[Lewis Thomson (OP)]

Mukul has posed the following question to TNS.

"What if our whole space is just like sheet having two sides? If we say that heavy mass objecs bend the space time fabric around it creating a trough then right if we look at the other side of the fabric we can imagine a hill. Similarily in the side of the fabric we live, there can be an uprising that is just a black area where we cant see anything. What I am trying to suggest is that there can be a parallel universe on the other side of the fabric and the black hole is just a hole in the fabric joining the two parallel universe on the opposite ends of the Fabric?"

What do you think of this theory? Discuss in the comments below...

[Eternal Student]

Hi.

   Firstly, we haven't got any hard evidence for parallel universe(s), just some theories that involve them.  Therefore,  the situation described by Mukul might be correct.   All I can do is talk about some of the theories that exist and discuss some of the differences and similarities to Mukul's idea.

What if our whole space is just like sheet having two sides?

    If there are parallel universes, then there's very little reason why there would be only two of them.   There could be many.   If there were many, then there's very little reason why we have some special relationship with just one of these, so that we are one side of a sheet and they are on the other side of the sheet.  This may reduce the usefullness of imagining a simple sheet which only has two sides.
   
   Example:   The "many worlds" interpretation for Quantum Mechanics.   
    The many-worlds interpretation implies that there are very many universes, perhaps infinitely many.[11] It is one of many multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realised.   

Taken from Wikipedia.   https://en.wikipedia.org/wiki/Many-worlds_interpretation

   
   In the many worlds interpreatation the relationship between the different "worlds" (or universes) may only be that we had a common history until the time and point of branching into two separate "worlds" or universes.   After the branching there's no requirement or expectation that one world would influence or be influenced by the other world.

If we say that heavy mass objecs bend the space time fabric around it creating a trough then right if we look at the other side of the fabric we can imagine a hill. Similarily in the side of the fabric we live, there can be an uprising that is just a black area where we cant see anything.

    This sounds a lot like you are considering General Relativity and considering the common visualisations of spacetime as some sort of fabric.
     Firstly, this entire concept of spacetime as some sort of fabric is intended only as a visualisation of what is happening and it's nothing more than that.   So we have to start by asking what is it that you think a "hill" or outward curvature of spacetime would do or represent in your model?  Presumably the opposite of a trough or inward curvature.   For the usual visualisation of spacetime as a fabric, a trough shows a distortion of spacetime so that an ordinary mass would be attracted to the centre of that trough.   So a hill would presumably be some area or region that repels ordinary matter or opposes conventional gravity.
    We haven't found a region of space where matter is obviously repelled from that point of space.    So we haven't found any obvious and isolated "hills" in the fabric of space.   However space is big and there would always be problems determining if something was repelled from one region or if it had just been attracted to some other region.
** See late editing below ***
    There are some situations where we might assume the existance of something that does work so as to oppose conventional gravity:  I'm mainly thinking of dark energy.  I suppose this could be explained or considered as something similar to your description of a hill formed by something on the other side of a sheet of fabric.  There are some ideas and theories which suggest that dark energy has a particular distribution through space.  However, it would tend to be reasonably uniform through space and not form isolated hills like the sort of thing you were describing.  So if this dark energy is caused by something "on the other side" of the fabric then the other side would seem to have a much more uniform spread of matter than in our universe (that's ok, it's possible, we can think of the other universe as having diffuse clouds of interstellar gas and very few concentrated lumps of matter like planets, for example).
     There are numerous sources of information about dark energy.   One recent thread from this forum includes this one:   "What do we know about dark energy?",  https://www.thenakedscientists.com/forum/index.php?topic=83661.0.


   

What I am trying to suggest is that there can be a parallel universe on the other side of the fabric and the black hole is just a hole in the fabric joining the two parallel universe on the opposite ends of the Fabric?"

    Well that does sound a lot like some existing theories on black holes.   The maximally extended Schwarzschild solution for a black hole allows for the existance of multiple regions of spacetime.   Note that this does NOT prove the existance of these other regions.  It is simply that we can extend the solution in a natural way and then these regions appear.   Just because we can extend the mathematics does not guarantee that there is any physical significance to the solution obtained.
    Anyway, when represented on a Kruskal diagram, we would be located in one region of spacetime (usually labelled and described as region I, which I know is quite a dull name).  What you would normally describe as the interior of the black hole is labelled as region II in the diagram below.   However the roman numeral you assign a region starts to show some variation between different sources of information here, there are 4 regions and no universal agreement on how you should label them. 
    There is also a region IV which is often described as a "white hole"  and, more interestingly a region III  which appears as a region of spacetime similar to the region I in which we are located.
    That region III could be thought of as a parallel universe,  a region of spacetime that is similar to our own.


     Kruskal diagram of a maximally extended Schwarzschild solution.   Taken from Wikipedia,   https://en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coordinates

    In some sense, the black hole would be a point of connection between these two regions, however passing from region I to region III should not be possible.   There are no time-like paths connecting the two regions, which just means that passing from one region to the other would require faster-than-light speed movement across the event horizon of the black hole.  So there are two things we should consider before we allow our imaginations to run wild and think of all those Sci Fi films and books that feature wormholes connecting two parallel universes:
    1.   There is no guarantee that there is a region III.  It is just an extension of some mathematics with no real world justification.
    2.     Even if region III exists in some sense, nothing should be able to pass from region I to region III or vice versa.  Specifically nothing in one region should be able to interact or influence anything in the other region.

  - - - - - - - -
   There are many other theories involving multiple universes.  This post is already too long, there's no need to try and discuss them all and I'm sure I don't know about all of them anyway.

   Let's just go back to Mukul's idea.  If it's right then it predicts the existance of "hills" in the fabric of spacetime.  So we should be trying to find these "hills".   The current research into badly behaved gravity and deviations from current models of gravity would seem to be most easily connected with what a "hill" should do:   So that will be theories about dark matter and especially dark energy.   

Best Wishes.

**Late editing**   Actually there are regions of space that could be considered to repel matter and indeed they are explained equally well as being equivalent to an attraction to other regions.     See Halc's post later.

[Halc]

Firstly, we haven't got any hard evidence for parallel universe(s), just some theories that involve them.

Tegmark enumerated four different 'levels' of multiverse. Only one of them might be described as 'parallel', which is, yes, MWI. For instance, the first level is other visible-universes which are 'elsewhere', not 'parallel'.

Mukal seems to be imagining a sort of literal interpretation of the Newtonian rubber-sheet analogy, which is a model of curved space, but not of spacetime at all.

Firstly, this entire concept of spacetime as some sort of fabric is intended only as a visualisation of what is happening and it's nothing more than that.

Yes. It is a cheap visualization of bent space, but not of bent spacetime since time is not represented.
The idea of a universe on the other side of it seems motivated by fictional descriptions of the 'upside down' in popular, if not scientifically sound, television series.

So a hill would presumably be some area or region that repels ordinary matter or opposes conventional gravity.
    We haven't found a region of space where matter is obviously repelled from that point of space.

We have!
The Dipole Repeller and Cold-spot Repeller are two such 'hills' which repel all matter (galaxies) near them.

This can actually be modeled with the rubber sheet. Given an infinite sheet will balls distributed perfectly evenly over it, none of the balls will be attracted in any particular direction. Take one ball away and all the ones nearby will be repelled from the 'hill' that forms in its absence.

However space is big and there would always be problems determining if something was repelled from one region or if it had just been attracted to some other region.

Same thing, literally.  Gravitational attraction one way can always be spun as an absence of as much attraction in the other direction.
Is wind attracted to the eye of a hurricane, or is it just pushed there from outside?  The push is always there, but the eye is not, and the fierce wind results only from the eye, the absence of pressure.

[Eternal Student]

Hi.

    Thanks Halc.  I've now found a few articles on repellers and attractors in our local bit of the universe.   I didn't know a lot about them and it's been interesting to consider the repellers as if they were repelling matter.

Same thing, literally.  Gravitational attraction one way can always be spun as an absence of as much attraction in the other direction.

     That does seem to be gist of the articles I've just read.    There is always some dense region of space which will then be called an attractor and these can usually be seen and detected by ordinary means.  The galaxies will be attracted to these areas more than they would be to regions that are just voids.  These voids are then called repellers since galaxies do seem to be drifting away from them.
   
    Anyway, I agree that it would make Mukul's hills very hard to identify.   If you were just studying the local movement of objects like galaxies through space then you have no way of knowing if a region of the hypothetical sheet was definitely deflected up like a hill, the effects are exactly the same as just being less down than everywhere else.

Best Wishes.

[Halc]

These voids are then called repellers since galaxies do seem to be drifting away from them.

... accelerating away, in all directions. It isn't random drift. I thought the lines in the picture I posted made that pretty clear, making it look sort of like those you see of magnetic fields and the paths of charged particles.
   

Anyway, I agree that it would make Mukul's hills very hard to identify.   If you were just studying the local movement of objects like galaxies through space then you have no way of knowing if a region of the hypothetical sheet was definitely deflected up like a hill, the effects are exactly the same as just being less down than everywhere else.

But they did identify them, precisely because of the recessive motion seen. They have maps which label the potentials of all these regions (the altitude of the rubber sheet so to speak), with the highest potential at the 'Cold Spot', and the lowest at the Shapley Attractor. Our local attractors (Virgo, which isn't even a blip on that map, and the Great Attractor) have higher potentials than Shapley. We're higher yet, but always going 'down hill' so to speak, picking up speed as we do so, but also losing speed due to the expansion of space, imparting a sort of terminal peculiar velocity as it were.

I suspect that our galaxy is losing speed at the moment, but mostly due to Andromeda throwing its hooks in as it catches up from behind. Once it passes up, we'll pick up speed again.  The combined mass of the two galaxies has a reasonably stable peculiar velocity but I think we'll never reach the Shapley Attractor.

Best Wishes.

[Eternal Student]

... accelerating away, in all directions. It isn't random drift.

   Don't worry Halc, I think I've got it and yes the diagram you presented does the job well.  I used the term "drift" because it's typically British to undetstate things.
   The repeller regions are less depressed (less down in the sheet analogy) than all other surrounding regions.  So it's not "random" drift that is seen:  In the local region around the repeller the net gravitational force points away from the repeller region.   So if you were on the left of the region the net force due to gravity is to the left and hence immediately away from the repeller region.   Similarly, if an object was to the right then net force is to the right.   A repeller location is a position in space that is a local maximum of the gravitational potential.

Thanks again and best wishes.

[Eternal Student]

Hi again.

   Seems quiet, not much else to comment on.

About the "sheet" analogy,  Halc said the following:

It is a cheap visualization of bent space, but not of bent spacetime since time is not represented.

    I would disagree with that.  The sheet with an indent caused by some massive object is used quite often to illustrate GR and curvature in 4-dimensional spacetime.
    Anyway, we both agree that it is just an analogy and that's all.  I guess we won't ever find out what Mukul had in mind because there's no way to ask him/her.

Best Wishes.

[yor_on]

You can only speak of repelling and attracting if you look at it as forces. And then you need something that forces the attraction and then something more that forces a repelling. And that would be? In relativity, and as per Pete, gravity is described as acting and getting acted upon. But I haven't seen it stated what is supposed to 'act'.  One answer might be 'gravitons' but doing so you will then need 'anti gravitions' too, to explain it, wouldn't you?

[wolfekeeper]

We know there are parallel universes other than our own, because we can see their effects in dual slit experiments. What we don't know is what happens to them when they stop interacting with ours.

[Halc]

We know there are parallel universes other than our own, because we can see their effects in dual slit experiments.

Only one quantum interpretation that I can think of suggests the existence of parallel universes. The other interpretations explain the double-slit results just fine without the extra universes. So we don't 'know' this at all since the other interpretations have not been falsified.

Hey, I have all the respect for MWI since it is probably one of the cleanest (simplest) interpretations, but I have issues with its ontology, which is just me. I have issue with the ontology of almost all the interpretations. Almost...

What we don't know is what happens to them when they stop interacting with ours.

We do actually. They evolve according to their wave functions.