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From: Androcles on 1 Aug 2010 22:34

"Darwin123" <drosen0000(a)yahoo.com> wrote in message
news:f9adfe78-4624-4ca0-9ff1-4810eb9459c9(a)f6g2000pro.googlegroups.com...
On Jul 31, 10:35 am, maxwell <s...(a)shaw.ca> wrote:
> 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 don't want to "confuse" the phenomenon with any theory. I want
a theoretical representation that can help me visualize the phenomena.
Visualization is a good short cut through mathematics.
As visualized in one theoretical representation that explain
electromagnetic phenomena:
1) Static magnetic fields are mediated by virtual photons that have a
time-like polarization state (i.e., spin).
2) Static electrical fields are mediated by virtual photons that have
a longitudinal polarization-state (i.e., spin).
3) Radio waves (light waves, etc.) are mediated by real photons that
have circular polarization-states (i.e., spin).
4) there are two circular polarization states: clockwise and
counterclockwise.
=========================================
So what is your problem with a photon being a single cycle of a wave
and what do your "virtual" photons do? Create "virtual" radio waves?




From: Darwin123 on 2 Aug 2010 13:21
On Aug 1, 10:34 pm, "Androcles" <Headmas...(a)Hogwarts.physics_z> wrote:
> "Darwin123" <drosen0...(a)yahoo.com> wrote in message

> So what is your problem with a photon being a single cycle of a wave
1) A "single cycle" of a wave can have a large amount of energy.
Real photons are caused by the quantization of the waves amplitude,
not the waves wavelength.
2) Single "cycles" of the wave have an uncertainty in position
determined by the wavelength of the cycle. A radio wave can have a
wavelength that is miles long. A single atom, maybe a nanometer wide,
can absorb a quantum of energy from a radi wave. That is what happens
in atomic clocks.
3) A single cycle of a wave has an uncertainty in frequency. If you
chop a single cycle out of a wave, it becomes a wave packet. The
frequency of this chopped out cycle has a bandwidth determined by the
classical uncertainty principle. The bandwidth of a single cycle of a
wave can be quite large. However, a real photon can exchange a fixed
amount of energy.


> and what do your "virtual" photons do? Create "virtual" radio waves?
Photons, both real and virtual, change the energy and momentum of
electrons they interact with. A virtual photon is annihilated or
created when it collides with an electron in the wire or antennae.
A virtual photon can "create" real photons. A virtual photon can
collide with an electron and be annihilated, passing changing the
energy of the electron. The electron can emit a real photon. Thus, the
electromagnetic energy stored by the antenna can become a radio wave.
There are classical analogues to each of these "quantum
processes." One can draw an equivalent circuit to visualize each of
these processes.
Circuit impedance has resistive and reactive components. The
difference is related to the phase lag between voltage and current. A
resistive element (e.g., resistors) has a voltage drop that has a zero
phase lag between current and voltage drop. Resistance corresponds to
loss due to "real photons" or some of type of "real excitation." A
reactance element (e.g., capacitors inductors) has voltage that has an
orthogonal phase lag (90 degrees or 270 degrees) to the current.
Reactance corresponds to "out of phase" effects of the electric and
magnetic fields in the device.
As has been mentioned by other posters, the connection between
QED and classical electronics is an unnecessary complication for
electrical work. However, statements about electronics being in a
different realm are so 1950. As circuits become more and more
miniaturized, they become more and more "quantum mechanical." Also,
electronic materials are becoming more and more "quantum mechanical."
Superconductors do not obey your classical ohms law. Many quantum
devices (SQUIDS, etc.) use superconductors.
The question on how the radio antenna relates to quantum
mechanics was an interesting one. I am not proposing that electricians
have to learn quantum mechanics if they don't want to. However, the
reply that QED exists in another realm was misleading.
From: Excognito on 2 Aug 2010 13:50
On Aug 1, 3:43 pm, Darwin123 <drosen0...(a)yahoo.com> wrote:
> 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.

Thanks for your considered reply. The lecture I refer to is the 1st
of the four he gave in NZ and available via http://vega.org.uk/video/subseries/8
- I'm afraid I can't remember where and it's likely I was reading more
into that he actually said (I can't access it from the computer I'm
working on). I suspect part of my problem is the overloading of some
terminology into both the classical and quantum domains. For
example, I'm not sure that the word 'oscillation' carries the same
connotation in QED as it does in classical physics. Another is,
perhaps, your use of the word 'collide' - I'm slowly forming the
impression that 'absorption' is perhaps a better working concept for
me (partly to make the point that the photon no longer exists, or at
least in the form it did).

Side question: Is there anything in theory (or practice) that gives an
estimate for how long it takes to make the transition from the states
'electron and photon' to the state '(more energetic) electron'?
From: Darwin123 on 2 Aug 2010 16:17
On Jul 29, 5:42 pm, Excognito <stuartbr...(a)gmail.com> wrote:

> Is there a good reference that explains these kind of issues from a
> "what's going on in this situation" perspective?
Here is a link comparing quantum mechanical to classical descriptions.
Note how virtual photons are needed to explain Faraday’s Law in a
quantum mechanical level. Y
http://www3.interscience.wiley.com/journal/55416/abstract?CRETRY=1&SRETRY=0
D. I. Hoult and B. Bhakar, “NMR signal reception: Virtual photons and
coherent spontaneous emission,” Concepts in Magnetic Resonance Part A
9 (5,) 277-297 (7 December 1997).
Here is part of the abstract.
“In portions of the magnetic resonance community, there is a
misunderstanding of the process of nuclear magnetic resonance (NMR)
signal generation and reception, and even in accepted texts, it is
frequently described in terms of absorption and emission of radio
waves, or radiation, by a two-level quantum system. … This difficulty
is examined, and an explanation of the signal given whereby Faraday's
law is explained simply in terms of an exchange of virtual photons. …
Mathematics in the article is kept to a minimum; proofs of the
Principle of Reciprocity description of Faraday's law for reception of
both signal and noise from a conducting sample are given in an
appendix. ©1997 John Wiley & Sons, Inc. Concepts Magn Reson”

From: Darwin123 on 2 Aug 2010 16:18
On Aug 1, 10:34 pm, "Androcles" <Headmas...(a)Hogwarts.physics_z> wrote:
what do your "virtual" photons do? Create "virtual" radio waves?
Here is a link comparing quantum mechanical to classical descriptions.
Note how virtual photons are needed to explain Faraday’s Law in a
quantum mechanical level. Y
http://www3.interscience.wiley.com/journal/55416/abstract?CRETRY=1&SRETRY=0
D. I. Hoult and B. Bhakar, “NMR signal reception: Virtual photons and
coherent spontaneous emission,” Concepts in Magnetic Resonance Part A
9 (5,) 277-297 (7 December 1997).
Here is part of the abstract.
“In portions of the magnetic resonance community, there is a
misunderstanding of the process of nuclear magnetic resonance (NMR)
signal generation and reception, and even in accepted texts, it is
frequently described in terms of absorption and emission of radio
waves, or radiation, by a two-level quantum system. … This difficulty
is examined, and an explanation of the signal given whereby Faraday's
law is explained simply in terms of an exchange of virtual photons. …
”


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