The
laws of science or
scientific laws are statements that describe, predict, and perhaps explain
why, a range of
phenomena behave as they appear to in
nature.
[1] The term "law" has diverse usage in many cases: approximate, accurate, broad or narrow theories, in all
natural scientific disciplines (
physics,
chemistry,
biology,
geology,
astronomy etc.). An analogous term for a scientific law is a
principle.
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"
Laws differ from
hypotheses and
postulates, which are proposed during the
scientific process
before and during validation by experiment and observation. These are
not laws since they have not been verified to the same degree and may
not be sufficiently general, although they may lead to the formulation
of laws. A law is a more solidified and formal statement, distilled from
repeated experiment.
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.
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:

where ρ is some quantity per unit volume,
J is the
flux of that quantity (change in quantity per unit time per unit area). Intuitively, the
divergence (denoted ∇•) of a
vector field
is a measure of flux diverging radially outwards from a point, so the
negative is the amount piling up at a point, hence the rate of change of
density in a region of space must be the amount of flux leaving or
collecting in some region (see main article for details). In the table
below, the fluxes, flows for various physical quantities in transport,
and their associated continuity equations, are collected for comparison.
-
More general equations are the
convection–diffusion equation and
Boltzmann transport equation, which have their roots in the continuity equation.
Laws of classical mechanics
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:

where

is the
action; the integral of the
Lagrangian

of the physical system between two times
t1 and
t2. The kinetic energy of the system is
T (a function of the rate of change of the
configuration of the system), and
potential energy is
V (a function of the configuration and its rate of change). The configuration of a system which has
N degrees of freedom is defined by
generalized coordinates q = (
q1,
q2, ...
qN).
There are
generalized momenta conjugate to these coordinates,
p = (
p1,
p2, ...,
pN), where:

The action and Lagrangian both contain the dynamics of the system for
all times. The term "path" simply refers to a curve traced out by the
system in terms of the
generalized coordinates in the
configuration space, i.e. the curve
q(
t), parameterized by time (see also
parametric equation for this concept).
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:

The following
[3][4]
general approaches to classical mechanics are summarized below in the
order of establishment. They are equivalent formulations, Newton's is
very commonly used due to simplicity, but Hamilton's and Lagrange's
equations are more general, and their range can extend into other
branches of physics with suitable modifications.
| Laws of motion |
Principle of least action:
 |
The Euler-Lagrange equations are:

Using the definition of generalized momentum, there is the symmetry:

|
Hamilton's equations


The Hamiltonian as a function of generalized coordinates and momenta has the general form:

|
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):

where p = momentum of body, Fij = force on body i by body j, Fij = force on body j by body i.
For a dynamical system the two equations (effectively) combine into one:

in which FE = resultant external force (due to any agent not part of system). Body i does not exert a force on itself. |
From the above, any equation of motion in classical mechanics can be derived.
- Corollaries in mechanics
- Corollaries in fluid mechanics
Equations describing fluid flow in various situations can be derived,
using the above classical equations of motion and often conservation of
mass, energy and momentum. Some elementary examples follow.
Laws of gravitation and relativity
Modern laws
- Special relativity
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

this replaces the
Galilean transformation
law from classical mechanics. The Lorentz transformations reduce to the
Galilean transformations for low velocities much less than the speed of
light
c.
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):

in which the (more famous)
mass-energy equivalence E =
mc2 is a special case.
- General relativity
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.
- Gravitomagnetism
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]
-
Classical laws
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:

For a non uniform mass distribution of local mass density ρ (r) of body of Volume V, this becomes:

|
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

where

is the eccentricity of the elliptic orbit, of semi-major axes a (planet aphelion) and semi-minor axes b (planet perihelion). This equation in itself is nothing physically fundamental; simply the polar equation of an ellipse
in vector form, in which the pole (origin of polar coordinate system)
is positioned at a focus of the ellipse, centred on the central star. |
Kepler's 2nd Law: equal areas are swept out in equal times (area bounded by two radial distances and the orbital circumference):

where L is the orbital angular momentum of the particle (i.e. planet) of mass m about the focus of orbit, |
Kepler's 3rd Law: The square of the orbital time period T is proportional to the mean radius a:

where M is the mass of the central body (i.e. star). |
Thermodynamics
-
- Ideal gas law, combines a number of separately developed gas laws;
- now improved by other equations of state
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.
-
These equations can be modified to include
magnetic monopoles,
and are consistent with our observations of monopoles either existing
or not existing; if they do not exist, the generalized equations reduce
to the ones above, if they do, the equations become fully symmetric in
electric and magnetic charges and currents. Indeed there is a duality
transformation where electric and magnetic charges can be "rotated into
one another", and still satisfy Maxwell's equations.
- Pre-Maxwell laws
These laws were found before the formulation of Maxwell's equations.
They are not fundamental, since they can be derived from Maxwell's
Equations. Coulomb's Law can be found from Gauss' Law (electrostatic
form) and the Biot-Savart Law can be deduced from Ampere's Law
(magnetostatic form). Lenz' Law and Faraday's Law can be incorporated
into the Maxwell-Faraday equation. Nonetheless they are still very
effective for simple calculations.
- 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.

The Hamiltonian (in quantum mechanics) H is a self-adjoint operator acting on the state space, (see Dirac notation) is the instantaneous quantum state vector at time t, position r, i is the unit imaginary number, ħ = h/2π is the reduced Planck's constant. |
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).

De Broglie wavelength: this laid the foundations of wave-particle duality, and was the key concept in the Schrödinger equation,

Heisenberg uncertainty principle: Uncertainty in position multiplied by uncertainty in momentum is at least half of the reduced Planck constant, similarly for time and energy;

The uncertainty principle can be generalized to any pair of observables - see main article. |
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.
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.
- Quantitative analysis
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.
- Reaction kinetics and Equilibria
- 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.
- Thermochemistry
- Gas laws
- Chemical transport
Geophysical laws
Biological laws
