Rules:
*You can teleport into and out of it at will
*It has a couple of plug sockets and can connect to internet from the region you teleported in from
*You can take objects and people with you
*As already stated, it is (3m)^3 (3m*3m*3m). The walls are plain plaster with a light in the middle of the ceiling. The pocket dimension is topologically toroidal, so if there weren’t walls and a ceiling/floor (which you can actually destroy) you would loop if you went more than 3m in any direction. Gravity, then, is artificial and can be altered to anywhere from 0 to 2g from a dial on the wall.
Edit: additional specifications
*You can only teleport out to where you teleported in from.
*Time proceeds at the same rate inside the pocket dimension
*There is an eject button for those inside to get out if something happens to you
I so wish we could experiment with this to see where it actually breaks down
That’s one of the things we are looking for with particle accelerators, like CERN.
Quantum Mechanics is ridiculously accurate, within its domain. However, it doesn’t predict, or allow for General Relativity.
GM is ridiculously accurate, within its domain, but doesn’t allow for quantum mechanics.
Therefore we know both must be wrong (or at least incomplete).
Unfortunately the overlap is when gravitational forces become significant on quantum scales. There’s 4 ways to study this.
We can pack a ridiculous amount of energy into a tiny space, in a controlled manner. This is the best method. We also can’t do it.
We can pack a ridiculous amount of energy into a tiny space, in an uncontrolled, brute force manner. We can then hope to get lucky, or do it enough to beat the odds. This is what particle accelerators like CERN do. We can’t control what hits when accurately, but we can do enough collisions that 1 in a trillion is useful, then sift through the data looking for it.
We can use tricks to ‘stretch’ the quantum realm. This method is limited, but interesting. Gravity wave detectors effectively do this. They can use a laser to create an effect quantum object measured in meters or more.
We can look for places where quantum gravity is dominant, and see what happens. This is what things like the web space telescope are good for. We can look closely at black holes, and neutron stars, and see what they do to space time. Unfortunately, we are also stuck with whatever the universe happens to have done.
In short, the problem is being chipped at. It’s painfully slow, and buried in ever more complex maths, but it’s being done. I would love to see this “solved” in my lifetime. It’s unlikely, but could happen.