Light is an essential source of energy for life on Earth and an effective means of communication. The role of light is significant, making it a subject of study not only in science but also in art. Numerous major scientific works have explored light, yielding important results. However, there remain phenomena of light that are yet to be explained or are still controversial, such as the nature of light and how it travels.
Currently, there are two theories about the movement of light: one posits that light travels in the form of waves, as proposed by Christiaan Huygens, while Isaac Newton and later Albert Einstein suggested that light travels in the form of particles. Each theory accounts for certain phenomena associated with light.
The wave theory of light, introduced by Christiaan Huygens, suggests that the stream of light is a propagation of waves. This theory explains many wave-like phenomena of light, such as interference and diffraction, and also accounts well for refraction and reflection. However, this theory does not explain the photoelectric effect.
The particle theory of light, proposed by Isaac Newton, posits that the stream of light is a flow of material particles. This theory explains reflection and some other properties of light. Albert Einstein also argued that light travels in the form of particles and provided an explanation for the photoelectric effect; however, this theory fails to explain many wave phenomena such as interference and diffraction. With the discovery that light travels at a constant speed, independent of the reference frame, and the wave theory of light, the hypothesis of a medium for light wave propagation, known as ether, was introduced, characterized as a fixed medium harder than diamond. However, the search for the ether medium failed, notably exemplified by the Michelson-Morley experiment. This further solidified the robustness of the particle theory of light.
The image is for illustration purposes only.
To explain the inconsistency between the Michelson-Morley experiment in 1887 and the hypothesis of ether as the medium for light propagation, Dutch physicist Hendrik Lorentz suggested that objects moving in the ether medium would contract and experience time dilation. The contraction of length and the slowing of clocks would ensure that the time for light rays moving from all directions would be the same, thus Lorentz still believed in the reality of the ether medium.
In 1905, Albert Einstein argued that the ether medium was unnecessary since people did not know how they were moving through space, and the speed of light is independent of the observer; they would measure the speed of light the same in all reference frames, regardless of their own speed. In other words, Einstein considered light to travel as particles without a medium for wave propagation. However, the wave nature of light is undeniable.
Moreover, as Huygens stated, if light travels as particles, then the light particles must collide with each other and change direction, making observation impossible if there is a beam of light crossing between the eye and the observed object. This viewpoint remains valid in the case of using light for communication in optical fibers. With the narrow cross-section of the fiber, if light travels as particles, information signals would be lost or distorted when two information streams move in opposite directions.
In practice, this does not occur, indicating that light travels in optical fibers as waves. With two different theories and phenomena showcasing both wave and particle properties, quantum theory considers light to have wave-particle duality. Once light is regarded as having wave-particle duality, using only one of the two theories—the wave theory or the particle theory—will be insufficient to explain this special characteristic of light.
The wave theory, as previously mentioned, does not explain the photoelectric effect, while the particle theory fails to account for the wave-like manifestations of light. Moreover, viewing light as traveling in particle form would reject the idea of light moving at a constant speed in a uniform medium since light particles would have mass (Einstein only considered light particles to be massless when at rest). If they have mass, light particles would exhibit inertia, causing them to accelerate when light changes its state of motion; hence, they could not instantly reach maximum speed upon emission, nor could their speed be constant. Conversely, if light particles are deemed massless even while moving to align with the observed actual speed of light, it would contradict the gravitational lensing hypothesis of relativity, since massless light particles wouldn’t experience gravitational interaction and would not bend near massive celestial bodies.
The constant speed of light in a uniform medium (through which light travels) is a property of wave-like motion. This speed can be achieved the moment light is emitted. When determining that light travels as waves (to align with observable phenomena), a resilient medium is required for light waves to propagate. However, if light is given a resilient medium, light particles would quickly lose energy due to friction with the medium, making it impossible to travel distances of billions of light-years. This is something proponents of light traveling as particles would prefer to avoid. So, what will the solution be?
There has been a proposed solution for wave-particle duality, introduced by Louis de Broglie in 1924, suggesting that light particles travel as waves. But if so, the speed of light particles must be greater than the speed of light since light travels in straight lines while light particles move in sine-like paths, which are longer than the straight path of light. The speed of light particles must be greater for them to keep pace with light, contradicting relativity, as Einstein considered the speed of light in a vacuum to be the maximum speed in the universe, with nothing able to exceed it.
A resolution exists that aligns with all observed phenomena concerning light, harmonizes both wave and particle theories, conforms to the Michelson-Morley experiment, and explains the wave-particle duality of light: the construction of a new hypothesis about the medium for light propagation. This medium is nothing but a medium formed from light particles (photons). It is a resilient medium, not fixed, thereby differing significantly from the so-called ether medium. When light particles undergo some stimulation or interaction, they will shift and interact with other light particles (and possibly with non-light particles). This interaction prevents light particles from moving over large distances and confines them to oscillate within a defined range. It is this oscillation of light particles that creates light waves. This is the answer to the questions of why the observable light exhibits both particle and wave characteristics.
Light has a particle nature because it is indeed made of particles. It exhibits wave characteristics due to the oscillation of light particles, which generates waves; the frequency of these oscillations corresponds to the frequency of light. Light propagates in the form of waves, while the light particles themselves move as oscillations around a certain position and are not entirely fixed in space. The energy in light is transmitted from one light particle to another when they interact, and it can also be transferred to non-light particles when the light particles collide with them. Thus, it is the light waves that carry energy away, not the light particles. The collisions between light particles and electrons are not continuous, and the frequency of these collisions depends on the frequency of light; the energy that electrons receive is contingent on the number of collisions. This explains the photoelectric effect, which occurs only under certain conditions. The energy is carried by the light waves, so it can be said that the nature of light is the movement of a flow of energy in the form of waves created by the oscillation of light particles.
The non-fixed nature of the light particle medium arises from the discrete nature of light particles. Because of this non-fixed characteristic, the light particle medium can be infiltrated by particles from other media, causing it to lose its homogeneity and vice versa. When subjected to the infiltration of other particles, the distance between the light particles expands, the interaction time between the light particles increases, and therefore the speed of light decreases. This is the cause of the phenomenon of refraction, where the speed of light decreases in certain transparent materials.
The non-fixed nature of the light particle medium also creates the possibility for a portion of the light medium to be contained within a sealed box. If that sealed box moves, it will carry that portion of the medium with it. Within this moving portion of the medium, the measured speed of light is no different from the speed of light in the stationary light particle medium and is unaffected by the speed of the moving medium. This scenario is similar to playing music in a moving enclosed train car. The sound will not be affected by the movement of the car but will change its propagation speed through the air inside the car, or be nullified by the movement speed of the air mass in the car.
The light beam in the Michelson-Morley experiment also travels within a moving light particle medium (this experiment was conducted in a sealed tunnel), and it is not influenced by the speed of Earth’s movement; consequently, it does not experience any speed change to create interference patterns in the interferometer. There is no contraction of distance, nor is there any dilation of time in this experiment. This has been demonstrated mathematically. Light waves are mechanical waves, similar to other mechanical waves like sound. They only differ in their wave propagation medium. When light travels in wave form, it will refract at the interfaces between two wave propagation media, and the path of light in the universe will bend due to the atmosphere of any celestial body that has a surrounding atmosphere. The sun also has an atmosphere, and thus the light from stars passing through this atmosphere is similarly bent, as if influenced by some force acting on that light beam. The nature of the bending of light rays near the sun is refraction, not due to the effect of gravitational lensing.
The light particle medium fills the universe, allowing light waves to propagate throughout space. Light particles have mass even in a stationary state. This will be confirmed when it is determined that one of the components of dark matter, which constitutes up to 70% of the universe’s mass, is light particles.
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