system
Given the following post, respond as if you're commenting on Reddit:
human
What allows humans to control gravity in an enclosed area?

I saw the video of Stephen Hawking on zero gravity. What do they have to do to make the conditions right for zero gravity?
gpt
We do not have the ability to control gravity. Stephen Hawking was accelerating towards the earth the entire time. However, since the plane he was in was also accelerating towards the earth, it looked and felt just like it would if there had been no gravity.

Imagine you're looking down a very long elevator shaft, and you dropped a penny down it. The penny would start out stationary and because of gravity it would accelerate constantly downwards, gathering more and more speed. Now imagine instead of a penny, you dropped an elevator car downwards. It would start out stationary like the penny, then drop in the same fashion. If there's no air in the shaft, it'll have exactly the same velocity at any point in time as the penny would have.

Now imagine you drop both at the same time, the penny in the centre of the elevator car. Both accelerate downwards: the penny matches the speed of the elevator precisely, and so remains in the centre of the car throughout. From inside the car, it looks and feels like the penny is not subject to gravity... but in reality it's just exactly as subject-to-gravity as the car is.

To reproduce this effect in a plane, we can fly it to a safe altitude, aim it somewhat upwards, and then follow the path and speed it would take if it all the air suddenly disappeared. Its motion would become purely ballistic, just like the motion of everyone in the plane. 

It would follow a parabola and eventually drop back down to earth. 

The motion of the people inside would follow a parabola and eventually drop back down to earth.

From the point of view of the people inside, they're floating in the plane!

Of course, we can only do this for a while, before we need to level the plane out again.