4 ms·

Officially it cannot be called a "geometry" at all if it does not satisfy the other axioms. So it's a "non-Euclidean non-geometry."

• jerf 2y ago
  Fair point.
• lupusreal 2y ago
  What if it satisfies an as-of-yet undiscovered set of axioms?
  • deleted 2y ago
    [deleted]
  • jerf 2y ago
    The math term "geometry" has a certain definition. Things that fit it are geometries, things that don't, aren't. The gee-whiz "what if" sort of question your asking doesn't really mean anything anymore. In about the mid-20th century math finally came to terms with basically anything you can define being a valid subject of investigation. Arguments about whether things like "imaginary numbers" are "real" and therefore worthy of study are largely gone now. (Whether they are "real" for some definition is now a philosophy question.) Since axioms define the system you are building, if you want "Euclidean except plus portals" you just need to write them down and start studying them. There are multiple possible sets that describe different systems; for a trivial example, you can assert in an axiom that area is conserved on passing through a portal, or you can write axioms that don't conserve that, and follow the implications from there. Video games have had both kinds of portals. Do portals need to be straight or can they be curved? What do curved portals do to other curves if they pass through them? Does "passing through them" even mean anything (note Euclidean geometry doesn't have "time" in it)? How crazy can I be with the portals? What happens if I define a portal as having a boundary corresponding to a Cantor set [1] and pass a line through it in an axiom system that has "time"? There is no one answer to that question, it depends on the axioms you write down, which may or may not even permit such portals. Mathematicians will accept any and all of these systems if you write them down. Some may prove to be "uninteresting". Some may prove to have contradictions in them. But it's been a long time since a mathematician would even consider berating you about any of those choices not being "realistic" or something. And I expect it is very likely some topologist somewhere could hear all these idle musings of mine and say "Oh, yes, you want to go to $TOPOLOGY_SUBDISCIPLINE for that." I don't know what the subdiscipline is or I'd name it, but I'm sure there's something already out there for all this. Since the mathematicians stopped worrying about "realness" they've really spread their wings as as discipline. [1]: https://en.wikipedia.org/wiki/Cantor_set https://en.wikipedia.org/wiki/Cantor_set
    • drdeca 2y ago
      Rather than including “portal” as a kind of thing in your space, I would think to just say, “around each point, there is an open ball such that the metric (not the metric tensor, just the d(x,y) thing) is the same as a ball in Euclidean space”. Oh, and then I guess you could add requirements that for any continuous path along with a, continuous choice of an orthonormal basis of the tangent space, along that path, that uh, transporting small volumes along it, not change the volume?
  • j2kun 2y ago
    It can be given an as-of-yet unchosen new name :)
  • mrighele 2y ago
    Axioms are not discovered but decided. A nice set of axioms gives birth to interesting anduseful interactions which you are the one that you discover, although in practice there's is a bit of back-and-forth, as you decides the axiom based on what you are trying to achieve and they get changed or tuned as time pass. Bonus point if the new axioms play well with existing ones, and are useful to work on our reality.