#学习笔记# 古希腊的哲学家的水平实在是屌炸天，记录两个例子来。第一个问题貌似小时候从我爸那儿听得印象深刻，第二个思考间接让我搞懂了原子弹的原理。具体描述一下，普拉图的一个不是那么出名的同事叫艾尔库塔斯（Archytas）用一个比喻思考宇宙究竟是无限还是有限，一个拿着长矛的勇士，站在宇宙的“边界”，掷出他的长矛，你觉得长矛会怎样？是继续无限飞下去，还是会碰到一个边界？边界的外面又是什么？包含这个边界的容器是什么？从他的逻辑出发考虑，艾尔库塔斯认为宇宙是没有无限的。这个思考对于现代人来讲似乎是家常便饭，2500年前的人的思维能到这种高度不是很惊人么。
再来说第二个思考，和对宇宙边界的思考类似，德谟克利特（Democritus）做了一个相反方向的思考。他提出如果有一把极其锋利的刀，把石头切开成小石头，更小的石头，直到变成沙子。他无法物理上继续试验继续切下去，但是思想上却可以把虚拟的试验继续下去，继续切下去会怎样？对于德谟克利特，这个思想试验只有两个结果，要么你可以无限的把石头切成更小的物质，要么你会达到一个看不见的，不可分割的最基础的本源，德谟克利特认为后者比较符合逻辑，他把这种想象中的东西称为原子（atom）。他甚至还独立分出了气味之类的东西不是由原子决定的，而是另一种属性。这种纯凭逻辑思维加上想象和推导，能够在人类证明原子存在两千年前就作出判断的能力，实在是惊人的很。
我稍微把这个思想试验（thought experiment）拓展了一下，如果硬切原子会怎么样，我猜弹性（抗力）会很大，我脑子中浮现的其实是做得很筋道的撒尿牛丸或者福州鱼丸用筷子硬夹丸子飞起来的画面，哈哈哈。言归正传，也就是说如果硬切，不管成功与否，应该都能产生大量的能量。于是我怀着对现代科技的期望，是不是有一把特别牛逼的刀可以做成这件事满足我的想象。搜了cut atom in half （把原子切成两半），你猜结果出来是什么，原！子！弹！哈哈哈哈哈。
再进一步研究了一下原子弹，链式反应的核裂变制造出来的，通俗的话说就是通过各种手段让原子核产生多米诺骨牌式的持续分裂释放能量，可是限制是只能用比较重的原子（化学术语叫高原子序数）的天生就带有裂变性质的原子来做原料，就是传说中的钚（plutonium），就是电影里各种大魔王爱偷的原材料。
再然后我的好奇心并没有得到满足，想如果硬要分裂不带天生裂变属性的原子会是啥结果，结果就发现氢弹了，哈哈哈。氢原子这么轻的东西需要极大的能量才能对原子结构做出任何改变，那么大的能量从哪儿来？你猜那帮小聪明们是怎么做到的？Bingo！用一个小型原子弹包在外面引爆，就可以引起氢原子的变化，aka氢弹。这个过程人类尚未能控制，于是与核裂变区分，叫做核聚变，也就是太阳工作的原理。氢弹的威力有多大？同质量的氢弹爆炸效果（TNT当量）几乎是原子弹的1000倍以上。从此人类有了自我毁灭的可能性，科~科。
话说我看历史书也能联系到原子弹的原理，真是一个奇怪的思想过程啊。
So, they were strikingly modern
in their views of space and time. They were also asking profound questions,
and two examples will give a sense of this. Plato had a colleague called Archytas. Archytas wondered about the universe. Was it finite or infinite? It’s a dichotomy and it’s very profound. In the analogy he used, he imagined there is a warrior, a spear carrier, who went to the edge of the universe as far as they could travel, and hurled their spear. What do you imagine Archytas said should happen? Should the spear travel forever, and into what? Or will the spear hit a boundary or a barrier or the limit, and then what is that substance, and what’s beyond that substance? So in this logic, Archytas knew that if there was a boundary, or a barrier, or a container, then something
has to contain the container. And so, in his logic, it was more rational to imagine that the universe was infinite. And so in Greek philosophy, at the time, the dominant idea was of an infinite universe.
A hundred or so years later, Democritus made an analogous thought experiment, this time going in the downward direction. He imagined a sharp knife in a stone. You cut the stone into smaller and smaller pebbles until it becomes a grain of sand, and then perhaps you can’t do the experiment,
but you imagine cutting it in half and half and half again, until it’s almost too small to see on the tip of your finger. Now you can’t do the experiment but the thought experiment continues, and how does it end? To Democritus there are only two outcomes. Either you reach a fundamental, invisible,
indivisible object, he called it an atom, or you carry on the process forever generating infinitely small sub-units, and to him it was apart logically that an infinite progression would continue, and so he imagined an indivisible element called an atom. He hypothesized atoms and also inferred that the properties of normal material like its texture, or its smell, or its taste would be secondary properties not held by the individual atoms themselves. Another strikingly modern idea, 2000 years before we had the ability to prove that atoms exist.