PHYSICS

HISTORY OF PHYSICS

Physics (from the Ancient Greek φύσις physis meaning "nature") is the fundamental branch of science that developed out of the study of nature and philosophy known, until around the end of the 19th century, as "natural philosophy". Today, physics is ultimately defined as the study of matter, energy and the relationships between them. Physics is, in some senses, the oldest and most basic pure science; its discoveries find applications throughout the natural sciences, since matter and energy are the basic constituents of the natural world. The other sciences are generally more limited in their scope and may be considered branches that have split off from physics to become sciences in their own right. Physics today may be divided loosely into classical physics and modern physics.

Ancient history Elements of what became physics were drawn primarily from the fields of astronomy, optics, and mechanics, which were methodologically united through the study of geometry. These mathematical disciplines began in antiquity with the Babylonians and with Hellenistic writers such as Archimedes and Ptolemy. Ancient philosophy, meanwhile – including what was called "physics" – focused on explaining nature through ideas such as Aristotle's four types of "cause".

MAJOR FIELDS
Branches of physics Physics deals with the combination of matter and energy. It also deals with a wide variety of systems, about which theories have been developed that are used by physicists. In general, theories are experimentally tested numerous times before they are accepted as correct as a description of Nature (within a certain domain of validity). For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These theories continue to be areas of active research: for instance, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after the original formulation of classical mechanics by Isaac Newton (1642–1727). These "central theories" are important tools for research in more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.

Classical mechanics Classical mechanics is a model of the physics of forces acting upon bodies. It is often referred to as "Newtonian mechanics" after Isaac Newton and his laws of motion. It also includes classical approach as given by Hamiltonian and Lagrange methods.

Thermodynamics and statistical mechanics The first chapter of The Feynman Lectures on Physics is about the existence of atoms, which Feynman considered to be the most compact statement of physics, from which science could easily result even if all other knowledge was lost. By modeling matter as collections of hard spheres, it is possible to describe the kinetic theory of gases, upon which classical thermodynamics is based. Thermodynamics studies the effects of changes in temperature, pressure, and volume on physical systems on the macroscopic scale, and the transfer of energy as heat. Historically, thermodynamics developed out of the desire to increase the efficiency of early steam engines. The starting point for most thermodynamic considerations is the laws of thermodynamics, which postulate that energy can be exchanged between physical systems as heat or work. They also postulate the existence of a quantity named entropy, which can be defined for any system. In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of system and surroundings. A system is composed of particles, whose average motions define its properties, which in turn are related to one another through equations of state. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes.

Electromagnetism Electromagnetism is a branch of physics which involves the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force usually shows electromagnetic fields, such as electric fields, magnetic fields, and light. The electromagnetic force is one of the four fundamental interactions in nature. The other three fundamental interactions are the strong interaction, the weak interaction, and gravitation. The word electromagnetism is a compound form of two Greek terms, ἤλεκτρον,ēlektron, "amber", and μαγνῆτις λίθος magnētis lithos, which means "magnesian stone", a type of iron ore. The science of electromagnetic phenomena is defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as elements of one phenomenon. The electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life. Ordinary matter takes its form as a result of intermolecular forces between individual molecules in matter. Electrons are bound by electromagnetic wave mechanics into orbitals around atomic nuclei to form atoms, which are the building blocks of molecules. This governs the processes involved in chemistry, which arise from interactions between the electrons of neighboring atoms, which are in turn determined by the interaction between electromagnetic force and the momentum of the electrons. There are numerous mathematical descriptions of the electromagnetic field. In classical electrodynamics, electric fields are described as electric potential and electric current in Ohm's law, magnetic fields are associated with electromagnetic induction and magnetism, and Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents. The theoretical implications of electromagnetism, in particular the establishment of the speed of light based on properties of the "medium" of propagation (permeability and permittivity), led to the development of special relativity by Albert Einstein in 1905. Although electromagnetism is considered one of the four fundamental forces, at high energy the weak force and electromagnetism are unified. In the history of the universe, during the quark epoch, the electroweak force split into the electromagnetic and weak forces.

Relativity The special theory of relativity enjoys a relationship with electromagnetism and mechanics; that is, the principle of relativity and the principle of stationary action in mechanics can be used to derive Maxwell's equations, and vice versa. The theory of special relativity was proposed in 1905 by Albert Einstein in his article "On the Electrodynamics of Moving Bodies". The title of the article refers to the fact that special relativity resolves an inconsistency between Maxwell's equations and classical mechanics. The theory is based on two postulates: (1) that the mathematical forms of the laws of physics are invariant in all inertial systems; and (2) that the speed of light in a vacuum is constant and independent of the source or observer. Reconciling the two postulates requires a unification of space and time into the frame-dependent concept of spacetime. General relativity is the geometrical theory of gravitation published by Albert Einstein in 1915/16. It unifies special relativity, Newton's law of universal gravitation, and the insight that gravitation can be described by the curvature of space and time. In general relativity, the curvature of spacetime is produced by the energy of matter and radiation.

Quantum mechanics Quantum mechanics is the branch of physics treating atomic and subatomic systems and their interaction with radiation. It is based on the observation that all forms of energy are released in discrete units or bundles called "quanta". Remarkably, quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wave functions. The Schrödinger equation plays the role in quantum mechanics that Newton's laws and conservation of energy serve in classical mechanics—i.e., it predicts the future behavior of a dynamic system—and is a wave equation that is used to solve for wave functions. For example, the light, or electromagnetic radiation emitted or absorbed by an atom has only certain frequencies (or wavelengths), as can be seen from the line spectrum associated with the chemical element represented by that atom. The quantum theory shows that those frequencies correspond to definite energies of the light quanta, or photons, and result from the fact that the electrons of the atom can have only certain allowed energy values, or levels; when an electron changes from one allowed level to another, a quantum of energy is emitted or absorbed whose frequency is directly proportional to the energy difference between the two levels. The photoelectric effect further confirmed the quantization of light. In 1924, Louis de Broglie proposed that not only do light waves sometimes exhibit particle-like properties, but particles may also exhibit wave-like properties. Two different formulations of quantum mechanics were presented following de Broglie's suggestion. The wave mechanics of Erwin Schrödinger (1926) involves the use of a mathematical entity, the wave function, which is related to the probability of finding a particle at a given point in space. The matrix mechanics of Werner Heisenberg (1925) makes no mention of wave functions or similar concepts but was shown to be mathematically equivalent to Schrödinger's theory. A particularly important discovery of the quantum theory is the uncertainty principle, enunciated by Heisenberg in 1927, which places an absolute theoretical limit on the accuracy of certain measurements; as a result, the assumption by earlier scientists that the physical state of a system could be measured exactly and used to predict future states had to be abandoned. Quantum mechanics was combined with the theory of relativity in the formulation of Paul Dirac. Other developments include quantum statistics, quantum electrodynamics, concerned with interactions between charged particles and electromagnetic fields; and its generalization, quantum field theory.

Interdisciplinary fields
To the interdisciplinary fields, which define partially sciences of their own, belong e.g. the * astrophysics, the physics in the universe, including the properties and interactions of celestial bodies in astronomy. * biophysics, studying the physical interactions of biological processes. * chemical physics, the science of physical relations in chemistry. * econophysics, dealing with physical processes and their relations in the science of economy. * geophysics, the sciences of physical relations on our planet. * medical physics, the application of physics to prevention, diagnosis, and treatment. physical chemistry, dealing with physical processes and their relations in the science of physical chemistry.

Summary
The table below lists the core theories along with many of the concepts they employ. Theory | Major subtopics | Concepts | Classical mechanics | Newton's laws of motion, Lagrangian mechanics,Hamiltonian mechanics,kinematics, statics,dynamics, chaos theory, acoustics,fluid dynamics,continuum mechanics | Density, dimension, gravity, space, time, motion, length, position, velocity,acceleration, Galilean invariance, mass, momentum, impulse, force, energy,angular velocity, angular momentum, moment of inertia, torque, conservation law, harmonic oscillator, wave, work, power, Lagrangian, Hamiltonian, Tait–Bryan angles, Euler angles, pneumatic, hydraulic | ElectromagnetisHYPERLINK "https://en.wikipedia.org/wiki/Electromagnetism"m | Electrostatics,electrodynamics,electricity,maHYPERLINK "https://en.wikipedia.org/wiki/Magnetism"gnetism,magnetostatics,Maxwell's equations,optics | Capacitance, electric HYPERLINK "https://en.wikipedia.org/wiki/Electric_charge"charge, current, electrical conductivity, electric field,electric permittivity, electric potential, electrical resistance, electromagnetic field, electromagnetic induction, electromagnetic radiation, Gaussian surface,magnetic field, magnetic flux, magnetic monopole, magnetic permeability | Thermodynamicsand statistical mechanics | Heat engine, kinetic theory | Boltzmann's constant, conjugate variables, enthalpy, entropy, equation of state, equipartition theorem, thermodynamic free energy, heat, ideal gas law,internal energy, laws of thermodynamics, Maxwell relations, irreversible process, Ising model, mechanical action, partition function, pressure, reversible process, spontaneous process, state function, statistical ensemble,temperature, thermodynamic equilibrium, thermodynamic potential,thermodynamic processes, thermodynamic state, thermodynamic system,viscosity, volume, work, granular material | Quantum mechanics | Path integral formulation,scattering theory,Schrödinger equation, quantum field HYPERLINK "https://en.wikipedia.org/wiki/Quantum_field_theory"theory, quantum statistical mechanics | Adiabatic approximation, black-body radiation, correspondence principle, free HYPERLINK "https://en.wikipedia.org/wiki/Free_particle"particle, Hamiltonian, Hilbert space, identical particles, matrix mechanics,Planck's constant, observer effect, operators, quanta, quantization, quantum entanglement, quantum harmonic oscillator, quantum number, quantum tunneling, Schrödinger's cat, Dirac equation, spin, wave function, wave mechanics, wave–particle duality, zero-point energy, Pauli exclusion principle,Heisenberg uncertainty principle | Relativity | Special relativity,general relativity,Einstein field equations | Covariance, Einstein manifold, equivalence principle, four-momentum, four-vector, general principle of relativity, geodesic motion, gravity,gravitoelectromagnetism, inertial frame of reference, invariance, length contraction, Lorentzian manifold, Lorentz transformation, mass–energy equivalence, metric, Minkowski diagram, Minkowski space, principle of relativity, proper length, proper time, reference frame, rest energy, rest mass,relativity of simultaneity, spacetime, special principle of relativity, speed of light,stress–energy tensor, time HYPERLINK "https://en.wikipedia.org/wiki/Time_dilation"dilation, twin paradox, world line |

PHYSICISTS William Gilbert | 1544-1603
English | hypothesized that the Earth is a giant magnet | Galileo Galilei | 1564-1642
Italian | performed fundamental observations, experiments, and mathematical analyses in astronomy and physics; discovered mountains and craters on the moon, the phases of Venus, and the four largest satellites of Jupiter: Io, Europa, Callisto, and Ganymede | Willebrod Snell | 1580-1626
Dutch | discovered law of refraction (Snell's law) | Blaise Pascal | 1623-1662
French | discovered that pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and to the walls of its container (Pascal's principle) | Christiaan Huygens | 1629-1695
Dutch | proposed a simple geometrical wave theory of light, now known as ``Huygen's principle''; pioneered use of the pendulum in clocks | Robert Hooke | 1635-1703
English | discovered Hooke's law of elasticity | Sir Isaac Newton | 1643-1727
English | developed theories of gravitation and mechanics, and invented differential calculus | Daniel Bernoulli | 1700-1782
Swiss | developed the fundamental relationship of fluid flow now known as Bernoulli's principle | Benjamin Franklin | 1706-1790
American | the first American physicist; characterized two kinds of electric charge, which he named ``positive'' and ``negative'' | Leonard Euler | 1707-1783
Swiss | made fundamental contributions to fluid dynamics, lunar orbit theory (tides), and mechanics; also contributed prolifically to all areas of classical mathematics | Henry Cavendish | 1731-1810
British | discovered and studied hydrogen; first to measure Newton's gravitational constant; calculated mass and mean density of Earth | Charles Augustin de Coulomb | 1736-1806
French | experiments on elasticity, electricity, and magnetism; established experimentally nature of the force between two charges | Joseph-Louis Lagrange | 1736-1813
French | developed new methods of analytical mechanics | James Watt | 1736-1819
Scottish | invented the modern condensing steam engine and a centrifugal governor | Count Alessandro Volta | 1745-1827
Italian | pioneer in study of electricity; invented the first electric battery | Joseph Fourier | 1768-1830
French | established the differential equation governing heat diffusion and solved it by devising an infinite series of sines and cosines capable of approximating a wide variety of functions | Thomas Young | 1773-1829
British | studied light and color; known for his double-slit experiment that demonstrated the wave nature of light | Jean-Babtiste Biot | 1774-1862
French | studied polarization of light; co-discovered that intensity of magnetic field set up by a current flowing through a wire varies inversely with the distance from the wire | André Marie Ampère | 1775-1836
French | father of electrodynamics | Amadeo Avogadro | 1776-1856
Italian | developed hypothesis that all gases at same volume, pressure, and temperature contain same number of atoms | Johann Carl Friedrich Gauss | 1777-1855
German | formulated separate electrostatic and electrodynamical laws, including ``Gauss' law''; contributed to development of number theory, differential geometry, potential theory, theory of terrestrial magnetism, and methods of calculating planetary orbits | Hans Christian Oersted | 1777-1851
Danish | discovered that a current in a wire can produce magnetic effects | Sir David Brewster | 1781-1868
English | deduced ``Brewster's law'' giving the angle of incidence that produces reflected light which is completely polarized; invented the kaleidoscope and the stereoscope, and improved the spectroscope | Augustin-Jean Fresnel | 1788-1827
French | studied transverse nature of light waves | Georg Ohm | 1789-1854
German | discovered that current flow is proportional to potential difference and inversely proportional to resistance (Ohm's law) | Michael Faraday | 1791-1867
English | discovered electromagnetic induction and devised first electrical transformer | Felix Savart | 1791-1841
French | co-discovered that intensity of magnetic field set up by a current flowing through a wire varies inversely with the distance from the wire | Sadi Carnot | 1796-1832
French | founded the science of thermodynamics | Joseph Henry | 1797-1878
American | performed extensive fundamental studies of electromagnetic phenomena; devised first practical electric motor | Christian Doppler | 1803-1853
Austrian | experimented with sound waves; derived an expression for the apparent change in wavelength of a wave due to relative motion between the source and observer | Wilhelm E. Weber | 1804-1891
German | developed sensitive magnetometers; worked in electrodynamics and the electrical structure of matter | Sir William Hamilton | 1805-1865
Irish | developed the principle of least action and the Hamiltonian form of classical mechanics | James Prescott Joule | 1818-1889
British | discovered mechanical equivalent of heat | Armand-Hippolyte-Louis Fizeau | 1819-1896
French | made the first terrestrial measurement of the speed of light; invented one of the first interferometers; took the first pictures of the Sun on daguerreotypes; argued that the Doppler effect with respect to sound should also apply to any wave motion, particularly that of light | Jean-Bernard-Léon Foucault | 1819-1868
French | accurately measured speed of light; invented the gyroscope; demonstrated the Earth's rotation | Sir George Gabriel Stokes | 1819-1903
British | described the motion of viscous fluids by independently discovering the Navier-Stokes equations of fluid mechanics (or hydrodynamics); developed Stokes theorem by which certain surface integrals may be reduced to line integrals; discovered fluorescence | Hermann von Helmholtz | 1821-1894
German | developed first law of thermodynamics, a statement of conservation of energy | Rudolf Clausius | 1822-1888
German | developed second law of thermodynamics, a statement that the entropy of the Universe always increases | Lord Kelvin
(born William Thomson) | 1824-1907
British | proposed absolute temperature scale, of essence to development of thermodynamics | Gustav Kirchhoff | 1824-1887
German | developed three laws of spectral analysis and three rules of electric circuit analysis; also contributed to optics | Johann Balmer | 1825-1898
Swiss | developed empirical formula to describe hydrogen spectrum | Sir Joseph Wilson Swan | 1828-1914
British | developed a carbon-filament incandescent light; patented the carbon process for printing photographs in permanent pigment | James Clerk Maxwell | 1831-1879
Scottish | propounded the theory of electromagnetism; developed the kinetic theory of gases | Josef Stefan | 1835-1893
Austrian | studied blackbody radiation | Ernst Mach | 1838-1916
Austrian | studied conditions that occur when an object moves through a fluid at high speed (the ``Mach number'' gives the ratio of the speed of the object to the speed of sound in the fluid); proposed ``Mach's principle,'' which states that the inertia of an object is due to the interaction between the object and the rest of the universe | Josiah Gibbs | 1839-1903
American | developed chemical thermodynamics; introduced concepts of free energy and chemical potential | James Dewar | 1842-1923
British | liquified nitrogen and invented the Dewar flask, which is critical for low-temperature work | Osborne Reynolds | 1842-1912
British | contributed to the fields of hydraulics and hydrodynamics; developed mathematical framework for turbulence and introduced the ``Reynolds number,'' which provides a criterion for dynamic similarity and correct modeling in many fluid-flow experiments | Ludwig Boltzmann | 1844-1906
Austrian | developed statistical mechanics and applied it to kinetic theory of gases | Roland Eötvös | 1848-1919
Hungarian | demonstrated equivalence of gravitational and inertial mass | Oliver Heaviside | 1850-1925
English | contributed to the development of electromagnetism; introduced operational calculus and invented the modern notation for vector calculus; predicted existence of the Heaviside layer (a layer of the Earth's ionosphere) | George Francis FitzGerald | 1851-1901
Irish | hypothesized foreshortening of moving bodies (Lorentz-FitzGerald contraction) to explain the result of the Michelson-Morley experiment | John Henry Poynting | 1852-1914
British | demonstrated that the energy flow of electromagnetic waves could be calculated by an equation (now called Poynting's vector) | Henri Poincaré | 1854-1912
French | founded qualitative dynamics (the mathematical theory of dynamical systems); created topology; contributed to solution of the three-body problem; first described many properties of deterministic chaos; contributed to the development of special relativity | Janne Rydberg | 1854-1919
Swedish | analyzed the spectra of many elements; discovered many line series were described by a formula that depended on a universal constant (the Rydberg constant) | Edwin H. Hall | 1855-1938
American | discovered the ``Hall effect,'' which occurs when charge carriers moving through a material are deflected because of an applied magnetic field - the deflection results in a potential difference across the side of the material that is transverse to both the magnetic field and the current direction | Heinrich Hertz | 1857-1894
German | worked on electromagnetic phenomena; discovered radio waves and the photoelectric effect | Nikola Tesla | 1857-1943
Serbian-born American | created alternating current |

DEFINITION OF PHYSICS phys·ics ˈfiziks/ noun * the branch of science concerned with the nature and properties of matter and energy. The subject matter of physics, distinguished from that of chemistry and biology, includes mechanics, heat, light and other radiation, sound, electricity, magnetism, and the structure of atoms.

SCIENTIFIC SKILLS IN ALL SCIENCES

Basic Science Process Skills

Observing - using the senses to gather information about an object or event. Example: Describing a pencil as yellow.

Inferring - making an "educated guess" about an object or event based on previously gathered data or information. Example: Saying that the person who used a pencil made a lot of mistakes because the eraser was well worn.

Measuring - using both standard and nonstandard measures or estimates to describe the dimensions of an object or event. Example: Using a meter stick to measure the length of a table in centimeters.

Communicating - using words or graphic symbols to describe an action, object or event. Example: Describing the change in height of a plant over time in writing or through a graph.

Classifying - grouping or ordering objects or events into categories based on properties or criteria. Example: Placing all rocks having certain grain size or hardness into one group.

Predicting - stating the outcome of a future event based on a pattern of evidence. Example: Predicting the height of a plant in two weeks time based on a graph of its growth during the previous four weeks.

Integrated Science Process Skills

Controlling variables - being able to identify variables that can affect an experimental outcome, keeping most constant while manipulating only the independent variable. Example: Realizing through past experiences that amount of light and water need to be controlled when testing to see how the addition of organic matter affects the growth of beans.

Defining operationally - stating how to measure a variable in an experiment. Example: Stating that bean growth will be measured in centimeters per week.

Formulating hypotheses - stating the expected outcome of an experiment. Example: The greater the amount of organic matter added to the soil, the greater the bean growth.

Interpreting data - organizing data and drawing conclusions from it. Example: Recording data from the experiment on bean growth in a data table and forming a conclusion which relates trends in the data to variables.

Experimenting - being able to conduct an experiment, including asking an appropriate question, stating a hypothesis, identifying and controlling variables, operationally defining those variables, designing a "fair" experiment, conducting the experiment, and interpreting the results of the experiment. Example: The entire process of conducting the experiment on the affect of organic matter on the growth of bean plants.

Formulating models - creating a mental or physical model of a process or event. Examples: The model of how the processes of evaporation and condensation interrelate in the water cycle.

SCIENTIFIC METHOD

The scientific method is a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry is commonly based onempirical or measurable evidence subject to specific principles of reasoning. The Oxford English Dictionary defines the scientific method as "a method or procedure that has characterized natural science since the 17th century, consisting in systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses."

Process The overall process involves making conjectures (hypotheses), deriving predictions from them as logical consequences, and then carrying out experiments based on those predictions to determine whether the original conjecture was correct. There are difficulties in a formulaic statement of method, however. Though the scientific method is often presented as a fixed sequence of steps, they are better considered as general principles. Not all steps take place in every scientific inquiry (or to the same degree), and are not always in the same order. As noted by William Whewell (1794–1866), "invention, sagacity, [and] genius" are required at every step.

Formulation of a question The question can refer to the explanation of a specific observation, as in "Why is the sky blue?", but can also be open-ended, as in "How can I design a drug to cure this particular disease?" This stage frequently involves looking up and evaluating evidence from previous experiments, personal scientific observations or assertions, and/or the work of other scientists. If the answer is already known, a different question that builds on the previous evidence can be posed. When applying the scientific method to scientific research, determining a good question can be very difficult and affects the final outcome of the investigation.

Hypothesis A hypothesis is a conjecture, based on knowledge obtained while formulating the question, that may explain the observed behavior of a part of our universe. The hypothesis might be very specific, e.g., Einstein's equivalence principle or Francis Crick's "DNA makes RNA makes protein", or it might be broad, e.g., unknown species of life dwell in the unexplored depths of the oceans. A statistical hypothesis is a conjecture about some population. For example, the population might be people with a particular disease. The conjecture might be that a new drug will cure the disease in some of those people. Terms commonly associated with statistical hypotheses are null hypothesis and alternative hypothesis. A null hypothesis is the conjecture that the statistical hypothesis is false, e.g., that the new drug does nothing and that any cures are due to chance effects. Researchers normally want to show that the null hypothesis is false. The alternative hypothesis is the desired outcome, e.g., that the drug does better than chance. A final point: a scientific hypothesis must be falsifiable, meaning that one can identify a possible outcome of an experiment that conflicts with predictions deduced from the hypothesis; otherwise, it cannot be meaningfully tested.

Prediction This step involves determining the logical consequences of the hypothesis. One or more predictions are then selected for further testing. The more unlikely that a prediction would be correct simply by coincidence, then the more convincing it would be if the prediction were fulfilled; evidence is also stronger if the answer to the prediction is not already known, due to the effects of hindsight bias (see also postdiction). Ideally, the prediction must also distinguish the hypothesis from likely alternatives; if two hypotheses make the same prediction, observing the prediction to be correct is not evidence for either one over the other. (These statements about the relative strength of evidence can be mathematically derived using Bayes' Theorem).

Testing This is an investigation of whether the real world behaves as predicted by the hypothesis. Scientists (and other people) test hypotheses by conducting experiments. The purpose of an experiment is to determine whether observations of the real world agree with or conflict with the predictions derived from a hypothesis. If they agree, confidence in the hypothesis increases; otherwise, it decreases. Agreement does not assure that the hypothesis is true; future experiments may reveal problems. Karl Popper advised scientists to try to falsify hypotheses, i.e., to search for and test those experiments that seem most doubtful. Large numbers of successful confirmations are not convincing if they arise from experiments that avoid risk. Experiments should be designed to minimize possible errors, especially through the use of appropriate scientific controls. For example, tests of medical treatments are commonly run as double-blind tests. Test personnel, who might unwittingly reveal to test subjects which samples are the desired test drugs and which are placebos, are kept ignorant of which are which. Such hints can bias the responses of the test subjects. Furthermore, failure of an experiment does not necessarily mean the hypothesis is false. Experiments always depend on several hypotheses, e.g., that the test equipment is working properly, and a failure may be a failure of one of the auxiliary hypotheses. (See the Duhem-Quine thesis.) Experiments can be conducted in a college lab, on a kitchen table, at CERN's Large Hadron Collider, at the bottom of an ocean, on Mars (using one of the working rovers), and so on. Astronomers do experiments, searching for planets around distant stars. Finally, most individual experiments address highly specific topics for reasons of practicality. As a result, evidence about broader topics is usually accumulated gradually.

Analysis This involves determining what the results of the experiment show and deciding on the next actions to take. The predictions of the hypothesis are compared to those of the null hypothesis, to determine which is better able to explain the data. In cases where an experiment is repeated many times, a statistical analysis such as a chi-squared test may be required. If the evidence has falsified the hypothesis, a new hypothesis is required; if the experiment supports the hypothesis but the evidence is not strong enough for high confidence, other predictions from the hypothesis must be tested. Once a hypothesis is strongly supported by evidence, a new question can be asked to provide further insight on the same topic. Evidence from other scientists and experience are frequently incorporated at any stage in the process. Depending on the complexity of the experiment, many iterations may be required to gather sufficient evidence to answer a question with confidence, or to build up many answers to highly specific questions in order to answer a single broader question.

DNA example | The basic elements of the scientific method are illustrated by the following example from the discovery of the structure of DNA: * Question: Previous investigation of DNA had determined its chemical composition (the four nucleotides), the structure of each individual nucleotide, and other properties. It had been identified as the carrier of genetic information by the Avery–MacLeod–McCarty experiment in 1944, but the mechanism of how genetic information was stored in DNA was unclear. * Hypothesis: Linus Pauling, Francis Crick and James D. Watson hypothesized that DNA had a helical structure. * Prediction: If DNA had a helical structure, its X-ray diffraction pattern would be X-shaped. This prediction was determined using the mathematics of the helix transform, which had been derived by Cochran, Crick and Vand (and independently by Stokes). This prediction was a mathematical construct, completely independent from the biological problem at hand. * Experiment: Rosalind Franklin crystallized pure DNA and performed X-ray diffraction to produce photo 51. The results showed an X-shape. * Analysis: When Watson saw the detailed diffraction pattern, he immediately recognized it as a helix. He and Crick then produced their model, using this information along with the previously known information about DNA's composition and about molecular interactions such as hydrogen bonds. | The discovery became the starting point for many further studies involving the genetic material, such as the field of molecular genetics, and it was awarded the Nobel Prize in 1962. Each step of the example is examined in more detail later in the article.

Other components
The scientific method also includes other components required even when all the iterations of the steps above have been completed:
Replication
If an experiment cannot be repeated to produce the same results, this implies that the original results might have been in error. As a result, it is common for a single experiment to be performed multiple times, especially when there are uncontrolled variables or other indications of experimental error. For significant or surprising results, other scientists may also attempt to replicate the results for themselves, especially if those results would be important to their own work.

External review The process of peer review involves evaluation of the experiment by experts, who typically give their opinions anonymously. Some journals request that the experimenter provide lists of possible peer reviewers, especially if the field is highly specialized. Peer review does not certify correctness of the results, only that, in the opinion of the reviewer, the experiments themselves were sound (based on the description supplied by the experimenter). If the work passes peer review, which occasionally may require new experiments requested by the reviewers, it will be published in a peer-reviewed scientific journal. The specific journal that publishes the results indicates the perceived quality of the work.

Data recording and sharing Scientists typically are careful in recording their data, a requirement promoted by Ludwik Fleck (1896–1961) and others. Though not typically required, they might be requested to supply this data to other scientists who wish to replicate their original results (or parts of their original results), extending to the sharing of any experimental samples that may be difficult to obtain.