Hear it pronouncedPhysics
Physics (Greek: physis – φύσις) is the science of matter[1] and its motion.[2] It is the science that seeks to understand very basic concepts such as force, energy, mass, and charge. More completely, it is the general analysis of nature, conducted in order to understand how the world around us and, more broadly, the universe, behaves.[3][4] Note that the term 'universe' is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. However, the term 'universe' may also be used in slightly different contextual senses, denoting concepts such as the cosmos, the world, and nature.
In one form or another, physics is one of the oldest academic disciplines, perhaps the oldest through its inclusion of astronomy.[5] Over the last two millennia, physics had been considered synonymous with philosophy, chemistry, and certain branches of mathematics and biology, but during the Scientific Revolution in the 16th century, it emerged to become a unique modern science in its own right.[6] However, in some subject areas such as in mathematical physics and quantum chemistry, the boundaries and the borderlines of physics remain difficult to distinguish.
Physics is both significant and influential, in part because advances in its understanding have often translated into new technologies, but also because new ideas in physics often resonate with the other sciences, mathematics and philosophy. For example, advances in the understanding of electromagnetism led directly to the development of new products which have dramatically transformed modern-day society (e.g., television, computers, and domestic appliances); advances in thermodynamics led to the development of motorized transport; and advances in mechanics inspired the development of the calculus, quantum chemistry, and the use of instruments like the electron microscope in microbiology.
Today, physics is both a broad and very deep subject that, in practical/fundamental terms, can be split into several subfields. It can also be divided into two conceptually different branches: Theoretical physics and experimental physics. The former deals with the inquiry and foundation of new theories while the latter deals with the experimental testing of these new, or existing, theories. Even though significant progress and important discoveries have been made in the field of physics during the last four centuries, many significant questions about nature and the universe still remain unanswered. In many areas of physics, it is still a continuing effort to try to gain a clearer understanding to the unknown and the outskirts of physics.
Contents |
[ Etymology
- Further information: Physis
The word 'physics' comes from the Greek words, φύσις (phúsis), meaning "nature", and φυσικός (phusikós), meaning "natural".
[ Introduction
- See also: List of basic physics topics and List of basic science topics
[ Scope and goals
Physics is the discipline devoted to understanding nature in a very general sense: the fundamental characteristic of physics is that it aims to gain knowledge, and hopefully understanding, of the general properties of the world around us. As an example, we can consider asking the following question on the nature of the Universe itself: how many dimensions do we need? Given that we know the Universe to consist of four dimensions (three space dimensions, and one time dimension), we can also ask why the universe picked those particular numbers: why not have four space dimensions? The fact that a choice was made out of a possibility of many means that questions like these fall under the scope of physics. Other general properties of nature include the existence of mass (as in Newton's laws of motion), charge (as in Maxwell's equations), and spin (in Quantum mechanics), amongst others.
However, whilst physics studies the general properties of nature, it will often also study the properties of certain objects within nature. Thus it is also physics whose job it is to describe what happens to, for example, planets whose motion is affected by nearby stars. Generally, the study of the specific objects in nature are shared between the three sciences: biology is roughly responsible for the living organisms, chemistry for the study of the chemical elements and molecules, and physics is given responsibility over all that remains (See the section Relation to mathematics and the other sciences for further information).
The fact that physics is delegated all objects besides those covered by biology and chemistry means that it is responsible for the study of a wide range objects and phenomena, from the smallest sub-atomic particles, to the largest galaxies. Included in this are the very most basic objects from which all other things are composed of, and therefore physics is sometimes said to be the "fundamental science".
Generalities aside, physics aims to describe the various phenomena that occur in nature in terms of simpler phenomena: that is, to find the mechanisms for why nature behaves the way it does. Thus, physics aims to both connect the things we see around us to a root cause, and then to try to connect these root causes together in the hope of finding an ultimate reason for why nature is as it is. For example, the ancient Chinese observed that certain rocks (lodestone) were attracted to one another by some invisible force. This effect was later called magnetism, and was first rigorously studied in the 17th century. A little earlier than the Chinese, the ancient Greeks knew of other objects (amber) that when rubbed with fur would cause a similar invisible attraction between the two. This was also first studied rigorously in the 17th century, and came to be called electricity. Thus, physics had come to understand two observations of nature in terms of some root cause (electricity and magnetism). However, further work in the 19th century revealed that these two forces were just two different aspects of one force – electromagnetism. This process of "unifying" forces continues today (see section Current research for more information).
[ The scientific method
Physics uses the scientific method to test the validity of a physical theory, using a methodical approach to compare the implications of the theory in question with the associated conclusions drawn from experiments and observations conducted to test it. Experiments and observations are to be collected and matched with the predictions and hypotheses made by a theory, thus aiding in the determination or the validity/invalidity of the theory.
Theories which are very well supported by data and which are regarded in observation as being almost certainly a naturally-occurring fact have been called scientific laws, or natural laws. Of course, all theories, including those called scientific laws, can always be replaced by more accurate, generalized statements if a disagreement of theory with observed data was to be found.[7]
[ Data collection and theory development
There are many approaches to studying physics, and many different kinds of activities in physics. There are two main types of activities in physics; the collection of data and the development of theories.
The data in some subfields of physics is amenable to experiment. For example, condensed matter physics and nuclear physics benefit from the ability to perform experiments. Sometimes experiments are done to explore nature, and in other cases experiments are performed to produce data to compare with the predictions of theories.
Some other fields in physics like astrophysics and geophysics are primarily observational sciences because most their data has to be collected passively instead of through experimentation. Nevertheless, observational programs in these fields uses many of the same tools and technology that are used in the experimental subfields of physics. The accumulated body of knowledge in some area of physics through experiment and observation is known as phenomenology.
Theoretical physics often uses quantitative approaches to develop the theories that attempt to explain the data. In this way, theoretical physics often relies heavily on tools from mathematics and computational technologies (particularly in the subfield known as computational physics). Theoretical physics often involves creating quantitative predictions of physical theories, and comparing these predictions quantitatively with data. Theoretical physics sometimes creates models of physical systems before data are available to test and validate these models.
These two main activities in physics, data collection and theory production and testing, draw on many different skills. This has lead to a lot of specialization in physics, and the introduction, development and use of tools from other fields. For example, theoretical physicists apply mathematics and numerical analysis and statistics and probability and computers and computer software in their work. Experimental physicists develop instruments and techniques for collecting data, drawing on engineering and computer technology and many other fields of technology. Often the tools from these other areas are not quite appropriate for the needs of physics, and need to be adapted or more advanced versions have to be produced.
The culture of physics research differs from the other sciences in the separation of theory from data collection through experiment and observation. Since the 20th century, most individual physicists have specialized in either theoretical physics or experimental physics. The great Italian physicist Enrico Fermi (1901–1954), who made fundamental contributions to both theory and experimentation in nuclear physics, was a notable exception. In contrast, almost all the successful theorists in biology and chemistry (e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, though this is changing as of late.
Although theory and experiment are usually performed by separate groups, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories. Likewise, ideas arising from theory often inspire new experiments. In the absence of experiment, theoretical research can go in the wrong direction; this is one of the criticisms that has been leveled against M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised.
[ Physics is quantitative
Physics is more quantitative than most other sciences. That is, many of the observations experimental results in physics are numerical measurements. Most of the theories in physics use mathematics to express their principles. Most of the predictions from these theories are numerical. This is because of the areas which physics has addressed are more amenable to quantitative approaches than other areas. Physical definitions, models and theories can often be expressed using mathematical relations, as early as 1638, when Galileo published the law of falling bodies in his Two New Sciences.
A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by comparing the predictions of its theories with data procured from observations and experimentation, whereas mathematics is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as mathematical physics.
[ Relation to mathematics and the other sciences
Physics relies on mathematics[8] to provide the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories can be succinctly expressed using mathematical relations.
Whenever analytic solutions are not feasible, numerical analysis and simulations can be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physics is an active area of research.
Beyond the known universe, the field of theoretical physics also deals with hypothetical issues[9], such as parallel universes, a multiverse, or whether the universe could have expanded as predominantly antimatter rather than matter.
In the Assayer (1622), Galileo noted that mathematics is the language in which Nature expresses laws, to be discovered by physicists. Physics is also intimately related to many other sciences,[10] as well as applied fields like engineering and medicine. The principles of physics find applications throughout the other natural sciences as they depend on the interactions of the fundamental constituents of the natural world. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to all material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other disciplines (biology, chemistry, geology, material science, engineering, medicine etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of collections of matter (such as gases and liquids formed of atoms and molecules) and the processes known as chemical reactions that result in the change of chemical substances. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (called in this case quantum chemistry), thermodynamics, and electromagnetism.
[ Philosophical implications
| Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things. Isaac Newton |
Physics in many ways stemmed from ancient Greek philosophy. From Thales' first attempt to characterize matter, to Democritus' deduction that matter ought to reduce to an invariant state, to the Ptolemaic astronomy of a crystalline firmament upon which the stars rested, our view of the universe seemed static. By the twentieth century, this picture became less certain, and now a static universe is only one possibility in an array of possible universes.
Aristotle's early observations in natural history, and natural philosophy usually did not involve much fact checking or detailed observation, which allowed errors to come to rest in our knowledge of the world. When closer investigation overturned this picture of the world, philosophers came to study other possible forms of reasoning. The use of a priori reasoning found a natural place in scientific method as well as the use of experiments and a posteriori reasoning came to be used in Bayesian inference[11]. By the 19th century physics was realized as a positive science and a distinct discipline separate from philosophy and the other sciences.
Study of the philosophical issues surrounding physics, the philosophy of physics can be encapsulated as empiricism, naturalism, and for some, realism[12]. The mathematical physicist Roger Penrose has been called a Platonist by Stephen Hawking[13], while Penrose continues to eschew quantum mechanics as a final theory about reality[14].
Ørsted (1811) noted that physicists readily make deductions about nature, based on their closer familiarity with experiments about nature,[15] whereas the mathematicians and philosophers must make do with fewer positive statements about nature.
There are certain statements, such as Newton's Third Law of Motion,[16] that can be generalized into the Principle of Equivalence. This principle is the logical basis for general relativity, whose solutions give metrics for spacetime. The success of general relativity influenced Einstein to eschew quantum theory, to which he made seminal contributions, and to eventually believe that all physical theory ought to be independent of observation.[17][18]
[ History
- See also: Famous physicists and Nobel Prize in physics
[ Ancient times
Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials in science have different properties, and so forth. Another mystery was the character of the universe, such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon. Several theories were proposed, the majority of which were disproved. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. On the other hand, the commonly accepted works of Ptolemy and Aristotle are not always found to match everyday observations. There were exceptions and there are anachronisms: for example, Indian philosophers and astronomers gave many correct descriptions in atomism and astronomy, and the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.
[ Middle Ages
The willingness to question previously held truths and search for new answers eventually resulted in a period of major scientific advancements, now known as the Scientific Revolution of the late 17th century. The precursors to the scientific revolution can be traced back to the important developments made in India and Persia, including the elliptical model of planetary orbits based on the heliocentric solar system of gravitation developed by Indian mathematician-astronomer Aryabhata; the basic ideas of atomic theory developed by Hindu and Jaina philosophers; the theory of light being equivalent to energy particles developed by the Indian Buddhist scholars Dignāga and Dharmakirti; the optical theory of light developed by Arab scientist Alhazen; the Astrolabe invented by the Persian Mohammad al-Fazari; and the significant flaws in the Ptolemaic system pointed out by Persian scientist Nasir al-Din al-Tusi. As the influence of the Islamic Caliphate expanded to Europe, the works of Aristotle preserved by the Arabs, and the works of the Indians and Persians, became known in Europe by the 12th and 13th centuries.
The Middle Ages saw the emergence of experimental physics with the development of an early scientific method emphasizing the role of experimentation and mathematics. Ibn al-Haytham (Alhazen, 965–1039) is considered a central figure in this shift in physics from a philosophical activity to an experimental one. In his Book of Optics (1021), he developed an early scientific method in order to prove the intromission theory of vision and discredit the emission theory of vision previously supported by Euclid and Ptolemy.[19][20][21] His most famous experiments involve his development and use of the camera obscura in order to test several hypotheses on light, such as light travelling in straight lines and whether different lights can mix in the air.[22] This experimental tradition in optics established by Ibn al-Haytham continued among his successors in both the Islamic world, with the likes of Qutb al-Din al-Shirazi, Kamāl al-Dīn al-Fārisī and Taqi al-Din, and in Europe, with the likes of Robert Grosseteste, Roger Bacon, Witelo, John Pecham, Theodoric of Freiberg, Johannes Kepler, Willebrord Snellius, René Descartes and Christiaan Huygens.
[ The Scientific Revolution
The Scientific Revolution is held by most historians (e.g., Howard Margolis) to have begun in 1543, when the first printed copy of Nicolaus Copernicus's De Revolutionibus (most of which had been written years prior but whose publication had been delayed) was brought from Nuremberg to the astronomer, who died soon after receiving the copy.
Further significant advances were made over the following century by Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise Pascal. During the early 17th century, Galileo pioneered the use of experimentation to validate physical theories, which is the key idea in modern scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia. In 1687, Newton published the Principia, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's Law of Gravitation, which describes the fundamental force of gravity. Both theories agreed well with experiment. The Principia also included several theories in fluid dynamics. Classical mechanics was re-formulated and extended by Leonhard Euler, French mathematician Joseph-Louis Comte de Lagrange, Irish mathematical physicist William Rowan Hamilton, and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.
After Newton defined classical mechanics, the next great field of inquiry within physics was the nature of electricity. Observations in the 17th and 18th century by scientists such as Robert Boyle, Stephen Gray, and Benjamin Franklin created a foundation for later work. These observations also established our basic understanding of electrical charge and electric current.
In 1821, the English physicist and chemist Michael Faraday integrated the study of magnetism with the study of electricity. This was done by demonstrating that a moving magnet induced an electric current in a conductor. Faraday also formulated a physical conception of electromagnetic fields. James Clerk Maxwell built upon this conception, in 1864, with an interlinked set of 20 equations that explained the interactions between electric and magnetic fields. These 20 equations were later reduced, using vector calculus, to a set of four equations, namely Maxwell's equations, by Oliver Heaviside.
In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe light. Confirmation of this observation was made with thee 1888 discovery of radio by Heinrich Hertz
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