Teleporting with paradox

You step into a booth of some sort. A machine scans your entire body and radioes your blueprint to Mars, where an identical body—a molecular duplicate—is created. This body immediately “comes to life” on Mars. Your body on Earth is meanwhile destroyed.

The person who comes to life on Mars looks and acts exactly like you. He (or she) has all of your beliefs, memories and character traits. He says that the last thing he remembers is stepping into a booth on Earth.

Is this person you? Have you just “materialized” on Mars?

Some people think not. Obviously, you died when your body was destroyed on Earth, they say! The person on Mars is just a physical and psychological duplicate who merely thinks that he is you.

Others disagree. They contend that the person on Mars really is you. This is a new way of travelling, they say. Anyone can get from Earth to Mars in a few minutes if they travel in this way!

This way of “travelling”—teleportation—is known to generate many paradoxes. Some of them are as baffling as the well-known ones of time travel. But there is one paradox in particular that bewilders me enough and it is a pretty basic one too. This paradox is suggested by some passing remarks of Derek Parfit in Reasons and Persons (1984), although Parfit does not mention the paradox explicitly and may not have noticed its existence. I don’t know of any discussion of it.

Imagine yourself on the cusp of entering the booth for the first time, wondering if you should really enter. Your main concern is whether you would find yourself “waking up” on Mars (in your duplicate body) at the end of the procedure. The alternative, given that your body on Earth will be destroyed, would be losing consciousness forever. So everything is at stake. Short of trying the booth out for yourself, is there some way of finding out whether you would really wake up on Mars?

It would be no use checking with the people who “come to life” at the Mars end because they would always awaken with the “memory” of having entered the booth on Earth. This would be so even if the booth actually killed anyone who entered it and replaced them with a freshly-minted person on Mars. This freshly-minted person would simply awaken with fake memories of having entered the booth. So you could learn nothing from questioning the people waking up on Mars. They would always say that they entered the booth on Earth and found themselves waking up on Mars and you would have no way of knowing whether this was really true.

It seems that the only way to know whether you would wake up on Mars is to try the booth out for yourself—if you dare!

This would seem like a fair conclusion to draw but Parfit actually says something quite unexpected at this point. Or maybe not so unexpected, I don’t know. He says that you could not find out whether you would wake up on Mars even if you tried the booth out for yourself!

This is where the basic paradox that bothers me starts to arise. For, at first sight, what Parfits says just seems to be wrong. Why could you not step into the booth, give it a go, and see for yourself whether you wake up on Mars? If you were willing to risk dying in order to find out what would happen, why could you not conduct the experiment and observe the result?

Let’s consider the possibilities. One thing that might happen if you enter the booth is that you simply lose consciousness forever, on account of your body having been destroyed. If so, then of course you wouldn’t find out anything: you’d be dead. But another thing that might happen is that you would find yourself waking up on Mars. In this case, it would be just like “coming to” in the morning, except that you would find yourself coming to on Mars. If this happens, why wouldn’t you thereby have found out what had happened? You enter the booth on Earth and find yourself awakening on Mars. You might say, “My God, this thing works.”

It seems to me that you stand some chance of finding out whether the booth works by trying it out for yourself. If it works, then you would find out. Why does Parfit think that you could not find out anything even if you tried the booth out for yourself?

I think it’s obvious what Parfit would say in reply. He would remind us that the person waking up at the Mars end—whoever this may be—would invariably think that he had stepped into the booth on Earth and woken up on Mars. So even if you survived the trip and found yourself waking up on Mars, you could not be sure that you were the one who had entered the booth. That person might have died and you might be a freshly-minted person with fake memories of having entered the booth. Not only would others be unable to tell, but you yourself would be unable to tell!

This point seems obvious once it sinks in and Parfit indeed makes it quickly before moving on. Parfit himself thinks that it is an “empty” question whether the person who wakes up on Mars is the same person as the one who entered the booth and seizes on the point just made to support this larger claim.

So now we have two lines of reasoning for contrary conclusions. On the one hand, Parfit seems to be right that no one waking up on Mars would be able to tell whether they were a freshly-minted person or the same person as the one who entered the booth. So even if the booth works and you really do wake up on Mars, you would have no way of knowing this. You could not find out what happened, as Parfit says.

On the other hand, it strikes me that the other line of reasoning remains unimpugned. Parfit does not discuss this other line of reasoning and nothing that he says indicates to me where it goes wrong. Thus, if the booth works, you could discover this simply by entering the booth and observing what happens.—You would find yourself waking up on Mars and would realize at once that you had survived. Indeed, if entering the booth does not kill you, it would be no more difficult to verify this than to verify the same of (say) ingesting rat poison. You could just try it and see. If it does not kill you, you would find out.

We can also see this in another way. Parfit’s point is that anyone waking up on Mars would find himself in one of two “indistinguishable” scenarios. On the one hand, he might really have arrived from a booth on Earth. On the other, he might be a freshly-minted person with the appropriate fake memories. He would have no way of telling which scenario was his. All of this sounds perfectly reasonable.

But then, standing at the door of the booth, considering whether to enter, notice how obvious it would be to you that you could never find yourself in the second of these two scenarios. How could you find yourself waking up on Mars in the mind of a freshly-minted person? You simply couldn’t. For, on the one hand, were you to die, you would just pass out. A freshly-minted person would wake up on Mars, thinking (falsely) that he had survived the trip. But you wouldn’t have any such thoughts: you’d be dead. On the other hand, were you to survive and wake up on Mars, there would be no freshly-minted person to speak of. So, whatever happens, you could never find yourself in Parfit’s second scenario. And you know this as you enter the booth. So, if you subsequently find yourself waking up on Mars, you would know that Parfit’s first scenario was the true one.

I can’t see anything wrong with either line of reasoning. Each seems entirely logical but they lead to contrary conclusions. Parfit’s reasoning shows that even if entering the booth causes you to awaken on Mars, you could not discover this by trying the booth out for yourself. The other reasoning shows that you could. Neither reasoning is refuted by trumpeting the other because each simply goes about establishing its own conclusion.

It is possible that both lines of reasoning are correct and that we are simply looking at a “veridical paradox.” In this case, the blame would lie with one of the unspoken assumptions that both sides share in common. The simplest one is the assumption that teleportation is even possible. Perhaps it is impossible that anyone should enter the booth and wake up on Mars! Perhaps you necessarily die.

Another possibility is that, while teleportation is not quite impossible, there is no real difference between your waking up on Mars and your being replaced with a freshly-minted person—as against what the preceding discussion has tended to assume. The two scenarios are essentially the same! Parfit himself would favour this conclusion. As mentioned, he thinks it is an “empty question” whether the person who wakes up on Mars is you.

My own feeling is guided by considering the even stranger case of “fission.” Suppose that two molecular duplicates of your body are created on Mars while the original is destroyed as before. Would you find yourself waking up on Mars? If so, in which of the two bodies? As before, it seems to me that you can potentially answer these questions by entering the booth and observing the results. But Parfit would not accept this. He would again claim that you could not find out anything even if you entered the booth yourself. Here is what he says of a case of this sort:

… we could not find out what happens even if we could actually perform this [experiment]. Suppose, for example, that I do survive as one of the resulting people. I would believe that I have survived. But I would know that the other resulting person falsely believes that he is me, and that he survived. Since I would know this, I could not trust my own belief. I might be the resulting person with the false belief. And, since we would both claim to be me, other people would have no reason to believe one of us rather than the other. Even if we performed this [experiment], we would therefore learn nothing.
  Whatever happened to me, we could not discover what happened … (Reasons and Persons, p. 258)

In this case, however, I suspect that Parfit is at best half right. Suppose that you do find yourself waking up on Mars. Parfit says that you would be unable to tell whether you were a freshly-minted person or the same person as the one who entered the booth. This claim still seems fair enough to me. But it also strikes me that you would be able to find out something else. You would be able to find out in which of the two bodies you have woken up. This would be true even if you could not answer the previous question. And what you discover here would not be negligible. E.g., if exactly one of the two bodies was shortly going to be in pain, then it would matter to you in which body you had woken up. Remarkably, Parfit says nothing about this. If he thinks that you could not find out even in which body you have awakened, as his remarks occasionally suggest, then I think that he is wrong. In this case, if you do wake up on Mars, then there is something that you could find out, even if Parfit is right that there is something else that you couldn’t.

If we bear this distinction in mind, then we may have a third way of resolving our basic paradox. Despite first appearances, the two lines of reasoning discussed previously may not conflict. Perhaps what Parfit claims you could not find out is not quite the same thing as what I believe you could find out. With the help of the fission case, we can begin to see that there are two different things here and what the difference might be. Despite the thorough nature of his wide-ranging discussion, there may be a whole dimension here that Parfit has left unexplored.

August 2017