Imagine an airplane flying one millimeter above the ground and circling the Earth once every 25 seconds while counting every blade of grass. Shrink all that down so that it fits in the palm of your hand, and you'd have something equivalent to a modern hard drive, an object that can likely hold more information than your local library. So how does it store so much information in such a small space? At the heart of every hard drive is a stack of high-speed spinning discs with a recording head flying over each surface. Each disc is coated with a film of microscopic magnetised metal grains, and your data doesn't live there in a form you can recognize. Instead, it is recorded as a magnetic pattern formed by groups of those tiny grains. In each group, also known as a bit, all of the grains have their magnetization's aligned in one of two possible states, which correspond to zeroes and ones. Data is written onto the disc by converting strings of bits into electrical current fed through an electromagnet. This magnet generates a field strong enough to change the direction of the metal grain's magnetization. Once this information is written onto the disc, the drive uses a magnetic reader to turn it back into a useful form, much like a phonograph needle translates a record's grooves into music. But how can you get so much information out of just zeroes and ones? Well, by putting lots of them together. For example, a letter is represented in one byte, or eight bits, and your average photo takes up several megabytes, each of which is 8 million bits. Because each bit must be written onto a physical area of the disc, we're always seeking to increase the disc's areal density, or how many bits can be squeezed into one square inch. The areal density of a modern hard drive is about 600 gigabits per square inch, 300 million times greater than that of IBM's first hard drive from 1957. This amazing advance in storage capacity wasn't just a matter of making everything smaller, but involved multiple innovations. A technique called the thin film lithography process allowed engineers to shrink the reader and writer. And despite its size, the reader became more sensitive by taking advantage of new discoveries in magnetic and quantum properties of matter. Bits could also be packed closer together thanks to mathematical algorithms that filter out noise from magnetic interference, and find the most likely bit sequences from each chunk of read-back signal. And thermal expansion control of the head, enabled by placing a heater under the magnetic writer, allowed it to fly less than five nanometers above the disc's surface, about the width of two strands of DNA. For the past several decades, the exponential growth in computer storage capacity and processing power has followed a pattern known as Moore's Law, which, in 1975, predicted that information density would double every two years. But at around 100 gigabits per square inch, shrinking the magnetic grains further or cramming them closer together posed a new risk called the superparamagnetic effect. When a magnetic grain volume is too small, its magnetization is easily disturbed by heat energy and can cause bits to switch unintentionally, leading to data loss. Scientists resolved this limitation in a remarkably simple way: by changing the direction of recording from longitudinal to perpendicular, allowing areal density to approach one terabit per square inch. Recently, the potential limit has been increased yet again through heat assisted magnetic recording. This uses an even more thermally stable recording medium, whose magnetic resistance is momentarily reduced by heating up a particular spot with a laser and allowing data to be written. And while those drives are currently in the prototype stage, scientists already have the next potential trick up their sleeves: bit-patterned media, where bit locations are arranged in separate, nano-sized structures, potentially allowing for areal densities of twenty terabits per square inch or more. So it's thanks to the combined efforts of generations of engineers, material scientists, and quantum physicists that this tool of incredible power and precision can spin in the palm of your hand.
想象一架飞机以离地面1毫米的高度飞行, 每25秒绕地球一圈, 还能覆盖每一寸表面。 再将其缩小成手掌大小, 你就会得到和现代硬盘差不多的东西, 它所包含的信息比你们当地图书馆还要多。 那么它是如何在这么小的空间 储存这么多的信息呢? 在每个硬盘的中心都有 大量高速旋转的磁盘, 每个磁盘的表面都有高速扫过的 记录磁头。 每个磁盘上都覆盖着一层薄薄的 微小的磁化金属粒, 数据以一种肉眼无法分辨的形式存在。 很多组微小颗粒形成的磁化图案 记录形成了数据。 每一组,又称之为比特(bit), 所有微粒都按照自身的磁性排列 形成两种状态之一, 对应0或者1。 将比特信息通过电磁铁 转换成电流,数据就能 被读写在硬盘上。 这块磁铁会产生一个强大磁场, 足以改变金属微粒的磁性。 当信息写入磁盘, 驱动使用磁读取器 将其还原成有意义的形式, 类似于留声机针将唱片纹路转化成音乐。 但是你是怎么从0和1中 得到这么多信息的呢? 其实是将很多很多个0和1组合在一起。 例如,一个字节(byte), 即8比特可以代表一个字母, 你平均每张相片有好几兆字节, 每一兆字节相当于800万比特。 由于每一比特必须写在磁盘的实体表面上, 所以我们总在寻求方法增加磁盘磁录密度, 或者说是增加每平方厘米能塞下的比特数。 现代硬盘的磁录密度大约是 每平方厘米93千兆比特, 是1957年IBM第一款硬盘的3亿倍。 储存容量的巨大提升 不仅仅是归因于将所有东西缩小, 而是包含了许多项创新技术。 一种称之为薄膜光刻的技术 使得工程师们可以缩小读写器。 除了尺寸,利用物质磁性和 量子特性上的新发现 可以让读取器变得更加敏感。 数学算法的出现可以让比特 被更紧凑地排列在一起 能过滤电磁干扰产生的噪音, 并且能从大量回读信息中 找到最有可能的比特顺序。 磁头热膨胀的控制是通过 在磁性记录器下面放上一个加热器, 使其能悬于磁盘表面5纳米以内, 大约是两条DNA链的宽度。 在过去的数十年, 电脑储存容量及性能的大幅度增长 遵循着一种模式,称为“摩尔定律”, 这一定律于1975年预测 信息密度每两年会增长一倍。 但是若每平方厘米超过15.5千兆, 继续缩小磁性颗粒, 或者将它们塞得更紧, 则会导致“超顺磁效应”。 即当磁粒体积过小, 它的磁性很容易受到热能干扰, 导致比特的朝向发生混乱, 从而引起数据丢失。 科学家们采用了一种 非常简单的方法解决了这个问题: 将磁记录方向由水平改为垂直, 这使得磁录密度增加到接近 每平方厘米0.155太(1000千兆)字节。 最近,通过热辅助磁记录技术, 磁录密度又提升了。 这种技术采用了一种热稳定记录介质, 通过在局部进行激光加热 来短暂减小磁阻力, 从而实现写入数据。 尽管这些驱动磁盘还处于原型阶段, 科学家们已经又玩出了新花样: 位元规则媒介, 比特对应的位置被安置于独立的 纳米大小的结构, 潜在地实现了磁录密度 至每平方厘米3.1太字节, 甚至更多。 多亏了一代又一代工程师, 材料科学家, 还有量子物理学家们的共同努力, 这个拥有不可思议的能量, 无比精确的小工具 才能在你手掌中旋转。