An Existential Problem in the Search for Alien Life
寻找外星生命的存在性问题

In 2020, a team of researchers found something surprising in the high clouds of Venus. Earth-based telescopes detected the spectral signature of phosphine, a simple molecule that should have no business persisting in those extremely acidic clouds. Cautiously excited, the researchers wrote that the phosphine could be the result of “unknown photochemistry or geochemistry”—or, they noted almost coyly, “possibly life.”
2020 年，一个研究小组在金星的高云层中发现了一些令人惊讶的东西。地基望远镜探测到了磷化氢的光谱特征，这种简单的分子不应该在酸性极强的云层中存在。研究人员谨慎而兴奋地写道，膦可能是 "未知光化学或地球化学 "的结果--或者，他们近乎腼腆地指出，"可能是生命"。

It was a thrilling possibility. “Signs of Life Found in the Clouds Surrounding Venus,” one headline blared; another, “Aliens Were on Venus This Whole Time?!” It was also, it turned out, a false alarm. The phosphine not only wasn’t a signal of life, but probably wasn’t even there at all, a swing-and-a-miss of data interpretation. The clouds of Venus were still, as far as anyone knew, as uninhabited as they’d always seemed.
这是一种令人兴奋的可能性。"在金星周围的云层中发现生命迹象"，一则新闻标题赫然醒目；另一则是 "外星人一直在金星上？"事实证明，这也是一场虚惊。磷化氢不仅不是生命的信号，甚至可能根本就不存在，这是数据解读的一次摇摆失误。据任何人所知，金星的云层仍然是无人居住的，一如既往。

Scientists took the false alarm in stride. Back to the drawing board, they seemed to say, shrugging—or back to their telescopes at least. This is how science works, after all: gradually, in small steps, in announcements and skepticism and reconsideration of the data. It’s even harder when the subject of study is extraterrestrial. Venus is the closest possible home to alien life, but there’s no way to go and scoop a sample of its atmosphere to put under a telescope. The search for alien life is done remotely, by interpretation and inference. The suspected phosphine—considered a “biosignature” by astrophysicists, because phosphine on Earth is only ever abundant when it’s a product of life—wasn’t even observed directly. Instead, the researchers detected it by analyzing wavelengths of light that could hint at what molecules might be in Venus’s atmosphere. The researchers were searching for a sign of a sign of life. There was a lot of room for error.
科学家们对虚惊一场泰然处之。他们耸耸肩，似乎在说，回到绘图板上去吧--或者至少回到他们的望远镜上去吧。毕竟，科学就是这样工作的：循序渐进，小步快跑，在宣布、怀疑和重新考虑数据中前进。当研究对象是外星生物时，就更加困难了。金星是最接近外星生命的可能家园，但我们无法去金星采集大气样本放在望远镜下观察。寻找外星生命的工作是通过遥控、解释和推理完成的。天体物理学家认为疑似磷化氢是一种 "生物特征"，因为地球上的磷化氢只有在作为生命产物时才会大量存在。相反，研究人员是通过分析光的波长探测到它的，这些波长可以暗示金星大气中可能存在哪些分子。研究人员正在寻找生命迹象。错误的余地很大。

The search for extraterrestrial life is not the kind that is likely to yield an aha moment—not in the sense that, with the tools currently available, scientists are going to look at data brought in from the cosmos and instantly declare, “Yes, this is life.” There are too many technical hurdles, too many variables that will need time to be sorted out. And even accounting for those issues, another obstacle exists—an enduring puzzle that tests the limits of science. The fact is, we still don’t know what life is.
寻找地外生命的工作不可能产生 "啊哈时刻"--从这个意义上说，科学家们不会利用现有的工具，看着从宇宙中带来的数据，立即宣布："是的，这就是生命"。有太多的技术障碍，太多的变数需要时间来解决。即使考虑到这些问题，还有另一个障碍存在--一个考验科学极限的永恒难题。事实上，我们仍然不知道生命是什么。

Hold a rock next to a flower and you’re probably confident you know the difference. But since the days of Aristotle, scientists and philosophers have struggled to draw a precise line between what is living and what is not, often returning to criteria such as self-organization, metabolism, and reproduction but never finding a definition that includes, and excludes, all the right things. If you say life consumes fuel to sustain itself with energy, you risk including fire; if you demand the ability to reproduce, you exclude mules. NASA hasn’t been able to do better than a working definition: “Life is a self-sustaining chemical system capable of Darwinian evolution.” It’s a decent way to describe life on Earth, but it lacks practical application. If humans found something on another planet that seemed to be alive, how much time would we have to sit around and wait for it to evolve?
把一块石头和一朵花放在一起，你可能会确信自己知道它们的区别。但是，自亚里士多德时代以来，科学家和哲学家们一直在努力为什么是生命、什么不是生命划出一条精确的界限，经常回到自我组织、新陈代谢和繁殖等标准，但却从未找到一个既包括、又排除所有正确事物的定义。如果你说生命消耗燃料来维持自身的能量，你就有可能把火也包括在内；如果你要求有繁殖能力，你就会把骡子排除在外。美国国家航空航天局（NASA）一直未能给出更好的工作定义："生命是一种能够进行达尔文进化的自我维持的化学系统"。用这个定义来描述地球上的生命还不错，但缺乏实际应用。如果人类在另一个星球上发现了似乎有生命的东西，我们还有多少时间坐等它进化呢？

The problem is that, in any attempt to define life, we’re inherently constrained by human intuition and the one example we have so far that informs it. The only life we know is life on Earth. Some scientists call this the n=1 problem, where n is the number of examples from which we can generalize. We have no idea if earthly life is average in the cosmos or some sort of freak outlier. With all the varied chemistries of other planets, all the contingencies that drive evolution, all the ways that matter and energy interact—who knows how strange life on another world might be? What if life as we know it is the wrong life to be looking for?
问题在于，在试图定义生命时，我们天生就受到人类直觉和迄今为止我们所掌握的唯一例子的限制。我们所知道的唯一生命就是地球上的生命。一些科学家将此称为 n=1 问题，其中 n 是我们可以归纳出的例子的数量。我们不知道地球上的生命是宇宙中的平均值，还是某种离群的怪胎。其他星球的化学成分千差万别，推动进化的各种偶然因素，物质和能量相互作用的各种方式--谁知道另一个世界的生命会有多么奇特呢？如果我们所知的生命是我们应该寻找的错误生命呢？

What we really want is more than a definition of life. We want to know what life, fundamentally, is. For that kind of understanding, scientists turn to theories. A theory is a scientific fundamental. It not only answers questions, but frames them, opening new lines of inquiry. It explains our observations and yields predictions for future experiments to test. Consider the difference between defining gravity as “the force that makes an apple fall to the ground” and explaining it, as Newton did, as the universal attraction between all particles in the universe, proportional to the product of their masses and so on. A definition tells us what we already know; a theory changes how we understand things.
我们真正想要的不仅仅是生命的定义。我们想知道生命的本质是什么。为了获得这样的认识，科学家们求助于理论。理论是科学的基础。它不仅能回答问题，还能为问题定下框架，开辟新的研究方向。它解释了我们的观测结果，并为未来的实验测试提供了预测。将万有引力定义为 "使苹果落地的力"，与像牛顿那样将其解释为宇宙中所有粒子之间的普遍吸引力，与粒子质量的乘积成正比，两者之间的区别就在于此。定义告诉我们已经知道的东西；而理论则改变了我们对事物的理解。

In recent years, the potential rewards of unlocking a theory of life have captivated a clutch of researchers from a diverse set of disciplines. “There are certain things in life that seem very hard to explain,” Sara Imari Walker, a physicist at Arizona State University who has been at the vanguard of this work, told me. “If you scratch under the surface, I think there is some structure that suggests formalization and mathematical laws.” It has long been presumed that although theories can explain physics and chemistry, biology is too messy, too contingent, to be boiled down to math and formulas. In 1997, the renowned biologist Ernst Mayr wrote that although the molecules that compose living organisms obey the laws of physics just as all molecules do, “organisms are fundamentally different from inert matter.” There is a threshold that matter can cross, beyond which the laws of physics do not explain or predict what happens; on the other side of that threshold is life.
近年来，揭示生命理论的潜在回报吸引了来自不同学科的研究人员。"亚利桑那州立大学（Arizona State University）的物理学家萨拉-伊马里-沃克（Sara Imari Walker）是这项工作的先驱，她告诉我："生命中有些事情似乎很难解释。"如果你从表面下看，我认为有一些结构暗示着形式化和数学规律。"长期以来，人们一直认为，虽然理论可以解释物理和化学，但生物学太混乱、太偶然，无法用数学和公式来概括。1997 年，著名生物学家恩斯特-迈尔（Ernst Mayr）写道，虽然组成生物体的分子与所有分子一样遵守物理定律，但 "生物体与惰性物质有着本质区别"。"物质可以跨越一个阈值，在这个阈值之外，物理定律无法解释或预测所发生的事情；在这个阈值的另一端，就是生命。

But Walker doesn’t think about life as a biologist—or an astrobiologist—does. When she talks about signs of life, she doesn’t talk about carbon, or water, or RNA, or phosphine. She reaches for different examples: a cup, a cellphone, a chair. These objects are not alive, of course, but they’re clearly products of life. In Walker’s view, this is because of their complexity. Life brings complexity into the universe, she says, in its own being and in its products, because it has memory: in DNA, in repeating molecular reactions, in the instructions for making a chair.
但沃克并不像生物学家或天体生物学家那样思考生命。当她谈到生命迹象时，她不会谈论碳、水、核糖核酸或磷化氢。她举出不同的例子：杯子、手机、椅子。当然，这些物体并不是有生命的，但它们显然是生命的产物。在沃克看来，这是因为它们的复杂性。她说，生命给宇宙带来了复杂性，无论是其自身的存在还是其产品，因为生命有记忆：在 DNA 中，在重复的分子反应中，在制作一把椅子的指令中。

Lee Cronin, a chemistry professor at the University of Glasgow and Walker’s main collaborator, told me that when Walker first explained to him her ideas for a theory of life, “I was like, ‘I have no clue what you’re talking about. But it feels like we are saying something super similar.’” Cronin studies the origin of life, also a major interest of Walker’s, and it turned out that, when expressed in math, their ideas were essentially the same. They had both zeroed in on complexity as a hallmark of life. Cronin is devising a way to systematize and measure complexity, which he calls Assembly Theory. He measures the complexity of an object—say, a molecule—by calculating the number of steps necessary to put the object’s smallest building blocks together in that certain way. His lab has found, for example, when testing a wide range of molecules, that those with an “assembly number” above 15 were exclusively the products of life. Life makes some simpler molecules, too, but only life seems to make molecules that are so complex.
李-克罗宁是格拉斯哥大学的化学教授，也是沃克的主要合作者，他告诉我，当沃克第一次向他解释她关于生命理论的想法时，"我当时想，'我完全不知道你在说什么。但感觉我们说的东西超级相似'。克罗宁研究生命起源，这也是沃克的主要兴趣所在，结果发现，如果用数学来表达，他们的想法基本上是一样的。他们都把复杂性作为生命的标志。克罗宁正在设计一种系统化和测量复杂性的方法，他称之为 "组装理论"。他通过计算将物体的最小构件以特定方式组合在一起所需的步骤数量，来衡量物体的复杂性，比如一个分子。例如，他的实验室在测试各种分子时发现，"组装数 "超过 15 的分子完全是生命的产物。生命也会制造一些更简单的分子，但似乎只有生命才会制造如此复杂的分子。

No one expects to find an alien cellphone in Mars’s Jezero Crater. But Walker’s whole notion is that it’s not only theoretically possible but genuinely achievable to identify something smaller—much smaller—that still nonetheless simply must be the result of life. The model would, in a sense, function like biosignatures as an indication of life that could be searched for. But it would drastically improve and expand the targets. Walker would use the theory to predict what life on a given planet might look like. It would require knowing a lot about the planet—information we might have about Venus, but not yet about a distant exoplanet—but, crucially, would not depend at all on how life on Earth works, what life on Earth might do with those materials. Without the ability to divorce the search for alien life from the example of life we know, Walker thinks, a search is almost pointless. “Any small fluctuations in simple chemistry can actually drive you down really radically different evolutionary pathways,” she told me. “I can’t imagine [life] inventing the same biochemistry on two worlds.”
没有人会指望在火星的杰泽罗环形山找到外星手机。但沃克的整个想法是，不仅理论上可能，而且真正可以实现识别更小的东西--小得多的东西--但仍然必须是生命的结果。从某种意义上说，这个模型的功能就像生物特征一样，是可以搜寻到的生命迹象。但它将极大地改进和扩大目标。沃克将利用这一理论来预测某个星球上的生命可能是什么样子的。这需要我们对该行星有很多了解--我们可能对金星有了解，但对遥远的系外行星还不了解--但关键是，这完全不取决于地球上的生命如何运作，也不取决于地球上的生命可能用这些物质做些什么。沃克认为，如果不能将寻找外星生命的工作与我们已知的生命实例区分开来，那么寻找工作几乎毫无意义。"她告诉我："简单化学反应中的任何微小波动，实际上都会促使你走上截然不同的进化道路。"我无法想象[生命]会在两个世界上发明相同的生物化学"。

Devising a universal theory for life is an ambitious project, to say the least. The scientists I’ve spoken with who are leading the search for biosignatures tend to welcome Walker’s unconventional approach, on the grounds that the more tools available, the merrier. Even so, no one is abandoning their search in the hopes that humanity will soon solve the mystery of life. After all, finding any examples of alien life—Earthlike or not, by whatever means possible—would radically advance our ability to understand the phenomenon.
至少可以说，为生命设计一个通用理论是一项雄心勃勃的计划。与我交谈过的科学家中，有一些人正在牵头寻找生物特征，他们倾向于欢迎沃克的非传统方法，理由是可用的工具越多越好。即便如此，也没有人放弃寻找，希望人类能尽快解开生命之谜。毕竟，通过任何可能的手段找到任何外星生命的例子--无论是否与地球相似--都会从根本上提高我们理解生命现象的能力。

Walker’s approach is grounded in the work of, among others, the philosopher of science Carol Cleland, who wrote The Quest for a Universal Theory of Life. But Cleland doesn’t share Walker’s ambitions that a theory may be within reach; instead she warns that any theory of life, just like a definition, cannot be constrained by the one example of life we currently know. “It’s a mistake to start theorizing on the basis of a single example, even if you’re trying hard not to be Earth-centric. Because you’re going to be Earth-centric,” Cleland told me. In other words, until we find other examples of life, we won’t have enough data from which to devise a theory. Abstracting away from Earthliness isn’t a way to be agnostic, Cleland argues. It’s a way to be too abstract.
沃克的研究方法以科学哲学家卡罗尔-克莱兰（Carol Cleland）等人的研究成果为基础，她曾撰写了《探索生命的普遍理论》一书。但克莱兰并不像沃克那样雄心勃勃，认为理论可能触手可及；相反，她警告说，任何生命理论，就像定义一样，不能受限于我们目前所知的一个生命实例。"根据一个例子就开始理论化是错误的，即使你努力不以地球为中心。因为你将会以地球为中心，"克莱兰告诉我。换句话说，在我们找到其他生命实例之前，我们没有足够的数据来设计理论。克莱兰认为，抽象出地球并不是一种不可知论。而是过于抽象。

It’s a knot easy to get tied up in: We don’t have a theory of life to guide the search for extraterrestrials, but we need to find extraterrestrial life before we can understand life with a theory. Instead of provincially looking for life as we know it, Cleland calls for a more flexible search guided by what she calls “tentative criteria.” Such a search would have a sense of what we’re looking for, but also be open to anomalies that challenge our preconceptions, detections that aren’t life as we expected but aren’t familiar not-life either—neither a flower nor a rock. It’s unsatisfying if you want a firm answer or a quick one, but it speaks to the hope that exploration and discovery might truly expand our understanding of the cosmos and our own world.
这是一个很容易打上的死结：我们没有生命理论来指导寻找外星人的工作，但我们又需要先找到外星生命，然后才能用理论来理解生命。克莱兰呼吁在她所称的 "暂定标准 "的指导下进行更加灵活的搜寻，而不是按照我们的认知去寻找生命。这样的搜索既要知道我们在寻找什么，又要对挑战我们先入之见的异常现象持开放态度，这些异常现象既不是我们想象中的生命，但也不是我们熟悉的非生命--既不是一朵花，也不是一块石头。如果你想要一个确定的答案或一个快速的答案，这是不令人满意的，但它表达了探索和发现可能真正扩大我们对宇宙和我们自己的世界的理解的希望。

Cleland’s approach ends up sounding much like a lot of the work being done hunting for biosignatures today. A true discovery of phosphine in the clouds of Venus would be an anomaly for sure, and absolutely challenge preconceptions about the kinds of chemistry happening in those clouds. Other work in the field seeks similar surprises. The astrobiologist Kimberley Warren-Rhodes studies life on Earth that lives at the borders of known habitability, such as in Chile’s Atacama Desert. The point of her experiments is to better understand how life might persist—and how it might be found—on Mars. “Biology follows some rules,” she told me. The more of those rules you observe, the better sense you have of where to look on other worlds. In this light, the most immediate concern in our search for extraterrestrial life might be less that we only know about life on Earth, and more that we don’t even know that much about life on Earth in the first place. “I would say we understand about 5 percent,” Warren-Rhodes estimates of our cumulative knowledge. N=1 is a problem, and we might be at more like n=.05.
克莱兰的方法听起来很像今天正在进行的许多寻找生物特征的工作。如果真的在金星云层中发现了磷化氢，那肯定会是一个反常现象，而且绝对会挑战人们对这些云层中发生的化学反应类型的成见。该领域的其他工作也在寻求类似的惊喜。天体生物学家金伯利-沃伦-罗兹（Kimberley Warren-Rhodes）研究地球上生活在已知可居住性边界的生命，比如智利的阿塔卡马沙漠。她的实验旨在更好地了解生命如何在火星上存活，以及如何在火星上发现生命。"她告诉我："生物学遵循一些规则。你观察到的这些规则越多，你就能更好地了解在其他世界应该去哪里寻找。有鉴于此，我们在寻找地外生命时最关心的问题可能不是我们只了解地球上的生命，而是我们对地球上的生命本来就不甚了解。"沃伦-罗兹（Warren-Rhodes）估计，"我们了解的知识大概只有 5%。N=1是个问题，我们可能更像是N=.05。

When I talk with people about a theory of life, which lately is my best attempt at small talk, I reach for the theory of gravity as a familiar parallel. Someone might ask, “Okay, so in terms of gravity, where are we in terms of our understanding of life? Like, Newton?” Further back, further back, I say. Walker compares us to pre-Copernican astronomers, reliant on epicycles, little orbits within orbits, to make sense of the motion we observe in the sky. Cleland has put it in terms of chemistry, in which case we’re alchemists, not even true chemists yet. We understand so little, and we think we’re ready to find other life?
当我与人谈论生命理论时（这是我最近在闲聊中的最佳尝试），我会把万有引力理论作为一个熟悉的平行理论。有人可能会问："好吧，就万有引力而言，我们对生命的理解处于什么阶段？比如牛顿？""我说，更远了，更远了。沃克把我们比作前哥白尼时期的天文学家，依赖于 "轨道"（epicycles）、"轨道中的小轨道"（little orbits within orbits）来理解我们在天空中观察到的运动。克莱兰则把它比作化学，在这种情况下，我们是炼金术士，甚至还不是真正的化学家。我们了解得如此之少，就认为我们已经准备好去寻找其他生命了？

Maybe we’ll never be ready. Yet how could we not search? Right now, the James Webb Space Telescope is peering into exoplanet atmospheres for spectral signatures. The Perseverance rover is bottling up soil samples on Mars for a future mission to bring to Earth to study. How could we not trawl the cosmos for anything that could help us understand our place in it, our kinship lines, and how this life, on Earth, came to be? And so we try. We scour other worlds, scan their clouds. And scratch beneath the surface, for the theory that might explain it all in equation and abstraction, to see the deeper truth beneath what we can see.
也许我们永远都不会准备好。然而，我们怎能不去寻找呢？现在，詹姆斯-韦伯太空望远镜正在窥探系外行星大气层的光谱特征。毅力号漫游车正在火星上采集土壤样本，以便未来的任务将其带回地球进行研究。我们怎能不在宇宙中寻找任何能够帮助我们了解我们在其中的位置、我们的亲缘关系以及地球上的生命是如何诞生的？于是我们开始尝试。我们搜索其他世界，扫描它们的云层。在表象之下寻找理论，用方程式和抽象概念来解释这一切，从而看到我们所能看到的表象之下更深层次的真相。