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On Space-Time

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  1. Nov 24, 2014 #1
    Hi all,

    I was wondering if anybody else here thinks the concept of space-time dilation/concentration (curvature) is a little bit funny, not in the sense of it having an effect on the neighbouring particles, but in the sense of actually stretching or contracting, as though it itself had certain properties and information, despite being nothing. Or am I missing something, and are the effects of general & special relativities only limited to the particles that experience them? The prime motive for this question, is, as I've said above, that it seems a wee bit difficult to prescribe empty space with information, its own internal set of rules, as, err, it doesn't have anything on which to 'stick' these rules to (it's nothing) - pardon the simplification.

    Thank you for the help,
    Adrian
     
    Last edited: Nov 24, 2014
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  3. Nov 24, 2014 #2

    phinds

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    Your confusion stems from your mistaken believe that time dilation, for example, is something that is "experienced". It is not. It is an observational phenomena.

    You, for example, right now as you are reading this, are traveling at .999999c (relative to an accelerated particle at CERN) and from the frame of reference of that particle, you are MASSIVELY time dilated. Do you feel any different?
     
  4. Nov 24, 2014 #3
    Sorry - I don't think you understood what I meant - I did not mean time dilation, I meant space-time dilation, and specifically the contortion of eucledian space to accomodate gravity. (I did not think of the 'time' aspect of space-time, as it's in a different category to the eucledian 'space')

    I also don't have a problem with the effects of space-time dilation, but I am scratching my head at the implication that actual space, nothingness, contracts in proportion to gravitational pull.
     
    Last edited: Nov 24, 2014
  5. Nov 24, 2014 #4

    phinds

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    Ah .... gotcha.

    I don't think it's helpful to think of space as a "thing" that gets "stretched/curved/whatever" but rather of spacetime as a framework in which things move according to the geometry caused by gravity.

    If space is a "thing" that IS stretched/curved/whatever, then you have to answer what IS this thing that gets contorted.
     
  6. Nov 24, 2014 #5
    Exactly! I was going crazy thinking that there's something wrong with me for not understanding Einstein's theoretical implications.

    And yet, is this not what contemporary Physics is based on? In the sense that if you are right at the edge of the singularity [black hole] about to fall in, you occupy an infitesimally smaller amount of space than you would if you were at the event horizon [despite having the exact same molecular structure, if we assume that you haven't been ripped to shreds by that point]?? Sorry if it's a bad example, but I think you know what I mean.
     
  7. Nov 24, 2014 #6
    Hi Adrian!

    - GR says nothing about space being "nothingness" or not. Einstein interpreted his predictions as that space has physical qualities. Einstein thus discussed the "metrical qualities of [..] space-time".

    - GR makes predictions about rods and clocks, not about invisible "space". And measuring rods are not predicted to stretch - nor is gravitational pull a factor!

    Instead, according to a distant observer measuring rods are predicted to shrink when moved downwards in a gravitational field, if they are pointing downwards/upwards. You can read it here:
    https://en.wikisource.org/wiki/The_Foundation_of_the_Generalised_Theory_of_Relativity
    Scroll immediately down to § 22. Behaviour of measuring rods and clocks in a statical gravitation-field - if you are allergic for the equations you can simply skip them, and only read the plain English conclusions just under (71) and (71a).
    Note: as you are there, you may also appreciate just under (72) ! :)

    PS: that gravitational pull is not a factor, is currently being explained by means of example D here:
    https://www.physicsforums.com/threads/time-dilations-on-confusing-situations.783613/
     
    Last edited: Nov 24, 2014
  8. Nov 24, 2014 #7
    Hi Harrylin!

    Yes, and this is what I meant - IF said measuring rod appears shorter to an outside observer, does it have to do with the actual object (rod) being physically shortened, or does it have to do with space contracting around a gravitational field? And if it is the latter, then clearly this space, nothing IS actually something, and has internal properties, does it not? (according to GR, whether explicitly or implicitly?)

    Adrian
     
  9. Nov 24, 2014 #8

    A.T.

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    Non contorted Euclidean geometry itself is also a set of rules. Doesn't it seem difficult to you, to prescribe empty space with that information?
     
  10. Nov 24, 2014 #9
    Not to me, though I could be missing something (which is why I'm here :) ) - prescribing empty space with a system of coordinates doesn't give that space any 'information' as it were, but if we say that that space will warp under certain conditions, hey presto, you have space that has certain internal properties, conditions, etc, that means that whenever it's subject to gravity, it will shrink. Which means that it's not just nothing, which seems a bit off to me.

    By the way, I hope I put this thread in the right section :/ (I didn't see a theoretical physics section)
     
    Last edited: Nov 24, 2014
  11. Nov 24, 2014 #10

    A.T.

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    Euclidean geometry is more than that. It tells you for example that the sum of inner angles in a triangle is 180°. Doesn't it seem difficult to you, to prescribe empty space with that information?
     
  12. Nov 24, 2014 #11
    Errr, I don't think so. The two examples aren't the same - the triangle doesn't grow to 181 degrees or 179 degrees based on its colour, or whatever external/internal factor you want to compare space-time concentration/dilation resulting from a gravitational field to. Can you elaborate?


    (To elaborate a bit myself, a triangle being 180 degrees isn't really native to eucledian space; it appears to me, at least off-the-cuff, to be nothing more than an geometric concept deduced through logic and maths - it doesn't actually amount to any information whatsoever in actual, physical eucledian space (not like you can turn eucledian space over and see a sticker saying 'FYI: here's a triangle, this is what it looks like, and its 180 degrees, no more no less!') - plus, we are the ones who decided what a triangle was, and how big a degree was, which means that we are the inventors and sole purveyors of the concept of a 'triangle' - I guess we can have a discussion whether the three dimensions is information in of itself, but it seems that space-time concentration/dilation according to gravity is a little bit different than identifying three axes in empty space. [P.S. And is XYZ also anthropogenic/centric? Hmm, need to read up on my maths.])
     
    Last edited: Nov 24, 2014
  13. Nov 24, 2014 #12

    A.T.

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    So you have no problem with empty space containing the information that triangle angles sum to 180° (Euclidean geometry)? But you have a problem with empty space containing the information for any other value than 180° (non-Euclidean geometry)?

    To clarify: Do you have difficulty with space containing...
    a) ...any information?
    b) ...just specific values?
     
  14. Nov 24, 2014 #13
    A.T. I'm not sure what your motivation is here, but all I'm asking is A. whether anybody else has a problem with space-time dilation/concentration (curvature) as it's commonly described in the literature, and/or B. can anybody confirm that I correctly understand said concept of space-time to affect not just the particles present within said 'plane' but also the space of said plane?
     
    Last edited: Nov 24, 2014
  15. Nov 24, 2014 #14

    A.T.

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    I'm trying to pin down where your difficulty lies, in order to address it. Answering my request for clarification would be helpful here.

    Is there any conceivable experiment that could tell the difference?
     
  16. Nov 24, 2014 #15

    Nugatory

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    How would you decide whether there even is an "actual physical Euclidean" space? There's no doubt that we live in some space, but is it in fact Euclidean?

    Select three points not all on the same line. Stretch strings tight between each pair of them to create a triangle. Then measure the three interior angles and add them up. This is not an abstract mathematical exercise, it's a direct measurement of something about the physical world that we live in.

    The interior angles will not, in general, add to exactly 180 degrees, and the discrepancy will be greater in the presence of stronger greater gravitational fields. Thus we conclude that we do not in fact live in an actual physical Euclidean space (although it's a pretty good approximation for daily life).
     
  17. Nov 24, 2014 #16
    Ok, with respect to both AT and Nugatory, enough with the bloody triangles - is there or is there not a warping of space in the presence of gravity? And if so, is that native to the particles that undergo said effects? Or is it native to the space in which said particles exist and they are merely 'passengers'? And if the latter is considered true, then how would one reconcile it with the idea that space is made up of nothing, and thus has no internal conditions (because there's nothing in said space to enforce said conditions)?
     
    Last edited: Nov 24, 2014
  18. Nov 24, 2014 #17

    A.T.

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    Is there any conceivable experiment that could tell the difference?

    How is Euclidean geometry reconciled with this idea?
     
  19. Nov 24, 2014 #18
    Jesus AT you can really take the life out of a discussion. Eucledian Geometry is reconciled with the idea of space as nothing in the way that I explained above, in an edit of my 2nd response [to yourself]. The only information that empty space could perhaps have are the three dimensions, and frankly even that sounds dubious, but I don't know sufficient maths to know for sure. Whatever the case, I think the original counter-example remains apt - a triangle doesn't grow to 181 or 179 degrees because it changes colour, or size - but empty space does appear to change if it's subject to gravity.
     
    Last edited: Nov 24, 2014
  20. Nov 24, 2014 #19

    A.T.

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    That alone doesn't imply Euclidean geometry. So how do you reconcile that specific geometry with your idea that space cannot have any information beyond the number of dimensions?
     
  21. Nov 24, 2014 #20
    AT, read what I wrote in my second response to you. Eucledian Geometry seems to be nothing more than a logical, mathematical, self-supporting construct existing outside of any actual 'eucledian', three-dimensional space. [read: it exists insofar that we know of it and understand it, like fiat currency (plus understanding)]
     
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