Who Are You?

Cosmic Variance
By Sean Carroll
Dec 15, 2009 9:12 PMNov 19, 2019 8:55 PM


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Last week I Twittered/Facebooked some provocative results from a poll of philosophers. In particular, this little tidbit:

Accept or lean toward: survival337 / 931 (36.1%)

Other304 / 931 (32.6%)

Accept or lean toward: death290 / 931 (31.1%)

Yes, that's all the detail presented in the question: "Teletransporter (new matter): survival or death?" As a professional philosopher, you're supposed to be familiar with the issue, which I reconstruct as follows. Imagine that someone has invented a working teleportation device. You step in the box, lights flash and sparks fly, and "you" rematerialize in another box, exactly the same in every way, but constructed out of a completely new collection of atoms. The original version of you is destroyed. Did you die? (And then, what if a million years passed in between the two events?) It would probably be annoying to real philosophers, but I personally put this question in the category of "Not that hard." And I would phrase my answer as: "Who cares?" What we should care about is how well the teleporter actually works -- is the reconstructed person really in exactly the same quantum state as the original one was in? Same memories, feelings, etc? That's an interesting technology question. But there's no interesting question associated with "Did you really die when you were teleported?", or "Are you really the same person after being teleported?" These are just bad questions. They assume a certain way of looking at the world that ceases to be useful once we've invented teleportation. Namely, they assume that there's a certain "essence of you-ness" that is (somehow) associated with your physical body and continues through time. That's a perfectly sensible way of talking in the real world, where we don't have access to duplicator devices or transporter machines. But if we did, that conception would no longer be very useful. There is a person who stepped into the first box, and a person who stepped out of the second box, and obviously they have a lot in common. But to sit down and demand that we decide whether they are "really" the same person is just a waste of time -- there is no such "really." Which isn't to say there aren't interesting questions along these lines, but they are operational questions -- how should I actually act, or what should I actually expect to happen, in these situations? -- rather than arid metaphysical ones. What if you murdered someone, and then teleported -- would the reconstructed person still be guilty of murder? That's not quite the right question, because it still relies on the slippery essence of continuous personhood, but there's a closely related sensible question -- should we treat the reconstructed person as if they had committed murder? And it seems to me that the answer is clearly "yes" -- whatever good reasons we had for treating the pre-teleportation person in a certain way, those reasons should still apply to the post-teleportation person. The issue of duplication seems much thornier to me than the issue of teleportation. If someone made an exact copy of a known murderer, should we treat both the original and the copy as murderers? (I vote "yes.") Fine, but what about the view from the inside? Let's say you have an offer to get paid $100 if you let yourself be copied, with the proviso that after being copied one of the two of you will randomly be chosen for immediate painless execution. Do you take that deal? I think problems like that are legitimately interesting, although to a great extent their mystery relies on the inadequacy of our conceptions of death. Most of us don't want to die, at least not right away. But if we did die, we'd be gone, and wouldn't have any wants or desires any more -- but it's very hard to consistently reason that way. Note that if we replaced "immediate painless execution" with "prolonged torture," it seems like a much more straightforward question. This showed up in our long-ago discussion of the quantum suicide experiment. In the many-worlds interpretation of quantum mechanics, you can make measurements that split the wave function of the universe into distinct branches. In some sense, then, you really do have a duplicator machine -- it's just that the whole universe gets duplicated, not just you. Some folks have tried to argue against this idea by pushing adherents into a logical cul de sac. You shouldn't (to make a long story short) be averse to bargains that leave you dead with large probability, as long as there exist branches of the wave function where you are alive and flourishing -- after all, in the branches where you are dead you don't care any more, right? My point in that earlier post -- a point I somehow managed to completely obscure -- was that these are misleading thought experiments, because very few of us would take seriously the corresponding classical suicide experiment. "Here, I'll flip a coin, and give you $100 if it's heads and shoot you instantly dead if it's tails. Deal?" Very little temptation to take that offer. But the logic is essentially the same -- if you're dead you don't care, right? (For purposes of these thought experiments we always assume you have no friends or loved ones who would miss you; it's just part of the philosophical game, not a comment on your actual social situation.) At some point in thinking about the many-worlds interpretation, issues like this inevitably do come up. That's what David Albert and I talked about a bit on Bloggingheads. There might be a certain measurement that yields result A 10% of the time, and result B 90% of the time. But in the MWI, the measurement splits the universe into two branches, and you end up either in the branch where you saw A or the branch where you saw B. What does it mean to say that you had a "10% chance of measuring A"? You either did or you didn't -- there is no ensemble of millions of you all doing the same experiment. People have made progress on these questions -- here's a talk by David Wallace on his work with David Deutsch in attacking this problem. (Don't ask me why everyone who thinks about these issues is named "David.") I haven't ever looked at this work closely enough to have an informed opinion. All I know is that being able to teleport around would be really cool.

Teletransporter (new matter): survival or death?

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