The Nature of Gravity
As perceived by
Have you ever been to a carnival where there is a ride that is a revolving cylinder? It holds about 20 people, who walk in to it and arrange themselves around the perimeter. Then when the carnie throws the switch the cylinder begins to rotate and as it picks up speed the riders feel themselves being pressed against the wall of the cylinder, and suspended with nothing to hold them but the centrifugal force of the turning cylinder. Demonstrating Newton’s 1st law of motion, an object at rest tends to stay at rest and an object in motion tends to stay in motion until an outside force acts to change its speed or direction. In this case it is the friction of the rider’s feet with the floor that starts them moving, and it is the wall of the cylinder that forces them to change direction. It is the same force at work in the centrifuge in chemistry class, and it is this same principle that causes the spin cycle on a washing machine to work.
Often times, in science we make models to help us to understand and explain difficult ideas. We make models of DNA, Atoms, Crystals, etc. we also make models of large things like the solar system, globes of the earth and so on.
Understanding gravity involves understanding our 4dimensional universe. Since we live in a 3dimensional world, a concept of the 4th dimension is difficult because we have no perception of it. So it helps to make a model that is scaled down by one dimension, and our model becomes a 2dimensional world in a 3dimensional universe.
The 2dimensional world might be the surface of a trampoline populated by 2D people that look like coins. And the massive object representing the world might be a large lead sphere. When we place the sphere on the trampoline, it causes the surface of the trampoline to bend down into the 3rd dimension. The people (coins) being 2dimensional, have no perception of the 3rd dimension, but they feel themselves being drawn to the sphere. They conclude that the sphere has a mysterious force called gravity that draws them to it. But we, being 3dimensional, can see that the sphere has no mysterious force, but rather, the coins are sliding down the slope of their 2dimensional space (the curved surface of the trampoline). Of course the problem with this model is that it requires the earth’s gravity to make it work. If we take our model up to the International Space Station (ISS), setup the trampoline, put the coins on it, and put the massive sphere on it, nothing happens, because the gravity from the earth is balanced by the orbiting motion of the ISS.
Remembering the carnival ride, however, we can cause the ISS to rotate (not as fast as the spin cycle of the washing machine of course) by attaching a couple of small rocket engines on opposite sides of the ISS, with one pointing toward the earth and one pointing away from the earth. The resulting centrifugal force creates an “artificial” gravity which causes the objects inside to be drawn to the sides of the ISS, away from the center of spin. And now we find that our model will work just like it did down on earth.
So now we scale everything back up by one dimension, and we have a rotating 4dimensional universe (where time is the 5th dimension) causing massive objects to warp our 3dimensional space into the 4th dimension, which in turn causes smaller massive objects, like 3dimensional people who have no perception of the 4th dimension, and rocks and apples and dust and the gas that we breathe, and even light, to be drawn towards the larger massive objects, such as stars and planets.
There are some excellent illustrations of the warping of space in “The Illustrated A Brief History of Time” p.19 and “The Universe in a Nutshell” p. 34 & 39, by Stephen Hawking.