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Thread: Can time/space exist where there is no gravitational field?

  1. #1 Can time/space exist where there is no gravitational field? 
    Forum Ph.D.
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    I've researched a lot, and i got really upset now that i know i've been studying on tons of those freaking crackpot sites.

    So i'll be asking here, please don't troll here thanks!

    1) Can space exist where there is no gravitational field?
    2) Is movement allowed where there is no gravitational field?


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  3. #2  
    Genius Duck Moderator Dywyddyr's Avatar
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    1) I don't see why not.
    2) Absolutely. (Although I'm not sure what you mean by "allowed".)


    And don't feel too bad, crackpot sites abound, the more you learn the easier you'll be able to discern what is and what isn't a nutter's page.


    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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  4. #3  
    Anti-Crank AlexG's Avatar
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    There is no where in the universe where there are not gravitational fields.
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    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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  5. #4  
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    In another words,
    1) Does space exist outside the universe?
    2) Can movement exist where there is no space?

    if answer for 1) is no, is it right to say that space cannot exist where there is no gravity?
    if answer for 2) is no, is it right to say that time cannot elapse where there is no gravity?


    finally, in this reply, is it right to say:
    1) gravity creates time?
    2) time, in another words, is "where movement is allowed"?
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  6. #5  
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    if answer for 1) is no, is it right to say that space cannot exist where there is no gravity?
    No, it is incorrect to say that space cannot exist where there is no gravity.

    if answer for 2) is no, is it right to say that time cannot elapse where there is no gravity?

    Again, it is incorrect to say that time cannot elapse where there is no gravity.

    finally, in this reply, is it right to say:
    1) gravity creates time?
    No.

    2) time, in another words, is "where movement is allowed"?
    No. Movement defines interval, and has nothing to do with gravity.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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  7. #6  
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    Quote Originally Posted by AlexG View Post
    it is incorrect to say that space cannot exist where there is no gravity.
    Quote Originally Posted by AlexG View Post
    it is incorrect to say that time cannot elapse where there is no gravity.
    and finally, referencing to what you said here,
    Quote Originally Posted by AlexG View Post
    There is no where in the universe where there are not gravitational fields.
    Basically, you are saying, Space and Time exists outside the universe?
    Am i right, or are you trolling me? :X please clarify. your making things messy here (at least thats what i think, sorry if im wrong haha)
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  8. #7  
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    Hm. I think question (1) from the OP has some merit, and may not be as clear-cut and straightforward as it initially appears. Firstly however let us be clear that we are talking about space-time, not just space; the two are part of the same manifold and cannot be separated.

    Let us consider the field equations of General Relativity in their most general form :



    What does this equation tell us ? The left hand side, , is called the Einstein tensor, and can be considered as representing the geometry of space-time. The right hand side represents any energy ( in whatever form ) present. The general form of the above equation simply says that the two are equal up to a constant, i.e. that energy and space-time geometry cannot be separated. The important thing to realize is that the absence of sources of energy ( ) does not mean that we are dealing with a space-time with trivial geometry / topology. For example, the Schwarzschild solution is based on the vacuum case where



    but does not automatically yield a flat space-time. The reason for this is primarily that these field equations are non-linear, i.e. that gravity is self-coupling. What I am trying to say here is that the field equations of GR tell us that energy and space-time are equivalent, they are really the same thing. If there is no energy at all, would it be meaningful to speak of space-time ? I think the question warrants further thought - I am not sure if the answer is a clear no, especially if we consider the cosmological case where a vacuum energy density is introduced into the equation in the form of a cosmological constant.
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  9. #8  
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    Quote Originally Posted by ryanawe123 View Post
    Quote Originally Posted by AlexG View Post
    it is incorrect to say that space cannot exist where there is no gravity.
    Quote Originally Posted by AlexG View Post
    it is incorrect to say that time cannot elapse where there is no gravity.
    and finally, referencing to what you said here,
    Quote Originally Posted by AlexG View Post
    There is no where in the universe where there are not gravitational fields.
    Basically, you are saying, Space and Time exists outside the universe?
    That does not logically follow.

    You could have space and time with no gravity. There is at least one solution of Einstein's field equations describing an empty universe.

    In our universe, there is nowhere with no gravity, because our universe contains mass and energy. And there is no "outside" of our universe.
    ei incumbit probatio qui dicit, non qui negat
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  10. #9  
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    Quote Originally Posted by Markus Hanke View Post
    Hm. I think question (1) from the OP has some merit, and may not be as clear-cut and straightforward as it initially appears. Firstly however let us be clear that we are talking about space-time, not just space; the two are part of the same manifold and cannot be separated.

    Let us consider the field equations of General Relativity in their most general form :



    What does this equation tell us ? The left hand side, , is called the Einstein tensor, and can be considered as representing the geometry of space-time.
    Is the term "tensor" not sufficient to illustrate a condition or state of differentials which theoretically are measurable in time? Or is the term tensor (potential) too vague to assign any values other than infinities, which leave us stranded again.

    I think the question warrants further thought - I am not sure if the answer is a clear no, especially if we consider the cosmological case where a vacuum energy density is introduced into the equation in the form of a cosmological constant.
    Intuitively I see no probblem with assigning any properties like "existing in time" as long as we can assume a dynamic condition. However I can't get my head around the concept of a dynamic state or condition in time before the cosmos had any properties, which is assumed by the term "there is nothing outside the universe".

    What about tensors or any kind of latent potential? Can there be a zero state of energy which is not subject to quantum ( the smallest increment in time).
    IMO, to solve this "infinite" time problem from before the BB", can we not say that, until the moment of the BB (coming into existence) there was no time, but at the "moment of the BB, universal potential instantaneously became space (inflation) and time (measurable expansion).
    Last edited by Write4U; February 28th, 2013 at 05:26 PM.
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  11. #10  
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    Quote Originally Posted by Strange View Post
    You could have space and time with no gravity
    so spacetime can exists if there is no gravity + matter/energy?
    and if yes, how fast/slow does time pass without gravity/energy?

    and also, could gravity/energy exist without space/time?
    if no, isn't it not wrong to say gravity/energy creates time?
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  12. #11  
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    Quote Originally Posted by ryanawe123 View Post
    Quote Originally Posted by Strange View Post
    You could have space and time with no gravity
    so spacetime can exists if there is no gravity + matter/energy?
    and if yes, how fast/slow does time pass without gravity/energy?

    and also, could gravity/energy exist without space/time?
    if no, isn't it not wrong to say gravity/energy creates time?
    IMO, it is better to say time emerges with the emergence of anything measurable.
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  13. #12  
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    if yes, how fast/slow does time pass without gravity/energy?
    At it's usual rate of 1 second per second.

    if no, isn't it not wrong to say gravity/energy creates time?
    I've got to parse these negatives.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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  14. #13  
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    Quote Originally Posted by ryanawe123 View Post
    and also, could gravity/energy exist without space/time?
    if no, isn't it not wrong to say gravity/energy creates time?
    Again, there is a logic problem here. Time, I think, for you to brush up on your syllogisms, given how often you've been tripped up by this sort of thing.

    Substitute "salt" for "gravity/energy" and "chlorine" for "space/time" in your sentences above, for example. Salt cannot exist without chlorine, but that does not in any way imply that salt creates chlorine.
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  15. #14  
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    Quote Originally Posted by ryanawe123 View Post
    so spacetime can exists if there is no gravity + matter/energy?
    and if yes, how fast/slow does time pass without gravity/energy?
    The same rate it always passes. (If it makes any sense to talk about time "passing". Or "rate" when you have nothing to measure it with. Or "fast/slow" when you have nothing to compare it to...)

    and also, could gravity/energy exist without space/time?
    Gravity couldn't because gravity is a side effect of the curvature of space-time.

    Could energy exist without space-time? Who knows. I doubt it but that sounds more like metaphysics.

    if no, isn't it not wrong to say gravity/energy creates time?
    I don't know why anyone would say gravity creates time. If anything, time (plus space plus matter-energy) creates gravity.
    ei incumbit probatio qui dicit, non qui negat
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    Personally, I don't see any reason for space time not to exist without gravity. This must be a force that is not entirely connected to line of time and the dimensions of space.
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  17. #16  
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    Quote Originally Posted by Write4U View Post
    Is the term "tensor" not sufficient to illustrate a condition or state of differentials which theoretically are measurable in time? Or is the term tensor (potential) too vague to assign any values other than infinities, which leave us stranded again.
    A tensor ( in this case a rank-2 tensor ) is simply a mathematical object, and doesn't really have much to do with potential or time measurements, or even infinities. You can picture a rank-2 tensor as a little machine; it takes as input two vectors, crunches them up, and spits out a scalar as a result. In more formal terms, it is a bilinear mapping of vectors and co-vectors into the real numbers.
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  18. #17  
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    Quote Originally Posted by SciFi-Real View Post
    Personally, I don't see any reason for space time not to exist without gravity. This must be a force that is not entirely connected to line of time and the dimensions of space.
    You see, here is the thing - according to the field equations of GR, space-time and the gravitational field are one and the same thing. You cannot separate them, and in physical terms you wouldn't need to as there is nowhere in the universe without gravity.

    I would argue that space-time cannot exist without gravity, and vice versa.
    Last edited by Markus Hanke; March 1st, 2013 at 06:33 AM.
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  19. #18  
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    I would argue that space-time cannot exist with gravity, and vice versa.
    Don't you mean "without"?
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    Take a particle moving through cos(x) nothing interacting with it just the most basic particle possible then nothing else acts on it only its rate of change of position in time
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  21. #20  
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    Quote Originally Posted by Dywyddyr View Post
    I would argue that space-time cannot exist with gravity, and vice versa.
    Don't you mean "without"?
    Of course, thank you. Corrected.
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  22. #21  
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    Quote Originally Posted by fiveworlds View Post
    Take a particle moving through cos(x) nothing interacting with it just the most basic particle possible then nothing else acts on it only its rate of change of position in time
    Why "cos(x)"? How does a particle "move through cos(x)"?
    If there's nothing BUT the particle you can't say it's moving. What does it change position relative to?
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  23. #22  
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    Quote Originally Posted by fiveworlds View Post
    Take a particle moving through cos(x) nothing interacting with it just the most basic particle possible then nothing else acts on it only its rate of change of position in time
    If nothing is interacting with it, it would continue in a straight line with constant velocity. (Newton's first law, and all that.) So where does cos(x) come into it?
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  24. #23  
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Write4U View Post
    Is the term "tensor" not sufficient to illustrate a condition or state of differentials
    which theoretically are measurable in time? Or is the term tensor (potential) too vague to assign any values other than infinities, which leave
    us stranded again.
    A tensor ( in this case a rank-2 tensor ) is simply a mathematical object, and doesn't really have much to do with potential or time measurements,
    or even infinities. You can picture a rank-2 tensor as a little machine; it takes as input two vectors, crunches them up, and spits out a scalar as a
    result. In more formal terms, it is a v bilinear mapping of vectors and co-vectors into the real numbers.
    Does a tensor have values or interactive dimensions ? Do they exist? If they do, they exist in time, no?
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  25. #24  
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    Quote Originally Posted by Write4U View Post
    Does a tensor have values or interactive dimensions ? Do they exist? If they do, they exist in time, no?
    I don't really know what you are asking. One of the defining characteristics of tensors is that they are independent of the choice of coordinate basis, so the above question makes little sense.
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  26. #25  
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    Ok, thank you,

    wiki,
    Because they express a relationship between vectors, tensors themselves must be independent of a particular choice of coordinate system. Taking a coordinate basis or frame of reference and applying the tensor to it results in an organized multidimensional array representing the tensor in that basis, or as it looks from that frame of reference.
    Would it be fair to say that tensors and vectors are mathematical potentials?
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  27. #26  
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    Quote Originally Posted by Write4U View Post
    Would it be fair to say that tensors and vectors are mathematical potentials?
    No. Vectors and tensors can be used to describe potentials, but that does not mean that all vectors and tensors are potentials. They aren't.
    Btw, a vector is also a tensor.
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  28. #27  
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    Quote Originally Posted by Markus Hanke View Post
    Btw, a vector is also a tensor.
    Hmm...we need to be a bit careful here - in general is not true to say that every vector is a tensor.

    It is somewhat vexing in physics.... here is my take. Given a manifold and the point , then there exists a set at this point that we may call the (tangent) vector space if the usual rules for vector addition and scalar multiplication are allowed (of course we need a bit more detail on what a "tangent" means in this context - park that for now). Suppose for simplicity I write , it being understood that this refers to this point and this point only, and write a tensor as , then I may call an element of this set, , say, as a type (2,0) tensor.

    Accordingly I may refer to an element in as a type (1,0) tensor. But since it is just an element in I can call it a (tangent) vector. Problems with terminology arise, however.

    The set of all tensors of a specified rank at a named pint is also a vector space, as is easily shown. Now if we agree to call the elements of a vector space as vectors, as seems reasonable, we have an immediate terminological problem......

    Every tensor of whatever rank at this point is a vector. But every tensor of a particular type, say type (p,q) where and is the dimension of is an element in a vector space of dimension {this is a much later editof a misleading typo}. But since (for reasons I have so far declined to give) the dimension of a tangent space cannot possibly exceed that of the manifold over which it is defined, we may say that a tensor is identified as a (tangent) (co-)vector depending on whether it is of type (0,q) or type (p,0) if and only if . Therefore an element in a vector space of dimension p + q > n cannot possibly be a (co)tangent vector

    Nonetheless, elements in a vector space of all tensors at a point are quite definitely vectors, but just not tangent to the "host" manifold. (They are tangent to some other manifold related to our host, but we have to park this one as well_

    It gets worse. Suppose we assume all the above to be true, and that elements in are tensors of type (1,0). Now for the different points I have a tangent vector space as outlined above and arbitrarily select a single element (rank 1 tensor of whatever type) at each of these points provided only they are all of the same type I may have a vector/tensor field. But here's the kicker....

    The set of all such arbitrary choices of vector fields is itself a vector space, elements of which - vectors surely? - are fields.

    I suggest this may be something to do with the reason that physicists frequently make no distinction between tensors and tensor fields.

    by edit Which is merely a roundabout and not completely transparent way of claiming the assertion
    Quote Originally Posted by Markus Hanke View Post
    , space-time and the gravitational field are one and the same thing. .
    is incorrect.

    When time permits I can explain. Perhaps.

    Gosh, what a wind-bag I am!
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  29. #28  
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    If anyone has read, or is thinking of reading my last excessively verbose post, let me alert them to the fact I made a seriously misleading typo which I have edited and flagged
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  30. #29  
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    Hm. As usual, Guitarist's mathematical rigour gets the better of me...for which I am glad, because these are opportunities to learn
    So I retract my earlier statement - not every vector is automatically a tensor. This is why it is crucial to have a mathematician on board, who can point out such errors.

    Quote Originally Posted by Guitarist View Post
    When time permits I can explain. Perhaps.
    Ok, I think it would be worthwhile to have a further discussion on this; I don't believe that it makes sense to talk about space-time in the complete absence of energy ( and thus gravitation ), i.e. an entirely empty universe. Unfortunately though I can't make that mathematically rigorous, so I might very well be wrong, and shall be glad to be shown that.
    I am aware of course that one can derive cosmological solutions from the GR field equations for an empty universe ( i.e. the Milne model ).

    Empirically we have no problem here, since gravitation is present everywhere in the universe.
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    Quote Originally Posted by Guitarist View Post
    Gosh, what a wind-bag I am!
    Keep on blowing.
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    Quote Originally Posted by Markus Hanke View Post
    Ok, I think it would be worthwhile to have a further discussion on this; I don't believe that it makes sense to talk about space-time in the complete absence of energy ( and thus gravitation ), i.e. an entirely empty universe.
    There is the Milne solution to Einstein's field equations, which describes an empty universe. Whether it makes sense to talk about or not is another thing! It would be a pretty boring universe with nothing to get bored). But it may be a useful approximation to ... something.
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    Does anyone have an opinion on David Bohm's "Holomovement"? He describes this "wholeness" in terms of current known physics.
    Borrowing ideas from holographic photography, the *hologram* is Bohm's favorite metaphor for conveying the structure of the Implicate Order. Holography relies upon wave interference. If two wavelengths of light are of differing frequencies, they will interfere with each other and create a pattern. "Because a hologram is recording detail down to the wavelength of light itself, it is also a dense *information* storage." Bohm notes that the hologram clearly reveals how a "total content--in principle extending over the whole of space and time--is enfolded in the movement of waves (electromagnetic and other kinds) in any given region." The hologram illustrates how "information about the entire holographed scene is enfolded into every part of the film." It resembles the Implicate Order in the sense that every point on the film is "completely determined by the overall configuration of the interference patterns." Even a tiny chunk of the holographic film will reveal the unfolded form of an entire three-dimensional object.

    Proceeding from his holographic analogy, Bohm proposes a new order--the Implicate Order where "everything is enfolded into everything." This is in contrast to the explicate order where things are unfolded. Bohm puts it thus:

    "The actual order (the Implicate Order) itself has been recorded in the complex movement of electromagnetic fields, in the form of light waves. Such movement of light waves is present everywhere and in principle enfolds the entire universe of space and time in each region. This enfoldment and unfoldment takes place not only in the movement of the electromagnetic field but also in that of other fields (electronic, protonic, etc.). These fields obey quantum-mechanical laws, implying the properties of discontinuity and non-locality. The totality of the movement of enfoldment and unfoldment may go immensely beyond what has revealed itself to our observations. We call this totality by the name *holomovement.*"
    Last edited by Write4U; March 3rd, 2013 at 09:21 AM.
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    Quote Originally Posted by ryanawe123 View Post
    I've researched a lot, and i got really upset now that i know i've been studying on tons of those freaking crackpot sites.

    So i'll be asking here, please don't troll here thanks!

    1) Can space exist where there is no gravitational field?
    2) Is movement allowed where there is no gravitational field?
    1) Not that we know, without space there would be no time.
    2) Yes absolutely, but gravity is never really absent, you could be in the middle of intergalactic space 3 million light years away from a galaxy and still be attracted to it, albeit on a small scale, and yes you could move where there is no gravitational field: F=ma

    finally, in this reply, is it right to say:
    1) gravity creates time?
    2) time, in another words, is "where movement is allowed"?
    1) Gravity is a distortion of time by mass, if by creates time you mean alters the perception of it via time dilation then perhaps.
    2) Time is the dimension needed for apparent motion as it is the transfer of energy, but time and space are the same so it's not practical to say 'time allows movement', space does as well; they aren't separate constructs.

    so spacetime can exists if there is no gravity + matter/energy?
    There is no way to know that, and it may be not observable because all degrees of matter and energy permetate the universe everywhere, even in seemingly voids you have particles and virtual particles so spacetime without matter or energy may seem very strange.

    and if yes, how fast/slow does time pass without gravity/energy?
    It passes the same for all inertial reference frames relative to themselves, for instance one traveling rather fast could view Earth passing by in 2 seconds for his own 1 second, yet for himself he measures 1 second every second and people on Earth recognizes 1 second per second for themselves also. Although this is true, your feet, hands and different parts of your body and slightly behind in time than your eyes or wherever you register 'time' consciously as it takes the photons a small amount of time to reach your eyes, the different parts of your body are behind in time, as much as stars in the sky are also, they are just a lot further behind than your feet or hands, BUT still on observation pass by at 1 second per second, more or less.

    This one second per second is also an generalization, specifically time at it's smallest is the time it takes a photon to travel across One Planck Length:

    Planck time - Wikipedia, the free encyclopedia

    and also, could gravity/energy exist without space/time?
    Well seeing as gravity is only the distortion of spacetime itself it would seem unlikely in the way we understand gravity but there is no way to know at this stage in our technological advancement.
    Energy probably could exist but it is not really known, if we limit down the dimensions to as small as possible to say the Hartle-Hawking state we might be able to say energy can exist in a seemingly infinitely small 'space' with no time, which is most commonly thought of as the big bang:

    http://en.wikipedia.org/wiki/Hartle-Hawking_state

    if no, isn't it not wrong to say gravity/energy creates time?
    Gravity is distortion of the spacetime continuum, this distortion is merely a distortion of space and time, it is interesting to think that energy could create space and time but we have never really observed such a vacuum void of all energy and matter to say so, the closest we have got is the Casimir effect which two plates are spaced apart a few micrometers which has been known to increase the speed of light by a small fraction, that increase would of course in an equation alter the results of energy equivalence I would think but that would have to be verified. This effect affects the virtual particles between the plates and cause a net forces between the two plates, this is delving into Quantum Mechanics.

    Again gravity and energy do not create time here is the process in a nutshell:

    Energy/Mass --> Distorts spacetime --> This is measured as gravity. The more mass/energy present the more the spacetime continuum is distorted until you reach the point where the mass becomes so great that it collapses in on itself past the point of a certain radius called the Schwarzschild radius; this forms a black hole. The time towards the black hole becomes slower and slower to outside observers until something closer towards the black hole disappears in time, well actually it redshifts out of the visible spectrum first but anyway, the mass and energy at the center seem to be infinite, but this is a failing of Einsteins field equations at this point. So to answer your question, energy does considerably alter the relative passage of time for outside observers but I don't think that it creates time, as even falling into a black hole consciously you would still recognize 1 second per second again relative to yourself.

    http://en.wikipedia.org/wiki/Gravitational_singularity

    Hope this helped:


    Singularities and Black Holes -- SEP
    Casimir effect - Wikipedia, the free encyclopedia
    Faster-than-light - Wikipedia, the free encyclopedia
    Schwarzschild radius - Wikipedia, the free encyclopedia
    Einstein field equations - Wikipedia, the free encyclopedia
    Einsteins field equations and Black Holes
    Gravitational redshift - Wikipedia, the free encyclopedia
    Gravitational time dilation - Wikipedia, the free encyclopedia
    Spacetime - Wikipedia, the free encyclopedia
    Mass & Energy
    http://en.wikipedia.org/wiki/Big_Bang
    Last edited by Quantime; March 3rd, 2013 at 10:30 AM.
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    Quote Originally Posted by Markus Hanke View Post
    Hm. As usual, Guitarist's mathematical rigour gets the better of me
    Ha!! Some, possibly including me, would call it pedantry
    Empirically we have no problem here, since gravitation is present everywhere in the universe.
    OK, the physical intuition of Markus, Strange and many others here exceeds mine by several orders of magnitude, so let me show you all my "thinking" (if that is not too grandiose a term for stumbling around in the dark)......

    We suppose first that by "Universe" we mean something rather less childish than "a load of black stuff with stars and planets in it". Let us suppose our "grown-up" version can be modelled as a 4-manifold which we may call spacetime. For now the only property of our manifold - I insist it is classically defined - is that any point in our manifold can be expressed in terms of the coordinate set where and the metric signature is

    Now if it it the case that the metric tensor (in matrix form) is everywhere then we may assume the curvature tensor everywhere i.e our spacetime is "flat". This very roughly describes a Minkowski 4-manifold, and this guy is the playground for SR.

    Does this mean that our spacetime 4-manifold does not exist? You tell me!!

    Now suppose that the curvature tensor everywhere. But by the definition of a manifold, at each and every point there must be a region (a neighbourhood) however small which is approximately Euclidean - in our case by "approximately" we may say Minkowskian.

    Now the non-linearity of the gravitational field eqns seem to say (and I am no expert) that the non-zero of the curvature tensor implies the existence of a gravitational field AND the existence of a gravitational field implies a non- zero curvature tensor. This is, as little as I understand it, roughly what GR says.

    But GR insists that SR is valid in any neighbourhood, however small; of our non-flat spacetime 4.manifold, that is SR is a subset of GR, then surely we may assume that at some scale. however small, we may assume our spacetime 4-manifold is Minkowski "flat" i.e. that gravity can be ignored, without cancelling the concept of spacetime.

    I am reasonably sure this is gibberish, and certainly sure I have been far too long-winded (as usual)
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    Hmm, I'd have maybe put a joke or two in there, but nice post nonetheless.
    Thanks Guitarist.
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    Does scientists/anyone have a reason of why mass and energy bends space-time?
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    Quote Originally Posted by ryanawe123 View Post
    Does scientists/anyone have a reason of why mass and energy bends space-time?
    Take your pick...

    a) We don't know.

    b) Because that is just the way the universe works.

    c) There may be some deeper explanation - quantum gravity, string theory, phase space, causal dynamical triangulation or whatver - but that will still leave you asking "why" that explanation works the way it does. Why is the universe made of (strings / 4-simplexes / space-time-momentum / whatever) and why do they behave the way they do? (And if someone comes up with an even deeper explanation, you will of course be able to ask "but why" about that as well.)
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    d) Maybe energy is space-time curvature or, equivalently, space-time curvature is energy (and mass is just a form of energy).
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    Quote Originally Posted by ryanawe123 View Post
    Does scientists/anyone have a reason of why mass and energy bends space-time?
    "Why" questions are perhaps impossible to answer, except at a superficial level. How deep down the rabbit hole do you want to go?

    For example, if you ask "why do opposite charges attract?" we could say, "because there is a raging exchange of virtual photons."

    But why is there a raging exchange of virtual photons? Etc.

    At some point, you realize there is no answer to the next why question, which to many means that the answers to the previous why questions in the series weren't "real" answers at all.

    So how deep do you want to go?
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    Quote Originally Posted by Strange View Post
    d) Maybe energy is space-time curvature
    Which is the possibility I am raising here, and the way I understand the situation when looking "behind" the equations.

    Now if it it the case that the metric tensor (in matrix form) is everywhere then we may assume the curvature tensor everywhere i.e our spacetime is "flat".
    Interestingly, the opposite does not appear to be true. We can start with and, given appropriate boundary conditions, arrive at the ( exterior ! ) Schwarzschild metric, which is definitely not globally flat. But then of course, the emphasis is on "appropriate boundary conditions"...

    Does this mean that our spacetime 4-manifold does not exist? You tell me!!
    No, it means that Minkowski space-time is unphysical, and only an approximation to reality. In the real world we never truly have an exact diag{1,-1,-1,-1} for the metric tensor, also because we cannot "zoom in" indefinitely, i.e. make the neighbourhood of our point as small as we like. There is a real, even quantifiable, limit ( the Planck length ). Interestingly, in order to "zoom in" further and further, we would have to invest more and more energy, thus effectively increasing curvature.

    At least that is my take on things, and I could very well be wrong. I just think there is more to this than initially meets the eye.
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    What does the string theory basically speak about the creation of time?
    Can someone enlighten me lol
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    Markus Hanke, March 4th, 2013, 10:59 AM

    Originally Posted by Strange,

    (d) Maybe energy is space-time curvature
    Which is the possibility I am raising here, and the way I understand the situation when looking "behind" the equations.
    In layman's terms, would it be fair to say that space-time curvature introduces a "tension" (potential energy)?
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    Quote Originally Posted by Write4U View Post
    In layman's terms, would it be fair to say that space-time curvature introduces a "tension" (potential energy)?
    There is no tension involved ( GR is not a mechanical theory ), but it is true of course that there is potential energy in the gravitational field. That field is itself a form of energy, and thus in itself also a source of gravitation - that is why the gravitational field is self-coupling.
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    1) Yes. A Gravitational Field exists within space. Not the other way around so technically, yes. Space is not dependent on the Forces of gravity. The bodies that exist within it do.

    2) Yes. There wouldn't be friction or anything to stop or slow you down but yes. Movement is not dependent on Gravity. Gravity affects movement but doesn't control it.
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    Quote Originally Posted by TheDoctor13 View Post
    1) Yes. A Gravitational Field exists within space. Not the other way around so technically, yes. Space is not dependent on the Forces of gravity. The bodies that exist within it do.
    Somehow that doesn't sound proper. To me it feels more like the spacetime duality. IOW, neither can exist without the other. Space creates gravity, gravity holds space together.

    2) Yes. There wouldn't be friction or anything to stop or slow you down but yes. Movement is not dependent on Gravity. Gravity affects movement but doesn't control it.
    Unless I misunderstand you, that seems incorrect. Movement is very much dependent on gravity in all respects, direction, speed, acceleration. Try to throw a ball toward space. It will slow down, reach a full stop and then fall back to earth at ever increasing speed, until terminal speed is reached.

    I understand that movement is also a result of propulsive forces. Perhaps one can say gravity is a negative (propulsive) force or rather it is an attractive force.
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    Quote Originally Posted by Write4U View Post
    Space creates gravity
    Nope.
    Mass creates gravity.

    Movement is very much dependent on gravity in all respects, direction, speed, acceleration.
    Nope.
    Gravity affects movement.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Write4U View Post
    Space creates gravity
    Nope.
    Mass creates gravity.

    Movement is very much dependent on gravity in all respects, direction, speed, acceleration.
    Nope.
    Gravity affects movement.

    True, mass creates gravity and gravity is directly proportional to amount of mass.

    I meant to say that space is the medium of gravity and spacetime distortion is responsible for the effects of gravity.

    wiki,
    Gravitation, or gravity, is the natural phenomenon by which physical bodies appear to attract each other with a force proportional to their masses. It is most commonly experienced as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. The phenomenon of gravitation itself, however, is a byproduct of a more fundamental phenomenon described by general relativity, which suggests that spacetime is curved according to the energy and momentum of whatever matter and radiation are present.
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    Well. That is true but Its not true to say Without gravity there'd be no motion. Gravity affects motion like you said but motion can still happen without gravity
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    Space should exist even without gravitational fields.
    Space is more fundamental and does not depend on matter .
    Even when there is nothing there is space.
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    Special & General Relativity Questions and Answers

    Can space exist by itself without matter or energy around?

    No. Experiments continue to show that there is no 'space' that stands apart from space-time itself...no arena in which matter, energy and gravity operate which is not affected by matter, energy and gravity. General relativity tells us that what we call space is just another feature of the gravitational field of the universe, so space and space-time can and do not exist apart from the matter and energy that creates the gravitational field. This is not speculation, but sound observation.

    --------------------------------------------------------------------------------
    Gravity Probe B - Special & General Relativity Questions and Answers
    Is this persuasive?
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    Quote Originally Posted by Write4U View Post
    Special & General Relativity Questions and Answers

    Can space exist by itself without matter or energy around?

    No. Experiments continue to show that there is no 'space' that stands apart from space-time itself...no arena in which matter, energy and gravity operate which is not affected by matter, energy and gravity. General relativity tells us that what we call space is just another feature of the gravitational field of the universe, so space and space-time can and do not exist apart from the matter and energy that creates the gravitational field. This is not speculation, but sound observation.

    --------------------------------------------------------------------------------
    Gravity Probe B - Special & General Relativity Questions and Answers
    Is this persuasive?
    I share this opinion as well. I don't think it makes any sense to talk about space-time in the absence of energy. But again, it is really just an opinion.
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    Either space, time and spacetime are all different things or may be same
    But space is most fundamental thing and its existence does not depend on any other thing.
    Yes its observation may depend.
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    Quote Originally Posted by RAJ_K View Post
    Either space, time and spacetime are all different things or may be same
    But space is most fundamental thing and its existence does not depend on any other thing.
    Yes its observation may depend.
    There's only space-time, you can't isolate space or time in the presence of gravitational fields.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by RAJ_K View Post
    Either space, time and spacetime are all different things or may be same
    But space is most fundamental thing and its existence does not depend on any other thing.
    Yes its observation may depend.
    There's only space-time, you can't isolate space or time in the presence of gravitational fields.
    No matter, we can call it space time but should exist even if there is no matter
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    When all is said and done the question is really a largely academic one, since there is no place in the universe where there is no gravitational field present. We therefore cannot experimentally test this one way or the other.
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    Quote Originally Posted by Markus Hanke View Post
    When all is said and done the question is really a largely academic one, since there is no place in the universe where there is no gravitational field present. We therefore cannot experimentally test this one way or the other.
    A test is not physically possible but it not necessary needed , we can make conclusion logically
    Even if there is nothing , space exists.
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    Gravity is just let say a force. Then the universe does not depend any force. Also the space can create a force if it is necessary. Means that there can be space if there is no gravitational force. And ıf the movement you ask if it is not working with gravitational force there can not be movement but like people they can move if there is no that force. To move just need free space and a little bit energy .
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    Quote Originally Posted by PhysicsEngineer View Post
    Also the space can create a force if it is necessary.
    What?

    To move just need free space and a little bit energy .
    Er, to move that energy needs to be applied as a force.
    No force - no movement.
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    I just looked that up a few days ago and I got the distinct impression that spacetime itself is the medium through which gravitational functions become expressed in reality.

    Gravitational forces are created by the fabric of spacetime, which in turn are created by massive objects. The spacetime distortion created by both objects will in turn affect the speed and direction of each, proportionally to their relative masses.

    The objects do not attract each other, they fall (inward) toward each other because they distort spacetime, distortions which must be followed physically and to which all other objects are subject.
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    Quote Originally Posted by Write4U View Post
    I just looked that up a few days ago and I got the distinct impression that spacetime itself is the medium through which gravitational functions become expressed in reality.

    Gravitational forces are created by the fabric of spacetime, which in turn are created by massive objects. The spacetime distortion created by both objects will in turn affect the speed and direction of each, proportionally to their relative masses.

    The objects do not attract each other, they fall (inward) toward each other because they distort spacetime, distortions which must be followed physically and to which all other objects are subject.
    Basically correct, but it is just as right to turn this upside down and say that energy ( mass ) is merely a manifestation of curved space-time; because the Einstein field equations say exactly that when re-arranged with the energy-momentum tensor on the left.
    That is the main idea - both points of view are equally valid.
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    Is question should not be "Can spacetime exist where there is no gravitational field or no matter"

    What we denote "space" and "time" may not exist physically or may be but yet there physically existence is not proved
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    what do forumers mean that "milne's model is invalid"?
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    "space" does not have any mass/energy/particles any other thing that physically exist "spacetime" may have So existence of "space" is clearly independent from matter I do not think GR or any other theory of physics condemns it
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    Quote Originally Posted by RAJ_K View Post
    "space" does not have any mass/energy/particles any other thing that physically exist "spacetime" may have So existence of "space" is clearly independent from matter I do not think GR or any other theory of physics condemns it
    Purely vaccumed space has no mass/particles/hardly any energy i guess.
    so do vaccuming space stop time?
    i am confused...
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    Quote Originally Posted by ryanawe123 View Post
    so do vaccuming space stop time?
    Nope. Time will flow at the same rate.
    The only difference would be how you perceive that rate relative to how an observer on Earth will perceive the rate. But either way, you will experience time just the same as you do now.
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    Quote Originally Posted by ryanawe123 View Post
    Quote Originally Posted by RAJ_K View Post
    "space" does not have any mass/energy/particles any other thing that physically exist "spacetime" may have So existence of "space" is clearly independent from matter I do not think GR or any other theory of physics condemns it
    Purely vaccumed space has no mass/particles/hardly any energy i guess.
    so do vaccuming space stop time?
    i am confused...
    May be time stop or run or may be time does not exist
    But time can only be shown in presence of matter or other physical existed objects
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    we confirm that space still exists in purely vaccumed space. since space exists, time must exist too?
    since one cant exist w/o the other

    its still confusing
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    If you listen to RAJ_K you will be confused.
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    Quote Originally Posted by ryanawe123 View Post
    we confirm that space still exists in purely vaccumed space. since space exists, time must exist too?
    since one cant exist w/o the other

    its still confusing
    One of three must true
    1. Time does not exist physically

    2. Time exists physically & is an effect / property of matter/energy like forces , In this case time would not exist in absence of matter/energy

    3. Time exists physically and its existence is independent from matter/energy , in this case time would exist in every corner of space whether there is matter/energy or not, but matter/energy makes effect on it or it makes effect on matter/energy ,So in the absence of matter/energy it is not possible to observe it
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    Quote Originally Posted by Strange View Post
    If you listen to RAJ_K you will be confused.
    If you have give better option and remove confusion
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    Quote Originally Posted by ryanawe123 View Post
    I've researched a lot, and i got really upset now that i know i've been studying on tons of those freaking crackpot sites.

    So i'll be asking here, please don't troll here thanks!

    1) Can space exist where there is no gravitational field?
    2) Is movement allowed where there is no gravitational field?
    Concentrate on what you believe space and movement to be. For instance: Is any change movement?
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    IMO, yes, even a quantum event is movement.
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    RyanAwe123:
    Quote Originally Posted by Guitarist View Post
    Now if it it the case that the metric tensor (in matrix form) is everywhere then we may assume the curvature tensor everywhere i.e our spacetime is "flat". This very roughly describes a Minkowski 4-manifold, and this guy is the playground for SR.
    This describes flat space-time. Because it is flat, we can say there is no curvature due to gravity.
    As a playground for S.R., this shows that time is perfectly valid in flat space-time.
    Quote Originally Posted by Guitarist View Post
    Does this mean that our spacetime 4-manifold does not exist? You tell me!!
    I spew forth, "no."
    Quote Originally Posted by Guitarist View Post
    Now suppose that the curvature tensor everywhere. But by the definition of a manifold, at each and every point there must be a region (a neighbourhood) however small which is approximately Euclidean - in our case by "approximately" we may say Minkowskian.

    Now the non-linearity of the gravitational field eqns seem to say (and I am no expert) that the non-zero of the curvature tensor implies the existence of a gravitational field AND the existence of a gravitational field implies a non- zero curvature tensor. This is, as little as I understand it, roughly what GR says.

    But GR insists that SR is valid in any neighbourhood, however small; of our non-flat spacetime 4.manifold, that is SR is a subset of GR, then surely we may assume that at some scale. however small, we may assume our spacetime 4-manifold is Minkowski "flat" i.e. that gravity can be ignored, without cancelling the concept of spacetime.
    Here, Guitarist says that space-time does not appear to be flat. However, using S.R. we can get away with treating small sections of space-time as flat and still arrive at a meaningful (if not fully accurate) result. Again, this implies that time can exist even if gravity is cancelled out.

    Smack me upside the head for mangling your post.
    Now, I wonder, if you sat yourself down at LaGrange 1 and held a stopwatch...
    Quote Originally Posted by Strange View Post
    (snip)We don't know.
    c) There may be some deeper explanation - quantum gravity, string theory, phase space, causal dynamical triangulation or whatver - but that will still leave you asking "why" that explanation works the way it does. Why is the universe made of (strings / 4-simplexes / space-time-momentum / whatever) and why do they behave the way they do? (And if someone comes up with an even deeper explanation, you will of course be able to ask "but why" about that as well.)
    Quantum Theory of Gravity includes force carriers and Gravitons. A graviton, though never directly observed, is predicted by the Standard Model.
    All other testing of the Standard Model has shown it to be very accurate. In spite of the elusive graviton. In this view, we can say that space is literally nothing and that any empty space is truly empty. Particles must interact in order for us to see the effect of that interaction.

    Relativity, on the other hand, shows gravity to be an effect caused by space-time being warped. One could argue that Space-Time is not exactly empty nothingness. That space-time itself has properties and may even have vacuum energy.
    So far, all testing of Relativity has shown it to be very accurate.

    Yet, these models do not agree with eachother, the bane of current cosmology.
    Something else that we do not yet know about is going on here. The answer may be found among the very small, smaller than electrons, even. Within the fabric of space-time itself.
    Or, it may be found in the Standard Model. We do not know, yet.
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  75. #74  
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    This describes flat space-time.
    Be careful, because this is true only locally. A vanishing Ricci tensor ( in 4 dimensions ) implies only that we are dealing with a vacuum, and that space-time can be considered locally Minkowskian, but it does not mean that space-time is flat globally. To show that a space-time is globally flat we need to prove that the Riemann curvature tensor vanishes everywhere; the Ricci tensor alone determines the full curvature only in 3 or less dimensions.
    Guitarist was able to state flat-ness as a fact because he also gave the additional information that the diagonal elements of his metric tensor were +/- 1.

    For example, the Ricci tensor vanishes for the Schwarzschild metric, even though that metric clearly does not describe a globally flat space-time.
    Neverfly likes this.
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    Scientific laws definitively allow certain portion of space empty
    Although practically there are some particles
    So space can definitely exist in absence of matter
    Space is more fundamental than anything else
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