If you're a criminal and you want to whack someone, why not just send an assassin into the past and kill them before they become a problem? That's especially true in 2074, when ultra-modern forensics make it just about impossible to hide dead bodies. That's the two-sentence synopsis of Rian Johnson's "Looper," which relies heavily on imaginary elements of time travel. It's also a pretty unlikely story.
It's not that every physicist believes that time travel is impossible. Based in Einstein's theories of the nature of our universe, time allows for some pretty bizarre twists and turns, and with the right insights, maybe humans could jump around in history, visiting ancient Aztecs and saddling up the dinosaurs (which as you already know, you'll never be able to recreate "Jurassic Park"-style).
However, even if we somehow figure out a way to zoom back into the past, we may not be able to interact with or alter anything or anyone there. That's because of the so-called "grandfather paradox." This idea based on the argument that if you traveled into the past and killed your grandfather, you simply would never exist. That's a pretty profound paradox.
There's also the notion of a causal loop. If a person travels back in time to effect an event, the time traveler eventually reaches a point in the timeline where she originally fired up the time machine ... resulting in a cycle that repeats itself endlessly. So although "Looper" makes for some fun banter, it's by no means a realistic portrayal of our multi-dimensional universe.