A sci-fi ponderable.
Let’s say easy personal time travel exists. Let’s further say that you can go back in time and change history however you like, then come back to a changed present (but still be the original “you”). At what point do you, or the Time Cops, or whoever, decide that tinkering is not to be allowed?
As an example: let’s say you could go back in time a few days and stop the Parkland school shooting. Would you do it without ethical qualms? Pretty sure most people would say some form of “yes,” because from the point of view of *right* *now* as I type this, the only changes in the timeline would be positive ones. But let’s say you were given the option of going back in time and popping a cap in Hitler or Lenin or Stalin or Mao or Castro or Proxmire or LBJ before they rose to power and caused a ruckus. Would you do it, even if you could see the rough outline of the new timeline and everything was awesome? This seems like it would be trickier. Because if you did, you would be committing a kind of genocide on a scale never imagined in human history.
Let’s say you whack Hitler just as he’s taking over the Nazi party, say, 1925. Let’s further suggest that the whackage is of such entertainingness that the rest of the Nazis get whacked with him. Huzzah! No Nazis, no Holocaust, no WWII. Assume that, somehow, this leads to an era of peace and prosperity never before imagined; the Soviets give up that Socialism nonsense and become free market capitalists, the Japanese skip past the Rape of Nanking and go straight to tentacle porn; FDR not only never gets elected and thus doesn’t turn a depression into the decades-long Great Depression, but instead goes down in flames such that he drags the Democrat party down with him and from then on US politics is split between the Republicans and the Libertarians. Huzzah! Everything is awesome! Star Trek goes for eleven seasons, Reagan wins three terms, Orions to Pluto by 1990. Huzzah indeed!
But here’s the thing. Unless you buy into nonsense like fate, destiny, predestination, the universe having some sort of plan… virtually *nobody* who was born much after 1926 or so in the “prime” timeline is actually born. By changing politics in Weimar Germany, you’ve set in motion a cascade of changes that lead to a “prime” mother and father not meeting, or meeting but not doing the deed on the specified date, or doing it thirty seconds later, or sperm #1,452,355,343 rather than #1,452,355,342 being the one that succeeds at the egg. And when that kid isn’t born in 1927, that kid can’t have the Prime offspring in 1952, who can’t make a kid in 1983, who cant reproduce in 2018. You will have eliminated from the timeline something along the lines of ten billion people. Granted, you will have created ten billion *other* people, but for the most part judicial systems are underwhelmed with the argument, “yes, you honor, I murdered my infant…but hey, I got right on to making a replacement, so… we’re cool, right?”
So: how does whoever has the power over time travel decide what changes can be made? “No changes allowed” is the easy answer.