【诺奖得主Wilczek科普专栏】“眼前”的量子力学

■ 作者

Frank Wilczek

■ 翻译

胡风、梁丁当

Frank Wilczek

中文版

眼睛能感知颜色，是因为光量子不可预测的振动改变了分子的形态。

提起“ 量子力学 ”，人们常常会想起很多神奇的悖论。确实，量子力学总是看上去高深莫测、遥不可及。但有时，就像我的好朋友、物理学家悉尼 · 科尔曼 (Sidney Coleman) 在美国哈佛大学的一次著名演讲中提到的：量子物理学就“在你眼前”。

我们能听见声音，是因为耳朵里的鼓膜（也称耳膜）感受到了压力波，也就是声波。人耳有着精妙的传输机械振动的通道，声波通过外耳道抵达中耳的鼓膜并引起振动。这两个耳膜就像一对反向钢琴（与钢琴通过手指“无声”地敲击琴键发出声音相对）的“琴键”，声波则是琴键上飞舞的手指！神经元随着琴键的跳动产生响应，向大脑发送电信号。我们的大脑再将这些信号转化为音乐、言语或者其他的声音。在这个过程中，有两点值得注意。首先，人耳能够天然地把接收到的声波分解，形成声谱。然而，直到19世纪，数学家才建立了能实现类似操作的数学方程，也就是傅立叶分析。

人耳的这项功能与光谱仪（分光仪）有异曲同工之处。光谱仪能把光分解为光谱，它有很多类型，从牛顿棱镜到各种复杂的现代仪器，但人眼不在其中。

其次，耳朵对声音的响应是分级的：某个音调越响，相应琴键的振动就越强。这就像在弹钢琴时，手指触键的力度决定了琴声是更响亮还是更柔和。与此形成鲜明对比的是羽管键琴（钢琴的前身），弹奏这种乐器时，羽管会以恒定的力度拨奏琴弦，因此手指触键的力度对琴声的影响甚微，音量也不会变化。

与听觉相比，视觉的形成在以上两个方面都非常不同。首先，光波的频率超出了所有生物机械结构的接收范围，因此人眼无法像耳朵分解声音频谱那样分解光的频谱。

我们的眼睛能感知光，本质上是因为光是由一份一份的能量——光子构成的，这可以触发分子形状的变化。从这里开始，我们要讲到量子物理。

对大多数人而言，彩色视觉需要依靠视网膜内三种视锥细胞中的受体蛋白质——视蛋白。当视蛋白感受到光子时，它的形状要么改变，要么不变，这个反应是开关式的，不是分级的。而根据量子力学，这种形变的发生与否还是概率性的。也就是说，我们无法准确地预测某个光子是否会触发特定的视蛋白形变，但能够知道它发生的概率。这个概率取决于光子的波长——光的颜色——以及触发的视蛋白种类。

如果说听觉的形成像是在反向钢琴上层次丰富地演奏，那么视觉神经元就是通过只有三个琴键且调音不准的羽管键琴来“看”这个世界。

由于不同的光子组合可能产生相同的概率模式，许多物理上不同的光源模式会使人眼感知到相同的颜色。从这个角度来看，我们所有人其实都是严重的色盲患者。

当光线昏暗时，光子的量子性迫使我们的视觉感知进入另一个极限。在只有几个光子时，基于视锥细胞的感知模式不再可靠，人眼将切换到基于视杆细胞的夜视模式。这种模式下的羽管键琴只有一个琴键，因此我们只能通过该琴键的触发频率感知到不同程度的灰色。

可以说，量子力学从根本上限制了我们的视觉感知能力。然而，当巨量的外界信息喷涌而至，即使我们只能感知其中的一小部分，也足以让我们的大脑加工出一部美轮美奂的电影。是的，量子力学既不遥远也不怪异，它就“在你的眼前”——更确切地说，它就在你的视网膜上。

英文版

We Need Quantum Physics to See

Our eyes register color because of molecular changes caused by the unpredictable vibrations of quantum particles of light

Many people, when they encounter the words“quantum mechanics,” go on the alert for esoteric paradoxes. And there are certainly plenty of those on offer. But sometimes, as my brilliant friend the physicist Sidney Coleman put it in a famous lecture at Harvard, quantum physics is “in your face.

To hear, we sense pressure waves, commonly called sound waves, which impinge on our eardrums. Channeled through some impressive natural mechanical engineering, sound waves set off vibrations on the membranes of our inner ears. Those membranes work like the keyboards of a pair of inverse pianos: The sounds play the keys! Neurons fire in response to the keys’ motion, generating the signals that our brains interpret as music,speech or whatever.

Two things are noteworthy in this process. First, we naturally deconstruct the incoming wave pattern into its component of pure tones. Mathematicians learned how to use equations to perform that feat in the 19th century and they call it Fourier analysis. It is similar to what spectrometers, ranging from Isaac Newton’s prisms to sophisticated modern instruments—but not our eyes—do to separate light into its component frequencies.

Second, the response is graded: The louder a tone, the more forceful the motion of the corresponding key. This is like a proper piano, where the pressure on a key determines whether it gives a louder or softer response,as opposed to a harpsichord, whose strings can only be plucked at a constant volume.

Vision differs radically from hearing in both ways. Light vibrates faster than mechanical engineering can handle, but our visual apparatus can exploit the fact that it comes in packets of energy—photons—which can trigger changes in the shapes of molecules. Now we’re talking quantum theory.

For most people, color vision involves three kinds of receptor proteins in the cone cells of the retina. Photons either induce shape changes or don’t; the effect is all-or-none, not graded. And, typically for quantum mechanics, they are chancy: We can’t predict exactly whether a given photon will trigger a given receptor, but only supply odds. Those odds depend on the photon’s wavelength—that is, the color tone it represents—and which type of receptor protein is involved.

What visual neurons get to “see,” compared with the robust dynamics of the inverse piano of hearing, is more like the keyboard of a poorly tuned harpsichord with only three keys.

Since many different combinations of photons can produce the same pattern of probabilities, many physically distinct patterns of illumination produce the same color perception. In this way, we are all profoundly colorblind.

In dim light, we run into another limit of our vision, stemming from the unpredictable behavior of photons. When there are only a few photons to work with, the cone cells become unreliable, and we switch over to night vision based on different cells, the rods. The nocturnal harpsichord has only one key, so we perceive only shades of gray, lighter or darker, according to how frequently that key triggers.

Fundamental limitations of vision follow from its reliance on quantum processes. Yet such is the gush of information from the external world that even an attenuated stream supplies enough material for our brains to manufacture a splendid motion picture. Far from being remote and esoteric,quantum mechanics is very much “in your face”—in your retina, to be precise.

本文纸质版每月在《环球科学》杂志刊登，网络电子版经作者授权于2023年7月10日发布于微信公众号 蔻享学术 (【诺奖得主Wilczek科普专栏】“眼前”的量子力学)，风云之声获授权转载。