Okay, guys (and I notice most people commenting here ARE guys) here's the problem with time travel. It's kind of gross, but be patient.
Any activity that changes a past event will trigger a change in the physical movement of the people who lived then. To be precise, spermatic fluid will be displaced. For example, all the people connected with the LHC and even those who have read of its problems will have shifted their physical positions.
The result? Children who would have been born won't be. Children who wouldn't have been born will be. Perhaps a girl rather than a boy, or perhaps a few months earlier or later. This in turn generates a cascade of similar changes affecting anyone who comes into contact with them (or fails to do so).
A similar effect can be expected from women, whose physical positions will later be altered at the time of conception (or non-conception).
Consequently, ANY time travel contact with past generations of Earth people or their habitat, constructs and so on will immediately terminate all future events, including the one that produced the individual who is going into the past. This could be a great explanation for all those UFOs that are visible for a short time, then suddenly disappear. Each of those UFOs could be the "first time" that a time-traveller has gone into Earth's past.
As for time travellers targeting the LHC, they would be better advised to reach back into the more distant past and zap the theoretical physicists whose original work formed the foundation for the LHC.
If the LHC is to be stopped (which I think it should be), it should be by people living here and now, not by assassins from the future.
