On the last day of class a few students, acting out of duty, politeness, or appreciation, begin to applaud hesitantly as the instructor gathers his materials to leave the room. If enough clap, the whole class may break into applause; if a few clap indecisively, it dwindles to an embarrassed silence. On all days except the last day of class, the instructor who keeps talking after the end of the hour notices that students... shuffle, put books away, occasionally stand up, hoping to start enough of an exodus to keep any departing students from being conspicuous. 
This is the phenomenon of 'critical mass'. It has to main components:
a) It's the determination of the minimum amount of material necessary before 
b) 'a point is reached at which the chain reaction can become self-sustaining.' 
Let's look at another trivial example:
Somebody puts up the volleyball net, gathers a few friends, starts a game, and attracts a few more players. Then one of two things happens. By the second or third day, a pretty good crowd has gathered to play volleyball; people begin to get acquainted; there’s discussion of what the best time to play is; there are bystanders willing to join the game; the enterprise is a success and may last until the snow comes. 
The term 'critical mass' originated from nuclear physics. Below, is more technical explanation. For more entertaining examples, skip to the next.
The name of the model, "critical mass," is derived from nuclear physics, where atomic energy is explained in the following general terms:
Radioactive decay occurs in a substance like uranium, which emits neutrons. These neutrons fly into space, unless they hit other uranium nuclei before leaving the mass of uranium. If they do hit other nuclei, the collisions cause energy to he released, and more neutrons to fly into space, unless those neutrons hit other nuclei, in which case more energy and more neutrons are released, and so on.
If the amount of uranium is large, there is a greater likelihood that a neutron will collide with other nuclei. There has to be enough uranium so that the released neutrons cause an equal or greater number of additional neutrons to be released for the reaction to carry on. This constitutes a "critical mass" of uranium.
When that amount is present, the process will cross the critical point when it becomes self-sustaining. A self-sustaining reaction has "gone critical." Any smaller amount will result in the chain reaction fizzling out. A larger amount with each neutron producing more than one neutron on the average creates a more explosive chain reaction. The difference between achieving or failing to achieve critical mass in an atomic reaction is obvious - it determines whether the bomb explodes, or whether the plant produces electric power. 
Some further examples:
1. The Transition to Minority Neighbourhoods:
It was observed that the entrance of a few members of a minority into a neighborhood often caused some among the formerly homogeneous population to leave, or to show signs of leaving.
Their departure left openings, so more members of the minority could enter; the increase in new residents induced more of the old to leave, and so forth in the familiar process. Some of the departures might be motivated by the minority entrants who had already arrived, some by the belief that the process, once started, would continue, and some by the fear that they might soon be selling their houses in panic.
Among early writers on the subject, the model was not explicit. The concept came to be applied to schools and school districts in the 1960’s, racial minorities again being the stimulus and white-pupil-departure the phenomenon.
The concept came to be applied to occupations, clubs and fraternities, medical schools and colleges, public beaches and tennis courts, restaurants, nightclubs and public parks. 
2. New Products
When new products are introduced, the purpose of the initial campaign is to establish a critical mass of users.
There are many marketing techniques for doing this, some involving claims about the product's superiority and how the consumer will be left behind if he does not purchase it. This can involve testimonials from famous athletes or others.
If a product has been on the market, claims are sometimes made that imply a critical mass has been achieved, so the appropriate thing for the consumer to do is to get on the bandwagon with everyone else. Such claims might involve statistics that claim "four out of five doctors surveyed recommend" or “this outsells all other brands." 
3. Presidential Elections
Critical mass is sometimes seen in politics when it is primary season before presidential elections and a candidate gathers momentum after early victories and voters “jump on the bandwagon" in support of the candidate who emerges as the favorite.
On the larger political field of international relations no example is more dramatic in recent history than the changes in modern history set in motion when Mikhail Gorbachev became leader of the Soviet Union's Politburo in 1985.
His policies of perestmika and glasnost opened the U.S.S.R. to many previously unknown freedoms and reforms, the discontent and dissatisfaction that existed below the surface began to gather voice and as some dared speak out and suffered no repercussions, others were encouraged to do so, which brought out others who were unhappy living under the communist system.
By 1989 the Soviets were participating in their first openly contested elections since 1917, and soon a critical mass demanding democracy was achieved in Eastern Europe. With Gorbachev’s encouragement of reform, democratic forces were unleashed in the Communist bloc. In 1989, to the continual shock of outside observers, revolutions overthrew the Communist regimes in Poland, Hungary, East Germany, Bulgaria, Czechoslovakia, and Romania.
The loosening of oppressive control within the Soviet Union allowed the sustained reaction to carry on in the fifteen Soviet Republics, and in 1990, Lithuania became the first to declare itself an independent country. Others soon followed, and the leading state, Russia, lacked economic resources to prevent the deterioration. In 1990, the Cold War that had molded international relations and held the world hostage to nuclear destruction came to an end.
The Communist bloc’s rival alliance to NATO, the Warsaw Pact, was officially dissolved the following year. 
 Modern Nuclear Chemistry by Walter D. Loveland, David J. Morrissey, Glenn T. Seaborg
 Britannica Concise Encyclopedia by Encyclopaedia Britannica Inc
 Micromotives and Macrobehavior by Thomas C. Schelling
 Schelling's Game Theory by Robert V. Dodge
 Diffusion of Innovations, 4th Ed by By Everett M. Rogers
Note: Also known as the ‘bandwagon effect’ and the ‘tipping point’.
Categorisation: physics → nuclear
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