If the people of Flatland didn't know they lived in a flat land, would we know if our Universe is flat?

We wouldn’t. I always thought that was the point of the book.

But why would our universe specifically be flat as a result of the multiverse? I only watched the first minute or so of the video – do they get into this later on? At any rate, I don’t think the concept of “flat” is applicable in the real world. Everything has dimension. Points and lines are useful ideas, but not physical objects.

We percieve three dimensions plus time. I had to read “Flatland” a couple times because I wasn’t ready for it the first time. The entities in the book see only one dimension so it is normal for them to see that individuals change when you move around them.

In our universe people change, as we walk around them. Their face disappears and the back of their head appears and we see that as normal. It is possible that there is another dimension where individuals are the same, no matter the position from which they are observed.

The point is that the flatlanders thought everything is normal, as do we. If there is an entity that sees in 4, 5, or 6 dimensions, it will be “normal”.

I love that book! And, as @wildpotato already said, I think the point of the book was to make us ponder that very thing…that just as the citizens of Flatland could not fathom a true third dimension, so would we be equally skeptical.

Flatlanders observe 2 spatial dimensions plus time. Their universe is a plane, and they are utterly incapable of visualizing anything in 3 dimensions as humans do. The lesson is that we humans are just as incapable of visualizing 4 dimensions, even though mathematicians describe the properties of higher-dimensional geometry in great detail using equations.

Clever flatlanders can still look for evidence of higher dimensions. For instance, they could precisely measure angles of a gigantic triangle to see if the sum deviates from 180 degrees, from which they would infer their universal plane has a slight curvature into the 3rd dimension even though they can’t visualize it. We humans, however, looking down on Flatland, plainly see that they live on a vast sphere – or doghnut, or saddle, or whatever 3-dimensional geometry it is. Locally, however, it still always appears flat to those embedded in it.

I’m not sure how the multiverse concept ties into this. Presumably Flatlanders might also wonder if they live in a 2-dimensional multiverse, but I think that’s a separate issue.

Clever flatlanders can still look for evidence of higher dimensions. For instance, they could precisely measure angles of a gigantic triangle to see if the sum deviates from 180 degrees, from which they would infer their universal plane has a slight curvature into the 3rd dimension even though they can’t visualize it. We humans, however, looking down on Flatland, plainly see that they live on a vast sphere – or doghnut, or saddle, or whatever 3-dimensional geometry it is. Locally, however, it still always appears flat to those embedded in it. I do not want to fight over or belabor the point but this clearly illustrates what I tell many (but they steadfastly fail to see) is the difference between the here and now (secular world) and the spiritual world is; it exist and there are clues and evidence we just cannot see above or outside of our own Flatland (secularness) to see the spiritual realm that can look down on us.

@wildpotato Yes, the video does get into possible geometries of multiverses. If you love cosmology as I do, it’s a long video but well worth watching through when you have the time.

@Ron_C Exactly. And if the brane theory(M-theory) turns out to be true, it may be that no beings experience all the dimensions there are because some are too tightly wound into one another to be perceived at anything above the quantum level.

@yankeetooter That was definitely the point of the book. But back when it was written, it was highly speculative science fiction. As cosmologists and quantum physicists are now closing in on answers, it appears it may have been an incredibly prescient work.

@gasman They might eventually wonder more. We see three spatial dimensions and time. Visualizing 4 or more dimensions in our mind’s eye eludes us, even though we can use equations to describe such space. But string theory works mathematically with 11 or 13 dimensions, so we are now looking for those. We have active experiments underway that will establish whether they exist or not. Surely advanced Flatlanders could do the same.

@Hypocrisy_Central Prove it.

Living in a physical universe, many find it difficult to conceive of a spiritual dimension that some recognize as both transcending physicality and being an inseparable part of it.

Seems like it depends on what you mean by the “know” part of ‘know we live in a flat land”.
No, if you mean with certainty and reproducible results involving the ‘extra’ dimension(s).
Yes/Maybe if it includes inexplicable phenomena.
E.g. it’s possible that ‘psychic’ phemonenon, or prescient impressions have extra-dimensional underpinnings but there does not seem to be any way to demonstrate that and a good scientist has to dismiss them as unexplainable.

@Bill1939 It seems that until the age of reason, a rather recent development in human existence, just about all of humanity found it impossible to not “conceive of a spiritual dimension that some recognize as both transcending physicality and being an inseparable part of it”.

@dabbler I have read that there are currently experiments underway that just might yield some compelling proof of a multiverse. Unfortunately, they are not yet the sort of experimental design which, should they fail to yield proof of any multiverse theory, would also rule out the possibility of such a cosmology.

@ETpro Those higher-dimensional string theories (11 or 26 however many dimensions) are connected to physical reality by assuming that most of the dimensions are compactified, i.e., rolled up so tightly they aren’t apparent, like the thickness of a 2-dimensional sheet of paper, or the thickness of a 1-dimensional string.

Trouble is, compactification at the Planck scale isn’t amenable to any experiment we can presently conceive of.

@gasman I do appreciate the difficulties involved in proving or disproving string theory, multiverses and the like.