© Fernando Caracena, 2016
Nicola Tesla raised the question of whether there can be a longitudinal component of electromagnetic waves. Classical theory, as embodied in Maxwell's Equations, is traditionally interpreted to say no. A previous discussion posted here several years ago discussed electromagnetic waves, which are also known as Hertzian waves. Tesla announced the possibility of longitudinal electromagnetic energy by claiming that he had found a method of transmitting power long distances unabated through "Non-Hertzian waves." Because such a claim violated traditional interpretations of Maxwell's equations, and the assumed form of gauge dependencies, Tesla's announcement of this unusual mode of power transmission tended to be dismissed by physicists as some kind of mistake, or at best, a misunderstanding of experimental results. Even now there are discussions of Tesla's observation as being correct, but his explanation's being off because of incorrect physical reasoning. See for example, the article by William Beaty, "TESLA'S BIG MISTAKE?", in which he argues that Tesla made a valid observation, but reasoned incorrectly.
Nikola Tesla was a genius physicist, inventor, and a great showman. Perhaps because of the latter characteristic of his personality, his results are met with skepticism by physicists. Matt Kelly, of the University of Virginia, wrote an article entitled, "Was Nikola Tesla a ‘crackpot’ or a genius?".
Tesla is a legendary figure, who has a large following of fans that think he discovered a means of obtaining free energy. He has become almost a cult figure. The movie, "The Prestige" (2006), features dueling magicians, one of which gains an edge over the other with the help of Tesla (played by David Bowie). Evidence of Tesla's international fame is that some people blamed him for the 10 to 15 megaton blast over Tunguska, Siberia in 1908 that flattened the Taiga for many miles around, but produced no impact crater, as a meteorite would have.
Tesla was one of my favorite physicists as I was growing up. During my high school years, I made and played with Tesla coils. Perhaps his notion of a longitudinal component to Electromagnetic radiation is not so far fetched from several points of view.
Plasma Waves
Consider the phenomenon of electromagnetic waves in plasmas. Although electrically neutral, the distribution of positive and negative charges that make up the plasma can oscillate, thereby producing an electric current and an oscillatory electric field, which is oriented in the direction of charge displacement that is, longitudinally. See the discussion at this link here.
There is physical evidence that the vacuum is electrically polarizable. Although electrically neutral and apparently empty, it is awash with positive and negatively charged virtual particles, which flick in and out of existence. The effects of this sea of virtual particles are real. One phenomenon associated with this is the Casimir effect, which manifests as a force of attraction exerted between two polished, flat, uncharged, metal plates that are brought very close together without touching. Another effect of electrical polarization is the screening of the electric field of the very small atomic nucleus, which is evidenced by the Lamb shift of spectral lines. (See the discussion of vacuum polarization here.)
The Vacuum is like a Plasma Therefore should support Longitudinal EM waves.
Although quantum field theory was not developed when Tesla was experimenting with high frequency, high voltage electromagnetism, he may have stumbled upon effects that could not be understood by physicists according to the state of electromagnetic theory at the time. Being a polarizable medium means that coupled longitudinal charge and electric field oscillations could be initiated in the vacuum.
In fact, Vladimir Onoochin has constructed a model of a spherical pulse of an electric wave generated by and expanding sphere of relativistic electrons that uses the Riemann–Lorenz concept in calculation of the longitudinal component of the E field, in which, the radiated field does not satisfy the Maxwell equation.
Modification of Maxwell's Equations
Koen van Vlaenderen, a Dutch electrical engineer, has proposed a modification of Maxwell's Equations that contains an extra scalar wave component, which represents a longitudinal wave of fluctuating charge propagating through the vacuum at the speed of light. The extra scalar field allows a plasma-like behavior of the vacuum through a charge fluctuation induced on its charged virtual particles. It is worth examining Vlaenderen's model in the light of the body of physics that is known about quantum electrodynamics and gauge theories. This, I am in the process of doing, and will report on it if I have anything significant to add.
Further speculations about what scalar waves might be are pursued in ExoNews.