Quorum sensing is the component of swarm intelligence that allows a swarm (or group) to settle on a decision and begin acting on that decision. It is used in a large variety of natural and artificial systems. I'll use a rock dwelling ant colony to illustrate the concept of quorum sensing:
A colony of ants happily dwells in their rocky home until the rocks shift causing extensive damage to their colony. The shifted rocks are no longer a suitable habitation for the colony and worker ants venture out looking for a new home. Every possible nook and cranny is explored until one large enough is found. The individual worker ant inspects the crevice and assesses its suitability for the colony including lighting, water flow and air ways. After the inspection the ant heads back to the colony and waits.
The waiting period is inversely tied to the quality of the new site. The poorer quality the site is the longer the ant waits, the better the site the less it waits. Once the waiting time is over the initial worker ant solicits other ants to follow him to inspect the new site. After the second inspection is complete the ants return and again wait, the same waiting rules apply, before soliciting yet another group of ants to inspect. At a critical point, the worker ants that remained in the colony realize that enough ants have approved a site and they pack up to follow them to the new site and the whole colony is relocated.
The timing of the waiting period is critical. The worker ant is basically waiting to see if another worker returns bursting into the colony with a 'must have' site. If no other ant solicits before they do then the ant can assume their selected site is the best site currently available. If a better site is located the returning worker ant would also start soliciting ants and the cascading effect ensues but at a faster rate because of the superior site.
The quorum part is the large number of ants going to the same prospective site. The sensing part is realizing that the quorum has reached a critical mass and thus a decision has been made. Quorum sensing is used in nearly every type of swarm including ants and bees, fireflies, light emitting bacteria, fish that swim in schools, even mobs and businesses. Each organization using quorum sensing differently, but the principles remain the same. Fireflies use quorums to determine where they should gather, light emitting bacteria to know when there are enough of them to make their light noticeable. Ants and bees use it to determine the most suitable location for the swarm without the time or danger of each member inspecting each site. Mobs use quorum sensing to determine why they are mobbing and what or whom they are going to mob.
Often in society we like to think that we are above such a fundamental practice of quorum sensing, but in reality we are steeped in it. No change can ever be effected unless a significant group, either is quantity or quality, approves the change. This is true in the corporate boardroom where leaders are appointed and even for the president of the United States who is elected. Swarms of people wait when new technology and products are introduced for a quorum, usually a select group of famous people, to endorse them new items before they themselves begin using them. Have you ever heard the phrase "I'm waiting until the next version to buy it" or "we'll see how well to works"? These are both cases of waiting until the quorum concludes that the change is acceptable. Another version implies that enough 'ants' approved the earlier version, usually by purchasing it, that the manufacturer could survive long enough to make the next version thus the quorum has been reached. Waiting for reviews of a new product is waiting for another 'ant' to solicit your use of the product; the early 'ant' is convincing you to inspect the new 'site' thus building the quorum.
Quorum dynamics are the governing principles behind quorum sensing. Quorum dynamics consists of two parts as briefly mentioned earlier: quality and quantity.
Quality is the voting power a particular member has. Some members have a lot of sway, unofficially and officially, while others do not. Some people can give a thousand word opinion and not convince another soul while others can give a single look and convince the whole quorum. In a family the mom would be expected to carry a higher quality rating than a child and an adult child more than a juvenile. Quality ratings aren't always along predictable or constant: an adult child would carry a higher quality rating than their mom on their wedding day.
The second attribute in quorum sensing is quantity, simply the number of agreeable members in the quorum. The total number of members is of little importance as few decisions rely on raw votes. For example, the decision over where a family will eat out likely rests with the parents not the four children. The parent's decision may be overruled if the children orchestrated a loud protest, thus exercising their voting power and the quorum decision would have achieved critical mass.
Every person with voting power, regardless of quality rating, is a "valid quorum member" and every "valid quorum member" has voting power. Valid quorum members can be adapted to members present and as the situation dictates. For example, when a visiting uncle arrives, though not a member of the immediate family, he becomes a valid quorum member and can affect policy decisions. On the other hand, if a grandfather dies, his grandchildren may have no voting power in funeral arrangements though they might have voting power in decisions about the family's summer vacation. It is also possible to be part of the quorum but to have no voting power. Parents to give a lot of advice to their newly married children and thus influence the outcome of the vote, but they themselves have no voting power.
Quorum sensing's critical mass is a function of quality times quantity. For a decision to be made enough people with enough voting power must reach consensus. In the example of the children going out to eat, their consensus overrode the decision of a single parent, but likely would not have withstood both parents' consensus. The threshold for each decision varies based on the importance of that decision. Children will are more likely to influence important decisions like where the family will move to but have less influence over what kind of car the dad buys for commuting to work.
Organizations use quorum sensing to sustain leaders and managers. A president may appoint a new department head, but the new department head will still need to get his department to 'sustain' him as a leader. While the quorum is vetting, the employees of the department will lack both trust and confidence in the leader. The quorum is likely to consist of key department staff, key in the sense of office politics and likeability not in the sense of job level or authority. A well liked janitor may have a higher quality rating than an annoying mid-level manager. It is important to remember that only valid quorum members can vote. The bimbo mail boy's vote won't be counted and neither will the department head even though he is part of the quorum. The length of the vetting depends on the perceived quality of the leader, just like the ant's delay in soliciting others to inspect the new site is determined by the perceived quality of the site. The more confident quorum members are in the new leader the more excited they will be to rally behind them, the less confident they are the less excited. Once a quorum member approves of the new leader they will solicit others to rally with them. This process will continue until a critical mass is reached, in both quality and quantity or until the vote fails by default because it takes too long.
If a sufficient quorum consensus is never reached then the quorum will either remain divided over the perceived competence level of the new leader, probably causing the larger quorum to divide into small quorums, or the quorum members who voted in the affirmative will retract their vote, and their support, until the new leader is left without a sustainable platform.