People did not really believe the world was flat hundreds of years ago.

Similarly the “new” world was in regular contact with the old for the last several thousand years. It’s only 53 miles from North America to Asia, even less if you include the two islands between them.

PS: We have a lot of ideas about how wrong people used to be. But, I suspect most people never really formed an option on most of this stuff because they never really considered the question.

If they were in regular contact, why did smallpox have such a devastating effect on Americans?

Population density. Smallpox dies out with lot’s of tiny isolated communities with minimal contact to each other.

Regular contact here means "there were people who could have made the journey between Asia and Americas, yet we don't have any proof and its effect on both societies is pretty much non-existent"

Specific people no, but genetics shows connections beyond culture.

"Similarly the “new” world was in regular contact with the old for the last several thousand years. It’s only 53 miles from North America to Asia, even less if you include the two islands between them."

That's an interesting idea. Do you have a reference or materials that talks about this?

> I, for example, if I lived a couple of hundred years back, would have probably believed the world is flat, since, well, it's the easiest oppinion to have by just looking around.

It's really not, which is why people have generally known that the Earth is round for thousands of years, and even had calculated it's approximate size before Christianity.

I know that there is pro-Columbus mythology that everyone thought he was insane because of a popular belief that the Earth was flat rather than the truth that everyone thought he was crazy because they knew how far away the East Indies actually were in the direction he was proposing to sail, but...

Anyone with even the slightest maritime background will immediately know the earth is round -- you can't see all the way across the atlantic (though this could be written off as being due to fog, etc), and more importantly, ships in the distance sink into the horizon until they are no longer visible.

Sure, but let's think about it. The average person didn't receive much schooling, and if they did it would have probably been on more practical matters.

It's quite possible that the average person could very likely never have even been exposed to the question of whether the earth is flat or not.

The solution is to remember how certain you are of any "facts", and speak and act appropriately.

If I've personally dug into X and have formed a very high level of certainty that X is true, then I will say "X is a fact" to my friends.

If I've merely read or heard assertions of X, from a source I somewhat trust, then I will tell my friends "Bob told me X" / "I remember reading X" / "I have the impression that X" / etc.

The vast majority of facts (e.g. did George Washington have a set of wooden teeth, or ivory teeth, or what?) are irrelevant to any decisions I or my friends might make. I will be fine leaving my level of certainty in the "I think I vaguely remember reading X" category indefinitely.

If X is a crucial fact impinging on some important decision that I have some part in, then I will prefer to raise my certainty about X to the "X is a fact" level, or at least to "several trustworthy sources have told me X". If, for some reason, this is impractical and I have to make a decision right now, then I guess I'll make the decision from the information I have (which may well be a cautious decision based on a 25% chance X is wrong), and I'll probably feel uncomfortable about it.

> you need a great deal of insight to reach some form of truth

And even then, it's often a matter of perspective where both people's experiences tell them that their truth is the correct truth, while still holding opposing truths from one another.

That doesn’t hold even without the equivocation on the meaning of “flat”. The universe may have a flat average curvature but in the neighborhood of a black hole it is very much not flat (both usages if “flat” to be taken in the differential geometry sense).

> differential geometry

Flat in the sense that at the largest scales the universe is extremely well-approximated by a dust that solves the Friedmann equations given an expanding Robertson-Walker background whose parameter k = 0.

If we zoom in on the “dust” we see that it is grainy/clumpy, and can resolve the grains/clumps into distant galaxies and clusters, and can in particular spot spiral structures that maintain a consistent set of shapes at various distances (or, if you like, at different solid-angle sizes, redshifts, and brightnesses of all of the gas clouds, fast pulsars, and supernovae in them). If we set k to the other allowed values, spiral galaxies at higher redshifts would distort compared to nearby spirals.

Within the clumps — clusters and galaxies themselves — the matter does not resemble a perfect fluid dust, and the background’s permitted trajectories are not those of Robertson-Walker with any value of k. The Friedmann equations thus are no help. To deal with that, we can use a Swiss Cheese model, in which we treat these large “clumps” as holes in the expanding Robertson-Walker cheese, and as necessary use Israel junctions to trace trajectories out of these holes, which are well-modelled by collapsing matter and typically an LTB metric. We can do this for select clumps, while leaving the rest as “grains” of Friedmann-solving dust.

However, unless you flush the Copernican principle (and have good evidence that it’s wise to do so), then we inhabit a nice flat universe in the sense above at length scales above Megaparsecs.

The flatness is technical and spatial. Galaxy clusters’ motion through the strongly curved spacetime gives us orientability: we can say “future” is the direction in which galaxy clusters develop greater mutual RADAR distances. But a pair of typical spiral galaxies at arbitrary RADAR distances will look like typical spiral galaxies to one another for many billions of years into the past or into the future.

Technically the flatness only applies where the expanding “cheese” metric in a Swiss Cheese model applies, and that’s far from large masses of collapsing matter.

Inside the “holes” one might find other holes and yet other holes and so on all the way down to stars and planets and people, all the non-relativistic examples of which look roughly like Schwarzschild from a great distance. In particular, the interaction between the generators of the “in-the-hole” metric and the “in-the-cheese” metric are amenable to perturbation theory in that the deviations from the respective metrics are pretty much vanishingly small. The expansion doesn’t stop galaxy clusters from collapsing inwards, and the galaxy clusters don’t stop the expansion.

Of course, if the hierarchical analytic approach turns you off so hard that you can’t bring yourself to appreciate a Swiss Cheese model as an effective description of the observed universe, you can always dive into non-perturbative inhomogeneous cosmology. In such models backreaction is the effect of inhomogeneities of matter and geometry (“… in the neighborhood of a black hole …” as you put it) on the average evolution of the cosmos. It’s been trendy for years to argue about whether backreaction is relevant (in the Wilson sense) or not, and if it is, whether it’s the traceless part or the trace of the effective stress-energy tensor that matters (pardon the pun) most. However, (a) you won’t rack up many real nerd points in HN comments sniping down comments like your parent’s because, for example, “it should be totally obvious to you that the universe is NOT flat R-W spacetime because there are procedural problems with contracting Ricci curvature functionals into a dust!”, and (b) someone who knows better, and there is allllways one of those, will just nuke you from a higher orbit.)