Entropy is the only quantity in the physical sciences (apart from certain rare interactions in particle physics; see below) that requires a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says, the entropy of an isolated system can increase, but not decrease. Hence, from one perspective, entropy measurement is a way of distinguishing the past from the future. However in thermodynamic systems that are not closed, entropy can decrease with time: many systems, including living systems, reduce local entropy at the expense of an environmental increase, resulting in a net increase in entropy. Examples of such systems and phenomena include the formation of typical crystals, the workings of a refrigerator and living organisms.
Entropy, like temperature, is an abstract concept, yet, like temperature, everyone has an intuitive sense of the effects of entropy. Watching a movie, it is usually easy to determine whether it is being run forward or in reverse. When run in reverse, broken glasses spontaneously reassemble, smoke goes down a chimney, wood "unburns", cooling the environment and ice "unmelts" warming the environment. No physical laws are broken in the reverse movie except the second law of thermodynamics, which reflects the time-asymmetry of entropy. An intuitive understanding of the irreversibility of certain physical phenomena (and subsequent creation of entropy) allows one to make this determination.
Additional relevant articles may be found in the following categories:
Thermodynamic entropy
Entropy and information
Quantum mechanical entropy
Entropy, in thermodynamics, is a state function originally introduced to explain why part of a thermodynamic system's total energy is unavailable to do useful work.
Entropy may also refer to:
Thermodynamics
Classical thermodynamic entropy, the original meaning, a state function introduced by Rudolph Clausius to explain why part of a thermodynamic system's total energy is unavailable to do useful work
Gibbs entropy Entropy in classical statistical physics, a measure of disorder, proportional to the logarithm of the number of microstates comprising a macrostate
von Neumann entropy Entropy in quantum statistical physics, the quantum analog of Gibbs entropy
A diversity index is a quantitative measure that reflects how many different types (such as species) there are in a dataset, and simultaneously takes into account how evenly the basic entities (such as individuals) are distributed among those types. The value of a diversity index increases both when the number of types increases and when evenness increases. For a given number of types, the value of a diversity index is maximized when all types are equally abundant.
When diversity indices are used in ecology, the types of interest are usually species, but they can also be other categories, such as genera, families, functional types or haplotypes. The entities of interest are usually individual plants or animals, and the measure of abundance can be, for example, number of individuals, biomass or coverage. In demography, the entities of interest can be people, and the types of interest various demographic groups. In information science, the entities can be characters and the types the different letters of the alphabet. The most commonly used diversity indices are simple transformations of the effective number of types (also known as 'true diversity'), but each diversity index can also be interpreted in its own right as a measure corresponding to some real phenomenon (but a different one for each diversity index).
Today, there is a lot, But frequent hints that this won't always be. We are invariably running out. The process seems slow because we forget, Time does not exist. Absolute zero stares back at us
On the top floor of Entropy – the art gallery, record and vintage store located downtown – Karen Mobley is showing off some “Big Doodles.” ... Due to the unique architecture of Entropy, Mobley created works that would complement the space ... .
MONTCLAIR, N.J., March 12, 2025 /PRNewswire/ -- Entropy Robotics, an AI-driven startup that develops autonomous software for robots and drones, announced today a strategic collaboration with SRI....
But a new study goes one step ahead; it not only links the two theories but also suggests that gravity emerges from quantum entropy ... This interaction is driven by entropy, which is a measure of disorder in a system. .
Many solutions have been proposed to get to the elusive quantum gravity, and the latest one, proposed by ProfessorGinestra Bianconi, uses the fascinating concept of entropy ...Entropy is a quantity that comes from thermodynamics.
The action for gravity is given by the quantum relative entropy between the metric of the manifold and the metric induced by the matter field and the geometry ... The role of entropy and the G-field.