If a version of you made a different choice, is that person still you?
A copy far away could have my memories without ever sharing my life. A version in another quantum branch could share my history until our experiences separated. Those are different ways to be another me.
Physics can describe how such copies might arise. Whether they count as the same person is a question about identity.
Another life
| Kind | What it would take to have another me |
|---|---|
| Level I | Space and its starting conditions must allow our arrangement, and variations of it, to recur |
| Level II | Another pocket needs suitable physics and enough variation to produce a copy |
| String landscape | A vacuum alone doesn't guarantee a copy; its physics and history must allow one |
| Braneworld | Another brane doesn't by itself imply another version of us |
| Level III, many worlds | Different allowed quantum histories must lead to different versions of the person |
What "chose differently" means
A version of me who took another path would accumulate other memories, friends, and habits. After enough years, we might have little in common beyond the lives we remember before that point.
In Level I, the copy grew up in its own region of space, on another Earth. Its past could match mine in every detail, but those events happened elsewhere.
In many worlds, the branches can share the same physical history before they separate. The later versions remember that common history from their respective branches.
Some other versions
- A version of me who never studied anything to do with computers, and has never written a line of code. Somewhere, that person does a job I can't picture, and has no idea what a payment reconciliation is. Lucky them.
- A version who grew up in a different city, with a different accent, and different friends.
- A version who plays for Chelsea. Physics doesn't forbid it. The fraction of such versions is another matter.
- A version who supports a different club. Physics doesn't forbid that either. I'd rather it did.
These are possible lives to imagine, subject to the conditions of each theory. The theories don't tell me whether those people would be happier, sadder, or better than me.
Which one is me?
The teleporter
Derek Parfit explored identity and psychological continuity in his 1984 book Reasons and Persons. One of his thought experiments uses a teleporter.
A machine scans every atom of your body on Earth, destroys the original, and builds an exact copy on Mars from new atoms. The person who steps out on Mars has your body, your memories, and your personality. They remember stepping into the machine on Earth. Did you travel to Mars, or did you die, while someone new was born?
Now change one detail. The machine builds the copy on Mars, but doesn't destroy the original. Two people now exist, one on Earth and one on Mars. Both remember being you yesterday. Both are sure they're you.
Parfit's answer
Parfit argued that psychological continuity, the connections between memories, intentions, and personality over time, could matter even when identity doesn't continue. In a case where one person has two equally continuous successors, both can preserve those connections.
That doesn't make two distinct people numerically one person. It separates the question of identity from the connections we value in survival. Caring about what happens to both successors needn't depend on declaring one the original.
Branching in many worlds
Many worlds raises a similar question if branches contain different versions of one earlier person. Each version can remember the common past, even though their later experiences differ.
Parfit's distinction gives me a way to think about those versions without deciding that one has a stronger claim to the shared memories.
David Lewis and counterparts
David Lewis argued that every possible world exists, as real as ours. His 1986 book On the Plurality of Worlds develops that view and his counterpart theory. These worlds describe logical possibilities; their existence is a philosophical claim independent of inflation or quantum physics.
For Lewis, the versions of me in other worlds aren't me. They're my counterparts: people in other worlds who resemble me closely enough. When I say "I could have been a teacher", Lewis reads that as "one of my counterparts is a teacher".
That fits Level I well. My copies in far away regions never shared anything with me, so it feels natural to call them counterparts, not me. It fits many worlds less well, because there the other versions did share my past.
Where that leaves me
I don't have a final answer. I think about the two cases differently:
- If the other versions never shared my past, like in Level I, they're counterparts: very similar people, but other people.
- If they did share my past, like in many worlds, they're as much me as I am. We split, and from that point we're different people with one past.
Do my choices matter?
If another version makes a different choice, does that make my own choice less important?
In Level I
In the Level I picture, far away copies could make different choices in matching circumstances. At the distances Tegmark estimates, light sent now would never reach them if the present accelerated expansion persists. My decision here wouldn't change theirs.
My choice still affects me and the people around me. A copy's different decision doesn't change the consequences here.
In many worlds
In many worlds, outcomes have different weights, calculated from their quantum amplitudes using the Born rule. Those weights depend on the physical processes that produce the outcomes. Counting imagined branches isn't a way to calculate them.
David Deutsch and David Wallace argue that a person expecting to branch should use those weights as probabilities when making decisions. Deutsch develops the argument in Quantum theory of probability and decisions. On that view, we still compare the likely consequences of our actions and choose accordingly.
Having several outcomes doesn't make their probabilities equal. It also doesn't remove the consequences experienced by the people in each branch.
Every Chelsea result
I like imagining a version of Chelsea that wins every match. Even if that history occurs somewhere, I still watch the matches in this one.
Can I meet the other me?
These proposals give us no established way to meet a copy.
| Kind | What prevents a visit in this picture |
|---|---|
| Level I | Copies at Tegmark's estimated distances lie beyond our event horizon if accelerated expansion persists |
| Level II and the landscape | Most pockets are separated by inflating space; possible collisions aren't a route for travel |
| Braneworld | Ordinary matter remains confined to its brane in the models discussed here |
| Level III, many worlds | Decoherence makes interference between macroscopic histories impractical |
Gravity connecting branes in a model doesn't give us the messages or journeys shown in Interstellar. The interaction and the ability to visit are separate questions.
What I think
I'm still unsure what to call a version who shares my past. If those versions exist, they're living their lives, and I'm living mine.
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