Wednesday, June 29, 2005
The James T. Kirk solution?: The New York Times has a very interesting article on time travel and the possibility of paradox. The article is accessible even for those who are not sciencephiles, but there's one thing about it that troubles me.
But what about killing your grandfather? In a well-ordered universe, that would be a paradox and shouldn't be able to happen, everybody agrees.
That was the challenge that Dr. Joe Polchinski, now at the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., issued to Dr. Thorne and his colleagues after their paper was published.
Being a good physicist, Dr. Polchinski phrased the problem in terms of billiard balls. A billiard ball, he suggested, could roll into one end of a time machine, come back out the other end a little earlier and collide with its earlier self, thereby preventing itself from entering the time machine to begin with.
Dr. Thorne and two students, Fernando Echeverria and Gunnar Klinkhammer, concluded after months of mathematical struggle that there was a logically consistent solution to the billiard matricide that Dr. Polchinski had set up. The ball would come back out of the time machine and deliver only a glancing blow to itself, altering its path just enough so that it would still hit the time machine. When it came back out, it would be aimed just so as to deflect itself rather than hitting full on. And so it would go like a movie with a circular plot.
In other words, it's not a paradox if you go back in time and save your grandfather. And, added Dr. Polchinski, "It's not a paradox if you try to shoot your grandfather and miss."
"The conclusion is somewhat satisfying," Dr. Thorne wrote in his book "Black Holes and Time Warps: Einstein's Outrageous Legacy." "It suggests that the laws of physics might accommodate themselves to time machines fairly nicely."
You can find an accompanying graphic here.
What troubles me about this "solution" is that it's not really much of a solution at all. Instead it is reminiscent of Star Trek's Capt. Kirk's "solution" to an insolvable problem. When confronted with a simulated battle where there is no possible way to win -- Kirk changes the conditions of the simulation to create a way for him to win.
The problem as posed is: "What would happen if you were to go back in time and kill your grandfather, thereby negating your own birth and so you couldn't have gone back in time to kill him in the first place?"
The Cal Tech "solution" is that you don't kill him, only wound. That's not a "somewhat satisfying" solution.
Let's bring it down a notch. Let's say that one train leaves Memphis at 3 p.m. going 40 mph en route for Pensacola and another leaves Cleveland at 2 p.m. going 60 mph destined for Phoenix. Which one arrives first? Is "the train from Boise" a satisfying solution? How about "there is no train?"
I'm sure there's probably some explanation for this, but I don't see how avoiding the question is the solution.