Gravity and the Fabric of Spacetime

We all know the story. One fateful day, Newton was sitting under a tree when an apple fell on his head. He proceeded to pull a piece of paper out of his pocket and write down the entire theory of gravity.
While that's not exactly how it worked, Newton's theory of gravity worked for quite a while. Gravity was like an invisible rope that tethered everything in the universe to everything else.

And then all of a sudden comes Einstein, who has to come over and change everything, right? He realizes something in Newton's plan that doesn't exactly fit. And in solving that problem, Einstein wound up overturning physics completely, destroying the established worldview, and creating the most (and the only) well-known physics equation - E=mc².

The Problem -

First, let's show you how Newton saw gravity.

Let's say that you're the the sun, and you have a long string. At the end of the string is a little ball...that's the Earth. Say you hold the string up above your head and start spinning it around.
So you're spinning the Earth around your head, like a helicopter. You realize that the Earth wants desperately to escape your tyrannical grip. After all, it is getting quite dizzy spinning around like that.
However, something keeps the Earth from flying away from you into oblivion and hitting someone nearby on the head: the string. The string is the only thing keeping the Earth from flying away from centrifugal force. As long as you hold onto that string, the Earth will stay forever tied to you.
That string is, of course, gravity. Newton's gravity, to be precise. Or as the fancy scientists call it, "Newtonian Gravity". With me so far?

Now let's say that you're spinning the Earth around your head, and all of a sudden you let go of the rope. The Earth, itching to fly off somewhere, leaves immediately after you let go (at least, it did for the purposes of this model). As soon as the gravity disappeared, the Earth flew off. The Earth flies off into nowhere immediately after the gravity is gone.

We can apply that to the solar system. The Earth is tied by a tether to the sun. If the sun were to all of a sudden disappear, then according to Newton, the Earth would immediately be free of the gravitational tyranny of the sun, and fly off into space immediately.

Wait...instantaneous reaction? I'm not sure if you know this, but in the early 1900's, Einstein proved that nothing traveled faster than light. So how can gravity (or the lack thereof) travel instantly, surpassing this universal speed barrier?

The Solution -

For now we'll bypass Einstein's roundabout path to solving this paradox (I might go back to this later, though), and go straight to his solution.

In the end, he found out that gravity wasn't an invisible tether...instead it was caused by the "warping of the fabric of spacetime".
...What? Well, allow me to elaborate.

Imagine that you have a piece of fabric stretched out taught, forming a sort of stage for the universe to play out on. Now place, say, a billiard ball on this fabric. You'd get something like the image to the right.

Excuse me for using a picture off of another site, but it helps illustrate the concept. The grid is the fabric of spacetime, and the globe would be the billiard ball...or a planet/star.
Let's go back to our fabric. Placing a billiard ball onto the sheet will curve our fabric. The fabric....of spacetime! Fancy, eh?
So now we have a good ol' fabric of spacetime that is curved by the weight of a billiard ball. We'll have that represent the Sun.

Now, take a marble (that's the Earth), and roll it along the fabric. You can see that the marble makes a curvature of its own, but it's not nearly as big as the Sun/billiard ball does. As you can see, this shows that Bigger Mass = Bigger Curvature, and Bigger Gravity.

Say that this marble were to roll somewhere near our Sun. What would happen?
The Earth would fall into the depression formed by the billiard ball. It would get "sucked into" the billiard ball's curvature, like a marble rolling into a hole in the ground. And that, my friends, is Einstein's Gravity. That is his theory of Special Relativity.


So does this solve the paradox of gravity instantaneously traveling across the universe? You bet it does. Say the fabric of space time were thick and foamy, like those new fancy TempurPedic Mattresses. Those are the neatest things ever. They mold to your body :)
Anyway, when you place a bowling ball on the mattress, it actually takes time to sink in before it curves the TempurPedic Mattress of Spacetime, and creates a depression of gravity.

So what happens when you remove the bowling ball? What happens when it just disappears?
The marble trapped in its gravity well won't all of a sudden be free. No, it takes time for the curvature of the TemperPedic Mattress of Spacetime to become flat and normal again. So it doesn't travel instantaneously -- instead, there's a speed limit, depending on how thick the matress/fabric is.

Surprise, surprise. That speed limit just happens to be the Speed of Light.


a quick summary - Gravity is created when objects are put into the "fabric" of spacetime, creating a little depression in the fabric. Other objects fall into the depression, and that sucking-in effect is gravity.
The fabric is sort of thick, and movement of gravity (the speed at which the depression forms) is at the speed of light, and no faster.


There you have it. This was Einstein's second major discovery about relativity (after Special Relativity, which has to deal with the speed of light and whatnot). Although he claimed it was more complicated, I actually think that it is much simpler. The two theories are closely intertwined. But both of them COMPLETELY re-wrote the science books and changed the way that scientists looked at the world.

If you have any questions or are unclear on one of the models, please leave a comment :)

EDIT: I would also like to point out that gravity warps space and time. It's hard to visualize how it warps time for now, so I'll just say the main effect - As gravity's pull on you gets stronger, time slows down around you. So a clock near a black hole (strong pull) would tick slower than a clock far away from a black hole (weak pull)