科学与数学en.wikipedia.org
玻尔兹曼大脑
Boltzmann Brain
2014 — 2024，它一次次回到首页
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导读
维基百科条目解释“玻尔兹曼脑”：在极长久或永恒的宇宙中，随机涨落可能自发形成带有记忆和意识的脑，其出现概率甚至可能高于演化出我们这样的观察者。这个思想实验被用来检验统计力学、熵与宇宙学模型中的概率论证及观察者选择问题。
为什么经久不衰：它用极简的反例把宇宙学、熵和意识联系起来，迫使人们反思“我们更可能是什么”这类概率论证，因此跨年代仍被反复讨论。
HN 评论精选
同一心理状态下的推理无法区分两种假设；而决策只在真实人类情形下有意义，因此玻尔兹曼脑概率不应影响行动。
我觉得这个问题很有趣：“我是一个玻尔兹曼脑，还是真实的人类？”如果存在一个真实人类和一个与你当前心理状态相同的玻尔兹曼脑，那么试图用单一答案回答这个问题就没有意义。你的推理在两者中会完全相同地执行，所以你的推理不可能得出一个单一且正确的答案。换个角度看，处理这种困惑的一种方式是意识到：如果你真是玻尔兹曼脑，你的任何想法或决定都根本不重要。即使你承认自己是玻尔兹曼脑有一定概率，除了当作好奇，也没有必要把它纳入决策过程。你的决定只有在你是真实人类的情况下才有意义，所以做决定时只值得考虑这一种可能。（类似地，如果马上要抛硬币，很多事取决于结果，包括硬币正面你就会被立即处决且无法逃脱，那么你对未来的所有计划都应当和你有100%把握硬币会反面时完全一样。因为那是你的决定唯一有意义的路径。）
AgentME查看英文原文
I find this question really fun: "Am I a Boltzmann brain, or a real human?". If there exists both a human and a Boltzmann brain with your current mental state, then it doesn't make sense to try to answer this question with a single answer. Your line of reasoning will be identically executed in both, so your line of reasoning can't result in a single answer and be correct. Instead, one way to address the confusion about whether you're a Boltzmann brain or not, is to realize that in the case you're a Boltzmann brain, none of your thoughts or decisions matter at all. Even if you admit there's some probability that you're a Boltzmann brain, there's no use factoring it into your decision processes besides as a curiosity. Your decisions only matter in the case that you're an actual human, so that's the only possibility worth considering while coming up with decisions. (Similarly, if a coin was going to be flipped in a moment, and a bunch of things depended on which way the coin landed, including that you were going to be immediately executed with no possibility of escape if the coin landed on heads, then all your plans for the future should be exactly the same as if you expected the coin to land on tails with 100% certainty. It's the only path that any of your decisions matter.)
把它与“上周四主义”这类关于世界年龄的怀疑论思想实验类比。
我一直很喜欢这个思想实验，以及和它松散相关的“上周四主义”[0]。
irjustin查看英文原文
I've always enjoyed this thought experiment and the loosely related one - Last Thursdayism[0]. [0] https://rationalwiki.org/wiki/Last_Thursdayism
把对象换成大脑会引入意识争论；用普通物体更能检验理论概率论证。
为什么不是玻尔兹曼石头、玻尔兹曼纪念币，或别的无生命东西？我以为重点是用一个简单比喻来衡量一个普通物体基于现代物理预测的公认宇宙历史出现的概率，而不是其组成原子通过极不可能的随机事件瞬间变成当前状态。换句话说（根据维基条目），如果某个现代物理理论的数学表明该物体更可能是随机涨落产生的，那就是反对采用该理论的一个理由。但如果你把关注对象设成大脑，就会招来一堆关于意识、以及普通大脑怎么知道自己不是玻尔兹曼脑的无谓争论。这就像介绍新编程语言语法时给我看 quine 的代码而不是“hello world”。这样做价值何在？
jancsika查看英文原文
Why is it not a Boltzmann rock, or a Boltzmann commemorative coin, or something else inanimate like that? I thought the whole point was to give a simple metaphor for a measuring the probability that a mundane object came into being based on the widely accepted history of the universe that modern physics predicts. As opposed to just coming into its current state instantaneously based on extremely unlikely random events among its constituent atoms. Or put differently (based on the Wikipedia article), if the math behind a modern physics theory means that it's more likely the given object's existence was due to random fluctuations then that's a strike against adoption of that theory. But if you make the object of interest a brain you get all this bikeshedding about consciousness and how do the regular brains know they're not Boltzmann brains, etc. It'd be like introducing me to your new programming language's syntax by showing me the code for a quine instead of "hello world." What is the value in doing such a thing?
该论证的永恒静态宇宙前提已过时；现代宇宙学更支持低熵起点。
大多数支持玻尔兹曼脑的论证写于1929年哈勃定律发现之前，当时多数人认为宇宙本质上是永恒且静态的。连爱因斯坦都试图用宇宙学常数来挽救永恒静态宇宙模型。在哈勃常数、大爆炸和暴胀被发现之后，低熵起点是更合理的解释。
gus_massa查看英文原文
Most of the arguments in favor of the Boltzmann Brain are written before the discovery of the Hubble's law (1929) when most people thought that the universe was essentially eternal and static. Even Einstein was trying to use the cosmological constant to save the model of a eternal static universe. After the discovery of the Hubble constant, the Big Bang and Inflation, it's more razonable a low entropy starting point.
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