Physics

Quantum time travel doesn't follow Back to the Future rules

Quantum time travel doesn't fo...
A "quantum time travel simulator" suggests that the butterfly effect doesn't apply to the quantum realm
A "quantum time travel simulator" suggests that the butterfly effect doesn't apply to the quantum realm
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A "quantum time travel simulator" suggests that the butterfly effect doesn't apply to the quantum realm
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A "quantum time travel simulator" suggests that the butterfly effect doesn't apply to the quantum realm
In this diagram, Bob's interference with the qubit in the deep past doesn't affect Alice's ability to read it in the future
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In this diagram, Bob's interference with the qubit in the deep past doesn't affect Alice's ability to read it in the future

Time travel movies have different rules about what happens when you start messing around with the timeline. If you’ve ever wondered which ones make the most sense, we may now have an answer. According to experiments using a quantum time travel simulator, reality is more or less “self-healing,” so changes made to the past won’t drastically alter the future you came from – at least, in the quantum realm.

The classic Back to the Future rules of time travel say that whatever you change in the past can have huge effects on the future. That’s why Marty McFly can almost erase his own existence by accidentally stopping his parents from meeting, and why Biff Tannen can get rich by giving his younger self a book of sports scores to bet on.

Other movies handle things differently. In Avengers: Endgame, the superheroes travel back in time to steal versions of the Infinity Stones out of different time periods to revive their fallen friends (look, it doesn’t make much sense unless you’ve seen all 20-something movies). Anyway, they can dabble in the past without ruining the future because the universe has a knack for correcting those paradoxes so that both versions of events did happen.

But which set of rules has more basis in science? According to a new study, quantum mechanics supports the Avengers model.

In a way, it all comes down to the butterfly effect, where a tiny action in one system can escalate into huge consequences. The name comes from the principle of chaos theory that a butterfly flapping its wings could set off a cascade of events that causes a hurricane in another part of the world.

In terms of time travel though, the term might have originated in reference to Ray Bradbury’s 1952 sci-fi story, A Sound of Thunder. In this story, time tourism is readily available, but carefully controlled so as not to affect the present. While on one of these trips into the distant past, a character steps off the beaten track and accidentally kills a butterfly, and when he returns to his own time, things have dramatically changed.

So how can we test these ideas? Since we can’t exactly jump into a TARDIS and start experimenting ourselves, researchers at Los Alamos National Laboratory used a quantum computer to develop a simulation of time travel. And their results were somewhat surprising.

“On a quantum computer, there is no problem simulating opposite-in-time evolution, or simulating running a process backwards into the past,” says Nikolai Sinitsyn, co-author of the study. “So we can actually see what happens with a complex quantum world if we travel back in time, add small damage, and return. We found that our world survives, which means there’s no butterfly effect in quantum mechanics.”

Using an IBM-Q quantum processor, the team created a complex system using quantum gates and demonstrating cause and effect, running both forwards and backwards in time. The simulation involved two hypothetical people, Alice and Bob, who each have a qubit – a quantum bit of information.

In the scenario, Alice prepares her qubit in the present, then sends it backwards in time. At some point in the past, Bob interferes with the qubit by measuring it. Then, the system is run forwards again to the present time, and Alice checks her qubit.

What you think “should” happen next depends on which time travel rules you subscribe to. The butterfly effect says that because the qubit is tied to so many variables, Bob’s small interference should completely change the system by the time we get back to the future (or present, to Alice).

In this diagram, Bob's interference with the qubit in the deep past doesn't affect Alice's ability to read it in the future
In this diagram, Bob's interference with the qubit in the deep past doesn't affect Alice's ability to read it in the future

But the team found that that wasn’t the case. Alice’s qubit comes back relatively unscathed, and she can recover the information on it. Interestingly, the fact that it’s tied to so many variables seems to be what actually saves it from damage – the information in the present qubit was hidden in the quantum correlations in the deep past. This web of connections isn’t so easily disturbed by Bob’s amateur efforts at timeline vandalism.

Stranger still, the further back in time the qubit travels, and the more complicated the whole system is, the less damage the qubit takes from interference. That might seem counterintuitive – since there are more things for the butterfly’s wings to knock over, so logically the effects should be more dramatic. But by the team’s reasoning, this just creates a stronger web of quantum correlations to protect the qubit from damage.

“We found that the notion of chaos in classical physics and in quantum mechanics must be understood differently,” says Sinitsyn.

If we want to take these findings to their logical conclusion, the argument could be made that Back to the Future represents time travel through classical physics, while Avengers: Endgame is a model of quantum time travel. Intriguingly, the latter movie justifies time travel by having the characters manipulate the “quantum realm.”

While the study may mostly seem like just a fun thought experiment, the team says that the simulation could have some real-world benefits. For one, since there’s no way a classical processor can handle this kind of simulation, it could be used as a way of testing whether a quantum computer is actually working on quantum principles.

The team also says that the concept could be used to create new security protocols for information in quantum systems. If a more sinister real-world Bob tries to mess with the qubits, the system can convert the information into a strongly entangled state to protect it.

“We found that even if an intruder performs state-damaging measurements on the strongly entangled state, we still can easily recover the useful information because this damage is not magnified by a decoding process,” says Bin Yan, co-author of the study. “This justifies talks about creating quantum hardware that will be used to hide information.”

The research was published in the journal Physical Review Letters.

Source: Los Alamos National Laboratory

18 comments
Brian M
Sort of makes sense - But does make a presumption that time is a travelable dimension (as we can and do in 3 dimensions). Unfortunately we really don't understand the nature of what time is or if it actually even exists other than in our own perceived concept of a historical timeline and relativity.

paul314
This seems to assume almost exactly what it claims to prove. In the experiment, the single Qbit gets changed, but nothing else does, and all the other entanglements essentially resurrect the Qbit (if I understand correctly). But in a more realistic model it seems to me that the changes made in that Qbit (or anything else) would propagate outward and change the rest of the system?
FB36
"reality is more or less “self-healing,” so changes made to the past won’t drastically alter the future you came from – at least, in the quantum realm."

IMHO, concept of spacetime in macro scale/world is an emergent property!
Just like concept of water/fluid (which only exists in macro scale/world & NOT in micro (atomic/molecular) scale/world)!

Would we expect removing a water molecule, could really make any noticeable difference in macro scale structure/movements of any piece of water/fluid?
(I think the answer is no & it can be easily proven w/ any computer fluid simulations!)

But, still, realize, we already know/proven that Butterfly Effect is quite real & powerful for any fluids/weather!

So, IMHO, this result does NOT really prove that time travel into past can be harmless!

(By the way, IMHO, spacetime is a superfluid created by gas-like dynamics of virtual particles of quantum vacuum!)
James Macaluso
To speculate, could it also be possible that a variable manipulated further in the past would simply have additional variables to affect along its now seperate time frame before we could register changes from our point of view of temporal incusion. To use the grandfather paradox to simplify; murdering grandfather 70 years ago and returning to the present wondering why I still exist... I now would require an additional 70 years before any of the results of my past meddling became apparent to myself since those altered elements now exist in a temporal vacuum and must ripple forward to affect all other variables. In short I propose that the results in the present would not be immediately apparent as we have now created an alternate time stream of cause and effect assuming a fluid singular universe model.
JeffK
There is a joke that asks "Do you know why God created time?" The correct response is "So everything doesn't happen at once!" Seems to me that makes about as much sense as anything else. I only hope that none of my tax money paid for this experiemnt.
christopher
So much for free will it seems...

@Brian - backwards time is a travelable dimension (experiments have succeed in changing changing the past).
Douglas Bennett Rogers
Maybe there are five extended dimensions, length, width, height, time, and probability. An entity perceives a volume, (dx, dy, dz, dt). The entity acts on the volume, producing an array. At time, t2, the entity picks an element of the array, which becomes the basis of the entity's universe. All of the other elements still exist and fulfill the requirements of past history alteration.
itsmeagain
A confusion I have about time travel to the past that is exemplified by the 'kill my grandfather' paradox is as follows: Suppose I travel back in time, taking a gun with me, and shoot and kill my grandfather a year before my father is conceived. That gun is made of a variety of atoms, mostly iron, that existed at that previous time. The iron atoms were probably in some magnetite iron ore on a mountainside at that time. So now all the molecules in my gun are at two places at once. This also applies to all the atoms in my body and in my time machine and anything else a took back in time with me. How does that work? Forget about my act of grandpapacide, won't this happen for any travel back in time, i. e., the instantaneous duplication of all the atoms going back in time? Has Calvin (of Calvin and Hobbes fame) figured this out?
Ralf Biernacki
@James: but by the time the disturbance has propagated the 70 years forward and reached the present, the present has moved---you are now another 70 years forward. In other words, if things work as you assume, the past will be rolling over, but the changes will never catch up with you---the "front" will always be 70 years in the past. That's very interesting: in a world that works like that, you can travel to the past and change it, but to experience the results of the change you must stay in the past. If you return, nothing will have changed for you. But if you travel into the past again, you will land in the altered past. What if someone else changes it in the meantime? It could end up so that if you travel to the past, you will find that it has no resemblance to the past that you expect, as recorded in history books---it is a completely different timeline resulting from many interventions. And the farther back you go, the more numerous the differences.
James Macaluso
@Ralf

What you propose is also very fascinating. I would never presume to be an expert on such things, I just enjoy thinking of such things. One place where I believe you missunderstood my supposition is that I do believe the future would be altered by our traveling to the past; however, I think not in the manner depicted in the movies.

For another example, I travel back in time 5 minutes and move my pen from my desk to the refrigerator. Then I return to the present to see my pen still on my desk. In another 5 minutes it will move to the refrigerator once the altered time stream catches up to me.

Now if multiple people begin time traveling and altering the past, then yes I believe any past we jump into would become quite chatoic and possibly unfamiliar, if I understand commenter Rogers postulation, as there would not only be our original 4th to contend with but the 4¹ that we altered as well as all others. I believe that each change to the past creates a mobile ripple of changes that eventually over writes the present.

On a completely different note, would we not be bringing diseases we are immune to into the past to a civilization unprepared for them and possibly wiping out massive swaths of life forms? Would our travel a great distance into the future be equally as dangerous to ourselves personally?