geordief

@iNow don't know .Everywhere? That would seem to be an answer in the spirit of "where is the centre of the universe ?" (answer : "everywhere") Do we know that black holes have "back holes" ,anyway? I thought we just knew as far as the event horizon and some have suggested all the matter could actually collect there -with there being nothing inside . Would the "back hole" be on the event horizon in that case? Obviously ,I know zilch about this Are you saying that the idea of the universe having an event horizon corresponding to what a black hole might is more or less a non starter ? Or maybe you were replying to someone else?

Is it at all possible that the observable universe is inside a black hole with the event horizon between the observable and the non observable regions?

Is there a limit to how large and how "undense" such an object can become? Less dense than the surrounding region ,for example?

We never got a breakdown**of covid mortality per hourly consumption of forms of social media vs mainstream medias vs party affiliation etc etc. ..by the looks of it. Hope we learned something from the Covid period **as suggested earlier in this thread.

Thanks.Probably a bit too hard for my brain to get around but I will accept it for now Not sure how it affects the point I was trying to make. Does it mean there isn't an unobservable universe beyond the observable universe? I think I found it here in physics stackexchange (by googling) https://physics.stackexchange.com/questions/552306/does-time-require-matter-to-exist#:~:text=In other words%2C GR states,without realistic physical initial conditions.

Don't really know what to think. But I don't think the observable universe is surrounded by a great void as in my understanding it is surrounded by "more of the same" but that we have no idea how much of this "more of the same" there or-or what it is doing. The only reason we don't see this "more of the same" is that it is receding from us faster than the speed of light and so has become invisible to us.

I am confused.Wouldn't it be extrinsic curvature if the boat fell off an imagined edge but intrinsic curvature if it just dropped out of view when it reached the horizon? Or does the light ray with which we see the boat disappear exist in an embedding 3rd dimension making the surface extrinsic after all?

Yes ,you draw a triangle on the flat water and measure the 3 angles. If they add up to 180 it is flat . Less and it is a positive curvature ;more and it is negative (like a saddle) And ,less obviously you can do the same in 3d(perhaps using a 2d surface embedded in the 3d in the same way) Not sure how you would do it in 4d spacetime.

OK .So can subsets of the physical universe (eg the solar system) be described as "local" or "global"? Does "local" only describe the model and not the physical objects it attempts to model? And are we talking about spacetime curvature or the topology of the physical (subsets of the) universe? I think I was talking about just topology in that last post(even if I used terminology that applies normally to spacetime curvature) ie is there such a thing as local topography versus global topography?

very distant= still local ? That wouldn't be evidence of a global topology, would it? The observable universe is still "local" isn't it? Does the maths you are referring to make any physical predictions or is it entirely " theoretical"? ("theoretical" in layman's speak)

How do we know that a global topology exists at all and that we don't just have a patchwork of local topologies?

Are there any physical models that do that with posited versions of an actual universe? Are there any models that picture such a universe "from a bird 's eye view"? Or is that just an illogical question to ask?(the "bird"/observer being part of the universe could not take a "bird's eye view" of itself as a part of the whole)

Can we populate the surface of that 2d model of a sphere with 3d objects of an imaginary universe which is posited to be edgeless and bounded? How would I see in the model that the objects were 3d if they were embedded in the 2d surface?

Is this a space with no time component? A space that is ready to be populated with objects? I think I am more familiar with a space that is created by objects reconfiguring themselves(in an overall expansionist way as per observations) I think that is the orthodox view even if I am not up to speed with it. It still seems difficult for me to imagine the 3d universe existing on the 2d surface of the sphere There doesn't seem to be room for the 3 dimensions. Is it just an analogy? And the universe is not hollowed out ,is it?)unless the "hollow" is somehow the past history-surely not that)

The difficulty I have is of picturing a topology for a universe that has no edge or boundary. It seems others don't have that problem or perhaps they have enough of a mathematical understanding of the possibilities that they don't need to have a mental picture Would I be right to think that a topology doesn't require any particular shape but it just describes the way parts of the whole connect with each other?

How many different kinds of topology are possible for the universe? Would these topologies depend on more than the spacetime curvature? Are they all expanding ,contacting ,steady state? Any other ways to distinguish between different candidate topologies?

Interesting, though that apparently Laplace and Mitchell (1815?) considered the idea of a Black Hole.They must have thought light had some mass,I suppose.

is the topology not a function of spacetime curvature? (perhaps the "curvature" description of the topology gave me the wrong idea) What might cause different topologies to arise?

Did/does Newtonian physics predict something similar?(light returning to sender) Wasn't there someone before Einstein who also predicted that light would be affected by gravity?

It is understood that the 7th out of every ten gave as a reason that number 6 had told them so. It is not known who was the original source of the information.

Not the Court system,I think. The Secretary of State in this case.

"You can't catch me" Nobody's fool.

Not really.I think I have heard this from time to time but can't remember where I did. Not recently ,anyway.

Especially Thanks.(I hope nobody took the use of the word literally ,as in time travel.I just used it as a turn of phrase) Thanks.It doesn't seem that extraordinary to my untrained ears. Would probability equations in QM normally have a time component?(it comes as a surprise to me that they would but I only aspire to a layman status )

I don't "mean" anything.I am asking a question. You seem to be suggesting that the question is trivial and might as well be considered in macro systems. I have no way of knowing if that is right or wrong as I don't have the expertise.