photons simulate time travel in the lab - physicsworld

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Photons simulate time travel in the labFeb 5, 2015 5 comments

Caught in a loop: has time travel been simulated?

Physicists in Australia claim to have simulated time travel using fairly standard optical

equipment on a lab bench. They say they have prepared photons that behave as if they are

travelling along short cuts in space–time known as "closed time-like curves", and add that their

work might help in the long-sought-after unification of quantum mechanics and gravity. Others,

however, argue that the research does little or nothing to establish whether time travel is

possible in nature.

Although everyday experience suggests the impossibility of travelling backwards or forwards in

time, Einstein's general theory of relativity does not rule it out. The theory allows for loops in

space–time called closed time-like curves that could be created by very powerful sources of

gravity such as black holes. These structures would bring an object back to a place and a time

that it had already passed through, typically via a short cut between the two separated regions

of space–time known as a wormhole.

Grandfather clause

In classical physics the existence of closed time-like curves would lead to a number of

paradoxes. One of the best known of these is the grandfather paradox, in which someone who

has travelled backwards in time kills their grandfather while he is still young, thereby preventing

their own birth. In quantum mechanics, however, such paradoxes can be avoided.

The quantum-mechanical equivalent of the grandfather paradox involves a subatomic particle

that has two states – one and zero – corresponding to "alive" and "dead". The paradox

emerges if the particle started out in state one, travelled backwards in time, met a younger

version of itself and then flipped the value of its earlier self to zero.

But in 1991 David Deutsch of Oxford University showed that the probabilistic nature of

quantum mechanics comes to the rescue. Deutsch found that there would always be a state

that a quantum particle could assume that would make the particle's trip back in time a safe

one. For example, if the particle were to start out in an equal mixture of one and zero, when

flipped it would remain in that state – a 50:50 mixture of one and zero.

Disappearing down a wormhole

In the latest work, Martin Ringbauer and colleagues at the University of Queensland in Brisbane

set out to reproduce Deutsch's model in the laboratory. But given the absence of any real

closed time-like curves in the vicinity of their lab, they were not able to directly study the

interaction between younger and older versions of the same quantum particle. Instead, they

used two separate particles. The idea is that the "younger" particle remains in normal

space–time, while the "older" one disappears down a simulated wormhole, reappears in the

"past" and then interacts with its junior partner.

To implement their scheme, the team generated pairs of single photons by shining a laser

beam through a nonlinear crystal. The younger photon was encoded by polarizing it – with

Photons simulate time travel in the lab - physicsworld.com http://physicsworld.com/cws/article/news/2015/feb/05/photons-simulat...

1 of 4 07/02/2015 21:52

1 duwayneaFeb 5, 2015 10:22 PMHillsboro, United States

5 commentsAdd your comments on this article

From the article: "Although everyday experience suggests the impossibility of travelling backwards or forwards intime,..."

Well, just this morning I traveled forward in time. When I got up it was 5:00 am, and now it's 2:13, so I traveledforward in time by 9 hours and 13 minutes. I expect to travel forward in time again, and leave work, go home and

horizontal polarization representing zero, vertical polarization representing one and

intermediate polarization representing superpositions. That photon then interfered with its older

partner in a beamsplitter, and the outcome was recorded by a pair of detectors.

Consistency condition

One of these detectors constitutes the entrance to the "wormhole" and is used to record the

state of the older photon to ensure that is in the same state as it is at the beginning of the

experiment – the point at which it emerges from the wormhole. In this way, the scheme satisfies

the "consistency condition" that Deutsch imposed on his model to remove the paradoxes from

time travel – that whatever goes into a wormhole emerges from it unchanged.

Encoding the younger photon arbitrarily with one of 32 different polarizations and fixing the

state of the older photon to satisfy the consistency condition, the researchers showed that they

could indeed meet this condition. They also found that the presence of a closed time-like curve

allows an observer to perfectly distinguish non-orthogonal states of the time-travelling photon,

such as horizontal and diagonal polarizations. This is something that cannot normally be done

in quantum-mechanical systems.

Encryption buster

According to project leader Tim Ralph, this result suggests a way to break quantum encryption,

since any eavesdropper with access to a closed time-like curve would in principle be able to

make a perfect copy of the secret key and so avoid revealing his or her presence via quantum

measurements. More broadly, he says, the research could provide an insight into the tension

between quantum mechanics and general relativity, given that closed time-like curves are only

possible with strong gravitational curvature.

Todd Brun of the University of Southern California describes the work as a "very nice

experimental demonstration of some of the bizarre consequences" of Deutsch's model,

although he says that the research is not able to test the model itself. Others, however, are

more critical.

Entirely predictable

Charles Bennett of IBM says that the results from the experiment are "entirely predictable from

well-established principles of quantum optics" and that what is instead needed is "continued

theoretical exploration of closed time-like curves' consistency with, and consequences for,

other parts of physics". He also believes that the experimental set-up "does not function as a

mechanism for reliably distinguishing non-orthogonal states". This is a view shared by Antoni

Wójcik of Adam Mickiewicz University in Poland, who says that the experiment "provides very

interesting confirmation of standard quantum mechanics but "does not answer any question"

concerning time-travelling quantum particles.

Also critical is Seth Lloyd of the Massachusetts Institute of Technology, who has developed a

rival model to Deutsch's. He points out that in the latest experiment there is no physical

connection between what comes out of and goes into the wormhole, and that as a result the

wormhole's output has to be classically computed and then manufactured. "This defeats the

purpose of quantum simulation," he says, "which is to predict what one can't simulate

classically."

The research was first described in Nature Communications and the paper is now available on

the arXiv preprint server.

About the authorEdwin Cartlidge is a science writer based in Rome


2 of 4 07/02/2015 21:52

2 skynrFeb 6, 2015 5:29 AM

3 M. AsgharFeb 6, 2015 11:33 AM

4 AntiGravFeb 6, 2015 9:44 PM

eat dinner. Then I'll travel forward once more, go to bed, arise tomorrow morning, and repeat the experiment.

Obviously, "everyday experience" certainly *does* suggest the very real possibility of moving "forward in time." Wedo it all the time.

With that said, I think it would be extremely helpful for someone to define exactly what moving backward in timewould involve. How does mass/energy conservation and information theory fit into the scenario of time travel? If atime traveler moves backward in time, do they retain the memories of the future? Does future information contenttravel to the past with them? If so, what does this mean for entropy, esp. as it relates to information theory?

To duwaynea,

There is a definite difference between going forward 'in' time and going forward 'with' time. Please check youranalytic wording and thus any conclusion from it before presenting any hypotheses. Thanks.

GR, QM-systems and time coordinate

Basically the QM-based equations represeted by a wavefunction, are symmetric as to the forwards and thebackwards changes in the time coordinate. In this case, there should be no fixed entropy, hence, no information.However, when one observes/ tests the system, it breaks down and this "irreversible" breaking down increass itsentropy and creates information and the forwards arrow of time. Hence, the possibilty of going back in time inlocal-classical GR via the closed spacetime curves, has to be a part of its inherent limitations, but a good story forthe scientific literature.

Your construct is gone...

You don't move forward in time. You move forward "with" time. You never move faster than the events that haventtaken place yet. Even in a time dilation event, time doesnt speed up just because you want it to. You are simplymoving faster in normal space-time so time "appears" to go by faster.

However, the "past" is a snapshot of the construct of space-time as it was at a particular previous moment in time.In order to go back to a past date in the construct, the construct would have to be spun backwards to how it was inthat previous moment in time.

There is no "past" to go to unless you have memorized the positions of the all the elements in the universe andthen sent them backwards along their paths that they have already taken. That would mean that you would haveto "undecay" elements that have decayed, move massive energies and gravity waves, planets, galaxies, etc., allbackwards. Once you move the universe backwards and have all the elements line up to where they were... thenyou have a place to go in the past. Until you can Tivo the universe you can not go backwards. In fact the Earth isnot in the same place it was 100 years ago. Neither is the solar system or the galaxy. They have all moved farfrom the place they were even 10 minutes ago. Remember that your time machine would not only have to have a"time" to go to but also a "point" in that time. That physical point would not contain the matter that you rememberbeing there as it has long been moving inside a constantly moving universe.

Entangling the universe and spinning its entanlged matter back along a previously taken path IS THE ONLY WAYto go back in time. Even then, entangled elements that decayed would not be able to be considered. Even if youlocalized a square mile area of the universe and entangled it, the rest of the universe would not be in the sameplace that was attached to that square mile. So I imagine the explosion of space-time snapping back to flood overthe missing square mile (in past and future) would be nothing short of "supernova". What would happen if youremoved a square mile of water from a mile deep ocean? The surrounding water would absolutely crush the openspace and send shockwaves through the ocean. Same would happen in space.

Time travel to the past is not possible and travel into the future is simply time dilation. The future is not movingfaster just because you are.

Einsteins "loop" only works until the closed loops "space-time" is considered. When the loops feedback isintroduced back into the same point in space, the origin (where it was ripped from) violently explodes.Consequently, when it arrives at its destination it is most likely arriving to the empty space where it was in thepast... but the past wont be there.

Edited by AntiGrav on Feb 6, 2015 9:59 PM.


3 of 4 07/02/2015 21:52

5 AntiGravFeb 7, 2015 3:11 PM

Would your old you and new you be entangled?

I think however, since we like thought experiments on impossible things, that an interesting possibility might bethat "if" you could go back and see your old you, would you be entangled? Perhaps simply going back in timeresets your current you into the state of the old you upon arrival. I simply say this because even if infinitedimension theory allows you to travel into a slighltly time-delayed dimension, just like this one, that again naturemight trump the possiblity of existing with identical copies of your atoms in another location. So, simply going backmight just cause a time loop. You get to the past but you lose your memory and then eventually go into the timemachine over and over again in a loop. Nature just put you in time-prison. What happens to time then? Does yourtime simply loop or does everyone around you loop too? No travel backwards people.


4 of 4 07/02/2015 21:52

photons simulate time travel in the lab - physicsworld

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