UFO? just a therm to say when you can't identify a flying object.
Extraterrestrial life: quite sure it's out there somewhere
Aliens on earth? Don't bet on it. Besides, if they are a bit smart, they'll use robot-landers first. You don't see us jumping around on Mars yet do ya? We send radio-controlled probes. Why would aliens take the risk of landing on a clearly habitted planet.
Time-travel: no idea, but i'll be back in a second to tell you if they managed to do it during my lifetime. Did you see me? hmmm....
Big things: an Einstein concept. Seems Einsteins theories where basically correct, but people like Stephen Hawkin make new discoveries on a regular basis, ever refining the basic theories and taking them into a whole new direction.
Reminds me, saw a documentary about neutrino's ( the universes most difficult to catch particles) yesterday. Last conclusion. Seems neutrino's could be the source of everything. Them beeing the most pure state of all things, then they decay into all sorts elements. Some scientist prooved they have to have a mass. if they do, they could well be a lot heavier when combined, than all visible masses in the univers ( starts, planets, gasses, ...)
It's kinda exciting that we could be on the brink of discovering a whole new view on what's out there.
... hmmm... I might have gone waaaaay out there... my 5 cents...
