There can be no other principles in physics that is more misunderstood or misstated than the principle of least time. Since this principle was originally discovered by Fermat in 1650 it is often known as Fermat’s principle. The same Fermat who was famous for his last theorem which was really a conjecture until 1996 when Andrew Wiles proved it correct and rightfully promoted it to the true status of a theorem. Now back to Fermat’s principle.
According to Richard Feynman the incorrect statement is the following: out of all possible paths that it might take to get from one point to another, light takes the path which requires the shortest time. Page 26-3, Volume I, the Feynman Lectures on Physics. On Page 26-7, Feynman gave the correct statement as the following: a ray going in a certain particular path has the property that if we make a small change (say a one percent shift) in the ray in any manner whatever, say in the location at which it comes to the mirror, or the shape of the curve, or anything, there will be no first-order change in the time; there will be only a second-order change in the time. In other words, the principle is that light takes a path such that there are many other paths nearby which take almost exactly the same time. More discussions can be found in Feynman’s book QED: The Strange Theory of Light and Matter. He was so convinced; he developed a new version of quantum mechanics using the theory of path integrals. On the other hand, according to Francis A. Jenkins and Harvey E. White, a correct and complete statement of Fermat’s principle is the following: The path taken by a light ray in going from one point to another through any set of media is such as to render its optical path equal, in the first approximation, to other paths closely adjacent to the actual one, Page 7 of their book: Fundamentals of Optics.
The keyword here is ‘optical path’ and in the book by Jenkins and White, it is defined as the refractive index multiplied by the length of the path. However, in this context, it can be defined as the wavelength of a single wave. If this is equal to the Planck length then it represents the highest frequency and consequently the highest energy photon. Moreover, if the rectilinear path is transformed into the curvilinear path then the photon changes its quantum state from pure energy to pure mass and the radiative EM wave becomes standing matter wave creating a particle of ½ integral spin that is the neutrino. This could provide a simple explanation why during a supernova; the neutrinos were detected before the optically visible photons. Somehow the neutrinos took less time from point A to point B since their de Broglie wavelengths were shorter than Planck length equivalent to half the unit length of quantum space-time as square of energy. In other words, instead of surfing the waves of the quantum vacuum fluctuations the neutrinos are tunneling into the space-time continuum of greater curvatures.


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