Physics is the branch of physical science that traditionally deals with matter, energy, force, and motion. These concepts can be applied to virtually any area of the physical sciences; therefore, physics is often considered to be the most fundamental branch of science. Indeed, according to reductionist thought, all other branches of science are specialized subdivisions of physics.
Classical physics generally encompasses all areas of physics that were well-understood by the end of the 19th century (i.e., before the events that led to the advent of quantum mechanics). It is applicable to problems on an "everyday" scale; that is, situations in which energies are large enough to permit one to neglect quantum effects, but small enough to neglect relativistic effects. Areas of study within classical physics include
- Mechanics, the study of forces acting on bodies
- Thermodynamics, the study of heat and entropy
- Electricity and magnetism, the study of electric and magnetic phenomena
Most kinds of wave behavior are also considered to lie within the classical domain. Therefore, the studies of sound (which may be considered a subset of mechanics) and the wavelike nature of light (a subset of electricity and magnetism) are classical pursuits.
Modern physics generally includes areas of study within physics that surfaced after 1900, the year in which Max Planck proposed a "quantum hypothesis" to explain the properties of light emitted by hot, dark objects. The two major aspects of modern physics are
- Relativity, a form of mechanics that is applicable in very high-energy situations
- Quantum mechanics, a probabilistic description of the discrete behavior of matter and energy at tiny length and energy scales
Modern physics was distinguished from classical physics mainly because these two ideas were so radically different from the classical viewpoint that they produced a paradigm shift in the way that physicists interpret the natural world.
Physics in Antiquity
Ancient Greek scholars, such as Democritus and Aristotle, thought of the material universe as being composed four different elements--earth, air, fire, and water. Motion and interactions between objects were explained by the concept of a "natural state" for each element. Bodies tended to move towards their natural state, and remained motionless when they attained it. For example, a rock fell to the ground after being tossed in the air because it was composed mostly of earth; as such, it moved toward its natural state at the center of the Earth as much as it was able. The Greeks also made advances in astronomy; Ptolemy developed a geocentric (Earth-centered) model of the solar system.
The ideas of the Greeks formed a paradigm that lasted for over 1000 years, making it one of the most successful scientific theories ever in terms of length of time spent as the dominant viewpoint.
The Scientific Revolution
As scientific methodology became further developed, the Aristotelian view came under scrutiny. By the 16th century, force and motion had become topics of interest among scientists of the day. Galileo Galilei performed a series of experiments involving rolling balls down inclined planes, and Johannes Kepler found mathematical relations governing the motion of the planets. Isaac Newton was able to explain the results of Galileo and Kepler through the use of three succinct laws of motion and an expression that accounted for gravitational force. Newton also made important contributions to the field of optics and suggested that light consisted of tiny corpuscles (particles).
As a result of the success of Newton's laws, the Aristotelian paradigm was rejected. It was replaced by a "clockwork universe" viewpoint, in which (theoretically) all future events could be predicted with arbitrary precision, given enough information about the present.
Further Development of Classical Physics
Newtonian mechanics was expanded by several other scientists, notably Pierre-Simon Laplace and Joseph Louis Lagrange. The motions of As Newton's laws became the dominant tool in physical science, they began to be applied to phenomena other than gravitational interactions. During the first half of the 19th century, several laws governing electricity and magnetism were discovered. In the mid-1860s, James Clerk Maxwell unified these individual laws into a single set of equations. Maxwell was able to derive a wave equation from his new set of equations, and found that the speed of the waves predicted by the equation was close to the measured speed of light. Thomas Young had demonstrated in 1801 that light had wavelike properties, but Maxwell's result provided the new insight that light waves were oscillating electric and magnetic fields.
Thermodynamics also came of age during roughly the same era. Advances by James Joule, Sadi Carnot, and Lord Kelvin, among many others, led to a better understanding of heat and entropy, and the formulation of the laws of thermodynamics. Later, attempts to relate the behavior of particles on the atomic and molecular level to the properties of large aggregates of such particles led to the development of statistical mechanics.
The Modern Era
By the close of the 19th century, the study of physics was widely thought to be essentially complete, with the exception of only a few "loose ends"--minor unsolved problems to be dealt with. As a solution to the so-called ultraviolet catastrophe problem, which involved light emission from hot, dark objects, Max Planck proposed in 1900 that the sources of light could be represented by small individual oscillators. Planck referred to the individual oscillators as quanta. At the time, Planck did not regard his finding as revolutionary; he thought that he was merely "fudging" the mathematics to arrive at a better solution. However, in 1905, Albert Einstein published a solution to the photoelectric effect problem: light could be thought of as being composed of very small, discrete packets (quanta), as opposed to the classical representation of light as a collection of waves. The discoveries of Planck and Einstein ushered in an entirely new era in physics. By the end of the 1930s, quantum mechanics--the name given to the new theory that applied to matter and energy on atomic scales--was well-established. Other physicists began applying the theory to more specific aspects of physics, resulting in quantum mechanical descriptions of electricity and magnetism, the weak and strong atomic forces, condensed matter physics, and many other areas.
In another 1905 publication, Einstein announced his discovery of what became known as the special theory of relativity, which revolutionized the way physicists thought about space and time. Einstein later extended the concept to non-inertial reference frames, which resulted in the general theory of relativity, an improved description of gravitational interactions, which was published in 1915.
The development of quantum mechanics and relativity induced a shift from the Newtonian "clockwork universe" paradigm. According to quantum mechanics, the fundamental laws of the universe are not deterministic, but probabilistic; one can't predict events with certainty, but can only find the probability of an event taking place. According to relativity, the universe has no preferred reference frame, and there is no such thing as absolute space and absolute time.
Like all other modern sciences, areas of interest in contemporary physics encompass an enormous variety of general and specialized subjects. Today's professional physicists can generally be categorized into two types: theoretical physicists, who are trained extensively in mathematics and work to develop or enhance physical theories; and experimental physicists, who obtain and analyze data in laboratory or laboratory-like settings. Theorists and experimentalists often collaborate in attempts to reconcile theoretical predictions with experimental results. Both types of physicists conduct research on a plethora of unsolved problems.
In terms of fundamental universal laws, there are several unsolved problems. Some are listed below.
- The Standard Model of particle physics is known to be incomplete; particle physicists are working to predict and discover unknown constituents of matter
- Evidence suggests that the matter in the universe that humans can currently detect directly is only a fraction of the matter that actually exists. Attempts to discover the nature of the as-yet-undetectable dark matter are underway.
- The universe is known to be expanding at an accelerated rate. Such acceleration requires energy, but the form and source of this dark energy is not well-understood.
- Quantum mechanics has not yet successfully been applied to the gravitational interaction. At huge energy scales, where both quantum mechanics and general relativity are required to describe the system, the known laws of physics break down. Attempts to discover a quantum theory of gravity have led to string theory and m-theory, but to date neither have permitted the design of a technologically feasible experiment to verify them.
- General relativity predicts the existence of gravitational waves, but none have been conclusively detected. Due to the weakness of the gravitational interaction, extremely sensitive detectors are required. The LIGO project is an example of current efforts to detect such waves.