I have a theory that can never be tested. I think changes to the past would change everything about the present (if the time traveler goes back far enough.) The reason has to do with sperm, but I’ll get there.
Let’s start with the version of the grandfather paradox explored in Back To The Future. Recall that Marty McFly went to the past and inadvertently interfered with his folks hooking up. He began to fade from a photograph (as the timeline caught up with changes that hadn’t happened “yet?” It’s insane.)
But after Marty’s interference, it is astronomically unlikely that he would EVER be born. Consider: an average man ejaculates hundreds of millions of sperm each time. Which of these fertilizes the egg is essentially random. If his parents had sex at a different time, or in a different position, the result if fertilization occurred would not be Marty.
You might be wondering: why should they have sex at a different time, if he gets them back together? Because he has altered the background circumstances. Their lives might be essentially the same as before his initial interference, but surely enough has changed that it would be very unlikely that they would have sex at exactly the same moment and in exactly the same way as they did before his interference.
I’m quite convinced of this, but it cannot be proven. Unless we could travel in time, we could never test my hypothesis. Perhaps there is some “conservation” principle which swings everything back into the “original” pattern as much as possible. I just don’t think so.
Let’s look at another example. I take a walk every night at midnight. I follow exactly the same route and I don’t intentionally go any faster or slower, or do anything differently. Yet my apple watch gives me differing numbers of steps. Some nights 3805, some nights 4123. (Perhaps the watch itself has a large error margin, but let’s suppose that it does not. It’s quite plausible that my total steps would vary somewhat.)
Why do my steps vary? Some nights I glance at the moon and, looking up, perhaps take somewhat shorter steps because I’m unconsciously nervous about falling. Some nights I jump over cracks for the sake of my mother’s spinal health. All sorts of things might happen. These choices – looking up at the moon, skipping a crack – surely result from neurological events. We probably can never trace them back to their ultimate origin, but it’s plausible that a change earlier the day – a bad social moment, some anger at a friend, my mind being distracted by a news story – would affect my brain in such a way that those exact choices would not be made. I conclude that, if you traveled back in time and assassinated someone, and I saw a news story about it, or saw my neighbors discussing it even if I didn’t know what they were talking about, or if traffic patterns changed and I was irritated on my way home, then my step count that night would change as a result. I would be thinking slightly different thoughts and forget to glance at the moon. I would have eaten dinner a little faster and as a result my brain state would be a little different. Something.
So, if someone actually went back in time and saved Kennedy (Umbrella Academy), I think as a result not a single person born since 1963 would exist today. Everyone would be affected, and as a result, their brain states would be altered. Maybe just a little in many cases. But enough so that the exact moment of ejaculation (even if a conception event occurs at all) would be different, and a different offspring would result.