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Containment and control would be tricky but probably solved for either of these by the time you can get these in useful quantities. For example, maybe stability is possible at a temperature or pressure we can realistically achieve, or specific forms or methods mean these are less of an issue. After all we dont know much right now except that they exist, but terms like "compact matter" cover many many kinds of physical entity, some more stable or amenable than others perhaps. when we get closer to them. That's about how room temperature superconductor research is progressing, and just when you thought that was an absolute limit, it turns out it wasnt. Even if we dont know an exact solution right now, hand waving the containment issue away or just about manageable or solved by that time, seems fair.

Containment and control would be tricky but probably solved for either of these by the time you can get these in useful quantities. For example, maybe stability is possible at a temperature or pressure we can realistically achieve, or specific forms or methods mean these are less of an issue. After all we dont know much right now except that they exist, but terms like "compact matter" cover many many kinds of physical entity, perhaps some will turn out to be more stable or amenable than others, when we get more able to research them in depth. That's about how room temperature superconductor research is progressing, and just when you thought that was an absolute limit, it turns out it wasnt. Even if we dont know an exact solution right now, hand waving the containment issue away or just about manageable or solved by that time, seems fair.

A comment below asked why these exotic materials and not more familiar dense items such as lead or gold. The trouble is these are rare, hard to find, require a lot of extra processes to extract and to break down compared to the others - and after all that they still aren't nearly dense enough (by a few orders of magnitude!) to solve the problem.

A comment below asked why these exotic materials and not more familiar dense items such as lead or gold. The trouble is these are rare, hard to find, require a lot of extra processes to extract and to break down compared to the others - and after all that they still aren't nearly dense enough (probably by a couple or more orders of magnitude depending on the scenario!) to be useful for solving the problem as described.

A comment below asked why these exotic materials and not more familiar dense items such as lead or gold. There are 4 reasons:

1. Elements with even lowish atomic numbers are quite rare in universal terms (hydrogen (74%) helium (24%), then the ratios drop off very quickly, and moreso for the many elements that don't occur in the common stellar fusion cycles: Source, notice the next common elements are O, C, Ne , Fe, N, So, Mg, S, all fusion products and none are "usefully" dense enough, you might as well store the actual supplies you need);
2. Dense elements also aren't normally readily available. You generally need to find workable ores, then large scale mining and extraction processes to get relatively small amounts. If everything's transmuted the supply need to be plentiful and easily obtained;
3. AFAIK they simply can't be compressed/stored to anything like a sufficient density to be practical for these purposes, compared to basic options like hydrogen and compact matter;
4. They are comparatively complex raw materials compared to hydrogen and compact matter, you have to break them down before you can use them.

A comment below asked why these exotic materials and not more familiar dense items such as lead or gold. The trouble is these are rare, hard to find, require a lot of extra processes to extract and to break down compared to the others - and after all that they still aren't nearly dense enough (by a few orders of magnitude!) to solve the problem.

1. Elements with even lowish atomic numbers are quite rare in universal terms (hydrogen (74%) helium (24%), then the ratios drop off very quickly, and moreso for the many elements that don't occur in the common stellar fusion cycles: Source. Notice the next common elements are O, C, Ne , Fe, N, Si, Mg, S, all fusion products and none are "usefully" dense, you might as well store the actual supplies you need);
2. Dense elements also aren't normally readily available. You generally need to find workable ores, then large scale mining and extraction processes to get relatively small amounts. If everything's transmuted the supply need to be plentiful and easily obtained;
3. AFAIK they simply can't be compressed/stored to anything like a sufficient density to be practical for these purposes, compared to basic options like hydrogen and compact matter;
4. They are comparatively complex raw materials compared to hydrogen and compact matter, you have to break them down before you can use them.