(Again, as with anything that gets infodumpy like this, I hope the final draft looks considerably different…)

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“The upgrades,” John said. “Freedom from old age, freedom from disease, resilience against trauma… that’s the baseline everyone gets, though some people go for more.  You’ll need that too, of course, because, well, you’ll be seeing a lot of disease and adverse circumstances, and we can’t go around losing you to smallpox or exposure to cold or anything.”

I felt my aversion to adventure rising.  Smallpox?

Ralph’s objection was different, though.  “Not,” he said, “Not that I have anything against the upgrades, but, but, if anyone can get an upgrade like that, why does it have to be me?”   Or not?

“The Great Firewall of Time,” John said. “Our infinite civilization on one side, and the old one with all its limits on the other.”

“Right, and?”

“Well, it’s there to protect everyone.  If only a fraction of 3↑↑↑3 people were interested in seeing, oh, the life of Christ, the old universe’d get so full you couldn’t move.  So nothing from this side can cross back. There are no special excuses, because over a long enough timeline, the number of people with the same excuse and worse would still be far too many.  It has to be a hard line.”

“3↑↑↑3?”

“Three to the third, to the third,” said John, pointing out the numbers in the air, “and keep on raising it to the third about seven or eight trillion times.  The eventual population is considerably larger than this. This end of time is considerably more durable, you see.”

“Anyway,” he went on, “the point is only people from your side of the wall can pass over it both ways.  And while there are a few other time travelers that could be upgraded to the task, none of them are particularly interested in stealing the thunder from your accomplishments.”