Scientific laws:
- summarize a large collection of facts determined by experiment into a single statement,
- can usually be formulated mathematically as one or several statements or equation, or at least stated in a single sentence, so that it can be used to predict the outcome of an experiment, given the initial, boundary, and other physical conditions of the processes which take place,
- are strongly supported by empirical evidence - they are scientific knowledge that experiments have repeatedly verified (and never falsified). Their accuracy does not change when new theories are worked out, but rather the scope of application, since the equation (if any) representing the law does not change. As with other scientific knowledge, they do not have absolute certainty like mathematical theorems or identities, and it is always possible for a law to be overturned by future observations.
- are often quoted as a fundamental controlling influence rather than a description of observed facts. I.e. "the laws of motion require that"
Although the nature of a scientific law is a question in philosophy and although scientific laws describe nature mathematically, scientific laws are practical conclusions reached by the scientific method; they are intended to be neither laden with ontological commitments nor statements of logical absolution.
Fundamentally, all scientific laws follow from physics, laws which occur in other sciences ultimately follow from physical laws. Often, from mathematically fundamental viewpoints, universal constants emerge from scientific laws.
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Conservation laws
Conservation and symmetry
Most significant laws in science are conservation laws. These fundamental laws follow from homogeneity of space, time and phase, in other words symmetry.- Noether's theorem: Any quantity which has a continuous differentiable symmetry in the action has an associated conservation law.
- Conservation of mass was the first law of this type to be understood, since most macroscopic physical processes involving masses, for example collisions of massive particles or fluid flow, provide the apparent belief that mass is conserved. Mass conservation was observed to be true for all chemical reactions. In general this is only approximative, because with the advent of relativity and experiments in nuclear and particle physics: mass can be transformed into energy and vice versa, so mass is not always conserved, but part of the more general conservation of mass-energy.
- Conservation of energy, momentum and angular momentum for isolated systems can be found to be symmetries in time, translation, and rotation.
- Conservation of charge was also realized since charge has never been observed to be created or destroyed, and only found to move from place to place.
Continuity and transfer
Conservation laws can be expressed using the general continuity equation (for a conserved quantity) can be written in differential form as:Physics, conserved quantity Conserved quantity q Volume density ρ (of q) Flux J (of q) Equation Hydrodynamics, fluids m = mass (kg) ρ = volume mass density (kg m−3) ρ u, where
u = velocity field of fluid (m s−1)Electromagnetism, electric charge q = electric charge (C) ρ = volume electric charge density (C m−3) J = electric current density (A m−2) Thermodynamics, energy E = energy (J) u = volume energy density (J m−3) q = heat flux (W m−2) Quantum mechanics, probability P = (r, t) = ∫|Ψ|2d3r = probability distribution ρ = ρ(r, t) = |Ψ|2 = probability density function (m−3),
Ψ = wavefunction of quantum systemj = probability current/flux
Laws of classical mechanics
Principle of least action
Main article: Principle of least action
All of classical mechanics, including Newton's laws, Lagrange's equations, Hamilton's equations, etc., can be derived from this very simple principle:There are generalized momenta conjugate to these coordinates, p = (p1, p2, ..., pN), where:
The action is a functional rather than a function, since it depends on the Lagrangian, and the Lagrangian depends on the path q(t), so the action depends on the entire "shape" of the path for all times (in the time interval from t1 to t2). Between two instants of time, there are infinitely many paths, but one for which the action is stationary (to first order) is the true path. The stationary value for the entire continuum of Lagrangian values corresponding to some path, not just one value of the Lagrangian, is required (in other words its not as simple as "differentiating a function and setting it to zero, then solving the equations to find the points of maxima and minima etc", rather this idea is applied to the entire "shape" of the function, see calculus of variations for more details on this procedure).[2]
Notice L is not the total energy E of the system due to the difference, rather than the sum:
Laws of motion Principle of least action: The Euler-Lagrange equations are:
Hamilton's equations
Hamilton-Jacobi equation
Newton's laws Newton's laws of motion
They are low-limit solutions to relativity. Alternative formulations of Newtonian mechanics are Lagrangian and Hamiltonian mechanics.
The laws can be summarized by two equations (since the 1st is a special case of the 2nd, zero resultant acceleration):
For a dynamical system the two equations (effectively) combine into one:
- Corollaries in mechanics
- Corollaries in fluid mechanics
- Archimedes' principle
- Bernoulli's principle
- Poiseuille's law
- Stoke's law
- Navier–Stokes equations
- Faxén's law
Laws of gravitation and relativity
Modern laws
Postulates of special relativity are not "laws" in themselves, but assumptions of their nature in terms of relative motion.Often two are stated as "the laws of physics are the same in all inertial frames" and "the speed of light is constant". However the second is redundant, since the speed of light is predicted by Maxwell's equations. Essentially there is only one.
The said posulate leads to the Lorentz transformations – the transformation law between two frame of references moving relative to each other. For any 4-vector
The magnitudes of 4-vectors are invariants - not "conserved", but the same for all inertial frames (i.e. every observer in an inertial frame will agree on the same value), in particular if A is the four-momentum, the magnitude can derive the famous invariant equation for mass-energy and momentum conservation (see invariant mass):
General relativity is governed by the Einstein field equations, which describe the curvature of space-time due to mass-energy equivalent to the gravitational field. Solving the equation for the geometry of space warped due to the mass distribution gives the metric tensor. Using the geodesic equation, the motion of masses falling along the geodesics can be calculated.
In a relatively flat spacetime due to weak gravitational fields, gravitational analogues of Maxwell's equations can be found; the GEM equations, to describe an analogous gravitomagnetic field. They are well established by the theory, and experimental tests form ongoing research.[5]
Einstein field equations (EFE):
Geodesic equation:
GEM Equations If g the gravitational field and H the gravitomagnetic field, the solutions in these limits are:
In addition there is the gravitomagnetic Lorentz force:
Classical laws
Main articles: Kepler's laws of planetary motion and Newton's law of gravitation
Kepler's Laws, though originally discovered from planetary observations (also due to Tycho Brahe), are true for any central forces.[6]Newton's law of universal gravitation: For two point masses:
Gauss' law for gravity: An equivalent statement to Newton's law is:
Kepler's 1st Law: Planets move in an ellipse, with the star at a focus
Kepler's 2nd Law: equal areas are swept out in equal times (area bounded by two radial distances and the orbital circumference):
Kepler's 3rd Law: The square of the orbital time period T is proportional to the mean radius a:
Thermodynamics
Laws of thermodynamics First law of thermodynamics: The change in internal energy dU in a closed system is accounted for entirely by the heat δQ absorbed by the system and the work δW done by the system:
Zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with one another.
- As the temperature T of a system approaches absolute zero, the entropy S approaches a minimum value C: as T → 0, S → C.
For homogeneous systems the first and second law can be combined into the Fundamental thermodynamic relation:
Onsager reciprocal relations: sometimes called the Fourth Law of Thermodynamics
- ;
- .
- Ideal gas law, combines a number of separately developed gas laws;
- Boyle's law
- Charles's law
- Gay-Lussac's law
- Avogadro's law, into one)
- now improved by other equations of state
- Dalton's law (of partial pressures)
- Boltzmann equation
- Carnot's theorem
- Kopp's law
Electromagnetism
Maxwell's equations give the time-evolution of the electric and magnetic fields due to electric charge and current distributions. Given the fields, the Lorentz force law is the equation of motion for charges in the fields.Maxwell's equations Gauss's law for electricity
Lorentz force law:
Quantum electrodynamics (QED): Maxwell's equations are generally true and consistent with relativity - but they do not predict some observed quantum phenomena (e.g. light propagation as EM waves, rather than photons, see Maxwell's equations for details). They are modified in QED theory.
- Pre-Maxwell laws
- Other laws
Photonics
Classically, optics is based on a variational principle: light travels from one point in space to another in the shortest time.In geometric optics laws are based on approximations in Euclidean geometry (such as the paraxial approximation).
In physical optics, laws are based on physical properties of materials.
In actuality, optical properties of matter are significantly more complex and require quantum mechanics.
Laws of quantum mechanics
Quantum mechanics has its roots in postulates, these lead to results which are not usually called "laws", but have the same status, in that all of quantum mechanics follows from them.One postulate that a particle (or a system of many particles) is described by a wavefunction, and this satisfies a quantum wave equation: namely the Schrödinger equation (which can be written as a non-relativistic wave equation, or a relativistic wave equation). Solving this wave equation predicts the time-evolution of the system's behaviour, analogous to solving Newton's laws in classical mechanics.
Other postulates change the idea of physical observables; using quantum operators; some measurements can't be made at the same instant of time (Uncertainty principles), particles are fundamentally indistinguishable. Another postulate; the wavefunction collapse postulate, counters the usual idea of a measurement in science.
Quantum mechanics, Quantum field theory Schrödinger equation (general form): Describes the time dependence of a quantum mechanical system.
Wave-particle duality Planck–Einstein law: the energy of photons is proportional to the frequency of the light (the constant is Planck's constant, h).
Wave mechanics Schrödinger equation (original form):
Pauli exclusion principle: No two identical fermions can occupy the same quantum state (bosons can). Mathematically, if two particles are interchanged, fermionic wavefunctions are anti-symmetric, while bosonic wavefunctions are symmetric:
where ri is the position of particle i, and s is the spin of the particle. There is no way to keep track of particles physically, labels are only used mathematically to prevent confusion.
Radiation laws
Applying electromagnetism, thermodynamics, and quantum mechanics, to atoms and molecules, some laws of electromagnetic radiation and light are as follows.- Stefan-Boltzmann law
- Planck's law of black body radiation
- Wien's displacement law
- Radioactive decay law
Laws of chemistry
Main article: Chemical law
Chemical laws are those laws of nature relevant to chemistry. Historically, observations lead to many empirical laws, though now it is known that chemistry has its foundations in quantum mechanics.The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction. Modern physics shows that it is actually energy that is conserved, and that energy and mass are related; a concept which becomes important in nuclear chemistry. Conservation of energy leads to the important concepts of equilibrium, thermodynamics, and kinetics.
Additional laws of chemistry elaborate on the law of conservation of mass. Joseph Proust's law of definite composition says that pure chemicals are composed of elements in a definite formulation; we now know that the structural arrangement of these elements is also important.
Dalton's law of multiple proportions says that these chemicals will present themselves in proportions that are small whole numbers (i.e. 1:2 for Oxygen:Hydrogen ratio in water); although in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction.
More modern laws of chemistry define the relationship between energy and its transformations.
- In equilibrium, molecules exist in mixture defined by the transformations possible on the timescale of the equilibrium, and are in a ratio defined by the intrinsic energy of the molecules—the lower the intrinsic energy, the more abundant the molecule. Le Chatelier's principle states that the system opposes changes in conditions from equilibrium states, i.e. there is an opposition to change the state of an equilibrium reaction.
- Transforming one structure to another requires the input of energy to cross an energy barrier; this can come from the intrinsic energy of the molecules themselves, or from an external source which will generally accelerate transformations. The higher the energy barrier, the slower the transformation occurs.
- There is a hypothetical intermediate, or transition structure, that corresponds to the structure at the top of the energy barrier. The Hammond–Leffler postulate states that this structure looks most similar to the product or starting material which has intrinsic energy closest to that of the energy barrier. Stabilizing this hypothetical intermediate through chemical interaction is one way to achieve catalysis.
- All chemical processes are reversible (law of microscopic reversibility) although some processes have such an energy bias, they are essentially irreversible.
- The reaction rate has the mathematical parameter known as the rate constant. The Arrhenius equation gives the temperature and activation energy dependence of the rate constant, an empirical law.
- Gas laws
- Chemical transport
Geophysical laws
Biological laws
- Life is based on cells.[7]
- All life has genes.[7]
- All life occurs through biochemistry.[7]
- Mendelian inheritance
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