Callista wrote:
Why in the world would you go back to prevent silly things that happened to you when you could do things like prevent major accidents, plane crashes, bombings, shooting sprees... jeez, compared to that, failing a year at college is really small peanuts!
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Mostly, it wouldn't work - by being in the past (even if you sat in a locked room) you're immediately influencing everything around you in a sphere of radius speed-of-light*time-you've-been-there, causing a gazillion zillion tiny changes in the way things move; as the time increased, the effect of your being there rapidly increases making everything different for everyone (like the so-called Butterfly Effect, writ large)...
...but you could certainly identify some people as having Serious Potential For Trouble, I suppose, and expose any plots or nastiness that were currently underway (assuming you knew about them.) You'd probably want to study up on some astronomical phenomena beforehand - precise times and locations of supernovae, gamma-ray bursts, that sort of thing (stuff which will be utterly unaffected by anything happening on Earth, and which could not have been secretly discovered - things like the structure of DNA or the orbital parameters of then-unknown minor planets are out) so that you could at least convince scientists of your veracity.
Oh, and you'd also need to find a way to match your present velocity and position with the velocity and position of Earth at your target time, or you'd either asphyxiate, burn in the atmosphere as you streaked across at 30kms-1 (or whatever speed) or cause a very large crater...
*ahem*
Leastways, that's
my plan.
_________________
No one has gone missing or died.
The year is still young.