Centauri Dreams on the Large Hadron Collider's possible utility as a time-machine:

But as to the argument that the LHC's operations could establish Year Zero for time travelers (creating the needed first instance of a time machine to which future travelers would be able to return), I'll take a pass. Surely if massive energies are what it takes to establish such a wormhole (itself a purely theoretical concept, and one that requires yet another theoretical idea -- phantom energy -- to hold it open), then the universe has supplied us with previous instances of 'closed time-like curves' in highly energetic events reaching back to the Big Bang. Does that mean a time traveler could only travel back 13.7 billion years? If so, that's plenty of temporal territory to play in, but the Fermi question equivalent for time travelers is, where are they?

Stephen Hawking has used the same argument. If time travel is possible, he maintains, we should see a steady influx of "tourists" from our own future. This reasoning is easily as limited as the assumption that ET visitors will "land on the White House lawn" if they happen to visit.