I think parallel universes exist, and I hope they do.
In 2014, I watched You Are the Apple of My Eye and heard a character ask whether, in another universe, two people might be together. I wondered whether physics had anything to say about that possibility. Twelve years later, I still want to know whether another version of a life can actually be happening somewhere.
Why I believe they exist
Level I is the idea I find easiest to accept. The other Earth would be made of ordinary matter, following the same laws as this one. What I have to accept is that space extends far enough, with conditions that allow arrangements like ours to repeat. That feels like a smaller step to me than adding new particles or extra dimensions.
I can imagine being wrong about the size of space. I can also see why the conditions for repetition matter. An enormous universe alone doesn't promise another me. What convinces me is the possibility that familiar physics, applied over enough space with the right conditions, allows something that sounds so unlikely from here.
With many worlds, the appeal is applying the same quantum evolution to the particle, the measuring device, and the person reading the result. The observer belongs inside the description too. I find that more convincing than putting a separate collapse rule at some point in the measurement. I want to understand what follows when the same rule applies all the way through.
The probability problem is still the part I can't resolve. If the allowed outcomes all persist, I want an explanation of why I should expect one more often than another. The interpretation needs to account for the probabilities people actually measure. I take that question seriously because it is part of the argument I find convincing, and I don't have an answer to it.
I don't find string landscapes and other branes as convincing as those two ideas. They ask for more assumptions about physics we haven't measured. I can picture the bulk from Interstellar, but picturing it doesn't give me the same reason to believe in it. Level I and many worlds have a stronger connection, for me, to physics that already explains things we can measure.
Who would the other me be?
I can imagine a version who never studied anything to do with computers. He would have a job I can't picture, and years of knowledge about something I might know nothing about. My work on payment systems could sound as unfamiliar to him as his work would to me. We might have trouble explaining an ordinary working day to each other.
A version who grew up in another city could have a different accent, different friends, and memories of places I have never visited. People who are strangers to me could be the people he knows best. There would be a whole history behind that life, including things he would consider too ordinary to mention.
Would I still call that person me? If he grew up on another Earth, I would think of him as another person. Even if his memories matched mine for a while, the events that produced them happened elsewhere. He would remember his own childhood. I would remember mine. The resemblance could be exact without either of us having lived the other's life.
I feel differently about a version in a quantum branch that shared my past. In that case, the earlier history belongs to both versions. I don't see why only the person in this branch should have the right to call that history his own. We would become different people as our experiences separated, each remembering the life before that point.
I'm still unsure what word fits that relationship. I can call the distant person a copy, but that feels less adequate for someone who shared my past. I think both versions would have an equal claim to those earlier memories. From then on, each would have to live with what happened in his own life.
My choices here
The possibility of another version making a different choice doesn't make my own decision disappear. My choice still affects me and the people around me. I experience what follows from it here, and the people involved have to live with those consequences too.
I work on payment systems, where a transfer must happen exactly once. Even if other branches existed, the people using a system would still need their balances to be correct. Another version of me writing correct code elsewhere wouldn't fix a bug in the code here. The work in front of me would still need to be done.
I think about personal choices in the same way. If another version took a different path, he would have to deal with what followed from his decision. I would have to deal with mine. The existence of both lives wouldn't make the experiences interchangeable. A good outcome for him wouldn't change what the people in this life experienced.
What I hope
I hope some of those other lives are being lived. The version who never learned to code, or grew up in a different city, would have his own memories of how he got there. He might be as unable to picture my life as I am to picture his. To him, his days would be the ones happening, and I would be the hypothetical other person.
I also like imagining the version of Chelsea that wins every match. Somewhere, perhaps, a version of me gets to watch that season. I would still be watching the matches here, with no benefit from his results, but I hope he enjoys it haha.
The question from that movie still interests me because of the lives it allows me to imagine. There could be people with a history like mine whose lives went in another direction, with friends and memories I will never share. I hope another version of me is living one of those lives somewhere.
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