原文：

Six degrees of separation

Six degrees of separation is the theory that anyone on the planet can be connected to any other person on the planet through a chain of acquaintances that has no more than five intermediaries. The theory was first proposed in 1929 by the Hungarian writer Frigyes Karinthy in a short story called "Chains."

In the 1950's, Ithiel de Sola Pool (MIT) and Manfred Kochen (IBM) set out to prove the theory mathematically. Although they were able to phrase the question (given a set N of people, what is the probability that each member of N is connected to another member via k_1, k_2, k_3...k_n links?), after twenty years they were still unable to solve the problem to their own satisfaction. In 1967, American sociologist Stanley Milgram devised a new way to test the theory, which he called "the small-world problem." He randomly selected people in the mid-West to send packages to a stranger located in Massachusetts. The senders knew the recipient's name, occupation, and general location. They were instructed to send the package to a person they knew on a first-name basis who they thought was most likely, out of all their friends, to know the target personally. That person would do the same, and so on, until the package was personally delivered to its target recipient.

Although the participants expected the chain to include at least a hundred intermediaries, it only took (on average) between five and seven intermediaries to get each package delivered. Milgram's findings were published in Psychology Today and inspired the phrase "six degrees of separation." Playwright John Guare popularized the phrase when he chose it as the title for his 1990 play of the same name. Although Milgram's findings were discounted after it was discovered that he based his conclusion on a very small number of packages, six degrees of separation became an accepted notion in pop culture after Brett C. Tjaden published a computer game on the University of Virginia's Web site based on the small-world problem. Tjaden used the Internet Movie Database (IMDB) to document connections between different actors. Time Magazine called his site, The Oracle of Bacon at Virginia, one of the "Ten Best Web Sites of 1996."

In 2001, Duncan Watts, a professor at Columbia University, continued his own earlier research into the phenomenon and recreated Milgram's experiment on the Internet. Watts used an e-mail message as the "package" that needed to be delivered, and surprisingly, after reviewing the data collected by 48,000 senders and 19 targets (in 157 countries), Watts found that the average number of intermediaries was indeed, six. Watts' research, and the advent of the computer age, has opened up new areas of inquiry related to six degrees of separation in diverse areas of network theory such as as power grid analysis, disease transmission, graph theory, corporate communication, and computer circuitry.

译文：

六度分割理论

六度分割理论指的是地球上的任何一个人通过不超过5个中间熟人可以取得和任何另外一个人的联系。这个理论是匈牙利作家Frigyes Karinthy于1929年在其小说“Chains”中首先提出的。

20世纪50年代，麻省理工学院的Ithiel de Sola Pool和IBM的Kochen试着从数学的角度证明这个理论。尽管他们可以表述出这个问题（有N个人，这N个人中任何两个通过另外的k_1,k_2,k3…k_n个人的关系取得联系的概率有多大），但是20年之后，他们还是无法满意地解决这个问题。1967年，美国的社会学家Stanley Milgram想出了个新的方法来测试这个理论，他将其称为“小世界难题”。他随机地从中西部区域选一些人给曼彻斯特的一个陌生人寄送包裹。寄送方知道接受方的名字，职业和大概位置。他们被指导将这个包裹送给他们的朋友中最可能认识那个接受方的人。收到包裹的人也要按照同样的要求来做，一直持续下去，直到包裹到达目的人手中。

尽管参与者认为至少要经过100个人才可以到达，但实际上平均只经过了5到7人就实现了。Milgram的发现被发表在《今日心理学》上，由此引来了“六度分割理论”的陈述。剧作家John Guare以此名作为其1900年戏剧的名字，这使得该理论更加流行了。尽管Milgram的发现被低估了，因为别人认为其事实依据不够充分，但是在Brett C. Tjaden在弗吉利亚大学的网站上放了个关于“小世界理论”的游戏后，六度分割理论则变的广泛接受了。Tjaden使用互联网电影数据库IMDB的数据来归档不同演员间的联系。时代杂志将这个网站称为弗吉利亚的Oracle of Bacon，其是1996年十大最佳网站之一。

2001年，哥伦比亚大学的一位教授Duncan Watts继续他自己关于这种现象的早期研究，并在互联网上重建了Milgram的实验。Watts使用电子邮件信息作为要传达的包裹，令人惊奇的是，在查看来自157个国家的48000位发送者和19名接受者的数据之后，Watts发现中间人的平均数量的确是6个。Watts的研究以及电脑时代的来临打开了关于六度分割理论调查的新领域 ，该理论应用于很多网络理论领域如电力网络分析，疾病传播，图论，合作通信和计算机电路等。