On quantum teleportation and continuity of self
A recent episode of NOVA‘s miniseries The Fabric of the Cosmos, based on Brian Greene’s book of that name and hosted by Greene, featured a discussion of quantum teleportation, and it got me thinking about the subject again. I’ve considered it as a possible far-future technology for my science fiction, but I’ve been resistant to the idea of using it as a means for teleporting sentient beings, because of the question of whether the self survives. The question is, if your body is destroyed and an exact duplicate is created elsewhere, is that really the same you? The being who steps out of the teleporter on the other end has all your memories and personality and considers herself to be the same person who stepped into the transmitting station, and nobody else might be able to tell the difference, but it might still be that the original person’s awareness ceased forever the moment she was destroyed by the teleporter to create her duplicate. Nobody else could tell the difference, but she could tell — or rather, she could if she hadn’t ceased to exist. The question is, is there an actual scientific way of resolving this dilemma, or is it doomed to be a matter of philosophy and personal belief forever? Because I’m not stepping in one of those things — or having one of my beloved characters step into one, at any rate — unless I can be persuaded that there’s a continuity of self-awareness from one end to the other.
But I’ve done a lot of reading about quantum theory over the past couple of years as research for my Star Trek: Department of Temporal Investigations books, and just other reading in general before that about the idea of decoherence as an explanation for how the quantum world becomes “classical” at a macroscopic scale. And upon reconsidering quantum teleportation within that context, I’ve realized there’s a possible way to resolve this question, one that actually allows defining the self and its continuity in scientific rather than metaphysical terms and thus allows getting a more concrete handle on the problem.
The key is to look within the question itself and evaluate its unexamined assumptions. The question is, will my own sense of myself as a continuous, self-aware consciousness be carried through the teleportation process? To answer that, we must ask the deeper question, what is that sense of self-continuity in the first place? How does it come about? A brain is actually an ensemble of trillions of quantum particles, each in its own superposition of multiple states. What makes all those separate particles function as a continuous whole? Current quantum theory suggests that it’s their interaction. By interacting with one another, they become quantum-entangled: that is, as their multiple overlapping states interact, some of those states will correlate with one another. As those particles interact with still more particles, their states will correlate as well, and you’ll eventually get a whole ensemble of entangled particles whose quantum states are correlated with one another, functioning as a single whole. (Those other states that don’t correlate either fizzle out to insignificance under the Quantum Darwinism model or branch off into alternate timelines under the Many-Worlds model, but that’s irrelevant here.) It’s that collective, mutual correlation among all the different particles that creates what we think of as a macroscopic, classical reality. (This is why it appears to us that measuring a quantum particle collapses its wavefunction into a single state. “Measurement” is simply a process by which we correlate the states of our own quantum particles with a particular state of the measured particle, so that the ensemble of particles that we call ourselves reacts only to that state and doesn’t perceive the others.)
So when I perceive my mind as a continuous whole, an entity with an unbroken existence in time and space (at least the space within my skull) rather than just a collection of quarks and leptons, that sense of continuity exists because of the interaction and entanglement among my brain’s particles creating a correlation among their states. Even when I’m not being teleported, I rely on quantum entanglement to give me a continuous sense of existence.
So what happens when someone is quantum teleported? Her particles are thoroughly scanned along with the particles of a reference object, both of their states measured precisely and defined in relation to one another. The Heisenberg Uncertainty Principle doesn’t let you measure the position and momentum of her particles exactly at the same time, but the reference object lets you get around that: you don’t have to know the specific positions and momenta, you just have to know how they differ from those of the reference particles. (In my Star Trek fiction, I’ve explained the “Heisenberg compensator” of the transporter as being based on this principle.) Now, presumably that interaction between our subject and the reference object creates a correlation, an entanglement, between them. (I’m actually not entirely sure of that part, but I think it stands to reason.) Now, the reference object is already pre-entangled with a matter supply at the receiving end, and some of the state information of our subject is transmitted instantaneously through that entanglement link while the remainder is transmitted “classically” (i.e. without direct entanglement) via a speed-of-light signal (although theoretically it could be stored and delivered on a hard drive, I imagine, if the storage capacity were high enough). Then that information — the exact quantum information of the original subject, who’s now been disintegrated (since you can’t record quantum states exactly without changing them) — is superimposed on the receiver station’s matter supply, transforming it into that exact, indistinguishable duplicate of the original.
So what this means, or so it seems to me, is that the subject who steps into the teleporter station to be disintegrated and the one who steps out after being assembled are not only exact, indistinguishable duplicates, but share a quantum entanglement with one another. Just like the different parts of her brain and body were entangled and correlated with each other all along, thereby giving her a sense of herself as a continuous being. So if entanglement/correlation is what creates our perception of continuous classical existence, then it stands to reason that by any meaningful definition, the subject who steps out of the receiving station is continuous with the subject who stepped into the transmitting station. Even though there’s a separation in space and time, they’re still the same unbroken whole, because it’s the quantum entanglement that creates a “whole” out of a bunch of individual particles in the first place, and entanglement is independent of separation in spacetime. Two entangled particles separated by 50 light years will still be just as directly linked as they would be if they were physically adjacent. So the classical view that the teleported person ceases to exist for a time and is then reconstructed somewhere else is irrelevant on the quantum level that underlies reality. It’s still the same continuous self for the same reason that the self exists as a continuous thing in the first place.
So for my purposes, at least as it applies to fiction, I’m now reasonably convinced that quantum teleportation would preserve the continuity of self — that your own self-awareness, your consciousness, would be preserved and unbroken through the process even if you were physically nonexistent for years between transmission and reception (say, if you were beamed across an interstellar distance, or if the hard drive containing your classical data got lost for a while). It may seem unlikely, but only until you realize that what we perceive as a continuous reality is something of a quantum illusion to begin with, simply a coherent, large-scale correlation among certain states of a bunch of interacting particles.
Although there are still some massive obstacles to consider. Like, how does the teleporter actually work? How does it measure all the 10-to-the-29th-power particles in a human body at once? Given that doing so would require completely disintegrating the subject, that would require a lot of energy, essentially a lot of heat. It would basically be vaporization. If it weren’t done instantaneously, wouldn’t that heat and energy rather radically affect the states of the adjacent particles you hadn’t gotten around to scanning yet? You’d need to scan fast enough to capture the complete information of an intact person. Conversely, if you did it more gradually, say, by using nanotech or pinpoint lasers to disintegrate a body bit by bit (a la TRON), there’s a question of timing. If you scan different parts of the brain at different times, then there will be a time lag between the different parts when you reassemble them, and that will alter their relation to one another. It would be like taking a blurred photo of a moving object because you left the shutter open too long. (Or more like using slit-scan photography to stretch out an image.) So even if the brain that came out the other end contained the same continuous mind that went in, the transition would still alter its mental state. Would that just be a minor hiccup in awareness, or would it actually alter your mind somehow? Probably the brain’s activity is dynamic enough that it would be temporary, but what about the physiological changes from one part of the body to the other if the scan takes too long, or the reassembly does? The human body is a jellylike, wibbly-wobbly thing on the inside. If the process is too slow, then the different ends of an neural connection or a capillary might not line up right because they moved during the scan or the reconstruction stage.
Still, I suppose if you had a powerful enough quantum computer, it might be able to record that amount of state information pretty quickly, and be able to employ corrective algorithms to compensate for any delays in the scanning or reconstruction process. It might be necessary to be sedated before transmission in order to avoid the discomfort of being disintegrated/reassembled or the sudden alterations of mental state that might occur as a result of scanning lag.
So I’m not going to jump right into using QT in my fiction without trying to get some handle on these practical questions first. It might not turn out to be better than the alternatives available in a given universe. It might be more practical or cost-efficient for teleporting small and simple objects or raw materials rather than sentient beings. But at least I no longer consider the continuity-of-self question to be insurmountable, and that was the most important objection I had. So the door is open now.