How does a radio antenna work quantum mechanically?
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From: Excognito on 29 Jul 2010 17:42 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?
From: Androcles on 29 Jul 2010 18:25 "Excognito" <stuartbruff(a)gmail.com> wrote in message news:ea5b1a7b-d9f2-4498-9615-5edfbd3259df(a)j8g2000yqd.googlegroups.com... | What are the physical processes, from a quantum perspective, involved | in receiving/transmitting radio waves? Molecules are quanta of matter. Quantum theory only applies at the atomic level, it does not apply to radio antennae. However, the principle of electromagnetic radiation is the same for both. The fundamental frequency of the antenna is governed by its length, the fundamental frequency of light is governed by the size of the molecule that emitted it. http://en.wikipedia.org/wiki/Humphreys_series | Eg, if an electron undergoes acceleration in a magnetic field, is the | magnetic force mediated by photons? It is mediated by the local magnetic field of its surroundings. This is usually referred to as "back emf", the force that opposes the acceleration. | When the accelerating electron | radiates, does it do so by emitting radio energy quanta? Think of dropping a ball on a drum. At the moment the ball strikes the drumskin, sound is emitted and the ball stops moving. But that is not the end of the story, the drum vibrates and it is the drum that emits the sound, not the ball. No matter what height you drop the ball from, the drum determines the frequency of the sound. Dropping the ball from a greater height only makes the sound louder, it doesn't change the pitch. 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?). http://www.androcles01.pwp.blueyonder.co.uk/AC/AC.htm A photon is one cycle of a wave, it is a pulse of energy. A train of photons, one after the other, is a wave. Same for a phonon. Some of the energy of the ball hitting the drum is radiated as sound. A phonon is just one cycle. For a very tight drum you'd hear a click. | 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? Radio is normally a train of photons, a wave. It sets the antenna oscillating the same way a generator works. One radio photon isn't much use to anyone. For example, a nearby lightning strike will be heard on your AM radio as an annoying click. | Is there a good reference that explains these kind of issues from a | "what's going on in this situation" perspective? There are too many to mention. Most will be wrong in some aspect or other.
From: Darwin123 on 29 Jul 2010 18:41 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.
From: Darwin123 on 29 Jul 2010 18:42 On Jul 29, 6:25 pm, "Androcles" <Headmas...(a)Hogwarts.physics_z> wrote: > "Excognito" <stuartbr...(a)gmail.com> wrote in message > > news:ea5b1a7b-d9f2-4498-9615-5edfbd3259df(a)j8g2000yqd.googlegroups.com... > | What are the physical processes, from a quantum perspective, involved > | in receiving/transmitting radio waves? > > Molecules are quanta of matter. Quantum theory only applies at the atomic > level, it does not apply to radio antennae. <LOL>
From: bert on 29 Jul 2010 18:49
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? Antenna is a transmitter in reverse. Both work on radio photon waves(vibrations) Radio waves are large so car antennas are a meter long. TreBert |