A
physical law or
scientific law
is a
scientific generalization based on
empirical observations of physical behavior (i.e. the
law of nature ). Laws of nature are observable.
Scientific laws are empirical, describing the observable laws.
Empirical laws are typically conclusions based on repeated
scientific experiments and
simple 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.
Laws of nature are distinct from
religious and
civil law,
and should not be confused with the concept of
natural law.Nor should 'physical law' be
confused with 'law of
physics' - the term
'physical law' usually covers laws in other sciences (e.g. biology)
as well.
Description
Several general properties of physical laws have been identified
(see Davies (1992) and Feynman (1965) as noted, although each of
the characterizations are not necessarily original to them.
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.
(Davies, 1992:82)
- Simple. They are typically expressed in terms of a single
mathematical equation. (Davies)
- Absolute. Nothing in the universe appears to affect them.
(Davies, 1992:82)
- 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). (Davies, 1992:83)
- Generally conservative of
quantity. (Feynman, 1965:59)
- Often expressions of existing homogeneities (symmetries) of space and
time. (Feynman)
- Typically theoretically reversible in time
(if non-quantum), although
time itself is irreversible.
(Feynman)
Often those who understand the mathematics and concepts well enough
to understand the essence of the physical laws also feel that they
possess an inherent intellectual
beauty. Many
scientists state that they use intuition as a guide in developing
hypotheses, since laws are reflection of symmetries and there is a
connection between beauty and
symmetry.
However, this has not always been the case; Newton himself
justified his belief in the asymmetry of the universe because his
laws appeared to imply it.
Physical laws are distinguished from scientific
theories by their simplicity. Scientific theories are
generally more complex than laws; they have many component parts,
and are more likely to be changed as the body of available
experimental data and analysis develops. This is because a physical
law is a summary observation of strictly empirical matters, whereas
a theory is a model that accounts for the observation, explains it,
relates it to other observations, and makes testable predictions
based upon it. Simply stated, while a
law notes
that something happens, a
theory explains
why and
how something happens.
Examples
Main article:
List of
laws in science.
See also:
scientific laws named after
people
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.
Laws as definitions
Some laws are correct purely by mathematical definition (e.g.,
Newton's Second law F =
\frac{dp}{dt}), or
uncertainty
principle, or
least action
principle, or
causality. They are
extremely useful, since they can be applied to
any
situation, and they cannot be violated.
Laws being consequences of mathematical symmetries
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
space-time). Laws
are constantly being checked experimentally to higher and higher
degrees of accuracy. The fact that they have never been seen
repeatably violated does not preclude testing them at increased
accuracy, which is one of the main goals of science. It is always
possible for them to be invalidated by repeatable, contradictory
experimental evidence; should any be seen. However, fundamental
changes to the laws are unlikely in the extreme, since this would
imply a change to experimental facts they were derived from in the
first place.
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.
Laws as approximations
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.
Physical laws derived from symmetry principles
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.
History and religious influence
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.
In all accounts of causality, the idea that there are underlying
regularities in nature dates to
prehistoric times, since even the recognition of
cause-and-effect relationships is an implicit recognition that
there are laws of nature.
Progress in identifying laws
per
se, though, was limited by the belief 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,
holy ghosts,
supernatural beings, etc. Early attempts
to formulate laws in material terms were made by ancient
philosophers, including
Aristotle, but
suffered both from lack of
definitions
and lack of accurate observations (experimenting), and hence had
various misconceptions - such as the assumption that observed
effects were due to intrinsic
properties of objects, e.g. "heaviness,"
"lightness," "wetness," etc - which were results lacking accurate
supporting experimental
data.
The precise formulation of what are today recognized as correct
statements of the laws of nature did not begin until the 17th
century in
Europe, with the beginning of
accurate experimentation and development of advanced form of
mathematics (see
scientific
method).
In essence, modern science aims at minimal speculation about
metaphysics. This results in spectacular
efficiency of science both in explaining how universe works and in
making our life better, longer and more interesting (via building
effective shelters, transportation, communication and entertainment
as well as helping to feed population, cure diseases, etc).
Significance, and renown of discoverers
Because of the understanding they permit regarding the nature of
our existence, and because of their above-mentioned power for
problem-solving and prediction, the discoveries or defining
(creation) of the new laws of nature are considered among the
greatest intellectual achievements of humanity. Due to their
subtlety, their discovery has typically required extraordinary
powers of observation and insight, and their discoverers are
typically considered among the best and brightest by others in
their fields, and, notably in the cases of
Newton and
Einstein by the general populace as
well.
Other fields
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.
See also
Notes
- E.g. an observable law relating to natural phenomena. - Oxford
Dictionary
References
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