Gravity

Suppose we had a magic gravity machine – a device with which we could control the Earth’s gravity, perhaps by turning a dial… Initially the dial is set at 1 g and everything behaves as we have grown up to expect.

1 g is the acceleration experienced by falling objects on the Earth, almost 10 meters per second every second. A falling rock will reach a speed of 10 meters per second after one second of all, 20 meters per second after two seconds, and so on until it strikes the ground or is slowed by friction with the air. On a world where the gravitational acceleration was much greater, falling bodies would increase their speed by correspondingly greater amounts. On a world with 10 g acceleration, a rock would travel 10×10 m/sec. or almost 100 m/sec after the first second, 200m/sec after the next second, and so on. A slight stumble could be fatal.

The acceleration due to gravity should always be written with a lowercase g, to distinguish it from the Newtonian gravitation constant, G, which is a measure of the strength of gravity everywhere in the universe, not merely on whatever world or sun we are discussing.

All the animals and plants on Earth and the structures of our buildings are all evolved and designed for 1g. If the gravity were much less, there might be tall, spindly shapes that would not be tumbled or crushed by their own weight. If the gravity were much more, plants and animals and architecture would have to be short and squat and sturdy on order not to collapse.

Consider a possibly typical group of Earth beings at the tea party from Alice in Wonderland. As we lower the gravity, things weigh less. Near 0 g the slightest motion sends our friend floating and tumbling up in the air.  Spilled tea – or and other liquid – forms throbbing spherical globs in the air: the surface tension of the liquid overwhelms gravity. Balls of tea are everywhere. If now we dial 1g again, we make a rain of tea. When we increase the gravity a little – from 1 g to, say, 3 or 4 g’s – everyone becomes immobilized: even moving a paw requires enormous effort.

As a kindness we remove our friends from the domain of the gravity machine before we dial higher gravities still. The beam from a lantern travels in a perfectly straight line (as nearly as we can see) at a few g’s, as it does at 0 g. At 1000 g’s, the beam is still straight, but trees have become squashed and flattened; at 100,000 g’s rocks are crushed by their own weight. Eventually, nothing at all survives except, through a special dispensation, the Cheshire cat.

When the gravity approaches a billion g’s, something still more strange happens. The beam of light, which has until now been heading straight up into the sky, if beginning to bend. Under extremely strong gravitational accelerations, even light is affected. If we increase the gravity still more, the light is pulled back to the ground near us. Now the cosmic Cheshire cat has vanished; only its gravitational grin remains…

(Carl Sagan, Cosmos)

cheshire cat

When the gravity approaches a billion g’s the cosmic Cheshire cat will vanish and nothing will be as it has been before. Image: © Megan Jorgensen

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