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Quote from Wikipedia, Photons: "Nevertheless, all semiclassical theories were refuted definitively in the 1970s and 1980s by photon-correlation experiments. Hence, Einstein's hypothesis that quantization is a property of light itself is considered to be proven."

From the Nobel archives: "The photon is the field quantum (particle) acting as the communicator of force between charged particles."

For about a century, visible light waves have been known to be quantized into discrete packets of electromagnetic energy with each packet containing an energy level of $E = hc/\lambda$ (where $h$ is Planck's constant, $c$ is the speed of light, and $\lambda$ is the wavelength of the electromagnetic wave). Visible light waves and RF waves are both electromagnetic waves with different wavelengths. Are RF waves also quantized into packets of energy with each packet containing an energy level of $E = hc/\lambda$? If so, considering that a free copper electron driven by one amp at 10 MHz vibrates back and forth by less than 1% of the width of its copper atom (2mm diameter copper wire) how do most of those discrete packets of energy, generated by the transmitter, get to the antenna?

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  • $\begingroup$ I might be wrong, but this seems like more of a physics question to me. $\endgroup$ – Zeiss Ikon Apr 11 at 14:33
  • $\begingroup$ @ZeissIkon When Cecil commented (in another thread) that he was going to ask a new question about this, I was a little concerned that it might belong on physics.stackexchange.com. However, I think that he nicely worded this so that it is still about the technology of radio, and therefore on-topic here. $\endgroup$ – Mike Waters♦ Apr 11 at 19:46
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Yes, RF radiation is quantized. Yes, the photon is the quantum of electromagnetic radiation.

However, this is not an excuse to revert to a particle theory of light. It is tempting to think of little "light bullets" shooting out of transmitters, through the air in straight lines, then hitting the receiving antenna. But you'd have difficulty explaining even basic radio phenomena with such a theory. How do you explain interference in antenna arrays? Or diffraction that limits the aperture efficiency of dish antennas? How do the "light bullets" change direction at the ionosphere?

If you want to think of photons as particles that go from one point to another, then you have to consider all possible paths a photon can get from point A to point B, even paths that make no sense classically, including curved, circuitous paths that wander for no reason at all. All possible paths must be considered.


By Matt McIrvin, CC BY-SA 3.0, Link

You must also account for the changing phase of the photon as it travels this path. Then all the possible paths combine in a path integral.

Since there are infinitely many possible paths, this is a very complicated problem, intractable except in the simplest cases. And while it does indeed explain how light can both be quantized while still exhibiting wave effects like interference and diffraction, it's extremely tedious, and we don't get the satisfaction of "light bullets" that travel in straight lines.

While this is an incredibly powerful tool for describing observable phenomena at very small scales, and it explains some things that classical electromagnetism could not such as the photovoltaic effect and the ultraviolet catastrophe, Maxwell's equations remain consistent with experimental results at more practical scales such as those encountered in radio communication.

Since QED provides none of a more intuitive understanding, a simpler mathematical model, or more accurate results for practical radio communication problems, I don't really see a reason to invoke it in any answer on this site.

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  • $\begingroup$ Thanks Phil, I've gone from being "wrong on so many levels (you) can't even decide how to begin to explain" to being less practical and more complicated which is true on some levels. However, as a purely conceptual model, I find some analogies of the particle theory easier to comprehend than some analogies of the classical model, e.g. electronic billiard balls. $\endgroup$ – Cecil - W5DXP Apr 11 at 17:17
  • $\begingroup$ @Cecil-W5DXP Could you share some of those analogies? EDIT: I know you are an IEEE member; just out of curiosity, does the IEEE favor the particle theory over the wave theory? Maybe they haven't taken a position? (Kindly don't ask why I asked about the IEEE. ;-) $\endgroup$ – Mike Waters♦ Apr 11 at 19:56
  • $\begingroup$ I just said the following to Scott Earle: "That physicists are still debating particles vs. waves tells us that there is more to discover. That double-slit experiment and the discussions both fascinates me and boggles my mind." $\endgroup$ – Mike Waters♦ Apr 11 at 19:56
  • $\begingroup$ @Mike Waters The useful analogy is a quantum of energy and momentum that can travel at the speed of light with or without the help of electrons. Maxwell's displacement current plus Ampere's law paved the way for photons traveling through space. The IEEE dictionary only defines "photon" in reference to fiber optics. The word "photon" throws so many EEs into an emotional tizzy that the FCC should probably put it on their list of forbidden dirty words. :-) $\endgroup$ – Cecil - W5DXP Apr 12 at 1:13
  • $\begingroup$ @MikeWaters what credible physicists are still debating particle versus wave theories? $\endgroup$ – Phil Frost - W8II Apr 12 at 13:00
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... considering that a free copper electron driven by one amp at 10 MHz vibrates back and forth by less than 1% of the width of its copper atom (2mm diameter copper wire) how do most of those discrete packets of energy, generated by the transmitter, get to the antenna?

Just as the molecules of $\mathrm{H_2O}$ carrying an (ideal) water wave aren't displaced in the direction the wave travels, so it is with the electrons whose oscillations give rise to electromagnetic waves. It is only the disturbance that travels. According to the Standard Model of particle physics, the energy that travels along a transmission line, along an antenna conductor and between two antennas is an electromagnetic wave, described by a field equation, that is carried by a messenger particle.

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    $\begingroup$ I'd agree but I think you leave this explanation too open to the misconception that the radio spews photons out the connector, and these very same photons are then launched into space by the antenna, or that photons are ordinary particles that behave like classical particles. People love any way of thinking about electromagnetism that doesn't involve waves. $\endgroup$ – Phil Frost - W8II Apr 11 at 19:25
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    $\begingroup$ @PhilFrost-W8II Hard not to do, Phil!! It took me a long time to get my head around the idea, but once I realized that the electrons in a conductor don't travel anywhere near the speed of light, I was finished thinking in terms of particles. $\endgroup$ – Brian K1LI Apr 11 at 19:28
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    $\begingroup$ Nearly everyone must have had that misconception at some point, but I think Glenn has resolved it by replacing "electron" with "photon" without attention to the very specific and odd definition of "particle" here. If you want to predict where the photons will go, you have to use something like the path integral, or Maxwell's equations, neither of which describe anything like a classical "particle". I think your last sentence leaves too much room for missing that very important point. $\endgroup$ – Phil Frost - W8II Apr 11 at 19:48
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    $\begingroup$ @MikeWaters As do I. I welcome suggested edits. $\endgroup$ – Brian K1LI Apr 11 at 20:07
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    $\begingroup$ @Cecil-W5DXP Why does there have to be a particle that travels at the speed of light? Just what behavior are you trying to explain? $\endgroup$ – Phil Frost - W8II Apr 12 at 20:53
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When I studied electromagnetic theory at Texas A&M during the 1950s, I was envious of my fellow classmates who could picture fields and waves in their mind's eye as I could never manage that feat. I noticed that my textbook used ink particles to illustrate fields and waves. The professor used chalk particles to illustrate fields and waves on a chalk board. I used carbon particles to illustrate fields and waves on a piece of paper. To this day, I have trouble picturing in my mind anything that doesn't consist of particles.

Particles do not necessarily contain matter. Anything, including pure energy, that has momentum is considered to be a particle. So I have finally been able to picture fields and waves as clouds of energy particles called photons. Just as each individual in a flock of birds or a school of fish does not have to be tracked, each individual photon doesn't have to be tracked to understand what the cloud of photons is doing, i.e. performing a path integral is not necessary. As long as coherent photons from a single source are being considered, knowing the phase of each photon is not necessary.

We know that electromagnetic RF energy moves from the source to the antenna at the speed of light where it is radiated as radio waves traveling through the air. We know that free electrons in the conductors are somehow involved in the transfer of energy although they simply vibrate in place at RF. Knowing the abilities and limitations of electrons and photons enables one (me in particular) to understand what is going on at a particle collective level.

The only thing that electrons and photons have in common is that they are both quantum particles in the standard model. The characteristics of both particles are available on Wikipedia. The velocity of an electron driven by a current is called the drift velocity which is very low. The velocity of a photon is the speed of light in the medium. The electron is a charged particle with mass. The photon has zero charge and zero rest mass. Both particles can transfer energy but the photon transfers energy at the speed of light which the electron cannot do. Electromagnetic fields and waves consist of photons as does Maxwell's displacement current. Photons can go where electrons cannot go. This difference in capabilities is what is responsible for skin effect in RF conductors.

Most of the photons in the wave in a transmission line travel at an average velocity of VF(c) where VF is the velocity factor of the transmission line and c is the speed of lignt. Most of the photons travel through the air or insulation surrounding the conductor. The free electrons in the conductor act like a sort of bucket brigade while guiding the photons to the antenna. The differential nature of the coherent photon energy flow in the two conductors tends to prevent radiation of photons from the feedline. When the photons reach the antenna some are free to radiate while others are involved in maintaining the standing waves on the antenna.

Some people are satisfied with their mental picture of electromagnetic waves using field and waves and/or Maxwell's equations and I remain envious of them. For others, like me, who have trouble picturing that model, the photon model might help. It goes without saying that the way I choose to picture fields and waves in my mind doesn't have any effect on reality.

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    $\begingroup$ If you're not tracking the phase of the photon, nor thinking of waves or fields, how do you describe the path the photons take? $\endgroup$ – Phil Frost - W8II Apr 14 at 16:47
  • $\begingroup$ @Phil Frost I don't try to describe the path the photons take. $\endgroup$ – Cecil - W5DXP Apr 15 at 4:56
  • $\begingroup$ It seems you do: "The velocity of a photon is the speed of light in the medium.", "Photons can go where electrons cannot go", "Most of the photons travel through the air or insulation surrounding the conductor", "guiding the photons to the antenna"... You seem to attribute quite a bit to how and where photons go, but if you can't explain how they move I don't think we can call this answer correct in any rigorous sense. $\endgroup$ – Phil Frost - W8II Apr 15 at 13:49
  • $\begingroup$ @PhilFrpst-W8II I never claimed it to be a rigorous analysis. Any more rigorous and one runs into the Heisenberg uncertainty principle. Of the four quotes in your comment above, exactly which ones are false? Do you believe that: photons don't travel at the speed of light, or photons cannot go where electrons cannot go, or photons are not traveling in the air or insulation, or transmission lines don't guide the photons to the antenna? Do we need to transfer this discussion to chat? $\endgroup$ – Cecil - W5DXP Apr 16 at 12:53
  • $\begingroup$ You've taken a superficial understanding of QED (which seems to stop at "photons exist") and used it to form a personal theory that satisfies your need to avoid thinking of waves. You've imbued photons with magic properties ("photons go where electrons cannot go") and made assumptions ("The velocity of a photon is the speed of light in the medium"), while expressly refusing to study the underlying theory ("performing a path integral is not necessary"). The burden is on you to prove your surmisations, not on me to disprove them. $\endgroup$ – Phil Frost - W8II Apr 16 at 16:43

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