A scientific law tells you what happens, never why it happens
Newton's laws, conservation of energy and the laws of thermodynamics all summarise what repeated observation shows, often in a single equation. Unlike theories, they offer no mechanism. And when a better theory arrives, an old law usually stays accurate; what shrinks is the range where it applies.
Laws are statements distilled from repeated experiments or observations that describe or predict natural phenomena, usually within a certain range. Many can be written mathematically: conservation of energy as a statement that the universe's total energy does not change, the first law of thermodynamics as a balance of heat and work. Hypotheses and postulates come earlier in the scientific process, proposed before and during testing; a law is a conclusion accepted across the scientific community after years of confirmation.
The difference from theory is crucial. A theory explains a phenomenon; a law merely condenses what has been seen, so it can only be trusted in conditions resembling those already observed. Every law implies a causal relationship in a physical system under repeated conditions. A well-confirmed fact such as mercury being liquid at standard temperature and pressure is too specific to count as a law.
Laws can be approximations of deeper ones. Newtonian dynamics is the low-speed limit of special relativity, still excellent within its restricted domain. Like theories, laws make predictions and can be falsified by contradicting data. That no violation has ever been seen does not stop scientists from testing them ever more precisely or under new conditions, and doing so is one of the central goals of science.
Physicists list typical traits of fundamental laws. They hold within their domain of validity, with no repeatable contradictions. They seem universal and absolute, applying everywhere and unaffected by anything. They are usually simple, often a single equation, and stable since discovery, though they may later turn out to approximate something more accurate. Many express symmetries of space and time, and outside quantum physics most are reversible in time even though time itself runs one way. In physics, laws cover the broad domain of matter, motion, energy and force rather than particular systems like the human body. Social sciences have laws too, such as Zipf's law in statistics, though economic laws generally predict far less reliably.
Source: Scientific law
