Silicon Valley Bitcoin Meetup; Sunnyvale, California; September 2016
硅谷比特币集会，加利福尼亚，2016年9 月
The topic of today’s talk is proof-of-work and the monument of immutability. Specifically, we’ll talk about immutability and what that means in this new era of digital currencies, what it means to have a digital system that is unchanging.
今天的主题是工作量证明机制和不可改变性。具体来讲，我们将会讨论不可改变性以及它对于新时代中数字货币的意义；拥有一种不可改变的数字货币系统到底意味着什么？
Immutability is a tricky concept—first of all, because it doesn’t really exist. Everything changes; there is nothing in nature that is forever unchangeable. The universe itself—the vacuum, the particles—everything changes. Nothing is immutable, so immutability is more a philosophical idea, but we do think of it in practical terms. What do we mean when we say "immutable" in practical terms? I like to think of it on a linear scale. On one end, you have something that’s very easy to change, and things get progressively harder to change, all the way to the thing that is most difficult to change, the most unchangeable thing; immutability is that side of the scale. So, for practical purposes, we’ll define immutability in any sense to be the maximum or endpoint of that scale.
不变性是一个棘手的概念——首先它并不是真的存在，一切都在改变，自然界中没有任何东西是永恒的。宇宙本身——空间，粒子——所有的一切都处于变化之中，没有什么是不可改变的，所以不变性像更是一种哲学思想，但我们从实际的角度思考：当我们说“不变性”的时候，实际上是什么意思？我喜欢从线性的尺度上思考它：一开始，你有一些很容易改变的事情，然后事情变得越来越难改变，一直到几乎不可能改变。因此，为了实用的目的，我们把“不变性”定义为最大程度上很难改变的东西。
On January 3rd, 2009, the scale expanded significantly, the endpoint changed. A new maximum was defined, a new maximum in terms of what it means to be immutable for a digital system. Nothing is as immutable as bitcoin; bitcoin defines the end of that scale at the moment, so it redefines the term immutable. That has some interesting implications, including that you can’t call the things to the left of that "immutable." You can’t call them "immutable-ish," you can’t call them "kind of immutable." "Immutable-ish" is like pregnant-ish; it only makes sense as the maximum value, not the maximum minus one. Immutable, once it’s redefined, prevents everything else from being called “immutable” any more.
2009年1月3日（比特币诞生的那天），对于数字货币系统来说，它定义了不变性的最大范围，没有什么东西能具有比特币那样的不可改变性。
Why is bitcoin immutable? What gives bitcoin the characteristics of immutability? What is it that makes it unchangeable? The first answer that comes to mind for most people is "the blockchain." The blockchain makes bitcoin immutable because every block depends on its predecessor, creating an unbreakable chain back to the genesis block, and if you change something it would be noticed. Therefore, it’s unchangeable.
为什么比特币是不可改变的？是什么给予了比特币不可改变的特性？是什么东西使它变得不可改变？大多数人想到的第一个答案是“区块链”。区块链使比特币变得不可改变，因为每个区块取决于它的前一个，它通过创建一条牢不可破的链条一直回溯到创世区块， 如果你改变了什么，就会被发现。因此，它是不可改变的。
That is the wrong answer, because it’s not really "the blockchain" that gives bitcoin its immutability. That’s a really important nuance to understand. The blockchain makes sure that you can’t change something without anyone noticing. In security we call that "tamper-evident": if you change it, it is evident. You cannot tamper with it without leaving evidence of your tampering. But there’s a higher standard in security. We call it "tamper-proof": something that cannot be tampered with. Not just “will be visible if it’s tampered with,” but “cannot be tampered with.” Immutable.
然而这是一个错误的答案，因为并不是“区块链”给予了比特币不可改变性。这是一个非常重要的细微差别。区块链确保你不能在没人注意的情况下篡改任何东西，如果你改动了，就会留下证据。但是这里有一个更高的安全标准，我们称之为“抗篡改性”：意思就是完全不能被篡改。不只是“如果被篡改就会被发现”，而是完全不可篡改，不可改变。
The characteristic that gives bitcoin its tamper-proof capability is not "the blockchain”; it’s proof-of-work. Proof-of-work is what makes bitcoin fundamentally immutable. That is a really important concept to understand, because a lot of people are throwing around the word "blockchain" and claiming that their alternative blockchains are immutable even though they don’t have a proof-of-work consensus algorithm, or any kind of consensus algorithm that gives them immutability. At best, they are tamper-evident, meaning someone will notice; but they are not unchangeable.
给予比特币抗篡改性的不是区块链，而是工作量证明机制。工作量证明机制使比特币从根本上不能被改变，这是我们必须理解的非常重要的一点，因为很多人四处宣传“区块链”这个概念，并且声称他们自己的替代性区块链系统也具有不可改变性，哪怕他们的“区块链”甚至都没有工作量证明算法，或者是没有任何一种可以保证其“不可篡改性”共识算法。他们充其量可以做到篡改会被发现，而做不到完全的不可改变性。
This distinction is going to become historically important.