Inverse Earth
This is a special world. In this world, you still have blue sky overhead and solid ground beneath your feet, seemingly no different—yet this world is completely different. The reason you haven’t noticed anything unusual is because every local region of this world appears identical to our familiar world.
However, in reality, every part of this world, even every molecule and every inch of vacuum, has properties completely different from our familiar world. Because in this world, even vacuum has mass! Moreover, vacuum has the highest density, gas has lower density, liquid even lower, and solid has the lowest density—exactly opposite to our world. These bizarre properties don’t prevent you from standing on this world’s ground. You still experience gravity, but the “gravity” you feel is actually “buoyancy” from the air—because you’re actually inside a planet!Your head points toward the planetary core, the planet’s interior is filled with high-density gas, and under the effect of buoyancy, you experience a net force equal in magnitude to Earth’s gravity. This force allows you to stand steadily on the inner surface of the solid planetary shell. When you ride this world’s “hot air balloon”—that is, a balloon filled with higher-density gas—the “hot air balloon” will carry you skyward, falling toward the planetary core. But don’t worry, you won’t fall all the way to the center. As the hot air balloon rises in altitude, the surrounding gas density increases until it eventually matches the balloon’s density, and you’ll stop descending. This world has oceans, and sometimes clouds in the sky, which float at certain heights above the ground, following similar principles to the hot air balloon.
This world’s sky and ground are inverted compared to ours, but as long as you don’t look far into the distance, you won’t notice anything unusual—because you can’t see the horizon. You’ll see the distant ground curving upward into the sky until atmospheric scattering makes the distant ground increasingly blurry, finally blending with the sky’s color. The gas at the planetary core, due to its increased density, has strong light absorption and scattering properties that intensify, causing the sky to still appear blue rather than directly showing the land on the opposite side of the hollow planet. There are even day and night here: a small sun orbits near the planetary core, powered by the rotation of the high-density gas at the center. When the small sun rotates to the far side of the core, the high-density gas at the center absorbs almost all of the sun’s light, leaving only moonlight-level brightness.
This world has strange physical laws, such as no gas can have density greater than or equal to vacuum density. It seems we cannot ride a hot air balloon to the planetary core. But curiosity drives us to ask: is there really no way to reach the center? Actually, you can use a rocket’s thrust to travel there. As the rocket gets farther from the surface and gradually approaches the center, the air experiences gravitational forces from all directions that cancel each other out, resulting in decreasing net force. The buoyancy you experience also decreases, and eventually you’ll be in a weightless state, just like in our space.
After taking a rocket to the planetary core, we can also go outside the planet. You must always remember that in this world, solids have the lowest density, but solid materials are still hard. Fortunately, we have modern excavation tools. Since air flows into the excavated tunnel, you can still feel gravity (actually buoyancy), feeling as if you’re in a very deep mine shaft. But as excavation continues deeper, we gradually move away from this planet’s interior composed of high-density gas. Like the law of universal gravitation in our world, its gravitational pull on us decreases with the inverse square of distance, causing the buoyancy we experience to gradually diminish. Eventually, we’ll experience weightlessness in the mine shaft, just like a mine shaft on Earth reaching near the core where gravity cancels out.
But the tunnel might never reach an end, because this planet is just a rare small bubble in the vast solid universe, unless you have enough time and luck to connect to another planet.
//The inspiration for this article came from Möbius inversion transformation (stereographic projection), Liu Cixin’s novel “The Mountain”, and an optical illusion video of a sinking bubble: a group of people standing underwater, but their exhaled bubbles appear to sink downward. Actually, the entire footage was shot upside down: those people weren’t standing on the water bottom, but were using water’s buoyancy to stand upside down beneath a lake’s ice layer, creating the feeling that buoyancy and gravity had exchanged roles, thus causing the illusion. (Original video name and source forgotten)