諾獎得主Wilczek：“眼前”的量子力學_風聞
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作者 | Frank Wilczek
翻譯 | 胡風、梁丁當
中文版
提起“ 量子力學 ”，人們常常會想起很多神奇的悖論。確實，量子力學總是看上去高深莫測、遙不可及。但有時，就像我的好朋友、物理學家悉尼 · 科爾曼 (Sidney Coleman) 在美國哈佛大學的一次著名演講中提到的：量子物理學就“在你眼前”。
我們能聽見聲音，是因為耳朵裏的鼓膜（也稱耳膜）感受到了壓力波，也就是聲波。人耳有着精妙的傳輸機械振動的通道，聲波通過外耳道抵達中耳的鼓膜並引起振動。這兩個耳膜就像一對反向鋼琴（與鋼琴通過手指“無聲”地敲擊琴鍵發出聲音相對）的“琴鍵”，聲波則是琴鍵上飛舞的手指！神經元隨着琴鍵的跳動產生響應，向大腦發送電信號。我們的大腦再將這些信號轉化為音樂、言語或者其他的聲音。在這個過程中，有兩點值得注意。首先，人耳能夠天然地把接收到的聲波分解，形成聲譜。然而，直到19世紀，數學家才建立了能實現類似操作的數學方程，也就是傅立葉分析。
人耳的這項功能與光譜儀（分光儀）有異曲同工之處。光譜儀能把光分解為光譜，它有很多類型，從牛頓稜鏡到各種複雜的現代儀器，但人眼不在其中。
其次，耳朵對聲音的響應是分級的 ：某個音調越響，相應琴鍵的振動就越強。這就像在彈鋼琴時，手指觸鍵的力度決定了琴聲是更響亮還是更柔和。與此形成鮮明對比的是羽管鍵琴（鋼琴的前身），彈奏這種樂器時，羽管會以恆定的力度撥奏琴絃，因此手指觸鍵的力度對琴聲的影響甚微，音量也不會變化。
與聽覺相比，視覺的形成在以上兩個方面都非常不同。首先，光波的頻率超出了所有生物機械結構的接收範圍，因此人眼無法像耳朵分解聲音頻譜那樣分解光的頻譜。
我們的眼睛能感知光，本質上是因為光是由一份一份的能量——光子構成的，這可以觸發分子形狀的變化。從這裏開始，我們要講到量子物理。
對大多數人而言，彩色視覺需要依靠視網膜內三種視錐細胞中的受體蛋白質——視蛋白。當視蛋白感受到光子時，它的形狀要麼改變，要麼不變，這個反應是開關式的，不是分級的。而根據量子力學，這種形變的發生與否還是概率性的。也就是説，我們無法準確地預測某個光子是否會觸發特定的視蛋白形變，但能夠知道它發生的概率。這個概率取決於光子的波長——光的顏色——以及觸發的視蛋白種類。
如果説聽覺的形成像是在反向鋼琴上層次豐富地演奏，那麼視覺神經元就是通過只有三個琴鍵且調音不準的羽管鍵琴來“看”這個世界。
由於不同的光子組合可能產生相同的概率模式，許多物理上不同的光源模式會使人眼感知到相同的顏色。從這個角度來看，我們所有人其實都是嚴重的色盲患者。
當光線昏暗時，光子的量子性迫使我們的視覺感知進入另一個極限。在只有幾個光子時，基於視錐細胞的感知模式不再可靠，人眼將切換到基於視杆細胞的夜視模式。這種模式下的羽管鍵琴只有一個琴鍵，因此我們只能通過該琴鍵的觸發頻率感知到不同程度的灰色。
可以説，量子力學從根本上限制了我們的視覺感知能力。然而，當巨量的外界信息噴湧而至，即使我們只能感知其中的一小部分，也足以讓我們的大腦加工出一部美輪美奐的電影。是的，量子力學既不遙遠也不怪異，它就“在你的眼前”——更確切地説，它就在你的視網膜上。
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.
Frank Wilczek
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