The little essays in this book are all about spacetime. So the cosmic view of gravity is implied by the title of this one. Although the words "gravity" and "grave" have other common meanings, they all seem to stem from the Latin root "gravis" which means heaviness or weight, what pulls things down to Earth.
In science, gravity is the source of physical weight. On a planet such as Earth for example. An object in freefall is said to be weightless. This idea only means that the weight of the object isn't being felt by the object. In other words, the object isn't pushing on something that can push back. At least not much.
We know that air offers some resistance to the motion of falling objects. But at least initially this is not felt very much. Gradually, though, the velocity of the object's fall is being accelerated by the effect of gravity. So as it falls, the air resistance increases. Although as they say, it's not the fall that hurts, it's the sudden stop when you hit the ground.
We know that if an object is in orbit around Earth there is no sudden stop. We also know that its speed stays pretty much the same. What happened to gravity? It's still there. But the object is going fast enough in its orbit that the accelerating effect of gravity isn't enough to make it fall to Earth. All Earth's gravity can do is bend the object's path into a circle so it keeps going around and around.
This illustrates the difference between speed and velocity. When an object is in orbit, all the acceleration is acting sideways to bend its path. There's none left to speed it up or slow it down. That's about it on the gravity story.
Unless you care about the mathematics. Isaac Newton accounted for gravity with a fairly simple formula that said all objects are attracted to each other by a force that acts between them. He called it action at a distance. He was not very happy with that idea because he couldn't explain it. But it worked so he stuck with it. And so did everyone else for several hundred years.
Then Albert Einstein decided that action at a distance was too spooky to be right. He worked out another equation that got rid of the distance problem by saying that the action was due to the curvature of space right where the object is. His equation worked better than Newton's for things that are moving very fast. But it was a lot more complicated, so Newton's is still used for things moving slow enough that it doesn't matter.
You might have wondered how these gravity equations help get rockets into orbit. They don't really help at all, except to tell you how much work it will take to get the rockets up there.
Gravity only works to pull things down. To push them up, you need atomic energy. So far, the atomic energy that's used for launching rockets is the kind you can get from chemistry. You put chemicals like liquid methane and liquid oxygen in your rocket, and get them to burn and produce hot gases.
This burning releases enough atomic energy to make the molecules move very fast. So the resulting gas has a very high pressure, and pushes the rocket upward with as much energy as the escaping gas has. Chemical energy. This means electrons pushing against electrons. The steady push of the exhaust gas on the rocket makes the rocket accelerate, until it's going fast enough to orbit around Earth instead of falling back down out of the sky.
What does all this have to do with spacetime? Like everything else that people make happen, a rocket launch is a way of choosing a possible future in space and turning it into an actual past.
The same kind of choosing makes the whole planet keep going around the sun. Except in that case the choosing is automatic. No one has to think about it. As far as we can tell, it's this same kind of choosing that makes time happen everywhere in space. And all this choosing is local, just as Einstein thought it should be.
Now it's pretty easy for us to get the idea of automatic choosing. We do it all the time. You don't have to tell your legs how to walk. But to have this happen everywhere in space? Come on. That's crazy, right?
Not as crazy as spooky action at a distance. Newton thought that was a crazy idea, but he didn't have a better one. If you asked him he might have said God did it. God set it up that way. God was always getting blamed for all sorts of spooky things.
Einstein didn't want to blame God for spooky things. He managed to explain how gravity didn't have to be spooky. But everything else? Those electrons that could push a rocket into space? More spooky stuff going on there that Einstein didn't like. How does that get explained?
If you've been reading this book, you already have the answer. What we call Spacetime is, for each of us, nothing more than a collection of ideas we think we understand, more or less. What we call reality is what we think all of those eaches adds up to. And each of those eaches is local to itself. No action at a distance needed. In other words, there's nothing spooky about it. Unless you want to call each of those eaches a spook.
