Around 1159 A.D., a mathematician called Bhaskara the Learned sketched a design for a wheel containing curved reservoirs of mercury. He reasoned that as the wheels spun, the mercury would flow to the bottom of each reservoir, leaving one side of the wheel perpetually heavier than the other. The imbalance would keep the wheel turning forever. Bhaskara's drawing was one of the earliest designs for a perpetual motion machine, a device that can do work indefinitely without any external energy source. Imagine a windmill that produced the breeze it needed to keep rotating. Or a lightbulb whose glow provided its own electricity. These devices have captured many inventors' imaginations because they could transform our relationship with energy. For example, if you could build a perpetual motion machine that included humans as part of its perfectly efficient system, it could sustain life indefinitely. There's just one problem. They don't work. Ideas for perpetual motion machines all violate one or more fundamental laws of thermodynamics, the branch of physics that describes the relationship between different forms of energy. The first law of thermodynamics says that energy can't be created or destroyed. You can't get out more energy than you put in. That rules out a useful perpetual motion machine right away because a machine could only ever produce as much energy as it consumed. There wouldn't be any left over to power a car or charge a phone. But what if you just wanted the machine to keep itself moving? Inventors have proposed plenty of ideas. Several of these have been variations on Bhaskara's over-balanced wheel with rolling balls or weights on swinging arms. None of them work. The moving parts that make one side of the wheel heavier also shift its center of mass downward below the axle. With a low center of mass, the wheel just swings back and forth like a pendulum, then stops. What about a different approach? In the 17th century, Robert Boyle came up with an idea for a self-watering pot. He theorized that capillary action, the attraction between liquids and surfaces that pulls water through thin tubes, might keep the water cycling around the bowl. But if the capillary action is strong enough to overcome gravity and draw the water up, it would also prevent it from falling back into the bowl. Then there are versions with magnets, like this set of ramps. The ball is supposed to be pulled upwards by the magnet at the top, fall back down through the hole, and repeat the cycle. This one fails because like the self-watering pot, the magnet would simply hold the ball at the top. Even if it somehow did keep moving, the magnet's strength would degrade over time and eventually stop working. For each of these machines to keep moving, they'd have to create some extra energy to nudge the system past its stopping point, breaking the first law of thermodynamics. There are ones that seem to keep going, but in reality, they invariably turn out to be drawing energy from some external source. Even if engineers could somehow design a machine that didn't violate the first law of thermodynamics, it still wouldn't work in the real world because of the second law. The second law of thermodynamics tells us that energy tends to spread out through processes like friction. Any real machine would have moving parts or interactions with air or liquid molecules that would generate tiny amounts of friction and heat, even in a vacuum. That heat is energy escaping, and it would keep leeching out, reducing the energy available to move the system itself until the machine inevitably stopped. So far, these two laws of thermodynamics have stymied every idea for perpetual motion and the dreams of perfectly efficient energy generation they imply. Yet it's hard to conclusively say we'll never discover a perpetual motion machine because there's still so much we don't understand about the universe. Perhaps we'll find new exotic forms of matter that'll force us to revisit the laws of thermodynamics. Or maybe there's perpetual motion on tiny quantum scales. What we can be reasonably sure about is that we'll never stop looking. For now, the one thing that seems truly perpetual is our search.
在公元 1159 年左右 印度一位名為巴斯卡拉的數學大師 草擬一個輪子 輪內有曲線凹槽盛裝水銀 他解釋當輪子旋轉時 水銀會流向每個凹槽的底部 這樣會讓輪子的一端永遠比另一端重 此不平衡現象會使輪子不停轉動 巴斯卡拉的設計圖 是永動機的最早雛形 在沒有任何外力協助下 機器能不斷運轉 想像風車製造使其不停轉動的微風 電燈泡自給自足發光的電力 這些裝置激發許多發明家的想像 因為這能改變人類與能源的關係 例如,若你能創造一個永動機 包含人類作為此完美 節能系統中的一部分 生命可能因此永續存在 但是有一個問題存在 就是這種機器行不通 永動機的想法 全都違反超過一項的熱力學基本定律 那是物理學的一支 解釋不同能量型態之間的關係 熱力學第一定律提到 能源無法被創造或毀滅 輸出的能量必定不多於輸入的能量 此定律立即排除永動機的運作 因一台機器最多能產生它消耗的能量 沒有多餘能量啟動車子或給手機充電 若只想讓機器不斷自行運轉呢 發明家提出許多想法 其中有幾個是巴斯卡拉 不平衡輪子的變體 水銀換成滾動球體或擺動的載重力臂 全起不了作用 移動的部分加重輪子的一端 也使輪子質量中心下墜低於輪軸 低質量中心 讓輪子像鐘擺般來回擺動 隨後停止 如果採取不同的方式呢 17 世紀的愛爾蘭 自然哲學家羅伯·鮑爾 提出一個自給水壺的想法 他解釋毛細管作用 即液體與管子表面的吸力 把水吸進細管裡 或許可讓水在容器循環流動 但若毛細管作用大到足以克服引力 將水吸引上來 同樣能阻止水注入容器裡 另個情況則運用磁鐵 就像這條斜坡 頂端的磁鐵把球吸上來 然後球掉進洞裡 重複循環 這想法也失敗,原因如同自給水壺 球就被磁鐵吸附在上面 就算球真的循環運動 磁力會愈趨減弱 最後停止運作 想讓這類機器持續運作 則必須製造額外的能量 促使整個運作系統超越停止點 打破熱力學的第一個定律 能量看似持續運作 但實際上它們總是 從一些外部來源獲取能量 就算工程師能設計一個機器 不違背熱力學第一定律 因第二定律這在現實中仍行不通 熱力學第二定律 說能量往往通過摩擦等過程而散失 任何真實機器都會有移動的部件 或與空氣和水分子產生交互作用 這會產生小量的摩擦與熱能 即便在真空狀態也是如此 熱能就是能量散逸 且持續散出 這會降低可供系統運作的能量 直到機器不可避免地停止 截至目前,這兩項熱力學定律 阻礙與永動機相關的任何想法 及所蘊含的完美節能美夢 是否永遠無法發現永動機則難以定論 因人類對宇宙的了解仍極為有限 也許人類會發現全新的運作法則 迫使我們重新審視熱力學定律 又或許永動運作 存在於渺小的量子尺度 唯一能確定的是人類不會停止探索 現在唯一確知的永動