A theoretical physicist at MIT shares what we actually know about toying with the arrow of time.
By Dave Mosher Posted 09.27.2012 at 1:05 pm 26 Comments

Looper's Time Machine Copyright of Looper, LLC, and courtesy of Sony Pictures

This weekend Looper, a gritty time-travel thriller from writer/director Rian Johnson (Brick, The Brothers Bloom) arrives in movie theaters across the United States. The story: Powerful crime organizations in the late 21st century can’t off their foes without getting caught, so they illegally send victims back in time to the year 2044 to be disposed of by hitmen called “loopers.” Looper's time travel, in short, is a futuristic version of the East River.

Though gunning down folks from the future is profitable, it comes at steep personal cost. Thirty years after Joe the looper (played by both Bruce Willis and Joseph Gordon-Levitt with some Bruce-Willis-like makeup) signs up, crime bosses nab him, send him into the past, and force his past self to kill his future self -- an attempt to close his “loop." This is the standard for loopers: live well, but live fast, and when your time comes, your younger self will be your killer. (Nothing could possibly go wrong with this scenario, nor introduce a series of mind-melting time travel paradoxes, right? Oh, wait.)

Moviemakers often turn to scientists to boost the plausibility of their plots, even when the plot involves a drug-addicted time-traveling hitman, but Johnson says he didn't consult with any researchers while making Looper.

“The science of real life time travel was less important than the science of using time travel in a story,” Johnson wrote in an email to Popular Science. “I studied other time travel movies to see how they put it at the service of the characters and story, and not the other way around.” Johnson has previously cited La Jetée as an influence, for example.

But the movie got us thinking about where the science of time travel stands. So we spoke with Edward Farhi, a theoretical physicist at MIT who has both seen the movie and intimately studied time travel.

INTO THE FUTURE

One of time travel's best-known facts is that going forward -- way, way forward -- is technically possible. Albert Einstein realized this as early as 1905, when he deduced the special theory of relativity.

"Einstein argued time is not an ever-flowing and immutable thing, but that an interval of time between events depends on the observer," Farhi says. In short, the faster you travel compared to some other object, the slower your clock ticks relative to the other object's.

For example, riding a rocket into space that's traveling close to light-speed -- then returning to Earth months or years later, according to your clock -- would elapse hundreds or even thousands of years on the planet. "You may meet your children's children's children's children and be younger than them, and that's weird," Farhi says. "But there's actually nothing paradoxical about this."

Einstein later developed the general theory of relativity, which argues intense gravitational fields are another way to skip ahead in time. Hanging around a black hole's point-of-no-return, where gravity warps space-time, the fabric of the universe, to an extreme degree, could make someone's clock run far slower than that of an observer near the gravity well's edge.

"If I hung you down there and you looked up at me, you'd see me in fast-motion and I'd see you in slow-motion," Farhi says. "That makes people wonder, could you build a gravity field strong enough to bend space-time to go backward? It's a pretty tempting idea."

Looper's future Joe meets the past Joe:  Photo by Alan Markfield, copyright of Looper, LLC, and courtesy of Sony Pictures

BACK TO THE PAST?

Looper doesn't propel characters into the future, except through cinematic montage sequences. But the movie does transport people into the past.

Farhi say most researchers discounted the possibility of backward time travel until the early 1990s, when astrophysicist J. Richard Gott published an intriguing study on theoretical warpings of spacetime called "closed timelike curves."

Gott solved for Einstein's equations in an imaginary 2-D universe to create the curves. Even accounting for the known laws of physics, the solution seemed to allow backward time travel.

Skeptical of this on-paper feat, Farhi and two other physicists then posed the question: what would it take to build this time machine?

The trio assumed they'd have infinite technology at their disposal, both to corral the necessary matter to warp 2-D space-time and fuel a spaceship to travel near the speed of light.

"In Gott's idealized universe, you could turn on the machine around [a dense piece of matter] and come back before you left," Farhi says.

After crunching their equations, the implausibility of the machine became clear. To warp space-time enough to create a closed timelike curve as well as power the spaceship would demand half of the fake universe's matter and energy. (Farhi noted that a similarly ridiculous requirement would also extend to such a machine in our own universe.)

That snag notwithstanding, theoretical physicist Gerard 't Hooft and his colleagues also pointed out that the extreme density would collapse the universe in a big crunch before the machine could begin to function.

"Einstein's theory of general relativity seems to conspire to end the universe before you're able to travel back in time and kill your grandfather before your parents were born," Farhi said. "This convinced me that traveling back in time is not possible."

Looper's Joseph Gordon-Levitt ("Joe") waits for a victim to execute.
Time to die:  Photo by Alan Markfield, copyright of Looper, LLC, and courtesy of Sony Pictures

TIME TRAVEL TODAY

Farhi has since moved on to study quantum computing, yet other ambitious researchers continue to probe the arrow of time. A group of researches in 2011, for example, suggested that quantum particles can be entangled across time, potentially allowing people to send data instantly into the future. Such a feat -- while impractical for now, as it may need to involve black holes to work -- might safeguard data important to humanity.

Another potentially helpful development is the proposed existence of space-time crystals. Such perpetually vibrating molecules might be key to the universe's most reliable clocks. Even in the coldest depths of space, which can halt atomic movement, magnetic forces within a properly assembled crystal would keep its atoms moving.

Most recently, physicists studying the decay of particles revealed that time's preferred direction -- forward -- really isn't a fluke.

Although Farhi's work in 1992 convinced him that paradoxes will always prevent backward time travel, contrary to Looper's worrisome depiction of the universe, he gave the possibility one last shot on May 7, 2005.

"The students [at MIT] decided to host a time traveler's convention. We advertised the event, put out milk and cookies for time travelers, and waited for some to show up. Maybe they were clever enough to remain incognito, but none came," Farhi says.

"So, as far as we know, reverse time travel can only exist in the movies."

Future Joe goes on a rampage:  Photo by Alan Markfield, copyright of Looper, LLC, and courtesy of Sony Pictures

26 Comments

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Robot
09/27/12 at 4:53 pm

I so much enjoy steak and eggs. Of course my comment has nothing to do with this artical, except it is the picture, as in this article has little to do with real science, except it talks about time travel in a fictional story.

Ok..... next!

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ghost
09/27/12 at 7:48 pm

NOT SO FAST ROBOT! this article is extremely scientific, in fact it goes deeper into it's subject "whether or not we can time travel like in this movie" than almost every other article on popsci!

as for the article: they are finally realizing that Einstein's equation (e=mc^2) works well for graphing energy and particles but maybe it doesn't quite account for time travel. the question i'm having is this; if a particle was moving backwards in time could we mistake it for moving forward in time simply because it has no way of us discerning which way it is in fact going?

say we have an electron moving in some direction at a speed of 2 centimeter's per second. now say that this electron suddenly jumped to the speed of light. remember the amount of energy required to go faster than the speed of light is not infinite but more accurately described as more energy in one spot than is physically possible. so the particle jumped towards the speed of light, maintaining it's speed until it expended the mass energy of one electron then it stopped moving completely. it would be in the past, in fact the electron would be able to see the reflections of itself before it even "jumped to light speed", the electron would get there before it left. now how would we view the electron's journey? would it exist as a blip, or would it more likely take the form of another particle that is going the opposite direction on a collision course with the original electron. this tachyon or the electron that's going backwards would have a negative energy but it's absolute energy would be equal to the mass and motion energy of the original electron. eventually the electron would meet the other at the exact moment that the electron "jumped to light speed" and annihilate both particles in the original reaction that caused more energy to be in one spot than is physically possible.

i'm thinking that the equation of the energy involved in this whole thing would look something like this: E=(MC^2)-|MC^2|+|MC^2|. the energy in order to move matter faster than the speed of light would be equal to double the mass energy and momentum energy of the original object. i could be wrong of course, I'm not a scientist, but i do think about these things... also it'd be pretty cool if i could send a particle back in time just by annihilating it with an anti particle.

to mars or bust!

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Robot
09/27/12 at 8:03 pm

Ghost,
Yea.... I do like a good science fiction movie too.
The more plausible delivery of the fantasy, the more fun the movie! ;)

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Robot
09/27/12 at 8:03 pm

Ghost,
Yea.... I do like a good science fiction movie too.
The more plausible delivery of the fantasy, the more fun the movie! ;)

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TeslasDisciple
09/28/12 at 9:58 am

Personally, I think if we ever travel back in time, we screw up the futur no matter what because of the butterfly efect.

Think about it, if we displace a pebble, that pebble might not be troddwen on by some poor whelp who slips on it, and he gets hit by a cart and dies, and his offspring is one of the people who invented timetravel, which means time itself gets mindf***ed, so we can't touch ANYTHING, not even the grass because we could probably screw the world over abou ten times before we even notice.

So basically, if time travel were possible, the problems arising would be WAY too big.

Get your facts first, then you can distort them as you please.
Mark Twain

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Robot
09/28/12 at 11:12 am

Real violence is bad. Pretending to be violent though is profitable and can make you famous too. And if it works well in one time, why not travel many times and span generations to make more opportunity.

GO Bruce Willis GURRRR!

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Oakspar77777
09/28/12 at 12:44 pm

Any time dialation is purely speculative - as we have never traveled at light speeds.

Perceptive time shift is, however, valid.

Lets say I travel 50 light years into space and look in my rearview mirror at Earth. My calander says 2100, but the Earth I am seeing is from 2050, because I am seeing light, and light travels one light year per year (duh).

Have I traveled back in time? No, but since I left Earth in 2025, time seemed to move at 1/3 my time, until today, 2100 where I am 50 years ahead of them (to my perception).

So, I go home, and get there in 2175 according to my calander - and their's. As I aproached home, their time would seem to speed up as the time delay in light shortened.

Thus, I could percieve that time stops in a light speed ship - but it is perception of them - just like I percieve the sound of a car changing pitch as it moves past me.

There is no evidence that time is not a constant force in the universe extending in one direction.

Still, such theories are fun to play with on the chalk board.

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Brian P.
09/28/12 at 2:06 pm

If anyone in the future reads this, please come get me. This time sucks.

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Dvmx
09/28/12 at 3:39 pm

Not sure about the grandfather paradox thing. After Everett's many worlds theory, if one goes back in time and interferes, one would be launching a different timeline, a parallel timeline.
And there we would have a different problem: having launched a new timeline, is one a part of that new timeline with no ability to "get back" to the one you came from?

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HBillyRufus
09/28/12 at 3:42 pm

I like ghost's ideas. But instead of thinking about sending a particle back in time by annihilating it with an anti-particle, you should think about it as the particle and the anti-particle being the same particle, one is the particle travelling forward in time and the other is the same particle going backward, and the energy released by annihilation is where it has reversed its time direction. Observers would see two particles colliding and disappearing in a big energy release. The problem is that we should be seeing a big absorption of energy instead of a release. Similarly if a backwards-in-time-travelling particle reversed its course and started going forward, observers would see a particle and an anti-particle spontaneously appear. And two particles appearing from nowhere and then colliding and disappearing would represent one particle travelling in a "time circle".

Taken to its extreme, the creation of the entire universe (where something came from nothing) could be explained by a whole lot of backward-in-time-travelling particles reversing course and going forward.

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Kromm
09/28/12 at 4:39 pm

The time travel paradox occurs when you have a concept of cause and effect. Abandon the concept of free will it is easy to consider a time traveler going backward in time and (often unwittingly) fulfilling a predetermined outcome. This often occurs in literature as the "twist" at the novel's end.

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berk99
09/28/12 at 9:29 pm

Didn't watch the movie but couldn't they have saved money by not hiring Loopers and just inject the guy with poison and quickly send them back? Just sayin'
or materialize them mid-ocean, or remote south pole, or over an active volcano, or....etc etc etc.

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TheRealArchAngel
09/29/12 at 8:15 am

"Fate" is actually a hard thing to change

Do anything to disrupt the possibility of your original reality,and you in it, and the event simply doesn't happen ("parodoxal reset") from the starting point's perspective but the Reality changer end's up in a alternate reality from that point on

For instance all attempts on Hitler's life failed keeping the current overall continuium,and the events that did result in Hitler's death caused a "parodoxal reset" from our perspective while the Assasin became a part of a alternate reality

Likewise, "killing your younger self" is possible but you will then be in a alternate reality where you are there but you "don't exist" any more and can no longer get back to the reality you came from (easiest thing would be to get rid of body and assume your own Identity..maybe fix the mistakes that made you want to kill yourself)..while back in your original reality you walked through a time door and never came back

Retired "grunt" scientist (don't guess,test it)

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Tigerflash
09/30/12 at 6:43 am

@Ghost

If a particle did accelerate to the speed of light and per chance stop at a point/time prior to its departure it would encounter a mass spatial event that would create a time eddy... In this event wouldn't it be likely to create a looping path for the particle that would in fact cause it to recurrently consume energy and expend mass particle deterioration resulting in a literal collapse of matter in space/time?

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redenegue
09/30/12 at 11:51 am

There is only one scenario where you could go back in time and kill your father, and that is if all of time is turned back. Not Possible! Otherwise it is a stupid idea to think that there is some paradox. There must be a place to go to and that place is separate from the place of origin. If you went back and killed your father, then returned to your own time, your father and you would still be alive. There would be a dead father, in the past chasing you into the future, but your time and the other father's would never meet. In a similar fashion, if you went to the past and removed something, and brought it to the future, it would be missing in the past and duplicated in the future. Thus conserving the mass and energy. If you went to the past and invested money, you would never see it in the future. Time in this version flows, and is in small incremental pieces. There are virtually an infinite number of pieces. Billions of people could be time traveling and you would never know it! The odds of someone landing on your specific space would be on the order 1 out of infinity. It is really obvious that a lot of people have not thought this out a little clearer. The key to this is simple, if this backward time travel is possible, time is in increments, time flows and the past and future coexist.

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GregN913
09/30/12 at 10:56 pm

The largest mistake is in thinking that time is linear. Sometimes to go backwards, you must go inwards.
Since I've been spouting this one off quite a bit and MOST other Earths would have figured this one out here it goes:
There are fractal-symmetrical sub-atomic particles that capture all physical input that goes into them in the form of an anti-fractal analogue compression wave. Not exactly like a little black hole, but close enough as you can only go in to it. Want to come back?
Thus, there are exact transcriptions of the "past" and the Earth as it is now, is another transcription of an even larger reiteration. That is, to go into the future, you must travel outwards at such a rate as to be immune to the forces that are shaping the Earth currently. To go backwards into the past, you must find a particle that has transcribed the present.
Nevermind the details, your physical form cannot make the trip. In either case it would appear that you need to travel faster than the speed of "light" to jump the reiterative barrier, but more than that you need to lose mass rather quickly (and intelligently) to obtain the correct transcription rate. With any luck, the redundant "safety net" keeps things on track. Without luck and you've doomed a infinite versions of yourself to life in oblivion where the universal constants just aren't the same...
Since it seems as if just any lunatic can have some say to how time travel works thought I'd just post it here in this SCI-FI forum. Cheers!

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African Rover
10/04/12 at 5:44 am

We recently read about this misunderstanding in Italy when particles from the hadron collider were thought to travel faster then light. That meant they arrived a little earlier than they were supposed to arrive, they did not arrive before they were sent. So going back in time by going very fast is really out. In my ( limited ) understanding time is a one way trip.

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theblackmist
10/04/12 at 11:03 pm

Why do I get a strange feeling that both being able to time travel backwards, and "freeze time" around you, are both just as possible as going forward in time...

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the cookie wookiee
12/29/12 at 2:34 pm

NO! It's the Alfred Bester theory. Time is a collection of filaments all pointed in the same direction, like a package of uncooked spaghetti. Each strand is the timeline of one individual. If you go back and kill George Washington, all you do is sever your own filament. Each butterfly effect you cause decreases the chance that you ever exist in someone else's "now". Eventually the odds catch up with you and you are no longer the pasta, but the sauce, able to visit any strand at any point along it, but not to be present in it or in any way affect it. In short, screw with time enough and you doom yourself to an eternity of watching, without being able to ever do anything about that which you observe. In even shorter, Hell.

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kippysparklebee
01/05/13 at 1:38 pm

Just saw Looper. I particularly like the cookie wookiee's post and have given alot of thought to the Alfred Bester theory.

If you travel physically or virtually beyond the speed of light (or whatever) and time slows down for you so that your "filament" is altered, then you might return to a point in the future but under what scenario would it be possible to return to a point in the past?

Not sure if I agree that you would only be an observer--why would that be? It seems like you would just exist in another time. Time itself wouldn't be altered, only your place in it.

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the cookie wookiee
01/12/13 at 6:59 pm

Well, an alternate explanation is that you would fade partially, depending on how unlikely it is for you to still exist. If its only 50%, then you could only half interact with matter, or antimatter or anything like that, due to time's attempt to resolve the paradox. Rather than an eternity of a consciousness without form, your consciousness would inhabit an ethereal form which could only only partially interact: a ghost.

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kippysparklebee
01/17/13 at 4:14 pm

the cookie wookie, you have just tied this to my thoughts on ghosts very neatly. Thank you!

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the cookie wookiee
02/05/13 at 2:58 pm

you're welcome.

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tioray
02/07/13 at 1:05 am

Oakspar77777 don't we see evidence of time dilation very day with satellite clocks having to be re-calibrated due to their speed in relation to all of those people living on the crust of the earth? Just as this article suggests, its accepted science that time dilation is caused by differences in either gravity or relative velocity.



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