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How does one get from point A to point B?
It's a simple question that gets asked often in our daily lives, but it is also the question that led to many important discoveries in physics. We'll never get Newton's famous laws of motion if Isaac Newton hadn't contemplated that question. Einstein's theory of relativity is another attempt at answering the question with better precision by expanding the question to include points in time, not just in space. But even with Albert Einstein's refinement, physics still doesn't have a complete answer to that question.
Because classical physics only considers a single path, usually the optimal one, between points A and B.
Enter quantum mechanics which allows very very very small things to follow more than one path from point A to point B in time. This is exhibited through the double slit experiment where a very very very small thing is sent to a barrier with two slits, which are separated by some distance, and somehow enters both slits without it being split into two. This inference was made from results which showed that the very very very small thing seems to interfere with itself after it has passed through the slits.
This strange phenomena is called quantum superposition.
Let's suppose quantum superposition applies to a normal sized thing, say a woman, who is taller than the average height of the white female population in the US, who has ridiculously perfect hair and ridiculously round eyes and ridiculously sweet smile despite the maniacal intent behind it.
Oh, and this woman's name is Root.
Point B is the moment a bullet lethally pierced through her.
Point A is…
Point A could be their first meeting in a hotel room, where Shaw, despite having just escaped a team of killing machines, is trapped by a seemingly defenseless Root who was fearful and anxious at first glance.
Or it could be the ten hours they spent together in a CIA safe house where they did none of the things that Root frequently suggested they did to make Harold uncomfortable.
Or it could be before they were even acquainted with each other. When Root found Shaw's file buried beneath firewall upon firewall in the Pentagon.
Or it could be further back when neither of them was even born yet.
Point A could be minutes before the bullet that killed Root flew out of the gun which housed it, when Shaw foolishly told her to leave.
Where(When) point A was isn't as important as point B. Because when(where)ever it was, there would always be an infinite number of paths between it and point B, one of which might just lead to a point B where(when) Root survives.
It has been 23,782.47 miles and six months since Root took her last breath. One minute away from when the average grief period should last.
But Shaw still catches herself thinking that her day will at any moment be ruined by the lamest flirtatious line ever uttered by any person in Root's voice.
Which still happens, sure, but it's not really Root, is it?
It's the Machine.
The interesting thing about the double slit experiment is that when you observe one of the slits, you would either find the very very very small thing at the slit you are observing, or it goes through the slit you've left unobserved. This time, the very very very small thing doesn't go through both slits. It only goes through one of them.
It is as if the very very very small thing knows that it is being observed and suddenly becomes very shy about displaying its strange behaviour. However, in doing so, it reveals another one of its quirks, that is;
The very very very small thing's present determines its past.
This quirk is more clearly exhibited by delaying the observation until just before the very very very small thing hits the screen that shows whether or not it has interfered with itself. Even when we don't insist on observing the position of the very very very small thing at the slits and instead observe it much later, the result is the same. The very very very small thing goes through only one of the slits.
Which means the very very very small thing's decision to go through which slit is made long after it has passed through the slit.
Shaw taps her earpiece.
"What kind of present would need to occur if I want a past in which Root survives that bullet?"
"I see you read the book I recommended," the Machine murmurs (with Root's voice) into her ear.
"Don't get too excited," Shaw says. "I just skimmed it."
"What an interesting paragraph to have landed on," the Machine replies.
Sometimes, Shaw pretends it's Root.
Sometimes, she actually believes it's Root. But then, sometimes she has a hard time believing that the barista who works at her coffee shop is real. So thinking that the voice in her ear comes from a Root who is flesh and blood instead of wires and plastic casings is not entirely unexpected.
"So do you have an answer to my question?" Shaw asks. "Otherwise, I'm going to bed."
"The ability to determine one's past isn't the same as being able to change it to our liking."
"Good night," Shaw says and shuts off her phone.
Without an observer, the very very very small particle is free to travel through all the possible paths it can take. It is only when it is witnessed that it is forced to take only one of those paths.
"Just think of it this way, sweetie," Root's voice blares through the radio. "The fact that you can't see me means that I'm able to live in multiple versions of events following my apparent demise, one of which I am alive and we're standing on some altar promising each other forever in front of our friends."
Shaw sniggers. "I guarantee you, a universe like that doesn't exist."