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Center of the universe

by superdave
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Hoku
#19
Mar12-10, 02:13 PM
P: 166
Mihael, you have misunderstood Chronos, who is saying that time didn't exist before the big bang.

I'm the one who suggested that spacetime is absolute, and since I'm a layman, I guess I won't get such harsh criticism for it. However, as I said in my last post, evidence is mounting in support of the absolute view and more scientists are begining to take it seriously. Check this out if you're interested - http://www.springerlink.com/content/k0htmwr32m4wd7kv/ . I will purchase the full article myself next week.

I know that the relational spacetime view compells us to say that "spacetime was irrelevant prior to the big bang", but since nobody actually knows what preceeded the big bang, that makes us unqualified to make such a statement.

Perhaps a way to bridge the absolute/relational spacetime controversy is by comparing it with the two kinds of energy, kinetic and potential. Both kinds of energy are very real, they're just in different phases of expression. Might this also apply to spacetime? Before the big bang was "potential spacetime" and afterwards it became "kinetic spacetime". Hmm...
Mihael@@/&
#20
Mar13-10, 06:37 AM
P: 37
It happens from time to time, not to understend somebody here:) Did not ment to be rude, sorry!
Kronos5253
#21
Mar13-10, 04:28 PM
P: 111
Quote Quote by mgb_phys View Post
The balloon analogy is for a 2d universe.
Imagine you are a 2d flat creature on the surface of the balloon there is no centre.
But it's 3D... So wouldn't it make more sense to say that you are a 3D creature inside the balloon? The way I see it, if the balloon is our universe, and the bug is us, shouldn't we be on the inside of the balloon, not on the outside? In that case everything expands from a central point, and you have a direct center of the "universe".
marcus
#22
Mar13-10, 05:14 PM
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Quote Quote by Hoku View Post
... evidence is mounting in support of the absolute view and more scientists are begining to take it seriously. Check this out if you're interested - http://www.springerlink.com/content/k0htmwr32m4wd7kv/ . I will purchase the full article myself next week.

I know that the relational spacetime view compells us to say that "spacetime was irrelevant prior to the big bang",...
Hoku that is a strange post! You refer to a 2001 article by James Overduin. If you want to know what Overduin was saying almost 10 years ago, you don't need to purchase that article. You can read this 2001 review article by him for nothing:
http://arxiv.org/abs/astro-ph/0101484

In any case how does what a not-too-prominent guy says in 2001 show that "evidence is mounting and more scientists are taking seriously"?

Why do you say that "relational spacetime view" entails believing that spacetime was irrelevant before the start of expansion?
It is people who explicitly adopt the relational view who have been constructing and studying models where spacetime goes back before the start of expansion. They say their view is relational and their computer models of evolving cosmic geometry go back before.

I'm talking about research proceeding at an increasing rate on the order of 100 papers a year. And current efforts to find ways to test the models.
This reality totally does not square with your statement.

Quote Quote by Mihael@@/& View Post
Time probably won't exist only in absolutely homogeneous universe.
Michael, that is an interesting statement. I think I understand. And I agree with what I think is the meaning you intended. The other issue is "does time go back before the big bang?"
On that question I think it is irrational, perhaps silly, to make statement Yes or No. We do not know. Do you perhaps agree with me here?

We have models where time stops and we have models where it continues on back before. The models have testable consequences and we should be able to test. But we have not yet tested. So there is no scientific reason to believe either one thing or the other.

Do you know the Einstein-Online website? I recommend reading A Tale of Two Big Bangs on the issue of does time have a beginning at the big bang.
http://www.einstein-online.info/en/s...ogy/index.html

http://www.einstein-online.info/en/s...ngs/index.html

It's a public outreach branch of the Max Planck Institute. Hope you check it out.
Hoku
#23
Mar13-10, 06:06 PM
P: 166
Marcus, thanks for your objections. Actually, the free article that you recommended is not the same as the one on Springerlink. I know this because I read the first page of the Springerlink article, which is free, and it is completely different from the arvix article. Still, it might have enough of the same info to save some money.

The arvix article was updated in 2002 but that doesn't mean the idea died out shortly thereafter. In fact, Stanford picked up on it and included the idea in their website http://einstein.stanford.edu/index.html, which is maintained to this day. Overduin wrote the "spacetime" section, which was current as of September 2007. His statement that "Spacetime behaves relationally but exists absolutely" can be found at the bottom of the "Einstein's Spacetime" subsection.

If there are any other objections to Overduin's publications let it be based on the specifics of what he is saying and not on date of publication or whether he's attained a high enough position at a prestigious enough university. An assistant professor position at a university in Maryland is respectable enough for a man as young as Overduin.

You are right about my incorrect use of the phrase, "relational spacetime view". I tend to incorrectly say that when the proper phrase I should be using is "emergent spacetime view". From an emergent spacetime view, spacetime IS irrelevant before the big bang because, based on that view, it did not yet exist.
marcus
#24
Mar13-10, 06:38 PM
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Quote Quote by Hoku View Post
... I tend to incorrectly say that when the proper phrase I should be using is "emergent spacetime view". From an emergent spacetime view, spacetime IS irrelevant before the big bang because, based on that view, it did not yet exist.
I don't see how that follows. There are theories where geometry emerges from some more fundamental degrees of freedom, which however can also exist before the beginning of expansion. I'm not sure what you mean by "emergent". It is a buzzword that people use various different ways. I don't see any reason why if spacetime emerges after the BB moment it could not emerge before the BB moment. No rational reason to believe that the same underlying stuff doesn't exist before. Fact is we don't know.

I'm not disrespecting James Overduin. As I say he's just not very prominent. In no way would I take him as an authority. And of course those two 2001 articles are different. I looked at the Springer sample too The free one is a long review article with lots of references and my guess is that it gives a good idea of what he thought and said around that time.

You cite a general audience outreach 2007 article by him at the GravityProbeB website. http://einstein.stanford.edu/SPACETIME/spacetime2.html
I don't altogether agree with it, but heck! Why not copy a sample excerpt here and see if anyone else has questions or objections?

==quote Overduin 2007==

Relational or Absolute?

In 1918, Einstein described Mach's principle as a philosophical pillar of general relativity, along with the physical principle of equivalence and the mathematical pillar of general covariance. This characterization is now widely regarded as wishful thinking. Einstein was undoubtedly inspired by Mach's relational views, and he hoped that his new theory of gravitation would "secure the relativization of inertia" by binding spacetime so tightly to matter that one could not exist without the other. In fact, however, the equations of general relativity are perfectly consistent with spacetimes that contain no matter at all. Flat (Minkowski) spacetime is a trivial example, but empty spacetime can also be curved, as demonstrated by Willem de Sitter in 1916. There are even spacetimes whose distant reaches rotate endlessly around the sky relative to an observer's local inertial frame (as discovered by Kurt Gödel in 1949). The bare existence of such solutions in Einstein's theory shows that it cannot be Machian in the strict sense; matter and spacetime remain logically independent. The term "general relativity" is thus something of a misnomer, as pointed out by Hermann Minkowski and others. The theory does not make spacetime more relative than it was in special relativity. Just the opposite is true: the absolute space and time of Newton are retained. They are merely amalgamated and endowed with a more flexible mathematical skeleton (the metric tensor).

Nevertheless, Einstein's theory of gravity represents a major swing back toward the relational view of space and time, in that it answers the objection of the ancient Stoics. Space and time do act on matter, by guiding the way it moves. And matter does act back on spacetime, by producing the curvature that we feel as gravity. Beyond that, matter can act on spacetime in a manner that is very much in the spirit of Mach's principle. Calculations by Hans Thirring (1888-1979), Josef Lense (1890-1985) and others have shown that a large rotating mass will "drag" an observer's inertial reference frame around with it. This is the phenomenon of frame-dragging, whose existence Gravity Probe B is designed to detect. The same calculations suggest that, if the entire contents of the universe were to rotate, our local inertial frame would undergo "perfect dragging" — that is, we would not notice it, because we would be rotating too! In that sense, general relativity is indeed nearly as relational as Mach might have wished. Some physicists (such as Julian Barbour) have gone further and asserted that general relativity is in fact perfectly Machian. If one goes beyond classical physics and into modern quantum field theory, then questions of absolute versus relational spacetime are rendered anachronistic by the fact that even "empty space" is populated by matter in the form of virtual particles, zero-point fields and more. Within the context of Einstein's universe, however, the majority view is perhaps best summed up as follows: Spacetime behaves relationally but exists absolutely.

==endquote==
Flatland
#25
Mar14-10, 04:43 AM
P: 152
Quote Quote by bannerman100 View Post
So where is the universe expanding from ? The concept of expansion requires a centre as a starting point.
In a closed universe space curves back itself. What this means is that if you travel in a straight line in any direction you will eventually end back at your starting point. Now tell me what is the center to that?
Mihael@@/&
#26
Mar15-10, 04:50 AM
P: 37
As Heraclitus and many others, we too try to comprehend the big stuff. Center of the universe and beginning of time is as big as it gets. My thoughts were in direction of black holes, and if their power can dissolve everything to a basic homogeneous state..no matter what that state would be.
Chronos
#27
Mar16-10, 12:08 AM
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I dislike arguing these issues, but, there is no compelling evidence of a 'center' or 'begiining' of the universe. I fail to see the relevance of black holes. They are quite ordinary residents of the universe unless you get too close.
Mihael@@/&
#28
Mar16-10, 02:37 AM
P: 37
As we are all free to imagine universe as we can, for me it is easiest to put a blackhole in role of our universe's mother :) As long as this depiction is any good:
http://www.instablogsimages.com/imag...on-model_9.jpg
marcus
#29
Mar16-10, 02:51 AM
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Quote Quote by Mihael@@/& View Post
As we are all free to imagine universe as we can, for me it is easiest to put a blackhole in role of our universe's mother :) ...
That's a good idea. Some quantum gravity models allow for a bounce, expanding to form a new spacetime region. The work is preliminary and some way needs to be found to derive testable predictions but there are QG models of collapse to black hole which do in fact bounce and initiate expansion.

Some authors (which can be looked up in arxiv) are Kevin Vandersloot, and Dah-wei Chiou.
Francesca Vidotto is working on something that might apply to this. Ashtekar has one or two recent papers that speculate along these lines. Leonardo Modesto is another who has studied the QG black hole. Sabine Hossenfelder too. Maybe I should get some links and try to organize the information.

Anyway it is remotely possible that a black hole collapse somewhere else can cause a new expanding tract of spacetime. We are, as you say, "free to imagine" this kind of thing. And some people actually study it. It's interesting and it hasn't been ruled out so far.
Dmitry67
#30
Mar16-10, 04:14 AM
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Quote Quote by Mihael@@/& View Post
As we are all free to imagine universe as we can, for me it is easiest to put a blackhole in role of our universe's mother :) As long as this depiction is any good:
http://www.instablogsimages.com/imag...on-model_9.jpg
Ah, its turtles all the way down... sorry, black holes :)
Mihael@@/&
#31
Mar16-10, 05:56 AM
P: 37
Most information about blackholes is quite limited, its known that it warps spacetime much more than any other object. But piercing through spacetime and creating another, new spacetime is something else, somthing I would bet on... Have no idea whats the meaning of Quasi-Geostrophic models, at firs look its to much for me, or is it Quantum Gravity models or Quantum Geometrodynamics :)
Mihael@@/&
#32
Mar17-10, 03:02 PM
P: 37
Could not edit my previous post again so I'll paraphrase:)

I know that some of you wisit PF just to relax, and that you have lots of intersting things to do and to think about. This particular topic shoud be intersting and for fun so I'll ask you to join.
Most information about blackholes is quite limited, its known that it warps spacetime much more than any other object. I never understood what is main stream idea about
the efect of BH on normal spacetime. In everyday life, anything with denser mass falls
through anithing with lower density. On the other hand if somthing that warps and falls through space and time it probably should be with higher densty of mass and also with higher densty of time.
If, in some moment, such object reaches critical potential of space and time I don't see any reason not pearcing througth and creating new spacetime..
Dmitry67
#33
Mar17-10, 03:34 PM
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I don't see how it "creates new spacetime"
For example, except the problem with the singularity, *ALL* properties of non-rotating black hole are visible on this wonderful picture (sorry no higher resolution):



So you can answer almost any question just by moving your fingure on the diagram.
Kronos5253
#34
Mar19-10, 02:31 PM
P: 111
Quote Quote by Kronos5253 View Post
The way I see it, if the balloon is our universe, and the bug is us, shouldn't we be on the inside of the balloon, not on the outside? In that case everything expands from a central point, and you have a direct center of the "universe".
Out of curiousity, can anyone explain to me why this doesn't work?
nutgeb
#35
Mar19-10, 03:09 PM
P: 294
Quote Quote by Kronos5253 View Post
Out of curiousity, can anyone explain to me why this doesn't work?
Kronos, the balloon analogy fails entirely once you depart from the surface of the balloon. Only the surface is included in the analogy. There is no analogy in the real universe for any point not located on the surface, such as the center of the balloon.

The same problem occurs whether the bug is inside or outside the balloon.

People have tripped over this conceptual problems many, many times.
ManyNames
#36
Mar21-10, 04:08 AM
P: 136
Quote Quote by WhoWee View Post
If a center could be identified, wouldn't it need to be plotted over time?

If you want a center to the universe, you can call every point on the spacetime map as the center of the universe, since big bang happened everywhere - but not at any single point.


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