In the new movie "Upside Down," Jim Sturgess and Kirsten Dunst are would-be lovers kept apart by -- wait for it -- different gravitational fields. Their unique dilemma ups the stakes on the ordinary obstacles, like mere feuding families, which Romeo and Juliet had to contend with.

For a lot of cinematic duos, being with the one you love is damn near impossible, especially if one (or both) of you aren't even human. "It's complicated" doesn't begin to cover some of the issues these movie lovers have faced when even time and space conspire to keep them apart. Some of them managed to make it work, while others faced a huge sacrifice or had to give up on love all together.

Below, we've rounded up 10 star-crossed lovers from sci-fi movies, who tried to make a go of it against all obstacles.

RELATED: Jim Sturgess on Kissing Upside Down and Acting With a Hangover

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  • Daisy and Benjamin in ‘The Curious Case of Benjamin Button’

    Talk about an age gap! Because Benjamin (Brad Pitt) is born old and ages in reverse, he's hardly a suitable match when he first meets Daisy (Cate Blanchett), who ages the usual way. They come together all too briefly in the middle. In the classic "Portrait of Jennie," Joseph Cotten <a href="http://www.youtube.com/watch?v=T4ViM1myZus" target="_blank">had a similar dilemma</a> when he fell for the mysterious Jennie (Jennifer Jones).

  • Superman and Lois Lane in ‘Superman II’

    The Man of Steel (Christopher Reeve) gives up his super powers to be with the woman he loves (Margot Kidder), but he quickly finds out he can't save the world without them. Turns out love's a luxury most superheroes -- including The Hulk and Spider-Man -- can't really afford.

  • Arwen and Aragorn in ‘The Lord of the Rings’ trilogy

    What's immortal elf Arwen (Liv Tyler) to do when she falls in love with noble mortal Aragorn (Viggo Mortsensen)? Her father warns her to give up her human love, since even if Aragorn triumphs over their enemy, he will still die long before she does. Arwen chooses a mortal life with him, even though it means certain heartbreak.

  • Decker and Ilia ‘Star Trek’

    It's hardly the wedding most lovers would imagine, but human Decker (Stephen Collins) and alien Ilia (Persis Khambatta) manage to take self-sacrifice to new heights as they find a way to be together forever.

  • Jake and Neyteri in ‘Avatar’

    In his alien alter-ego, Jake woos beautiful Na'vi Neyteri, who doesn't suspect he's really a human. Once she learns of his epic betrayal, we'd say that counts as a deal breaker, except that true love -- and a certain Tree of Souls -- work wonders.

  • Madison and Allen in ‘Splash’

    Allen instantly falls for Madison, who saved his life as a boy, but he doesn't realize that she's a mermaid until late in the game. He's faced with a choice: Let her go back to the sea or give up everything he's known and go with her.

  • Irene and Vincent in ‘Gattaca’

    In this stylish futuristic noir, Irene (Uma Thurman) is one of the genetically bred elite, while Vincent (Ethan Hawke) is a lowly "in-valid," meaning his parents had sex the old-fashioned way. He manages to pass as a "valid" and win her heart, but the truth -- and a murder -- threaten to separate them.

  • Selene and Michael in ‘Underworld’

    Selene (Kate Beckinsale) is a vampire and Michael (Scott Speedman) is her sworn enemy, a werewolf. She's under orders to kill him, but instead protects him. They learn they weren't the first vampire-lycan lovers to defy the age-old ban, a match that had no happy ending.

  • Navarre and Isabeau in ‘Ladyhawke’

    It's hard to find two more star-crossed lovers in movies: These two are cursed by a man in love with Isabeau (a never more beautiful Michelle Pfeiffer). By day, she is doomed to be a hawk, while by night Navarre (Rutger Hauer) is a wolf. The moment at sunrise when they almost touch in their human forms is haunting.

  • Rick and Rachael in ‘Blade Runner’

    He hunts replicants, she's a replicant! Future cop Rick Deckard (Harrison Ford) finds himself falling for Rachael (Sean Young), even though his job is to kill her, despite a pre-programmed, tragically short life. As the character who cynically gives them his blessing says, "She won't live, but then again, who does?"