About planetary gravity

Discussion in 'Science and Technology' started by RB_Kandy, Sep 14, 2012.

  1. Tiberius

    Tiberius Commodore Commodore

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    Well, I was responding to RB Kandy's suggestion about gravity being the force of the universe expanding outward. I don't see how it is possible to have this force coming from inside the Earth if Kandy's postulation is correct.
     
  2. RoJoHen

    RoJoHen Awesome Premium Member

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    You've pretty much got it. The annoying thing about being humans is that we really can't quite comprehend the existence of things beyond out 3-dimensional space. Things like, how can the universe be "expanding" if it's already infinite? What exactly does an infinite thing expand into? Even the balloon analogy is flawed because we can clearly see the balloon getting larger and expanding into the air around it.

    It's the same reason we have a problem with the question "If God created the universe, then who created God?" We can't grasp concepts like "forever" or "infinity" because we ourselves live such finite existences. My brain is too small to comprehend such big things.
     
  3. The Borgified Corpse

    The Borgified Corpse Admiral Admiral

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    Agreed. The balloon analogy does help me wrap my head around the idea of everything expanding away from each other at an equal rate without anything moving towards anything else. But at first, I did have trouble, just with the way it was phrased, understanding the point; i.e. that while the balloon is expanding, the center has nothing to do with the surface of the balloon.

    Mentally, I tidy that up by defining God as the thing that predates all things, including the universe. God is the ultimate original source of all creation and anything that was itself created cannot be called "God." If "God" created the universe but something else created "God," the something else would be God, not this "God" named thing. I have no problem with the idea that there is some kind of universal constant that exists before, after, and separate from the universe as a sort of bedrock upon which all other things exist.
     
  4. Christopher

    Christopher Writer Admiral

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    Close enough for a BBS post. ;) There are plenty of books on the subject if you want to examine it further.


    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.


    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.


    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.
     
  5. The Borgified Corpse

    The Borgified Corpse Admiral Admiral

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    Suppose the universe were closed, but still edgeless? What if the universe as we perceive it were really the 4th dimensional equivalent of a sphere? If you travelled forever along its 3-dimensional "surface," you'd never reach the edge, although you might eventually come back to the point from which you started.
     
  6. sojourner

    sojourner Admiral Admiral

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    moebius strip universe.
     
  7. gturner

    gturner Admiral

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    If the universe was a moebius strip we'd be able to detect large traces of interstellar and intergalactic cellulose, yet we don't.
     
  8. Maurice

    Maurice Vice Admiral Admiral

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    Here's a topic [link] on the issue of the elevator in Total Recall
     
  9. iguana_tonante

    iguana_tonante Admiral Admiral

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    And that's why we have maths. :techman:
     
  10. Christopher

    Christopher Writer Admiral

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    Missed this before:

    Gardner didn't offer any specific equations in his article, I'm afraid. But your father must've been thinking of terminal velocity, the maximum allowed by air friction. The scenario Martin Gardner described in his essay assumed the tube was in vacuum and the train/elevator was magnetically suspended apart from the walls, so there was no friction and no terminal velocity limit.

    Also, terminal velocity would be different depending on the density and surface area of the object and the density of the air it falls through (which is why a sheet of paper falls faster when crumpled than when flat). The 150 mph figure is terminal velocity for a human adult falling under 1g acceleration through the lower atmosphere of Earth, not a large elevator car. So if the car in the Fall wasn't in vacuum, there would be a terminal velocity, but we don't know what the actual value of it would be.
     
  11. Deckerd

    Deckerd Fleet Arse Premium Member

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    You could calculate it, provided you know the values of the car.
     
  12. Christopher

    Christopher Writer Admiral

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    You'd also need to know the air pressure and the gravity, both of which would be changing throughout the length of the shaft, so there wouldn't be a single result.
     
  13. RoJoHen

    RoJoHen Awesome Premium Member

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    Unless you're in the US. Then you just have math. :p
     
  14. iguana_tonante

    iguana_tonante Admiral Admiral

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    So your educational system is so poor they only teach you one? That's a shame.

    ;)
     
  15. gturner

    gturner Admiral

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    No, we just take as a postulate that the system of thought that establishes the existence of plurals can't itself be plural because it results in an inifinite recursion and what's known as the British English paradox, whereby they commit pluralization errors that are actually correct, which can't be true becaus they're also errors (thus the paradox).

    Okay, I just made that up.
     
  16. RoJoHen

    RoJoHen Awesome Premium Member

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    You totally made that up, but it sounded good.
     
  17. iguana_tonante

    iguana_tonante Admiral Admiral

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    Well, you always do that.