The problem with things being so small is that we can't ever see them clearly. Or, if we try to see them, the photons themselves corrupt what we're seeing. This is known as the Uncertainty Principle. It introduces randomness, and the best we can do is is derive probability distributions to describe what's happening.
In scientific experiments, we can always look back and see what happened, but there's no way to predict beforehand what will happen. One could ask, "But how does nature decide what actually happens?"
One answer is the Many Worlds Interpretation (MWI). It says that at any given time, ALL possibilities happen, but in different parallel universes. Or in other words, every fraction of a second, a very large number of parallel universes branch off, something like this:
This picture shows just one universe at the beginning, and then after two branching events, a whole bunch of similar but different universes arise. As time progresses forward, we could end up living any of these lines while copies of ourselves follow other lines. We have no idea that other universes exist, and we can never visit them, so to us our timeline looks simple ... like this (just like the one-world model):
Okay, you ready for this? What if you get in a time machine and jump ahead in time? In the middle while you're gone, the universe is going to continue branching out in its own millions of timelines, and when you return, you could end up on any of those timelines ... like this:
At the same time, copies of you will end up in all the other timelines. Isn't this crazy? Remember, it's just a theory.
Also, remember, from your POV, your time travel trip is going to look like this (the simple version -- again, it looks like the one-world model):
What happens when you go back in time? This is when things start getting weird.
In the one-world model, a trip backwards looks like this:
Looks simple enough? There's just one problem with this. In the one-world model, you can't change the past, as the past is already set in stone. For example, you can't kill your own grandpa, else you'll cease to be born, creating a paradox. And if you've never met yourself in the past, you can never meet yourself when you go back in time. It's not just that you would break the universe or anything -- you'd just simply be unable to change what has already occurred, no matter how hard you try, and so it goes. Example: the movie 12 Monkeys, and the book Slaughterhouse-Five.
But in MWI theory, going backwards in time does something interesting. You can see this easily by reversing our earlier forward jump, like so:
Going backwards is easy--just follow the timeline backwards--all the way back to the original timeline. But then as you move forward naturally in time, you're going to be subject to random fluctuations again. You could end up where that red dot is. In fact, the chances of returning back to your original timeline is practically zero.
In short, going back in time creates a new timeline for you. The simplified picture looks like this:
In this picture, everything to the left of the branch represents the history of what happened leading up to that point in time. Then at the point where you arrive from your time travel trip, the two universes diverge. The top branch represents your original timeline, and the bottom branch is your new timeline after you traveled back.
And get this: a copy of your original self also moves along that bottom branch. Since this copy is in a different universe, he can do entirely different things. That is, you could meet yourself, and you could even kill that copy without causing any paradoxes. As the original You still lives his life exactly as you remembered along the upper original branch.
But then what happens if you travel forward in time from the red dot? You can only end up in universes that branch off from that dot. In other words, you can never return to the original timeline. Doing so would break casualty rules. A universe's branch can never ever return to another earlier-created branch.
Perhaps in an attempt to return to that original timeline, you try going backwards in time to an earlier part of that branch. Alas, random fluctuations would ensure that you just create another (third) timeline from that point, and you just make matters worse!
The one exception to this "can't return" rule is the concept of a "tether," which is an entirely made-up sci-fi concept that I don't think is supported by science, but it still sounds feasible. Before you leave your timeline and embark on a trip, you can mark your point so that you can return exactly where you left from. Several shows use this tactic.
Now that you are armed with the basics of Multiple Worlds Time Travel, which is mostly supported by real scientific theories, I'll demonstrate tomorrow how time travel can be explained as presented in Avengers: Endgame.
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