That's why we have her put a ribbon around the soul gem while it's in this position. Then we translate via grief both the soul gem and the ribbon. Then Mami can just pull it towards her, and even if we are using k and she's using j, it'll still get pulled towards the right position.
Hn. I'm still somewhat worried that moving something along the
k-axis away from its local origin where it intersects our ability to perceive it in
xyz-space will also move it away from where it intersects Mami's ability to perceive or manipulate it in
j-space, thus causing her to loose her grip on it or something.
So, I suppose the first test would just be to wave things around in our hammerspace, and see if we Mami can locate and grab onto them, and vice versa; if we're using the same dimensional axis this renders a lot of this moot. (Though it does also decrease a bit of defensive utility against multidimensional enemies.)
If we're not using the same dimensional axis, the next test would I guess be to try your, @Arafan's, idea about Mami being able to keep holding onto something (not our soul gem) she's tagged in the
j-axis while we move it in the
k-axis, and then have her try tugging it back into
xyz-space when we drop it. And if that works, then we could be sure our soul gem or anything else is retrievable from either of our hammerspaces by one another, so long as we've tagged it properly.
And that's all if, having had Mami put our soul gem into her four dimensional hammerspace herself, we determine that being moved along a fourth dimensional axis does cut off our soul gem-body Bluetooth. If it doesn't, and we don't get gemmed when our soul gem is moved along a fourth dimensional axis, then all these questions become less relevant for the protect-our-gem-with-hammerspace idea.
Though, of course, learning whether meguca use the same fourth dimensional axis to store things and whether they can access eachothers' hammerspace along that dimension, or if they use different axes, whether they can access items they've previously tagged even if they've been moved along another axis, is still pretty important.
Possibly, see if anyone's (by which I mean Sayaka's, at the moment) teleport powers interact with any extra dimensions we're capable of sensing.
Also, we only really experimented with moving our grief along a single new dimensional axis, to check that we could. We should check if we can also access other dimensional axes in the same way; we should check if Mami can do the same; pursuant to the above, we should check if any of these axes on our lists overlap (even if we can't access all the same axes to use as hammerspace, having even one shared would be good to know about). Also, we should probably give official names to any dimensional axes we discover we can interact with in this manner, that way we can stop using nonspecific pointers like
i, j, k, q that have to be initialized at the start of each conversation like this, because this could get confusing if we're not all using the same dimensional axis to store things.
Also, I think a high priority might be to
make sure we know how Homura's shield storage interacts with our and anyone else's fourth dimensional manipulation (preferably during a private science session where we don't need to worry about infohazards), because if someone can figure out how to get access to Homura's shield's hammerspace, I think we may have a problem. Like, maybe all meguca use their own dimensional axes, and it's fine, or maybe Homura is one of a few that have a specialized dimensional axis with particular properties (i.e. persisting through time loops) to work with so no one else is liable to be able to touch it, or maybe Homura's hammerspace just has the security protocol of "must be accessed from this shield accesspoint". (Do we know if someone else can reach into Homura's shield grab something, like, as in if they're standing right next to her as she accesses it? Or if she can grant them access to do so? Or, perhaps if her shield simply has the security feature known as "must be accessed by Akemi Homura".)
Either way, I suspect there's rather a lot of sensitive things in that shield, and while I suspect it's got at least enough security that the Incubators haven't gotten in to go looking around inside (
or haven't let on that they've found anything? Homura was keeping the Book Of Witches in her shield before she gave it to us, right? I suspect there are other major info security breaches? It's also possible that most timelines, the Incubators don't actually know they should be looking, and by the time they learn, Homura has looped out; it's possible that it would be trivial for them to access the shield and it's only infosec that ensures they don't. Alternatively, magic is bullshit that even the Incubators don't fully understand, so even if they can do stuff on multi dimensional axes, whatever magical security is on the shield keeps them out. Planning for the former is untenable because it would throw all our current actions out the window anyway, and planning for the latter isn't cautious; probably just be careful about infosec and don't assume anything until it's proven, is the only way to productively proceed from there... Er. That was a tangent.), making absolutely sure some sufficiently advanced experimentation won't just let random meguca peruse its contents is probably a good idea.
*Note: I am not an expert in multidimensional things. Or even an apprentice, heh. These are just sort of points I can see maybe being of relevance?