Physics is the

science that studies and describes the fundamental nature and behavior of physical systems. Physical systems, with their measurable properties and observable phenomena, abound in the world of everyday experience, such as the billiard balls on a table or the current in a wire, as well as in realms ranging from the sub-atomic to the super-galactic, and indeed the aggregate universe as a whole. Thus physics as a science studies, and its body of knowledge applies to, all systems on all scales everywhere in the universe at all times. It is therefore the most fundamental of the sciences, the most profound, and the most philosophically rich.

The basic premise is that the laws of nature can be expressed mathematically, that they are universal, and that their effects are observed consistently. The laws of nature as formulated strive to describe everything in the universe from the sub-atomic realm to the nature of space-time itself, and in doing so predict how systems will evolve. Thus, physics overlaps mathematics and philosophy on one side, and chemistry, materials science, and electrical and mechanical engineering on the other. The science is both experimental and theoretical. Physics constructs laws of nature based on observations and mathematical consistency, with laws being tested and modified as observational results dictate.

**The Realms of Mechanics:**

The prediction of system evolution with a knowledge the interactions involved is known as **Mechanics**. In everyday usage the word brings to mind the trajectories of billiard balls or the study of forces on an I-beam, and indeed, it was these human-scale phenomena that were first systematically studied. The body of knowledge that describes such things is called **Newtonian Mechanics**, and encompasses such studies as kinematics, statics, ballistics, and fluid mechanics. The essence of Newtonian Mechanics is that forces cause changes in motion in a continuous manner with time as a universal parameter, all of which is concisely stated by Newton’s Laws.

However, it was revealed in the 20th century, with the birth of modern physics, that Newtonian Mechanics is only an approximation to the true laws of nature, and fails when the scale is too small, the velocities too fast, or the energy, mass, or momentum too large. On a small scale, when the actions of individual molecules, atoms, or sub-atomic components are relevant, we must use **Quantum Mechanics** to describe the system. Newtonian Mechanics can then be derived as the large scale limit of Quantum Mechanics. The essence of quantum mechanics is that matter has wavelike properties, that identical particles, being merely waves in some sense, are fundamentally indistinguishable, that quantum systems exist in definite states which can be discrete or continuous, with the need for waves not to destructively interfere often leading to the discrete case, and that what can be said predictively about a system is its probability to be in a certain state. The implications of this, such as the lack of determinacy inherent in the quantum world, have troubled physicists, philosophers, and lay people for 80 years. Quantum mechanical systems evolve according to the Schrodinger Equation, or the equivalent Heisenberg Equation.

It also turns out that when the velocities of system components are large enough, the mechanics of **Special Relativity** properly describes the system, and again Newtonian Mechanics can be derived as the low velocity limit of Special Relativity. Special relativity, in order to preserve the speed of light as a universal constant, maintains that different observers will observe different time and space intervals between events, thus getting rid of the idea of time as a universal parameter and linking space and time into a single object, spacetime. The space and time intervals as measured by different observers are related by Lorentz transformations. A major consequence of special relativity is that there is energy associated with matter at rest.

For the fast *and* small, consistent with both Special Relativity and Quantum Mechanics, **Quantum Field Theory** describes systems where the small scale is relevant and the velocities are relativistic. The procedure is to describe particles as manifestations of fields that exist throughout spacetime. Quantum Field Theory has provided a successful model of the known sub-atomic particles and their interactions. It should, however, trouble philosophers even more than quantum mechanics, for it imbues empty space with infinite energy that conveniently cancels out for any observable quantity.

However, the fact that yet another generalization, **General Relativity**, is needed to describe systems where sufficiently large masses, momenta, or energies curve spacetime itself means that Quantum Field Theory is as well not a complete theory of nature. In fact that complete theory, whatever it may be, is not yet known. General Relativity states that energy curves spacetime, in a manner described by the Einstein Equation, which alters the trajectories of particles, giving rise to what we observe as gravity.

Even given the predictions of mechanics, if there are vary many system components, it is often not useful or even possible to know how individual components behave, but rather what is useful to know is how components behave on average. This is **Statistical Mechanics**, which can be derived based on assumptions from the appropriate quantum or classical mechanical theory.

**Thermodynamics** describes how different components of a system share energy and particles. It is derived from straight-forward mathematical considerations and the idea that systems exist in denumerable states, and is related to Statistical Mechanics in that knowledge of the behavior of individual particles is not relevant. From thermodynamics we can know the equilibrium point of complicated processes.

**The interactions:**

Physical system evolve because of interactions, and it has been traditional to view nature as having four fundamental interactions, electromagnetism, gravity, and the so called strong and weak interactions. Physics contains the fundamental theories of all of these interactions.

**Electrodynamics** describes electric and magnetic fields and the electromagnetic radiation that carries them. Classical electrodynamics, that is electrodynamics on scales larger than the sub-atomic, is formulated rather succinctly by Maxwell’s Equations, which relate the fields and their sources, and summarize in shorthand many laws discovered over the 18th and 19th centuries, along with the Lorentz Force Law, which gives the effect of the fields on a point charge. Optics refers to specifically the study of electromagnetic radiation and Electronics is the study of electric circuits of various complexities. The quantum field theory Quantum Electrodynamics describes the electromagnetic interaction on the quantum level, where it is revealed to be a manifestation of the Electroweak interaction, which includes the weak interaction as well.

**Gravity** is the interaction that couples all matter and energy in the universe. Because of this universality, gravity is believed to be a manifestation of the curvature of space-time caused by matter and energy, and is described by the Einstein Equation of General Relativity. In the scale of everyday experience, gravity can be accurately described by the more familiar Newton Law of Gravitation.

The **Weak Interaction** occurs on the level of sub-atomic particles, and is a manifestation of the Electroweak interaction as stated above. The **Strong Interaction** occurs between quarks, a class of sub-atomic particles, and is described by the quantum field theory Quantum Chromodynamics.

**The underlying universal truths:**

In the face of all of this, then, it turns out then that the most fundamental formulation of the laws of nature seems to be that invoking symmetries and the least action principle. The essence of **Symmetry**, in the context of physics, is that when a system is invariant with respect to a parameter, there is a associated with that parameter a quantity that remains unchanged. This is often called Noether’s Theorem. The **Principle of Least Action** (also known as Hamilton’s Principle), states that systems evolve on a path through space and time in such a way as to extremize the action, which is the integral over the path of a function of the system called the Lagrangian which contains information about the system components and the interactions. In quantum theories, systems have some small probability to evolve on paths other the one of minimum action, and this is known as the **Path Integral** formulation of the least action principle.

**The historical development of physics:**

Physics is certainly the most ancient of the sciences, and maybe the oldest line of inquiry in human culture. The study of physics began millennia ago with the dawn of thoughtful inquiry itself, with the realization that the behavior of objects, for instance the trajectories of arrows, is consistent and predictable. In the ancient civilizations, especially Egypt and Greece, it was discovered that patterns in nature could be described with mathematics and geometry, and this was explored to a high level of sophistication. From the periods of motion of celestial bodies to the flow rate of fluids in a certain volume, the first formulations of physics were essentially mathematical, and they remain so to this day.

In the middle ages, indispensable discoveries in mathematics and astronomical measurement were made in the Islamic world and in India, but the important advances in physics would come about in the west. Exhaustive sky surveys by late medieval astronomers, many of them monks, led in the 16th century to the first major advances since the classical era, the Copernican model of the solar system, and shortly thereafter Keppler’s laws of planetary motion. These laws were significant in that they confirmed gravity as a universal and invariant phenomenon, challenged the perception of preferred locations by reinvisioning the earth as just one of several planets, and reduced voluminous astronomical data of angles and periods to relatively simple mathematical truths.

In the following century, Galileo, in timing the fall of objects and studying collisions, as well as attempting to measure the speed of light, reconceived physics as an experimental science. By turning his telescope to the heavens and seeing the moons of Jupiter and rings of Saturn, he revealed an exotic and beautiful cosmos waiting to be understood, as did Robert Hooke with his microscope. The work of Keppler and Galileo was used by Issac Newton to formulate his equations of motion in the late 17th century. Newton’s equations brought about a revolution in thought as they both postulated instantaneous action at a distance by forces and reduced the workings of the universe to something that could be governed by a precise ‘Clock-maker.’ In addition, by developing the calculus to express his theories, Newton severed the millennia-old link between physics and simple mathematics. The laws of the universe were now and forever more in a more complicated language.

The end of 18th century saw the beginnings of the study of electricity with experiments such as Benjamin Franklin’s kite. However, the most important experimental discovery was August Coulomb’s inverse square law for the electric force, which, mirroring Newton’s inverse square law for gravitation, brought the study of electricity to a firm theoretical footing. The first half of the 19th century then saw rapid advances in the understanding of electromagnetism. Ampere’s discovery that an electric current effects a compass needle and his subsequent law relating electric current to the magnetic field it induced, combined with Michael Faraday’s law, which quantified how magnetic fields could produce a current, unified the formerly separate studies of electricity and magnetism. These combined with Gauss’ Law, which introduced the concept of an electric field and related it to its charge sources, and experiments on light which confirmed it as wave-like electromagnetic radiation, were succinctly summarized by James Clerk Mawell in Maxwell’s equations, which contain our complete theory of electromagnetism and electromagnetic radiation, one of the great triumphs of physics.

Much of the sophisticated mathematics that the increasingly sophisticated physics needed and would need in the 19th and 20th centuries was developed in the early 1800s by the likes of Karl Friedrich Gauss, Leonhard Euler, and others. There was also a reformulation of mechanics, inspired directly by the notion in Christian theology of ‘least action.’ The laws developed by Hamilton and Lagrange, applying a physicist’s definition of notions such as action, revealed themselves as complete descriptions of mechanics, and have come to be seen as part of the most fundamental formulation of physics.

The other great realm of advancement in the 19th century was Statistical Mechanics and Thermodynamics, with the theoretical work of chemists and engineers such as Josiah Willard Gibbs and Jean Carnot, and culminating in the work of Ludwig Von Boltzmann and Maxwell. The principles of physics as understood by the 19th century were the basis for many of the sweeping technological innovations that transformed human society.

The theories of Mechanics, Electricity and Magnetism, and Statistical Mechanics that had been articulated by the end of the 19th century form the body of knowledge often known as ‘classical physics.’ Many saw physics as a solved problem, with maybe a few details still outstanding. One physicist, Lord Rayleigh, remarked that the only things left to do were to determine the nature of the ether and explain the photo-electric effect. As described below, he was right in a way, although he probably didn’t realize the extent to which answering these two questions would overturn our view of the universe.

The opening decades of the 20th century saw three dramatic and simultaneous revolutions in physics that constitute probably the greatest reworking of mankind’s view of the universe ever. One was the advent of relativity theory, first Special Relativity and then General Relativity, promulgated by Albert Einstein with the help of Hermann Minkowski and others, which removed the concept of preferred reference frames from science, and introduced abstract mathematical ideas of space-time as physical reality. Special Relativity, proposed by Einstein in 1905, resolved the failure of the Michaelson-Morely experiment to detect an ether, the fixed medium in which light and other electromagnetic waves were assumed to travel, by introducing the speed of light as a universal constant to all observers. Einstein’s thought experiments over the next ten years about the universality of gravity led to General Relativity.

The second great scientific revolution of the early 20th century was the development of the quantum theory of sub-atomic matter. Experiments in the first decade of the century by Pierre Curie, James Chadwick and Ernest Rutherford led to models of tiny atoms involving even smaller constituents like electrons and positive nuclei. Concurrently, prompted by the results of measurements of emission spectra, and of the ability of light radiation to stimulate current in the so-called ‘photo-electric effect,’ Einstein and others hypothesized that radiation was only available in distinct quanta of energy. These results led to an effort to explain the optical emission spectra of elements by changes in the internal structure of the atom, and resulted in the ‘early quantum theory,’ proposed by Neils Bohr in 1911, in which electrons occupied orbits with distinct energies around the nuclei of atoms. A correct quantum theory of matter, Quantum Mechanics, was developed in the early 1920s by Erwin Shrodinger, Warner Heisenberg, Paul Dirac, Enrico Fermi, and many others. Quantum Mechanics provides the best theoretical picture of the atom, and in doing so explained phenomena from radioactive decay to the periodic table.

The third early 20th century discovery with vast implications came with advances in astronomy. Astronomers such as Edwin Hubble, using the radiation ‘dopplar effect’ predicted by relativity, concluded that the universe, now known to consist of many galaxies in addition to ours, was expanding. This led to the ‘big bang’ theory of cosmic origins, the first time science had a coherent theory for how the universe began.

On the heels of these amazing developments, which were and are often incomprehensible to those not trained as physicists, as well as the general rapid advancement of technology of the early 20th century, much of it a result of all physics had discovered over the centuries, came the second world war and the atomic bomb, followed shortly by the transistor, developed with quantum theory, and space travel. The world was awed and intimidated by the power of physics.

During the post-war era scientific research was increasingly dedicated to technological applications, such as the development of semi-conductors, new materials, nuclear power generation, and rocketry, all of which required extensive contribution from classical and modern physics. However, there were also many theoretical and experimental advancements built on the foundations of early 20th century physics. Quantum Field Theories, such as Quantum Electrodynamics and Quantum Chromodynamics, developed by minds like Richard Feynman, described the sub-atomic processes discovered before and after the war, with Quantum Electrodynamics matching experimental observations to an extent that made it the most accurately verified theory ever. The statistical mechanics of quantum systems predicted intriguing new phenomena such Bose-Einstein Condensation and superfluidity, which were experimentally verified. The BCS Theory of superconductivity, also drawing on quantum statistical mechanics, was a major triumph of the 1950s. In the early 1960s, first masers and then lasers were developed with quantum theory.

In the last forty years of the 20th century, astronomers discovered phenomena such as black holes, quasars, and neutron stars, all of which were predicted with quantum theory and relativity, and more observations, including the discovery of the cosmic background radiation, introduced new questions about the structure of the universe. Experiments with particle accelerators revealed a zoo of exotic sub-atomic particles, suggesting that even the then observed sub-atomic particles were themselves composits of other particles. This led by the 1970s to the current standard model of elementary particles involving quarks and leptons, developed by Murray Gell-Mann and others. The experiments of particle physics in the last 30 years have been oriented toward testing the predictions of the standard model.

In other areas of study, the investigation of non-linear dynamics, physical systems whose evolution, governed by non-linear differential equations, is highly sensitive to initial conditions, led to the development of chaos theory. It dawned on others that electron spin, a feature of quantum mechanics, could be used in place of the presence or absence of a voltage to represent a data bit, giving rise to the possibility of so-called quantum computers. Continued experimental and theoretical work in semi-conductors and superconductors, and the discovery of nuclear magnetic spin resonance have spurned technological advancement, and physical laws and physicists themselves have been increasingly used to model everything from the stock market to bacterial evolution.

There remain great unsolved problems in physics. Astronomers tell us that as much as 90% of the matter in the universe is not in a form that the standard model of elementary particles can account for. There has yet to be a complete or verified quantum theory of gravity, although it is believed that the many variants of String Theory are a start. There are many questions regarding the validity of the standard model, and if String Theory is verified, they will have to be reconciled. There is also the question of the shape and ultimate fate of the Universe, and how, if at all, this ties in with the eventual fundamental theory of particle interactions. The technology to make quantum computers possible will take decades of development. Additionally, there is still much to be discovered about many human scale physical phenomena are incompletely understood, such as the flow of air due to a bee’s wings.

__Current Research:__

Current research in physics, and therefore the sub-specialties of it, can roughly be divided into the following areas. Each area of research consists of both **Experimental** and **Theoretical** work. Experimentalists perform physical experiments to test existing theories or theoretical constructs, or provide data with which new theories can be developed. Theoreticians develop and tweak the theories of physical processes. In the past 50 years in physics, there has been a sharp divide in training and temperament between those doing experimental work and those doing theoretical work.

- **High Energy Physics or Particle Physics** investigates the fundamental constituents of matter. Very high energies, such as those obtained by particle accelerators, are required to reveal the presence of these particles. Experiments performed at accelerators such as Fermilab, CERN, SLAC, and CLEO are testing aspects of the current standard model and looking for physics beyond it, and current theoretical work is examining the standard model and developing string theory.

- **Astrophysics** observes the cosmos for data. Experiments and theoretical work seek to come up with a coherent theory of cosmology, especially one that is consistent with particle physics. There are experimental and theoretical investigations into the macroscopic structure of the universe, the nature of the dark matter, and the nature of observed phenomena such as black holes neutron starts, and the Cosmic background radiation. These realms often overlap in energy and length scale with particle physics, and there is much overlap among the two disciplines.

- **Condensed Matter Physics**, while perhaps the least known specialty outside of physics, employs over half of the research physicists in the world. It is the investigation of materials and other systems above the atomic scale. The work of Condensed Matter Physics, especially, of late, in superconductors, semiconductors, optical films, and nanotubes, currently has the most technological implications, and indeed much of the experimental research is done in concert with electrical or materials engineering. Condensed matter research is often divided into ‘hard condensed matter’ realms, where quantum mechanical effects are important, and ‘soft condensed matter’ realms, in which they are not.

- **Nuclear Physics** studies the properties of atomic nuclei and their nucleon components. Because these components reveal themselves at high energies, but energies lower than those at which the sub-nucleon components of particle physics can be observed, nuclear physics is often called ‘medium energy physics.’ Nuclear physics experiments are performed both in small laboratories, and at particle accelerators such as Brookhaven National Laboratory. Current work is aimed at understanding the exact structure of nucleons, as well as developing a quark-gluon plasma, a new state of matter consisting of quarks not bound into ordinary nucleons.

- **Atomic Physics** deals with the physics of atoms and atomic length scales. While the most fundamental questions about the structure of the atom are a solved problem, there is still much to be learned about the precise properties of many atomic systems. Atomic physics can have a great deal of overlap with chemistry, and experiments can be performed on a table-top.

- **Optics** uses lasers to investigate material, molecular, atomic or nuclear properties, and is often seen as indistinct from atomic physics or condensed matter physics.

- **Biophysics** is a hybrid discipline that uses the tools of physics to investigate biological processes. Some biophysics work uses the theories and mathematical rigor of physics to model biological processes, much as it can be used to model the stock market, while other biophysics work utilizes the equipment and know-how of physics labs to perform experiments on organic systems.

The study of physics has brought mankind from a state of knowing and understanding little about the fundamental nature of the world around us, to a position where we know the properties of distant galaxies and investigate and manipulate the mini-universe of the tiny atom. In many ways physics is the ultimate expression of our curiosity, creativity, and intelligence – indeed our very humanity.

*This entry was compiled from the knowlege absorbed during the undergraduate and (ongoing) graduate education of the author, as well as from experiences and conversations stemming from the resulting immersion in the discipline. As a result, providing a list of references would be an impossible task, and not an accurate reflection of how the article was put together. *