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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Humans have no way to visualise four-dimensional objects, so the best way to explain them is by analogy.
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Imagine the world of pac man.
Pac man lives in a 2d world
There is up and down, left and right. No third direction of travel. Only two
Our experience of pac man is on a computer screen.
But a 2d world could be represented by any flat object. Like a piece of paper.
Take a piece of paper now and hold it in your hand.
In some pac man games, if you go off the top of the screen, you appear at the bottom.
And if you go off the left side you appear the right side.
Take your piece of paper, and bend it, so that the top and bottom sides are touching each other.
The little 2nd pac man living on the piece of paper doesn’t know the paper has been bent. All it knows is that if it walks off the top end, it appears at the bottom.
From its perspective, it is still living in a 2nd world.
But from our perspective, its world now seems to be a 3d object, a tube.
———-
Now take a real football
Wrap the piece of paper around the football, and flat it down.
Now if the Pacman living on the paper, walks off the top end of its world it will appear at the bottom. And if walks off the left side it appears at the right.
The Pacman still thinks it’s living in a 2d world, but the world is actually curved in a third dimension that it can’t perceive.
—————
Now imagine the real universe.
Imagine we shot a probe off into space at some stupidly high speed
It flies off, out of sight, and its gone.
Then, ten years later, it appears heading towards the earth, from the opposite side that it disappeared from.
It has looped around the universe, and come out the other side, like the pac man does in it’s curved world.
Scientists then fire off probes in all directions, and the same thing happens. Whatever direction we fire probes in, they appear on the other side of the universe, heading towards the earth.
That would inform us that our 3d universe is actually curved in a 4th spatial dimension that we can not perceive.
Humans have no way to visualise four-dimensional objects, so the best way to explain them is by analogy.
——
Imagine the world of pac man.
Pac man lives in a 2d world
There is up and down, left and right. No third direction of travel. Only two
Our experience of pac man is on a computer screen.
But a 2d world could be represented by any flat object. Like a piece of paper.
Take a piece of paper now and hold it in your hand.
In some pac man games, if you go off the top of the screen, you appear at the bottom.
And if you go off the left side you appear the right side.
Take your piece of paper, and bend it, so that the top and bottom sides are touching each other.
The little 2nd pac man living on the piece of paper doesn’t know the paper has been bent. All it knows is that if it walks off the top end, it appears at the bottom.
From its perspective, it is still living in a 2nd world.
But from our perspective, its world now seems to be a 3d object, a tube.
———-
Now take a real football
Wrap the piece of paper around the football, and flat it down.
Now if the Pacman living on the paper, walks off the top end of its world it will appear at the bottom. And if walks off the left side it appears at the right.
The Pacman still thinks it’s living in a 2d world, but the world is actually curved in a third dimension that it can’t perceive.
—————
Now imagine the real universe.
Imagine we shot a probe off into space at some stupidly high speed
It flies off, out of sight, and its gone.
Then, ten years later, it appears heading towards the earth, from the opposite side that it disappeared from.
It has looped around the universe, and come out the other side, like the pac man does in it’s curved world.
Scientists then fire off probes in all directions, and the same thing happens. Whatever direction we fire probes in, they appear on the other side of the universe, heading towards the earth.
That would inform us that our 3d universe is actually curved in a 4th spatial dimension that we can not perceive.
The easy ELI5 explanation is that a dimension is just a word for a number (or direction) that can’t mean anything else
On Earth you can talk about North vs South, which is just 1 dimension describing how far away from the equator
East vs West is a second dimension describing how far away from a certain line you are. Even though it’s a second dimension it’s still one number
Then you have altitude, the one number that describes how high you are (measured against sea level but that’s kind of arbitrary. Could be measured against anything as long as it’s consistent)
A 4th dimension is just another number that describes something that you can’t describe elsewhere.
In the case of time, the when is something you need without realizing it. If I’m going to meet up with you, I need a time to meet up with you otherwise I might be waiting at a place for a decade.
The fact I need a time (which is just some kind of number) is what makes it a dimension
The easy ELI5 explanation is that a dimension is just a word for a number (or direction) that can’t mean anything else
On Earth you can talk about North vs South, which is just 1 dimension describing how far away from the equator
East vs West is a second dimension describing how far away from a certain line you are. Even though it’s a second dimension it’s still one number
Then you have altitude, the one number that describes how high you are (measured against sea level but that’s kind of arbitrary. Could be measured against anything as long as it’s consistent)
A 4th dimension is just another number that describes something that you can’t describe elsewhere.
In the case of time, the when is something you need without realizing it. If I’m going to meet up with you, I need a time to meet up with you otherwise I might be waiting at a place for a decade.
The fact I need a time (which is just some kind of number) is what makes it a dimension
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