【诺奖得主Wilczek科普专栏】宇宙弹球的启示｜蔻享学术

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2022-11-08 02:11

■ 作者：Frank Wilczek

■ 翻译：胡风、梁丁当

FrankWilczek

弗兰克·维尔切克是麻省理工学院物理学教授、量子色动力学的奠基人之一。因发现了量子色动力学的渐近自由现象，他在2004年获得了诺贝尔物理学奖。

中文版

行星的倾斜与旋转揭示了太阳系复杂混乱的历史，了解这段历史可以让我们更好地认识人类在宇宙中的位置。

当9月秋分过后，在远离赤道的地方，人们开始觉察到昼夜长短的变化。在北半球的我，有更多的时间徜徉夜空繁星，在由行星倾斜和偶尔交汇的天体轨迹铸就的看台上，思索宇宙的韵律。

太阳系中每个行星都沿着同样的方向绕太阳公转，运行轨道几乎都在黄道面上。它们的自转方向一般与公转方向相同。金星是个例外。计算表明，在强大的太阳潮汐力的影响下，金星的自转方向可能在某个时刻发生了“翻转”。

太阳和几乎所有行星的自转轴都接近垂直于黄道面。天王星又是一个令人瞩目的异类：它的自转轴是躺平在黄道面上的，所以天王星基本上是滚动着围绕太阳运行。

为什么大部分行星的公转和自转的方向是一样的？

这与太阳系是如何形成的有关。太阳系形成于一团气体云。这团云或许有些像哈勃空间望远镜和韦布空间望远镜探测到的那些令人惊叹的恒星孵化场。当云逐渐凝结的时候，角动量守恒效应变得突出，越靠近中心的部分旋转越快，就像龙卷风或者浴缸放水时形成的漩涡那样。从云中凝结出来的物体会从原始云团继承一部分旋转性。

和地球一样，天王星也绕着一个轴自转，“一天”的时间约为17小时。但是天王星的自转轴倾斜度很大，因此它的昼夜更替和季节变换与地球的规律截然不同。

冬至的时候，天王星的自转轴几乎正对着太阳，导致面向太阳的半球整个夏季几乎都是极昼，另一半球整个冬天几乎都是极夜。在靠近两极的地方，一个夏日（从日照的意义上讲）就长达42个地球年，也就是天王星年的一半。随着漫长的极地日逐渐过去，太阳缓缓地向地平线落下。

显然，如果地球也像天王星一样倾斜，地球上的生态将变得很不一样，温血动物的生存将会非常艰难。

可为什么天王星如此不同呢？

太阳系的形成是一个混乱的过程。物质在碰撞过程中发生聚集，进而形成行星。早期，小的天体之间频繁发生碰撞。但是随着时间的推移，天体越来越大，碰撞次数也越来越少。碰撞虽然是随机的，却会产生剧烈而持久的影响。天文学家通过计算发现，如果一个天王星的原型与一个质量约为地球两倍的天体发生碰撞，可以导致它倾斜到如今的角度。

地球也曾经历过一次大碰撞。通过对月球成分的分析，有令人信服的证据表明，月球可能是地球的原型与火星大小的天体碰撞后从地球剥离出来的。这次碰撞形成了今天的地球，具有恰到好处的23度倾斜，和较为温和的四季。只要碰撞稍微有些不同，后果就会大不一样。

早期太阳系的混乱风暴大多已平息，但并没有完全结束。（问问恐龙就知道了！）随着科学的进步，人类现在能够防御、甚至消除许多潜在的碰撞危险。

今年9月26日（当地时间），NASA实施了DART任务，将一艘半吨重的航天器故意撞向一颗较小（50亿吨）的小行星，以测试我们改变其轨道的能力。

天文学给我们上了一堂好的哲学课。天王星和月球的存在，无情地嘲讽了人类宣扬的“万物理论”的矫情和不切实际。无论是历史的还是物理的“万物理论”，都认为我们可以完全地掌握这个复杂、混乱的宇宙。

我们不能。但是科学的理解和明智的决策可以帮助我们在与命运的抗争中至少取得一些胜局。

英文版

With the passing of the autumn equinox late last month, those of us who live far from the equator feel thechange in the balance of day and night. In the northern hemisphere, where I am, there’s more occasion to contemplate the night sky and to ponder cosmic rhythms set by the tilt of planets and the sometimes-colliding paths of celestial objects.

The orbits of all the planets in our solar system lie very nearly in the same plane, called the ecliptic, and they allproceed in the same direction around the sun. They tend to spin in the direction of their orbits. (Venus is one exception; calculations show it may have “flflipped” at some point due to strong tidal forces from the sun.) Almost all the planets and the sun also spin around axes that are nearly perpendicular to the ecliptic. The glaring outlier is Uranus, which is essentially spinning on its side with its axis in the ecliptic, a profound anomaly within the solar system.

Why do almost all the revolutions and spins go the same way? It is because of how the solar system formed. It started as a gas cloud, probably resembling the stellar hatcheries captured in stunning images from the Hubble and Webb telescopes. As the cloud condensed, its angular momentum got focused. As in tornadoes or draining bathtub whirlpools, we fifind rapid spinning near the middle. The objects that condensed out of the cloud inherited parts of that overall whirling motion.

Uranus spins around an axis just as Earth does, and the length of its rotational “day” is not so difffferent, about 17 hours. But because of its big tilt, the daily and seasonal rhythms on Uranus follow a difffferent, more drastic pattern. At the solstices, the axis of rotation points directly at the sun. All of summer is sunlight, all of winter is darkness. Near the poles, a single summer day (in the sense of sunlight) lasts 42 Earth years, which is half the Uranian year. As that long polar day wears on, the sun sinks ever so slowly toward the horizon.

Clearly, life on Earth would be drastically difffferent-and, for warm-blooded animals, much tougher-if Earth were as tilted as Uranus.

But why is Uranus difffferent? The formation of the solar system was a chaotic affffair. The planets were formed by the aggregation of material through collisions. Early on, there were frequent collisions among small bodies, but as time went on, we’ve had larger bodies and fewer collisions. Those events are essentially random, yet they can have drastic long-term effffects. Astronomers have calculated that a collision between proto-Uranus and a body with about twice the mass of Earth could have jolted its axis into its present tilt.

Earth once had a big collision, too. There is convincing evidence from the composition of our moon that it was ripped out of proto-Earth, likely by a collision with a Mars-sized body. Earth emerged from this encounter with a not-drastic tilt of 23 degrees and relatively benign seasons. That outcome might easily have been difffferent.

The chaotic violence of the early solar system has calmed considerably, but it’s not over yet. (Ask the dinosaurs!) Thanks to science, humankind is now able to watch for, and possibly defuse, many potential sources of impact. On Sept. 26, NASA’s DART mission deliberately crashed a 1,200-pound spacecraft into a relatively small (11 billion-pound) asteroid, to test our ability to nudge its orbit.

Astronomy teaches philosophy. Uranus and the moon mock the human pretension of “theories of everything,” whether historical or physical, that suggest we can fully master our complex, chaotic universe. We can’t, but scientifific understanding and wise decision-making can help us to score at least some victories over fate.

本文2022年10月01日发表于微信公众号 蔻享学术《【诺奖得主Wilczek科普专栏】宇宙弹球的启示》，风云之声获授权转载。