This is about another of my favorite subjects: Gravity. Some quick, interesting facts about gravity:
1. No one really knows what it is. There's been many theories, and while we can predict the effects of gravity real well, no one really knows what it is at all. There are three current models of gravity in use, and each has a place: There's old school Newtonian Gravity, which works very well in our day to day life, but begins failing when dealing with more extreme conditions, like in space - where the next theory takes over: 2. Einsteinian gravity via Space Time. Einstein proposed the idea of SpaceTime - the fabric of the Universe. The picture above is a representation of star bending Spacetime, causing a near by photon to bend  - we have confirmed beyond doubt that light does indeed bend around heavy objects. Many other proofs of this theory have been produced, and so it seems quite solid. And yet, at the most extreme conditions, Einsteinian gravity fails as well, since it cannot explain the Singularity, which leads to the third theory, currently still a work in progress: 3. Quantum gravity, which uses a force particle called the Graviton (never detected) to convey the force of gravity at the particle level.

A large task underway in physics is to combine the wildly successful concept of Spacetime with quantum gravity. Get to it if you're looking to make a name for yourself.

Now, most depictions of SpaceTime show it as a 2-D graph, as above. It's just easier to visualize. But something never made sense to me - if it is the bending of space which causes gravity, then isn't it still just "mass falling down"? Sure looks like it above, right? However, that is a problem with the graph. A true 3-d representation of SpaceTime looks like this:
It might be a bit hard to conceptualize, but picture the circle above as our sun, and the square as SpaceTime surrounding the sun in all directions - Spacetime will be stretched in all directions around the sun, with the more extreme stretching the closer to the object. As you might now see, gravity is a result of SpaceTime pushing down on you, NOT something attracting you to it. Any object that bends SpaceTime (i.e. any object with mass) will have a deformed sphere of SpaceTime around it, and it's that deformation pushing down on you which we measure in units of gravity. Big concept here - gravity is not a force but a measurement.

BUT! That could be all wrong. Quantum theories of gravity think so.

Now, I've posted this before, but it's germane to this discussion - a 2-d representation of SpaceTime in our Solar System:
The deeper the depression, the "stronger" the gravity. But this is just the start of course, since our solar system is but a speck of dust in the Galaxy, and our Galaxy is just another average Galaxy in a Universe of billions of galaxies, such that one can extend this 2-d representation of SpaceTime gravity wells to the galactic level, as such:
And this goes on even further, of course. And I suspect if we do indeed reside in a Multiverse, than Universes no doubt orbit other Universes. Mind boggling, really.

The point? Two points: We sit at the near top of a mountain (the moon is above us), and we're looking/falling(being pushed) down, down, down a hole that never ends.

And/or 2: This theory is wrong, and a quantum theory of gravity will explain why gravity exists and how it works.

String theory is the work-in-progress attempting to synthesize the theories of Einstein with Quantum Mechanics. More on string theory at another time, alas!

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