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Uniformity of distance

  1. Feb 4, 2012 #1

    i dont know if this is the correct place to post this question, but i have been crushing my head with something i thought about. Its making me insane. How can i know that distance is uniform across the universe? what i mean is, what guarantees me that a metre measured from A to B is the same distance as a metre from B to C? Is it possible to prove in some way that our measure device is not affected by some space distortion or anything like that? Is there actually any way to know it? If not, does humanity assume it is? Isnt it dangerous?

    Thanks in advance
  2. jcsd
  3. Feb 4, 2012 #2
    A unit of length, by the most basic definition, is the distance between two distinct, identifiable points on a rigid body. If we move the rigid body from one region of space to another and look at the same two points we are still looking at the same distance.

    We know that a meter stick is the same length here as it is over there because our definition of a meter is the length of that stick.
  4. Feb 4, 2012 #3
    But can we guarantee that the stick will have/represent the same length in distinct zones of the universe? Is there some physical law or concept that is guaranteed to be true that confirms it
  5. Feb 4, 2012 #4
    But we don't know(=can't prove) that this is constant, any more than we know that the charge on alpha Centauri electrons is the same as on ours.

    All we can show is that it is a sensible, reasonable assumption, in the abscence of any good reason to thing otherwise or supporting evidence either.
    Also that as far as our space probes have travelled, experiments have supported this assumption.

    However there are systems of maths and physics theories based on them that do not make this assumption.

    You should study the Poincare disk
  6. Feb 4, 2012 #5
    Then its an assumption until it fails. Just like almost everything we know about the universe, right? Since this question could be placed concerning any other property of view about the universe
  7. Feb 4, 2012 #6
    Some assumptions are better than others.
  8. Feb 4, 2012 #7
    But none is certain, right? I'm getting depressed...i'm arriving at the conclusion that we can't know anything for sure =(
  9. Feb 4, 2012 #8


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    Scientific knowledge is always malleable, absolutes are the realm of philosophy.
    But, take heart. Most scientific theories have symmetries allowing for scalability (relativity will really bake your noodle on this account). So, if the definition of a meter (e.g., length) changes from one location in space to another, so too must other things change accordingly (e.g., time, motion, force, energy, momentum, etc.) because there is an inter-dependency in units of measurement defining these quantities.
  10. Feb 4, 2012 #9
    =S when we start wondering about everything being a fake, i imagine how would it be if i wonder about the existence of the world around me and how i see it. Its too bad to think about this =(
  11. Feb 6, 2012 #10
    Would our eye be able to detect such phenomenon?
  12. Feb 6, 2012 #11


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    You are starting from a position that Plato would have understood and agreed with: there is "something" out there that really is "distance", but when you try to measure it you might be measuring it wrong.

    But science doesn't work like that. If you propose a procedure to measure something (in your case, this presumably involves carry one particular meter stick around the universe, but that's OK, it doesn't have to be a practical procedure, provided you can do it in principle), then what you measure is what you measure. If you decide to call what you measured "distance", it doesn't make any sense to ask the question whether "distance" is really something else.

    Of course if you discover that your procedure sometimes gives different measurements for the same thing, that's a different (and serious) problem that needs to be fixed - but that doesn't seem to be what your OP is talking about.
  13. Feb 6, 2012 #12


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    I think we may be selling Science a bit short here. When we look at the spectrum of light reaching us from a very long way away, we can see the same structure of lines - although red-shifted. This is quite good evidence that things must be much the same over there as here and that the relative masses, spacings and charges are not different from what me measure on Earth.

    So you either have to assume that 'everything' is basically the same or else it's all different - yet, (coincidentally and with a lot of ingenuity on someone's part) with the same relationships between all of the quantities. The "it's the same" argument is simpler so it's a more attractive proposition. But you could say it's only a matter of the way you look at it.

    It puts me in mind of the argument between Evolution and a very ancient Universe, on the one hand and, on the other, the careful planting of all that false evolutionary evidence to make a four thousand year old Universe 'look as if' it's much older. Not that I'd insult anyone who writes in this forum by comparing them with Evolution Deniers.
  14. Feb 6, 2012 #13

    i agree with the meter stick analogy, yet it makes me wonder if some hidden physical law could make the stick change its length in a different space zone without us noticing it
  15. Feb 6, 2012 #14

    I understand your argument, still, will we ever be able to be 100% sure that everything is basically the same? Can't hidden physical laws exist and be fooling us? I've thought that if i measured the universe using my foot, no matter how the space behaves, i would always end up walking a number of steps based on my foot size, and i would get tired proportionally to it and without noticing changes in space. But is this evidence or way of thinking also a good proof? What if my assumption that i tire accordingly to my foot size and not to space itself is wrong?
  16. Feb 6, 2012 #15


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    I dabbled in Solipsism (http://en.wikipedia.org/wiki/Solipsism) myself a few decades ago. It's fun, but a logical impossibility.
  17. Feb 6, 2012 #16


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    In the absence of more knowledge, we can only either assume that either option is possible or do the more satisfying intellectual thing and go for the simpler one. Experience would sort of justify that option.
  18. Feb 6, 2012 #17
    Nothing guarantees this because it isn't true! More particularly, distance is not uniform over time: we can observe this through the red shift of distant galaxies. The light we observe from these galaxies originated a long time ago when space was more compact: by the time the light gets to us the wavelength appears to be longer and so the light is shifted towards the red end of the spectrum.

    The constancy of apparent length you are searching for is only relevant in classical physics: it falls apart under relativity - google the pole barn paradox if you want to start on a long trail of discovery!
  19. Feb 6, 2012 #18

    what about the length of an object, a stick for instance, is it constant over time? Or our bodies
  20. Feb 6, 2012 #19
    That depends what you mean by length and who is measuring it.

    If a one metre stick is travelling towards the measurer she will measure it as less than a metre, if it is travelling away more than a metre.

    If you measure the stick with another stick then you will detect no change.

    If you take two sticks the same length, accelerate one to near light speed along a circular path (so time passes slower for the accelerated stick) and compare them again they will still be the same length.

    If you measure the stick by seeing how long it takes for light to travel along it, you will detect no change. This is how we define the SI unit of length (metre) so yes I suppose you can say that the length of an object that is stationary relative to the measurer is constant over time.
  21. Feb 6, 2012 #20
    Also through the whole space?
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