
    Last edited: Mar 4, 2010
  11. Mar 4, 2010 #10
    Naty1:

    Ha, that's funny. Yes, I'm sure a speeding ticket could be considered empirical enough proof for me! ;)

    Thanks for the links. The Doppler radar did answer my question about the red-shift. That example didn't occur to me.

    Do you have any advice about what would happen if an antenna created frequencies within the visible light spectrum? Would they be visible?

    Thanks.
     
  12. Mar 4, 2010 #11
    jtbell:

    Thanks. That answers that question. Thank you!
     
  13. Mar 4, 2010 #12

    Born2bwire

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    If you already know the textbook information than you already know the answers. We are not going to tell you anything different.
     
  14. Mar 5, 2010 #13
    Okay, I don't think anyone here has noticed this but OP, what level physics are you in? I'm honestly not trying to insult you. I felt this exact same way at the beginning of physics two last semester and ALL OF YOUR QUESTIONS WILL BE ANSWERED IN THIS AND NEXT SEMESTER!!!!

    I think your just a bit confused homie on what exactly light is. I think you may be misunderstanding with some of the vocabulary and terms here so lets start with instead of EM we write things out ONLY because i feel that its going to clear up some confusion. The way your using EM leads me to believe that you're thinking electromagnetism and confusing it all with electromagnetic radiation which is also called electromagnetic waves which is also known as light

    electromagnetic waves = electromagnetic radiation = light

    what makes these up are a sinusoidally varying (varies in a back and forth oscillation like a sine or cosine function) electric field and magnetic field perpendicular to each-other

    They teach you this at the end of physics two:
    First they teach simple harmonic motion, then jump to the laws/principles of electric forces/phenomena, and then as if it was a completely separate thing they teach you the laws/principles of magnetic forces/phenomena. Then they explain this guy, Maxwell, took Gauss's Law for electric fields & Gauss's law for magnetic fields (initially completely separate things before Maxwell!!) Faradays law (electromagnetic induction) and Ampere's law (and added his own thing into ampere's law all of which shows that a time varying electric field acts as a source of magnetic field i.e. check out solenoids and alternators)

    Most of these guys happened to study both electricity AND magnetism at the same time.....conveniently :) I say conveniently because At the time, when these guys were discovering and studying these phenomena it was initially thought that electric and magnetic phenomena were essentially two separate things.....UNTIL!!!! TADA!!! This guy Maxwell linked them all together and brought two of the biggggggg sections in science together into one!

    This eventually led to us to understand that those time varying (sinusoidal) electric and magnetic fields we were talking about earlier that were perfectly perpendicular to each other, can sustain each other and travel (propagate) through space (and material) REALLY FAST! and round about that time we figured out that this is what light is. In fact it is these electromagnetic waves that we explain in physics 3 within which we begin with waves traveling along a string or slinky like back in high school physics. While we continue to develop the idea that light (electromagnetic radiation) is actually two wavering fields that are perpendicular to each other (now called electromagnetic waves or radiation) also, well its hard to explain right quick but essentially they travel in waves as well. That’s double wavy-ness and its pretty nuts when you get all deep into it and where it took us(all explained just before Einstein's Relativity and the end/blur area of classical type physics and on into Modern Physics and quantum mechanics and all more fun!)

    anyway long history lesson short

    All these things you're talking about are classified as electromagnetic waves (or radiation).

    Light is another word but where I think a lot of confusion ensues is a crap ton of people (that don't take these classes or haven't learned this stuff until they take these classes) think of light as what we see. AND DON"T WORRY!! THATS NORMAL!!!

    It truly is which sadly frustrates me a tiny bit that not enough people learn this awesome stuff! :)

    Physicists use the word "light" to mean anything that has that sinusoidally time varying, 90 degree intersecting, in phase (don't worry if you don't know what it is :) ), electric and magnetic fields oscillating and sustaining each other and traveling through space (and materials like glass!!! but some move faster and better though different materials! Relatively simple rules learned in OPTICS section of physics 3 like reflection and refraction and rainbows and fun stuff relating to VISIBLE LIGHT. I know everyone knows ROYGBIV. Also still in classical...I'm pretty sure.. anyway)

    SO

    now were clear.


    Radio/TV waves>Microwaves>Infrared>R.O.Y.G.B.V>ultraviolet>x-rays>gamma rays

    are ALL LIGHT!!! all are electromagnetic waves/electromagnetic radiation/light.

    k good :)


    now next thing.... I really hope you read all this because I’m taking a ton of time to summarize a whole lot of class and BOOK LEARNING, because that’s where all this stuff is kept as a data base and means to spread the knowledge of the way things work, but, I guess its like studying for my test on this tomorrow :)

    the actual thing that defines and individualizes each of these different types EM waves are their individual frequency AND wavelength, I presume, in a vacuum.
    <<<<BIGGER ------------------- Wavelength------------------SMALLER >>>>>>>
    Radio/TV waves>Microwaves>Infrared>ROYGBV>ultraviolet>x-rays>gamma rays
    <<<<LOWER--------------Frequency------------------------HIGHER>>>>>>>>>>

    The relationship is direct. All of those things listed above are able to travel at c (2.998E8 m/s) in a vacuum. Actually, c=(wavelength in meters)*(frequency in hertz)

    anyway in optics you will learn that all these light waves can penetrate their fair share of materials. Some EM waves can penetrate some materials only a very little bit and these same materials, other EM waves, say with a longer wavelength (and correspondingly lower frequency) in a vacuum can penetrate through diamond better BECAUSE OF ITS FREQUENCY THOUGH. When the light hits the surface of the diamond or other materials with higher refractive index (look up definition for even more clarity to complicated to go into now) and refracted the actual individual colors in the small portion of the EM spectrum that is visible to us will separate out. Hence how we have rainbows in the rainy sky or out of prisms.

    This should lead you to answer your glass question yourself!

    and hopefully that one to! once you learn all of this thoroughly from a lecture and reading the text like is expected of you in these classes as there is never enough lecture time to cover everyyyyything

    I don't know the logistics or anything and I am not 100% sure but if you could in fact get a radio antenna to have its charges oscillate at that frequency I'd think it should light up as that’s pretty crazy fast. This is kinda similar to the filament in your light bulb I’d suspect (I’d have to think for a minute...a little burned out from studying books for my test :( )

    I think something many professors don't teach enough of is that the HISTORY and way we walked through discovering all these things we’re learning plays an integral part and an aid in learning the material and understanding how we got to where we are today in the realm of scientific knowledge. All of which offers deep understanding to the properties themselves.
     
  15. Mar 5, 2010 #14

    Matterwave

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    As far as I know, no one has done this exact experiment...it is very hard to shift radio wavelengths into light wavelengths (e.g. by doppler blue shift) because the difference in wavelength is so large (several meters vs 400-700 nanometers).

    But let's just back up and ask what you are actually looking for.

    You suspect that radio waves are in some way fundamentally different from visible light, other than a simple frequency/wavelength shift as all textbooks would say. May I ask, how? Do you not believe both are electro-magnetic waves in nature? Do you not believe both are waves in nature?

    If you accept that both are electro-magnetic waves, then there are only several properties which can define such a wave. There's amplitude (or intensity), there's frequency, there's wavelength, and that's about it. The only other possible difference is the shape of the waves (e.g. sine waves or sawtooth waves); however, by Fourier analysis we know that all the shapes we could think of can be created out of a series of sine waves. What other properties of these waves could possibly be different?


    The entire EM spectrum are solutions to the Maxwell Equations. Maxwell equations describe electric and magnetic fields and how they interact with each other (There are 4 in total, which basically say that electric fields can be created by a changing magnetic field and vice versa, as well as by static charges and static magnetic North-South poles). You find that a certain solution to these equations is waves propagating out in space. We call these waves "electro-magnetic waves" because they are composed of varying electric and magnetic fields. We believe that radio waves, light, UV rays, gamma rays, etc., are electro-magnetic waves because they behave exactly as Maxwell's equations would have them behave. They all travel at the speed of light (c), and they are all composed of varying electric and magnetic fields.

    If you have specific properties of these waves that you think are different, feel free to expound on them, and maybe we can make things clearer. Simply saying "radio and light waves are different" doesn't help us help you.
     
  16. Mar 5, 2010 #15
    This is really the crux of the question, is it not? Are light waves and radio waves the "same" thing? We could ask it another way: Are radio waves photons? Or yet another way: can radio waves be polarized?

    The standard answers are that all of these waves are simply portions of an electromagnetic spectrum and hence are the "same". Tesla on the other hand fervently believed that radio waves were longitudinal in nature. And I really know of no definitive experiments proving otherwise for low frequency radio. One would have to set up some large antennas in space away from all other objects to really be sure. We do know that light is clearly polarized to a high degree and microwaves are clearly "sort of" polarized. This has lead some scientists in the past to believe (and theoretically calculate) that all EM radiation is BOTH transverse and longitudinal. The ratio being determined by frequency. Low frequencies being largely longitudinal and high frequencies (light) being largely transverse with only a tiny amount longitudinal. As far as I know none of this has been demonstrated by experiment.

    But that is classical physics theory. We then progress to modern physics where we need to ask the key question: are radio waves photons? The question being how can CW phenomena such as E and M fields generate photons at radio energies? What does that mean? How can one measure individual radio photons as one can do with light? Does this even make sense?

    As you can see the issues are far from settled. Personally I'd like to see the space polarization experiment to settle the longitudinal question once and for all.
     
  17. Mar 5, 2010 #16

    ZapperZ

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    I would like to add that in accelerator physics, we deal with "rf" wave all the time. In fact, in the early days, the MHz range of rf is a common source. Now, we have increase that into the GHz range. This is the source of power for conventional accelerating structures. I myself work with 1.3 GHz rf sources.

    The point here is that, in addition to the already-established antennas and radios, etc., this range of EM frequencies is so well-tested, it is not even funny anymore. We make it do some very exotic gymnastics, such as putting it through various waveguide geometry, etc.

    BTW, if the rf in this range has a "longitudinal" component, than many of our modeling of such accelerating structures will be wrong, and our particle accelerator should not work the way they do now. This is especially true for the current generation of FEL, where beam emittance parameter is extremely tight! Just look at the one required at the LCLS!

    Zz.
     
  18. Mar 5, 2010 #17
    This question still stands unanswered:

    1) I understand that increasing the frequency would 'classify' them as no longer being radio waves however I am referring to a situation where we still emit them from an antenna - but with a much higher frequency (created by a circuit board). In other words, would a radio tower start to glow visibly if you increased the frequency.


    Actually, according to what has been said here so far, the answer seem to be - YES.

    Now, if I understood that correctly, I would be particularly interested in practical approach and real experimental setup of this 'luminous radio antenna' - how would one go about to modify a standard radio emitter to increase the energy (frequency) of EM waves it emits to the point of visible spectrum, if possible. If not possible, then what are the limits?



    I propose that light emitting quality is closely related with the 'temperature' of the radiating body, i.e. kinetics of the material itself, so my first guess is that 'glowing radio antenna' would most likely melt before it actually starts glowing, but on the other hand, some other materials, perhaps crystal antenna, might emit light rather than radio waves when plugged in the same radio circuit.

    Furthermore, this light waves could then be captured by photo-sensitive receiver antenna and so electromagnetic radiation in visible spectrum should be able to carry radio or tv signal just the same as normal radio waves do. I suppose this is actually implemented with optical cables and similar light-communication technology, though this may be digital rather than analog.


    My questions:
    a) Is radio or television signal encoded as digital or analog information?
    b) High-definition picture, how the same waves carry more information?
    c) What is the relation between information transmitted and energy used?
     
    Last edited: Mar 5, 2010
  19. Mar 5, 2010 #18

    ZapperZ

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    I don't understand this "glow" part. What is "glowing"?

    For example, in an incandescent light bulb, the "glow" part is the filament, whereby thermal agitation causes it to heat up, and then emit visible light. A source of light need not glow! Example: lasers. It simply produces visible light that you see when this light scatters off other objects (air dusts, walls, etc.). I can also produce the same thing from bunches of electrons moving very fast and oscillating in an alternating magnetic field, such as those done in many synchrotron light sources. Do you think they "glow" when they do that?

    So why would an antenna glow unless one is attributing the lights to the same process in the filament of a light bulb? If this is all there is, then what's the mystery here again?

    Zz.
     
  20. Mar 5, 2010 #19

    ZapperZ

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    Addendum: please note that the speed of light at such low frequencies (i.e. 5 to 50 Hz) has been tested. See, for example,

    M. Fullekrug, PRL v.93, p.043901 (2004).

    There's is nothing inhere to indicate of any frequency-dependent on the speed of light.

    Zz.
     
  21. Mar 5, 2010 #20
    glowing = emitting visible light


    How do you define "glow"? The question is about the possibility of some standard radio antenna emitting visible light, that's all.




    I did say the light emitting quality is closely related to kinetics of the material itself. Please, what is your point? According to the definition above anything that radiates visible light "glows", as opposed to just reflecting the light. I'm not sure if lasers and electron beams emit photons or they become visible due to some interaction with the medium they propagate through, but in any case I guess we could say they "glow" since we can see them, just like Lightsaber glows. Why do you ask?



    The question is whether or not it is possible to make some radio antenna emit visible light.

    Diode would probably emit light if plugged in radio circuit instead of the transmitter antenna, whether or not you consider that as the same as is in light-bulb the real question is actually if light photons can carry the information in the same way radio waves can?

    Therefore, the real essence of the OP is that visible light photons appear to be more digital in nature, easily obstructed and absorbed, while radio waves seem more "everywhere" they seem more analog and persistent. It seems that visible light might not be able to carry analog encoding like radio waves do, what do you think?



    Considering some real devices mostly everyone has already in their laptops, like Wi-Fi and IR port, the question then boils down to whether Infrared communication can do the same thing as Wi-Fi or not, in terms of numbers of em waves their frequency and information encoding/decoding? Opposite question would be whether we can send radio waves down some optical cable and encode information digitally like is done with the visible light photons?
     
  22. Mar 5, 2010 #21

    ZapperZ

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    You need to carefully consider the description and words that you are using here. A filament "glows" AND emit light. A laser does NOT glow, but still emit light. If you mean something to be a "light source", then say that rather than asking it to "glow". Those two are not mutually inclusive.

    If all you define "glow" as being a "light source", then what are we debating here? An antenna ALREADY "glows" since it is a source of EM wave! The klystron that I have that emits the 1.3 GHz rf is already "glowing", by your definition. Or do you simply restrict the use of that word to light sources within the visible spectrum only? This would be rather puzzling because, what is so special about that frequency range, especially considering how NARROW of a band it is within the known EM spectrum.

    I fail to see the significance of the issue you are having in this thread.

    Zz.
     
  23. Mar 5, 2010 #22

    Born2bwire

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    These are completely non-issues as far as I can see. The idea of longitudinal waves for is not something I have seen seriously discussed in literature since around Sommerfeld's papers over a hundred years ago. Tesla can believe all he wants but people need to realize that Tesla was one of the pioneering scientists in electromagnetics. He was doing research around the same time that Maxwell first published the completed theory, that Hertz first proved the existence of electromagnetic waves, and when Marconi first proved the long distance transmission of radio waves. All of these events, spanning from around the mid-1870's to the early 1900's encompass much of Tesla's career and it should be no wonder that not all of his ideas proved to be correct or even sane by today's theory.

    Do radio waves follow the same theory as waves in visible light? Yes, of course. We use the same quantum theory to create terahertz lasers and quantum wells. We can measure the granularity in signals that arise due to the fact that energy is transferred via photons. Indeed, the shot noise and Johnson noise are all important effects that require that the energy be discretized. The measurement of black-body radiators is also a excellent proof for the quantization of light and every antenna engineer is very much aware of the noise that results from black-body radiation. And of course radio waves can be polarized, this is something demonstrated easily in a high school physics lab and again is one of the most basic elements of antenna engineering.

    All of this lies at the base of one of the most widely verified theories of science.

    You can't get an antenna to emit visible light, it is not a physically feasible phenomonen. The wavelength for visible light is in the hundreds of nanometers and you cannot hope to construct a classical antenna in such lengths scales let alone create a voltage source that would oscillate at such frequencies. HOWEVER, the theory is still perfectly sound but we just have to redesign our "antenna" in a more feasible structure. ZapperZ I believe has already mentioned synchrotrons. They can emit visible light via the same basic mechanisms as a wire antenna, via the acceleration of charges.

    We can encode information the same way in visible light as we can in radio waves. Again, the physics doesn't change. The only thing that changes is the feasibility of adapting the same principles and techniques. An optical cable is essentially the same thing as a coax cable, except that they are designed for different bandwidths and frequency spectrum. Material properties change over the electromagnetic spectrum. In addition, quantum effects become more prominent at around the terahertz range and above. The techniques and materials that are efficient below the terahertz are not useful for infrared and above. Still, the basics are more or less the same. A coaxial cable is a waveguide that guides the electromagnetic wave between the signal core and the surrounding shield. An optical fiber is a dielectric waveguide that guides the electromagnetic waves inside the dielectric via reflections at the dielectric-air interface..

    Information is encoded via amplitude and/or frequency modulation and by encoding bits in the phase space. To do this with light means that we need to be able to maintain coherent light, because if the phase or frequencies wander then the information is destroyed. This is easy with low frequency waves but much more difficult with the visible spectrum. This is also compounded by the non-linear behavior of most optics. RF frequency waveguides are also non-linear in that they are dispersive. Dispersive means that the group velocity of a wave is frequency dependent. So if I send in a sharp pulse, it may come out as a wide pulse since different frequency spectra arrive at different times. Dispersion is one problem that is more prominent at higher frequencies and makes optical communication more difficult. Still it can be done as evidenced by the fact that most telecommunication networks are fiber optic lines which operate in the infrared I believe.
     
    Last edited: Mar 5, 2010
  24. Mar 5, 2010 #23
    Glowing object is the one you can see in the dark. Ok, I see what is your definition and I can only say the key word for something to "glow" is to be VISIBLE, according to my dictionary, but that's fine, I accept "electromagnetic glow" in general. So, tell us, do electron beams and laser beams "glow" in radio-wave spectrum at all, ever? In other words, can radio antenna sometimes register whether some laser or electron beam is passing nearby?



    [EDIT, deleted repeated questions]: Oops, all the questions seem to be answered by Born2bwire, thanks for nice explanation, I accept that, and agree.
     
    Last edited: Mar 5, 2010
  25. Mar 5, 2010 #24

    Born2bwire

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    One of the biggest problems when it comes to understanding electromagnetics is that most people, for better or for worse, are familiar with a quasi-quantum/classical explanation of light. They understand the classical theory of light being waves and they combine this with the quantum idea of light being quantized into photons. The mistake lies in mixing the two (though frustrating this does work with some simple problems like the photo-electric effect, black-body radiation and some other simple quantum problems). The full quantum theory of light does not assume that light is made up of photons that are emitted at point A and travel along some path to point B. It is probably more accurate to say that light consists of photons being created at point A, annihilated at point B, and in between the light is a field.

    In truth, there is one theory, quantum electrodynamics which uses quantum field theory. It is a very complicated theory but what it boils down to is that the basic constituents of light are fields. These fields behave more or less like a classical field (add more caveats here because this is still a quantum field). So I would hesitantly hazard to say that you could say that the field nature helps encompass the basic classical wave properties of diffraction and interference. This field is not made up of photons however. Photons only exist as the quantization of the field's energy. When this field interacts however, it can only do so through a quanta, the photon. The photon only comes into play during an interaction which appears to be done via a point-like classical particle (again it's still a quantum particle, a bunch more baggage comes with that description). In this theory, we are not shooting out real physical particles called photons when we emit light. Instead, we add energy into a field that is representative of the light and when this field interacts, it does so through a photon. So we do not visualize these classical particles bending around objects and stuff like that because we do not consider that photons follow a real trajectory between a source and a sink.

    The classical wave description is very accurate for below terahertz frequencies. Above it however, the quantum effects and scales make quantum or pseudo-quantum theory to be more accurate. In addition, when it comes to very low power and small distance scales quantum effects become very prominent even for low frequencies (Casimir force, shot noise, etc.). However, QED still as a full and complete theory that encompasses the whole spectrum of the electromagnetic radiation. It is just far easier to use classical theory when we can. Though currently QED is a quantum theory of light that includes special relativity. Right now, there isn't a quantum theory of light that includes general relativity. That is the stuff that the more esoteric theories like string theory are trying to solve. So there is room for the theory to develop but within its scope of application, QED is probably one of the most accurately verified theories.
     
  26. Mar 5, 2010 #25

    ZapperZ

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    Again, what is SO SPECIAL about something you can see with your eyes? Why is this a strict requirement, considering how limited your "eye" is as a detector? Are you aware of HOW SMALL the bandwidth is for the visible spectrum? What makes it so special about it that you are basing everything about EM radiation on this particular range?

    Zz.
     
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