The domain of physics extends from the infinitesimal to the infinite and is largely undefined. At one end of the scale, there are quarks composing nucleons (neutrons and protons) and on the other end, there are galaxies, with sun-like stars as its constituents and a universe that we do not know much about.

In physics, domains are also defined in terms of various important attributes like speed, temperature and other physical quantities. In the domain defined by speed, we study both stationary objects and objects moving at very high speed, perhaps three – fourths of the speed of light. It is thanks to the extraordinary efforts of scientists in the last two centuries that we now know some of the important bounds of nature. For example, the upper limit of speed is the speed of light in a vacuum. Similarly, the lower limit of temperature is 0 K. These are some of the highlights of the development of our basic understanding of nature and its extent.

The uncertainty about the domain of physics stems from the fact that new experiments and discoveries continuously break the bounds (limits) set before. An example: for many years, the charge on the electron was considered the smallest amount of charge, but today after the discovery of quarks, we know that these carry lesser amounts of charge than that carried by electrons. Thus, the extent of physics is actually changing as we learn more and more about nature.

Theories of physics are extremely general, being the underlying governing principles of natural events extending to the whole universe. This aspect contrasts physics from other streams of science, which are often specific and sometimes localized. Generality of physics and its theories render physical laws to form the basic scientific framework upon which other branches of science are developed. Take the example of charged molecules called "ions" - a subject of investigation in Chemistry. Oppositely charged ions are glued together under the influence of an electrostatic force irrespective of the nature and type of ions and the atoms or molecules involved. The magnitude of this electrostatic force is secular in that its magnitude is determined by an inverse square law – whatever be the context and location.

Simplification and unification have emerged as the basic trait of physics. There are only a few laws to define a wide variety of natural phenomena – a fact that underlines the simplification of governing laws in physics. On the other hand, unification of physical quantities and concepts is also prevalent. Take the example of matter and energy. They are now considered equivalent. The Special Theory of Relativity establishes the equivalence of these two quantities as E = m c 2 E=m c 2 . Further, this dual nature of matter highlights the wave (E energy) nature of particles (m mass), which underlies the concept of mass-energy equivalence. Similarly, the treatment of magnetism in terms of electrical charge is an example of unification of physical concepts.

Simplification can also be seen in the laws governing gravitation and electrostatics. Gravitational and electrical forces are conservative forces, determined by inverse square laws. The similarity of the forms of mathematical expressions is no coincidence, but a sure indication of the underlying nature of the universe, which emphasizes simplification :

F G = G m 1 m 2 4 π r 2 .......... Gravitation Force F G = G m 1 m 2 4 π r 2 .......... Gravitation Force

F E = q 1 q 2 4 π ɛ 0 r 2 .......... Electrostatic Force F E = q 1 q 2 4 π ɛ 0 r 2 .......... Electrostatic Force

Simplification and unification of physical quantities, concepts and laws are remarkable, suggesting more such cases – which are yet to be discovered. Consider the physical quantities: “mass” and “charge”. There is as yet no relationship connecting these two fundamental quantities of physics. Similarly, the two major categories of forces known as nuclear and weak forces are not yet fully understood. Scientists are working to examine these unknown territories.

The fundamental laws of physics are set against either too big or too small quantities, presenting a peculiar problem in establishing direct validation of basic theories in physics. Even today, there is not a single experimental set up which could directly verify Einstein’s theory of relativity. For example, mass of an electron moving at two – third of the speed of light can not be measured directly. As we do not see the atom and its constituents, theories based on them are also not directly verifiable. We can not even verify Newton’s first law of motion, which states that an object in the absence of net external force shall keep moving! We have seen all objects come to rest in the earth's frame of reference, when left unaided. This peculiarity, however, does not mean that these laws have not been validated as required for scientific studies.

It should be amply clear that scientific method for validation also includes inferences based on indirect measurements. In that sense, Einstein’s special theory of relativity has been tested and verified by results obtained from the experiments involving motions of charged particles at great speed. Surprising is the exactness and accuracy of the results obtained. In the same context, accuracy of predications involving solar systems, satellites etc. have validated laws of gravitation.

Studies and experiments continue to bring new details and dimensions to our understanding of natural phenomena. New revelations recast old facts, hypotheses and theories. The relativity theory propounded by Albert Einstein, for example, revealed that Newton’s laws of motions are basically a subset of more general theory. Similarly, Newton’s hypothesis regarding velocity of sound was recast by Laplace, arguing that propagation of sound is adiabatic and not isothermal process as considered by Newton. His assertion was based on the experimental result and was correct. There are many such occasions when an incomplete or erroneous understanding of natural event is recast or validated when new facts are analyzed.

There is an irresistible perception that physical phenomena are governed by a universal law - a fundamental law, which is valid at all dimensions and at all speeds. As against this historically evasive natural conjecture, our understanding and formulation are limited to domains of applicability. Newton’s law works fine in our world, where dimensions are bigger than atomic size and speed is not exceeding 0.17c (speed of light, “c”). If the speed of an object exceeds this limit, the relativistic effects can not be ignored.

Consequently, we are currently left with a set of laws, which are domain specific. One law resigns in favor of other as we switch from one domain to another. The plot below approximately defines the domains of four major physical laws in terms of dimension and speed. Though, there are further subdivisions proposed, but this broader classification of applicability of natural laws is a good approximation of our current understanding about natural phenomena.

Figure 1: Approximation of domains in terms of speed and dimension.

Domains of physical laws

The reduction of the special theory of relativity into classical mechanics at smaller speeds gives an indication that there may be a law which is the most general and, therefore, universal. At present, however, there is no such clear cut “deducibility” among other domain specific laws, which involves “quantum mechanics” or “general relativity”. Unquestionably, unification of physical laws is the most fundamental question eluding all scientific investigation to this date.

Our knowledge about nature is improving progressively with every passing day. Yet our so called understanding at any point of time is subject to new revelation and meaning. Most of the time, we come to realize that our understanding is mostly limited to our surrounding and the context of application. It may sound bizarre but it is a fact that we have yet not fully understood even the basic concepts like distance, mass and time. Theory of relativity attached new meaning to these terms. It may not be totally brazen to think that even relativistic improvisations be ultimately incomplete and inaccurate. Who knows ?