Reference: The Universe and Physical Elements
The word “energy,” as used by Newton meant, the “capacity for doing work imparted by matter in motion.” Moving matter had momentum that was transferred upon impact. The change in momentum of an object constituted force acting upon that object. When that force produced movement against some resistance, it was seen as work being performed. The capacity for doing work was then converted into actual work. Thus, we have mechanical energy. Heat was also recognized as energy. Heat involves motion of matter at atomic level. Thus, the basis of energy is momentum resulting from matter in motion.
In other words, to have energy we must have both matter (substance) and motion.
We usually recognize matter as a substance because it is substantial enough to be sensed; but when it comes to light, we recognize it only as energy. Light is recognized to have momentum, even when it is minuscule compared to a material particle.
Light is energy. Is light a substance with motion?
Newton did consider light to be a substance with motion. He considered light to be made of particles.
Light being a substance, that was very different from matter, was considered by Newton.
But, the existence of light particles could not be proved. Not everyone looked at light as a substance. Many looked at light as a wave traveling in space, and a wave theory for light was advanced. This theory did not require light to be a substance, but it did require a substantial medium for the propagation of light.
The alternative of light being a wave that was traveling in some medium was considered by most physicists.
Meanwhile, electricity was discovered and was intensely studied. Electricity was found to be a substance that was very different from matter. Based on his very precise experiments, Faraday forwarded the idea that electricity consisted of lines of force. These lines of force, when concentrated, formed the atoms of matter. Faraday supported Newton’s idea of light having substance. He suggested that light consisted of much finer lines of force.
Faraday suggested that matter got loosened into lines of force in electricity and light, and no medium was required for the propagation of light.
Maxwell applied the idea of lines of force to the wave theory of light. Using brilliant mathematics, Maxwell developed an electromagnetic theory, from which he could derive the “speed of light”. This was a wonderful validation of the electromagnetic theory. But Maxwell had to assume certain electromagnetic properties for space. Maxwell essentially declared space to be the medium for light waves. Later when very precise experimentation was carried out, no such medium for light could be detected. This was shocking for most physicists.
Light, more likely, was a substance that spread out in wave form.
Soon after the above discovery, Max Planck came up with the mathematical concept of “quantum” to resolve the problem of Black Body radiation. This idea basically proposed a proportionality between radiant energy and its frequency. This was a radical departure from classical physics because, in wave theory, energy is proportional to the amplitude. Einstein then applied this idea of quantum to successfully explain the photoelectric phenomenon, proving that the idea of quantum was quite practical. Based on this understanding, Einstein proposed the concept of “photon” as a particle of light.
Einstein finally provided the proof of light being a substance.
Einstein also showed that a particle of radiative energy became more condensed as it increased in frequency. Thus, the electromagnetic spectrum was also a spectrum of substance that, ultimately, included matter. Furthermore, with increase in frequency the speed of quantum reduced from the speed of light to a fraction of that speed for much condensed electrons.
Energy at the level of light could now be separated into substance and motion as in the case of matter.
A substance may now be defined more broadly as “that which is substantial enough to be detected or sensed.” Substance shall range from gradually thickening radiative energy to rigid matter. Radiative energy was viscous rather than absolutely rigid like matter. Both viscosity and rigidity were now relatable to the property of mass inherent in any substance.
The substance may be quantifiable on a scale of mass.
Experimentally, no variations in the speed of light could be detected, making motion an inherent property of photon. Similarly, motion could be looked upon as an inherent property of quantum throughout the spectrum of substance, even when it could be affected by external conditions within a certain range. Evidently, such inherent speed decreased with increasing viscosity, rigidity or mass.
Motion is inherent to substance to some degree. It may be quantifiable on the scale of mass within a certain range.
Material particles are discrete and they are made of discrete atoms. But the quantum of radiative energy is a continuous wave-like flow of extremely thin consistency. This quantum may be compared to a certain concentration of Faraday’s lines of forces. The consistency of quantum thickens as its frequency increases.
A quantum may be viewed as a concentration of lines of force.
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ADDITIONAL THOUGHTS
Energy is proportional to frequency actually means that energy is proportional to its “mass density.” The idea of frequency comes from partially successful wave-theory.
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