15 is probably not the minimum. There are some ideas floating around for getting down to 14, and some wilder ideas that might get to 13 or 12.
Below that we'd need some significantly different mechanism that nobody has thought of yet. It doesn't seem likely that anyone will be able to prove that universal construction is impossible with a single-digit number of gliders -- but if a solution exists it might take an omniscient being to find it.
... Or maybe some clever hacker will figure it out tomorrow! That's what happened to get us to the current minimum. We were stuck at a minimum of 32 for quite a while, until Daniel Vargas (MathAndCode) suddenly showed up with a new idea.
Below that we'd need some significantly different mechanism that nobody has thought of yet. It doesn't seem likely that anyone will be able to prove that universal construction is impossible with a single-digit number of gliders -- but if a solution exists it might take an omniscient being to find it.
... Or maybe some clever hacker will figure it out tomorrow! That's what happened to get us to the current minimum. We were stuck at a minimum of 32 for quite a while, until Daniel Vargas (MathAndCode) suddenly showed up with a new idea.