# What exactly do people mean when they say the universe is 4th dimensional?

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I frequently hear people say that the universe is based off of a 4th-dimensional space-time continuum. As a result, it could hypothetically do things like loop on itself if you went in one direction long enough. However, people often point out that the 4th dimension is not time, but rather an independent spacial dimension. At the same time though, they might talk about how the 4th dimension actually is time in the space-time continuum, which makes no sense to me at all. Is the 4th dimension time in the space-time continuum, or something else?

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Imagine, if you can, living in a 2-dimensional world. You’re a square, among many other flat shapes that can only move or perceive their world in 2 dimensions, “edge” on, right? What would happen if I took this hypothetical piece of paper all the 2 dimensional people lived on, and plopped a cube on it. What would they see? Looks kind of like a square when you can only see the edge-on 2D face of a cube. But me and everyone else out here in the third dimension knows it’s a cube, and there’s a whole facet of this universe above and below the piece of paper the 2D people conceive as their whole world that they aren’t even aware of.

This is *kind of* what you’re hearing when you see people discussing the higher dimensions. We’re just a bunch of flat shapes speculating about what depth would look like. Unfortunately, the prevailing theories are that there are actually more than 4 dimensions to the universe that are even harder to perceive and measure outside of complex mathematical models I don’t understand at all. Which is why the answer changes, depending on who you ask, at what level and even how much “time” as a concept enters the equation.

Dimensions are just ways to measure things at their core. You can define a 3D object in terms of length, width and height. The universe has a rather obvious 4th dimension in time- to properly place something in the universe, you have to specify the time, because it changes over that axis.

There is also rather compelling mathematical and observational reason to think that there are other spatial dimensions as well. For one the math on string theory demands it. These dimensions are rather small and usually tightly roll themselves up, but they should be there if string theory is right. The math itself is a bit above the paygrade of eli5 but the observational reasons are useful as well: gravity is a fundamental force, but it’s dwarfed by the other 3 fundamental forces. Like you wouldn’t even see it if placed on a graph at the same scale as the others. This has bothered scientists for ages. I mean, really think about it: you can resist the gravitational force of an entire planet with a 2 cent fridge magnet. That’s pretty weird. A popular, plausible hypothesis is that gravity as a force is much more pronounced (i.e. ‘leaking’) into those other dimensions. There’s still no consensus about that, I don’t want to paint it as fact, but it gives you an idea of the thought process.

This often comes up in the context of trying to explain how objects move through space, and especially how gravity works to cause smaller (less massive) objects to “fall” toward larger objects.

Picture a 2D world in which “space” consists of a flat sheet of stretchy material, like a trampoline.

You place a really heavy object (your mom, perhaps) in the center of the trampoline. Then you place a small ball, like a marble, off to one side. What happens when you let go? It rolls inward toward the center.

Why does the ball roll toward the middle? From our perspective, because we are living in three visible spatial dimensions, we can see that it is because the presence of a heavy object in the middle of the trampoline is curving the fabric of “space” (the trampoline), which causes the ball to roll “downhill” even though nothing is obviously pushing it that direction.

But the ball only exists in 2D; from its perspective it looks like the trampoline is still flat, because it only knows about the directions forward/backward, and left/right. It has no idea what “up/down” means. But it still rolls in toward the middle, and we, as 3 dimensional beings, can see that it is because the fabric of the 2D world the ball is living in is curved.

Our 3D world is similar. When you put a big object in 3D space, it curves the fabric of space time in a very similar way. It creates a curvature in an unseen 4th dimension, that creates an apparent force of gravity, pulling other objects toward it. Just like your mom standing on the trampoline causes the ball to roll “downhill” toward her because the trampoline is curved due to her mass, the Earth existing in our 3D space causes us to fall “downhill” toward it because space is curved due to its mass.

We just can’t see that curvature because, like with the ball, it exists in an “extra” dimension that is outside of what we are capable of perceiving directly. But, like the ball, we can still tell that the curvature is there, because we feel its effects (gravity).

Dimensions are just ways to measure things at their core. You can define a 3D object in terms of length, width and height. The universe has a rather obvious 4th dimension in time- to properly place something in the universe, you have to specify the time, because it changes over that axis.

There is also rather compelling mathematical and observational reason to think that there are other spatial dimensions as well. For one the math on string theory demands it. These dimensions are rather small and usually tightly roll themselves up, but they should be there if string theory is right. The math itself is a bit above the paygrade of eli5 but the observational reasons are useful as well: gravity is a fundamental force, but it’s dwarfed by the other 3 fundamental forces. Like you wouldn’t even see it if placed on a graph at the same scale as the others. This has bothered scientists for ages. I mean, really think about it: you can resist the gravitational force of an entire planet with a 2 cent fridge magnet. That’s pretty weird. A popular, plausible hypothesis is that gravity as a force is much more pronounced (i.e. ‘leaking’) into those other dimensions. There’s still no consensus about that, I don’t want to paint it as fact, but it gives you an idea of the thought process.

Imagine, if you can, living in a 2-dimensional world. You’re a square, among many other flat shapes that can only move or perceive their world in 2 dimensions, “edge” on, right? What would happen if I took this hypothetical piece of paper all the 2 dimensional people lived on, and plopped a cube on it. What would they see? Looks kind of like a square when you can only see the edge-on 2D face of a cube. But me and everyone else out here in the third dimension knows it’s a cube, and there’s a whole facet of this universe above and below the piece of paper the 2D people conceive as their whole world that they aren’t even aware of.

This is *kind of* what you’re hearing when you see people discussing the higher dimensions. We’re just a bunch of flat shapes speculating about what depth would look like. Unfortunately, the prevailing theories are that there are actually more than 4 dimensions to the universe that are even harder to perceive and measure outside of complex mathematical models I don’t understand at all. Which is why the answer changes, depending on who you ask, at what level and even how much “time” as a concept enters the equation.

This often comes up in the context of trying to explain how objects move through space, and especially how gravity works to cause smaller (less massive) objects to “fall” toward larger objects.

Picture a 2D world in which “space” consists of a flat sheet of stretchy material, like a trampoline.

You place a really heavy object (your mom, perhaps) in the center of the trampoline. Then you place a small ball, like a marble, off to one side. What happens when you let go? It rolls inward toward the center.

Why does the ball roll toward the middle? From our perspective, because we are living in three visible spatial dimensions, we can see that it is because the presence of a heavy object in the middle of the trampoline is curving the fabric of “space” (the trampoline), which causes the ball to roll “downhill” even though nothing is obviously pushing it that direction.

But the ball only exists in 2D; from its perspective it looks like the trampoline is still flat, because it only knows about the directions forward/backward, and left/right. It has no idea what “up/down” means. But it still rolls in toward the middle, and we, as 3 dimensional beings, can see that it is because the fabric of the 2D world the ball is living in is curved.

Our 3D world is similar. When you put a big object in 3D space, it curves the fabric of space time in a very similar way. It creates a curvature in an unseen 4th dimension, that creates an apparent force of gravity, pulling other objects toward it. Just like your mom standing on the trampoline causes the ball to roll “downhill” toward her because the trampoline is curved due to her mass, the Earth existing in our 3D space causes us to fall “downhill” toward it because space is curved due to its mass.

We just can’t see that curvature because, like with the ball, it exists in an “extra” dimension that is outside of what we are capable of perceiving directly. But, like the ball, we can still tell that the curvature is there, because we feel its effects (gravity).