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A naive question about irrational numbers

  1. Oct 20, 2012 #1
    I've been thinking about this recently and couldn't find the answer to my question (even though I assume it's a really simple one, so forgive me if it's too trivial).

    Let's say we have two rods of length 1 meter and we put them at right angles to each other. Then we cut a third rod just long enough to connect the non-touching ends of the first two rods - now we have a right isosceles triangle. Question: what's the length of the hypotenuse? Clearly the answer is √2

    Now suppose we physically measured the length of the rod which is the hypotenuse (maybe by multiplying the number of atoms on top of each other times the length of each atom?) Aren't we going to get a rational number as a result?
  2. jcsd
  3. Oct 20, 2012 #2


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    "Now suppose we physically measured the length of the rod which is the hypotenuse (maybe by multiplying the number of atoms on top of each other times the length of each atom?) Aren't we going to get a rational number as a result?"
    the relevance being?
    For example, a mathematical triangle is characterized by that "points" on two sides of the triangle will get ARBITRARILY closer to each other as they converge to their commmon joint. (Agreed?)

    No physical "triangle" will achieve that, so the only insights from this example is that
    A) maths do not attempt to describe reality, but elucidates logical structures
    B) In the studies of reality, maths will be an extremely useful tool for model building and prediction.
  4. Oct 20, 2012 #3
    Yeah, I get the overall point. Could you talk about B) a bit more? I completely agree with the statement in general, but I can't see how it follows from my example...
  5. Oct 20, 2012 #4


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    Simple. Even though the mathematics may not give you the exact answer, it gives you a good idea what the answer should be near. So, let's say you were only told those two rods were both 1 meter and the rod used to make the hypotenuse was 3 meters, then you should be able to say, "hey wait, that sounds like the third rod is to long and that there were will be a lot of excess."
  6. Oct 20, 2012 #5
    In the same regard you can always plot an irrational number on a real number line and you can always measure the distance of that point from the zero point even though it has no exact value.
  7. Oct 20, 2012 #6


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    Be careful of your wording! Every real number has an "exact value" whether it is rational or irrational.
  8. Oct 20, 2012 #7
    Is it accurate to say that if we had two rods which are exactly 1 meters long, it would be impossible to connect them with a third rod without a slight residual (of, say, 1 atom)?
  9. Oct 20, 2012 #8


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    What does exact mean?
  10. Oct 20, 2012 #9
    With no residual :)
  11. Oct 20, 2012 #10
    True, indeed. My mistake! Sorry for that. Here's what i meant to say- Irrational numbers have a non-terminating, non- repeating decimal expansion and taking that definition into account, we can have a value that is approximately close to the exact value and not the exact value can be represented coz we don't know it.
  12. Oct 20, 2012 #11
    this doubt of yours is solved by the concept of unit cells in chemistry. no atom exists of its own but in a lattice and when you calculate the edge length of that lattice, many a times the result is in terms of irrational numbers. So, that would suffice cause, an irrational multiplied by a suitable rational will give a number that will show us the exact number of atoms.

    For more, search for Bravis lattice on wiki
  13. Oct 20, 2012 #12


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    The real world does not have an exactly 1 meter anything. The comparison relies on the way we measure and the size of the measuring stick used for reference. Take the coast line of any country. If our measuring stick is a kilometer, we leave out out a few inlets, so shrink the stick and the coast line gets larger. If the measuring stick gets down to an atom, the coastline becomes very large. That is why it is impossible to have a theoretical mathematical value placed on a real world item.
  14. Oct 20, 2012 #13
    I think the answer is simple, if you create an exactly 90 degree triangle with exactly 1 meter legs, the hypotenuse would be exactly the square root of two if you were to measure it (keeping in mind not to round the measurement of the hypotenuse, but measuring as many decimal points as you like with your absolutely exact measurement tool and verifying that all the decimal points are the same as for the square root of two). Any difference from the square root of two is because of measurement error, you just have to measure more exactly.
  15. Oct 20, 2012 #14


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    It isn't possible to do any of the things you said. We can't create an angle that measures exactly 90 °, and we can't create legs that measure exactly 1 meter. Furthermore, whatever the length of the hypotenuse, it is not physically possible to measure it exactly. There is no "absolutely exact measurement tool." The triangles that we use in geometry and trigonometry are idealized figures, not things we can create in the real world. When we measure a physical triangle, any such measurement is an approximation. We can get values that are closer by using more precise measurement tools, but we can't get exact values.

    How do you propose to do that?
  16. Oct 20, 2012 #15
    I would like to ask a clarifying question. Is it the case that space is continuous and that is why we can't ever measure exact distances, or we don't know if space is discrete or continuous but we simply don't have the instruments to make the exact measurements (like to pick an object with a length of exactly 1m)?
  17. Oct 20, 2012 #16


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    The case is that maths and the study of reality are logically separate areas, and that you shouldn't go about presuming that the conditions and premises set up in maths (and leads to some results there) are present in "reality".
    Thus, it is utterly irrelevant how the world behaves, that has no bearing whatsoever on whether a mathematical proposition is true or not.
  18. Oct 21, 2012 #17
    Excuse my lack of understanding, I'm not a physicist and don't have any related background. I'm not sure how the last post answered my last question.

    I understand that math deals with idealized objects and not everything that's true about math is also true about the physical world. But why is it that lengths of exactly 1m don't exist in the physical world? Is it possible they exist but we simply don't have the proper instruments to measure them?
  19. Oct 21, 2012 #18
    if you are a physicist, you might already know that we take all units and measurements relative to something. like weight of 1 kg is taken as weight of Ir-Pt cylinder kept in sealed jar and all measures of weight are taken relative to the same. similarly about all other 7 base units.
  20. Oct 21, 2012 #19
    Hmm, I said I am NOT a physicist :)
  21. Oct 21, 2012 #20
    sorry, i read it wrong. the other thing i told of is true. you may refer any college level or class 11 or class 12 level physics books and then you'll know. the seven base units- second, kg, candela, metre, ampere, mole and Kelvin are defined as base units and all other units can be derived using these 7. for more, refer-

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