Computing with time travel

December 9, 2015
If the universe allows 'open timelike curves', particles travelling back in time along them could help to perform currently intractable computations. Even though such curves don't allow for interaction with anything in the past, researchers writing in npj Quantum Information show there is a gain computational power as long as the time-travelling particle is entangled with one kept in the present. Credit: Adapted from npj Quantum Information, doi:10.1038/npjqi.2015.7 (2015)

Why send a message back in time, but lock it so that no one can ever read the contents? Because it may be the key to solving currently intractable problems. That's the claim of an international collaboration who have just published a paper in npj Quantum Information.

It turns out that an unopened message can be exceedingly useful. This is true if the experimenter entangles the message with some other system in the laboratory before sending it. Entanglement, a strange effect only possible in the realm of , creates correlations between the time-travelling message and the laboratory system. These correlations can fuel a quantum computation.

Around ten years ago researcher Dave Bacon, now at Google, showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped together as being hard.

The problem was, Bacon's quantum computer was travelling around 'closed timelike curves'. These are paths through the fabric of spacetime that loop back on themselves. General relativity allows such paths to exist through contortions in spacetime known as wormholes.

Physicists argue something must stop such opportunities arising because it would threaten 'causality' - in the classic example, someone could travel back in time and kill their grandfather, negating their own existence.

And it's not only family ties that are threatened. Breaking the causal flow of time has consequences for quantum physics too. Over the past two decades, researchers have shown that foundational principles of quantum physics break in the presence of closed timelike curves: you can beat the uncertainty principle, an inherent fuzziness of quantum properties, and the no-cloning theorem, which says quantum states can't be copied.

However, the new work shows that a quantum computer can solve insoluble problems even if it is travelling along "open timelike curves", which don't create causality problems. That's because they don't allow direct interaction with anything in the object's own past: the time travelling particles (or data they contain) never interact with themselves. Nevertheless, the strange quantum properties that permit "impossible" computations are left intact. "We avoid 'classical' paradoxes, like the grandfathers paradox, but you still get all these weird results," says Mile Gu, who led the work.

Gu is at the Centre for Quantum Technologies (CQT) at the National University of Singapore and Tsinghua University in Beijing. His eight other coauthors come from these institutions, the University of Oxford, UK, Australian National University in Canberra, the University of Queensland in St Lucia, Australia, and QKD Corp in Toronto, Canada.

"Whenever we present the idea, people say no way can this have an effect" says Jayne Thompson, a co-author at CQT. But it does: quantum particles sent on a timeloop could gain super computational power, even though the particles never interact with anything in the past. "The reason there is an effect is because some information is stored in the entangling correlations: this is what we're harnessing," Thompson says.

There is a caveat - not all physicists think that these open timeline curves are any more likely to be realisable in the physical universe than the closed ones. One argument against closed timelike curves is that no-one from the future has ever visited us. That argument, at least, doesn't apply to the open kind, because any messages from the future would be locked.

Explore further: Curves in spacetime violate Heisenberg's uncertainty principle

More information: Xiao Yuan et al. Replicating the benefits of Deutschian closed timelike curves without breaking causality, npj Quantum Information (2015). DOI: 10.1038/npjqi.2015.7

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LariAnn
2.3 / 5 (3) Dec 09, 2015
One argument against closed timelike curves is that no-one from the future has ever visited us.


Well, what would be the response if someone came to you and told you they were from the future and had something to tell you about? You'd probably route them to the nearest psychiatric facility, that's what! So my bet would be that even if someone visited us from the future, we'd never believe it, especially if they didn't arrive in a vehicle (a la Terminator).
baudrunner
2.3 / 5 (3) Dec 09, 2015
If the universe allows 'open timelike curves', particles travelling back in time along them could help to perform currently intractable computations.
Excuse me, but I laughed when I read that.

In the far-flung future, students will be knuckle-rapped for using "recursive mathematics".
Bongstar420
5 / 5 (1) Dec 09, 2015
Aren't the "barriers" to time travel more logical than actual measured barriers?
Lets hear about actual measurements that are actual barriers rather than suppositions by mathematicians.

I could just as easily look to Futurama to tell me about "time paradox" solutions
Bloodyorphan
3 / 5 (2) Dec 09, 2015
So the computer sits there and waits for parts of qubits to arrive, which where processed in the same computer and sent back through time therefore eliminating the need for the said computer to actually process the bits in the first place.

Virtual quantum computing takes on a whole new meaning.

This would change science forever, models would be invalidated without ever having to run any calculations, it would all be taken care of by the "virtual" wibbly wobbly timey wimey stuff.

Might as well just call it God and go home 8-)
ichisan
5 / 5 (3) Dec 09, 2015
I guess the intractable problem of time travel has already been solved while I was not looking. Physorg is quickly changing into a pseudoscience site.
Osiris1
4 / 5 (1) Dec 10, 2015
Causality may be a critter of comic book thinking....but even worse. It assumes only one time dimension, like a point in cartesian real space.

Suppose there are three temporal dimensions! Suppose the prohibition on reverse time travel is only enforced in quantum temporal mechanics if it is on a timeline vector function exactly congruent to the one defining YOUR past. Einstein DID say that the universe was not only strange, but stranger than many could imagine!

So you could have an infinite number of grandfathers on OTHER timelines, but they would not technically be YOUR grandfathers for they would not be back on YOUR timeline. Before you imagine crimes in other continuums, however, consider that those other existences on other timelines would have their own police and courts, and the real grandson in THAT line may not like YOU very well.
Spaced out Engineer
not rated yet Dec 11, 2015
Address is probably more of a problem if closed loop curves exist. Causality is not a problem in a multiverse scenario, but accurately making such a curve across such an expanse would take an enormous amount of energy and infeasible computation. Closed loop curves make sense from a traversal of shapeness and cosmological self referentiality for further complexity thesis, but it is still unknown if mathematics is used as a representation of reality or is reality.
antialias_physorg
5 / 5 (1) Dec 11, 2015
So the computer sits there and waits for parts of qubits to arrive, which where processed in the same computer and sent back through time therefore eliminating the need for the said computer to actually process the bits in the first place.

That would be the closed timeline curve. They are talking about open timelike curves here.

I guess the intractable problem of time travel has already been solved while I was not looking.

It's expressly statetd in the article that it hasn't. Just that there is currently no known theory that forbids it (this is not the same as saying "time travel must exist". It's like "playing to win" and "playing not to lose". Sounds the same but is a totally different startegy)
Bloodyorphan
3 / 5 (2) Dec 11, 2015
You forgot the entangled particle antialias, you still need to interpret a result otherwise the whole exercise is pointless isn't it ?
Colbourne
not rated yet Dec 14, 2015
I think you will find that if a person goes back in time killing their own grandfather would have no effect on them as they would be creating a new branch in the universe.
Everything from the point when they arrived in the past would be new and the resulting universe in their own (pre-existing future) time zone would no longer be relevant .
antialias_physorg
3 / 5 (2) Dec 14, 2015
You forgot the entangled particle antialias, you still need to interpret a result otherwise the whole exercise is pointless isn't it ?

The don't look at the state - only at the fact that there is a message (an entanglement in this case). There's no "interpretation of the result" happening, because that would be a closed timelike curve, then.
Bloodyorphan
3 / 5 (2) Dec 14, 2015
Please explain why an Open Time Curve gives us a supercomputer in that case.
antialias_physorg
3 / 5 (2) Dec 14, 2015
Check out the paper (it's linked in full at the end of the article). If you don't feel like slogging through the entire thing go to the last paragraphs of the results section where they give an example of how this works.
SuperThunder
1 / 5 (2) Dec 14, 2015
the no-cloning theorem, which says quantum states can't be copied.

Suits me, you'd never get me to step into a transporter anyway.

"That's because they don't allow direct interaction with anything in the object's own past: the time travelling particles (or data they contain) never interact with themselves. Nevertheless, the strange quantum properties that permit 'impossible' computations are left intact. "We avoid 'classical' paradoxes, like the grandfathers paradox, but you still get all these weird results," says Mile Gu, transporter technician first class, as Super Thunder walked from the transporter room still saying "nooope" oh his way to the shuttle bay.

Jokes aside, this is neat as all get out. I hope I (and we all) live to see quantum computers become dominant.

Bloodyorphan
3 / 5 (2) Dec 14, 2015
The only thing required for an open time curve is a displacement of spacial position, the qubits are still interpreted. just in a different physical location.

The causality problem is only caused by a qubit interacting with itself, not interpretation.

For this to work, you still have to manipulate and interpret the Qubit, otherwise the entire exercise is pointless.
SuperThunder
2.3 / 5 (3) Dec 14, 2015
The only thing required for an open time curve is a displacement of spacial position, the qubits are still interpreted. just in a different physical location.

Is that all it takes!? Why do we even have physicists?

There is a caveat - not all physicists think that these open timeline curves are any more likely to be realisable in the physical universe than the closed ones.

Didn't you hear the news, phycisists...!?

It's just a jump to the left.
And then a step to the right.
With your hand on your hips.
You bring your knees in tight.
But it's the pelvic thrust.
They really drive you insane.
Let's do the time warp again!

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