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A physical law or scientific law "is a theoretical principle deduced 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." 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''.〔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 ().〕 == Description == 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. * Simple. They are typically expressed in terms of a single mathematical equation. * Absolute. Nothing in the universe appears to affect them.〔 * 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).〔 * Generally conservative of quantity. * Often expressions of existing homogeneities (symmetries) of space and time.〔 * Typically theoretically reversible in time (if non-quantum), although time itself is irreversible.〔 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「physical law」の詳細全文を読む スポンサード リンク
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