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.
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:
- Suppose that devices one and three have been supplied by space aliens, and nobody understands how they work. The phone lines to Australia are down, and it's just possible that the whole thing is a practical joke. Device number one is a Great Big Lie -- it's just a phone booth with a button in it -- and device number three doesn't exist at all. What do you experience when you walk into the booth and press the button? If no-one can tell the difference between the real and fake devices, would your experience of the fake device differ from your experience of the real one?
- If the question, `what do you experience?' seems ambiguous, can you rephrase it as, `at the moment that you press the button in the booth, what would you believe would happen next?'
- Suppose that devices one and three work as described, but the internet connection to Australia is very slow. It will take a week for all the information read by device one to reach device three. Suppose further that the shotgun-wielding maniac is having an off day, and he merely clips you; you manage to escape before he can reload. A week after pressing the button in the booth, you are recovering in hospital; in Australia, a copy of you is made in device number three. What do you experience?
- Suppose that, rather than in Australia, device number three is actually located at the bottom of the deep ocean. The person who materialises in device three immediately drowns. Does this make any difference to your experience? What if device three is in the space aliens' secret (oxygen-filled) base on the Moon? What if it's on a planet orbiting Alpha Centauri?
- Would you use such a machine? And if, rather than a shotgun-wielding maniac, device number two were replaced with some means of killing which was guaranteed to be painless and instant?
- (If applicable.) What happens to your soul?
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.