How does a radio antenna work quantum mechanically?
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From: Androcles on 31 Jul 2010 18:01 "Excognito" <stuartbruff(a)gmail.com> wrote in message news:1355cb2a-59b1-4084-b515-dcf92fbbbac7(a)q22g2000yqm.googlegroups.com... Another one that bugs me is this: I've just watched one of the Feynman NZ lectures, in which he describes the probability amplitude of the photon as rotating. Under SR, AFAICT, a moving body's time (as measured by 'stationary' me) tends to zero as its speed approaches c. Given that photons travel at c (in any frame), how can the photon have any property that varies in observer time? ============================================== You are trying to use logic. Blind faith in the Prophet Ayatollah Rabbi Saint Einstein the Divine is required for any understanding.
From: maxwell on 31 Jul 2010 19:37 On Jul 31, 7:49 am, "Androcles" <Headmas...(a)Hogwarts.physics_z> wrote: > "maxwell" <s...(a)shaw.ca> wrote in message > > news:ff372e74-f833-44f9-ae24-a545bbc5d367(a)v32g2000prd.googlegroups.com... > On Jul 30, 11:20 am, "Androcles" <Headmas...(a)Hogwarts.physics_z> > wrote: > > > > > > > "maxwell" <s...(a)shaw.ca> wrote in message > > >news:6e1e8404-40b2-4da9-b0df-230bcef461ac(a)o7g2000prg.googlegroups.com... > > On Jul 29, 3:41 pm, Darwin123 <drosen0...(a)yahoo.com> wrote: > > > > On Jul 29, 5:42 pm, Excognito <stuartbr...(a)gmail.com> wrote:> What are > > > the > > > physical processes, from a quantum perspective, involved > > > > in receiving/transmitting radio waves? > > > > There are rather easy rules of thumb that connect classical > > > electrodynamics (CED) to quantum electrodynamics (QED). I will assume > > > that you know classical electrodynamics rather well, so that you are > > > comfortable analyzing a classical antennae. I will also assume that > > > you don't know QED but for a few popular images. In other words, I > > > assume that you have heard the phrases "real photon" and "virtual > > > photon". > > > The electromagnetic field of an antennae can be divided into a > > > near-field component and a far-field component. > > > Far-field component: What are generally called "radio waves" are > > > the far field component. Radio waves carry energy a large distance > > > from the antennae (i.e., many antennae lengths). In QED, radio waves > > > are modeled as "real photons". > > > Near-field component: The near-field component consists of static > > > and near static fields that exist only near or inside the antennae. In > > > other words, the energy inside the antennae is mostly stored in near- > > > field component. In QED, the near-field component is modeled as > > > virtual photons. > > > > > Eg, if an electron undergoes acceleration in a magnetic field, >is the > > > > magnetic force mediated by photons? > > > > Very close to the accelerating electron, the electric and magnetic > > > fields are distinguishable. So most of the force close to the electron > > > is mediated by virtual photons. The virtual photons disappear at a > > > certain distance from the electron by a distance determined by > > > Heisenberg's uncertainty principle. Some of the virtual photons become > > > real photons, and some just disappear. Virtual photons are equivalent > > > to the near-fields studied by electrical engineers. > > > At large distances from the accelerating electron, there are no > > > virtual photons. However, all the energy is traveling as real photons.. > > > Real photons are equivalent to the "radio waves" studied by electrical > > > engineers.> When the accelerating electron > > > > radiates, does it do so by emitting radio energy quanta? > > > > The electron is always surrounded by virtual photons which are > > > close to the electron. When the electron is accelerated, energy is > > > added to the virtual photons. The virtual photons change into real > > > photons when they acquire a sufficient amount of energy from the > > > accelerating electron. Of course, the accelerating electron loses > > > energy. In order to accelerate, an electron requires a continuous > > > input of energy.> If so, does > > > > that mean that the electron's trajectory is a sequence of linear > > > > >steps > > > > rather than a continuous curve? > > > > Virtual photons are not quantized the way real photons are > > > quantized. The energy of a virtual photon is constrained by > > > Heisenberg's uncertainty principle. In other words, the energy of a > > > virtual photon is not quantized. > > > The trajectory of the electron is not so much continuous as fuzzy. > > > The exact position of the electron is unknown. The trajectory is more > > > like a fuzzy band than a precise curve. > > > Under the conditions that radio engineers usually work at, the > > > fuzziness caused by the uncertainty principle is unimportant. The band > > > is narrow enough to be called a line curve for pruposes of the radio > > > engineer. QED is generally not important for understanding the > > > spectrum of radio antennae. However, there are some special conditions > > > where the uncertainty principle can not be ignored. > > > > > Assume a conducting wire antenna lying normal to the direction of > > > > propagation of a radio 'wave' (what is the structure of this 'wave' in > > > > terms of a photon model?). > > > > There are two complications involved with a photon model for > > > energy traveling in an electrical conductor. > > > Complication #1: Radio waves don't penetrate deeply into > > > conductors. They are rapidly turned to heat energy. That is why there > > > is a skin depth to conductors. In the classical picture of the case > > > you are envisioning, there are radio waves just outside the wire and a > > > heating in the wire caused by electric currents. > > > Complication #2: Pauli's exclusion principle. The electrons in a > > > conductor aren't isolated from each other. According to quantum > > > mechanics, there can't be two electrons in the same state. So you > > > can't pretend that a single electron interacts with the radio wave > > > without shaking up other electrons. > > > Solution to both complications: Don't treat either photons or > > > electrons as individual particles. Pretend that electrons and photons > > > combine inside the conductor as a strange hybrid particle called a > > > plasmon. > > > There is a coupled excitation called a plasmon. Inside the > > > conductor, photons lose their status as individual particles. Inside > > > the conductor, photons lose their status as individual particles. > > > Instead, there are these strange composite particles called plasmons. > > > What you want to know is how photons become plasmons as they enter > > > the conductor. You would like to study the properties of plasmons. You > > > don't want to know how photons behave inside the conductor, because > > > the photon doesn't behave as such in a conductor.> When a radio photon > > > interacts with an > > > > electron in a conductor, how does the (linear?) momentum of the > > > > >photon > > > > get converted into electron motion in a specific direction >along the > > > > antenna? > > > > The photon becomes a plasmon inside the conductor. The momentum > > > of the photons is transferred into the plasmons inside the conductor. > > > The plasmon has a finite half life, and decays into smaller plasmons. > > > The momentum gets redistributed into smaller plasmons. > > > > > Is there a good reference that explains these kind of issues >from a > > > > "what's going on in this situation" perspective? > > > > No. I have not found a book that explains these kind of issues > > > from a "what's going on in this situation" perspective. I have looked.. > > > However, there are books that explain the mathematics of quantum > > > mechanics as applied to solids. > > > This post is based on my personal intuition concerning the > > > mathematical descriptions that I have read. I have gotten into > > > advanced courses and research involving solid state. To me, it is > > > fairly obvious "what is going on" once I understand the mathematics. > > > I, personally, have a knack for taking abstract mathematics and > > > turning it into pictures and images. I can not be sure if I am doing > > > it "right" or not. > > > Books on solid state physics do describe the quantum mechanics of > > > what happens inside an electrical conductor. I don't know your level. > > > However, if you understand CED really well and if you have studied > > > rudimentary quantum mechanics, I suggest the next step is studying > > > solid state physics. I think that once you understand the mathematics, > > > you may find your own pictures of what is going on. > > > Gentlemen: We are looking at a part of reality from two different > > scales - macro & micro. At the macro level we have electrical > > currents moving backwards & forwards and from the micro scale, > > electrons forming the currents. With two antennae, one sending & one > > receiving energy: we have induction (remote interaction) between the > > sources & sinks. We have also two mathematical schemes, again at > > different scales, to describe this situation. Neither Maxwell (CED) > > nor his field theory successors (QED) wanted to focus on the real > > physics (inside the conductors: very complicated) so they invented > > simple math schemes to "describe" what they imagined might be going on > > between them; i.e. in the empty space in between. > > Do not fall into the ancient scholastic trap of thinking the symbols > > in these math schemes describe any form of reality - there are no > > magnetic fields or photons. Where is Newton when we need him? > > ============================================= > > Last I heard he was scratching his head and then laughing at virtual > > photons inside a transformer. Since there are no magnetic fields I'll > > inform my fridge to let go of the magnets holding my shopping notes > > up and go back to using licky sticky stuff, shall I? > > Magnetism is a real phenomenon: it is the interaction between > electrons in motion. Do not confuse the phenomena with the theories > that are used to explain them. > ================================= > I'll be careful not to confuse real magnetism with the interaction between > electrons in motion, then. > Electrostatic fields are a real phenomenon: they are the interaction between > magtrons in motion, whizzing around the ferrite loop of a transformer, > at right angles to the copper loop that the electrons whizz round. Do not > confuse the phenomena with the theories that are used to explain them. > http://www.androcles01.pwp.blueyonder.co.uk/AC/oscillator.JPG The key difference here is that there is no such thing as instantaneous inter-electron interactions; in other words, the Coulomb force is a macroscopic, long-time average of many electrons interacting. The delay introduced by the finite time between the two ends of an interaction means that the target electron reacts differently when there is relative motion before the interaction or not. At the human scale we have called these distinctions: electric & magnetic forces - nature is a unity.
From: spudnik on 1 Aug 2010 00:28 the photon is some kind of oddball Copenhagenskooler reification -- is that a Bohmian word? -- of Newton's corpuscle, courtesy of Einstein's cutesy coinage per the photo-electrical effect, thanks to the Nobel cmte. decision, although it is not a neccesary impediment to using Schroedinger's *wave* equation, for light (iff it can be used for that; I don't actually know .-) we might owe the quantumization of light to Planck, but we do not owe the rock o'light to Einstein, or de Broglie, or Dirac; pioneers don't have to be perfect! > I've just watched one of the Feynman bongo-drum & throat-singing > NZ lectures, in which he describes the probability amplitude of the > photon as rotating.... > Given that photons travel at c (in any frame), how can the photon have > any property that varies in observer time? thus: one example of pedagogy is enough; thank *you*. well, Bucky liked to use "Universe," since there is only one by definition, duh ... unless you're in the Copenhagen cat-joke school of "a multiverse" (because, there could be more than one .-) thus quoth: Basic English, which had a vocabulary of only 850 words, but which he wished to become the world language. In Wells' ... www.larouchepub.com/eiw/public/2007/2007.../22-26_747-48.pdf > > A "force" is a net pressure > > measured independently of the area of application. A "net pressure" is > > the average amount of pressure in a given direction.) > http://esp.wookeepoopeeya.org/wookee/Presher#Definishun thus: I just found abok that addresses many of the concerns -- from a brief perusal of about three "random" openings, and of the index -- of the Truthers; it's from 2005, by a couple of NYTimes reporters, _102 Minutes_, which was the time from the attack of the north tower (WTC1) to its fall (as you know, the first to be hit & last to fall). thus: I'll huff and I'll puff.... have you ever proven a theorem in (say) constructive geometry? > Ahahahaha... thus: there are two 3d versions of the pythag.thm., each with different dimensional attributes.... iff you don't study Fermat's numbertheorie, you're up Shitz Creek without a paddle; however, it is better to start with his "reconstruction of Euclid's porisms," although they are just planar (synthetic geometry: see "Geometrical Fragments," belowsville .-) thus: and, the other half d'oil evaporates, as has been shown of late (again) in the newspapers. Congress and the Administration are a bit behind, in using Iran Oil's big blow-out in the Gulf, to leverage BP's cap&trade nostrum; eh? a-yup: Such microbes have been found in every ocean of the world sampled, from the Arctic Antarctica. But there are reasons to think that the process may occur more quickly in the Gulf than in other oceans. --les ducs d'oil! http://tarpley.net/online-books/ --Light, A History! http://wlym.com/~animations/fermat/index.html
From: spudnik on 1 Aug 2010 00:35 jus a God-am femtosecond; your question is nonsequiter! > > Given that photons travel at c (in any frame), how > > can the photon have any property that varies in observer time? --les ducs d'oil! http://tarpley.net/online-books/ --Light, A History! http://wlym.com/~animations/fermat/index.html
From: Darwin123 on 1 Aug 2010 10:43
On Jul 31, 5:52 pm, Excognito <stuartbr...(a)gmail.com> wrote: > On 31 July, 20:01, Darwin123 <drosen0...(a)yahoo.com> wrote: > > > On Jul 30, 8:00 pm, Marvin the Martian <mar...(a)ontomars.org> wrote:> On Thu, 29 Jul 2010 14:42:35 -0700, Excognito wrote: > Another one that bugs me is this: I've just watched one of the Feynman > NZ lectures, in which he describes the probability amplitude of the > photon as rotating. I am not entirely clear what you saw in these lectures. There are a series of his lectures presented in the book, Richard P. Feynmann, "QED: The Strange Theory of Light and Matter" (Princeton University Press, 1985) I read "QED". I don't highly recommend this book as an introduction to QED, but it makes some good points. I suspect the lecture you saw may be written in this book. What you are saying looks it came from Chapter 2: "Photons" Particles of Light". Maybe I can improve on Feynmann (oh, the arrogance) as far as explaining things on a heuristic level. > Under SR, AFAICT, a moving body's time (as > measured by 'stationary' me) tends to zero as its speed approaches c. > Given that photons travel at c (in any frame), how can the >photon have any property that varies in observer time? The spin does not vary in observer time. None of the properties of a photon varies in observer time. In the QED way of looking at things, the photon is created, destroyed, and does not change any of its properties in between. If the photon hits and electron, it could be destroyed (i.e., absorbed) and a new photon with different properties created (i.e., a photon is emitted). However, the properties of the photon can not change during its travel. This is one reason that I say that Newton's Laws can not be applied, even as an approximation, to a photon. The photon is moving at the speed of light. The photon is going at the speed of light. Newton's Law's fail at the speed of light. If the photon were an observer, time would stop for that observer. The observers clock are stopped. So the observer can't see anything change. Now I have to conjecture as to what your question means. Allow me to speculate as to what you were imagining while watching Feynmann's lecture. You are imagining the probability amplitude as an actual displacement vector. You are imagining the probability amplitude as an arrow with a definite length. This unit has actual units of length (kilometers, meters, miles, inches, etc.). However, this is not a good analogy. The probability amplitude does not have any units. It is not the displacement of an aether, as certain people like to visualize it. You are seeing light as consisting of finer particles moving back and forth. How can these particles be moving faster than the speed of light? The direction of the probability amplitude vector is analogous to the polarization of the radio wave. It has nothing with the displacement of actual particles. Hence, the oscillation of the probability amplitude does not move any particles faster than the speed of light. The probability amplitude is more closely analogous to the electromagnetic field. I will not distinguish between electric fields and magnetic fields here, since QED is Lorentz invariant. The spin of the photon relates to the polarization of the radio wave. The polarization of the radio wave relates to the direction of the electric field. Thus, Feynmann is when he talks about spin is referring to the direction of the electromagnetic field. In the far field, the electric field and the magnetic field are always perpendicular to direction of propagation. Thus, there are only two polarization states. I use the circularly polarized basis, which is a reasonable choice. The electric field is spinning around the axis defined by the flow of energy (the Stokes vector). Thus, there are The is the clockwise polarization state, and the counterclockwise polarization state. This corresponds to the two spin states of a photon, clockwise and counterclockwise. The photon is spinning because the electrical field of the wave is spinning. The polarization of the radio wave can change. However, that is caused by photons being created and destroyed. The photon can not change its spin state. Now, the polarization in the near field is somewhat more complicated. Inside the cavity of the antenna, the electric field can be parallel to the direction of energy propagation. The electric field can be changing its frequency of spin. The virtual photons exist in this antenna cavity. The properties of the virtual photons correspond the the properties of the electromagnetic field inside the cavity. Virtual photons can come in four polarization states. The virtual photons have more polarization states than real photons. Well, maybe not exactly. There is a mathematical feature that resolves this discrepancy. However, I am not sure you would be interested. The four polarization states Virtual photons can be circularly polarized. For circularly polarized modes, both electric and magnetic fields are perpendicular to the direction of energy propagation. Thus, there are clockwise polarized virtual photons and counterclockwise virtual photons. However, there are also two more polarization states. There are oscillation states where the electric field is parallel to the direction of energy propagation. These are called transverse magnetic polarization states. There are also oscillation states where the magnetic field is parallel to the direction of energy propagation. These are called the transverse electric polarization states. These correspond to the four spin states of a virtual photon. I used electrical engineer language to describe the four polarization states of the electromagnetic field inside an antenna. Let me use covariant QED language. The two circular polarization states are unchanged in QED language. Clockwise and counterclockwise are good words. However, the transverse magnetic mode is called the longitudinal spin state. The transverse electric mode is called the time-like spin state. To summarize: I described a good heuristic relating QED to classical antenna theory. The spin state of the photon relates to the polarization state of the radio wave or radio field. There are two polarization/spin states in the far field (far from the antennae) and four polarization/spin states in the near field (close to the antennae). The classical electromagnetic field can be described as a collection of photons. Photons don't change spin states once they are created. However, the number of photons with any particular property can change in the presence of electric charges. The same electric charges can change the state of a classical electromagnetic field. This is only a heuristic. It has a finite range of applicability. However, I think you can carry it a very long way. |