A **physical law** or a **law of physics** is a statement "inferred from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present."^{[1]} Physical laws are typically conclusions based on repeated scientific experiments and observations over many years and which have become accepted universally within the scientific community. The production of a summary description of our environment in the form of such laws is a fundamental aim of science. These terms are not used the same way by all authors.

The distinction between natural law in the political-legal sense and law of nature or physical law in the scientific sense is a modern one, both concepts being equally derived from *physis*, the Greek word (translated into Latin as *natura*) for *nature*.^{[2]}

Several general properties of physical laws have been identified. Physical laws are:

- True, at least within their regime of validity. By definition, there have never been repeatable contradicting observations.
- Universal. They appear to apply everywhere in the universe.
^{[3]}^{:82} - Simple. They are typically expressed in terms of a single mathematical equation.
- Absolute. Nothing in the universe appears to affect them.
^{[3]}^{:82} - Stable. Unchanged since first discovered (although they may have been shown to be approximations of more accurate laws—see "Laws as approximations" below),
- Omnipotent. Everything in the universe apparently must comply with them (according to observations).
^{[3]}^{:83} - Generally conservative of quantity.
^{[4]}^{:59} - Often expressions of existing homogeneities (symmetries) of space and time.
^{[4]} - Typically theoretically reversible in time (if non-quantum), although time itself is irreversible.
^{[4]}

Some of the more famous laws of nature are found in Isaac Newton's theories of (now) classical mechanics, presented in his *Philosophiae Naturalis Principia Mathematica*, and in Albert Einstein's theory of relativity. Other examples of laws of nature include Boyle's law of gases, conservation laws, the four laws of thermodynamics, etc.

Many scientific laws are couched in mathematical terms (e.g. Newton's Second law *F* = ^{dp}⁄_{dt}, or the uncertainty principle, or the principle of least action, or causality). While these scientific laws explain what our senses perceive, they are still empirical, and so are not "mathematical" laws. (Mathematical laws can be proved purely by mathematics and not by scientific experiment.)

Other laws reflect mathematical symmetries found in Nature (say, Pauli exclusion principle reflects identity of electrons, conservation laws reflect homogeneity of space, time, Lorentz transformations reflect rotational symmetry of spacetime). Laws are constantly being checked experimentally to higher and higher degrees of precision. This is one of the main goals of science. Just because laws have never been observed to be violated does not preclude testing them at increased accuracy or in new kinds of conditions to confirm whether they continue to hold, or whether they break, and what can be discovered in the process. It is always possible for laws to be invalidated or proven to have limitations, by repeatable experimental evidence, should any be observed.

Well-established laws have indeed been invalidated in some special cases, but the new formulations created to explain the discrepancies can be said to generalize upon, rather than overthrow, the originals. That is, the invalidated laws have been found to be only close approximations (see below), to which other terms or factors must be added to cover previously unaccounted-for conditions, e.g. very large or very small scales of time or space, enormous speeds or masses, etc. Thus, rather than unchanging knowledge, physical laws are better viewed as a series of improving and more precise generalizations.

Many fundamental physical laws are mathematical consequences of various symmetries of space, time, or other aspects of nature. Specifically, Noether's theorem connects some conservation laws to certain symmetries. For example, conservation of energy is a consequence of the shift symmetry of time (no moment of time is different from any other), while conservation of momentum is a consequence of the symmetry (homogeneity) of space (no place in space is special, or different than any other). The indistinguishability of all particles of each fundamental type (say, electrons, or photons) results in the Dirac and Bose quantum statistics which in turn result in the Pauli exclusion principle for fermions and in Bose–Einstein condensation for bosons. The rotational symmetry between time and space coordinate axes (when one is taken as imaginary, another as real) results in Lorentz transformations which in turn result in special relativity theory. Symmetry between inertial and gravitational mass results in general relativity.

The inverse square law of interactions mediated by massless bosons is the mathematical consequence of the 3-dimensionality of space.

One strategy in the search for the most fundamental laws of nature is to search for the most general mathematical symmetry group that can be applied to the fundamental interactions.

Some laws are only approximations of other more general laws, and are good approximations with a restricted domain of applicability. For example, Newtonian dynamics (which is based on Galilean transformations) is the low-speed limit of special relativity (since the Galilean transformation is the low-speed approximation to the Lorentz transformation). Similarly, the Newtonian gravitation law is a low-mass approximation of general relativity, and Coulomb's law is an approximation to Quantum Electrodynamics at large distances (compared to the range of weak interactions). In such cases it is common to use the simpler, approximate versions of the laws, instead of the more accurate general laws.

According to a positivist view, when compared to pre-modern accounts of causality, laws of nature fill the role played by divine causality on the one hand, and accounts such as Plato's theory of forms on the other.

The observation that there are underlying regularities in nature dates from prehistoric times, since the recognition of cause-and-effect relationships is an implicit recognition that there are laws of nature. The recognition of such regularities as independent scientific laws *per se*, though, was limited by their entanglement in animism, and by the attribution of many effects that do not have readily obvious causes—such as meteorological, astronomical and biological phenomena—to the actions of various gods, spirits, supernatural beings, etc. Observation and speculation about nature were intimately bound up with metaphysics and morality.

In Europe, systematic theorizing about nature (*physis*) began with the early Greek philosophers and scientists and continued into the Hellenistic and Roman imperial periods, during which times the intellectual influence of Roman law increasingly became paramount.

The formula "law of nature" first appears as "a live metaphor" favored by Latin poets Lucretius, Virgil, Ovid, Manilius, in time gaining a firm theoretical presence in the prose treatises of Seneca and Pliny. Why this Roman origin? According to [historian and classicist Daryn] Lehoux's persuasive narrative,

^{[5]}the idea was made possible by the pivotal role of codified law and forensic argument in Roman life and culture.

For the Romans . . . the place par excellence where ethics, law, nature, religion and politics overlap is the law court. When we read Seneca's

Natural Questions, and watch again and again just how he applies standards of evidence, witness evaluation, argument and proof, we can recognize that we are reading one of the great Roman rhetoricians of the age, thoroughly immersed in forensic method. And not Seneca alone. Legal models of scientific judgment turn up all over the place, and for example prove equally integral to Ptolemy's approach to verification, where the mind is assigned the role of magistrate, the senses that of disclosure of evidence, and dialectical reason that of the law itself.^{[6]}

The precise formulation of what are now recognized as modern and valid statements of the laws of nature dates from the 17th century in Europe, with the beginning of accurate experimentation and development of advanced forms of mathematics. During this period, natural philosophers such as Isaac Newton were influenced by a religious view which held that God had instituted absolute, universal and immutable physical laws.^{[7]}^{[8]} In chapter 7 of *The World*, René Descartes described "nature" as matter itself, unchanging as created by God, thus changes in parts "are to be attributed to nature. The rules according to which these changes take place I call the 'laws of nature'."^{[9]} The modern scientific method which took shape at this time (with Francis Bacon and Galileo) aimed at total separation of science from theology, with minimal speculation about metaphysics and ethics. Natural law in the political sense, conceived as universal (i.e., divorced from sectarian religion and accidents of place), was also elaborated in this period (by Grotius, Spinoza, and Hobbes, to name a few).

Some mathematical theorems and axioms are referred to as laws because they provide logical foundation to empirical laws.

Examples of other observed phenomena sometimes described as laws include the Titius–Bode law of planetary positions, Zipf's law of linguistics, Moore's law of technological growth. Many of these laws fall within the scope of uncomfortable science. Other laws are pragmatic and observational, such as the law of unintended consequences. By analogy, principles in other fields of study are sometimes loosely referred to as "laws". These include Occam's razor as a principle of philosophy and the Pareto principle of economics.

**^**"Law of nature".*Oxford English Dictionary*(3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)**^**Some modern philosophers, e.g. Norman Swartz, use "physical law" to mean the laws of nature as they truly are and not as they are inferred by scientists. See Norman Swartz,*The Concept of Physical Law*(New York: Cambridge University Press), 1985. Second edition available online [1].- ^
^{a}^{b}^{c}Davies, Paul (2005).*The mind of God : the scientific basis for a rational world*(1st Simon & Schuster pbk. ed.). New York: Simon & Schuster. ISBN 978-0-671-79718-8. - ^
^{a}^{b}^{c}Feynman, Richard (1994).*The character of physical law*(Modern Library ed.). New York: Modern Library. ISBN 978-0-679-60127-2. **^**in Daryn Lehoux,*What Did the Romans Know? An Inquiry into Science and Worldmaking*(Chicago: University of Chicago Press, 2012), reviewed by David Sedley, "When Nature Got its Laws",*Times Literary Supplement*(12 October 2012).**^**Sedley, "When Nature Got Its Laws",*Times Literary Supplement*(12 October 2012).**^**Davies, Paul (2007-11-24). "Taking Science on Faith".*The New York Times*. ISSN 0362-4331. Retrieved 2016-10-07.**^**Harrison, Peter (8 May 2012). "Christianity and the rise of western science".*ABC*.**^**"Cosmological Revolution V: Descartes and Newton".*bertie.ccsu.edu*. Retrieved 2016-11-17.

- Francis Bacon (1620).
*Novum Organum*. - John Barrow (1991).
*Theories of Everything: The Quest for Ultimate Explanations*. (ISBN 0-449-90738-4) - Daryn Lehoux (2012).
*What Did the Romans Know? An Inquiry into Science and Worldmaking*. University of Chicago Press. (ISBN 9780226471143)

- Stanford Encyclopedia of Philosophy: "Laws of Nature" by John W. Carroll.
- Baaquie, Belal E. "Laws of Physics : A Primer". Core Curriculum, National University of Singapore.
- Francis, Erik Max. "The laws list".. Physics. Alcyone Systems
- Pazameta, Zoran. "The laws of nature". Committee for the scientific investigation of Claims of the Paranormal.
- The Internet Encyclopedia of Philosophy. "Laws of Nature" – By Norman Swartz
- "Laws of Nature",
*In Our Time*, BBC Radio 4 discussion with Mark Buchanan, Frank Close and Nancy Cartwright (Oct. 19, 2000)

The **Born rule** (also called the **Born law**, **Born's rule**, or **Born's law**), formulated by German physicist Max Born in 1926, is a physical law^{[citation needed]} of quantum mechanics giving the probability that a measurement on a quantum system will yield a given result. In its simplest form it states that the probability density of finding the particle at a given point is proportional to the square of the magnitude of the particle's wavefunction at that point. The Born rule is one of the key principles of quantum mechanics.

The Born rule states that if an observable corresponding to a self-adjoint operator with discrete spectrum is measured in a system with normalized wave function (*see* Bra–ket notation), then

- (In the case where the eigenspace of corresponding to is one-dimensional and spanned by the normalized eigenvector , is equal to , so the probability is equal to . Since the complex number is known as the
*probability amplitude*that the state vector assigns to the eigenvector , it is common to describe the Born rule as telling us that probability is equal to the amplitude-squared (really the amplitude times its own complex conjugate). Equivalently, the probability can be written as .)

In the case where the spectrum of is not wholly discrete, the spectral theorem proves the existence of a certain projection-valued measure , the spectral measure of . In this case,

Given a wave function for a single structureless particle in position space, this reduces to saying that the probability density function for a measurement of the position at time will be given by

- .

In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman-Smoluchowski ratchet is an apparent perpetual motion machine first analysed in 1912 as a thought experiment by Polish physicist Marian Smoluchowski and popularised by American Nobel laureate physicist Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962, during his Messenger Lectures series The Character of Physical Law in Cornell University in 1964 and in his text The Feynman Lectures on Physics as an illustration of the laws of thermodynamics. The simple machine, consisting of a tiny paddle wheel and a ratchet, appears to be an example of a Maxwell's demon, able to extract useful work from random fluctuations (heat) in a system at thermal equilibrium in violation of the second law of thermodynamics. Detailed analysis by Feynman and others showed why it cannot actually do this.

Conservation lawIn physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves over time. Exact conservation laws include conservation of energy, conservation of linear momentum, conservation of angular momentum, and conservation of electric charge. There are also many approximate conservation laws, which apply to such quantities as mass, parity, lepton number, baryon number, strangeness, hypercharge, etc. These quantities are conserved in certain classes of physics processes, but not in all.

A local conservation law is usually expressed mathematically as a continuity equation, a partial differential equation which gives a relation between the amount of the quantity and the "transport" of that quantity. It states that the amount of the conserved quantity at a point or within a volume can only change by the amount of the quantity which flows in or out of the volume.

From Noether's theorem, each conservation law is associated with a symmetry in the underlying physics.

Coulomb's law

**Coulomb's law**, or **Coulomb's inverse-square law**, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is conventionally called *electrostatic force* or *Coulomb force*. The quantity of electrostatic force between stationary charges is always described by Coulomb’s law. The law was first published in 1785 by French physicist Charles-Augustin de Coulomb, and was essential to the development of the theory of electromagnetism, maybe even its starting point, because it was now possible to discuss quantity of electric charge in a meaningful way.

In its scalar form, the law is:

where *k*_{e} is Coulomb's constant (*k*_{e} ≈ ×10^{9} N⋅m^{2}⋅C^{−2}), 9*q*_{1} and *q*_{2} are the signed magnitudes of the charges, and the scalar *r* is the distance between the charges. The force of the interaction between the charges is attractive if the charges have opposite signs (i.e., *F* is negative) and repulsive if like-signed (i.e., *F* is positive).

Being an inverse-square law, the law is analogous to Isaac Newton's inverse-square law of universal gravitation, but gravitational forces are always attractive, while electrostatic forces can be attractive or repulsive. Coulomb's law can be used to derive Gauss's law, and vice versa. The two laws are equivalent, expressing the same physical law in different ways. The law has been tested extensively, and observations have upheld the law on a scale from 10^{-16} m to 10^{8} m.

In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, and from this, the volume of the immersed object can be deduced (the volume of the immersed object will be exactly equal to the volume of the displaced fluid).

An object that sinks displaces an amount of fluid equal to the object's volume. Thus buoyancy is expressed through Archimedes' principle, which states that the weight of the object is reduced by its volume multiplied by the density of the fluid. If the weight of the object is less than this displaced quantity, the object floats; if more, it sinks. The amount of fluid displaced is directly related (via Archimedes' Principle) to its volume.

In the case of an object that sinks (is totally submerged), the volume of the object is displaced. In the case of an object that floats, the amount of fluid displaced will be equal in weight to the displacing object.

Archimedes' principle, a physical law of buoyancy, states that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body. The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface of an object partially submerged in a liquid. The weight of the displaced portion of the fluid is equivalent to the magnitude of the buoyant force. The buoyant force on a body floating in a liquid or gas is also equivalent in magnitude to the weight of the floating object and is opposite in direction; the object neither rises nor sinks. If the weight of an object is less than that of the displaced fluid, the object rises, as in the case of a block of wood that is released beneath the surface of water or a helium-filled balloon that is let loose in the air. An object heavier than the amount of the fluid it displaces, though it sinks when released, has an apparent weight loss equal to the weight of the fluid displaced. In fact, in some accurate weighing, a correction must be made in order to compensate for the buoyancy effect of the surrounding air. The buoyant force, which always opposes gravity, is nevertheless caused by gravity. Fluid pressure increases with depth because of the (gravitational) weight of the fluid above. This increasing pressure applies a force on a submerged object that increases with depth. The result is buoyancy.

Hagen–Poiseuille equationIn nonideal fluid dynamics, the Hagen–Poiseuille equation, also known as the Hagen–Poiseuille law, Poiseuille law or Poiseuille equation, is a physical law that gives the pressure drop in an incompressible and Newtonian fluid in laminar flow flowing through a long cylindrical pipe of constant cross section.

It can be successfully applied to air flow in lung alveoli, or the flow through a drinking straw or through a hypodermic needle. It was experimentally derived independently by Jean Léonard Marie Poiseuille in 1838 and Gotthilf Heinrich Ludwig Hagen, and published by Poiseuille in 1840–41 and 1846.The assumptions of the equation are that the fluid is incompressible and Newtonian; the flow is laminar through a pipe of constant circular cross-section that is substantially longer than its diameter; and there is no acceleration of fluid in the pipe. For velocities and pipe diameters above a threshold, actual fluid flow is not laminar but turbulent, leading to larger pressure drops than calculated by the Hagen–Poiseuille equation.

Inverse-square lawThe inverse-square law, in physics, is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space (see diagram).

Radar energy expands during both the signal transmission and also on the reflected return, so the inverse square for both paths means that the radar will receive energy according to the inverse fourth power of the range.

In order to prevent dilution of energy while propagating a signal, certain methods can be used such as a waveguide, which acts like a canal does for water, or how a gun barrel restricts hot gas expansion to one dimension in order to prevent loss of energy transfer to a bullet.

Joule effectJoule effect and Joule's law are any of several different physical effects discovered or characterized by English physicist James Prescott Joule. These physical effects are not the same, but all are frequently or occasionally referred to in literature as the "Joule effect" or "Joule law" These physical effects include:

"Joule's first law" (Joule heating), a physical law expressing the relationship between the heat generated and current flowing through a conductor.

Joule's second law states that the internal energy of an ideal gas is independent of its volume and pressure, depending only on its temperature.

Magnetostriction, a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field.

The Joule–Thomson effect (during Joule expansion), the temperature change of a gas (usually cooling) when it is allowed to expand freely.

The Gough–Joule effect or the Gow–Joule effect, which is the tendency of elastomers to contract if heated while they are under tension.

Le Chatelier's principleLe Châtelier's principle (UK: , US: ), also called Châtelier's principle or "The Equilibrium Law", can be used to predict the effect of a change in conditions on some chemical equilibria. The principle is named after Henry Louis Le Châtelier and sometimes Karl Ferdinand Braun who discovered it independently. It can be stated as:

When any system at equilibrium for a long period of time is subjected to change in concentration, temperature, volume, or pressure, (1) the system changes to a new equilibrium and (2) this change partly counteracts the applied change.It is common to treat the principle as a more general observation, such as

When a settled system is disturbed, it will adjust to diminish the change that has been made to it,or, “roughly stated”,

Any change in status quo prompts an opposing reaction in the responding systemor simply The System always kicks back.

The principle has a variety of names, depending upon the discipline using it (see homeostasis, a term commonly used in biology).

In chemistry, the principle is used to manipulate the outcomes of reversible reactions, often to increase the yield of reactions. In pharmacology, the binding of ligands to the receptor may shift the equilibrium according to Le Châtelier's principle, thereby explaining the diverse phenomena of receptor activation and desensitization. In economics, the principle has been generalized to help explain the price equilibrium of efficient economic systems.

Phenomena in apparent contradiction to Le Châtelier's principle can arise in systems of simultaneous equilibrium: see the article on the theory of response reactions.

MIT PressThe MIT Press is a university press affiliated with the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts (United States).

Mach's principleIn theoretical physics, particularly in discussions of gravitation theories, Mach's principle (or Mach's conjecture) is the name given by Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach. The idea is that the existence of absolute rotation (the distinction of local inertial frames vs. rotating reference frames) is determined by the large-scale distribution of matter, as exemplified by this anecdote:

You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?

Mach's principle says that this is not a coincidence—that there is a physical law that relates the motion of the distant stars to the local inertial frame. If you see all the stars whirling around you, Mach suggests that there is some physical law which would make it so you would feel a centrifugal force. There are a number of rival formulations of the principle. It is often stated in vague ways, like "mass out there influences inertia here". A very general statement of Mach's principle is "local physical laws are determined by the large-scale structure of the universe".This concept was a guiding factor in Einstein's development of the general theory of relativity. Einstein realized that the overall distribution of matter would determine the metric tensor, which tells you which frame is rotationally stationary. Frame-dragging and conservation of gravitational angular momentum makes this into a true statement in the general theory in certain solutions. But because the principle is so vague, many distinct statements can be (and have been) made that would qualify as a Mach principle, and some of these are false. The Gödel rotating universe is a solution of the field equations that is designed to disobey Mach's principle in the worst possible way. In this example, the distant stars seem to be revolving faster and faster as one moves further away. This example doesn't completely settle the question, because it has closed timelike curves.

Magic systemA magic system, which might also be referred to as a magical system, is a set of rules that regulate the magical effects that can be produced in a fictional setting. Magic systems are most elaborate in both video games and role-playing games, because of the necessity to balance the game itself. A common feature of magical systems is either abide by its own environmental or physical law of nature, or use a method of limiting both the quantity and quality of spells that can be cast by a magic user.

Newton's law of universal gravitation**Newton's law of universal gravitation** states that every particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This is a general physical law derived from empirical observations by what Isaac Newton called inductive reasoning. It is a part of classical mechanics and was formulated in Newton's work *Philosophiæ Naturalis Principia Mathematica* ("the *Principia*"), first published on 5 July 1687. When Newton presented Book 1 of the unpublished text in April 1686 to the Royal Society, Robert Hooke made a claim that Newton had obtained the inverse square law from him.

In today's language, the law states that every point mass attracts every other point mass by a force acting along the line intersecting the two points. The force is proportional to the product of the two masses, and inversely proportional to the square of the distance between them.

The equation for universal gravitation thus takes the form:

where *F* is the gravitational force acting between two objects, *m _{1}* and

The first test of Newton's theory of gravitation between masses in the laboratory was the Cavendish experiment conducted by the British scientist Henry Cavendish in 1798. It took place 111 years after the publication of Newton's *Principia* and approximately 71 years after his death.

Newton's law of gravitation resembles Coulomb's law of electrical forces, which is used to calculate the magnitude of the electrical force arising between two charged bodies. Both are inverse-square laws, where force is inversely proportional to the square of the distance between the bodies. Coulomb's law has the product of two charges in place of the product of the masses, and the electrostatic constant in place of the gravitational constant.

Newton's law has since been superseded by Albert Einstein's theory of general relativity, but it continues to be used as an excellent approximation of the effects of gravity in most applications. Relativity is required only when there is a need for extreme accuracy, or when dealing with very strong gravitational fields, such as those found near extremely massive and dense objects, or at very close distances (such as Mercury's orbit around the Sun).

Norman SwartzNorman Swartz (born 1939) is a professor emeritus (retired 1998) of philosophy, Simon Fraser University. He is the author or co-author of multiple books and multiple articles on the Internet Encyclopedia of Philosophy. He earned a B.A. in physics from Harvard University in 1961, an M.A. in history and philosophy of science from Indiana University in 1965 and a Ph.D. in history of philosophy of science in 1971 also from Indiana University. He uses the term physical law to mean the laws of nature as they truly are and not as they are inferred and described in the practice of science.

Oersted's lawIn electromagnetism, Ørsted's law, also spelled Oersted's law, is the physical law stating that an electric current creates a magnetic field.This was discovered on 21 April 1820 by Danish physicist Hans Christian Ørsted (1777–1851), when he noticed that the needle of a compass next to a wire carrying current turned so that the needle was perpendicular to the wire.

Ørsted investigated and found the physical law describing the magnetic field, now known as Ørsted's law.

Ørsted's discovery was the first connection found between electricity and magnetism, and the first of two laws that link the two; the other is Faraday's law of induction.

These two laws became part of the equations that govern electromagnetism, Maxwell's equations.

PhysicalismIn philosophy, physicalism is the metaphysical thesis that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. Physicalism is a form of ontological monism—a "one substance" view of the nature of reality as opposed to a "two-substance" (dualism) or "many-substance" (pluralism) view. Both the definition of "physical" and the meaning of physicalism have been debated.

Physicalism is closely related to materialism. Physicalism grew out of materialism with advancements of the physical sciences in explaining observed phenomena. The terms are often used interchangeably, although they are sometimes distinguished, for example on the basis of physics describing more than just matter (including energy and physical law). Common arguments against physicalism include both the philosophical zombie argument and the multiple observers argument, that the existence of a physical being may imply zero or more distinct conscious entities.

Supernatural horror filmSupernatural horror film is a film genre that combines aspects of horror film and supernatural film. Supernatural occurrences in such films often include ghosts and demons, and many supernatural horror films have elements of religion. Common themes in the genre are the afterlife, the Devil, and demonic possession. Not all supernatural horror films focus on religion and can have "more vivid and gruesome violence".

T-symmetry**T-symmetry** or **time reversal symmetry** is the theoretical symmetry of physical laws under the transformation of time reversal:

T-symmetry implies the conservation of entropy. Since the second law of thermodynamics means that entropy increases as time flows toward the future, the macroscopic universe does not in general show symmetry under time reversal. In other words, time is said to be non-symmetric, or asymmetric, except for special equilibrium states when the second law of thermodynamics predicts the time symmetry to hold. However, quantum noninvasive measurements are predicted to violate time symmetry even in equilibrium, contrary to their classical counterparts, although this has not yet been experimentally confirmed.

Time *asymmetries* are generally distinguished as among those...

The Character of Physical Law is a series of seven lectures by physicist Richard Feynman concerning the nature of the laws of physics. Feynman delivered the lectures in 1964 at Cornell University, as part of the Messenger Lectures series. The BBC recorded the lectures, and published a book under the same title the following year; Cornell published the BBC's recordings online in September 2015.

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