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Old September 28 2012, 04:04 AM   #64
Christopher
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Re: About planetary gravity

The Borgified Corpse wrote: View Post
So, let me see if I've got this straight? In the beginning, the universe was infinitely dense in an infinite space. That infinite space then expanded, leading to the level of density that we are now familiar with in the universe. But since space is infinite and always has been (even though infinity seems to be getting bigger), it means that there can be no center to it. Am I grasping the concept so far?
Close enough for a BBS post. There are plenty of books on the subject if you want to examine it further.


I've certainly never been able to wrap my head around the idea that the universe is really a 2-dimensional or 4-dimensional object and that the 3-dimensional space that we interact with on a day-to-day basis is merely an illusion.
Well, that's not what I'm saying here; it's just often helpful to use 2-dimensional surfaces as analogies for 3-dimensional space, because it's easier for people to grasp a "Flatland" in 3D space than to grasp a 3D volume in a 4D hyperspace.


The fundamental existence of 3-dimensional space as the only space with any practical applications in the universe in which we live seems undeniable.
Well, except that there must be at least one additional dimension, time. It's not space, it's spacetime.

And if there are higher dimensions, they certainly could influence how things work in 3D space. String theory proposes that the differences between particles and forces are influenced by how the strings they're made of vibrate in other dimensions, and that the reason gravity is such a weak force is that most of it "leaks" out into other dimensions. Although I'm becoming increasingly skeptical that string theory is right, since there's still no real way to test it and it's basically an abstract exercise without evidence.


I suppose, if I'm to interpret the balloon analogy correctly, what you're saying is that, if there is a center to the universe, the center exists in a 4th dimension that cannot be measured and that is the thing that is actually expanding, causing the rest of the universe to become more spread out without changing the actual infinity of it all. Is that right?
Not necessarily. That's what would be the case if the universe were closed, as I said, but evidence suggests it's flat. Again, the balloon description is just an analogy; you're not supposed to treat every part of it as meaningful, just to focus on the particular aspect that the analogy is meant to get across. For instance, I'm not suggesting that galaxies are really microbes, or that some big kid inflated the universe. The balloon-surface analogy is just one relatable example of a type of expansion without a center. Another that's often used in talking about the universe's expansion is a loaf of raisin bread rising in the oven, with all the raisins moving apart from each other uniformly. Of course in that case there's a center to the loaf of bread because it's finite in size, but the bread isn't expanding outward from that center point; rather, every part of it is getting farther away from every other part uniformly, discounting edge effects. If the loaf of bread were infinitely large, or at least larger than your ability to measure, you could not find a unique point that everything was expanding outward from, because every point within it fits that description equally well. The reason no point is the center of the universe is because every point is.
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