The Keys of Marinus

I wouldn’t think of asking you to travel in such an absurd way.

Teleportation – moving from place to place near-instantly, without having to travel through the intervening space – has long had a hold on the human imagination. From the Arabian Nights to the Ring Cycle, it appears as a magical ability to disappear here and reappear there, and is still invoked in this way by various mystics to this day (as well as, bizarrely, being studied seriously by US military intelligence). Even when it comes into science fiction, it is at first as a mystical or psychic power, whether as John Carter’s sudden trip to Mars or Gully Foyle’s jaunting.

But science fiction inevitably seeks to translate mystical marvels into technological devices, and teleportation is no exception. It’s most famous from Star Trek, of course, and apart from a few sad fans no one knows or cares that Doctor Who got there first.

So how could the technology of teleportation work?

Naively, you could imagine doing teleportation by measuring the position and all the other properties of every particle in the body, then transmitting that information to somewhere else where the body is reassembled. This is the usual explanation of Star Trek-style teleportation. It is, unfortunately, impossible. It’s generally said that this impossibility is due to the Heisenberg Uncertainty Principle, which says that physical variables at the quantum level come in matched pairs, such as position and momentum, and the more accurately you measure one the less accurately you can know the other. This is quite true, and in itself a fatal blow to this model of teleportation (one which later Trek series hilariously handwaved away by invoking “Heisenberg compensators”), but there’s a deeper version of this idea that we need to understand before going on to see how quantum teleportation can work.

In quantum mechanics, systems of particles exist in quantum states, which cannot be measured directly. A single measurement only gives us partial information, and it destroys the quantum state in the process. If you have a load of systems in the same quantum state you can measure all of them, and build up an approximate description of the underlying state – the more of these systems you measure, the more accurate the description. What you can’t do is directly measure the complete quantum state of a single system, such that you could then transmit that information somewhere else and recreate the system.

Quantum teleportation solves these problems, but with some restrictions and subtleties. It involves the use of particles that have been made to interact in some way so that they are each part of the same quantum system, then separated such that they are still part of the same quantum state even though they are some distance apart. This is called entanglement.

Imagine a setup where two people, let’s call them Arbitan and Barbara, share in advance a pair of particles that have been put into an entangled state. Now Arbitan has a third particle, that is in some quantum state of its own. This is the particle he wishes to teleport. By making certain cunningly-contrived measurements on this third particle in conjunction with his entangled particle, Arbitan manages to extract a set of information about his combination of particles, which he sends to Barbara by conventional means. Barbara can then use this information to put her half of the entangled pair into the same state as the particle that Arbitan wanted to teleport. So the net effect is that the quantum state of Arbitan’s particle is destroyed, and transferred to Barbara’s particle. Crucially, it is the complete quantum state that is transferred, not just the partial information that Arbitan could glean by measuring his particle’s quantum state directly. That’s really the clever bit.

An interesting philosophical wrinkle here is that it is not quite right to say that a quantum state is transferred from one particle in Arbitan’s possession to a different particle in Barbara’s possession. Elementary particles are indistinguishable from one another. Electrons aren’t like cars. Even though cars are mass-produced and come in production runs of apparently identical cars, there is a real sense in which my dark blue Vauxhall Astra is not the same as your dark blue Vauxhall Astra, even before they get scratched and grimy and the passenger sides covered in the muddy footprints of our respective spouses. Electrons are different. They don’t have number plates or identifying marks. If we each have an electron, and we swap them, the electrons remain in the same physical state: as far as the laws of physics are concerned, nothing has changed. This is really, really important. The behaviour of matter depends on how electrons and other particles behave as a statistical aggregate, and those statistics become very different if this isn’t true. Among the many, many things that depend on this are semiconductors, such as the chips that drive the computer or phone or whatever device you’re using to read this blog.

The upshot of this, as far as teleportation is concerned, is that there’s no sense in saying “you haven’t teleported the particle, you’ve just transferred its quantum state to another particle far away”. These two things are identical.

We also don’t need to worry about this apparent duplication process giving rise to multiple identical copies. Arbitan’s measurement destroys the quantum information in his version of the system – there is only ever one copy at a time.

There is still the question of  – assuming we can scale this up from the spin state of one particle to the entire quantum ensemble of 1029 particles that make up your typical living, breathing human in such a way that the teleported person is still living and breathing at the end of the process – whether the teleported person (let’s call her Susan) is copied as a single, continuous entity or whether she is killed by Arbitan and resurrected by Barbara as a new person with only the memories of the original Susan. The argument that the particles are indistinguishable, so she should just chill out, might not seem so compelling to the Susan in Arbitan’s clutches, as she experiences her quantum information being destroyed. It’s as much a question of philosophy as physics, and it’s philosophers we turn to for an answer.

In a recent survey of 931 philosophers, one of the questions they were asked was precisely this: does teleporting Susan result in her death and the creation of a copy, or her survival in Barbara’s far-off location? The results were as follows:

Survival: 36.2%



I guess that’s why they get paid the big bucks.

Now there are three big restrictions on this kind of teleportation. The first is that Arbitan still has to send the results of his measurements to Barbara before she can perform the teleportation at her end. That’s maybe not such a big deal, but it does mean that you can’t use this to travel faster than light. The second is that Barbara has to have a suitable supply of appropriate particles to complete the teleportation. Easy enough if we’re talking about individual electrons, but quite how you would store and use the raw material for a complete Susan is a trickier question.

The biggest problem of all, though, is that this can only work at all if Arbitan and Barbara have previously shared between them enough particles in entangled quantum states to allow them to do the teleport at all. And each entangled pair is a one-use, disposable item – when they’re gone, they’re gone and Barbara has to go back to Arbitan the slow way so they can share another batch. This means you can only teleport between pre-arranged locations that have been visited by someone carrying entangled particles from the home station, and these need to be resupplied or else they will run out of entangled particles and become useless.

Let’s be honest, it’s starting to sound a bit shit.

Could there be another way?

In the post for An Unearthly Child, we talked about distorting spacetime with the use of exotic matter. We can do something similar for teleportation.

The idea, basically, is to cut out a small region of spacetime at the departure point, and an identical region of spacetime at the arrival point, and join them together so that they become one. You then have a portal in spacetime through which you can simply step from one region into another.

Physicist Matt Visser has done a lot of work on these sorts of traversable wormholes. In one of his papers he lays out a simple design: a cuboidal frame into which the traveller can step and be instantly transported to another place. The edges of the frame are made of exotic matter, and the clever bit is that all the immense stress-energy needed to rupture spacetime in this way is concentrated along these edges: as long as you just step through the faces of the cuboid, you should feel no ill effects.

This is a crucial piece of progress. Most wormholes, such as those that may be created by rotating black holes, subject anyone who comes near them to such overpowering tidal forces that the hapless traveller becomes, in general relativity jargon, spaghettified. Which is about as pleasant as it sounds. If any wormhole is to be actually useful for travel, it must be set up so as to avoid this danger.

That said, it’s still not something that we have any idea how to set up in practice. How to manufacture exotic matter with negative mass is still an open question (though one that we may return to for The Evil of the Daleks), as is the amount of such matter that would be needed to create this frame. Visser’s earlier calculations suggest that making a human-sized frame would require a quantity of exotic matter roughly comparable to the mass of Jupiter, though he reckons he has since come up with a way to do it with much less.

These niggling technical details aside, this kind of travel through wormholes – let’s call it “classical teleportation” – has real advantages over the trendier quantum teleportation. There are no questions of whether you are killed in the process, for a start: you simply step through the portal as if you were stepping through a door, and any philosophical questions about whether you are the same person on the other side of the teleporter become no more pressing than the question of whether the you that gets off a bus is the same as the you that got on it. (In other words, actually quite a tricky philosophical problem if you think about it, but not one that keeps most people awake at night.) Also, we don’t have to worry about continually replenishing the supply of entangled particles to keep the process going: once the wormhole is set up, you can go back and forth as much as you please, and if you want to close it and reopen it somewhere else you just need your original supply of exotic matter.

So perhaps we should assume that the travel dials that Arbitan provides to our time travellers somehow generate a frame of exotic matter that punches a hole in spacetime that opens out onto the destination. To my mind it’s a more pleasing solution: having teleportation work along similar scientific principles to the Tardis gives a pleasing sense of coherence to this science-fictional world. Which, let’s face it, is more than can be said for Terry Nation’s plots.


4 thoughts on “The Keys of Marinus

  1. Funnily enough, I just recently wrote about personal identity theory and concept of self issues for an upcoming Vaka Rangi blog post. I did a little of that at the undergrad level and it’s one of those things that really gets philosophers fired up (I think quantum teleportation was even one of the examples that came up).

    Unfortunately, or maybe not depending on how you see things, my class was taught by a pan-Buddhist who didn’t believe in ego and free will and enjoyed trolling students such that he turned the entire seminar into a shaggy dog story by concluding personal identity theory was really rather a bit overrated.

  2. I’ll look forward to that post.

    For me, one of the most fascinating things about that issue is that the feeling of continuity of self differs from person to person. Some people feel very strongly that they are the same person as ten, twenty, thirty years ago, while other regard their earlier selves as being quite distinct from their current self.

    Personally, I don’t feel a great sense of continuity with the Iain Coleman of a decade or two ago. Whether that’s an innate tendency, or whether it’s because I’ve had very different jobs/vocations in different places at various times in my life, I don’t know. I do know that I respond to the Doctor’s regenerations as metaphorically expressing the discontinuity I feel about my past. The only difference is, the Doctor generally gets younger and better-looking.

    • My experiences are very similar: I feel just about zero continuity with the person I was a decade or two ago aside from a few shared interests and defining character traits, and sometimes it’s hard for me to feel contiguous with the person I apparently was only a *year* or two ago. I generally find this to be a good thing.

      Another thing that professor liked to bring up was the fact that every cell in our bodies is replaced every seven years. Our bodies are constantly shedding and growing new cells, but apparently every seven years a cycle resets where there are literally no cells left in our bodies that were there seven years ago. So in a very real sense we are quite literally not the same person anymore.

  3. Pingback: The Sensorites | Relative Dimension

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