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Thread: spacetime itslef can move faster than light?

  1. #1
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    spacetime itslef can move faster than light?

    so nothing can move faster than light..
    however i understand that spacetime (the architecture that electromagnetic waves move freely in) can itself move faster than the speed of light.

    this is what happened in the inflation phase of the early universe?

    does this explain why far red-shifted galaxies seem to be moving faster than c?

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    Quote Originally Posted by sabianq View Post
    so nothing can move faster than light..
    however i understand that spacetime (the architecture that electromagnetic waves move freely in) can itself move faster than the speed of light.

    this is what happened in the inflation phase of the early universe?

    does this explain why far red-shifted galaxies seem to be moving faster than c?
    Yes, it does. Spacetime can expand faster than c.

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    Quote Originally Posted by sabianq View Post
    so nothing can move faster than light..
    however i understand that spacetime (the architecture that electromagnetic waves move freely in) can itself move faster than the speed of light.

    this is what happened in the inflation phase of the early universe?

    does this explain why far red-shifted galaxies seem to be moving faster than c?
    Things with rest mass cannot travel faster than light. And it also appears that information can't either. But yes it does appear that the expansion of spacetime can exceed C.

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    Quote Originally Posted by cosmocrazy View Post
    Things with rest mass cannot travel faster than light. And it also appears that information can't either. But yes it does appear that the expansion of spacetime can exceed C.
    We don't know a lot about the inflationary period, but as far as I know, no one has ever proposed that two adjacent points of space ever moved apart faster than c.... It is only the accumulation of much space over a wide area that distant points can said to be separating faster than c.

    I'm not sure if the units work out for spacetime to have velocity.
    Forming opinions as we speak

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    Quote Originally Posted by sabianq View Post
    so nothing can move faster than light..
    however i understand that spacetime (the architecture that electromagnetic waves move freely in) can itself move faster than the speed of light.

    this is what happened in the inflation phase of the early universe?

    does this explain why far red-shifted galaxies seem to be moving faster than c?
    I've never heard or read that spacetime "moves" (but then, there are lot of things I've never heard of)
    My understanding of high redshifts of very distant objects (e.g. galaxies), is that they result from the expansion of space, and these high redshifts may mimic separation-speeds >c

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    Quote Originally Posted by antoniseb View Post
    We don't know a lot about the inflationary period, but as far as I know, no one has ever proposed that two adjacent points of space ever moved apart faster than c.... It is only the accumulation of much space over a wide area that distant points can said to be separating faster than c.

    I'm not sure if the units work out for spacetime to have velocity.
    yes exactly, thanks for elaborating on what I was trying to say. That is why, from current theory, I stated that the "expansion" of spacetime appears to be able to expand at a rate faster than c.

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    Hold on now. Does space-time expand, or does space expand? Both have been mentioned here.

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    Quote Originally Posted by Roobydo View Post
    Hold on now. Does space-time expand, or does space expand? Both have been mentioned here.
    It is the expansion of the space between objects.

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    Quote Originally Posted by Roobydo View Post
    Hold on now. Does space-time expand, or does space expand? Both have been mentioned here.
    The idea is that the standard coordinization of space-time is a frame co-moving with the universal expansion. When you do that space is flat and distances are expanding metrically.

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    Quote Originally Posted by antoniseb View Post
    We don't know a lot about the inflationary period, but as far as I know, no one has ever proposed that two adjacent points of space ever moved apart faster than c....
    It may be a matter of "how adjacent" you're talking about. My guess is that the proposed model of inflation has it occurring so quickly, most all "points" would be "receding" from each other at > c. (100 doublings in 10-32 sec.)

    Otherwise, you are right, of course, that....
    Quote Originally Posted by antoniseb View Post
    It is only the accumulation of much space over a wide area that distant points can [be] said to be separating faster than c.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by sabianq View Post
    so nothing can move faster than light..
    however i understand that spacetime (the architecture that electromagnetic waves move freely in) can itself move faster than the speed of light.

    this is what happened in the inflation phase of the early universe?

    does this explain why far red-shifted galaxies seem to be moving faster than c?
    Spacetime cannot move at all. Where would it go ? There is no other place, it is the whole enchilada.

    Neither can spacetime expand. Spacetime includes all of space and all of time. GR is a completely deterministic theory and spacetime includes everything. past, present, future, and all of "space", all mixed together.

    However, under the assumption of homogeneity and isotropy, assumptions that are made in cosmology, spacetime decomposes into a one-parameter foliation by space-like hypersurfaces of constant curvature. Those space-like hypersurfaces play the role of "space" and the parameter plays the role of "time". In that setting "space" is expanding in the sense that points related by the time-like parameter are seen to increase in distance from one another, as measured in the metric that is inherited from the full space-time manifold. So what is expanding is "space", not spacetime, and one must be a bit careful about what one means by either "space" or "time", since in the presence of matter and the gravity that comes with it, spacetime has (non-constant) curvature and neither space nor time have global meanings.

    With that definition in mind, what we see is that this large scale expansion of space, over time, is such that the speed with which distant objects are receding from us can exceed the speed of light. That is permitted in relativity. What is not permitted is for anything to move throught space (i.e. in local coordinates) faster than light. That proscription does not extend to effects attributed to effects of the metric of spacetime itself -- i.e. it does not extend to the expansion of space which is a statement about slices of the entire spacetime manifold and not a statement about any dynamic change of the manifold (which would make no sense since time itself is part of spacetime and not a parameterization).

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    Quote Originally Posted by loglo View Post
    The idea is that the standard coordinization of space-time is a frame co-moving with the universal expansion. When you do that space is flat and distances are expanding metrically.
    "Space" may or may not be flat. Nobody knows. It is a major open question.

    The fact that "space" is expanding has nothing to do with the curvature, although the mechanism may. If inflation is a correct theory, then it helps to explain why the curvature is nearly zero. But that is another story, which you can read here, direct from the horse's mouth.


    Experimental evidence is that space is very close to flat, but close is not good enough to be able to answer some important questions. A very large sphere is "almost flat", and that is why the earth was widely thought to be flat, complete with edges, until the 15th century.

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    Quote Originally Posted by Roobydo
    Hold on now. Does space-time expand, or does space expand? Both have been mentioned here.
    In the midst of writing this, I see Dr. Rocket has made a distinction here, and he is likely right. I had written the following, which is apparently more about general usage as opposed to technical usage....

    I could be wrong, but I think 'space' is generally just shorthand for spacetime. With a finite speed of light, space (in astronomical terms) necessarily involves this time delay between what you are observing and when it actually happened.

    But as loglo's point implies, current understanding of the 'spatial medium' is apparently so free of fundamental observations, not much can really be said or claimed about it. So meanwhile, space is simply coordinatized to match observations. The more distant an object, the more is its distance increasing. (Well, technically, there is some variation there. Early on, expansion was slowing, then constant, now accelerating.)

    At any rate, does that say anything about the space between here and there? Is it 'stretching' or 'reproducing', or is there any 'materiality' to space at all? Since independent observations confirm spacetime is very close to flat, modern scientists have pinned ~70% of the entire mass of the universe as coming from... 'empty' space. So there's apparently some materiality there, but what that materiality is, is rather up in the air, so to speak, especially with respect to how it can cause the expansion to accelerate.
    Everyone is entitled to his own opinion, but not his own facts.

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    What do we know ?

    An initial rapid expansion that apparently subsided and has since sped up and still is doing so... That the apparent velocity of distant galaxies is greatly distorted by the expansion of the void between us... Space time.

    Why is this not well understood ?

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    Quote Originally Posted by DrRocket View Post
    "Space" may or may not be flat. Nobody knows. It is a major open question.

    The fact that "space" is expanding has nothing to do with the curvature, although the mechanism may. If inflation is a correct theory, then it helps to explain why the curvature is nearly zero. But that is another story, which you can read here, direct from the horse's mouth.


    Experimental evidence is that space is very close to flat, but close is not good enough to be able to answer some important questions. A very large sphere is "almost flat", and that is why the earth was widely thought to be flat, complete with edges, until the 15th century.
    Well, flat-ish then.

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    Quote Originally Posted by loglo View Post
    Well, flat-ish then.
    Yes, but flat-ish, is not good enough to tell if the universe is open (aka infinite) or closed (aka finite). In fact to even discriminate on the basis of curvature takes some additional assumptions.

    For local questions away from large bodies, the curvature is small enough to ignore. But when you look at the structure of the entire universe and ask global questions, a little curvature goes a long way.

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    Quote Originally Posted by astromark View Post
    What do we know ?

    An initial rapid expansion that apparently subsided and has since sped up and still is doing so... That the apparent velocity of distant galaxies is greatly distorted by the expansion of the void between us... Space time.

    Why is this not well understood ?
    Apparently you don't understand. Spacetime is NOT expanding.

    It is only space-like 3-dimensional slices of 4-dimensional spacetime that can be considered to be expanding.

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    Quote Originally Posted by DrRocket View Post
    Spacetime cannot move at all. .... Neither can spacetime expand....

    So what is expanding is "space", not spacetime, ...

    With that definition in mind, what we see is that this large scale expansion of space, over time, is such that the speed with which distant objects are receding from us can exceed the speed of light. That is permitted in relativity. What is not permitted is for anything to move throught space (i.e. in local coordinates) faster than light .....
    (my bold)
    That's how I "understand" it, from a layman's point of view. Still, I have difficulties to grasp why redshifts due to "expansion of space" can't be looked at as Doppler redshifts. Where's the problem?

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    Doppler shifts arise from the relative motion of source and observer through space, whereas astronomical redshifts are 'expansion redshifts' due to the expansion of space itself.

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    Redshifts due to the "expansion of space" can be viewed in a similar way to Doppler shifts.

    From Expanding Space: the Root of All Evil?

    The explanation of redshift is a crucial link that needs to be made between cosmological observations and theory. A derivation of the balloon analogy is often employed in the teaching of this concept; a wave is sketched on a balloon and as it is blown up the wavelength is seen to increase as the sketch is stretched along with the expansion of the underlying space. This is largely uncontroversial, but care must be taken in ensuring that the analogy does not mislead.

    Since we have shown how bodies held together by electromagnetic forces do not expand with the expansion of space, why should electromagnetic waves be affected? The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied. The difference between frames relates to a changing background metric rather than a differing velocity.

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    Quote Originally Posted by speedfreek View Post
    ...From Expanding Space: the Root of All Evil?

    ...a wave is sketched on a balloon and as it is blown up the wavelength is seen to increase as the sketch is stretched along with the expansion of the underlying space. This is largely uncontroversial, but care must be taken in ensuring that the analogy does not mislead... Since we have shown how bodies held together by electromagnetic forces do not expand with the expansion of space, why should electromagnetic waves be affected?
    Well, for one, the local electromagnetic forces have not been traveling at 'c' through billions of lightyears of... whatever to reach us, arriving highly redshifted.

    Quote Originally Posted by Expanding Space
    The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through.
    I thought they just said that was uncontroversial?

    Quote Originally Posted by Expanding Space
    Rather, cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted.... The difference between frames relates to a changing background metric rather than a differing velocity.
    Well, of course not a differing 'velocity'. But simply detecting a signal in a "different frame to that which it is emitted" would seem to skirt the question of what may or may not have been happening to that electromagnetic wave on its long journey through a variable 'underlying background metric'. And I agree, the 'signal' we're talking about was emitted long ago, when the Universe was considerably younger and more dense; I expect the background metric had distances between things increasing a lot more quickly back then. So yes, that signal is coming from not just a different frame, but from a very different frame.

    So when the background metric implies cosmological distances between objects are increasing, what does that seem to imply about the 'underlying space' between those objects? Is space considered wholly insubstantial, a mere imaginary spatial blackboard upon which to place points and curves?
    Everyone is entitled to his own opinion, but not his own facts.

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    Mr Rocket...

    Quote Originally Posted by DrRocket View Post
    Apparently you don't understand. Spacetime is NOT expanding.

    It is only space-like 3-dimensional slices of 4-dimensional spacetime that can be considered to be expanding.
    "What do we know ?" and, finished with.
    " Why is this not well understood ?"

    You Mr Rocket are perfectly untitled to a opinion. I only used the words 'Space time' because separating time out of space does not seem to say enough. The void (space) that is seemingly expanding at a ever faster rate (time related). To me is expanding space time. The universe is getting bigger and older. I can not take time away from space.Space is meaningless without time. I do not care that you can. You may have a better understanding of this than I. If my understanding is so wrong ? please expand your explanation so as I might be enlightened.

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    Quote Originally Posted by astromark View Post
    " If my understanding is so wrong ? please expand your explanation so as I might be enlightened.
    Yes, your understanding is so wrong. If you do not have some notion of time separated from space (and it takes some work to make sense of this) what can the "ing" in "expanding" possibly mean ?

    Try reading my earlier post which explains this in some detail.

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    Now I am convinced... I was never wrong...but understand that point is open for interpretations different than mine... I like to think that I am a student of mainstream science,. Always learning and listening. Wanting to have a better understanding is not wrong. Yes I have revisited posts 11 and 12 and all of the others you have added here. Your method of explanation is as subtle as a taser. Please stop making this personal. All I want is a better explanation ?... THAT IS (was) A QUESTION.
    Last edited by astromark; 2010-Feb-16 at 07:42 AM. Reason: added was for clarity.

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    Quote Originally Posted by astromark View Post
    All I want is a better explanation ?... THAT IS A QUESTION.
    To quote Dirac, "That is not a question."

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    I concede that I should have used 'was' and have now added it...Can you please explain to me why and how time as in 'space time' is not in your thinking of the explanation of what we are asking of here. The expansion of the Universe at velocity greater than c.
    Understanding that mass can not do that. So its got to be the space between all the mass. That is. Do I dare add that ?

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    Quote Originally Posted by gzhpcu View Post
    Doppler shifts arise from the relative motion of source and observer through space, whereas astronomical redshifts are 'expansion redshifts' due to the expansion of space itself.
    Yes, that's the "standard" explanation I'm aware of. But why, or how, can a photon "know" (or care) about the background of separating objects, i.e., whether the separation is due to relative motion or due to the expansion of space? Look at the analogy of an expanding rubber sheet (I prefer this to the balloon analogy, because no curvature is complicating the situation). Two objects on the expanding rubber sheet will certainly(?) be unable to tell apart whether they move ON the rubber sheet, or whether the rubber sheet is expanding.
    I know that the situation in the "real" universe is said to be different, because we have this fundamental law c= const. But still ... we have also propeller-driven aircraft that "separate" at v > vsonic without any supersonic boooom

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    Quote Originally Posted by DrRocket View Post
    "Space" may or may not be flat. Nobody knows. It is a major open question.

    The fact that "space" is expanding has nothing to do with the curvature, although the mechanism may. If inflation is a correct theory, then it helps to explain why the curvature is nearly zero. But that is another story, which you can read here, direct from the horse's mouth.


    Experimental evidence is that space is very close to flat, but close is not good enough to be able to answer some important questions. A very large sphere is "almost flat", and that is why the earth was widely thought to be flat, complete with edges, until the 15th century.
    Dr. I think anybody who ever sat on a hill by the ocean and watched a boat disappear over the horizon realizes the Earth is not flat but slightly rounded, and Eratosthenes put the geometry to it B.C. with a fair error bar. Columbus having been shipwrecked into the hands of a Portuguese cartographer, carried a map of the Indies by some regards. pete



    map:http://en.wikipedia.org/wiki/Piri_Reis_map

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    Quote Originally Posted by astromark View Post
    I concede that I should have used 'was' and have now added it...Can you please explain to me why and how time as in 'space time' is not in your thinking of the explanation of what we are asking of here. The expansion of the Universe at velocity greater than c.
    Understanding that mass can not do that. So its got to be the space between all the mass. That is. Do I dare add that ?
    Try reading the earlier detailed explanation of what "time", "space" and "spacetime" mean. These terms are not interchangeable. I'm sorry if the explanation is obscure to you, but in fact the explanation is very precise and that precision is required to clearly define the terms. It is a sad fact that use of the usual popularized terms and explanations is the ultimate source of much confusion.

    Your statement that "its go to be the space between all the mass" is basically correct, but it is indeed "space" and not "spacetime", and one has to be quite careful about what one means by "space" when talking in the context of general relativity and spacetime.

    Your confusion is understandable. In trying, and eventually succeeding, in understanding what is meant by "expansion of the Universe" and "age of the Universe" in the context of general relativity, I had a conversation on the subject with a rather well-known experimental particle physicist. He did not understand the meaning of the terms either and recommended a discussion with another well-known physicist who specialized in general relativity. The message here being that the constructs are not widely known and understood and that it is important to get into the details in order to make sense of a fundamentally contradictory situation.

    Much of the confusion is the result of your use of the terms "space" and "spacetime" incorectly and more or less interchangeably. They are not the same thing.

    Similarly one must be careful with the use of "time" in the context of spacetime and the earlier posts do address that issue as well. In order for something to be expanding, an implied ongoing process, one has to have some notion of time, and not just spacetime. So to have such a discussion one must have a decomposition of spacetime into spatial and time coordinates. Such a decomposition, on a global basis is not a part of general relativity, and indeed GR tells us that no such decomposition is natural. In order to make sense of such ideas on a global scale one is forced to resort to the implications of some relatively sophisticated mathematics and to invoke the assumptions of homogeneity and isotropy -- assumptions that are clearly not justified as anything other than approximations on the largest of scales.

    One has to be very careful with terminology and concepts in order to make statements that make sense and not be misleading or downright incorrect. The universe, for instance, not expanding at a velocity greater than c. In fact neither the universe nor expansion have a velocity. What is occurring is that the volume of a parameterized set of 3-dimensional space-like hypersurfaces is increasing, related points are receding from one another at speeds, relative to the time-like parameter and the metric on the hypersurfaces that is inhereted from the full spacetime manifold, that is in excess of c.

    If this is confusing that is simply part of the overall package. General relativity tells us very clearly that time and distance are coordinate effects, valid only for a specified observer and only near that observer. There is no universal meaning to either time or space. But in cosmological discussions it is necessary that there be some universal notion of time and a universal notion of space. Moreover, those same cosmological discussions rely in a fundamental way on general reltivity. That requires some work and some subtlety in order to make sense of the constructs being used. It is confusing. But it is essential that the ideas and constructs be clearly and precisely defined. Everyday words and concepts are misleading at best, and really just plain wrong.

    Adding "was" does not create a question.

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    Quote Originally Posted by trinitree88 View Post
    Dr. I think anybody who ever sat on a hill by the ocean and watched a boat disappear over the horizon realizes the Earth is not flat but slightly rounded, and Eratosthenes put the geometry to it B.C. with a fair error bar. Columbus having been shipwrecked into the hands of a Portuguese cartographer, carried a map of the Indies by some regards. pete



    map:http://en.wikipedia.org/wiki/Piri_Reis_map
    You are probably historically correct. However, that has little to do with the point that I was trying to make, which is that being "nearly flat" locally, doesn't tell you much.

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