## What is Gravity

One of the first things we learn about shapes and geometry is the concept of parallel lines. Two lines that both travel in the same direction and their paths never cross. Well, let me tell you, the reality is a lot more complex. All the lives we’ve been told we live in a three-dimensional space or the third dimension. Those dimensions being height, width, and depth. Now, this is superficially correct, and thinking any different won’t change your life, but the reality is we exist within a 4-dimensional realm called spacetime.

What is Gravity

We have freedom of movement within three of these dimensions, space, but the fourth, time, is restricted. This means I can move in any direction within space at any speed or any acceleration, my only constraint is the energy needed to change my momentum. But in the fourth dimension, time, I can only go one direction at one speed. (What is Gravity) For that reason we are considered three-dimensional beings, we don’t possess the ability to comprehend the fourth dimension. But that doesn’t mean it doesn’t exist and believe it or not, it’s why things are attracted to each other by what we call gravity. To comprehend spacetime and how the fourth dimension creates gravity. we need to be able to comprehend how higher dimensions affect lower dimensions.

What is Gravity? So to begin, let’s pretend we have a pair of one-dimensional friends, Friend A and B, and these friends get in a fight and decide, I’m done with you and start walking away from one another in opposite directions. Then after a while, all of a sudden friend B pops up out of nowhere and both of them lose their minds. How is this possible? There’s no conceivable notion, other than teleportation, of how these two friends could meet up again. Their one-dimensional minds are just unable to comprehend what is happening. Because their world is actually a circle within two dimensions. However, Without information from the second dimension, it’s impossible to know the true direction of their path or any path for that matter. Therefore the one-dimensional friends are bounded by the **second dimension**. What is Gravity

## What is Gravity

The same thing happens for those living in two dimensions. These friends walk away from each other, turn 90 degrees to their north. Keep walking in parallel lines, and yet somehow they run into each other. That is because they actually exist upon a sphere. Again, without information on the third dimension, they can’t conceive the true nature of the path they are taking. That brings us to our world.

The third dimension, and yes we too are bounded by a higher, the fourth, dimension. Just like our friends in the previous two examples, there’s no way for us to conceive exactly how we can travel in straight parallel lines and meet up again. But we can understand that our plane of existence is curved in the next higher dimension. What does that mean? We have no idea. Just like our friends from before, but that doesn’t mean we can’t understand the consequences of it. This curvature in the fourth dimension is what we experience as gravity and just like how everything in the lower dimensions that travel in straight parallel lines eventually meets up, the same thing happens here in the third dimension. However, here things differ a little from our friends before. Unlike our lower dimensional friends, we don’t have the freedom to choose how fast to move through our 4dimensional spacetime.

What is Gravity

Whether you are running, sitting, or falling, you are moving through spacetime at the notorious speed of C.roughly 300,000km/s. This is of course the same C from the famous e=mc^2 postulated by einstein and is equally distributed between space and time. So if you decided to move faster through space you must move slower through time and if you were to somehow slow down in space, you’d move faster through time. Now, you may be thinking, well I’m just sitting here why isn’t time passing faster? But remember the surface of the planet is moving, the earth is moving, the solar system is moving, and the galaxy is moving. It’s impossible to stop moving through space. Plus your changes in velocity are so small compared to C the difference is imperceptible. Even rockets that travel at 8km/s are only moving 2 ten thousandths of a percent of the speed of light.

So we are constantly moving through spacetime at a constant speed. And since spacetime is curved, just like the previous examples, our perceived straight paths appear to “attract” or converge with one another. If this still isn’t making sense don’t get discouraged we are talking about a very complex concept, normally outside our realm of comprehension. But bear with me, we’re going to start visualizing this and I think you’ll have the same eureka moment I had. Okay! Let’s start visualizing this. To begin we’ll start with the most common visualization of gravity, but it’s not my favorite. We’ll visualize the fabric of space-time as a trampoline. Now, normally empty spacetime just looks like a trampoline. But of course, spacetime is not empty, it’s filled with matter, normal and dark alike, and this matter has a very strange effect on spacetime. What is Gravity

## What is Gravity

Now, Although they exist within space-time, the elementary particles that make up all matter are not made up of spacetime themselves. They are for lack of a better term and as far as we know today, pure energy, and this pure energy occupies volume within spacetime or disturbs it. But this volume that should be spacetime doesn’t simply disappear because energy or matter is occupying it, it is displaced. So the more energy or mass that exists, the greater a disturbance and displacement it has on the surrounding spacetime. So planets and stars and all the energy within them warp the spacetime around them. That means spacetime is curved by matter. This is very important because straight lines on curves have unique behaviors. A straight line along a curve is called a geodesic and an easy way to identify geodesics on spheres is that they always pass through the point on the exact opposite side of the sphere. The only reason it wouldn’t is if some force was applied to it along its path, but then it would no longer be a straight line. Now two geodesics, no matter where you put them on a sphere, will cross each other’s paths.

Now whether objects following those paths hit each other is a matter of timing and relative velocities, but they are constantly moving to intercept the other path. So, just like how moving along a curve in the third dimension can be viewed as moving along a straight path in the second dimension, moving along this now curved fourth dimension means it still feels like we are moving in a straight line in the third dimension. And just like how two-dimensional beings moving in parallel lines eventually meet one another along the surface of a sphere, us moving in straight parallel lines in space means we still end up meeting each other or crossing one’s path within spacetime. Think about how crazy that is! If we suspend a ball above the surface of the earth, it’s moving in the same direction as the earth through space. Let’s even freeze the rotation of the earth so we don’t worry about the angular velocity affecting our ball’s movement. Now when we look at the ball and the earth they are both clearly moving along two parallel paths through three-dimensional space. So there’s no reason for them to cross each other’s path. What is Gravity

But when I let go, as they travel through spacetime, just like how we saw with the friends walking on the sphere, because they are traveling along a curve in a higher dimension the ball and the earth end up running into each other despite traveling parallel with each other. And this is the fundamental concept of gravity. This becomes abundantly clear if we change our perspective. If we look at our two friends walking along in two dimensions it looks like they walk into each other, however, if we remove any reference to their velocity because they are walking at the same speed it simply looks like they are gravitating towards each other for no reason. The same thing applies to our ball and earth! As they both whizz past it’s clear that, oh okay they’re moving along a curve on the fourth dimension and thus converge. We can see both of their horizontal and vertical movements. But to someone standing on the earth, the ball appears motionless as if it’s not moving through space at all.

The only difference is that the ball’s path through spacetime converges with the path of the earth. Even though it appears as if the ball is motionless on the ground since both the earth and the ball are still moving through spacetime, they’re still constantly colliding. Finally, let’s go back to our trampoline example and I want to explain my original confusion with this model, in case anyone else had or has it. As we roll a ball around our **spacetime it’s clear that we see the curve of orbits** and we see that oh yea things fall down “the well” as it’s called. But why does it need to? If gravity isn’t a tangible force, why if I place a motionless ball here on the trampoline does it have to roll down towards our planet? And this is where my mistake was. What is Gravity

You see before when we talked about lower-dimensional beings moving about, nothing is stopping them from stopping themselves or just decided to sit down. But remember, everything is always moving through spacetime, so the ball placed on the trampoline has to move because everything in spacetime moves. And our planet in the middle of the trampoline isn’t motionless either, it’s also moving through spacetime at the same speed as the ball. And once you see the trampoline-like this. Well, I hope it clicks and it makes sense why something would just fall as we perceive it, straight down. So remember, if it feels like you are stuck in a rut or not making progress, the universe is always there behind you moving and pushing you forward.