“Earth’s core is a solid iron sphere surrounded by molten iron and nickel. The churning movements of that liquid metal turn the entire planet into a giant bar magnet. Its magnetic field can be depicted in the style of a school textbook: Lines emerge near the south pole, curve around the globe, and renter near the north pole. This geomagnetic field is always present. It doesn’t change across the day or through the seasons. It’s not affected by weather or obstacles. Consequently, it is a boon for travelers, who can always use it to establish their bearings. Humans have done so for more than a thousand years, using compasses. Other animals- sea turtles, spiny lobsters, songbirds, and many others have done so for millions of years, without help.
Their ability, known as magnetoreception, allows them to navigate even when celestial bodies are obscured by clouds or darkness, when large landmarks are wreathed in fog or murk, and when the skies and oceans are devoid of telltale scents…”*
地核是流質的鐵水及䤼液所包裹的實心鐵。隨著自𨍭，地球就變成一個巨大的棒狀磁鐵。就像教科書上畫的，磁力線從南極出發，沿著地表的曲線往北，在北極附近再進入地球。這個地理磁場一直存在著，日日夜夜如此，也不隨著季節而改變。它不受天氣或地面阻障的影響。地磁是對旅人的恩賜，讓他們可以掌握自己所在的方位。人們已經利用這個功能，透過羅盤超過一千年。其它的動物，諸如海龜、刺鰲蝦(龍蝦)、鳴禽等等及其它很多生物，也已經自行利用地磁超過好幾百萬年。
這種能力，叫做「磁感應」(magnetoreception)，讓動物在空中的身體順利導航，縱使有雲的遮蔽或在暗夜之中，或地標被霧或濃雲所圍繞，或天空、海中聞不到熟悉的氣味⋯⋯
“At the time of writing, magnetoreception remains the only sense without a known sensor…Researchers have amassed many important clues about their identity and whereabouts but also several false leads. And without knowing for sure what these receptors are, or even where they are, it is fiendishly difficult to know how they might work. There are, however, three plausible ideas.”
截至本書出版為止，我們對於「磁感應」的感應器依然一無所知。至今，倒是有三個值得注意的想法。
“The first involves a magnetic iron mineral known as magnetite. In the 1970s, scientists discovered that some bacteria turn themselves into living compass needles by growing chains of magnetite crystals inside their cells. When these microbes are shaken, they tend to swim either north or south. Animals could theoretically build their own magnetite compasses, too. Imagine a magnetite needle that’s tethered to a sensory cell. As the animal turns, the needle tugs upon its tether. The cell registers that tension and triggers a nervous signal. In this way, cells could turn an abstract magnetic stimulus into something more tangible a physical yank….”
第一種想法，是動物身體內有某種磁鐵礦(megnetite)。
這些磁鐵礦在震動的時候，形成類似羅盤指北針的效果，連結到感覺神經，拉扯身體進而調整姿勢、因此決定了方向。
“The second hypothesis for how magnetoreceptors could work involves a phenomenon called electromagnetic induction, which mostly applies to sharks and rays. As a shark swims, it induces weak electric currents in the surrounding water, and the strength of those currents changes depending on the shark’s angle relative to the geomagnetic field. By sensing these tiny variations with the electroreceptors…, the shark could potentially determine its heading….”
第二種想法，是電磁感應(electromagnetic induction)。
譬如鯊魚(和魟魚)在游動的時候，身旁的水中會引發弱電流，而這個弱電流的強度會隨著游動方向與地磁的角度而改變。從弱電流強度的改變，鯊魚(和魟魚)就可能由此決定要游往的方向。
“The third explanation for magnetoreception is the most complicated, but also the one that has gained the most momentum. It involves two molecules known as a radical pair, whose chemical reaction can be influenced by magnetic fields. To understand this deeply, you must delve into the strange realm of quantum physics. But to understand it well enough, you need only to imagine that the two molecules are dancing. Light triggers the dance, cuing the partners to take hold of each other. Once in this excited state, they can be affected by magnetic fields, which alter the tempo of their dance, and thus its final steps. The partners’ final positions offer a record of the magnetic fields that shaped their previous movements. Through their dance, the radical pair transforms a magnetic stimulus that is hard to detect into a chemical stimulus that is simple to assess.”
第三種想法，是自由基對(radical pair)。
這是量子物理的範疇。想像一對自由基分子的共舞，受到光的驅動，在激發的狀態下受到磁場的影響，進一步影響到自由基分子對相對的位置，動物因此可以把磁力轉化為生化反應，也就有了方向感。
“I wonder if this is partly why the radical pair explanation has gained so much traction. Complicated though it is, it brings magnetoreception into the realm of vision, a sense that we can readily appreciate. Similarly, we talk of compasses because they offer a familiar gateway into the abstract world of magnetism. But the compass metaphor can be misleading. Compasses are precise and dependable. They have to point north, and they cannot waver. But Sonke Johnsen, Ken Loh-mann, and Eric Warrant suspect that biological compasses are inherently noisy. That is, it might be impossible for them to instantly get a precise, accurate read on Earth’s magnetic field because that field is so weak. Animals might have to keep a running average of the signals from their magnetoreceptors over long periods of time. This limitation makes magnetoreception slow, cumbersome, and deeply paradoxical. It detects one of the most pervasive and reliable stimuli on the planet–the geomagnetic field- but does so in an inherently unreliable way. This might explain why so many magnetoreception studies have been hard to replicate…”
「自由基對」的想法之所以那麼令人矚目，可能是該想法把「磁感應」變得具像化，就像羅盤的功能一樣，把抽象的磁力變得具體可靠的現象。
但是有專家們懷疑，生化的羅盤本質上是充滿雜訊的。地磁的強度是如此的微弱，生物要即時偵測到精確的磁力，是困難的，它們可能是在某個平均磁力水準上下，透過長時間不斷的調整，才逐漸獲得正確的方位。
這個限制，使得「磁感應」變得是緩慢的、麻煩的、甚至是看來是互相予盾的能力。「磁感應」雖然能偵測到無所不在的、可靠的地磁，但本質上卻是一種不太可靠的能力。這也解釋了為什麼有很多「磁感應」的研究，都無法再次重覆進行一樣的實驗而得到相同的結果。
“The noisy and erratic nature of magnetoreception might also explain why no animal relies on it alone. Instead, they seem to use it as a backup sense in case more reliable ones like vision fail. “If you’re a migrating animal, magnetoreception is probably the least important sense, unless you’re completely lost,” Keays says. In the absence of magnetic cues, bogong moths can still navigate by looking at the pattern of stars in the night sky. Turtle hatchlings ignore magnetic fields when they first enter the water and use the direction of the waves to guide them out to sea.
Animals never use a single sense exclusively. “They use every damn piece of information they can get their hands on,” Warrant tells me. “They are multisensory in every possible way.”
「磁感應」充滿雜訊及易變的性質，解釋了為什麼沒有動物只單一倚賴這種感應能。反而，似乎動物是在其他更可靠的感應能力，譬如視覺，只有無法運作的時候，才靠備用的「磁感應」。
對於遷徙性的動物而言，「磁感應」也許是最不重要的一種感應。
縱使沒有磁力的線索，布岡夜蛾在晚上也可以透過星座的位置來導航。
幼龜剛出生來到水裏，並不透過磁場來決定方向。它們離水回到陸地，則是依照海浪前進的方向而定。
動物從來不倚靠單一一種感應能力。它們會善用各種感官所蒐集到手的各種資訊。
*：ED Yong, “An Immense World,” 2022, Random House/New York
2023/11/2 They know the way, Magnetic Fuelds Damakey
