In 1977, the physicist Edward Purcell calculated that if you push a bacteria and then let go, it will stop in about a millionth of a second. In that time, it will have traveled less than the width of a single atom. The same holds true for a sperm and many other microbes. It all has to do with being really small. Microscopic creatures inhabit a world alien to us, where making it through an inch of water is an incredible endeavor. But why does size matter so much for a swimmer? What makes the world of a sperm so fundamentally different from that of a sperm whale? To find out, we need to dive into the physics of fluids. Here's a way to think about it. Imagine you are swimming in a pool. It's you and a whole bunch of water molecules. Water molecules outnumber you a thousand trillion trillion to one. So, pushing past them with your gigantic body is easy, but if you were really small, say you were about the size of a water molecule, all of a sudden, it's like you're swimming in a pool of people. Rather than simply swishing by all the teeny, tiny molecules, now every single water molecule is like another person you have to push past to get anywhere. In 1883, the physicist Osborne Reynolds figured out that there is one simple number that can predict how a fluid will behave. It's called the Reynolds number, and it depends on simple properties like the size of the swimmer, its speed, the density of the fluid, and the stickiness, or the viscosity, of the fluid. What this means is that creatures of very different sizes inhabit vastly different worlds. For example, because of its huge size, a sperm whale inhabits the large Reynolds number world. If it flaps its tail once, it can coast ahead for an incredible distance. Meanwhile, sperm live in a low Reynolds number world. If a sperm were to stop flapping its tail, it wouldn't even coast past a single atom. To imagine what it would feel like to be a sperm, you need to bring yourself down to its Reynolds number. Picture yourself in a tub of molasses with your arms moving about as slow as the minute hand of a clock, and you'd have a pretty good idea of what a sperm is up against. So, how do microbes manage to get anywhere? Well, many don't bother swimming at all. They just let the food drift to them. This is somewhat like a lazy cow that waits for the grass under its mouth to grow back. But many microbes do swim, and this is where those incredible adaptations come in. One trick they can use is to deform the shape of their paddle. By cleverly flexing their paddle to create more drag on the power stroke than on the recovery stroke, single-celled organisms like paramecia manage to inch their way through the crowd of water molecules. But there's an even more ingenious solution arrived at by bacteria and sperm. Instead of wagging their paddles back and forth, they wind them like a cork screw. Just as a cork screw on a wine bottle converts winding motion into forward motion, these tiny creatures spin their helical tails to push themselves forward in a world where water feels as thick as cork. Other strategies are even stranger. Some bacteria take Batman's approach. They use grappling hooks to pull themselves along. They can even use this grappling hook like a sling shot and fling themselves forward. Others use chemical engineering. H. pylori lives only in the slimy, acidic mucus inside our stomachs. It releases a chemical that thins out the surrounding mucus, allowing it to glide through slime. Maybe it's no surprise that these guys are also responsible for stomach ulcers. So, when you look really closely at our bodies and the world around us, you can see all sorts of tiny creatures finding clever ways to get around in a sticky situation. Without these adaptations, bacteria would never find their hosts, and sperms would never make it to their eggs, which means you would never get stomach ulcers, but you would also never be born in the first place. (Pop)
1977年,物理学家爱德华·珀塞尔 (Edward Purcell) 计算出,如果你推动一个细菌 然后任它而去, 它将在大概一百万分之一秒内停止运动。 在这样短的时间内,它运动的距离 小于一个原子的宽度。 这种情况适用于精子 和其它很多微生物。 它们都必须非常小。 微生物栖生在一个完全不同于我们的世界, 对它们来说,渡过一英尺水 都将极其困难。 不过为何大小对于“游泳者”如此重要呢? 是什么使得精子的世界 与抹香鲸的世界 有着完全不同呢? 为了找到答案,我们需要潜入 流体的物理世界。 试着从这个角度来想一下 想象你在一个游泳池里游泳 有你和一大堆的水分子 水分子的数量级是你的 千万亿万亿倍 所以,你庞大的身躯很容易 在它们中间前行 但是如果你非常小的话 小到和水分子差不多的话, 突然之间,就变成你在一游泳池的人 中间游泳。 不同于轻松地在一堆 小小的分子中挥动游弋, 现在每个水分子 都像另一个人,你必须推动划过他们 才能到达别的地方 1883年,物理学家奥斯本·雷诺 (Osborne Reynolds) 计算出一个简易的常数, 可以预知流体的表现能力。 这个常数叫做“雷诺数”, 它适用于简单的个体, 比如“游泳者”的大小、 游的速度、 流体的密度、 以及流体的 黏度或稠度。 这意味着 大小不同的生物 生活在完全不同的世界。 比如,由于极其巨大 抹香鲸生活在 大雷诺数世界。 它拍打一下尾巴, 可以向前游很远。 同时,精子生活在 低雷诺数世界。 如果精子停止摇动尾巴, 它向前的距离不会超过一个原子。 想象一下变成精子是什么样的 你需要把自己放到 它所在的雷诺数中。 想象自己在一罐蜜糖中, 你挥动手臂的速度 和分针一样慢, 你就能明白 精子对抗的是什么 那么,微生物是如何移动的呢? 许多根本不考虑游泳。 它们让食物带着前行。 就像一头懒牛 等着它嘴下的那块草长回来。 但有些微生物会游泳, 它们有惊人的适应性。 其中一个方式就是 改变纤毛的形状。 灵巧地弯曲纤毛 可以使动力 大于阻力。 单细胞生物,比如草履虫 就可以在 水分子中前行。 还有更绝的, 就是细菌和精子的方法。 它们并不把前后摆动纤毛, 而是旋转着,就像螺旋拔塞。 同酒瓶中的螺旋拔塞 将旋转运动转化成向前运动一样, 这些微小的生物旋转着螺旋状的尾巴 使自己向前,它们世界中的水 它们世界中的水同瓶塞一样硬。 另一种办法更加奇怪。 一些病毒选择了蝙蝠侠的方法。 它们用抓钩将自己拉起。 甚至还能 像弹弓一样将自己弹过去。 另一些用化学方法。 幽门螺旋杆菌只在胃中的 酸性粘液中生活。 它们释放一种化学物质 使周围的粘液变薄, 从而划过粘液。 并不奇怪, 这些家伙也要对 胃溃疡负责。 所以,当你近距离观察 我们的身体和这个世界, 你可以看到各种各样的小生物 用聪明的方法 应对棘手的情况。 如果不去适应, 细菌找不到寄主, 精子也遇不到卵子, 这意味着你也不会得胃溃疡, 但你最初也不会出生。