Problems with time machines
I've seen a few articles on this recently, and got to see Stephen Hawking in person recently (due to a screwup on the seats, we and a few others got seated on stage! I could actually read what was on his computer screen from where I was sitting).
As I understand it, one of these spinning mass dealies for time travel can only be used to travel into the past to the point where the machine is created.
Given that, even if the technology was feasible, who on Earth would be stupid enough to build one of these? God knows what would come pouring into our timeline the second it was activated.