How does a radio antenna work quantum mechanically?
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From: Androcles on 31 Jul 2010 10:49 "maxwell" <spsi(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
From: Excognito on 31 Jul 2010 11:05 Thanks for the replies so far. I'm working my way through Feynman's Lectures, Penrose's Road to Reality and a couple of other texts, plus revising my conventional EM stuff. Part of the process is trying to unlearn things I 'know' and question assumptions. As for what I am - a systems engineer (mainly working at customer requirement level), but educated as a physicist many decades ago.
From: Darwin123 on 31 Jul 2010 15:01 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: > > What are the physical processes, from a quantum perspective, involved in > > receiving/transmitting radio waves? > > You don't need QM to understand radio antennas any more than you need QM > to do planetary orbits. It's just silly. The internal consistency of a scientific theory is seldom silly. You don't need QM to calculate planetary orbits. However, the differences between an electrons orbit in the atom and a planets orbit around the sun are very important. The orbits of an electron in a cyclotron, or the orbits of an electron in a Rydberg atom, are for the most part classical. Yet, quantum mechanics is very important in understanding the spectrum of what comes out. Understanding the border between classical EM and quantum EM is also important in the new fields of quantum communications. Most improtant: some people just like to know stuff. The connections between different topics in science are fascinating from an aesthetic and philosophical level. The OP just wanted to know, for his own satisfaction, how classical electronics is related to quantum mechanics. > > Look up Ehrenfest's theorem. An excellent beginning to the topic, but hardly the last word. Ehrenfests theorem explains how Newton's Three Laws for particles relates to Schroedingers equation for waves. However, the situation in an antenna doesn't correspond to Newton's Three Laws. Yes, I do know Ehrenfest's theorem. Have you tried applying Ehrenfest's theorem to photons, recently? Newton's Three Laws of Motion don't apply to photons, as I have told many people here again and again. Photons are not equivalent to the corpusales described by Newton to describe light. Therefore, Ehrenfest's theorem tells us nothing about photons. Ehrenfest's theorem tells us nothing about electromagnetic fields. Ehrenfest's theorem is marginally useful for describing the trajectories of electrons under semiclassical conditions. It is useful in describing the evolution of electron wave packets. However, it is worthless for describing light wave packets, electric fields, and magnetic fields. I think the OP intuited this. He was right. The OP wanted a heuristic model to understand QEDto understand how electromagnetic fields and radio waves behave under quantum mechanical conditions. For this, one needs to understand photons as particles that don't obey Newton's Laws. Electrons are different from photons. Electrons approximately obey Newton's Laws. Photons do not obey Newton's Laws even approximately. Therefore, the classical limits of electron and photon behavior are different. Quantum electrodynamics describes both electrons and photons. Therefore, the connection between quantum electrodynamics and classical mechanics is not trivial. The connection is interesting.
From: Igor on 31 Jul 2010 15:38 Darwin123 wrote: > On Jul 30, 10:01 am, Igor <thoov...(a)excite.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? > > > > > Eg, if an electron undergoes acceleration in a magnetic field, is the > > > magnetic force mediated by photons? When the accelerating electron > > > radiates, does it do so by emitting radio energy quanta? If so, does > > > that mean that the electron's trajectory is a sequence of linear steps > > > rather than a continuous curve? > > > > > 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?). 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? > > > > > Is there a good reference that explains these kind of issues from a > > > "what's going on in this situation" perspective? > > > > I'm not aware that a radio antenna is capable of doing anything in the > > quantum realm. The energy of radio wave quanta are simply too small > > to affect matter in any significant way. > Counterexamples > 1) Josephson Junctions show quantization effects at radio frequencies. > This is the basis of SQUIDS and other quantum mechanical devices. > 2) Nuclear magnetic resonance is based on the resonance between radio > waves and a spin-flip transition. This is the basis of Magnetic > Resonance Imaging. > 3) There are spin-flip resonances used in atomic clocks. The Cesium > band used in atomic clocks is at radio frequencies. > 4) The radio spectrum of a cold object follows a Planck distribution, > with finite peak frequency, only because the energy of radio waves is > quantized. The quanta of the radio waves interacts with the charged > particles in the system. > -Special case: The 3 K Cosmic Black Body Radiation follows a > Planck distribution. It wouldn't follow the Planck distribution unless > the energy of radio waves were quantized. The quanta interacted with > the hydrogen plasma way back in the Great Transparency transition. > > >That's why we usually use > > classical EM when considering antennas. > This is wrong. The reason we use classical EM when considering > antennas is that the radio devices usually involve large electric > charges involving an Avogrados number of electrons. To generate such > electric charges, we need an Avogrados number of radio frequency > quanta. > Classical EM generally applies as an approximation under certain > conditions. Classical EM often applies in the limit of large quantum > numbers. Classical EM also applies best in the limit where the quanta > are not entangled. > Quanta always have the potential of interacting with matter, no > matter how small the quanta. The "classical limit" is based on other > things. Very good counter-examples. My reply was off the cuff and not very thought out.
From: Excognito on 31 Jul 2010 17:52
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: > > > What are the physical processes, from a quantum perspective, involved in > > > receiving/transmitting radio waves? > > > You don't need QM to understand radio antennas any more than you need QM > > to do planetary orbits. It's just silly. > > The internal consistency of a scientific theory is seldom silly. > You don't need QM to calculate planetary orbits. However, the > differences between an electrons orbit in the atom and a planets orbit > around the sun are very important. > The orbits of an electron in a cyclotron, or the orbits of an > electron in a Rydberg atom, are for the most part classical. Yet, > quantum mechanics is very important in understanding the spectrum of > what comes out. > Understanding the border between classical EM and quantum EM is > also important in the new fields of quantum communications. > Most improtant: some people just like to know stuff. The > connections between different topics in science are fascinating from > an aesthetic and philosophical level. The OP just wanted to know, for > his own satisfaction, how classical electronics is related to quantum > mechanics. > > > Look up Ehrenfest's theorem. > > An excellent beginning to the topic, but hardly the last word.. > Ehrenfests theorem explains how Newton's Three Laws for particles > relates to Schroedingers equation for waves. However, the situation in > an antenna doesn't correspond to Newton's Three Laws. Yes, I do know > Ehrenfest's theorem. Have you tried applying Ehrenfest's theorem to > photons, recently? > Newton's Three Laws of Motion don't apply to photons, as I have > told many people here again and again. Photons are not equivalent to > the corpusales described by Newton to describe light. Therefore, > Ehrenfest's theorem tells us nothing about photons. Ehrenfest's > theorem tells us nothing about electromagnetic fields. > Ehrenfest's theorem is marginally useful for describing the > trajectories of electrons under semiclassical conditions. It is useful > in describing the evolution of electron wave packets. However, it is > worthless for describing light wave packets, electric fields, and > magnetic fields. I think the OP intuited this. He was right. > The OP wanted a heuristic model to understand QEDto understand how > electromagnetic fields and radio waves behave under quantum mechanical > conditions. For this, one needs to understand photons as particles > that don't obey Newton's Laws. > Electrons are different from photons. Electrons approximately obey > Newton's Laws. Photons do not obey Newton's Laws even approximately. > Therefore, the classical limits of electron and photon behavior are > different. > Quantum electrodynamics describes both electrons and photons. > Therefore, the connection between quantum electrodynamics and > classical mechanics is not trivial. The connection is interesting. .... == The OP wanted a heuristic model to understand QED to understand how electromagnetic fields and radio waves behave under quantum mechanical conditions. For this, one needs to understand photons as particles that don't obey Newton's Laws. .... == The OP just wanted to know, for his own satisfaction, how classical electronics is related to quantum mechanics. Spot on. Every now and again, I make the mistake of trying to actually 'understand' something. I have yet to learn that this is normally a bad policy as my neuron often confuses itself with questions it can't answer. 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? |