Classical mechanics is the physics of forces, acting upon bodies. It is often referred to as "Newtonian mechanics" after Newton and his laws of motion. Classical mechanics is subdivided into statics (which deals with objects in equilibrium) and dynamics (which deals with objects in motion).

Classical mechanics produces very accurate results within the domain of everyday experience. It is superseded by relativistic mechanics for systems moving at large velocities near the speed of light, quantum mechanics for systems at small distance scales, and relativistic quantum field theory for systems with both properties. Nevertheless, classical mechanics is still very useful, because (i) it is much simpler and easier to apply than these other theories, and (ii) it has a very large range of approximate validity. Classical mechanics can be used to describe the motion of human-sized objects (such as tops and baseballs), many astronomical objects (such as planets and galaxies), and even certain microscopic objects (such as organic molecules.)

Although classical mechanics is roughly compatible with other "classical" theories such as classical electrodynamics and thermodynamics, there are inconsistencies that were discovered in the late 19th century that can only be resolved by more modern physics. In particular, classical nonrelativistic electrodynamics predicts that the speed of light is a constant relative to an aether medium, a prediction that is difficult to reconcile with classical mechanics and which led to the development of special relativity. When combined with classical thermodynamics, classical mechanics leads to the Gibbs paradox in which entropy is not a well-defined quantity and to the ultraviolet catastrophe in which a blackbody is predicted to emit infinite amounts of energy. The effort at resolving these problems led to the development of quantum mechanics.

Description of the theory

We will now introduce the basic concepts of classical mechanics. For simplicity, we only deal with a point particle, which is an object with negligible size. The motion of a point particle is characterized by a small number of parameters: its position, mass, and the forces applied on it. We will discuss each of these parameters in turn.

In reality, the kind of objects which classical mechanics can describe always have a non-zero size. True point particles, such as the electron, are properly described by quantum mechanics. Objects with non-zero size have more complicated behavior than our hypothetical point particles, because their internal configuration can change - for example, a baseball can spin while it is moving. However, we will be able to use our results for point particles to study such objects by treating them as composite objects, made up of a large number of interacting point particles. We can then show that such composite objects behave like point particles, provided they are small compared to the distance scales of the problem, which indicates that our use of point particles is self-consistent.

Position and its derivatives

The position of a point particle is defined with respect to an arbitrary fixed point in space, which is sometimes called the origin, O. It is defined as the vector r from O to the particle. In general, the point particle need not be stationary, so r is a function of t, the time elapsed since an arbitrary initial time. The velocity, or the rate of change of position with time, is defined as

.

The acceleration, or rate of change of velocity, is

.

The acceleration vector can be changed by changing its magnitude, changing its direction, or both. If the magnitude of v decreases, this is sometimes referred to as deceleration; but generally any change in the velocity, including deceleration, is simply referred to as acceleration.

Forces; Newton's Second Law

Newton's second law relates the mass and velocity of a particle to a vector quantity known as the force. Suppose m is the mass of a particle and F is the vector sum of all applied forces (i.e. the net applied force.) Then Newton's second law states that

.

The quantity mv is called the momentum. Typically, the mass m is constant in time, and Newton's law can be written in the simplified form

where a is the acceleration, as defined above. It is not always the case that m is independent of t. For example, the mass of a rocket decreases as its propellant is ejected. Under such circumstances, the above equation is incorrect and the full form of Newton's second law must be used.

Newton's second law is insufficient to describe the motion of a particle. In addition, we require a description of F, which is to be obtained by considering the particular physical entities with which our particle is interacting. For example, a typical resistive force may be modelled as a function of the velocity of the particle, say

with λ a positive constant. Once we have independent relations for each force acting on a particle, we can substitute it into Newton's second law to obtain an ordinary differential equation, which is called the equation of motion. Continuing our example, suppose that friction is the only force acting on the particle. Then the equation of motion is

.

This can be integrated to obtain

where v₀ is the initial velocity. This means that the velocity of this particle decays exponentially to zero as time progresses. This expression can be further integrated to obtain the position r of the particle as a function of time.

Important forces include the gravitational force and the Lorentz force for electromagnetism. In addition, Newton's third law can sometimes be used to deduce the forces acting on a particle: if we know that particle A exerts a force F on another particle B, it follows that B must exert an equal and opposite reaction force, -F, on A.

Energy

If a force F is applied to a particle that achieves a displacement δr, the work done by the force is the scalar quantity

.

Suppose the mass of the particle is constant, and δW_total is the total work done on the particle, which we obtain by summing the work done by each applied force. From Newton's second law, we can show that

δW_total = δT,

where T is called the kinetic energy. For a point particle, it is defined as

.

For extended objects composed of many particles, the kinetic energy of the composite body is the sum of the individual particles' kinetic energies.

A particular class of forces, known as conservative forces, can be expressed as the gradient of a scalar function, known as the potential energy and denoted V:

.

Suppose all the forces acting on a particle are conservative, and V is the total potential energy, obtained by summing the potential energies corresponding to each force. Then

.

This result is known as the conservation of energy, and states that the total energy, E = T + V, is constant in time. It is often useful, because most commonly encountered forces are conservative.

Further results

Newton's laws provide many important results for composite bodies.

There are two important alternative formulations of classical mechanics: Lagrangian mechanics and Hamiltonian mechanics. They are equivalent to Newtonian mechanics, but are often more useful for solving problems. These, and other modern formulations, usually bypass the concept of "force", instead referring to other physical quantities, such as energy, for describing mechanical systems.

History

The Greeks and Aristotle in particular were the first to propose that there are abstract principles governing nature.

One of the first scientists who suggested abstract laws was Galileo Galilei who also performed the famous experiment of dropping two canon balls from the tower of Pisa (The theory, and the practice showed that they both hit the ground at the same time).

Sir Isaac Newton was the first to propose the three laws of motion (the law of inertia, the second law mentioned above, and the law of action and reaction), and to prove that these laws govern both everyday objects and celestial objects.

Newton also developed the calculus which is necessary to perform the mathematical calculations involved in classical mechanics.

After Newton the field became more mathematical and more abstract.

Further Reading

• Feynman, R., Six Easy Pieces.
• ---, Six Not So Easy Pieces.
• ---, Lectures on Physics.
• Kleppner, D. and Kolenkow, R. J., An Introduction to Mechanics, McGraw-Hill (1973).

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Thermodynamics is the study of energy, its conversions between various forms such as heat, and the ability of energy to do work. It is closely related to statistical mechanics from which many themodynamic relationships can be derived.

It can be argued that thermodynamics was misnamed as it does not actually relate to rates of change as such and therefore would probably have been better called thermostatics as a field. Thermodynamics relates to whether certain chemical reactions are possible but not how quickly they occur.

The field covers a wide range of topics including, but not limited to: efficiency of heat engines and turbines, phase equilibria, PVT relationships. gas laws (both ideal and non ideal), energy balances, heats of reactions, and combustion reactions. It is governed by 4 basic laws (in brief):

The Laws of Thermodynamics

Alternative statements are given for each law. These statements are, for the most part, mathematically equivalent.

• Zeroth law: A fundamental concept within thermodynamics, however, it was not termed a law until after the first three laws were already widely in use, hence the zero numbering. There is some discussion about its status. Stated as:

  • If each of two systems is in thermal equilibrium with a third system, all must be in equilibrium with each other.

• 1st Law: Is stated as follows:

  • Energy can neither be created nor destroyed only changed.

  • The heat flowing into a system equals the sum of change in internal energy plus the work done by the system.

    • The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.

    • The sum of heat flowing into a system and work done by the system is zero.

• 2nd Law: A far reaching and powerful law, it can be stated many ways, the most popular of which is:

  • It is impossible to obtain a process such that the unique effect is the subtraction of a positive heat from a reservoir and the production of a positive work.

    • A system operating in contact with a thermal reservoir cannot produce positive work in its surroundings (Kelvin)

    • A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body (Clausius)

  • The entropy of a closed system never decreases (see Maxwell's demon)

• 3rd Law: This law explains why it is so hard to cool something to absolute zero:

  • All processes cease as temperature approaches zero.

  • As temperature goes to 0, the entropy of a system approaches a constant

The three original laws have been humorously summarised as: (1) you can't win; (2) you can't break even; (3) you can't get out of the game.

Basics

This is a brief summary and collection of the major concepts in thermodynamics. To learn more about each, just click on the corresponding links:

U stands for the internal energy, T stands for temperature, S stands for entropy, P stands for pressure, V stands for volume, ρ stands for density, F stands for Helmholtz free energy, H stands for enthalpy, G stands for Gibbs free energy, μ stands for chemical potential and N stands for particle number.

The rest of this discussion is about systems in equilibrium only. For nonequilibrium thermodynamics, see ...

Substances describable by temperature alone

Blackbody radiation is an example. The reason why this is the case is because photon number isn't conserved. The state is completely described by its temperature except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase. given the internal energy as a function of temperature, we can define F=U-TS.

Substances describable by temperature and pressure alone

Most "pure" nonmagnetic substances fall into this category. This state is completely described by its temperature and pressure, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase. Given U and V (or the density ρ) as a function of T and P, we can define the Helmholtz energy as before and the Gibbs energy as G=U-TS+PV and the enthalpy as H=U+PV.

Substances describable by temperature, pressure and chemical potential

If there are more than one kind of atom/molecule, a substance would fall into this category. This state is completely described by its temperature, pressure and chemical potentials, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.

Substances describable by temperature and magnetic field

If a substance is a ferromagnet or a superconductor, for example, it would fall into this category. It is completely described by its temperature and magnetic field, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.

Thermodynamic Systems

A thermodynamic system is that part of the universe that is under consideration. A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the surroundings. Often thermodynamic systems are characterized by the nature of this boundary as follows:

• Isolated systems are completely isolated from their surroundings. Neither heat nor matter can be exchanged between the system and the surroundings. An example of an isolated system would be an insulated container, such as an insulated gas cylinder. (In reality, a system can never be absolutely isolated from its environment, because there is always at least some slight coupling, even if only via minimal gravitational attraction).

• Closed systems are separated from the surroundings by an impermeable barrier. Heat can be exchanged between the system and the surroundings, but matter cannot. A greenhouse is an example of a closed system.

• Open systems can exchange both heat and matter with their surroundings. Portions of the boundary between the open system and its surroundings may be impermeable and/or adiabatic, however at least part of this boundary is subject to heat and mass exchange with the surroundings. The ocean would be an example of an open system.

Thermodynamic State

A key concept in thermodynamics is the state of a system. When a system is at equilibrium under a given set of conditions, it is said to be in a definite state. For a given thermodynamic state, many of the system's properties have a specific value corresponding to that state. The values of these properties are a function of the state of the system and are independent of the path by which the system arrived at that state. The number of properties that must be specified to describe the state of a given system is given by Gibbs phase rule. Since the state can be described by specifying a small number of properties, while the values of many properties are determined by the state of the system, it is possible to develop relationships between the various state properties. One of the main goals of Thermodynamics is to understand these relationships between the various state properties of a system. Equations of State are examples of some of these relationships.

Thermodynamics also touches upon the fields of:

• Fluid mechanics

• Calorimetry

• Thermal Analysis

• Thermochemistry also known as chemical thermodynamics

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Statistical mechanics is the application of statistics, which includes mathematical tools for dealing with large populations, to the field of Mechanics, which is concerned with the motion of particles or objects when subjected to a force. It provides a framework for relating the microscopic properties of individual atoms and molecules to the macroscopic or bulk properties of materials that can be observed in every day life, therefore explaining thermodynamics as a natural result of statistics and mechanics (classical and quantum). In particular, it can be used to calculate the thermodynamic properties of bulk materials from the spectroscopic data of individual molecules.

At the heart of statistical mechanics is the partition function:

where k is Boltzmann's constant, T is the temperature and E_i reflects each possible energetic state of the system. This is the version for systems which don't allow an exchange of matter.

The partition function provides a measure of the total number of energetic states available to the system at a given temperature.

It is often useful to consider the energy of a given molecule to be distributed among a number of modes. For example, translational energy refers to that portion of energy associated with the motion of the center of mass of the molecule. Configurational energy refers to that portion of energy associated with the various attractive and repulsive forces between molecules in a system. The other modes are all considered to be internal to each molecule. They include rotational, vibrational, electronic and nuclear modes.

A partition function can be defined for each mode. Simple expressions have been derived relating each of the various modes to various measurable molecular properties, such as the characteristic rotational or vibrational frequencies.

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Electromagnetism is a theory unified by James Clerk Maxwell to explain the interrelationship between electricity and magnetism. At the heart of this theory is the notion of an electromagnetic field.

A stationary electromagnetic field stays bound to its origin. Examples of stationary fields are: the magnetic field around a wire carrying current or the electric field between the plates of a capacitor.

A changing electromagnetic field propagates away from its origin in the form of a wave. These waves travel in vacuum at the speed of light and exist in a wide spectrum of wavelengths. Examples of the dynamic fields of electromagnetic radiation (in order of increasing frequency): radio waves, microwaves, light (infrared, visible light and ultraviolet), x-rays and gamma rays. In the field of particle physics this electromagnetic radiation is the manifestation of the electromagnetic interaction between charged particles.

The subfield of electromagnetism dealing specifically with the rapidly changing electric and magnetic fields which constitute light, is called electrodynamics.

The whole of electromagnetism is governed by Maxwell's equations, which are compatible with and served as a motivation for the theory of relativity.

Electromagnetic Method

A geophysical method in which the magnetic and or electric fields resulting from generated surface currents are measured. Measurements may be made in the frequency domain at a number of frequencies, or the time domain at several time intervals after a transient pulse. Natural field methods such as magnetotellurics (MT) use natural magnetic and electromagnetic field as the source.

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Light cone

The special theory of relativity (SR) is the physical theory published in 1905 by Albert Einstein that modified Newtonian physics to incorporate electromagnetism as represented by Maxwell's equations. The theory is called "special" because the theory applies only to the special case of measurements made when both the observer and that which is being observed are not affected by gravity. Ten years later, Einstein published the theory of General Relativity, or GR for short, which is the extension of special relativity to incorporate gravitation.

Motivation for the theory of special relativity

Before the formulation of special relativity, Hendrik Lorentz and others had already noted that electromagnetics differed from Newtonian physics in that observations by one of some phenomenon can differ from those of a person moving relative to that person at speeds nearing the speed of light. For example, one may observe no magnetic field, yet another observes a magnetic field in the same physical area. Lorentz suggested an aether theory in which objects and observers travelling with respect to a stationary aether underwent a physical shortening (Lorentz-Fitzgerald contraction) and a change in temporal rate (time dilation). This allowed the partial reconciliation of electromagnetics and Newtonian physics. When the velocities involved are much less than speed of light, the resulting laws simplify to Newton's laws. The theory, known as Lorentz Ether Theory (LET) was criticized (even by Lorentz himself) because of its ad hoc nature.

While Lorentz suggested the Lorentz transformation equations as a mathematical description that accurately described the results of measurements, Einstein's contribution was to derive these equations from a more fundamental theory. Einstein wanted to know what was invariant (the same) for all observers. His original title for his theory was (translated from German) "Theory of Invariants". It was Max Planck who suggested the term "relativity" to highlight the notion of transforming the laws of physics between observers moving relative to one another.

Special relativity is usually concerned with the behaviour of objects and observers which remain at rest or are moving at a constant velocity. In this case, the observer is said to be in an inertial frame of reference or simply inertial. Comparison of the position and time of events as recorded by different inertial observers can be done by using the Lorentz transformation equations. A common misstatement about relativity is that SR cannot be used to handle the case of objects and observers who are undergoing acceleration (non-inertial reference frames), but this is incorrect. For an example, see the relativistic rocket problem. SR can correctly predict the behaviour of accelerating bodies as long as the acceleration is not due to gravity, in which case general relativity must be used.

Invariance of the speed of light

SR postulated that the speed of light in vacuum is the same to all inertial observers, and said that every physical theory should be shaped or reshaped so that it is the same mathematically for every inertial observer. This postulate (which comes from Maxwell's equations for electromagnetics) together with the requirement, successfully reproduces the Lorentz transformation equations, and has several consequences that struck many people as bizarre, among which are:

• The time lapse between two events is not invariant from observer to another, but is dependent on the relative speeds of the observers' reference frames.

• The twin paradox is the "story" of a twin who flies off in a spaceship travelling near the speed of light. When he returns he discovers that his twin has aged much more rapidly than he has (or he aged more slowly).

• Two events that occur simultaneously in different places in one reference frame may occur one after the other in another reference frame (relativity of simultaneity).

• The dimensions (e.g. length) of an object as measured by an observer may differ from those by another.

• The mass of a particle increases as it's velocity increases. This led to the famous equation E = mc². See below.

Lack of an absolute reference frame

Another radical consequence is the rejection of the notion of an absolute, unique, frame of reference. Previously it had been suggested that the universe was filled with a substance known as "aether" (absolute space), against which speeds could be measured. Aether had some wonderful properties: it was sufficiently elastic that it could support electromagnetc waves, those waves could interact with matter, yet it offered no resistance to bodies passing through it. The results of various experiments, culminating in the famous Michelson-Morley experiment, suggested that either the Earth was always stationary, or the notion of an absolute frame of reference was mistaken and must be discarded.

Equivalence of mass and energy

Perhaps most far reaching, it also showed that energy and mass, previously considered separate, were equivalent, and related by the most famous expression from the theory:

E = mc²

where E is the energy of the body (at rest), m is the mass and c is the speed of light.

The most practical implication of this theory is that it puts an upper limit to the laws (see Law of nature) of Classical Mechanics and gravity formed by Isaac Newton at the speed of light. Nothing carrying mass can move faster than this speed. As an object's velocity approaches the speed of light, the amount of energy required to accelerate it approaches infinity, making it impossible to reach the speed of light. Only particles with no mass, such as photons, can actually achieve this speed (and in fact they must always travel at this speed in all frames of reference), which is approximately 300,000 kilometers per second or 186,300 miles per second.

The name "tachyon" has been used for hypothetical particles which would move faster than the speed of light, but to date evidence of the actual existence of tachyons has not been produced.

Simultaneity

Special relativity also holds that the concept of simultaneity is relative to the observer: A 'time-like interval' has end-points separated by a path along which it is possible for a hypothetical matter or light to travel. A 'space-like interval' has end-points separated by a path in space-time along which neither light nor any slower-than-light signal could travel. No information can pass between points separated by a space-like interval. Events along a space-like interval cannot influence one another by transmitting light or matter, and would appear simultaneous to an observer in the right frame of reference. To observers in different frames of reference, event A could seem to come before event B or vice-versa; this does not apply to events separated by time-like intervals.

Status of Special Relativity

Special relativity is now universally accepted by the physics community, unlike General Relativity which is still insufficiently confirmed by experiment to exclude certain alternative theories of gravitation. However, there are a handful of people opposed to relativity on various grounds and who have proposed various alternatives, mainly Aether theories. One alternative theory is doubly-special relativity, where a characteristic length is added to the list of invariant quantities.

The Geometry of Space-time in Special Relativity

SR uses a 'flat' 4 dimensional space, usually referred to as space-time. This space, however, is very similar to the standard 3 dimensional Euclidean space, and fortunately by that fact, very easy to work with.

Tests of postulates of special relativity

• Michelson-Morley experiment - ether drift

• Hamar experiment - obstruction of ether flow

• Trouton-Noble experiment - torque on a capacitor

• Kennedy-Thorndike experiment - time contraction

• Forms of the emission theory experiment

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Video: Visualization of Einstein's special relativity

The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way. Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the universe might be expanding in contrast to the static universe model advocated by Einstein. In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe.

In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. As such, at this point, the fabric of time and space had not yet come into existence. This perhaps echoed previous speculations about the cosmic egg origin of the universe.

Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law. Lemaître had already shown that this was expected, given the Cosmological Principle.

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory universe (originally suggested by Friedmann, but advocated by Einstein and Richard Tolman) and Fritz Zwicky's tired light hypothesis.

After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the universe seemed to expand. In this model, the universe is roughly the same at any point in time. The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it derisively as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor the latter. The discovery and confirmation of the cosmic microwave background radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory.

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE, the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise measurement of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.

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Timeline of the Big Bang

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang", and is considered the "birth" of our universe. Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.73 ± 0.12 billion years. The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the universe.

The earliest phases of the Big Bang are subject to much speculation. In the most common models, the universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. Approximately 10−35 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially. After inflation stopped, the universe consisted of a quark-gluon plasma, as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and anti-leptons—of the order of 1 part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.

The universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton-antiproton pairs (similarly for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion (one thousand million; 10⁹; SI prefix giga) Kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.

Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types of matter are known as cold dark matter, hot dark matter and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the universe.

Independent lines of evidence from Type Ia supernovae and the CMB imply the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 72% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10−15 seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics.

Big bang theory assumptions

The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic.

These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10−5. Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.

If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10⁻⁵ via observations of the CMB. The universe has been measured to be homogeneous on the largest scales at the 10% level.

FLRW metric

General relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves are specified using a coordinate chart or "grid" that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann-Lemaître-Robertson-Walker metric (FLRW metric). This metric contains a scale factor, which describes how the size of the universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system, the grid expands along with the universe, and objects that are moving only due to the expansion of the universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the universe.

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.

Horizons

An important feature of the Big Bang spacetime is the presence of horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. Our understanding of the universe back to very early times suggests that there was a past horizon, though in practice our view is limited by the opacity of the universe at early times. If the expansion of the universe continues to accelerate, there is a future horizon as well.

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The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements (see Big Bang nucleosynthesis). These are sometimes called the three pillars of the big bang theory. Many other lines of evidence now support the picture, notably various properties of the large-scale structure of the cosmos which are predicted to occur due to gravitational growth of structure in the standard Big Bang theory.

Hubble's law and the expansion of space

Observations of distant galaxies and quasars show that these objects are redshifted—the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed:

v=H₀D

where

v is the recessional velocity of the galaxy or other distant object

D is the comoving proper distance to the object and

H₀ is Hubble's constant, measured to be 70.1 ± 1.3 km/s/Mpc by the WMAP probe.

Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the Copernican Principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander Friedman in 1922 and Georges Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson and Walker.

The theory requires the relation v = HD to hold at all times, where D is the proper distance, v = dD / dt, and v, H, and D all vary as the universe expands (hence we write H₀ to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected.

That space is undergoing metric expansion is shown by direct observational evidence of the Cosmological Principle and the Copernican Principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogenous, supporting the Cosmological Principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican Principle, that the Earth is not in a central position, on a cosmological scale. Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

Cosmic microwave background radiation

WMAP image of the cosmic microwave background radiation

During the first few days of the universe, the universe was in full thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. As the universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering. Because of this repeated scattering, the early universe was opaque to light.

When the temperature fell to a few thousand Kelvin, electrons and nuclei began to combine to form atoms, a process known as recombination. Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering, 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the microwave region of the electromagnetic spectrum. The radiation is thought to be observable at every point in the universe, and comes from all directions with (almost) the same intensity.

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories. Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a Nobel Prize for their discovery.

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 10⁵. John C. Mather and George Smoot were awarded Nobels for their leadership in this work. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies.

In early 2003, the first results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general, it confirms too that a sea of cosmic neutrinos permeates the universe, a clear evidence that the first stars took more than a half-billion years to create a cosmic fog. Another satellite like it will be launched within the next few years, the Planck Surveyor, which will provide even more accurate measurements of the CMB anisotropies. Many other ground- and balloon-based experiments are also currently running.

The background radiation is exceptionally smooth, which presented a problem in that conventional expansion would mean that photons coming from opposite directions in the sky were coming from regions that had never been in contact with each other. The leading explanation for this far reaching equilibrium is that the universe had a brief period of rapid exponential expansion, called inflation. This would have the effect of driving apart regions that had been in equilibrium, so that all the observable universe was from the same equilibrated region.

Abundance of primordial elements

Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium and lithium-7 in the universe as ratios to the amount of ordinary hydrogen, H. All the abundances depend on a single parameter, the ratio of photons to baryons, which itself can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for ⁴He/H, about 10⁻³ for ²H/H, about 10⁻⁴ for ³He/H and about 10⁻⁹ for ⁷Li/H.

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for ⁴He, and a factor of two off for ⁷Li; in the latter two cases there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too.

Galactic evolution and distribution

Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures agree well with Big Bang simulations of the formation of structure in the universe and are helping to complete details of the theory.

Other lines of evidence

After some controversy, the age of universe as estimated from the Hubble expansion and the CMB is now in good agreement with (i.e., slightly larger than) the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of temperature-sensitive emission lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev-Zel'dovich effect in clusters of galaxies does not depend directly on redshift; this seems to be roughly true, but unfortunately the amplitude does depend on cluster properties which do change substantially over cosmic time, so a precise test is impossible.

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While very few researchers now doubt the Big Bang occurred, the scientific community was once divided between supporters of the Big Bang and those of alternative cosmological models. Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations (see the history section above). With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory.

The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many independent observations including Big Bang nucleosynthesis, the cosmic microwave background, large scale structure and Type Ia supernovae, and can hardly be doubted as important and real features of our universe.

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark energy and dark matter are considered the most secure, while inflation and baryogenesis remain speculative: they provide satisfying explanations for important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory. Explanations for such phenomena remain at the frontiers of inquiry in physics.

Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a universe of finite age, this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact. The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause these regions to have the same temperature.

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.

Flatness/oldness problem

  The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. From top to bottom: a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature.

The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann-Lemaître-Robertson-Walker metric.[41] The universe may have positive, negative or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density, positive if greater, and zero at the critical density, in which case space is said to be flat. The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat. Given that a natural timescale for departure from flatness might be the Planck time, 10−43 seconds, the fact that the universe has reached neither a Heat Death nor a Big Crunch after billions of years requires some explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the universe must have been within one part in 10¹⁴ of the critical density, or it would not exist as it does today.

A resolution to this problem is offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is believed that inflation drove the universe to a very nearly spatially flat state, with almost exactly the critical density.

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by cosmic inflation, which removes all point defects from the observable universe in the same way that it drives the geometry to flatness.

A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature hypothesis.

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called "baryogenesis" created the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium. All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.

Globular cluster age

In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7-billion-year age of the universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the universe.

Dark matter

A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best ΛCDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy

During the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.

The evidence for dark matter comes from its gravitational influence on other matter, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.

Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae have revealed that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2008, which combined data from the CMB and other sources, indicate that the universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less then 1% of neutrinos. The energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

In the ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general theory of relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.

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Simulated view of a black hole in front of the Milky Way. The hole has 10 solar masses and is viewed from a distance of 600 km.

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch. Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero—a Big Freeze. Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.

Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe expands and cools. Other explanations of dark energy—so-called phantom energy theories—suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.

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Video: History of Everything

All types of hadrons always have zero total color charge.

Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. An atom of anti-hydrogen, for instance, is composed of a negatively-charged antiproton being orbited by a positively-charged positron. If a particle/antiparticle pair comes in contact with each other, the two annihilate in a burst of electromagnetic radiation.

With antimatter, the entire possible energy of the matter could be harnessed, instead of the very small chemical energies or nuclear energies that can be extracted today. The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×10¹⁷ J of energy (by the equation E=mc²). In contrast, burning a kilogram of petrol produces 4.2×10⁷ J, and nuclear fusion of a kilogram of hydrogen would produce 2.6×10¹⁵ J.

Since the energy density is vastly higher than these other forms, the thrust to weight equation used in antimatter rocketry and spacecraft would be very different. In fact, the energy in a few grams of antimatter is enough to transport a small ship to the moon. It is hoped that antimatter could be used as fuel for interplanetary travel or possibly interstellar travel, but it is also feared that if humanity ever gets the capabilities to do so, there could be the construction of antimatter weapons.

Scientists succeeded in 1995 to produce anti-atoms of hydrogen, and also anti-deuteron nuclei, made out of an anti-proton plus an anti-neutron, but not yet more complex antimatter. Also, they exist for a very short time, they can not be stored. As far as we know there are no antimatter atoms in existence in this universe outside of our particle physics labs. This is a great mystery since one would expect matter and antimatter to have been generated in equal amounts after the Big Bang. The scarcity of antimatter has given us a stable universe, however, without which life could not have evolved.

The scarcity of antimatter means that it is not readily available to be used as fuel. Generating a single atom of antimatter is immensely difficult and requires particle accelerators and vast amounts of energy - millions of times more than is released after it is annihilated with ordinary matter, due to inefficiencies in the process. No more than a handful of antimatter atoms have ever been made. Therefore, unless substantial quantities from some as-yet unimagined natural source of antimatter are found, or ways to generate antimatter more efficiently are determined, antimatter will remain a curiosity rather than a viable propulsion system.

The symbol used to denote an antiparticle is the same symbol used to denote its normal matter counterpart, but with an overstrike. For example, a proton is denoted with a "p", and an antiproton is denoted by a "p" with a line over its top (P).

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Video: Particle Physics: Antimatter

Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. As a result, they obey Bose-Einstein statistics. The spin-statistics theorem states that bosons have integer spin.

All elementary particles are either bosons or fermions.

Guage bosons are elementary particles which act as the carriers of the fundamental forces.

Particles composed of a number of other particles (such as protons or nuclei) can be either fermions or bosons, depending on their total spin. Hence, many nuclei are in fact bosons. While fermions obey the Pauli exclusion principle: "no more than one fermion can occupy a single quantum state", there is no exclusion property for bosons, which are free to (and indeed, other things being equal, tend to) crowd into the same quantum state. This explains the spectrum of black-body radiation and the operation of lasers, the properties of liquid Helium-4 and superconductors and the possibility of bosons to form Bose-Einstein condensates, a particular state of matter.

Examples of bosons:

• photons, which mediate the electromagnetic force

• W and Z bosons, which mediate the weak nuclear force

• liquid Helium

• Cooper pair

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Standard Model of Elementary Particles

In particle physics, an elementary particle refers to a particle of which other, larger particles are composed. For example, atoms are made up of smaller particles known as electrons, protons, and neutrons. The proton and neutron, in turn, are composed of more elementary particles known as quarks. One of the outstanding problems of particle physics is to find the most elementary particles - or the so-called fundamental particles - which make up all the other particles found in Nature, and are not themselves made up of smaller particles.

The Standard Model of particle physics contains 12 species of elementary fermions ("matter particles") and 12 species of elementary bosons ("radiation particles"), plus their corresponding antiparticles. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, and it is possible that some or all of its "elementary" particles are actually composite particles. There might also be other elementary particles not described by the Standard Model, the most prominent being the graviton, the hypothetical particle that carryies the gravitational force.

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Godfrey Kneller's 1689 portrait of Isaac Newton (aged 46)

Newton explains, "Every object in the Universe attracts every other object with a force directed along the line of centers for the two objects that is proportional to the product of their masses and inversely proportional to the square of the separation between the two objects."

Newton eventually published his still famous law of universal gravitation in his Principia Mathematica as follows:

F = Gm₁m₂/r²

where:

• F = gravitational force between two objects

• m₁ = mass of first object

• m₂ = mass of second object

• r = distance between the objects

• G = universal constant of gravitation

Strictly speaking, this law applies only to point-like objects. If the objects have spatial extent, the true force has to be found by integrating the forces between the various points.

Vector Form

The above form is a simplified version. It is more properly expressed as vector equation. (All quantities in bold represent vector quantities in what follows.) The form below is vectorially complete:

F₁₂ = (Gm₁m₂(r₂-r₁))/(|r₂-r₁|³)

where:

• F₁₂ is the force on m₁ by m₂

• m₁ and m₂ are the masses

• r₁ and r₂ are the position vectors of their respective masses

• G is the gravitational constant

For the force on mass two, simply multiply F₁₂ by -1.

The primary difference between the two formulations is that the second form uses the difference in position to construct a vector that points from one mass to the other, and then divides that vector by its length to prevent it from changing the magnitude of the force.

Newton's Reservations

It's important to understand that while Newton was able to formulate his law of gravity in his monumental work, he was not comfortable with it because he never, in his words, "assigned the cause of this power." In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity. Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science.

He lamented the fact that 'philosophers have hitherto attempted the search of nature in vain' for the source of the gravitational force, as he was convinced 'by many reasons' that there were 'causes hitherto unknown' that were fundamental to all the 'phenomena of nature.' These fundamental phenomena are still under investigation and, though hypotheses abound, the definitive answer is yet to be found. While it is true that Einstein's hypotheses (see below) are successful in explaining the effects of gravitational forces more precisely than Newton's in certain cases, he too never 'assigned the cause of this power,' in his theories. It is said that in Einstein's equations, 'matter tells space how to curve, and space tells matter how to move,' but this new idea, completely foreign to the world of Newton, does not enable Einstein to assign the 'cause of this power' to curve space anymore than the Law of Universal Gravitation enabled Newton to assign its cause. In his own words:

I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto attempted the search of nature in vain.

If science is eventually able to discover the cause of the gravitational force, Newton's wish could eventually be fullfiled as well.

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The gravitational attraction of protons is approximately a factor 1036 weaker than the electromagnetic repulsion. This factor is independent of distance, because both forces are inversely proportional to the square of the distance. Therefore on an atomic scale mutual gravity is negligible. However, the main force beween common objects and the earth and between celestial bodies is gravity; this is due to the fact that they (at least one of the two) are electrically neutral to a high degree: even if in both bodies there were a surplus or deficit of only one electron for every 1018 protons and neutrons this would already be enough to cancel gravity (or in the case of a surplus in one and a deficit in the other: double the attraction).

The relative weakness of gravity can be demonstrated with a small magnet picking up pieces of iron. The small magnet is able to overwhelm the gravitational force of the entire earth.

Gravity is small unless at least one of the two bodies is large, but the small gravitational force exerted by bodies of ordinary size can fairly easily be detected through experiments such as the Cavendish torsion bar experiment.

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The convention establishing CERN was signed on 29 September 1954 by 12 countries in Western Europe. The acronym CERN originally stood, in French, for Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research), which was a provisional council for setting up the laboratory, established by 11 European governments in 1952. The acronym was retained for the new laboratory after the provisional council was dissolved, even though the name changed to the current Organisation Européenne pour la Recherche Nucléaire (European Organization for Nuclear Research) in 1954. According to Lew Kowarski, a former director of CERN, when the name was changed, the acronym could have become the awkward OERN, and Heisenberg said "But the acronym can still be CERN even if the name is [not]".

Soon after its establishment, the work at the laboratory went beyond the study of the atomic nucleus, into higher-energy physics, an activity which is mainly concerned with the study of interactions between particles. Therefore the laboratory operated by CERN is commonly referred to as the European laboratory for particle physics (Laboratoire européen pour la physique des particules) which better describes the current research being performed at CERN.

Scientific achievements

Several important achievements in particle physics have been made during experiments at CERN. These include, but are not limited to, the following.

• 1973: The discovery of neutral currents in the Gargamelle bubble chamber.
• 1983: The discovery of W and Z bosons in the UA1 and UA2 experiments.
• 1989: The determination of the number of neutrino families at the Large Electron Positron Collider (LEP) operating on the Z boson peak.
• 1995: The first creation of antihydrogen atoms in the PS210 experiment.
• 1999: The discovery of the direct CP-violation in the NA48 experiment.

The 1984 Nobel Prize in physics was awarded to Carlo Rubbia and Simon van der Meer for the developments that led to the discoveries of the W and Z bosons.

The 1992 Nobel Prize in physics was awarded to CERN staff researcher Georges Charpak "for his invention and development of particle detectors, in particular the multiwire proportional chamber."

Computer science

This NeXTcube used by British scientist Sir Tim Berners-Lee at CERN became the first Web server.

The World Wide Web began as a CERN project called ENQUIRE, initiated by Sir Tim Berners-Lee and Robert Cailliau in 1989. Berners-Lee and Cailliau were jointly honored by the ACM in 1995 for their contributions to the development of the World Wide Web.

Based on the concept of hypertext, the project was aimed at facilitating sharing information among researchers. The first website went on-line in 1991. On 30 April 1993, CERN announced that the World Wide Web would be free to anyone. A copy of the original first webpage, created by Berners-Lee, is still published on the World Wide Web Consortium website as a historical document.

This Cisco Systems router at CERN was probably one of the first IP routers deployed in Europe.

Prior to the Web's development, CERN had been a pioneer in the introduction of Internet technology, beginning in the early 1980s. A short history of this period can be found here.

More recently, CERN has become a centre for the development of Grid computing, hosting among others the Enabling Grids for E-sciencE (EGEE) and LHC Computing Grid projects. It also hosts the CERN Internet Exchange Point (CIXP), one of the two main Internet Exchange Points in Switzerland. CERN's computer network is connected to JANET (formerly UKERNA), the research and education network, JANET aids CERN to disperse large data over a network grid for closer analysis.

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Current complex

CERN operates a network of six accelerators and a decelerator. Each machine in the chain increases the energy of particle beams before delivering them to experiments or to the next more powerful accelerator. Currently active machines are:

• Two linear accelerators generate low energy particles for injection into the Proton Synchrotron. The 50 MeV Linac2 is for protons, and the 4.2 MeV/u Linac3 is for heavy ions.
• The Proton Synchrotron Booster increases the energy of particles generated by the proton linear accelerator before they are transferred to the other accelerators.
• The Low Energy Ion Ring (LEIR) accelerates the ions from the ion linear accelerator, before transferring them to the Proton Synchrotron (PS). This accelerator was commissioned in 2005, after having been reconfigured from the previous Low Energy Anti-proton Ring (LEAR).
• The 28 GeV Proton Synchrotron (PS), built in 1959 and still operating as a feeder to the more powerful SPS.
• The Super Proton Synchrotron (SPS), a circular accelerator with a diameter of 2 kilometres built in a tunnel, which started operation in 1976. It was designed to deliver an energy of 300 GeV and was gradually upgraded to 450 GeV. As well as having its own beamlines for fixed-target experiments, it has been operated as a proton-antiproton collider, and for accelerating high energy electrons and positrons which were injected into the Large Electron-Positron Collider (LEP). From 2008 onwards, it will inject protons and heavy ions into the Large Hadron Collider (LHC).
• The On-Line Isotope Mass Separator (ISOLDE), which is used to study unstable nuclei. Particles are initially accelerated in the PS Booster before entering ISOLDE. It was first commissioned in 1967 and was rebuilt with major upgrades in 1974 and 1992.
• The Antiproton Decelerator (AD), which reduces the velocity of antiprotons to about 10% the speed of light for research into antimatter.

The Large Hadron Collider

Most of the activities at CERN are currently directed towards building a new collider, the Large Hadron Collider (LHC) and the experiments for it. The LHC represents a large-scale, worldwide scientific cooperation project.

The LHC tunnel is located 100 metres underground, in the region between the Geneva airport and the nearby Jura mountains. It uses the 27 km circumference circular tunnel previously occupied by LEP which was closed down in November 2000. CERN's existing PS/SPS accelerator complexes will be used to pre-accelerate protons which will then be injected into the LHC.

Six experiments (CMS, ATLAS, LHCb, TOTEM, LHC-forward and ALICE) are currently being built, and will be running on the collider; each of them will study particle collisions under a different point of view, and with different technologies. Construction for these experiments required an extraordinary engineering effort. Just as an example, to lower the pieces for the CMS experiment into the underground cavern which will host it, a special crane will have to be rented from Belgium, which will be able to lift the almost 2000 tons for each piece. The first of the approximately 5,000 magnets necessary for construction was lowered down a special shaft at 13:00 GMT on 7 March 2005.

This accelerator will generate vast quantities of computer data, which CERN will stream to laboratories around the world for distributed processing (making use of a specialised GRID infrastructure, the LHC Computing Grid). In April 2005, a trial successfully streamed 600 MB per second to seven different sites across the world. If all the data generated by the LHC is to be analysed, then scientists must achieve 1,800 MB per second before 2008.

As of August 2008 the majority of the LHC ring is now cooled to the 1.9 K (−271.25 °C) operating temperature. The initial particle beams are due for injection in August 2008, the first attempt to circulate a beam through the entire LHC is scheduled for September 10, 2008, and the first high-energy collisions are planned to take place after the LHC is officially unveiled, on October 21, 2008.

Decommissioned accelerators

• The original linear accelerator (Linac1).
• The 600 MeV Synchro-Cyclotron (SC) which started operation in 1957 and was shut down in 1991.
• The Intersecting Storage Rings (ISR), an early collider built from 1966 to 1971 and operated until 1984.
• The Large Electron-Positron Collider (LEP), which operated from 1989 to 2000 and was the largest machine of its kind, housed in a 27 km-long circular tunnel which now houses the Large Hadron Collider.
• The Low Energy Antiproton Ring (LEAR), commissioned in 1982, which assembled the first pieces of true antimatter, in 1995, consisting of nine atoms of antihydrogen. It was closed in 1996, and superseded by the Antiproton Decelerator.

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

The smaller accelerators are located on the main Meyrin site (also known as the West Area), which was originally built in Switzerland alongside the French border, but has been extended to span the border since 1965. The French side is under Swiss jurisdiction and so there is no obvious border within the site, apart from a line of marker stones. There are six entrances to the Meyrin site:

• A, in Switzerland. Open for all CERN personnel at specific times.
• B, in Switzerland. Open for all CERN personnel at all times. Often referred to as the main entrance.
• C, in Switzerland. Open for all CERN personnel at specific times.
• D, in Switzerland. Open for goods reception at specific times.
• E, in France. Open for French-resident CERN personnel at specific times. Controlled by customs personnel. Named "Porte Charles de Gaulle" in recognition of his role in the creation of the CERN.
• Tunnel entrance, in France. Open for equipment transfer to and from CERN sites in France by personnel with a specific permit. This is the only permitted route for such transfers. Under the CERN treaty, no taxes are payable when such transfers are made. Controlled by customs personnel.

The SPS and LEP/LHC tunnels are located underground almost entirely outside the main site, and are mostly buried under French farmland and invisible from the surface. However they have surface sites at various points around them, either as the location of buildings associated with experiments or other facilities needed to operate the colliders such as cryogenic plants and access shafts. The experiments themselves are located at the same underground level as the tunnels at these sites.

Three of these experimental sites are in France, with ATLAS in Switzerland, although some of the ancillary cryogenic and access sites are in Switzerland. The largest of the experimental sites is the Prévessin site, also known as the North Area, which is the target station for non-collider experiments on the SPS accelerator. Other sites are the ones which were used for the UA1, UA2 and the LEP experiments (the latter which will be used for LHC experiments).

Outside of the LEP and LHC experiments, most are officially named and numbered after the site where they were located. For example, NA32 was an experiment looking at the production of charmed particles and located at the Prévessin (North Area) site while WA22 used the BEBC bubble chamber at the Meyrin (West Area) site to examine neutrino interactions. The UA1 and UA2 experiments were considered to be in the Underground Area, i.e. situated underground at sites on the SPS accelerator.

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

LHC experiments

ATLAS: A Toroidal LHC Apparatus
CMS: Compact Muon Solenoid
LHCb: LHC-beauty
ALICE: A Large Ion Collider Experiment
TOTEM: Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf: LHC-forward

LHC preaccelerators

p and Pb: Linear accelerators for protons (Linac 2) and Lead (Linac 3)
(not marked): Proton Synchrotron Booster
PS: Proton Synchrotron
SPS: Super Proton Synchrotron

Hadron Colliders

Intersecting Storage Rings: CERN, 1971–1984
Super Proton Synchrotron: CERN, 1981–1984
ISABELLE: BNL, cancelled in 1983
Tevatron: Fermilab, 1987–2009
Relativistic Heavy Ion Collider: BNL, operational since 2000
Superconducting Super Collider: Cancelled in 1993
Large Hadron Collider: CERN, 2008–
Very Large Hadron Collider: Theoretical

The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator complex, intended to collide opposing beams of protons (one of several types of hadrons) with very high kinetic energy. Its main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. It is theorized that the collider will confirm the existence of the Higgs boson, the observation of which could confirm the predictions and missing links in the Standard Model, and could explain how other elementary particles acquire properties such as mass.

The LHC was built by the European Organization for Nuclear Research (CERN), and lies underneath the Franco-Swiss border between the Jura Mountains and the Alps near Geneva, Switzerland. It is funded by and built in collaboration with over eight thousand physicists from over eighty-five countries as well as hundreds of universities and laboratories. The LHC is operational and is presently in the process of being prepared for collisions. The first beams were circulated through the collider on 10 September 2008, and the first high-energy collisions are expected to take place after 6-8 weeks.

Although there have been questions concerning the safety of the Large Hadron Collider in the media and even through the courts, the consensus in the scientific community is that there is no conceivable threat from the LHC particle collisions.

References

• Energising the quest for 'big theory'
• symmetry magazine LHC special issue August 2006, special issue December 2007
• BBC Horizon, The six billion dollar experiment
• New Yorker: Crash Course. The world’s largest particle accelerator.
• NYTimes: A Giant Takes On Physics’ Biggest Questions.
• Large Hadron Collider, Ingenia magazine
• Birth of a Giant, CNRS magazine
• Why a Large Hadron Collider? Seed Magazine interviews with physicists.

Links

• Official website
• Lyndon Evans and Philip Bryant (editors) (2008-08-14), "LHC Machine", Journal of Instrumentation 3(S08001), doi:10.1088/1748-0221/3/08/S08001, <http://www.iop.org/EJ/journal/-page=extra.lhc/jinst> . Full documentation for design and construction of the LHC and its six detectors (1600 p).
• Official Timeline of LHC Milestones
• TED talks (video) - Brian Cox: What really goes on at the Large Hadron Collider
• Overview of the LHC at CERN's public webpage
• ATLAS Experiment - Virtual Reality photography panoramas

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Climate change is any long-term significant change in the “average weather” that a given region experiences. Average weather may include average temperature, precipitation and wind patterns. It involves changes in the variability or average state of the atmosphere over durations ranging from decades to millions of years. These changes can be caused by dynamic processes on Earth, external forces including variations in sunlight intensity, and more recently by human activities.

In recent usage, especially in the context of environmental policy, the term "climate change" often refers to changes in modern climate .

Notes

• Emanuel, K. A. (2005) Increasing destructiveness of tropical cyclones over the past 30 years., Nature, 436; 686-688 ftp://texmex.mit.edu/pub/emanuel/PAPERS/NATURE03906.pdfPDF
• IPCC. (2007) Climate change 2007: the physical science basis (summary for policy makers), IPCC.
• Miller, C. and Edwards, P. N. (ed.)(2001) Changing the Atmosphere: Expert Knowledge and Environmental Governance, MIT Press
• Ruddiman, W. F. (2003) The anthropogenic greenhouse era began thousands of years ago, Climate Change 61 (3): 261-293
• Ruddiman, W. F. (2005) Plows, Plagues and Petroleum: How Humans Took Control of Climate, Princeton University Press
• Ruddiman, W. F., Vavrus, S. J. and Kutzbach, J. E. (2005) A test of the overdue-glaciation hypothesis, Quaternary Science Review, 24:11
• Schmidt, G. A., Shindel, D. T. and Harder, S. (2004) A note of the relationship between ice core methane concentrations and insolation GRL v31 L23206

References

1. ^ Royer DL, Berner RA, Park J (2007). "Climate sensitivity constrained by CO₂ concentrations over the past 420 million years". Nature 446 (7135): 530–2. doi:10.1038/nature05699. 
2. ^ Peter Bruckschen, Susanne Oesmanna and Ján Veizer (1999-09-30). "Isotope stratigraphy of the European Carboniferous: proxy signals for ocean chemistry, climate and tectonics". Chemical Geology 161 (1-3): 127. doi:10.1016/S0009-2541(99)00084-4. 
3. ^ "Volcanic Gases and Their Effects". U.S. Department of the Interior (2006-01-10). Retrieved on 2008-01-21.
4. ^ IPCC. (2007) Climate change 2007: the physical science basis (summary for policy makers), IPCC.
5. ^ See for example emissions trading, cap and share, personal carbon trading, UNFCCC
6. ^ ^abSteinfeld, H.; P. Gerber, T. Wassenaar, V. Castel, M. Rosales, C. de Haan (2006). Livestock’s long shadow. 
7. ^ "World CO2 levels at record high, scientists warn", The Guardian (2008-05-12). 
8. ^ | Future Atmosphere Changes in Greenhouse Gas and Aerosol Concentrations
9. ^ Amos, Jonathan (2006-09-04). "Deep ice tells long climate story", BBC. Retrieved on 2008-01-21. 
10. ^ Charlson, R. J.; S. E. SCHWARTZ, J. M. HALES, R. D. CESS, J. A. COAKLEY JR., J. E. HANSEN, and D. J. HOFMANN (1992-01-24). "Climate Forcing by Anthropogenic Aerosols". Science 255 (5043): 423–430. doi:10.1126/science.255.5043.423. PMID 17842894. Retrieved on 2008-01-28. 
11. ^ Leenhouts, B. 1998. Assessment of biomass burning in the conterminous United States. Conservation Ecology [online] 2(1): 1. [1]
12. ^ Ruddiman, William (2005-12-05). "Debate over the Early Anthropogenic Hypothesis". RealClimate. Retrieved on 2008-01-21.
13. ^ California Warming Attributed to Growth by Mandalit del Barco. Day to Day, National Public Radio. 30 Mar 2007.
14. ^ Ahlenius, Hugo (June 2007). "Climate feedbacks". United Nations Environment Programme/GRID-Arendal. Retrieved on 2008-01-21.
15. ^ Climate Change 2001: The Scientific Basis
16. ^ For additional discussion of feedbacks relevant to ongoing climate change, see http://www.grida.no/climate/ipcc_tar/wg1/260.htm
17. ^ Arctic Change Indicators
18. ^ Bering Sea Climate and Ecosystem Indicators
19. ^ How scientists study climate change: Some important research concepts used by scientists to study climate variations
20. ^ Baxter, JM & Buckley PJ and Wallace CJ, eds. (2008), Marine Climate Change Impacts Annual Report Card 2007–2008, Lowestoft: Marine Climate Change Impacts Partnership, <http://www.mccip.org.uk/arc/2007/default.htm> 
21. ^ Petit, J. R.; J. Jouzel, D. Raynaud, N. I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V. M. Kotlyakov, M. Legrand, V. Y. Lipenkov, C. Lorius, L. PÉpin, C. Ritz, E. Saltzman and M. Stievenard (1999-06-03). "Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica". Nature 399: 429–436. doi:10.1038/20859. Retrieved on 2008-01-22. 
22. ^ Langdon, PG; Barber KE, Lomas-Clarke SH (August 2004). "Reconstructing climate and environmental change in northern England through chironomid and pollen analyses: evidence from Talkin Tarn, Cumbria". Journal of Paleolimnology 32 (2): 197–213. doi:10.1023/B:JOPL.0000029433.85764.a5. Retrieved on 2008-01-28. 
23. ^ Birks, HH (March 2003). "The importance of plant macrofossils in the reconstruction of Lateglacial vegetation and climate: examples from Scotland, western Norway, and Minnesota, USA". Quarternary Science Reviews 22 (5-7): 453–473. doi:10.1016/S0277-3791(02)00248-2. Retrieved on 2008-01-28. 
24. ^ Coope, G.R.; Lemdahl, G.; Lowe, J.J.; Walkling, A. (1999-05-04). "Temperature gradients in northern Europe during the last glacial--Holocene transition(14--9 14 C kyr BP) interpreted from coleopteran assemblages". Journal of Quaternary Science 13 (5): 419–433. John Wiley & Sons, Ltd.. doi:10.1002/(SICI)1099-1417(1998090)13:5<419::AID-JQS410>3.0.CO;2-D. Retrieved on 2008-02-18. 
25. ^ Fox, R.; Warren, M.S., Asher, J., Brereton, T.M. and Roy (2007). "The state of Britain’s butterflies 2007". Butterfly Conservation and the Centre for Ecology and Hydrology, Wareham, Dorset. Retrieved on 2008-01-21.
26. ^ McGuirk, Rod; Bernard Lagan, Joseph Kerr (2007-01-30). "Australian Drought". Retrieved on 2008-01-21.
27. ^ Welbergen, J. A.; Klose, S. M., Markus, N. & Eby, P. (2008-02-22). "Climate change and the effects of temperature extremes on Australian flying-foxes". Proceedings of the Royal Society B 275 (1633): 419–425. Royal Society Publishing. doi:10.1098/rspb.2007.1385. Retrieved on 2008-01-21. 
28. ^ "Biodiversity and climate change". United Nations Environment Programme. UNEP-WCMC. Retrieved on 2008-01-28.

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Precession of the perihelion

The essential elements of the scientific method are traditionally described as follows:

• Observe: Observe or read about a phenomenon.

• Hypothesize: Wonder about your observations, and invent a hypothesis, a 'guess', which could explain the phenomenon or set of facts that you have observed.

• Test

  • Predict: Use the logical consequences of your hypothesis to predict observations of new phenomena or results of new measurements.

  • Experiment: Perform experiments to test the accuracy of these predictions.

• Conclude: Accept or refute hypothesis

  • Evaluate: Search for other possible explanations of the result until you can show that your guess was indeed the explanation, with confidence.

  • Formulate new hypothesis

These activities do not describe all that scientists do. This simplified method is useful for teaching, since it describes the way in which scientists often think of themselves as acting.

This idealised process is often misinterpreted as applying to scientists individually rather than to the scientific enterprise as a whole. Science is a social activity, and one scientist's theory or proposal cannot become accepted unless it has been published, peer reviewed, criticised, and finally accepted by the scientific community.

Observation

The scientific method begins with observation. Observation often demands careful measurement. It also requires the establishment of operational definitions of measurements and other relevant concepts. Definitions are not scientific hypotheses; they are not "falsifiable"; they are always true or tautological. Definitions condense a number of ideas into a single word or phrase. That being said, an observer's definition could differ significantly from commonly understood concepts of a term, and still be correct. Such a definition, however, would carry greater risk of being misunderstood. These definitions are operational in that they may differ with the context of a hypothesis, and they may be refined when the hypothesis is refined.

For example, the term "day" is useful in ordinary life and its meaning may vary with the context. (Do we mean a 24 hour period or do we mean the time between sunrise and sunset?) We don't have to define it precisely to make use of it. In many sciences it is precisely 86,400 atomic seconds. In studying the motion of the Earth, we may use two distinct operational definitions: a solar day is the time between two successive observations of the sun at the same position in the sky; a sidereal day is the time between two successive observations a specific star sky at the same position. The length of these two kinds of day differs by about four minutes.

Slight differences between operational definitions are often important, as they are needed to make experiments precise enough to distinguish subtle underlying phenomena. An example of this lies in choosing the appropriate segmentation in the statistical analysis of data. Distinctions in operational definitions can also reflect important conceptual differences: for example, mass and weight are regarded as quite different concepts in science, but the distinction is often ignored in everyday life.

Hypothesis

To explain the observation, scientists use whatever they can (their own creativity (currently not well understood), ideas from other fields, or even systematic guessing, or any other methods available) to come up with possible explanations for the phenomenon under study.

In the twentieth century Karl Popper introduced the idea that a hypothesis must be falsifiable; that is, it must be capable of being demonstrated wrong. Paul Feyerabend argued against this position, providing examples of falsified scientific theories that nevertheless had a vital role in the progress of scientific understanding.

Of course, it is impossible for the scientist to be impartial, considering all known evidence, and not merely evidence which supports the hypothesis under development. But by submitting their theories for peer review, scientists can at least make it more likely that the hypotheses formed will be relevant and useful, or at least get others to agree with it.

In the extremely rare cases where no better grounds for discriminating between rival hypotheses can be found, the bias scientists almost always follow is the principle of Occam's Razor; one chooses the simplest explanation for all the available evidence, in whatever sense "simple" is chosen to be defined (is it that which takes the fewest steps, or combines the smallest number of scientific facts, or takes the fewest words to express, or is the easiest to understand, or is the most predictable, or simply seems the most like common sense, or the average person's idea of common sense, to the scientist(s) judging the model?)

Prediction

A hypothesis must make specific predictions; these predictions can be tested with concrete measurements to support or refute the hypothesis. For instance, Albert Einstein's General Relativity makes a few specific predictions about the structure of space-time, such as the prediction that light bends in a strong gravitational field, and the amount of bending depends in a precise way on the strength of the gravitational field. Observations made during a 1919 solar eclipse supported the hypothesis (i.e., General Relativity) as against those of the other possible hypotheses which predicted different results. (Later experiments confirmed this even further.)

Deductive reasoning is the way in which predictions are used to test a hypothesis.

Verification

Probably the most important aspect of scientific reasoning is verification: The results of one's experiments must be verified. Verification is the process of determining whether the hypothesis is in accord with empirical evidence, and whether it will continue to be in accord with a more generally expanded body of evidence.

Ideally, the experiments performed should be fully described so that anyone can reproduce them, and many scientists should independently verify every hypothesis. Results which can be obtained from experiments performed by many are termed reproducible and are given much greater weight in evaluating hypotheses than non reproducible results.

Scientists must design their experiments carefully. For example, if the measurements are difficult to make, or subject to observer bias, one must be careful to avoid distorting the results by the experimenter's wishes. When experimenting on complex systems, one must be careful to isolate the effect being tested from other possible causes of the intended effect (this results in a controlled experiment). In testing a drug, for example, it is important to carefully test that the supposed effect of the drug is produced only by the drug itself, and not by the placebo effect or by random chance. Doctors do this with what is called a double-blind study: two groups of patients are compared, one of which receives the drug and one of which receives a placebo. No patient in either group knows whether or not they are getting the real drug; even the doctors or other personnel who interact with the patients don't know which patient is getting the drug under test and which is getting a fake drug (often sugar pills), so their knowledge can't influence the patients either.

Evaluation

Falsificationism argues that any hypothesis, no matter how respected or time-honoured, must be discarded once it is contradicted by new reliable evidence. This is of course an oversimplification, since individual scientists inevitably hold on to their pet theory long after contrary evidence has been found. This is not always a bad thing. Any theory can be made to correspond to the facts, simply by making a few adjustments—called "auxiliary hypothesis"—so as to bring it into correspondence with the accepted observations. The choice of when to reject one theory and accept another is inevitably up to the individual scientist, rather than some methodical law.

Hence all scientific knowledge is always in a state of flux, for at any time new evidence could be present that contradicts long-held hypotheses. A classic example is the explanation of light. Isaac Newton's particle paradigm was overturned by the wave theory of light, which explained diffraction, and which was held to be incontrovertible for many decades.The wave paradigm, in turn was refuted by the discovery of the photoelectric effect. The currently held theory of light holds that photons (the 'particles' of light) are both waves and particles; experiments have been performed which demonstrate that light has both particle and wave properties.

The experiments that reject a hypothesis should be performed by many different scientists to guard against bias, mistake, misunderstanding, and fraud. Scientific journals use a process of peer review, in which scientists submit their results to a panel of fellow scientists (who may or may not know the identity of the writer) for evaluation. Scientists are rightly suspicious of results that do not go through this process; for example, the cold fusion experiments of Fleischmann and Pons were never peer reviewed—they were announced directly to the press, before any other scientists had tried to reproduce the results or evaluate their efforts. They have not been reproduced elsewhere as yet; and the press announcement was regarded, by most nuclear physicists, as very likely wrong. Peer review may well have turned up problems and led to a closer examination of the experimental evidence Fleischmann, Pons, et al believed they had. Much embarrassment, and wasted effort worldwide, would have been avoided.

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