More than 300 years after Newton thought about falling apples, gravity is on Stephen Hawking’s mind. Come April 26, the wheelchair-bound icon of modern physics is to board a modified Boeing 727 for a roller-coaster flight that will give him the sense of walking on the moon and the ability to fly like Superman. The aircraft will climb at a 45 degree angle to 32,000 feet before diving 10,000 feet, letting him experience 25-second snippets of ‘zero gravity’. Because of the steep angle of climb, twice Earth’s normal ‘pull’ pushes down on passengers, which is replaced by a feeling of ‘lightness’ as the plane levels off before diving. The dive creates one of three levels of simulated space conditions: Martian (one-third Earth’s gravity, where you can do one-armed push-ups), lunar (one-sixth Earth’s gravity that makes you bounce off the walls), or zero gravity (where you float in mid-air like astronauts). On Earth, gravity pulls us down, while the ground’s upward ‘push’ balances it. When the plane dives at the same rate as gravity tugs at it, we ‘float’ as there’s no equal and opposite force to balance gravity.
The vestibular cortex in the brain has a built-in model of gravity that handles information from balance organs in the inner ear. Place a baby on a glass table where she can see the floor below and she becomes fearful, even without any experience of falling. This ‘expectation’ of downward acceleration helps the brain calculate and compensate for Earth’s gravity, so that we don’t fall.
Next time you play catch, watch your hand ‘instinctively’ grab the ball thrown at you. The brain tells the muscles to tighten exactly one-tenth of a second before the ball’s impact so that it doesn’t knock away the hand. Astronauts in space proved this by catching balls released from a spring-loaded cannon, which moved at a constant speed (unlike the constant acceleration on Earth). Initially, they found it difficult to catch the ball as their brains anticipated its acceleration as on Earth. And back on Earth, they had to readjust to the ball moving faster. This shows that the brain retains multiple models of acceleration. It chooses which one to apply. Perhaps Newton would never have bothered to discover the laws of gravity had he known that his brain knew about them all along.