It is currently accepted that light behaves simultaneously as a particle and as a wave. It is the purpose of this paper to demonstrate that such duality does not exist.
The nature of light has been a topic of study for philosophers and scientists since antiquity. A list of all the theories about the nature of light will fill several large books. This background reviews only the most important developments.
- Newton considered that light was formed by corpuscules, particles, that now are called photons. He considered that such photons traveled in a stright line from source to destination. His theory explains satisfactorily what was known about light at his time. It explains reflection, refraction, lenses, and similar phenomena; what is called geometrical optics.
- Huygens studied light and found phenomena that could not be explained if light was formed by particles, and propossed that light acted as a wave. This is the case of what became to be known as Huygens Law, the fact that when an object interrupts a beam of light, the edges of the object act as sources in the forward direction. His wave theory also explains why the edges of a shadow are never sharp, and similar phenomena.
- Maxwell developed his theory of electromagnetism, which included light and all other radiations as an electromagnetic wave. The work of Hertz discovering the radio waves and of Röntgen discovering X-rays, proved his theory correct.
- The experiments by Young on interference convinced everybody that light was a wave.
- Plank discovered what is now called the Plank Constant h, which is a basic constant of nature. A radiation with a frequency ν has an energy E = h ν, which is the smallest packet of energy in nature. Although it has been tried to take Plank's work as a proof that light behaves as a particle, Plank never accepted that his discovery told anything about the nature of light. It should be added that the Nobel Prize Committee has never accepted the particle nature of light, or its duality.
- Einstein used Wein's work in the wave length distribution of heat radiation trying to explain photoelectricity, but he was not able to come up with a solution. He used Plank's work and applied it to photoelectricity. He insisted that light behaved as particles. Millikan experiments proved, with high precision, that Plank's equation applies to photoelectricity.
- Bohr's study of the atoms produced the conclusion that atoms emit or receive energy in packets following Plank's law. When an electron makes a transition from one energy level to another, it produces a photon of frequency ν, if E1 - E2 = h ν.
- Compton experiments with X-rays showed that X-rays were scattered by matter. He found that this scattering was different for free electrons and for electrons attached to an atom. He was not able to explain this effect using the wave equation and, for a while, the Compton effect was considered a prove that X-rays act as particles. Gordon and Klein used wave theory to explain the Compton effect in much better way than considering the X-rays as particles.
- Electrons and atoms were considered as particles until de Broglie proved that they also behave as waves. His work was experimentally proved by Davidson and Thompson.
- Schöedinger discovered the nonrelativistic wave equation for the electron, which put the wave theory of matter in a firm base.
- Dirac developed the mathematical theory for the interaction of electromagnetic fields and charged particles. He embedded the duality of light as a particle and as a wave in the theory. Pauli, Tomonaga and Feynman expanded the work into Quantum Electrodynamics.
- Heisenberg developed the Principle of Uncertainty, which converted Quantum Mechanics into one of the most productive branches of Physics.
- The Double Slit Experiment, first developed by Young, is considered to prove both natures of light. When the intensity of light is high, an interference pattern develops which can only be explained by the wave nature of light. When the intensity is low, that photons are created one by one, the results of the experiment are interpreted as a proof that light also behaves as a particle. This experiment has been reproduced many times, using not only light, but electrons and other particles, extending the results to matter.
This analysis needs to start with an important clarification. The quick survey presented above shows that the concept of the duality in the nature of light is based more on interpretations than on facts. The fact that a particular phenomenon can be explained if light behaves as a particle is not a proof that light behaves as a particle; it is necessary that, additionally, the phenomenon cannot be explained considering that light is a wave. If a phenomenon can be explained either way, such phenomenon cannot be taken as a proof that light has a certain behavior. In order to accept the duality of light it is necessary to find phenomena that can be explained only considering light as a wave, and other phenomena that can be explained only if light is a particle. The case for light being a wave is clear, all phenomena involving interference can be explained only if light is formed by waves. The purpose of this paragraph is to make an analysis, as unbiased as possible, of the phenomena that is considered to prove that light behaves as a particle, to see if there is an explanation considering light as a wave.
Reflection can be explained considering that the photons are particles that travel in a straight line. Consider a source of light, a mirror and a detector. According to Newton's theory the photons move from source to mirror and to the detector in such a way that the angle of insidence is equal to the angle of reflection. This is a simple explanation and can be veryfied experimentally.
Quantum Mechanics explains the mirror in a similar way, but with the additional constrain that the photons move from source to mirror and to the detector following all possible paths. There is a probability that a photon follows a particular path. It is found that the probability is larger for the parths close to Newton's interpretation, but that there is a real probability of the photons following all other paths. This is proved experimentally in the reflection grating, where part of the mirror has been removed. The explanation of the whole mirror can be based on the assumption of the photons being particles or waves. The explanation of the reflective grating requires that the photons are waves. Consequently, the explanation of the mirror is really a proof that light behaves as waves.
The experiments done by Huygens can only be explained if light behaves as waves. There is no way to explain them if light are particles.
Photoelectricity is normally considered the most important proof that light behaves as particles. The logic is that only a particle can carry the energy required to produce the photoelectric effect. The work of Bohr with atoms proved that atoms accept and emit energy in quantum packets following Plank's Law. Such results do not say anything about the characteristics of the energy being received or radiated. When the atom radiates energy, it is accepted that it is in the form of a wave of frequency ν. There is reason to accept that when the atom absorves energy will do it from a wave of frequency ν. Consequently, there is no reason for considering that light striking an atom in the photoelectric effect should behave any different than under any other condition. Thus, if the energy of light comes in the form of a wave of the proper frequency, the photoelectric effect will happen. This has been proved experimentally many times.
All other effects mentioned in the background are explained when light is considered to be a wave. The conclusion is simple: There are many phenomena, those involving interference for example, that can only be explained if light behaves as a wave. There are a few phenomena that can be explained if light behaves as a particle, but in all those cases, there is an explanation when light is considered as a wave.
The Double Slit Experiment
The double slit experiment, as developed by Young and repeated many times with different settings, can only be explained when considering light as a wave, except for the case when the source produces photons one at a time, which is considered to prove that light behaves as a particle. In order to analize this experiment, it will be divided into two sets of experiments.
The first set of experiments requires a source of light and a set of detectors. CCD cells will be assumed with a very small area and arranged as a continuous line of detectors. For this set of experiments the detectors will be arranged such that the distance from source to detector is the same for all of them. This set of experiments consists of five experiments:
- First experiment: The source of light is turned ON at full intensity for a certain time. It is clear that the detectors will receive a uniform illumination;
- Second experiment: The source of light is turned ON with a lower intensity than before, and for the same time as before. It is clear that the detectors will receive a uniform illumination of a lower value;
- Third experiment: The light source is turned ON at the lower intensity, but for a longer time such that the number of photons produced is the same as in case 1. It is clear that in this case the detectors will receive a uniform illimination of the same value as in the first case;
- Fourth experiment: The light source is turned ON in a way that it produces photons one at a time and at a very low rate, and for the same time as in 1 and 2. It is known that in this case some of the detectors will receive photons and other will receive nothing;
- Fifth experiment: The light source is turned ON producing photons individually at a low rate as in 4, but for a longer time such that the number of photons produced is the same as in 1 and 3. It is logical to expect that in this case, all the detectors will receive the same illumination and that the value will be the same as in case 1 and 3.
The consequence of this set of experiments should be clear: This set of experiments do not say anything conclusive about the nature of the photons. The results depend only on the number of photons, not on their nature.
The second set of experiments use the same set up as above, but including a screen with the two slits in the path of the light from source to detectors, in a way that light cannot go directly from source to detectors. This set of experiments has also five experiments:
- First experiment: This is the same as the first one above, but with the slits. This is the basic Young experiment. It is well known that the detectors will show a pattern of interference that can only be explained if light is a wave;
- Second experiment: This is also commonly done. It is similar to the second above, but with the slits illuminated at lower intensity, but for the same time as in 1. It is well known that a pattern of interference develops, but at a lower intensite than in 1;
- Third experiment: This is never done. It is the same as the second experiment, but the source of light is left ON for as long as necessary so it produces the same number of photons as in 1. It is logical that the pattern of interference will form with the same intensity as in 1;
- Fourth experiment: This is the one that it is considered proves the particle nature of the photon. The light is turned ON producing photons, one at a time at a low rate, but for the same time as in 1 and 2. Naturally, since there are not enough photons, a pattern of interference does not form;
- Fifth experiment: This is never done. Similarly as the fifth above, the light is left ON producing photons one at a time, at a low rate, but for a time long enough so the same number of photons is produced as in 1 and 3. It is logical that a pattern of interference will now be formed, with the same intensity as 1 and 3.
Since the pattern of interference is formed wherever the same number of photons is produced, the explanation of the experiments can only be done if light is considered to be formed by waves.
The Production of Light
This is a problem that is never considered and it is very important when analysing the nature of light. Let us consider that production of light in a very simple setting. Consider a totally dark room, a single source of light, and a large screen illuminated by the light. Let us analyze what happens under the assumption that light is formed by particles.
The screen is illuminated by the source of light. The screen is formed by a very large number of very small areas, all of them illuminated by the source of light. If light is formed by particles, the source of light needs to generate a particle in the direction of each one of the small, elementary areas of the screen. Each of these elementary areas of the screen receives a continuous flow of photons; then, the source must generate a very large number of particles continuously, to produce all those photons in each of those directions. Since each one of those particles carry an energy E = h ν, the source would be required to generate such energy. A very large number of particles would requiere a very large amount of energy. It is well know that a normal source of light, say a lamp bulb, does not require a large amount of energy to produce the light needed to illuminate a normal room.
The analysis above, produce illogical results when a star is considered. Any star would be required to produce continuously a very large number of particles to be sent in all directions of the universe. This would require an enormous amount of energy radiated each moment. A star would produce light only for a moment and its energy source would be exhausted; that is, the stars would shine for only a moment if light is formed by particles. Since it is known that starts shine for millions of years, the explanation of the production of light under the assumption that light is formed by particles produces unreasonable results. On the other hand, if light is assumed to be formed by waves, no problem exist to explain the production of light in all directions, whether in a room or by a star.
The Propagation of Light
The analyses above show that, when considering that light is formed by waves, it is possible to explain all phenomena of light except for one: The propagation of light. The point is that, if light is formed by particles, a particle propagates in a straight line, independent of the media; actually, a particle, like a bullet, does not require a media to propagate. It has been shown above that the production of light by the stars cannot be explained unless light is a wave. The problem comes from the fact that a wave requires a medium in which to propagate.
Most scientists today accept that the space between stars and galaxies is formed by material particles. This idea has existed since the begining of time. The name given to the particles that are considered to form space has been changing with time, but the idea has remained the same: space is formed by matter in some form. For some strange reason, today scientists do not want to accept that the matter they consider forms space could be the media in which light propagates as a wave. This attitude probably comes from the fact that some time ago, there was a theory, using the name of ether for space, that considered light as a vibration of the ether. The Michelson-Morley experiment tried to prove this fact. The scientists of the time concentrated on the fact that the null result was a consequence of the failure of the experiment. Nobody ever considered that the null result was also the right result if the experiment proved what it was supposed to prove. The efforts of many scientists to change the results to satisfy what they considered should be the right result, confused the issue even more. When Einstein propossed to change the concepts of time and space so the null result satisfied what he considered the right result, created even more confusion, that subsists until today. All those explanations were not needed because the Michelson-Morley experiment did not prove anything about the ether or the nature of light. As mentioned above, a superficial analysis of the experiment shows that the null result was expected if the assumptions of the experiment where right and the same result was also expected if the assumptions of the experiment were wrong.
As a consequence of the above, the logical explanation for the propagation of light as a wave is that it propagates in whatever name is given to what constitutes space. A further analysis of the redshift, from the point of view of Plank's Law, leads to the same conclusion, because Plank's Law indicates that there is a loss of energy as the photons propagate for a long time in space.
In summary, considering that light is a wave explains all possible phenomena of light, while considering that light is formed by particles leaves many phenomena without an explanation. There are phenomena that can be forced to accept an explanation with light as a particle. This indicates clearly that light always behaves as a wave and never as a particle. That the idea of considering light as a particle does not have a justification. Thus, there is no duality in the nature of light.