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What is Spacetime?

  1. Jun 4, 2005 #1
    What is Spacetime? I think this is an essential question in Cosmology. Some will accept, others don't.
    Has spacetime a hidden property, is it a sub-material elastic object/background or ...? Is there one spacetime or are there spacetimes? Is spacetime the "Field" (das Feld) Einstein referred to?

    The Stanford link http://plato.stanford.edu/entries/spacetime-holearg/ shows some approaches.

    Marcus pointed out in post https://www.physicsforums.com/showpost.php?p=588977&postcount=7 - referring to an article with Renate Loll - that: "According to Einstein, spacetime is flexible (malleable, ductile), but also at the same time continuous and smooth-----for example it does not have breaks (fractures, gaps, rips....)".

    So:
    1. Flexible (thus elastic),
    2. Continuous and smooth,
    3. No breaks (non-breakable).

    In my speculative opinion spacetime is indeed a hidden object and it is non-breakable.
    But how can we come from these properties to local, discrete packages (which seems independent)?
    Well, by bending and penetrating parts of spacetime with each other you can create locally discrete zones (holons) where two or more parts of that object interact - and influence - each other. That creates locally friction (and kinetics effects) between the spacetime layers. Interaction in such system is a feedback approach: The local parts influence the larger hidden system and the hidden system influences the local packages.
    The consequence of this approach is that we can not make a distinction between matter (fields) and energy (fields) because they all come from the same properties. Interconnectedness in incorporated in everything, and that's called "gravity". Our Universe is thus "restructured gravity'.

    Attached image shows you the basic concept of the multi-layers spacetime. If the image is not loaded you can find that on this link http://mu6.com/holons_2/genderless_to_duality.jpg .

    But ... what is spacetime in your opinion?
     

    Attached Files:

  2. jcsd
  3. Jun 8, 2005 #2
  4. Jun 8, 2005 #3
    I've theorised a bit on the nature of space/time but no one can actually say what spacetime fabric is as of yet because there is no way of actually proving anything conclusively.
     
  5. Jun 8, 2005 #4

    selfAdjoint

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  6. Jun 8, 2005 #5
    first I found the old one afterwards (via Google!), secondly since 2003 there are lot new members, thirdly I was touched by the way Renate Loll expressed her points in the article (in Dutch).
    BTW she stays on the level of spacetime "ripples" ('plooien' in Dutch). If something in non-breakable then there can be also other geometric concepts like topogical caps like I explained. I wonder why she stays in the one dimensional thinking of ripples. Why not multi-layers created by only one layer?
     
  7. Jun 8, 2005 #6

    selfAdjoint

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    Maybe because she doesn't find any need for multiple layers!
     
  8. Jun 8, 2005 #7
    :biggrin: Touché.
    yes probably. Or she didn't imagined the concept. I e-mailed her last weekend and asked. No reply yet. But maybe she doesn't find any need to answer. :wink:
     
  9. Jun 8, 2005 #8

    marcus

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    I was hoping someone who can read Dutch would read the original. I was guessing about some of the words and sometimes only made a very rough approximation in English. Even the title I did not translate exactly, I think! Any help you can give would be appreciated Pela.

    I wonder if the dutch word 'plooien' is related to the english word DEPLOY (unfold) and EMPLOY (fold in) and maybe also the PLEATS in the fabric of a curtain or dress. havent checked, just guess.

    Here is the Dutch text
    www.phys.uu.nl/~loll/Web/press/knutselen.pdf

    here is a thread with partial rough English translation
    https://www.physicsforums.com/showthread.php?p=588977&postcount=6
     
    Last edited: Jun 8, 2005
  10. Jun 8, 2005 #9

    marcus

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    thanks for linking us back to that Good Old Thread of 2003! I had forgotten that thread. Somebody says that scientists have discovered that the color of spacetime is beige and that he thinks it is made of caramel pudding----one may refer to it not as the spacetime continuum but as "the spacetime FLAN'


    In this thread a poster named Eh gave a germane quote from Einstein
    https://www.physicsforums.com/showpost.php?p=69491&postcount=4


    "Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."

    At one time I found a link to a source for that quote which gives context, but I posted the link and forgot where. Does anyone have a source?
     
    Last edited: Jun 8, 2005
  11. Jun 8, 2005 #10
    OK Marcus I will translate this night some parts and later the others.
    Indeed I believe "plooien' is related. We also use 'plooien' in the curtain or dress. Very similar toe 'plooien' is 'vouwen' (like folding something as an action, but also the ripples in the elbow or skin.)
     
  12. Jun 8, 2005 #11

    marcus

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    thanks, any amount of improvement, substantial, or even just a small correction, would help. but please do not work too hard, unless you enjoy translating! (it is not to lose sleep over :smile:)

    the photograph of Loll that they put in that Handelsblad article looks very serious to me. I like this one better
    http://perimeterinstitute.ca/images/marseille/marseille103.JPG
    where she is out for a walk with friends. Lee Smolin took it.
     
  13. Jun 8, 2005 #12

    marcus

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    I found the old post with google,
    https://www.physicsforums.com/showthread.php?t=5768
    it has a link to a longer quote:


     
    Last edited: Jun 8, 2005
  14. Jun 8, 2005 #13

    Hans de Vries

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    Marcus, The German version you have is an exact translation of the original
    Dutch version. There's not that much physics in it since it's for a daily journal.

    dutch: http://www.phys.uu.nl/~loll/Web/press/knutselen.pdf
    german: http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm

    The essential points (for english readers) can be found here:

    http://focus.aps.org/story/v14/st13

    Regards, Hans
     
  15. Jun 8, 2005 #14

    marcus

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    Hans, thanks for your interest! I've looked at the three sources you give here and must say the Focus article by Adrian Cho is good science journalism. Worth recommending as a quick introduction.

    I'm interested in the popular press coverage too, for various reasons. I'd like to understand those articles better although there is not much science content. [for example: What is the feeling attached to the dutch word "knutselen" and the german translation of it "basteln"? My dictionary does not do such a good job with colloquialisms and nuances. It just says that "basteln" means to devote hard work to something.]
     
  16. Jun 8, 2005 #15

    Hans de Vries

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    "knutselen" and "basteln" is a very good 1:1 translation. To English the title
    would translate best I think to "Playing with quantum foam". However, the
    word "playing" misses the aspect that you're actually trying to make/create
    something while playing, like in "basteln" and "knutselen"

    Regards, Hans
     
  17. Jun 8, 2005 #16

    selfAdjoint

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    Mmm, just to get the sense rather than anything literal, how about "tossing"? You can toss a ball, or a salad, or a bowl on a potter's wheel.
     
  18. Jun 8, 2005 #17

    marcus

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    Another possible English title or headline for the story could be

    "Building with quantum foam"

    I gather that basteln and knutselen have the sense of

    "to do handcrafts, to make stuff or tinker with stuff for fun"

    Arts-and-crafts handwork is certainly popular in the United States, there is currently a craze for knitting. Retired people take classes in ceramics and woodworking and weaving and there's all those Martha Stewart glue-gun decoration projects.

    but I dont know any general verb for "to do recreational handcrafts".

    So maybe the best is just "building with quantum foam"

    BTW in the interview Loll says SOME PEOPLE talk about quantum foam, but that is not her choice of words. The journalist put the foam image in the title for his own journalistic reasons, not because that is a central analogy in Loll's work. Yes the title of the popular article is misleading, so what's new.

    [MUCH LATER EDIT: this is an afterthought, how about

    "Crafting the Quantum Foam"]
     
    Last edited: Jun 9, 2005
  19. Jun 8, 2005 #18

    marcus

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    to take another look at the thread topic: what is spacetime?

    if we go by the accepted wisdom (Einstein) spacetime does not exist, what exists is the GEOMETRY of it (the "field" is just a name for the geometry of spacetime)

    so we have come again to the essential question: how can something have a shape if it doesnt exist.

    or, as AE put it, if it cannot claim separate existence IN ITS OWN RIGHT. I am not sure what that means but anyway, if something does not exist in its own right as a real something, then how can it have a shape.

    how can disembodied geometry exist of its own accord?


    so then people go and put the responsibility for existing on MATTER and they say that spacetime geometry (the "field") is just the geometric relationships between bits of matter or between events involving bits of matter, like the collision between my foot and the floor when I take a step.

    maybe it is too frustrating to think along these lines.

    anyway this thread is supposed to be about what is spacetime, and I think
    the people making the most progress now are the Triangulations (CDT) people and they say, if I understand right, that

    Spacetime is NOT made of triangles. It just helps to use triangles when you quantize its geometry. And afterwards shrink the triangles down in size, if you can get some more computer time and it is practical to shrink them some more. But spacetime is not made of them, one just uses them because they work.

    And they do not say it is "foam" or "fabric" or "fluid" or any kind of substance AFAIK. I didnt hear them use a material analogy. They just want to do the best they can to making a quantum dynamics of the GEOMETRY and not ask, for the time being AFAICS, what IT it is the geometry of.

    Please let me know if you got a different impression about the CDT approach.
     
    Last edited: Jun 9, 2005
  20. Jun 9, 2005 #19

    hellfire

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    I guess this depends on what one defines with “remove the gravitational field”. It seams that Einstein assumed that the removal of the gravitational field is equivalent to the removal of guv. Based on this assumption, the identification between gravitational field and spacetime seams to be a meaningful step, as guv = 0 has no physical meaning. I have no knowledge to question Einstein’s claims, but it seams not very obvious to me that the removal of the gravitational field is equivalent to the removal of the guv. As a first guess I would take “removal of gravitational field” equal to taking no matter sources Tuv = 0, or as a second guess taking zero Christoffel symbols, or may be a zero Riemann curvature tensor. None of them lead to guv = 0. I have to admit that it is unclear to me what “gravitational field” actually means in general relativity, which seams to be the key issue to arrive to such a conclusion:

     
  21. Jun 9, 2005 #20
    Hi Marcus

    We have an intuitive idea of space and time from experiments we performed in the cradle with fingers and toes. You remember. There is no such intuitive understanding of gravitational field. I wonder why we should now accept the idea, put forward by Einstein in his years as a self-professed curiosity of socklessness, that the concept of gravitational field should be more fundamental than the intuitive concepts of space and time?

    I can and do accept gratefully the notion, due to Einstein and Minkowski, that space and time are equivalent. This is not intuitively obveous. However I have found that space-time equivalence yields fairly easily to the following analysis: an increment of time is required to measure an increment of space. Hence there is no space without time. It remains to wonder if there is any time without space.

    Curiously, this seems to reduce in mathematical terms to the meaninglessness of zero in the denominator of rational expressions for motion. I suggest that nothing exists which does not move (this from the idea due to Einstein that there is no preferred reference frame). So, any object has to possess a velocity, expressed as change in spatial position divided by change in temporal position. From this I deduce that taking the increment of time to zero results in the disappearance of the notion of "object." Or, equivalently, that zero time results in the notion of object being generalized to the notion of the universal set. If time is zero, nothing is everything. Everything is nothing.

    Since we must posit that we exist, there is something. Hence time cannot go to zero. The notion of instantaneous velocity is a convenient and useful fiction. We do of course take change in distance to zero, meaning only that the measured object is moving in the same frame as the observer. Does this allow us to say that space-time equivalence means only that space and time are proportional, and not that they are identical?

    I think not. (Please do not let that be my epitaph!) The notion of point-like particles is, I do think, like the notion of zero time, a useful fiction. I imagine that it was thoughts like these that led Planck to declare the action potential, and Heisenburg to declare the uncertainty principle.

    That's my coffee.

    On a personal note, if anyone wonders where I have been lately, the garden is planted and the cottage now has windows and a door to go with the floor, walls and roof. None of this is my doing, but it has been interesting to watch. Heat, telephone, and electricity may arrive eventually. Plumbing is that bucket in the corner.

    Be well,

    Richard
     
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