catallaxy files

catallaxy in technical exile

Time travel paradoxes

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I’ve recently been reading Michio Kaku’s new book, Parallel Worlds. Don’t get me wrong, it’s fascinating, brainteasing stuff and I don’t feel I’ve wasted time at all trying to come to grips with the latest physics and cosmology theories so engagingly explained by superstring theorist Kaku, who has his own website here. But aren’t these people getting a little bit carried away by their mathematical fictions given the scarcity of expermental evidence to date ? Especially this stuff about the universe being a three dimensional membrane floating in a five dimensional surface and subatomic particles that make us up being disturbances on this brane surface.

Anyway, enough of that short aside. What I’d like to share and paraphrase is Kaku’s short but comprehensive list of time travel paradoxes.

1) Grandfather paradox – this is the one that almost everyone knows about. What happens if you go back in time and kill your grandfather?

2) Information paradox e.g. a scientist creates a time machine and then goes back to the past and gives the formula for building a time machine to himself as a youth. Where does the information for building the time machine come from?

3) Bilker’s paradox (don’t know why it’s called that) – a person knows what the future will be and does something to make it impossible e.g. Jack uses a time machine to go to the future where he sees he is destined to marry Jill so he comes back to his present and marries Jane instead

4) The sexual paradox – this is the most mindblowing of all and is illustrated by Kaku using the plot of a story written by British philosopher Jonathan Harrison. It goes something like this (note that it also incorporates some of the other paradoxes discussed so far but the main element here is where someone can be his own father and eat himself):

Jill one day finds an old deep freezer in a cave, within which is a man named Jack cyrogenically frozen. Jill defrosts him and he comes back to life and tell Jill that he has with him a book which describes how to build a deep freezer to cyrogenically preserve humans as well as how to build a time machine. Jack and Jill subsequently fall in love, get married and have a son named John.

When John becomes a young man, he follows in his father’s footsteps and builds a time machine. He and his father Jack then decide to go on a time travel expedition together and they take along the book that his father originally had with him when he was defrosted. But the trip ends tragically as they find themselves trapped in the distant past and running out of food (assume the time machine broke down and they don’t have enough materials in the distant past to rebuild it). John decides then that the only way to stay alive is to kill his father and eat him, which he subsequently does.

Armed with this new stock of food to keep him going, John finds he does have enough raw materials to at least build this deep freezer according to the instructions in the book. To save himself he enters the freezer and goes into a state of suspended animation.

Many, many years later, Jill comes across this freezer and thaws out John. To disguise himself, he calls himself Jack. Jack and Jill fall in love and get married …

In addition to the ‘self creation’ and ‘self eating’ paradox in the last story and the information paradox, there’s also a genetic paradox. John is the son of Jack and Jill so half his genes must come from Jill. But John is actually Jack therefore John and Jack must have exactly the same genes …

Anyway, feel free to discuss these paradoxes and their potential resolution (I know, one resolution is simply to adopt the many worlds interpretation of quantum physics, another is to posit that time machines can never be built, and I’m sure you can think of cleverer ones) or feel free to come up with or point me to even more ingeniously constructed ones.


Written by Admin

May 12, 2006 at 11:08 pm

Posted in Uncategorized

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