what is gravity?

What really happens when something "falls"? 

One of the greatest misconceptions we have about gravity is the idea that objects are somehow "attracted" to the earth by the "force" of gravity.   This is sometimes thought of as a magnetic-like attraction, which is far from the truth.   If you hold an object, say a "bean bag", and let it go, it will appear, from your perspective, to travel in a straight line directly to the floor.  But what has really happened here?

Einstein's remarkable thought was to imagine that space itself is somehow warped by massive objects, and that objects moving through space simply follow a "path of least resistance".  This was an incredibly bizarre concept for most people to comprehend and many physicists could not bring themselves to believe it.  But once Einstein imagined curved space as being the source of gravitation, he knew he was right.  It was simply a matter of time before he could struggle through the complex mathematics that describe the behavior.

Finally, in late 1915, with the help of his close friend Marcel Grossman, he had it.  Then, in 1916 he published, in the German "Annals of Physics" (Annalen der Physik), the most momentous scientific revelation of our time, "The Theory of General Relativity".

To understand gravity, there are a couple fundamental concepts to understand:

1. 1.  WE ARE IN CONSTANT MOTION

2. The earth is rotating and we are all therefore constantly moving.  Even if you happen to be at home sitting at your computer, you are rotating along with the earth at a speed of about 800 miles per hour (depending on you location) to the east.  Since we rotate, we see the Sun rise in the east and set in the west.

3. 2.  THE SPACE Around us is "curved"

4. Even though you cannot see it, the space around the earth (and the space around you) is "curved".   To demonstrate this concept, imagine a large rubber sheet upon which you place a heavy (or "massive") object.    You can emulate this by placing a heavy object, like a bowling ball, on your mattress and observe the depression it makes. 
6. This effect is a "curvature" of the fabric (mattress) due to the mass (bowling ball).  As you might imagine, the heavier the object you use, the more significant this curvature will be.  Similarly, any object with mass will have a similar effect on the invisible space around it.

Einstein's visualizes gravitation

The idea above is exactly what Einstein was thinking when he tried to imagine why things move they way they do.   He knew the earth was rotating and that he, along with the atmosphere and everything around him, was in constant motion.

Now imagine that you are standing in your living room holding a bean bag outstretched in your hand.   Without doing anything, remember that you and the bean bag are moving at a constant speed of about 800 mph to the east.  When you let go of the bean bag, it is still moving with you at 800 mph to the east.  You observe that the bean bag travels directly to the floor.  Why is this?

First, let's pretend it takes 1 second for the bean bag to hit the ground.  Let's also assume we travel 700 feet to the east in that one second.   If this sounds odd, just remember that you, the bean bag, the furniture, and the house are all moving together at the same speed.  Nothing seems out of place because everything is rotating together - including the atmosphere around your house.

So 1/10th of a second after letting the bean bag go, both you and the bean bag have traveled 70 feet to the east.  But something else interesting has happened here.  The bean bag is some small distance below your hand.  It has traveled away from your hand.  Let's think about the path that it has traveled.

From your perspective (or "relative to you"), the bean bag has moved downward a couple inches.   Imagine that you were at "point A" (in space) when you released the bag.   1/10th of a second later, you are now at "point B" (70 feet to the east).  While the bag has traveled with you, it has also begun to move downward (from your perspective) to the floor.

Now let's think about the path the bean bag has traveled from the perspective of some observer in space (an astronaut) with a great set of binoculars and a bit of x-ray vision.  :-).  He looks down and sees you, the bean bag, and your house rotating at about 800 mph to the east.   When you are at point A he sees you release the bean bag.    He continues watching and observes that the bean bag not only travels with you, but also begins to move downward toward the floor.  After 1/10th of a second, he sees that you have moved 70 feet to the east, and the bag is a couple inches closer to the earth.   From his perspective (i.e., "relative to him"), it appears as though this bean bag is following a curve - from point A to point B.  As he continues to watch, the bean bag follows a curved path (or arc) from point A to point J (where there are 10 points from point A through point J, each 70 feet apart).  But from your perspective (again, "relative to you"), this bean bag has simply appeared to travel straight down to the ground.

So what is the "true" path traveled by the bean bag?  A curve, or a straight line?  Well, it depends.  It depends on who is the observer.  With this example, you have learned that there is no so-called "absolute motion".   No one can say what is the "true path" of the bean bag.  The motion or path an object takes must always be described relative to some so-called "frame of reference".  In the first case, where the bag appeared to follow a straight line, the frame of reference was with you, where you were standing.  In the second case, where the bean bag followed a parabolic curve, the frame of reference (or observation point) was somewhere above the earths atmosphere.

a gravitational field on your mattress

Let's return to our mattress and bowling ball example.  You can try this at home and see the effect that curve space has on objects that are passing nearby.   Get your bowling ball, or any dense, heavy object and place it on the middle of your mattress.  It helps to have a rather "flat" top on your mattress.

Now imagine that the bowling ball is the Sun, and take a small marble (simulating the Earth, or an asteroid), and roll it past the bowing ball.  Depending on how close the marble is to the massive object, the more it is effected by the warpage of the fabric of space.  It's quite interesting to actually try this with a marble and bowling ball on your mattress.   Both the speed of the marble and the distance from the massive object will dictate whether it is "captured" by the gravitational warpage and follow the contours of space in a catastrophic collision with your bowling ball.  

[ If you don't have a bowling ball or other heavy object, you can simply have someone take their fist and push downward on the the mattress to cause the curvature you need. ]

As you observe the behavior of the marble with various speeds and distances from the massive object, it becomes obvious that the path traveled has nothing to do with a "magnetic" attraction or "force" of any sort.   The objects are simply following the contours of space - the path of least resistance.  There is no "gravitational force", per se.

In truth, objects simply move through space (or more accurately through "space-time"). 

Greg's comments on Curved Space (from Yahoo! Answers)