I bet you must have had such an experience: on a bored afternoon, you were sitting in a cafe, watching the light rain outside the window, and the stirring stick in your hand unconsciously drew a circle in the cup. The blending of coffee and milk has formed a constantly changing spiral picture under your stirring. You may think it's just a boring moment, but you know what? This humble vortex before your eyes is actually the epitome of vortex, one of the most grand and mysterious laws of the universe.

We live in a world full of rotation. From the drainage vortex formed when the bathtub is drained, to the typhoon hovering over the ocean to accumulate energy, and then to the vast universe, galaxies are rotated into beautiful spiral arms under the action of gravity, and even your DNA double helix structure follows the law of "rotation" without exception. Eddy current, a seemingly simple physical phenomenon, contains mathematical and physical principles, which are far more complicated and profound than we thought.

In fluid mechanics, vortex is defined as a region with rotating motion in fluid. It is not only visual aesthetics, but also the key to energy transfer and material mixing. For example, when an airplane flies over the sky, there will be strong tip vortices at the end of its wing, which can last for several minutes, posing a potential danger to the following airplane. Engineers study and simulate these vortices to design more efficient and safer aircraft. Similarly, in the ocean, huge ocean currents and eddies play the role of "planetary mixers", which transport heat and nutrients from one sea area to another, and have a profound impact on the global climate and ecosystem.

You may ask, why is rotation everywhere? This goes back to an ancient physics principle: conservation of angular momentum. Imagine a figure skater. When she pulls her arms back, her rotation speed will increase dramatically. This is because her total angular momentum (rotating inertia multiplied by rotating speed) must remain constant, and when her inertia decreases, her speed must increase. This principle also applies on the cosmic scale. When the giant nebula collapses under the action of gravity to form a star, in order to keep the conservation of angular momentum, the collapsed matter will begin to accelerate its rotation and form a disk-shaped structure around the star, which is the cradle of the birth of the planet.

From the vortex in the coffee cup to the galaxy in the deep space, the vortex exists in various forms, connecting the macro and micro worlds. It tells us that even the most inconspicuous daily phenomenon may hide the deepest mystery of the universe. Next time, when you feel bored, you might as well stare at the coffee in your hand, or observe the smoke in the air, and try to understand the running rules of the world from the rotation. You will find that boredom is no longer boredom, it is more like a door leading to a vast universe full of miracles and unsolved mysteries. So, don't underestimate the little whirlpool in your cup. It may be the closest daily connection between you and the mysteries of the universe.

（Writer：Ganny）