Physical law

From Free net encyclopedia

A physical law, scientific law, or a law of nature is a scientific generalization based on empirical observations of physical behavior. They are typically conclusions based on the confirmation of hypotheses through repeated scientific experiments over many years, and which have become accepted universally within the scientific community. However, there are no strict guidelines as to how or when a scientific hypothesis becomes a scientific law.

The production of a summary description of nature in the form of such laws is the fundamental aim of science. Laws of nature are distinct from the law, either religious or civil, and should not be confused with the concept of natural law.

1 Description
2 Examples
3 Laws as approximations
4 Necessity, origin, and existence
5 History, and religious influence
6 Significance, and renown of discoverers
7 Other fields
8 See also
9 References
10 External links

[edit]

Description

Several general properties of physical laws have been identified (see Davies (1992) and Feynman (1965) as noted, although each of the characterizations is not necessarily original to them). Physical laws are:

true (a.k.a. valid). By definition, there have never been repeatable contradicting observations.
universal. They appear to apply everywhere in the universe. (Davies)
simple. They are typically expressed in terms of a single mathematical equation. (Davies)
absolute. Nothing in the universe appears to affect them. (Davies)
stable. Unchanged since first discovered (although they may have been shown to be approximations of more accurate laws—see "Laws as approximations" below),
eternal. they appear unchanged since the beginning of the universe (according to observations). It is thus presumed that they will remain unchanged in the future. (Davies)
omnipotent. Everything in the universe apparently must comply with them (according to observations). (Davies)
generally conservative of quantity. (Feynman)
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.

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 it happens.

[edit]

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 Principia Mathematica, and Albert Einstein's theory of relativity. Other examples of laws of nature include Boyle's law of gases, conservation laws, Ohm's law, the four laws of thermodynamics, etc.

[edit]

Laws as approximations

Some laws are low (or high) limits of others, more general laws (or as scientists say, of more fundamental laws). For example, Newtonian dynamics (which is based on Galilean transformations) is just low speed limit of laws of special relativity (simply because Galilean transformation follow from Lorentz transformation at the limit of low speed). Similar, Newtonian gravitation law follows from general realtivity at the limit of low gravitational potential (compared to square of speed of light), or Coulomb law follows from QED at large (compared to range of weak interactions) distances. In such cases we understandably use more simple laws-approximations instead of more accurate fundamental laws.

Those laws which are just mathematical definitions (say, fundamental law of mechanics - second Newton's law <math> F = \frac{dp}{dt}</math>), or uncertainty principle, least action principle, etc. - are absolutely correct (simply by definition). Others which reflect symmetries found in Nature (say, identity of electrons or homogenuity of space and time) are constantly being checked experimentally to higher and higher degree of accuracy. The fact that they have never been seen repeatably violated does not preclude testing them at increased accuracy which is one of 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 above examples), 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 actually better viewed as a series of improving generalisations.

[edit]

Necessity, origin, and existence

If the universe were purely chaotic, the existence of life as it is known would be impossible, since organized complexity is a defining characteristic of life. The laws of nature create order in the universe (especially those laws which are just definitions and thus are absolutely correct ones), and result in a generally stable environment that, in accordance with the anthropic principle, is permissive of life, including humanity.

Most laws are mathematical consequenses of various mathematical symmetries (see Emmy Noether theorem as a proof of this). For example, conservation of energy is a consequence of the shift symmetry of time (no time moment is different than any other), while conservation of momentum is a consequence of the symmetry (homogeniety) of space (no place in space is different than any other). Indistinguishability of similar particles (say, electrons) results in Dirac and Bose statistics which in turn results in Pauli exclusion principle for fermions and in Bose condensation for bosons as well as in second law of thermodynamics (law of increase of entropy in closed system).

Some laws are not laws at all but simply definitions. For example, central law of mechanics F = dp/dt (Newton's second "law" of mechanics) is not a law at all but is a mathemetical definition of force (introduced first by Newton himself). The principle of least action (or principle of stationary action) , Heizenberg uncertainty principle and few other laws fall into this category.

So to large extent laws of nature are not laws of nature per se, but mathematical expressions of certain simplicities (symmetries) of existing Universe. The application of these expressions to our needs resulted in spectacular efficacy of science – its power to solve otherwise intractable problems, and make accurate predictions (because the more accurate is a definition, the more accurate prediction it can make).

[edit]

History, and religious influence

The idea that there are underlying laws 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, seems to have been hampered by belief in animism, and by the attribution of many effects that do not have readily identifiable causes—such as meteorological and astronomical phenomena—to the actions of gods. Early attempts to formulate laws in material terms were made by ancient philosophers, including Aristotle, but suffered from various misconceptions, such as the assumption that observed effects were due to intrinsic properties of objects, e.g. "heaviness", "lightness", etc - which were results of lack of accurate experimental data.

The formal and 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 institution of the use of the scientific method.

Despite laymen belief that laws of nature are somehow god(s) given, there is no scientific evidence of that. On the contrary, practically all laws are either mathematical definitions or restatements of mathematical symmetries of space and time (see above).

[edit]

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 of the 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 , Einstein, Emmy Noether, in the general populace as well.

[edit]

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 often 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.

[edit]