It depends on what is real and how some things are achieved. I suppose reaching an alternate universe could gain access to either of those things, but its possible that alternate universes have barriers preventing universes from crossing, or maybe you need some really fantastic event to happen (like in Half Life).
Ghosts may not be real, but it is possible I suppose, though I don't believe there are ghosts (unless you went to a universe where there were).
Aliens, I'd like to believe that we aren't the only living things in the universe or even the galaxy, but its more likely we may just catch a glimpse or such through a telescope rather than meet them.
Time travle is different, its not really anything as much as an idea, thus it cannot really exist. Of course, alternate universe come up again, as does the possibility of reversing the flow of all molecules and atoms to previous states (ala Singularity, an upcoming game from Raven), although traditional time travle may be too hard considering the paradoxes and chaos theory, plus the fact that the extra mass you add could cause some sort of collapse or tug along strings across the universe affecting things you didn't even know anyone could ever possibly affect.
Ugh, I'd guess aliens since if they don't exist in our universe (unlikely), we could find them in another.
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