Though we mightn't ever have the technology for it, couldn't we go back in time by just travelling faster than the speed of light? Granted we wouldn't be able to go back to the present a similar way, but there could be a different way to do that. I speak of freezing the body and having it thawed out at whichever point in the future you want to be at. That actually strikes me as possibly being doable sometime in the not too distant future as advances in cryopreservation seem to be going along quite well.

As with the above, this next bit is pure speculation, but I think if someone did try to change the past, they'd **** up somehow and the events would be as we know them today. When time travel is possible (if ever) perhaps humans will be wise enough to not screw around with things that have the potential to possibly bite them in the ass so bad.