【诺奖得主Wilczek科普专栏】宇宙弹球的启示

作者 | Frank Wilczek

With the passing of the autumn equinox late last month, those of us who live far from the equator feel the change 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.