"...with the ability to get here."
Maybe it's the ability to get 'now'.
What lies just ahead and just behind us in time? And beyond those 'layers'? Other existences? Is the veil thin, requiring reasonable levels of energy to traverse?
At our present state of knowledge the energy requirement would be a real barrier. I say “present state”, as an intelligent educated natural philosopher of a couple millennia ago would have denied the practicality of moving a 97,000 ton ship at 30+ knots (Nimitz Class carrier) for the same reason.