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Apple Inc. is expected next week to introduce new versions of its digital music players that have more features, but cost the same.
Chief Blingologist -- 49% sacred, 51% profane
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08.30.07
:
Every answer asks a thousand questions.
The three most important words in science are not "I've got it!" but "Gee, that's odd."
(overheard on /.)
The three most important words in science are not "I've got it!" but "Gee, that's odd."
(overheard on /.)
: cool.
> If things started out as a big bang, on some scale,
> we will find a "center" of the universe.
But understanding why this is so is what makes all of this fun.
Remember that modern metric theories, of which General Relativity is just one, posit that the universe is four dimensional. Three space dimensions and one time dimension make up a four dimensional "spacetime". Unless you have seen an explaination of exactly what this means, it's just words, like "the universe is gizifa". This can lead to misunderstandings.
I'll try to explain what this means, using a model I'm sure you've seen before, but likely poorly explained. Consider a balloon, partially inflated. The surface of the balloon, the "skin", is effectively a two dimensional object. The balloon as a whole is three dimensional. You have a two dimensional surface enclosing a three dimensional volume. Still with me?
The reason we use this model is because it is very similar to our model of the universe. In this model everything you see around you, the three dimensional world, is the "surface" of a larger four dimensional construct. Just as the skin of a balloon is a 2D surface of a 3D space, everything you see around you is in the 3D skin of a 4D space. Still with me?
Consider the balloon again. Critically, there is no "center" to the surface. Where is the middle of the surface of a sphere? Where is the middle of the surface of the Earth? The question itself is just "wrong". In the case of the Earth we arbitrarily decided to draw lines on it in certain placed, latitude and longitude. You could do the same with a balloon, make the neck the "north pole" for instance. By the same token we could have chosen some other coordinate system entirely, let's put the "west pole" in Ecuador!
There is a point of the balloon as a whole that can be thought of as the center, through. Its in the space "below" the surface that's filled with air. The same is true of the Earth, the center is down below us, about 6400 km away. But, critically, that point does not lie on the surface.
Now one more thing to consider. Draw some dots on the outside of the balloon. Label one of them "milky way". Now start inflating the balloon. You'll notice that the dots will move away from each other as you inflate them. In fact, from the point of view of the "milky way", all the other dots are moving away from it. But the same is true of all the other dots too. No matter which one you pick to observe, you'll see that everything moves away from it. And that's because, for lack of a better way to put it, space itself is getting bigger. In fact, the dots aren't really moving at all relative to their original locations on the surface of the balloon, their real motion is along a line drawn into the middle of the volume, that "real center".
In the case of the universe the same thing applies. We look out in space and we see that everything is moving away from us. This is surprising if the universe is a 3D space, but complete expected if it's 4D. So where is the center of the universe? It's "down" somewhere. And what is that missing direction? Well we already said it, it's time. So what does that mean?
That means the center of the universe is a point in time, not space.
As soon as you really grasp this model you'll see why everyone likes it. For one, it trivially answers lots of different questions:
1) why is everything moving away from us?
it's not, everything is just "inflating"
2) why do we appear to be in the middle?
its just the way it looks, and it looks the same way everywhere else too
3) why are we moving apart at all?
because time is going forward (just look at your watch)
Hope this helps!
Maury
> If things started out as a big bang, on some scale,
> we will find a "center" of the universe.
But understanding why this is so is what makes all of this fun.
Remember that modern metric theories, of which General Relativity is just one, posit that the universe is four dimensional. Three space dimensions and one time dimension make up a four dimensional "spacetime". Unless you have seen an explaination of exactly what this means, it's just words, like "the universe is gizifa". This can lead to misunderstandings.
I'll try to explain what this means, using a model I'm sure you've seen before, but likely poorly explained. Consider a balloon, partially inflated. The surface of the balloon, the "skin", is effectively a two dimensional object. The balloon as a whole is three dimensional. You have a two dimensional surface enclosing a three dimensional volume. Still with me?
The reason we use this model is because it is very similar to our model of the universe. In this model everything you see around you, the three dimensional world, is the "surface" of a larger four dimensional construct. Just as the skin of a balloon is a 2D surface of a 3D space, everything you see around you is in the 3D skin of a 4D space. Still with me?
Consider the balloon again. Critically, there is no "center" to the surface. Where is the middle of the surface of a sphere? Where is the middle of the surface of the Earth? The question itself is just "wrong". In the case of the Earth we arbitrarily decided to draw lines on it in certain placed, latitude and longitude. You could do the same with a balloon, make the neck the "north pole" for instance. By the same token we could have chosen some other coordinate system entirely, let's put the "west pole" in Ecuador!
There is a point of the balloon as a whole that can be thought of as the center, through. Its in the space "below" the surface that's filled with air. The same is true of the Earth, the center is down below us, about 6400 km away. But, critically, that point does not lie on the surface.
Now one more thing to consider. Draw some dots on the outside of the balloon. Label one of them "milky way". Now start inflating the balloon. You'll notice that the dots will move away from each other as you inflate them. In fact, from the point of view of the "milky way", all the other dots are moving away from it. But the same is true of all the other dots too. No matter which one you pick to observe, you'll see that everything moves away from it. And that's because, for lack of a better way to put it, space itself is getting bigger. In fact, the dots aren't really moving at all relative to their original locations on the surface of the balloon, their real motion is along a line drawn into the middle of the volume, that "real center".
In the case of the universe the same thing applies. We look out in space and we see that everything is moving away from us. This is surprising if the universe is a 3D space, but complete expected if it's 4D. So where is the center of the universe? It's "down" somewhere. And what is that missing direction? Well we already said it, it's time. So what does that mean?
That means the center of the universe is a point in time, not space.
As soon as you really grasp this model you'll see why everyone likes it. For one, it trivially answers lots of different questions:
1) why is everything moving away from us?
it's not, everything is just "inflating"
2) why do we appear to be in the middle?
its just the way it looks, and it looks the same way everywhere else too
3) why are we moving apart at all?
because time is going forward (just look at your watch)
Hope this helps!
Maury
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