The unity tutorial is just to get up and running as fast as possible in the simplest way possible, not to make the perfect decisions for a networked physics based game.
I however do think they should provide an actual reference to prove the claim,
If that is the case I would really like to see some internals of how the physics engine has been implemented in their pattern. I'm not asking for the rapier port source code, but it is hard to think in terms of a new advertised programming paradgim when there's no working examples.
For example I am very curious to see how a constraint solve is implemented in a SQL-like fashion. You would need various math operations and an efficient matrix representation off the top of my head, and I can't think of how that maps to a SQL-like interface.
So my understanding is rapier is just compiled into the wasm and run like normal while the results are stored into the table for clients to receive via subscription. I won't say it's a perfect solution, but efforts are being made by the small community to see how we can really push the db.
Note: while it's all very procedure like you can just store data normally in the rust side, you just have to push the information to the table for it to be query-able.
"What engine does BitCraft use? The BitCraft client is developed using the Unity game engine. For the server, we have developed a sophisticated distributed system called SpacetimeDB, which hosts the entirety of BitCraft’s server architecture. "
Correct me if wrong, but this suggests the physics engine is Unity's, and not something implemented in SpacetimeDB.
I guess making the claim that is was more complicated might have not been right for me to do, but it would be hard to believe that they did the degenerate case of O(n^2).
Speculation: I have only written a physics engine once during college (so please excuse my ignorance), but I think for a basic mmo a simple chunk and AABB approach would work. It would be easy to query for the surrounding chunks and just run the collision on that subset. I know one of the team members was working on porting a minecraft 1.7.3 server at one point, but I don't know if they got up to the point of moving collision off of the server and into the database.
Speculation as well: Looking at the trailer for Bitcraft, it looks like they have articulated characters and cloth physics. This raises more questions for me.
Game physics involves a lot of things (solvers, collision detection, numerical stability, etc.). I am skeptical of their claims of being able to run physics in what I understand are stored procedures for their database.
I looked at their docs for physics https://spacetimedb.com/docs/unity/part-4 where they demonstrate the simplest form of collision check (sphere overlap). I fail to see how that is an improvement or speedup over existing methods. Some quotes: "This may not be the most efficient way to do collision checking (building a quad tree or doing spatial hashing might be better), but SpacetimeDB is very fast so for this number of entities it'll be a breeze for SpacetimeDB." >> Nothing is quantified with numbers.
"For every circle, we look at all other entities." >> This is the most inefficient N^2 way you could do collision detection.
And not to mention networked physics is a whole additional layer of complexity, where you have to use some form of prediction techniques and very likely end up changing your core physics code to accommodate this.
All of this suggests to me much thought has not been put into the claim "you can also do physics with it!" and its implications. Perhaps it is enough for extremely simple physics, as demonstrated with their demo game. If the author is reading this, I suggest spending some time understanding what this claim implies and qualifying it better.
However I must mention that I applaud their courage to try something so outlandish. If you truly believe your claims are possible, I encourage you to keep working on it.
But I'll be convinced when extraordinary evidence backs extraordinary claims.
Source: I work on a commercial game physics engine and related netcode.