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What is spacetime made of?

  1. Apr 15, 2010 #1
    Does it have a sense to ask what is Spacetime made of?

    Because we know:

    Spacetime can be bent, which results in gravity and gravitational lensing effects etc. If it was made of "nothing" it could'be bent, right?

    Spacetime "flows" into a black hole and at the event horizon the speed of its flow is faster than c. Again, to speak about flow of "nothing" wouldn't make sense.

    So Spacetime is clearly "something" it has an objective and observable existence and it's probably meaningful to ask what it is made of.

    Do we have any clue or this question doesn't make sense?
  2. jcsd
  3. Apr 15, 2010 #2


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    No, not right.

    Why not?

    You are making assumptions about what is "possible". Perhaps the problem is that you do not understand what "bent" and "flow" mean here.
  4. Apr 15, 2010 #3

    Vanadium 50

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    The question doesn't make sense. You are ascribing properties to spacetime that it doesn't have ("Spacetime "flows" into a black hole"), and also arguing based on popularizations "Spacetime can be bent, which results in gravity and gravitational lensing effects etc. If it was made of "nothing" it could'be bent, right?")
  5. Apr 15, 2010 #4
    Great, then I would love to understand the correct meaning of "bent" and "flow".

    1. Bent - when I look at picture of any gravitational lens like http://en.wikipedia.org/wiki/File:Gravitational_lens-full.jpg I can see, well a lens of deformed space. If there was not a physical bent of trajectories of light how else could you explain the phenomena?

    2. Flow - you hit the nail. I have just watched a BBC Horizon epizode on Black holes and Prof. Max Tegmark explains the event horizon exactly as I have described - standing by a waterfall he says that even when you swim as fast as you can the water (meaning the Spacetime) will flow faster than you can swin. So this is not true explanation?
  6. Apr 15, 2010 #5
    Are "bent" and "curved" synonymous here?

    1. The trajectories of objects (geodesics in space-time) are the straightest possible lines in space-time. If space-time is flat, these are straight lines in space. Otherwise, they may be curved in space. Objects are simply flowing along the "gradient" of space-time, and their path is given by the geodesic equation.

    2. If you watched it on BBC it was probably dumbed down for the general audience, which generally does not have sufficient command of tensor analysis to understand the technical explanation. The reason is simply that going in a straight line in space-time implies a spatial acceleration towards masses, as explained by Einstain's equation.
  7. Apr 15, 2010 #6


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    Spacetime possesses geometrical properties, distances, curvature, etc. (In fact, in a purely relational theory spacetime would be the geometric relationship between different material objects) It does not seem to possess material properties, composition, velocity, etc. I would think that the question "what is spacetime made of" is asking for material properties.
  8. Apr 15, 2010 #7
    The paradox I'm struggling with is how there can be something with geometrical properties, yet without any material properties of some sort (these do not have to be material in the classical sense)?

    I also heard that empty space has some virtual particles in it and that its total energy even when its "empty" doesn't have to be zero (one possible explanation of Dark energy). Could this virtual field of particles/energy be a base of the Spacetime and give rise to its geometrical properties?
  9. Apr 15, 2010 #8


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    That seems like an odd connection to make. Do you expect that something with material properties must have personal properties like preferences or feelings? Or, perhaps a more related question would be, do you expect that even fundamental (material) particles must have a (geometrical) size?
  10. Apr 15, 2010 #9
    The part about the waterfall must be a misunderstanding of what the BBC show was trying to explain. Perhaps they were referring to in-falling matter or maybe they were just using an analogy so as not to confuse the viewer with concepts such as severely distorted spacetime. In any case, spacetime is distorted (curved) around the black hole, but doesn't "flow" into the gravity well.

    I think, however, that the second part of ZirkMan's question is perfectly valid. I don't think there's much question as to the distortion of spacetime by matter. I wouldn't refer to this concept as a "popularization" since it's a well accepted theory among the scientific community. Since matter appears to distort spacetime, this would imply that there is some interaction between matter (energy) and what we refer to as spacetime. If interaction exists, then there must be something there for the matter to interact with.

    I haven't delved into this to any great extent, but I do remember reading about some concept in quantum mechanics that was called "Quantum Foam" where the foam represented subatomic spacetime turbulence.
  11. Apr 15, 2010 #10


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    What is a circle or a triangle made of?
  12. Apr 15, 2010 #11

    George Jones

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    I think Tegmark based his analogy on the more quantitative American Journal of Physics paper

  13. Apr 15, 2010 #12


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    AFAIK, the river model works for Schwarzschild and Kerr black holes, but not in general spacetimes like pp-waves or the FLRW metric. It seems to be a limited but defensible model specifically developed for popularizations.
  14. Apr 15, 2010 #13
    I don't think my expectations are my problem (I try not to have any) but my imagination.

    It's really hard to look at a picture of a gravitational lensing effect and not see these multiple flares of the same object as a result of some invisible but still clearly defined lens like object around a cluster of galaxies and when light from the distant source comes closer to it it splits and follows its curved shape creating these multiple images or Einstein rings.

    But I really like your relational theory definition. It says that you can get these effects simply out of pure geometrical relations. I can imagine that, makes more sense now, thanks.
  15. Apr 15, 2010 #14


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    I am glad it helped. The distinction between geometrical and material properties is the idea which has made the most sense to me so far.
  16. Apr 16, 2010 #15
    I would like to answer this question and test the new geometrical insight to find out more about the nature of Spacetime.

    Circle is a result of two different points and a rule that moves one of the points on a flat 2D plane so that their distance doesn't change.

    Triangle is a result of straight connection of 3 different points on a flat 2D plane.

    Well, this could be one of valid definitions. But how about this one in the same style:

    Spacetime is a result of straight connections between two or more points in 4D (x,y,z,t) and a rule that states that more mass/energy each point has the more the connecting line will be curved in 3D (x,y,z) but will remain straight in 4D (x,y,z,t) (the last part I'm not sure).
  17. Apr 17, 2010 #16


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    That is an oversimplification. Between two spatial locations there are an infinite number of 4D paths, whether the spacetime is flat or not. Like road racers, who all follow (roughly) the same 3D path but have different 4D trajectories. From experience it seems that a straight line is the path followed by light, which has the distinction of winning every race. I think a spacetime might well be described by all the possible paths between masses.

    But spacetime has to remain an abstraction. You can't touch or feel space nor time like you can matter and energy.
  18. Apr 17, 2010 #17
    I would be fine with this answer if only that abstraction remained abstract and left me alone. But now it pulls me down every time I try to jump up. Can you tell me how?
  19. Apr 17, 2010 #18


    Staff: Mentor

    I like this reply. :smile:

    All theoretical constructs are mathematical abstractions so Mentz114 is correct in one sense, but since they are also subject to experimental validation I am fundamentally uncomfortable with statements like Mentz114's. I think that space and time are on equal scientific footing to matter and energy: they are measurable quantities in an experimentally validated physical theory.
  20. Apr 17, 2010 #19


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    The clue obviously can be found within the theory of special relativity. The theory concerns about the fundamentals: the space, time, energy, and matter. The special relativity has unified those fundamentals into two distinct entities: the spacetime as the unification of space and time and the energy substance from which matter is derived (E=mc2).

    The spacetime and energy are not two separate entities as we think. The spacetime is not like a sort of container and energy something that fills the container. On the contrary, they are inextricable just like water substance and its spherical form in a drop of water. The spacetime is merely the geometrical quality of energy. In plain English, energy is noun and spacetime adjective.

    The spacetime does not have existence on its own. Energy is the one and the only independent reality in nature. It is primordial; neither can be destroyed nor created, omnipresent; permeates throughout every part of an object or a place. Everything else is derived from it. The concept of empty spacetime loses its meaning. This is the deepest meaning of the energy conservation, the most primitive law of nature.
  21. Apr 18, 2010 #20
    Nice thread! Just posting to keep track of it.
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