Visualize a sheet made of rubber, stretched tightly in all directions - flat, smooth, essentially featureless.
Now imagine a heavy sphere, like a marble, placed on the sheet. Imagine the smooth, uniform, gradual depression in the sheet, the gentle curve in the material.
Juggling this set of images, now add another: another marble, shooting across the surface of the sheet, leaving its own impression on the surface as it moves across. See in your mind this second sphere roll close to the first - just glancing off the very edge of the transformed sheet.
Replay this in your head, sending the second marble closer and closer to the first, until the second marble can no longer escape the impression of the first, instead finding a circular path about the first.
If in your head, you can conceptually extrapolate this image into three dimensions, you will have a vague picture of our current understanding of gravity; the marbles are massive bodies, like stars and planets, and the sheet is a two-dimensional slice of space-time. Gravity, as we understand it, is a distortion in space-time caused by these massive bodies.
A conceptual framework like this is not necessarily a practical or necessary framework for everyday use, though. Einstein's elegant space-time distortion is still taught years after students learn the Newtonian model, because while Newton's model has the fundamental failing that it says nothing about what gravity actually is
, it does give a simple mathematical framework for calculating the effects two bodies will have on each other as they pass. The formula essentially says that the magnitude of the gravitational force between two objects is proportional to the mass of both objects, but inversely proportional to the square of the distance they are from each other.
The effect is a function of two quantities - the masses - and distance.
Both models of gravity tell us that two masses at a sufficient distance from each other will have essentially no effect on each other. They have very little way, even supposing a sudden dose of sentience, of determining that the other even exists
. The events required to make these two bodies aware of each other are simple, straightforward, and yet desperately unlikely. The two masses must simply travel close enough to each other to move through the other's sphere of distortion, or sphere of influence.
For an orbit to exist, the two must travel close enough for one to become trapped in the circling path about the other.
For an orbit to be broken, some external force, strong enough to overcome the distortion, the mutual attraction, must push one object at an appropriate angle, such that it is not simply immediately recaught in a circular path about the first.
And if that should happen, freely moving through essentially empty space, the two semi-sentient objects should eventually move out of range such that they are essentially where they began - without knowing that the other truly exists; out of influence range, out of touch.
Think about that for a moment.
Every single day that we make a phone call, or jump on one of these magic internet boxes, or watch a television show from the other side of the world, we violate in a limited, human way one of the most elegant laws of the universe. When we write a letter, we confirm the continued existence of our mass with one a thousand time smaller than ours.
To be human is to have incredible power; the simple facts of our memory and indomitable will allow us to continuously confirm that which we have seen - that which has frightened us, that which inspires us, that which we reject, and most importantly, that which we love.
Distance does not equal absence.
(Cross-posted from All Write Already
. Also, credit is owed to Brian Greene's The Elegant Universe
for a good deal of the visualization/metaphor for the physics bits.)