René Descartes defined human existence in terms of its thinking character: “I think, therefore I am.” Converse is also true: “I am, therefore I think.” Thus, humans by nature are endowed with a thinking character. Thus, it would be natural for one to think about one’s environment and one’s place in it. A major part of our environment is the physical nature, usually referred to as the “Nature”. Human thinking pertaining to the nature came to be known as the Natural Philosophy, and Physics, together with its metaphysical component is the foundation of all natural philosophy. In Descartes’ words:
“All (Natural) Philosophy is like a tree, of which Metaphysics is the root, Physics the trunk, and all the other sciences, the branches that grow out of this trunk, ….”
With the development of the quantum theories, the realist view of reality suffered a fatal blow and the “musings of philosophers” like, “Does a particle exist until it is observed?” …, became the burning issues for the practicing physicists to resolve.
Broadly speaking, physics is a general analysis of the nature conducted in order to understand its workings. Since these workings take place in the setting of the space-time complex, e.g., the motion of a material body, their understanding and that of the space-time are interlinked, constituting together the perceptions of “physical reality.” Various issues pertaining to such perceptions have occupied the thinking minds from the ancient times to the modern. Contributions of the ancient natural philosophers, including the Indian, since the era dating back to the Indus Valley Civilization, even earlier; Sumerian and Egyptian dating back to about 3000 BCE; Greek, dating back to about 600 BCE; and relatively recent Arab but earlier than the modern, contain a wealth of knowledge. A great deal of their understanding has permeated the modern theories at a fundamental level providing their underpinnings. Also, their concrete studies including the experimental observations as well as the methods of predictive calculations and developments in the related mathematics, have provided the foundations for the later studies.
Natural philosophers and the practicing physicists have had differing views about the meaning of “physical reality,” as well as about the processes involved in developing its understanding. This article provides a brief overview of some such views.
Nature and Observer
As stated above, the natural philosophy developed from human attempts to understand nature. Clearly, the nature of interaction between the “Nature” and “Knower” or the “Observer” would impact upon the processes involved as well as the understanding. In the ancient times, the Samkhya and Vaisheshika systems, two of the six basic Hindu philosophies, systematically developed the concepts pertaining to metaphysics and physics in considerable detail. In the matter of interrelation between the nature and observer, in the Samkhya system, the mind and then Purush, the Observer, are considered to have emerged out of Prakriti, the nature, but they acquired independent significance. Both, in the Samkhya and Vaisheshika system, the mind and through it, the observer is considered to play a pivotal role in determining the meaning of an observation (S. Kak, Indian Physics: Outline of early history). Later, Descartes expressed a similar view:
“And so something that I thought I was seeing with my eyes is in fact grasped solely by the faculty of judgment, which is in my mind.”
The “Idealist” position goes even further as expressed in George Berkeley’s assertion that material objects do not exist unless perceived and exist only as perceptions. This view is shared, e.g., by the German philosophers Fichte, Hegel and Schopenhauer, and to an extent by Kant. The ancient thinking alluded to above holds that the nature and observer exist in an interactive state and thus, an observer actively participates in the workings of nature impacting upon the observations, which are essentially the perceptions of observer.
There are two main approaches for understanding nature: “Positivism” overlapping with Idealism, and “Realism.” The positivist view is that the only scientific knowledge is the one that can be expressed in logical statements. This is essentially the same view as the one expressed in the Nyaya philosophy, another one of the six basic Hindu philosophies, and the one held by the Rationalists, e.g., Zeno, that originated in the fifth century BCE in Italy, as well as the Greek philosopher Plato. Since logic is developed from the observations involving an active participation of the observer and their interpretations involving mind, this places the observer in a central position, impacting upon the meaning of reality. In contradistinction, the realists hold that there exists a reality independent of the observer. In brief, the realists believe that the reality is objective, whereas the positivists believe that it is subjective. Positivists point out that the realist view involves a logical contradiction, since there is no way of observing an observer-independent reality and hence, we cannot verify that such a reality exists. A weaker form of objectivity is identified with the positivist position though, an empirical reality that science uncovers, which although not independent of the observer, is the same for all observers, i.e., different observers following the same steps would arrive at the same observations. On the other hand, a weaker form of subjectivity is attributed to the realist position in that the role of the mind and observer is acknowledged in deciphering the reality but their impact upon the essential meaning of reality and observation is rejected. These mutually exclusive views have persisted throughout the history of physics.
In the modern classical thinking that prevailed from about the middle of the second millennium to the beginning of the quantum concept in 1900, realism was the unquestionable truth, i.e., the reality; nature and its workings; was assumed to exist independent of the observer. According to the classical theories, a particle is a particle and a wave is a wave, always and everywhere. In 1900 AD, Planck found that light, which was considered a waveform, exhibited particle-like properties in some experiments. Later it was found that the physical entities like the electrons, which were considered particles, exhibited wave-like properties in some experiments. This conclusion was formalized into the doctrine of the wave-particle duality, i.e., each of the physical entities was both, a wave and a particle exhibiting one of its two complementary characters depending on the experiment, i.e., how it is observed, implying a direct impact of the observer on the observation (J. Horgan, Quantum philosophy, Scientific American, July 1992, 94-104). Subsequently, the role of observer in our perceptions of reality became and still remains a burning issue to be resolved; so does the wave-particle duality.
In the modern classical times, it did not make much difference whether the reality was objective or subjective, as long as it was clear, consistent and the same for all observers. Physicists of that period studied the macroscopic phenomena with macroscopic means where no observation produced results that indicated an active intervention of the observer in reality. In case of the quantum theories, which were developed to describe the microscopic phenomena, the situation changed fundamentally as indicated above. As is understood now, in case of the macroscopic phenomena, the quantum effects were so miniscule that they escaped detection and with them, certain metaphysical issues escaped consideration, which although were considered by some earlier thinkers, which is a rather curious circumstance. Einstein, a committed realist, spent most of his later years attempting to discredit the quantum theories for their positivist underpinnings and for their inability to describe the objective reality, which he held, does exist. He failed miserably and continues to fail to this day, i.e., the newer observations continue to establish the correctness of the quantum theories.
Meaning of Understanding
A copy of the universe is not what we need
one of the damned things is ample
– Rebecca West
What do we mean by understanding the nature and its workings? Both, in the positivist and realist thinking, mind plays a crucial role in deciphering an observation and grasping its essential content, which is then formalized in the form of a theory. This process entails a sequence of mappings, each one creating an image in the mind, not a copy, starting with what is observed, seen, to finally creating one that is grasped, understood, by the mind. As the inquiry continues, one has a collection of many final images, each one corresponding to an observation. For the mind, these images are essentially equivalent to the observations and are themselves often referred to as the “Observations.” The next process, that of grasping, i.e., understanding, is greatly facilitated by organizing them on a compact set of assumptions, principles, laws, which enable one to chart one’s course in a forest of the grasped images, observations. Von Neumann, a renowned mathematician and physicist has said:
“I have been convinced several times humiliatingly simply that theoretical physics is just to describe a large set of observations by a smaller set of assumptions.”
Thus, the sciences attempt to organize the observations based on their common traits formulating some rules. If the scope of these rules does not transcend the set of observations, then they are termed the empirical laws; otherwise, they acquire a fundamental character; although the boundary between the two gets fuzzy at times. For example, one of Kepler’s laws of planetary motion states that a line joining a planet and the Sun sweeps out equal areas during equal intervals of time. The scope of this and the others of Kepler’s laws is limited to describing the motion of bodies with the same properties and in the same configurations as the planets. On the other hand, Newton’s laws describing the motion of a body acted upon by a force, which in this case is the gravitational force, and the law quantifying the gravitational force between each pair of massive bodies, enlarge the scope considerably beyond the planet-like systems and thus, acquire a fundamental character. However, they still retain some empirical character for their limited metaphysical content.
A set of the fundamental laws would not be of much value unless there is some method to reproduce from them the observations underlying their development and more; i.e., a deductive methodology. The laws and the methods together should have a predictive ability. In physics, various methods of mathematics are used for deductions. In earlier times, the Indian scientists attempted to describe the perceived reality in terms of the numbers and the Greek, in terms of the geometrical concepts. Both of these formulations complementing each other constitute the founding underpinning of the methods of mathematics in modern physics.
It should be understood that mathematics by itself “has nothing to do with reality;” as Bertrand Russell pointed out. Mathematics is an art, not a science; essentially a concoction of the human mind. It is true that various mathematical concepts and methods have found applications in the sciences and the development of some areas in mathematics has been spurred and stimulated by the issues faced in the sciences, but this does not alter the basic character of mathematics, including that of such applied areas. In mathematics, one starts with a set of assumptions and endeavors to unravel their implications, i.e., their complete content, following certain definite rules of deduction. To be meaningful, the set of assumptions should be inherently self-consistent, i.e., it should not yield any contradictory results. Gödel proved that no consistent mathematical system can be complete, i.e., if a mathematical system is consistent, there will always be meaningful mathematical statements, which can neither be proven, nor disproven. In view of this result, the mathematicians had to abandon their dream of developing a complete and consistent system. Self-consistency is required of the laws of sciences also for the associated theories to be meaningful, including the consistency with the observations, in addition; i.e., for a scientific theory to be meaningful and thus, to be acceptable, it should not only be inherently self-consistent but also no result deduced from it should contradict any observation. Further, no scientific theory can be proven to be true; i.e., no matter how many observations are satisfactorily described by a theory, one cannot conclude that there will never be an observation contradicting the theory, while one such disagreement is sufficient to disprove it. Thus, scientific truth is always tentative that may be superseded.
Due to their broader scope, and also the appeal inherent in their relatively abstract nature, physicists have always preferred and endeavored to develop the laws of fundamental nature. Empirical laws are used as the stepping stones to arrive at such laws. To keep the empiricism out of them, the tendency is to formulate them in terms of the metaphysical concepts. For this reason, making an observation an assumption of a theory is not looked upon favorably. In some cases, observations have been taken as the founding assumptions, laws, of the theories, but it led to controversies. For example, one of the two founding assumptions of Einstein’s special theory of relativity is that the speed of light in vacuum is the same for all observers moving at a constant speed with respect to each other; observers meaning the abstract frames of reference. This peculiar property of light, not shared by the material bodies, had experimentally been established. Objections were raised over this assumption. Einstein had taken the position that certain experimental facts should be allowed to be the founding assumptions of theories in physics. Later on, Heisenberg developed his uncertainty principle essentially on the experimental grounds. As an example of this principle, the momentum and position of a particle cannot be determined completely accurately simultaneously. Einstein then argued that an experimental observation should not be made into an assumption of a theory; instead, all observations should be deducible from the theories independent of the observations. Heisenberg reminded Einstein that he had taken completely opposite position in defending his one such assumption underlying the special theory of relativity. Incidentally, attempts at deducing the special theory of relativity by eliminating or replacing the assumption of the constancy of speed of light are still continuing; so are the attempts at placing the quantum theory, the uncertainty principle being one of its underpinnings, at a more rational footing.
Space, Time and Causality
Without any special effort, we notice space around us. Things have relative positions imparting cognition of space. We observe the configurations of things and thus, acquire the pertaining experience, i.e., we collect a set of observations and their understanding, which can be ordered. For example, the position of a rising Sun gets stored in mind; this observation gets augmented by an additional observation, that of the Sun in a different position. This induces an order in the collection of observations, which is a set included in a larger set, which is included in yet another larger set, and so on. In the present example, the set having a single member, the observation of the rising Sun, is included in the set with two members: Observation of the rising Sun and the Sun in another position; and so on. This is conveniently grasped as an evolution with respect to a parameter, termed the Time. Then the Present is understood in terms of the existing set; Past, in terms of the order of its evolution; and Future, as the order of evolution that may be but is not part of our experience at present. The past is known in the present but the future is unknown. This description of space based on our common experience characterizes it as an absolute relational extent in which objects exist and events occur; time also appears as an absolute entity defined by the order of events. Also, this assigns a single direction to the time: Forward. Eddington termed the unidirectional character of time, the “Arrow of Time,” although the concept was developed from the evolution properties of the physical systems. The space and time, although interconnected through the concept of evolution of a physical system, e.g., the motion of a particle, they appear essentially as independent entities. Ordering of events with respect to the time defines Causality, i.e., there is a Cause for an event, its Effect, and the cause precedes the effect. Common thinking of the cause and effect relation for simultaneous events, e.g., kicking a stone is a cause for the stone to move, is excluded from the definition of causality.
Although, this way of cognizing the space-time appears to be quite natural, it has not been a universally held view, and as will be indicated, its scope has been found to be too limited for an adequate understanding of the nature and its workings. Resolving various issues pertaining to the essential nature of space-time has been a matter of concern throughout the history of the natural philosophy. The issues to resolve have been whether the space and time are themselves entities, relationships between the entities, or part of a conceptual framework. An understanding of their concrete, empirical nature has also been a matter of concern, which has differed from time to time.
Ancient Indian scientists considered the position in space and change in time fundamental to all reality. The space and time were considered to be interlinked and relative, not absolute, nonmaterial “substances.” The nature works on this background, i.e., reality takes shape in the space-time complex. Among many, the Greek philosophers Plato and Aristotle have delved into the issues pertaining to the space-time in considerable depth, particularly, Aristotle. Although a disciple of Plato, Aristotle differed from him in his views about space-time and motion, as was the case with various other views about the workings of nature. In general, Plato is considered to be a rationalist, while Aristotle, an empiricist. Aristotle argued that since things exist in it, space exists as an entity different from the things, i.e., a nonmaterial entity, defining the positions of things. Time is defined as “a number of change in respect of the before and after,” i.e., a quantifier of evolution, movement. The space is an order of coexistence; time is an order of succession. In Newton’s view, the space is absolute, i.e., it exists permanently and independently of whether there is anything in it or not. Similarly, the time is independent of events; events occur in it in sequence. Leibnitz, a contemporary of Newton contradicted this view; instead asserted that the space and time are the elements of a fundamental intellectual structure used to position things and sequence events and thus, structure experiences, which agrees closely with Kant’s view expressed earlier. Kant also maintained that neither space nor time can be empirically perceived. In any case, the concept of causality, which played a pivotal role in the earlier concepts of how the nature works, later became a fundamental requirement in physics and remains so.
Need to quantify the positions and events in space and time requires some measure of distance and time. This can be done by an arbitrarily selected standard measuring rod, i.e., a mathematical line segment, and the duration of time calibrated with the distance travelled by some moving object, e.g., the Sun or needle of a clock. In the Newtonian thinking, which prevailed in the beginning of the modern physics, space and time were considered independent entities and the length of a rod and the ticking rate of a clock, be they standard or not, remain the same for all observers regardless of their positions and states of motion. This view suffered a setback with the development of the special theory of relativity. Accordingly, it was determined and experimentally verified, that the moving rods shrink and the moving clocks slowdown, which is a consequence of the observed fact that the speed of light in vacuum remains the same for all observers moving at constant speeds with respect to each other. Here “observers” mean the “frames of reference,” i.e., their consciousness or the act of observation plays no role in reaching these conclusions; reality remains objective. If the Newtonian view holds, then the speed of light would be different for the observers moving with respect to each other as is the case with, e.g., a moving car observed by a stationary observer and by another one chasing it. In order to accommodate the changes in the lengths of line-segments and durations of time, time was attached to space as another dimension, i.e., the position of an object and the time it is attained at were considered together, i.e., an event. This forges the space and time into one four-dimensional space-time manifold, spacetime, a mathematical “space.” The “length” of a four-dimensional vector in this spacetime manifold was defined in such a way that the changes in their components, three-dimensional line-segments and durations of time, cancel out, which requires just a simple subtraction. Thus, the length of a four-dimensional vector remains constant as it is moved in the spacetime manifold, as is the case with a Newtonian three-dimensional measuring rod as it is moved in space. Length in the spacetime manifold was initially called the “proper time,” which can be positive, negative or zero. However, this terminology can be confusing in certain general situations; therefore the terminology of the three-dimensional space such as “length,” is normally preferred now for the higher dimensional spaces with its extended meaning understood.
As another consequence of the special theory of relativity, no material body can travel faster than the speed of light for it would violate the causality. Preservation of causality was and is so dear to the physicists that an existence of the tachyons, the particles moving faster than light, was excluded for this reason. However, the issue remains far from being settled for the proponents of the tachyons assert that the objections based on causality have been adequately addressed. Reversible phenomena, which are not encountered at the macroscopic level, are quite common at the microscopic level invalidating the concept of the arrow of time also, but the causality is still maintained. The concept of the arrow of time remains valid for the bulk items with the pertaining phenomena understood as the aggregates of the corresponding microscopic phenomena, as the behavior of the gases is understood in terms of the motion of their constituent molecules.
As indicated above, the length of a vector in the relativistic spacetime manifold remains constant as it is transported along an arbitrary trajectory in this space, which was imposed in constructing this space in the first place. Weyl argued that there is no a priory reason to impose this restriction. Also, that in order to compare the lengths of two vectors at different locations, i.e., events, one vector must be transported to the other, and asserted that as a vector is transported, its length can change depending on the trajectory it is transported along. He prescribed a mathematical rule for this change. Weyl asserted further that a measure of length, gauge, can be assigned at each point in spacetime, essentially arbitrarily. One of the basic requirements of physics that different observers must be able to communicate with each other in an unambiguous way without leading to any contradiction, i.e., they should arrive at the same description of the phenomena, maintaining the weaker form of objectivity, was managed satisfactorily by developing the precise rules for the gauge transformations. This concept can be pictured better by suppressing the time components of the four-dimensional vectors, e.g., by projecting it in three-dimensional space or by considering the time-independent phenomena. In such cases, the length of a three-dimensional rod, i.e., a line-segment, changes as it is mathematically transported from one point to the other, and different measures of unit length can be used at two space points. Weyl’s view drew criticism on various grounds, the foremost opponent being Einstein because of its positivistic underpinnings impacting adversely upon his realist view. However, Weyl’s this concept of the gauge transformations came to be at the heart of modern physics.
Mathematical construction of the Weyl geometry requires no restriction on the gauge, i.e., a gauge can be assigned to a point in spacetime essentially arbitrarily. It has been argued recently that for the physical systems, the gauge should be the same at two physically equivalent points so that the weaker form of objectivity can be maintained. For example, the gauge at every point of a trajectory in spacetime traversed by a free particle should be the same. However, if this particle encounters a particle detector, it interacts physically with the detector, i.e., the terminal point of the trajectory is not physically equivalent to the other points on it, resulting in a different gauge at this point. For a given particle, this gauge is determined by the interaction of the particle and detector. In more complicated experimental setups, e.g., involving beam splitters, which partially reflect and partially transmit the beams of light or particles, and the complete reflectors, there would be several physically inequivalent points, with a commensurate distribution of the gauges, e.g., the gauges at the reflector, beam-splitter and at a point along the trajectory of a free particle would be different. According to some recent studies incorporating Weyl’s these ideas, the entities, which are detected as particles in some experiments while exhibiting the wave-like properties in the others, can all be considered particles, eliminating the menacing wave-particle duality. The calculations show that if there is no net gauge in an experimental setup, i.e., if different gauges cancel each other out, which does happen in appropriate experimental setups, then a particle exhibits wave-like properties; otherwise, it exhibits particle-like properties; resulting in an understanding of the observed impact of an observer on the observations.
Some of the basic issues and thinking underlying the nature of the scientific inquiries impacting upon the perceptions of physical reality outlined above were of little concern in the modern classical theories with their realist view of reality and the prevailing pragmatic thinking. Accordingly, such issues were considered to be the musings of philosophers of little practical value. With the development of the theory of relativity, the assumption that the space and time are absolute and independent entities, had to be abandoned but the realist view that the reality is objective, survived intact. With the development of the quantum theories, the realist view of reality suffered a fatal blow and the “musings of philosophers” like, “Does a particle exist until it is observed?” “Is moon there when nobody looks;” “Does a tree falling in a forest make a sound if there is nobody to hear it?” …, became the burning issues for the practicing physicists to resolve. Pragmatic thinking still has it that such questions are of little value as long as there is a reliable predictive mechanism, i.e., the methods to calculate, is available; such questions are often responded with “Shut up and calculate,” as the phrase goes. But many are not satisfied by this empiricist thinking maintaining that it goes against the grain of sciences, which endeavor to formulate the workings of nature in terms of the sets of self-consistent laws of fundamental nature. Furthermore, that an understanding at the fundamental level is known to provide a deeper understanding of reality leading to a better theory, impacting at all levels, including the pragmatic phenomenology. Recent formulations incorporating Weyl’s concept of the spacetime constitute a step forward in this direction. However, the roads are always long, and there are still “miles to go.”
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