The **microscopic scale** (from Greek : μικρός, *mikrós*, "small" and σκοπέω, *skopéō* "look") is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly.^{ [1] } In physics, the microscopic scale is sometimes regarded as the scale between the macroscopic scale and the quantum scale.^{ [2] }^{ [3] } Microscopic units and measurements are used to classify and describe very small objects. One common microscopic length scale unit is the micrometre (also called a *micron*) (symbol: μm), which is one millionth of a metre.

By convention, the microscopic scale also includes classes of objects that are most commonly too small to see but of which some members are large enough to be observed with the eye. Such groups include the * Cladocera *, planktonic green algae of which * Volvox * is readily observable, and the protozoa of which * stentor * can be easily seen without aid. The submicroscopic scale similarly includes objects that are too small to see with an optical microscope.^{ [4] }

In thermodynamics and statistical mechanics, the microscopic scale is the scale at which we do not measure or directly observe the precise state of a thermodynamic system – such detailed states of a system are called microstates. We instead measure thermodynamic variables at a macroscopic scale, i.e. the *macrostate*.

The **holographic principle** is a tenet of string theories and a supposed property of quantum gravity that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region—such as a light-like boundary like a gravitational horizon. First proposed by Gerard 't Hooft, it was given a precise string-theory interpretation by Leonard Susskind who combined his ideas with previous ones of 't Hooft and Charles Thorn. As pointed out by Raphael Bousso, Thorn observed in 1978 that string theory admits a lower-dimensional description in which gravity emerges from it in what would now be called a holographic way. The prime example of holography is the AdS/CFT correspondence.

**Quantum mechanics**, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest – including atomic and subatomic – scales.

**Statistical mechanics** is one of the pillars of modern physics. It is necessary for the fundamental study of any physical system that has many degrees of freedom. The approach is based on statistical methods, probability theory and the microscopic physical laws.

**Wave–particle duality** is the concept in quantum mechanics that every particle or quantum entity may be described as either a particle or a wave. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behaviour of quantum-scale objects. As Albert Einstein wrote:

It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.

In mathematical physics, especially as introduced into statistical mechanics and thermodynamics by J. Willard Gibbs in 1902, an **ensemble** is an idealization consisting of a large number of virtual copies of a system, considered all at once, each of which represents a possible state that the real system might be in. In other words, a statistical ensemble is a probability distribution for the state of the system.

In physics, the **correspondence principle** states that the behavior of systems described by the theory of quantum mechanics reproduces classical physics in the limit of large quantum numbers. In other words, it says that for large orbits and for large energies, quantum calculations must agree with classical calculations.

**T-symmetry** or **time reversal symmetry** is the theoretical symmetry of physical laws under the transformation of time reversal:

In theoretical physics, the **renormalization group** (**RG**) refers to a mathematical apparatus that allows systematic investigation of the changes of a physical system as viewed at different scales. In particle physics, it reflects the changes in the underlying force laws as the energy scale at which physical processes occur varies, energy/momentum and resolution distance scales being effectively conjugate under the uncertainty principle.

The **macroscopic scale** is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic

The **measurement problem** in quantum mechanics is the problem of how wave function collapse occurs. The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key set of questions that each interpretation must answer.

**Dephasing** is a mechanism that recovers classical behaviour from a quantum system. It refers to the ways in which coherence caused by perturbation decays over time, and the system returns to the state before perturbation. It is an important effect in molecular and atomic spectroscopy, and in the condensed matter physics of mesoscopic devices.

The **thermodynamic limit**, or **macroscopic limit**, of a system in statistical mechanics is the limit for a large number *N* of particles where the volume is taken to grow in proportion with the number of particles. The thermodynamic limit is defined as the limit of a system with a large volume, with the particle density held fixed.

Classical *Newtonian* physics has, formally, been replaced by quantum mechanics on the small scale and relativity on the large scale. Because most humans continue to think in terms of the kind of events we perceive in the human scale of daily life, it became necessary to provide a new **philosophical interpretation of classical physics**. Classical mechanics worked extremely well within its domain of observation but made inaccurate predictions at very small scale – atomic scale systems – and when objects moved very fast or were very massive. Viewed through the lens of quantum mechanics or relativity, we can now see that classical physics, imported from the world of our everyday experience, includes notions for which there is no actual evidence. For example, one commonly held idea is that there exists one absolute time shared by all observers. Another is the idea that electrons are discrete entities like miniature planets that circle the nucleus in definite orbits.

In physics, **maximum entropy thermodynamics** views equilibrium thermodynamics and statistical mechanics as inference processes. More specifically, MaxEnt applies inference techniques rooted in Shannon information theory, Bayesian probability, and the principle of maximum entropy. These techniques are relevant to any situation requiring prediction from incomplete or insufficient data. MaxEnt thermodynamics began with two papers by Edwin T. Jaynes published in the 1957 *Physical Review*.

In physics the term **exchange force** has been used to describe two distinct concepts which should not be confused.

**Mesoscopic physics** is a subdiscipline of condensed matter physics that deals with materials of an intermediate length. The scale of these materials can be described as being between the nanoscale size of a quantity of atoms and of materials measuring micrometres. The lower limit can also be defined as being the size of individual atoms. At the micrometre level are bulk materials. Both mesoscopic and macroscopic objects contain many atoms. Whereas average properties derived from its constituent materials describe macroscopic objects, as they usually obey the laws of classical mechanics, a mesoscopic object, by contrast, is affected by thermal fluctuations around the average, and its electronic behavior may require modeling at the level of quantum mechanics.

**Scale relativity** is a geometrical and fractal space-time physical theory.

A **macroscopic quantum state** is a state of matter in which macroscopic properties, such as mechanical motion, thermal conductivity, electrical conductivity and viscosity, can be described only by quantum mechanics rather than merely classical mechanics. This occurs primarily at low temperatures where there is little thermal motion present to mask the quantum nature of a substance.

**Temperature** is a physical property of matter that quantitatively expresses hot and cold. Temperature is measured with a thermometer.

**Macroscopic quantum phenomena** refer to processes showing quantum behavior at the macroscopic scale, rather than at the atomic scale where quantum effects are prevalent. The best-known examples of macroscopic quantum phenomena are superfluidity and superconductivity; other examples include the quantum Hall effect, giant magnetoresistance and topological order. Since 2000 there has been extensive experimental work on quantum gases, particularly Bose–Einstein condensates.

- ↑ "The microscopic scale".
*Science Learning Hub*. The University of Waikato. Archived from the original on 20 April 2016. Retrieved 31 March 2016. - ↑ Jaeger, Gregg (September 2014). "What in the (quantum) world is macroscopic?".
*American Journal of Physics*.**82**(9): 896–905. Bibcode:2014AmJPh..82..896J. doi:10.1119/1.4878358. - ↑ Reif, F. (1965).
*Fundamentals of Statistical and Thermal Physics*(International student edition. ed.). Boston: McGraw-Hill. p. 2. ISBN 007-051800-9.We shall call a system '

*micro*scopic' (i.e., '*small*scale') if it is roughly of atomic dimensions or smaller (say of the order of 10 Å or less). - ↑ Jaeger, Gregg (September 2014). "What in the (quantum) world is macroscopic?".
*American Journal of Physics*.**82**(9): 896–905. Bibcode:2014AmJPh..82..896J. doi:10.1119/1.4878358.

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