This would be an earth-shattering event. Literally.
We like to think of the ground beneath our feet as solid and permanent. For everyday purposes, this is a useful — and reassuring! — approximation. But on the scale of the entire planet, it’s quite misleading. In fact, the vast majority of the earth is made up of red-hot metal and silicates that behave very differently from the stiff, rocky crust. The outer core is composed of fully liquid metal flowing freely under the influence of heat and magnetism. Even the solid mantle is warm and gooey; over geological time scales, it too behaves as a very viscous liquid. Under the enormous pressures in the earth’s interior, these materials get squashed down like a spring. For example, the iron and nickel in the solid inner core are over 50% denser than they would be on the surface.
Fortunately for us, this giant “spring” is presently held in place by billions of tons of rock that aren’t going anywhere. But when meddlesome Quorans start changing the laws of physics, all sorts of weird things can happen. Under weaker gravity, every rock in the earth suddenly weighs 5% less. That means that the planet’s interior no longer has enough pressure to hold it in place, causing it to expand outwards.
The crust, only a few tens of kilometers thick at best, is unable to bear the immense force. It shatters like a glass window.
The weakest points, the faults between plates, break first. Then the great continental plates begin to fracture at hot spots and along long-forgotten ancient fault lines. The world shudders at the injury. Earthquakes, far stronger than any ever recorded, ripple through the ground. The expanding interior lifts the entire planet’s surface by several kilometers; the crust fragments simply float upwards atop the rising mantle, while gigantic volcanic eruptions fill in the gaps with hot lava.
In the depths of the ocean, the shaking seabed displaces billions of tons of water. Tsunamis demolish every coastal city. In many low-lying areas, salty floodwaters drown even places far inland.
Billions die in the first few hours of the disaster. The greatest tragedy, though, is the fate of the survivors. Over the coming months to years, most of them face slow deaths by starvation as Earth freezes into eternal winter.
Three separate effects combine to cool the planet. First of all, the sun’s weaker inward pull allows the planets, including Earth, to climb into more distant, elliptical orbits. That means less sunlight reaches Earth’s surface. At the same time, the sun cools down and darkens noticeably. Its feebler gravity is less able to drag hydrogen atoms into its core to fuse together and release energy. And finally, to make matters even worse, Earth itself spends the next few years cloaked with thick layers of long-lasting volcanic ash, reflecting terawatts of life-giving sunlight into outer space.
It’s tempting to imagine that the tropics might still stay warm enough to support life. After all, we only changed gravity by 5% — how bad can it be? Unfortunately, Earth’s climate is a rather fragile system. Once the polar ice caps start to grow, the planet falls into a vicious cycle: more ice reflects more sunlight, which cools the ocean more, which leads to more ice forming. These effects snowball until the entire planet freezes over. This isn’t just an ordinary ice age; it’s much more like the Snowball Earth that happened about 650 million years ago.
Will any humans survive? I think so, but it’s going to be a miserable existence. With surface temperatures falling to somewhere around -50° C in most places, we’ll have to spend almost all of our time inside heated buildings. From a survival standpoint, though, that’s OK as long as we can produce enough food. We should be able to grow crops by clearing a small area of the surface and using it as a greenhouse. If needed, we can supplement our diet by fishing under the ice. Importantly, we will have many years to build the infrastructure we’ll need, since the planet will take a long time to cool down. Overall, I predict we’ll find a way to carry on; humans are pretty resilient.