15 May, 2004: And one of the fingers on the button will be...

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Imagine a universe which is exactly like our own, except that all three of the following devices could be built. Device number one looks at your body and writes down the position and state of every particle in it instantaneously, or at least so quickly that it seems instantaneous. Device number two kills you, either at the same time as device number one is doing its thing, or at some later stage. Device number three -- which might be located anywhere -- takes the information written down by device number one, and uses it to reconstruct a copy of you which is indistinguishable to an outside observer from the person who was examined by device number one.


(In our universe, devices one and three cannot be built: they violate the uncertainty principle of quantum mechanics. Even if they didn't, the amount of data which would have to be transferred is impossibly big. The human body contains something like 5,400,000,000,000,000,000,000,000,000 atoms; accurately recording the position and velocity of each of these would require -- let's say -- twenty-four bytes of information. A terabyte of data can be stored in hard disks weighing a kilogram or two; storing all this data would require hard disks weighing about as much as the British Isles. By contrast, device number two is easy to build, and throughout recorded history human beings have shown considerable ingenuity in making numerous versions of it. For our purposes, it may as well be completely comical: a man with a shotgun standing outside the booth containing device number one will suffice.)

With devices one, two and three, I can build a teleporter. You walk into a booth and press a button. Device number one records your body, and transmits it to device number three, which is located somewhere else -- let's say Australia. Leaving the booth, device number two does its stuff: a shotgun-wielding maniac blows you away. This is not a neat teleporter; if forced to use it, the crew of the Starship Enterprise would have spent a lot of their time cleaning blood off their decks. But it clearly is a teleporter: to a witness in Australia, the person who steps out of device three is the same as the person who stepped in to device one; and when asked their experiences, they will explain that they walked into a booth in Britain -- ignoring the shotgun-wielding maniac as they passed -- pressed a button, and found themselves in Australia.

But what do you experience when you walk in to device number one and press the button?

To my surprise, this question has generated considerable discussion among people I've asked. (I was also very surprised that there are people who haven't heard this question -- or something very like it -- before.) For those who aren't sure, some further questions:

If you want to know what I think, read this; but make up your own mind before you do. At this point I should also apologise for inverting the purpose of this web log; I've already bored people in the pub with this question, but every time it comes up in real-life conversation an argument starts, so I may as well get some web log mileage out of it. Also, a special request for the comments: I'm not a great fan of brand-name philosophy, so if you feel it's necessary to invoke a brand-name philosopher to explain your answer, then please either (a) reconsider and phrase your point in English; or (b) post a holiday photograph to compensate, as with my ID cards strange attractor. And, again, sorry: no graph (next week...). At least this piece isn't about ID cards, though please do come to Mistaken Identity on Wednesday, especially if you're David Blunkett.

Copyright (c) 2004 Chris Lightfoot; available under a Creative Commons License.