4 ms·

Just as difficult a question: Is it possible for piece of string to be 2 units long? In the real world, you always run into limits on how finely you can measu

• joejohnson 15y ago
  I think something could be two units long. Two is discrete, and at a microscopic level, things can be exactly discrete values (i.e. two angstroms...). I think asking if something could be exactly pi long is a different question.
  • ars 15y ago
    Actually, no they can't. At a quantum level things do not have definitive sizes. They have sort of "clouds", where the center of the cloud is more likely to be their size, and the edges are less likely - but still possible.
    • nitrogen 15y ago
      So a string of 24 carbon atoms is not 2 dozen carbon atoms long? Unit=dozen carbon atoms, length=2?
      • ars 15y ago
        No. A carbon atom does not have a fixed size. And especially when you attach atoms together the spacing between them varies depending on the strength and type of the bond. Additionally, molecules are constantly in motion (heat) - one of the motions is changing the size of the spacing between them. But even if you froze it to absolute zero (which is impossible), it doesn't matter, the size is not determinate, it's only probabilistic. So a real physical object can never have a fixed size.
  • fexl 15y ago
    Yeah I gotta think about that one. If we assume that there is some smallest discrete base unit of space (which I think is plausible), then all string lengths could only be some integer multiple of that base unit. Therefore length pi is out of the question. Now presumably 1 meter will be equivalent to some integer multiple of base units, though that may turn out not to be the case. Perhaps the current definition of a meter in terms of certain wavelengths of light will turn out to have a remainder of 1/7 of a base unit. ;) In that case you'd just define a slightly adjusted meter as a whole number of base units. So for simplicity let's assume that a meter is a whole number of base units. Then it's clear that a string of any integer meter length would be possible. A string of length 1/3 meters might be possible, assuming that the number of base units in a meter was divisible by 3. But a string of length pi meters would be impossible. As a computer programmer I tend to think of transcendental numbers in terms of processes which relentlessly converge, and that of course is the theory of limits. But I also recognize the financial constraints on running processes. So there will always be a "good enough" aspect to any physical measurement.
    • ars 15y ago
      See my reply here: http://news.ycombinator.com/item?id=2667461 http://news.ycombinator.com/item?id=2667461 There is no discrete base unit of space, and on top of that objects do not have determinate sizes.
      • fexl 15y ago
        I was just taking the gambit and running with it. Certainly when you start zeroing in on the end of the string, things start to get very fuzzy indeed.
      • jarin 15y ago
        Isn't the Planck length a theoretical base unit?
        • ars 15y ago
          Theoretical, yes. No (current) evidence for it to be a base unit though. But it doesn't matter, actual physical object do not have definitive sizes.
• derleth 15y ago
  > number theory How does number theory relate to this? (I think you just misused the term, and are thinking more about the philosophy of mathematics and not about that specific discipline.)
  • fexl 15y ago
    Number theory relates because his question suggests that a transcendental length might be especially problematic. I don't think it is, because of the hard limits on our ability to measure physical phenomena. So it doesn't matter if we're talking about lengths that are integer, rational, algebraic, transcendental, computable, or otherwise. The end of the string is fuzzy at many different scales, so even defining its length at high precision becomes a problem, and at very high precisions, you actually change the length when you measure it. If that sounds like me ducking the question it's because nature itself ducks the question.
    • derleth 15y ago
      > Number theory relates because his question suggests that a transcendental length That isn't what the field of number theory concerns itself with.
      • fexl 15y ago
        I understand that. I'm just saying that the original post drags in the concept of transcendental numbers, a key aspect of real number theory, and attempts to mash it into the real world where it doesn't fit. Recall what I said here: "In the real world, you always run into limits on how finely you can measure things before you run into quandaries about number theory." In other words, when you measure entities and processes in the real world, it is unlikely that you'll ever have to think about the nature of the continuum, or transcendental numbers, or even for that matter precise integers. For example, back in the '80s when I was using an Apple II computer to collect measurements from a microwave dish, it was obvious that we should measure amplitudes to four decimal places (or whatever, I forgot), and not concern ourselves with the impossible task of counting a precise integer number of energy quanta at each frequency. Certainly when you're measuring energies in a particle accelerator you'll have different standards, but you will still always come face to face with the economics of "good enough" and even Heisenberg's hard limits on observability in principle. So to reiterate: the constraints of the real physical world will always bind you long before you ever have to care about the vagaries of number theory. Nevertheless, the abstract theory of real numbers is definitely useful even in the physical sciences, because that theory transcends all physical constraints and therefore imposes no a priori limits on observations. So it is not wise to fetter your mathematics with the chains of physical constraints.