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Zeno's paradoxes

  1. Apr 24, 2004 #1

    honestrosewater

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    Just curious.
    Is everyone except me satisfied with the solutions to Zeno's paradoxes? see
    http://plato.stanford.edu/entries/paradox-zeno/ for info.
    It's been a while since I thought about these at any length, but I certainly remember never being satisfied with any solution to or refutation of them. If you are convinced they're rubbish, or quasiparadoxes, what convinced you?
    Perhaps this belongs in a different forum, if so, please relocate :)
    Happy thoughts
    Rachel

    EDIT- I didn't write what's on that site. I mentioned it because it's one of the more extensive. You can find brief explanations by just googling zeno's paradoxes.
     
    Last edited: Apr 24, 2004
  2. jcsd
  3. Apr 24, 2004 #2
    Honestrosewater:

    You have done great work with the pirate :biggrin: problem.
    I will study it tomorrow and replay to you there.

    Please share with us your thinking about Zenon paradox.
    My belive it that everting is open even a mathematics true.

    Thank you
    Moshek
     
  4. Apr 24, 2004 #3

    pig

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    That is a very long text and I am far too lazy to read it at the moment. Sorry about that :) But I will write something about one of the paradoxes mentioned - Achilles and the tortoise.

    Let's consider a hippopotamus walking 2 m/s following an armadillo walking 1 m/s. The armadillo is 1 meter ahead of the hippopotamus.

    What Zeno would say:

    By the time the hippo reaches the dillo's original position d0, the dillo will have travelled to a position d1, half a meter ahead. By the time the hippo reaches d1, the dillo will be at d2, a quarter of a meter ahead, and so on. No matter how many times we do this, the dillo is always a little bit ahead. Therefore, the hippo will never reach him, and the dillo is safe from being crushed under its tremendous weight.

    Where Zeno is right:

    No matter how many times we do this, we will not reach the point where they are in the same spot.

    What Zeno doesn't say:

    No matter how many times we do this, we will not reach the point in time where 1 second has elapsed. Therefore, this doesn't prove that the hippo will never reach the dillo, since by Zeno's method, we never look far enough ahead in time. It only proves that at any moment in time before 1 second has elapsed, now matter how close, the dillo is ahead, which is true :)

    That's why we say - As time tends to 1 second, the dillo tends to get stepped on. :) If we want to get to the 1 second mark Zeno's way, we must do it infinitely many times, and as we approach 1 second, the distance approaches zero. If you argue that it "doesn't make sense", and that the distance will "never actually reach zero" - then the time will also never actually reach one second.

    I apologize if this point of view was mentioned in the text, or if the paradox still holds in some way in spite of it.
     
    Last edited: Apr 24, 2004
  5. Apr 24, 2004 #4

    Hurkyl

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    To make an analogy... if you only look at red M&M's, does it make sense to conclude that no green M&M's exist?

    That's the kind of impression I get from Zeno's paradoxes. You'll never see the end result in the sequence of events he studies, but why should one then conclude that the end result doesn't happen?
     
  6. Apr 24, 2004 #5

    arildno

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    As to the arrow paradox:
    If we set a distance travelled s equal to the product of velocity and time elapsed (s=v*t), I can't see that Zeno's observations amount to more than saying if the time elapsed is zero, then s is zero (which is true, since v*0=0 for any number v).
    He then goes on to say this observation implies that v=0 (the so-called paradox).
    I really can't see that implication..
     
  7. Apr 24, 2004 #6

    honestrosewater

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    To be fair to Zeno, we don't know exactly how he stated the paradoxes, so any criticism of him must bear that in mind.

    pig, yes, nice effort :) So if you can define a (dimensionless) point on the hippo, for its position, the point the hippo must reach, for the limit, and measure infinitely small intervals of time, you have three less problems :)
    How would you translate the absract mathematics to the physical world?

    Hurkyl, my analogy- if you call a tail a leg, how many legs does a mule have? 4. Calling a tail a leg doesn't make it one. (before someone jumps on me, 4, assuming the obvious) That's the kind of impression I get from the solutions- they may be correct in themselves, but do not address the issue.

    arildno, I think I missed something else in your argument, but can't put my finger on it yet.
    Afraid I'm too tired now to get into a long post. Nothing moves until it moves. Case closed :) No, tomorrow...
    Happy thoughts
    Rachel
     
  8. Apr 24, 2004 #7
    Funny thing about Zeno is that... You can actually see the paradox in the summation of Zeno's paradox... Its 1!!! Hehe...
     
  9. Apr 24, 2004 #8

    pig

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    Rachel,

    If you wish to avoid dimensionless points, let's define the distance between the two as the distance between the hippo's nose and the tip of the dillo's tail.

    If you wish to avoid limits and working with infinitely small numbers, then no matter how many times you divide, the distance to be travelled is always strictly greater than zero, but note that the elapsed time is then also always strictly less than 1 second, so the hippo doesn't reach the dillo within a second which is correct.

    You get the paradox when you try to put infinity into this in an incomplete way - if you divide their movement into infinitely many steps, but don't divide the time proportionally. The possible reason is that it takes you a couple of seconds to imagine each step and would take forever to imagine the whole movement, but it doesn't work that way for them - in reality required time gets smaller and smaller for each step.

    What Zeno does is basically divide a finite quantity into infinitely many parts, and say that because there are infinitely many parts, their sum is infinite. :)
     
    Last edited: Apr 24, 2004
  10. Apr 24, 2004 #9
    The implication is that you can't determine distance travelled by adding up the distance travelled at each individual instant in time. If you assume that you should be able to calculate distance in that manner than this is a paradox.

    The problem with Zeno's paradoxes is that solving them generally involves changing definitions of terms like distance and velocity. That seems like "cheating" since you haven't really solved the paradox, just changed your definitions to avoid it; people are often very attached to their definitions of basic terms like distance.
     
  11. Apr 25, 2004 #10

    pig

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    By this same logic, you couldn't have divided a nonzero time interval into "each individual instant in time" in the first place:

    If by summing the distances you get 0 meters because the travelled distance is 0 in each individual moment, then by summing the intervals you also get 0 seconds because each individual interval is 0 seconds, and since the original interval was nonzero, the division is obviously incorrect.

    In both of these paradoxes, as Zeno divides distance, he also divides time, so he should treat them the same. He doesn't. You cannot ignore the concept of time when dealing with speed, because speed is defined as distance in time. In the hippo case, Zeno says "never" on the basis of infinite number of steps required, but that is irrelevant, what is relevant is the time all those steps take.
     
    Last edited: Apr 25, 2004
  12. Apr 25, 2004 #11
    Even more generally: take an interval of real numbers [a,b] with b>a. It is composed of points. Those points have zero length. Therefore the sum of all those lengths is zero. Therefore the interval has zero length. But the interval has length b-a>0 so this is a paradox.

    Zeno's paradoxes rely on this paradox a great deal: take a concept that is well defined on intervals, extend that concept to points, then demonstrate a paradox.
     
  13. Apr 25, 2004 #12

    honestrosewater

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    But exactly! Where exactly does the hippo's nose end and the "not hippo's nose" begin? You cannot avoid dimensionless points because the distance between the hippo's nose and the dillo's tail will eventually be smaller than any dimensional "point" you put on them. So you must then make your "point" smaller, only to eventually have to make it smaller again... this has nothing to do with time. It has to do with infinitely divisible space.

    Even if you say: these two atoms, x and y, are touching; x is on the hippos nose and y is not on the hippos nose. And use the point where the two atoms touch as your "point". But then you have only changed the problem to the distance between any two "touching" atoms. Where do two atoms touch? You must now define where an atom ends and "not an atom" begins.

    I don't think the paradoxes can be so easily discarded.

    Happy thoughts
    Rachel
     
  14. Apr 25, 2004 #13

    honestrosewater

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    Thank you! There is no "next" real number! There is no "next" rational number either. The integers have "next" numbers, because they are constructed from the unit 1. You cannot define length until you define length. This is Zeno's Dichotomy paradox. There is no "next" position.

    Remember Russell's paradox? I think it is related. If you want to define a set by listing its elements, you cannot define a set that contains itself. After every definition, you must extend the definition.

    Happy thoughts
    Rachel
     
  15. Apr 25, 2004 #14

    honestrosewater

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    Perhaps Zeno saw the difficulties in dividing space infinitely and in time infinitely and decided to combine them, using motion. He also has plurality paradoxes which treat only space.

    Happy thoughts
    Rachel
     
  16. Apr 25, 2004 #15
    But expressing the paradoxes in terms of space and time attaches useless baggage to them. These aren't paradoxes within space and time, only our attempts to define them with mathematics.

    Russell's paradox isn't about defining sets as lists of their elements. Nor is it a problem with sets that contain themselves. It's a problem with unrestricted comprehension...the idea that all we need to define a set is to provide a rule for determining what is contained in that set.

    I don't see how the fact that there is no next position is interesting. It's rather trivial to demonstrate without Zeno's paradoxes.
     
  17. Apr 25, 2004 #16

    honestrosewater

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    But Zeno couldn't express them by saying "take an interval of real numbers [a,b] with b>a..." I don't see what you mean.


    What about the idea that all we need to define a path is to provide a rule for determining what is contained in that path?
    You must also specify the range, what can be contained in that set, or to what the rule will apply?
    The path is a set of points. But is the path not also a set of intervals? Think of a point as an element (an element is not a set). Then an interval is a set (an interval contains points). So how can you switch from points to intervals in the same rule? Wouldn't you need two different rules, one that applies to points, and one that applies to intervals? And even to intervals that contain intervals? Not to mention applying the same rules to physical objects and mathematical objects. Perhaps I am not saying this correctly. Or maybe I'm just wrong.

    Same as above, Zeno didn't have modern mathematics. I think he is expressing the same concept that is expressed using mathematics. But the mathematical solution or explanation does not make sense for space and time, for the physical world.

    Happy thoughts
    Rachel
     
  18. Apr 25, 2004 #17
    Yes, this is the idea...we have to be careful not to extend definitions beyond the point where they make sense. We can find the distance travelled over two intervals of time by adding together the distance travelled over each interval. That doesn't mean that we can find the distance travelled over an interval by summing the distance travelled at each point.

    Zeno's paradoxes aren't really paradoxes in nature...they're paradoxes in mathematics. When we try and use that inconsistent math to define nature then the illusion is created that there are paradoxes in nature itself. The solution is to use math which is not inconsistent.

    This might seem unsatisfactory. After all, we can't "redefine nature". But the abstractions that we use to describe nature can be redefined.


    I certainly don't fault Zeno for not being able to apply the knowledge accumulated in the many years after he died. But that doesn't mean we can't apply that knowledge now.

    When we express the problems Zeno brought up in terms of space and time, we complicate the issue. Everyone has preconceived notions of what space and time are, and those notions get in the way of discussing the core issue "how do we measure infinite sets?" We can discuss the physical world after we fix up the math.
     
  19. Apr 26, 2004 #18

    honestrosewater

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    So how do we measure infinite sets? :biggrin:

    I also remember running into problems when "learning" about air resistance on a falling body- elementary, right? As I recall, an object can encounter n meters/second of resistance without ever reaching a speed of n m/s (unless it was travelling n m/s while at rest, before it was dropped).
    i.e., it slowed down this much because it would have encountered this much resistance if it hadn't slowed down ;) Maybe I just didn't get it. Seems like a lot of information being exchanged instantaneously. Guess it boils down to instantaneous velocity which I never liked. Maybe I just don't get lots of stuff. Maybe it's because I saw this strange French film last night and am feeling rather pessimistic and broody.

    Happy thoughts (if it matters) :rolleyes:
    Rachel
     
  20. Apr 26, 2004 #19

    pig

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    I now understand what you are talking about. From my experience, most people conclude that mathematically, one object will not reach another no matter how much time passes - they keep slowing down the hippo in their "thought experiment", but not time. :) And that's not where the paradox is. I hope you understand what I'm trying to say. I originally assumed you were making that mistake.

    I don't really have anything smart to add here since master coda explained it better than I could. The problem is how we deal with infinity, not in nature itself. :)

    [edited to remove something stupid and irrelevant.. note to self - read, comprehend, post]
     
    Last edited: Apr 26, 2004
  21. Apr 26, 2004 #20

    arildno

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    Excellent discussion of the meaning of Zeno's paradoxes, master_coda and honestrosewater!
    Clearly, I've been much too dismissive of his thoughts.
    Thx a lot!
     
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