"Looper" stars Joseph Gordon-Levitt as Joe, a hitman who finds himself in the exceedingly awkward position of being contracted to kill his older self, played by Bruce Willis, the kind of dilemma made possible only by time travel.
"This time travel crap, just fries your brain like a egg," says Joe's boss, Abe, played by Jeff Daniels. Spend a little time talking to physicists and mathematicians about this stuff, and you quickly realize Abe is right.
First, it's important to understand that time travel is possible. People have done it (albeit only 1/48 of a second into the future), it's real. Being able to do it over any great distance is, however, at this point, a massive engineering problem.
And "Looper," like all great time-travel films, still bumps up against science.
You've likely seen by now (say, in the trailer above) that when Bruce Willis is sent back in time to be killed, he appears out of thin air in the middle of a cornfield. Talk to Princeton professor Richard Gott, author of "Time Travel in Einstein's Universe," and he'll tell you his problem with the scene isn't that Willis travels back in time.
"It looks like Bruce Willis just—POOF!—appears in the middle of this field, that would violate local energy conservation. Mass equals energy, so when he pops into field, suddenly you've violated energy conservation, by the weight of Bruce Willis, with E=mc squared—well that's quite a lot. That's a big atomic bomb of energy."
Matter can't simply appear out of thin air--if Willis had come zooming in via a DeLorean, Gott would find it more believable. "Looper" also violates a rule on which seemingly every time travel expert seems to agree.
"The earliest (in time) you can reach is the point the machine first started making time run slowly," said Brian Clegg, via email from his office in London, author of "How to Build a Time Machine."
"The only way to get around this is if an alien race built them before we were capable of doing so and left them lying around, rather like the monoliths in the '2001' movie," added Clegg.
That is to say, if the time machine were invented today, it wouldn’t be of any help in diverting what befell the Packers on Monday. And not just because there was no time machine back then, but because what happened happened.
"What has happened in the past is already passed, it can't be undone," says Prof. Thomas Roman, who co-wrote "Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space," along with Prof. Allen Everett.
Everett says that for travel back in time to work, it has to obey one of two rules, self-consistency or parallel universes, to avoid the Grandfather Paradox, which says you can't kill your own ancestors.
"If you go back in time and try to murder your grandfather, you slip on a banana peel as you go to pull the trigger and you miss. The laws of physics always work out in such a way that behavior is consistent," explains Everett. "That theory avoids the problem of paradoxes, but the problem is it's a little hard to imagine how the laws of physics manage to do that. That's what we call the consistency theory."
Everett prefers another theory, on which he's written extensively, but it's one that's no more satisfying if you're hoping to assassinate Hitler.
"The other idea for getting around it is the idea of parallel universes. If you go back and murder Hitler, you can do it all right, but you do it in a different universe. So in one universe you have a holocaust, and in the second universe you don’t have a holocaust. You haven't prevented anything."
Further, Everett's work has brought him to a far grizzlier conclusion.
"I was able to demonstrate it appeared probable that if you sent (a person) back in time, not only (would) it wind up in a different world, but the various pieces—the atoms—actually wound up in different worlds. The conclusion is that backwards time travel would be hazardous to your health."
While all these men agree that time travel forward is, to one degree or another, possible, some have serious doubts about going backwards.
UConn professor Ronald Mallett is perhaps the most outspoken proponent of the technology, a boyhood quest sparked by his father's death, one that he recounts in his book "Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality."
But even Mallett sees sending people back in time as being beyond his reach.
"One of the most important uses of a time machine, at least to me, is for use as an early warning device," said Mallett. "Imagine if you have a way you can send information back in the past, and warn ourselves of disasters, like Katrina… the thousands of lives you can save."
Mallett envisions a tabletop time machine that opens loops in space-time by rotating light, and sends a kind of Morse code, substituting dots and dashes with neutrons spinning clockwise or counter-clockwise.
But even if time travel is possible, and you manage to kill Hitler, and the scientists are wrong about self-consistency and splitting universes, are you sure it's the right move? What if Hitler were killed, and Stalin saw his chance to roll across a softened up Europe and push the Iron Curtain all the way to the shores of the Atlantic—would that be better?
"Looper" opens Friday, Sept. 28