You probably know that traveling back in time to kill your grandfather is not only unethical, it’s also prohibited by the laws of nature. This isn’t because the laws prohibit travel to the past (in fact, there are several speculative models of current physics that allow for it) but because killing someone who fathered your father means you aren’t born (and thus not in a position to travel back in time and do the dirty deed). What you may not know is why this restriction on your action seems especially onerous. In a recent Discover article, CalTech physicist Sean Carroll argues that the difference can be explained by an appeal to boundary conditions.
Why should boundary conditions matter? The clearest answer is provided by Columbia University philosopher David Albert, who argues that it is the initial boundary condition of the low entropy big bang that explains why the future seems (at least somewhat) under our control while the past does not. Since there are vastly more ways to have high entropy than low entropy, a universe that begins in a low-entropy state is overwhelmingly likely to evolve to a state with higher entropy. This means that the past is more restricted than the future (there are fewer states the universe could have been in yesterday than states it could be in tomorrow), so the past seems fixed while the future seems under our control.
However, Carroll argues that time travel adds another boundary condition. Someone traveling to the past can know what the future holds and this introduces a restriction on what state the universe will soon be in. Everything done during the trip to the past must result in that very future state (any attempt to kill your grandfather bungles and results in your grandfather fathering your father, etc.). The upshot is that while we are used to restrictions on what we can control in the past (the initial boundary condition guarantees we cannot control the past at all), we are not used to restrictions on what we can control in the future (we know of no analogous future boundary condition).
Metaphysics Time is normally thought to be directed from the past toward the future, but we can raise …
By Frank Arntzenius , Rutgers University
(Vol. 1, October 2006)