From the history of science I have chosen the exploration of the nature of light. From the history of philosophy I have chosen the work of Rene Descartes, the man who laid before us what has been called ever since his writings, the mind-body problem.
First to Western Civilization’s proudest achievement, the field of the physical science. Here we will consider object X as being light itself. In this example Attribute A is the attribute of being a particle. The attribute of Non-A is defined as the attribute of being a wave. Or vice a versa, the attribute A is defined as being a wave and hence non-A is the attribute of being a particle. This was the reality of definitions under the rules of Classic pre-Newtonian and Newtonian physics. Particles and Waves were considered as mutually exclusive, contradictory, and oppositional possibilities. An object could only be defines as being either a wave or a particle.
Waves and particles have completely different properties and rules of interaction. Water exhibits the characteristics of waves those being diffraction, interference patterns and its energy and its subsequent effects of that energy being spread out continuously in space and time. Billiard balls exhibit the characteristics of particles, those being localization, they exchanges energy in a lump, that energy exchange obeys the laws of conservation of energy and momentum in collisions. Underpinning Classical pre-Newtonian and Newtonian physics is A-logic with its foundational three laws of definition.
‘…[I]n classical physics, the concepts of waves and particles are mutually exclusive. A classical particle behaves like a BB shot. It can be localized and scattered, it exchanges energy suddenly in a lump, and it obeys the laws of conservation of energy and momentum in collisions; but it does not exhibit interference and diffraction. A classical wave behaves like a water wave. It exhibits diffraction and interference patterns and has its energy spread out continuously in space and time. Nothing can be both a classical particle and a classical wave.’
Using A-logic object X, light, must either have the attribute of being a particle all of the time and in all contexts or it must have the attribute of being a wave all of the time and in all contexts. Interestingly enough scientific theory on the nature of light vacillated throughout Western History as defining light as a particle or as a wave. According to A-logic a billiard ball can not and does not ever act like it is a wave, and it is also true that water can not and does not act like a collection of distinct particles. The two are different and distinct and A-Logic system of physics would state that nothing can have both properties. Each property, waveness and particleness, is mutually exclusive and mutually opposite.
Niels Bohr had to formulate the theory of complementariness to explain the dual and simultaneous potential properties of atomic and subatomic objects. Light is always both a particle and a wave. Light/photon is always A and non-A, depending on how you define A, a particle or a wave. A-logic and A-logic based physics considers particles and waves as opposites and mutually exclusive. Reality demonstrates that particles and waves are potential states depending on the context of how the observer is observing the phenomenon; waveness and particleness are not mutually exclusive. Two observers using different means to conduct their observations can conclude that a photon has wave properties and particles properties at the same time. Any single means of observation will only detect one property exclusively but to do so it must ignore the conditions that could demonstrate the other property.
Niels Bohr presented the idea of complementariness in physics to explain the seemingly paradoxical experimental results relating to photons. We need to go back into the history of science to understand what the ‘big deal’ is, why this idea of complementariness was such a radical and revolutionary idea in the history of Western physics. I will digress for awhile into the early ideas of the underlying structure of our physical reality and then present the famous 1801 dual slit experiment and the problem with photons of light.
Classical physics as known in Western Civilization arose out of the study of nature that began in Ancient Greece around 700 – 600 BCE. It is not the case that prior to this time the ancient world lacked practical knowledge of the physical world. The building of cities, irrigation channels, building of tunnels, aqueducts, fabric dyes, astronomical observations of the stars and planets, agriculture, metallurgy, instruments to record the passage of time, the pyramids, the building of ships to navigate the waters of the Mediterranean Sea, etc., all of this demonstrates a remarkable practical knowledge. Knowledge that existed without really being studied to discover the why and how of what was being accomplished.
it was only that prior to this time, somewhere around the 700 BCE, there were no references to people trying to articulate and understanding in verbal symbol systems the how and why of things. The process of describing how things work and interacted in nature and as the result of human action, this activity of devising explanations, this was a new occurrence. Technology, the practical application of knowledge, existed before science, the constructing of mental maps and models to explain the workings of things, existed.
In this the beginning of what will be known in the Western civilizations history of science as the dawn of the Classical understanding of the world, all the things of the world were considered to be made from four things, four elements. The underlying principle and the explanation of why all things act the way they do was because all things could be classified as deriving from four basic things, these were the primal elements. These four elements were: Earth, Air, Fire, and Water , . Varying ideas were put worth by these early Classic Greek ‘scientists’ attempting to establish either that one of these as the primary or the underlying cause of the others, or to establish whether they all co-existed at the same time and none of them being the cause of the other.
Perhaps noticing the changeable states of water, Thales of Miletus (born in 624 – died in 545 BCE), had suggested that since water could exist as a solid, liquid or even a gas under natural conditions, it must be the single principal element in the universe from which all other substances are made. But not all of the early Greek thinkers considered Water as the originating element. Anaximander of Miletus (about 611-547 BCE) assumed as the first principle an undefined, unlimited substance (to apeiron) itself without qualities, out of which the primary opposites, hot and cold, moist and dry, became differentiated. Anaximenes of Miletus (about 586 to 526 BCE), pupil of Anaximander of Miletus, took for his principle primal element Air, conceiving it as modified, by thickening and thinning, into Fire, wind, clouds, Water, and Earth.
But Heraclitus of Ephesus (around 500 BCE) believed that Fire was the originating element. From Fire all things originate, and return to it again by a never-resting process of development. All things, therefore, are in a perpetual flux. However, this perpetual flux is structured by Logos-- which most basically means 'word,' but can also designate 'argument,' 'logic,' or 'reason' more generally. The Logos which structures the human soul mirrors the Logos which structures the ever-changing processes of the universe. Heraclitus believed that the world was a place of perpetual change, eternal Becoming, and the static form that things seem to have was an illusion due to our limited temporal view of the world. In contrast, Parmenides of Elea (around 515 BCE) believed that Being was the basic principle and Being was unique and invariable, change was and illusion caused by our senses. Being was a continuous indivisible plenum which is also immovable.
All this was observed and taken for granted and none had found a way of reconciling it all.
Until Aristotle came along and established his own school called the Lyceum around 335 BCE. Aristotle developed a system of formal analysis. This was the master set of tools to analyze and describe everything. His system was so thorough, complete and all encompassing it captured the imagination. It became The Tool. Aristotelian system of classification became the basis for Classical physics, biology, and actually the basis upon which all of what was to become known as science. Aristotle using his system took all things that were able to be separately known and distinguished and chopped them all up an fitted them within his system. Science became the practice of using Aristotelian logic system to study and explain the natural world.
But what started out by Aristotle as a series of ideas became taught as a system of beliefs. Jose Ortega Y’ Gasset noted: ‘Beliefs, to be sure, begin as ideas. But in the process of slowly pervading the minds of the multitude they lose the character of ideas and establish themselves as “unquestionable realities”.’ Once an idea becomes ‘unquestionable’, once it becomes an obvious truth as to how things are, it changes form and becomes a belief. Aristotelian logic system as a belief began to shape what was observed. Whatever didn’t fit in accordance with that system was assumed not to exist and was not seen, or at least having been seen was quickly forgotten and ignored. Aristotelian system classified things by set rigid definitions which were originally derived by Aristotle from his careful observation of the natural world around him. But once he established the classification schemes they became a belief that pre-determined expectations of what was to be observed. Even Aristotle himself could became tool trapped , , .
All things, material and immaterial, objects or ideas, could be and were chopped up and separated and in doing so were considered by A-logic to be two opposing things. This assumption of opposition under pins and underlies much of the problems of A-logic.
An amazing idea came to a Greek philosopher/scientist Democritus (born 460 BCE died 370(?) BCE). He imagined that there existed the tiniest of things, a thing which was so small as not to be able to be seen. Such a tiny thing was indivisible. It was the building block out of which all things could be made. This tiny object he called an atom. The record of this idea were lost for a long time. It was only recovered at the time of the Renaissance (the 1400’s CE) in the poem of Lucretius’ De Rerum Natura (On the Nature of Things written around 55 -60 BCE.), outlining the Epicurean system named after its founder Epicurus. The epicurean system was first developed as an attempt to meet the logical objections against change and motion put forward by Parmenides of Ella. These atoms moved freely in a proposed neutral void – a place with no absolute up or down, and no specific universal center. From the collisions of these atoms and the variety of ways they combined with each other was created all of our seen and experienced physical reality.
The modern re-statement of Atomic theory began with John Dalton around 1800. This theory was fleshed out by Ernest Rutherford in 1911, who described Atoms as being composed of two sub-atomic particles, Electrons – first discovered by J. J. Thomson in 1897 and Protons – first described by Eugen Goldstein in 1886. It was in 1932 that James Chadwick who actually discovered a third sub-atomic particle which was the Neutron which Rutherford in 1911 had theorized as existing. Everything according to the Atomic system could be described as being made up of these three.
As noted in Classic Aristotelian based physics, who disagreed with the atomist ideas of Democritus and the Epicurean system, particles were always in a specific location, they could be chopped up into smaller and smaller bits, they could interact in collisions and thus create energy- which was the increase in the objects mass and rate of travel according to the what was called the laws of conservation of energy. Now liquids like water move in waves. Waves of water interact not as a localized bit but as diffuse things that can interfere with other waves of water in what is describe as diffraction and interference patterns, and its energy to effect other objects, things that are particles, is equally distributed throughout the wave in its movement through space.
Eventually the scientific description of our world went beyond the Aristotelian based system and was derived using a revised atomic theory. From the framework of the modern atomic system the world was made up of material things which were composed of three building blocks, the proton, the neutron and the electron. In trying to describe where Electromagnetic energy, such as visible light came from in terms of the Atomic System, the photon, the fourth sub-atomic ‘particle’ was recognized and described by Niels Bohr in 1913. Electromagnetic energy was emitted when atoms interact and electrons jump out of and return to their normal orbits around the nucleus of the atom made up of the protons and neutrons. A ‘particle’ of electromagnetic energy such as light was a single photon.
Everything was going along well until more and more scientists were observing light in more and more ways with more and more ingenious instruments. As I am about to present to you, in Western civilization the theories about light’s nature shifted back and forth, like the swinging of a pendulum. My question is: Why? And just and importantly why was this constant shifting ignored?
To appreciate those questions, let us go back again to the beginning, back to ancient Greece. In 55 BCE Lucretius, of the Epicurean school of thought which continued the ideas of earlier atomists, wrote that light and heat from the Sun were composed of minute particles. These ideas of the atomists were abandoned and ignored by Plato and Aristotle and so these ideas were forgotten and had no impact on shaping the ideas of light for later generations.
In approximately 1040 CE an Arabic scientist Abu Ali al-Hasan ibn al-Haytham, also known as Alhazen, developed a broad theory that explained vision using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. Alhazen held light rays to be streams of minute particles that traveled at a finite speed. He improved Ptolemy’s theory of the refraction of light. Unfortunately for the history of Western civilization and science Alhazen's writings did not become known in Europe until the late 16th century.
Rene Descartes held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light which wrongly assumed that light traveled faster in a denser medium, by analogy with the behavior of sound waves. Descartes' theory is often regarded as the forerunner of the wave theory of light.
Pierre Gassendi, an atomist, proposed a particle theory of light which was published posthumously in the 1660’s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. Newton stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light traveled only in straight lines. He did, however, explain the phenomenon of the diffraction of light by allowing that a light particle could create a localized wave in the aether. This diffraction of light had also been observed and noted by Francesco Grimaldi in his work published after his death in 1665: Physico-mathesis de lumine.
Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to dominate physics during the 18th century.
Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the aether . As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.
The wave theory predicted that light waves could interfere with each other like sound waves (as noted by Thomas Young in his famous double slit experiment of 1801), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye. The importance of this experiment will be revisited in the 1900’s by Niels Bohr and others in their theories of light.
Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum , published in 1746, that diffraction could more easily be explained by a wave theory. Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Academie des Sciences in 1817. Simeon Poisson added to Fresnel's mathematical work to produce a convincing argument in favor of the wave theory, helping to overturn Newton's corpuscular theory.
Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Leon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.
As noted earlier, the weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminferous aether had been proposed, but its existence was cast into strong doubt by the famous experiment by Albert Abraham Michelson and Edward Morley conducted in 1887.
‘In 1845 Michael Farady discovered what is now called the Faraday effect and the phenomenon that he named diamagnetism. The plane of polarization of linearly polarized light propagated through a material medium can be rotated by the application of an external magnetic field aligned in the propagation direction. He wrote in his notebook, "I have at last succeeded in illuminating a magnetic curve or line of force and in magnetising a ray of light". This established that magnetic force and light were related.’ Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the aether.
Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behavior of electric and magnetic fields, still known as Maxwell’s equations. The technology of radio transmission was, and still is, based on this theory.
Then in 1900, Max Planck described quantum theory, in which he proposed that light be considered as a particle that could exist in discrete amounts of energy only. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron, neutron, and proton. As it originally stood, this theory did not explain the simultaneous wave-like nature of light, though Planck would later work on theories that did. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory.
‘Albert Einstein's contribution to this discussion was a theoretical explanation of a certain phenomenon, which showed that light was a stream of particles (which would later be named photons). This contribution in a 1905 published paper entitled: On a Heuristic Viewpoint Concerning the Production and Transformation of Light, won for Einstein a Nobel Prize.
This phenomenon is called the photoelectric effect. When you shine a light upon certain metals, a stream of particles (later found to be electrons) is emitted from that metal. The emission has been found to have certain properties.
1. The number of electrons emitted by the metal depends on the intensity of the light beam applied on the metal; more intense the beam, higher the number of electrons emitted.
2. The emitted electrons move with greater speed if the applied light has a higher frequency.
3. No electron is emitted until the light has a threshold frequency, no matter how intense the light is.
These observations baffled physicists for many decades, since they cannot be explained if light is thought of only as a wave. If light were to be a wave, both the energy and the number of the electrons emitted from the metal should increase with an increase in the intensity of light. Observations contradicted this prediction; only the number, and not the energy, of the electrons increased with the increase of the intensity of the light.
What Einstein showed was that the photoelectric effect as it had been observed could be explained if individual particles (or quanta) of light were penetrating the metal and knocking electrons loose from atoms. According to Einstein's paper, increasing the intensity of the light increased the number of photons, while the energy of each individual photon remained the same, as long as the frequency of the light remained the same. Therefore the number of electrons emitted would increase, but the energy transmitted to them by the particles of light would remain the same.
In one stroke Einstein showed that light is a stream of particles, and also that there was solid evidence for the existence of quanta. His theory could satisfactorily explain all the known properties of the photoelectric effect, and was the first result derived from quantum theory of the interaction between radiation and matter.’
This clearly contradicted the exclusivity of the wave theory of light, and for years physicists, including Einstein, tried to rectify this contradiction without success.
Let us now return to the year 1801 and Thomas Young’s famous experiment. Young believed he would put the matter to experimental test by doing his double slit experiment for light. , , , Let me describe this experiment in detail.
An obstruction is put in front of the light source. The obstruction has two doors or slits which can be opened or closed. If we shoot off a series of particles at the two slits they will pass through the slits and collects on a screen behind the obstruction. This pattern of hits and the amount of hits on the target is the sum of the particles which managed to pass through both the two slits. Now if we use water and let it splash through the slits we end up with not a sum of two amounts but what appears to be a series of collections – the water is crashing into each other as it passes through the two slits and preventing some of it from collecting on the other side. This result pattern is called a wave interference pattern.
Now when Young used light and sent it through the two slits the collected light interacts with each other in a pattern that resembles two crashing waves of water. This result on the photographic plate is evidence of an interference pattern which could only be created by waves. Thus the light going through two slits must clearly be considered a wave but the light going through only one slit appear to have particle attributes.
But scientist were ingenious and curious. The technology became refined and now they could take measurements of light photons as they ‘hit’ the slits and before they were collected at the target behind the slits. Everyone expected the results to still demonstrate wave-like properties. ‘When we place the detectors near the slits to measure the particle properties of light, the wave properties of interference cannot be observed. With no detectors near the slit, the experiment is designed to measure the wave properties of light. We cannot, then, say that a photon passed through one slit or the other.’ What we have now observed is that the photons, or electrons, as they hit and pass through only one of the slits, can be counted and measured as if they not waves but separate particles like our pellets. They show no wave like properties. If we have a photon counter which could count the number of photons being fired from the flashlight we would record, for example, that 50 photons hit and passed through one slot and 50 photons hit and passed through the other slot. But that not all 100 would reach the target beyond the slots because now the photons can interact with each other and in doing so they interfere with each other like two waves. Now with pellets you could determine where they came from and in attempting to do so does not alter the results of hitting the target. But with photons any and all attempts to measure location and trajectory of a specific photon will disrupt the wave interference patterns. The act of measuring a photon as a single particle removes from it all wave like properties.
‘We…see that the concepts of classical waves and classical particles do not adequately describe either phenomenon. Each behaves like a classical wave when propagation is considered and like a classical particle when its energy exchange is considered.’ So, what was recorded was that the light photons acted just like particles and light photons acted like waves. This made no sense. Light photons could not be both a particle and a wave.
Even more problematic was that the same experiment could be and has been done with electrons. Electrons have long been ‘known’ to be particles. One of the defining attributes of being a particle is that it can be measured, it has mass. It was known that a photon does not have any mass. Neither does a wave have any mass. Now as the results of the experiment previously described, we have both photons of light and electrons acting like waves and particles. This appeared to be a complete contradiction of Classical and Newtonian/Pre-Einstein physics.
The history has been recounted and it is a long one of very brilliant and careful thinkers and observers oscillating between the two possibilities of wave and particles as being the model for photons that make up light. Why was there such a dilemma? Why was this so perplexing? The ‘obvious’ answer would be to accept all the data and just build a theory that explains all the data. It was clear that they had data which showed light having properties of waveness and properties of particleness. Hence the ‘obvious’ solution would be to accept both sets of data and propose that light had both properties under certain specific circumstances. But this seemingly ‘obvious’ solution was not obvious to them. Instead over those long years of history the scientists kept going back to either wave or particle, one or the other. Why?
