If the universe has a flat structure because we perceive light rays straight, why is general relativity not debunked (because it says that spacetime is curved). I am confused please help me out or tell me the gap in my understanding.

If the universe has a flat structure because we perceive light rays straight, why is general relativity not debunked (because it says that spacetime is curved). I am confused please help me out or tell me the gap in my understanding.
Flat universe is a global description. Curved space is local. Eddington 1919 experiment showed this.
https://en.wikipedia.org/wiki/Tests_...ral_relativity
How does this have an effect on light then?
Light follows the curved path resulting from the effect of general relativity.
When it is said that the universe is flat, it is the threedimensional spaces such that all locations are of equal age that are flat. By contrast, the curvature of general relativity is the curvature of fourdimensional spacetime.
A decidedly bended universe is portrayed by elliptic geometry, and can be thought of as a threedimensional hypersphere, or some other circular 3complex, (for example, the Poincaré dodecahedral space), which are all remainders of the 3circle. The shape of the universe is the nearby and worldwide geometry of the universe. The neighborhood highlights of the geometry of the universe are basically depicted by its ebb and flow, while the topology of the universe portrays general worldwide properties of its shape starting at a persistent protest. The shape of the universe is identified with general relativity, which depicts how space time is bended and bowed by mass and vitality.
If the universe is flat, does that mean there is something above and below it?
If it is expanding then why is it expanding, and into what?
Would a neighbouring universe provide a gravitational attraction?
While I'm here, if one of space, time and matter were to disappear would the other two still exist?
One more...what exactly is the universe and why should it even bother to exist?
If we're in a hologram that might be the start of an explanation.
The volume is increasing (and hence the density is decreasing and therefore the temperature decreasing). But if the universe is infinite, then it is still infinite even if it doubles in size!
But if it is not infinite, there is nothing "outside" the universe (there is no outside) and so it is not expanding "into" anything even if it is getting bigger.
I don't know if total energy of the universe can be defined. And I'm not sure how (or if) the increasing amount of dark energy is consistent with the zero energy hypothesis.If total energy was constant that might make sense to me (but total energy is thought to be zero anyway,isn't it  so perhaps that is not a consideration)
I'm looking at a 3D object now and there is something above and below it.
Could there be unknown higher dimensions?
What is driving the expansion? If we can only say dark energy, we don't know.It is not expanding into anything. Things are just getting further apart from one another.
But if the universe is infinite, then it is still infinite even if it doubles in size!
If finite, not also?
We don't understand infinity, dark energy, dark matter. What we do know is actually very little.
There could be. But there is no evidence or need for any such thing. (String theory adds extra spatial dimensions, but they are purely hypothetical)
We know that an even distribution of mass (which is approximately Tre for the universe on large scales) must either contract or expand (doing neither would be rather like balancing a pencil on the tip  very unstable; it would quickly start expanding or contracting). The expansion was driven by the initial conditions of the universe. But we don't know what they were. Depending on the density one would expect the expansion to either slow (because of gravity) and possibly reverse or keep going or accelerate (in the presence of dark energy).What is driving the expansion? If we can only say dark energy, we don't know.
We understand a lot about dark matter, just not what it is made of. We probably know less about dark energy. Not sure what is not understood about infinity. It seems a pretty straightforward concept: "without end".We don't understand infinity, dark energy, dark matter. What we do know is actually very little.
If infinity means without end and the universe is infinite then it could not have been originally if it started with a singularity. Once it was finite, if even for an instant.
Does that suggest that infinity is variable depending on the size of the universe?
Marcus de Sautoy in his book What We Cannot Know raises the question if the universe is finite or infinite. He doesn't know, either.
Ask the question again  if matter ceased to exist would space and time still remain, and if time ceased would space and matter remain?
We do not know how it started. We only "know" as far back as 10^43 seconds after what may have been been called the Big Bang.
I do not think it is believed that a "singularity" actually existed it is more or less a term for something we do not understand (mathematically at any rate)
I was under the impression that there is a singularity in a black hole which is the reverse of big bang cosmology.
What do you mean by mathematically? Are you saying the universe is not a mathematical construct, and the shape of it could not be described as such.
Einstein's wife was supposed to have said that her husband could work out the shape of the universe on the back of an envelope, implying that the space telescope was a waste of money.
I think my use of "mathematically"was fairly clear. (we don't have a valid mathematical description for certain scenarios such as what existed before T^43 seconds the earliest time after which we can make predictions using mathematics)
I am not saying that the "universe is a mathematical construct" Are you?
For what it is worth ,I would say that our mathematical constructs attempt to describe the universe ,but always imperfectly (they are idealizations).
You are implying that maths is only a tool to describe it. Tegmark might disagree.
https://www.scientificamerican.com/a...mathexcerpt/
It is such a basic idea that I am entitled to my approach. Perhaps you can point out where in that article ** (or any of that man's or another's writings) a cogent argument is made to support the idea that mathematics is so essential a phenomenon that the universe would fall apart if it did not exist except as a method to describe it?
My own (biased) view is that maths (and ideas) are a subset of the universe but not essential to its functioning.
To repeat "I am not saying that the "universe is a mathematical construct" Are you?"
** whilst interesting I do not have the time to read it at the moment.
I wasn't expecting you to read Tegmark's book Our Mathematical Universe in its entirety.
So that's why I posted the fragment only. Should take about 3 minutes to read.
I don't like the hypothesis that the external reality exists separate from the observer.I prefer to assume that the observer is an integral part of the overall arrangement.
But that is a philosophical question and an unscientific one unless consequences can be shown to flow from it.
I don't see though how this affects how we understand (or fail to understand) what lies inside a black hole (at its "centre" if that is a useful idea)
Do you still say that there is an actual "singularity" inside a black hole ,rather than a region where our present physical theories break down?
It is almost certain that neither singularity has any physical reality. It probably needs a theory of quantum gravity to explain why the singularity doesn't exist.
It is possible it can be described mathematically, but probably not with the math we currently have!What do you mean by mathematically? Are you saying the universe is not a mathematical construct, and the shape of it could not be described as such.
Has anyone provided a fairly rigorous definition of “invented” vs “discovered”?
I consider app/user interface schemes to be whole cloth invention, generally very poor at that.
I would be interested in a condensed introduction/overview of that 3rd stated view of mathematics.
I was led to believe it is a place of infinite energy, but not a place of spatial coordinates. Inside a black hole it could result in the birth of a new universe.
It would confirm that the universe began with some sort of mathematical beginning if we did have the maths, but how about the Schwarzschild radius.
https://en.wikipedia.org/wiki/Gravitational_singularity
So we don't know what's inside the BH just as we don't know if the universe has a boundary. You are never going to meet either.
Think of the universe as an expanding loaf of bread. The surface expands into the space outside of its surface. But you can't see the surface of the universe. If you can't see the surface how can you know if there is one? If there isn't one the universe in infinite. If there is one the universe isn't. To be scientifically honest, and accurate we can't say its expanding into anything. That would imply a surface and a finite universe. We don't know that so we don't say that. We can only say its expanding because that's all we see.
The idea of flatness comes from the idea is that if it's flat (uniformly flat motion) there is no clue that there may be a surface. We look for a bit of rotation (non flatness) at the extreme of distance. If we detect rotation (we don't) it's a clue there may be a surface. We don't see rotation so no clue as to if there is a surface and no clue if it's infinite or not.
You may ask; what if the rotation is so little that it's only detectable beyond the limits of what we can see A fair question but; If the rotation is beyond what we can see then it's in the same category as a surface we cant see. We don't know if it's there or not.
Last edited by doitright; December 13th, 2018 at 05:52 AM.
What type of bread? Is the universe being cooked in an oven?
What's in the space?The surface expands into the space outside of its surface.
If it's 100 billion light years across, I'm not surprised.But you can't see the surface of the universe.
Nice piece of logic.We don't know if it's there or not.
For the folks who think space is a liquid and the galaxies some type of vortex. .
The Niagara River Whirlpool. This pic doesn't do it any justice because it is really enormous. It's a bulge in the Niagara River downstream from the Falls. So big that a cable car runs across it and because it's a long trip from side to side people think they're going from Canada to the USA when in fact the famous vortex is entirely within Canadian territory. No person has ever successfully swam across it. It's where most suicide victims, the ones who leap over the Falls, end up.
That analogy does not correspond to current cosmology.
Our models say there is no boundary whether the universe is infinite or finite.If there isn't one the universe in infinite. If there is one the universe isn't.
Flatness has nothing to do with rotation. (There is no evidence the universe is rotating, either.)The idea of flatness comes from the idea is that if it's flat (uniformly flat motion) there is no clue that there may be a surface. We look for a bit of rotation (non flatness) at the extreme of distance. If we detect rotation (we don't) it's a clue there may be a surface. We don't see rotation so no clue as to if there is a surface and no clue if it's infinite or not.
Not that you are saying or implying this ,but I wonder whether a boundary can actually be dynamic or is it static by definition ? As the universe is dynamic everywhere presumably any "boundary" could only be an approximation.
Separately is there any sense in which we can take a part of the universe (eg me here typing on this keyboard) and give it an age wrt the earliest times that we have knowledge of ?(ie T>^43sec) ?
Can I say that the particular event that I am presently a part of is some 13 billion years "old".?
If I was to pick another location at random ,say somewhere inside a Black Hole would the "age" of that region be more or less the same (13 billion years) even though for an external observer it might seem that time had stopped there after the Black Hole had formed?
As I wrote earlier in the thread of Black Hole problems, from outside perspective time never stops at event horizon. From outside we can only operate with Schwarzschild frozen stars. It is because of what is called coordinate singularity. Coordinate singularity is true singularity from outside i.e. for us. So time does not stop from the outside perspective of notBlack Hole universe, and the objects are younger. The authors (Kenichi Nakao, ChulMoon Yoo, Tomohiro Harada in their work On gravastar formation: What can be the evidence of a black hole? arxiv.org/abs/1809.00124) the Schwarzschild frozen stars call "gravitationally contracting objects". The authors also stress one fact about Black Holes on p. 33 "It is a profound fact that general relativity has predicted the advent of domains of which the existence can not be confirmed through any observation." I will clarify. Because Schwarzschild frozen stars and Black Holes are predicted both by the same GR, we cannot make any conclusion which one is true inside GR working framework from the outside perspective.
We can call for help a quantum mechanics. Now I will be attacked as fringe. Because Hawking radiation from outside point of view necessarily invites existence of event horizon it is a wrong concept, because from outside event horizon does not exist. From the outside, the event horizon does not exist because saying that it does exist means a breakdown of law of physics. The breakdown of law of physics from the outside is implied in GR by coordinate singularity. Gravastar quantum gravity model does not have event horizon from both sides (in and out). To settle these questions we can only hope that gravastar or some other nonhorizon model will give some early predictions (e.g. gravitational waves ringdown or something else) which are not so close to the time of Black Hole "event horizon formation", so that our civilization can caught it in reasonable time. Working in the framework of GR only is lost of time. Or the great GR will need serious reformulations.
I wish to tribute the pass of my good friend Obvious Leo who was a member of this and daughter forum before site breakdown (sciencechatforum.com/viewtopic.php?f=117&t=32053). I did not know until very recently. Rest in peace my friend.
Zlatan
Well, if you take the observable universe, for example, then that has a "boundary" or horizon which can be precisely defined and is continuously changing.
But the problem with the universe as a whole, as it is described by GR in the Big Bang model the spacetime manifold has to be smooth and continuous, so it cannot have an edge (a discontinuity).
Yes, you can say that the universe has an age, based on the notional singularity if you trace things back. And different people will measure that age differently in their frame of reference. In general that difference is too small to worry about, but I suppose you could consider an observer just outside a black hole (you can't meaningfully compare time inside a black hole) who says that the universe is significantly younger. But they (and we) would know that was using a different frame of reference and so could convert between the measurements and show they were consistent.Separately is there any sense in which we can take a part of the universe (eg me here typing on this keyboard) and give it an age wrt the earliest times that we have knowledge of ?(ie T>^43sec) ?
Can I say that the particular event that I am presently a part of is some 13 billion years "old".?
If I was to pick another location at random ,say somewhere inside a Black Hole would the "age" of that region be more or less the same (13 billion years) even though for an external observer it might seem that time had stopped there after the Black Hole had formed?
The singularity only exists if you use Schwarzschild coordinates (because they only apply to the region outside the black hole). If you use an alternative coordinate system, then the singularity disappears. So you can't use this singularity as an argument that black holes do't exist.
There is a reason for that.Now I will be attacked as fringe.
I have referred the argument to my earlier post in the thread in Black Hole Problems. But the site makes problems to send links as well direct link of original post of forum freshman.
thescienceforum.com/pseudoscience/48000blackholeproblems.html#post617973
Also, you should read more carefully.
Zlatan
It is not a problem with the site. It is the fact you haven't posted a link. You didn't post a link to the arXiv article you posted either.
I assume you link should be: Black Hole Problems as the post you linked to is by some nutcase.
From that link:
This is just nonsense. One can choose different coordinate systems and they are equally valid. You even go on to mention Kruskal–Szekeres, which are exactly equivalent and just as much "the very essence of GR" because they are exactly equivalent to Schwarzschild coordinates. The same is true of, for example, GullstrandPainlevé coordinates. But they have no singularity at the horizon, so maybe they are even more "the essence". Or maybe this "essence" is just bollocks.
Also, the Schwarzschild metric only describes an eternal black hole in an empty universe, so it hardly seems like the "essence of GR". What about all the other matter in the universe.
So do all equivalent coordinate systems.but the Schwarzschild coordinates capture the behavior of the matter and energy!
Yes Schwarzschild coordinates are equally valid, and the coordinates all stem from GR, so you cannot use it to prove that the one is more right than the others. That is an argument why you cannot disregard my or others' frozen stars stance. I did mention in that post some other arguments which you do not mention and which are related.
The all coordinates only agree from outside perspective, or you are claiming that in Kruskal–Szekeres coordinates for instance you do not calculate gravitational time dilation blow up from outside perspective. You simply do not want to see my argumentation. The prediction of central singularity of GR; from outside i.e. our perspective i.e. stationary observers for all coordinates is not falsifiable by that same GR, and event horizon cannot exist. You are familiar with the significance of falsifiability in science.
I have put my argumentation. Throw it with words like nonsense.
As some members of this forum have already said this could go indefinitely. But it will not.
Regards
Zlatan
Didn't we block this guy at some point?
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