Science fiction author Larry Niven (best known for his creation of Ringworld) has published and promulgated numerous "laws" describing the universe.  Among time travel fans, however, one emerges specifically as Niven's Law, a statement concerning time travel, which reads:

If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.

In simple terms, Niven suggests that if it is possible to change the past by sending information to the past, eventually the past will be altered to an ideal state and no one will have any reason to change it further, and thus the ability to send information to the past will never be developed, there being no need for it.  For example, if it were determined that the world of the future would be better if Ford had put an electric car into mass production in 2008, those in the future would send that information to 2008, and Ford would put the car into production; this would then change the future such that there would be no need for anyone to send that message to 2008, and the message which was received would never be sent, the world being the better version created by the delivery of that information.

Two challenges may be raised against Niven's Law, one entirely theoretical the other entirely practical.

The theoretical problem is related to causal chains. Niven's Law would seem to work in a divergent dimension theory universe, in that each sending of information to the past would create a universe in which that information was received; but in a fixed time theory world it does not apply at all (you cannot change the past), and in a replacement theory world there is a serious question as to whether an effect in the past can be maintained if it undoes its own cause in the future--the problem of killing your own grandfather, in that the cause in the future of the change in the past has been undone.

The theoretical problem in relation to divergent dimension theory is that the changes to the past will always have been made in a different universe.  The inventors of the time machine will never see it function, as the changes they make are actually made in someone else's past.  In such universes, it is extremely difficult to demonstrate that any time machine works.

The practical problem lies in the notion that humanity in the future will be in complete agreement concerning what the best past would have been.  There will always be some who believe that the world would have been better had Kennedy (or perhaps Lincoln) not been assassinated, and so might attempt to alter that event; however, as the Red Dwarf episode Tikka to Ride humorously demonstrates, not everyone will agree that such a world would have been better, and someone might well attempt to restore the original history or something like it.  Bill and Ted's Bogus Journey similarly shows that while Rufus might think the legacy of the band has left a future paradise, De Nomolos hopes to replace it with something more to his liking.

Thus even if it is proposed that undoing a future cause does not undo a past effect (that you can kill your own grandfather) it does not follow that the world would ever reach a state which everyone agreed was the best possible history.  Niven's Law is thus probably incorrect.

This concludes our series on temporal theory.  As always, questions may be addressed in the comments section or by e-mail.

Next time:  Star Trek.

The author would like to apologize to anyone who attempted to send e-mail to this address prior to the end of August; there were technical problems which prevented delivery of such e-mail, which have now been resolved.