What is Light? What are Radio Waves?

  • A talk given at a Santa Fe Science Cafe, 2013 Jan. 16
  • Abstract of the talk
  • Video of the talk
  • Interview on KSFR radio (15 minutes; choose the 3rd audio option)

The great discovery by Maxwell about 150 years ago of the real nature of light stands as one of the greatest discoveries in all of human history. The goal of this talk was to share with people what light really is, because its nature is not widely understood. I also wanted to demistify “electromagnetic radiation” and “electric fields”, terms that for many people are rather scary due to a lack of understanding of what the terms really mean.

Technical comment for physicists: As a result of preparing and giving this talk, I had a minor insight about the physics of light. A colleague has argued that magnetic fields are merely electric fields seen from a different reference frame. I’ve argued that this isn’t the whole story. I offer several examples that show that magnetic fields are not simply relativistic manifestations of electric fields.

(1) All experiments on electrons to date are consistent with them being true point particles, with zero radius, yet they have a magnetic moment even when at rest. There is no reference frame moving with constant velocity in which the magnetic field of the electron vanishes.

(2) Light consists of electric and magnetic fields that are perpendicular to each other, propagating at the speed of light. There is no physically realizable reference frame in which it is possible to transform away the magnetic field.

(3) Here is my minor recent insight: In the classical wave picture, light is produced by accelerated charges. Because the velocity is constantly changing, there is no constant-velocity reference frame in which the charge is at rest, and in which the magnetic field of the charge vanishes.

Bruce Sherwood

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7 Responses to What is Light? What are Radio Waves?

  1. Jane Jackson says:

    I wonder if David Hestenes’ recent work on his model of the electron can add insights. David Hestenes, Emeritus Professor of Physics at Arizona State University, was an invited speaker at the 7th Conference on Applied Geometric Algebras in Computer Science and Engineering, in Campinas (Brazil), from 23rd to 27th July 2018 at the University of Campinas (IMECC – UNICAMP). His talk is entitled, “Deconstructing the Electron Clock”. An abstract and two lengthy, mathematically-intensive preprints are at http://www.ime.unicamp.br/~agacse2018/guests

    The AGACSE 2018 has three main goals:
    * To promote the development of the geometric algebra and geometric calculus.
    * To spread geometric algebra technologies within industry and business by showing a range of applications (robotics, image processing, computer vision, flight navigation, molecular geometry, etc.).
    * To make progress on the inclusion of geometric algebra ideas into the mathematics, physics, computer science and engineering curricula.

  2. Daniel says:

    Hi professor, I still don’t get why accelerated charges produce radiation. The accelerating charge produces a “kink” in the electric field but where does the magnetic field come from? (the radiating magnetic field need not have the same the direction as the magnetic field of the moving charge as in Matter and Interactions book)

    Also won’t the oscillating electric and magnetic fields (flux) induce curly E and B around it in a circle instead of just in the direction of propagation?

    • BruceSherwood says:

      As is discussed in detail in Chapter 23 of our textbook, when there is a time-varying electric field there is found to be an accompanying magnetic field, and when there is a time-varying magnetic field there is found to be an accompanying electric field (study again sections 1, 2, and 3 of Chapter 23). For the full details see the inevitably complicated explanation of how this plays out in the case of an accelerated charge in the last section of Chapter 23. It’s not possible for me to repeat all of Chapter 23 here.

      • Daniel says:

        I’ve re-read all the sections that you mentioned, professor. But the moving slab that contains the fields already has both E and B inside it instead of just the electric field in the case of an accelerating charge. Is the transverse electric field alone enough to start off the oscillating fields(E and B)?

        Also why do the fields need to be confined inside the slab? (the path integral not taking into account for the path outside the slab)

  3. BruceSherwood says:

    In sections 1-2 of Chapter 23 the fields are NOT “confined inside the slab”. What is shown in those sections is that if we make a guess that a pulse of ExB fields moves in the direction of ExB, with E and B present throughout a large volume of space, we are able to show that such a configuration of E and B fields is consistent with the 4 Maxwell equations, but only if the propagation speed is c (3e8 m/s). We study a section of the moving fields to analyze. This discussion does not prove that such waves exist in nature. Section 3 provides a qualitative way to see how a transverse E could be produced by an accelerated charge, and the analysis in sections 1-2 shows that a moving transverse E would be accompanied by a moving transverse B, perpendicular to E. Then in the final section of Chapter 23 we give an actual proof of the formula for E.

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