What if someone invented a teleportation machine, a machine that would take an object, destroy it, and recreate it somewhere else, possibly miles or light years away? If that person demonstrated that this machine would work on all manner of inanimate objects (suppose they even used it on a corpse successfully) would you be willing to try it? When you came out the other side, would you still be you? What if instead of making one copy on the other side it made two, would they both be you? Which set of eyes would you be seeing out of?
These are representative of the issues of self that arise when you start thinking about the idea of teleportation. Many of these issues revolve around what philosophers call the mind/body problem, which is the question of how our subjective experience of having a mind, the place where we hear our own thoughts, relates to or physical bodies. When Descartes considered the question in the seventeenth century, the connection between the brain and mental abilities was little understood. These days, many people are strict materialists, who believe that the mind is accounted for entirely by the physical processes that go on inside the brain. People who believe this would be inclined to think that after going through the teleportation machine you would still be just as much you. Moreover, they would probably have to concede that if a second, physically identical copy were made it would be just as much you, having all the same thoughts and memories, but the minds of the two copies of would begin to diverge the second they began having different experiences. There are others who believe that the body is only a container or conduit for a soul, which is truly where the mind resides, and then the question becomes whether the soul will enter the new copy of the body or be separated from it. From this vantage, the question of making two identical copies becomes more perplexing; one might believe that only one would retain a soul, neither would have one, or that they might share one soul and one mind. It is these differences that make the question of teleportation and self interesting, because it provides a way for each of us to probe his beliefs about the mind and the self. While you might be willing to offer some guesses as to the answers to these questions, would you be willing to step into the teleporter and put your ideas to the test?
In addition to questions relating to the nature of the mind, there are further metaphysical questions about whether we can meaningfully say that after being transported we have the same object (as opposed to a facsimile). Clearly, we could take a mass produced Ford Escort, blow it up, and produce a new one in a factory miles away. We would hardly consider this teleportation, even if the car were produced to the same specifications. In this case, though, the new car and the original would have measurable differences, as long as one made detailed enough measurements. How similar then, must the new version be? The fictional teleporter in Star Trek is supposed to disassemble objects and convert them into energy that is then beamed to another starship and converted back into the original object. Must the new object be made of the same "stuff" as the original, as in Star Trek? Why should it matter, as long as the new one is identical? The considerations in answering these questions can then relate back to the mind/body problem and questions, for example, about the existence of a soul. If all we require is that the new version of the object be physically indistinguishable from the original, then it would seem reasonable to think that any teleportation device could also make many copies of an object, or person; however, if we require the copies to really be completely indistinguishable, down to the quantum level, then it turns out this is not the case.
Interestingly, a sort of actual teleportation has been achieved, quantum teleportation. This process teleports the quantum state of a system between two places by sending only a classical signal, which contains less information than the quantum state itself, but allows the state to be reconstructed on the other side using entangled pairs. This technique has been used to teleport the polarization of a beam of light so far, but it could in principle transport any quantum state, even that of a person. Of course, to be able to teleport an object in this fashion we already must be able to construct an object of that sort and manipulate its state at the quantum level, so it's very difficult to imagine that we could ever use this technique on any macroscopic object, especially a human being. However, we may still consider the philosophical implications.
The strange thing about quantum teleportation is that in order to create the state on the receiving end you have to destroy the state of the original on the sender's end by measuring it. What's more, there's a general result called the no cloning theorem that says you can't have a device that can make exact copies of arbitrary quantum states while retaining the original. So, it seems that nature has sidestepped at least part of the quandary. If we believe that the self is just encoded in the quantum state of a person's body, then it seems that our device can't produce any ambiguity about which is the real you. It should be noted, however, that quantum mechanics does not disallow two systems from being identical; indeed, identical particles abound in nature leading to results like the Pauli Exclusion Principle. The no cloning theorem just says it's impossible to build a device that makes completely identical copies of anything you put in.
Sometimes in discussions of the mind and self, the topic of cloning comes up. Cloning (in the biological sense) is a process that ideally produces a second organism with the same genetic code as the original. A person and her clone will have essentially the same genetic relationship as identical twins, except that they may not be the same age. Experience both with identical twins and more recently will cloned animals shows that they have different physical characteristics (presumably due to environmental factors), and clones certainly do not gain the memories of the original. As a result, clones are probably not a very useful tool in thinking about issues of self, because they would be both physically and mentally different from the original subject.
A response to Conflict's WU below:
Quantum mechanics is indeed unintuitive, but it does not follow that anything can happen. Subject to constraints like causality and results like the no cloning theorem, I know of nothing in modern physics that precludes the possibility of teleportation in principle, but it looks like in practice it would require such precise measurements, exquisite control, and staggering computing power that it's hard to see how it would ever be manageable, even for a more advanced civilization. As I've written, quantum teleportation has already been achieved experimentally in a number of simple systems, though this isn't necessarily exactly that sort of thing people think of when they talk about teleporting objects.
Where they are correct, many of the statements Conflict makes about physics have to do with the fact that single particles are not localized to a point and cannot be accurately thought of as following a single trajectory from one point to another. There is some question as to how then you know it's the same particle you're looking at, and, indeed, when you put multiple, identical particles together in quantum mechanics new behavior results from the fact that there is no way you can keep track of which is which. However, Conflict makes a number of incorrect statements: First of all, information is not passed instantaneously between different places. Likewise, tunneling is not instantaneous. A wave packet will spread through a barrier at the group velocity, which is finite. Additionally, whenever quantum tunneling occurs there is a non-zero probability of finding the system in the classically forbidden area, so it's not really fair to say that the particle goes from one place to the other without being in between. Optical tunneling, while mathematically similar, is just based upon the existence of evanescent waves and can be understood completely classically without reference to quantum tunneling or teleportation. Neither radioactive decay nor energy level transitions in atoms are instantaneous. The system gradually shifts into a superposition of the two states, and it is only a fast measurement that gives the appearance of an instantaneous transition. In the end, I think few of these phenomena have much baring on the issue of teleportation.
- "Thus we conclude that no measurement in the Klein-Gordon theory can affect another measurement outside the light-cone." p. 29, An Introduction to Quantum Field Theory, Peskin and Schroeder.
- p. 48, Introduction to Quantum Mechanics, J. D. Griffiths.
- See, for example, Griffiths.