In a cave known on some planets as "Altamira", where there are prehistoric drawings on the walls across many worlds, there is an event.
It'd be sort of hard to miss. There's a lightshow, some harmless radiation spikes, a resonant high-pitched noise that lasts for several hours and is really annoying for miles around, basically it was thoroughly obvious. The interesting thing was that the cave then proved to exist in several universes as a single cave where previously it was one cave to a customer.
We will elide here the frantic linguistic nerdery, the protocols necessary to ensure that no one brought a flu home, the physics experiments for determining how things split back into their own universes (you can go to someone else's, provided their world is currently connected and you leave through their entrance), the security arrangements each world undertook at the aperture, and the installation of conference furniture and a water cooler and everybody's respective Internet access. Instead we will open on the Summit: contingents of diplomats and whatever auxiliary personnel each world found meet, assembling in the cave. (Please don't touch the paintings, some cultures care a lot about those.)
Things which are common knowledge to all delegations:
- The translation protocols are much better at translating things which are common across the cultures present. Certain specific vocabulary used to express niche concepts may not come through clearly – delegations may request translation checks.
- Units default to IRL Earth units with the translation protocols unless you specify that you are referring to "our" units, like "I am 22 of our years old, which would be 88 (standard) years."
- It is known that there are more than four worlds that the cave connects to – it's just that it lets in only four at a time, since it only has four entrances.
- Cables and infrastructure installed by one world intended to transport things (whether physical or energetic e.g. light in fiber optic cables) become disconnected and stop working if the cave does not connect to their world, but which seamlessly reactivate once the cave does connect. Infrastructure installed in the cave which are not intended to transport things work regardless of the installing-world's connection status.
- The cave shifts which worlds are connected to it at an irregular schedule – the gaps between each shift lasting hours to days. It is possible for the same world to remain connected during a shift. It is possible for one world to be refused connection for several shifts in a row. There is no discernable pattern. Before a shift, the cave will glow and make noise like earlier, but only internally and at a lower intensity. Delegations will have about an hour to an hour and a half to leave. There is no physical danger over being present during a shift, but there is the risk that the delegation's homeworld would become disconnected over the shift, leaving them stuck in the cave (or being forced to go to another world) until their world is connected again.
- Translated recordings and transcripts of meeting proceedings (but only recording speech which is said to the whole group, not private or one-on-one conversations) are stored and are sent to all delegations, including those which were not present during that meeting.