Can we reverse time to before this hypefest started?

The purpose of this post is mostly just to signal-boost Konstantin Kakaes’s article in MIT Technology Review, entitled “No, scientists didn’t just ‘reverse time’ with a quantum computer.” The title pretty much says it all—but if you want more, you should read the piece, which includes the following droll quote from some guy calling himself “Director of the Quantum Information Center at the University of Texas at Austin”:

If you’re simulating a time-reversible process on your computer, then you can ‘reverse the direction of time’ by simply reversing the direction of your simulation. From a quick look at the paper, I confess that I didn’t understand how this becomes more profound if the simulation is being done on IBM’s quantum computer.

Incredibly, the time-reversal claim has now gotten uncritical attention in Newsweek, Discover, Cosmopolitan, my Facebook feed, and elsewhere—hence this blog post, which has basically no content except “the claim to have ‘reversed time,’ by running a simulation backwards, is exactly as true and as earth-shattering as a layperson might think it is.”

If there’s anything interesting here, I suppose it’s just that “scientists use a quantum computer to reverse time” is one of the purest examples I’ve ever seen of a scientific claim that basically amounts to a mind-virus or meme optimized for sharing on social media—discarding all nontrivial “science payload” as irrelevant to its propagation.

35 Responses to “Can we reverse time to before this hypefest started?”

  1. Ori Vandewalle Says:

    I think the issue is that “scientists use quantum computer to…” can be followed by pretty much anything, and the average person will accept it (either uncritically or as “those crazy scientists”) because they don’t have a good concept of what to expect from quantum computers.

  2. Sanketh Says:

    I am surprised that this headline caught on more than Wired’s “Quantum computers could be the ultimate defence against the next global financial crisis.”

    I blame Springer Nature for all this hype. Over the years we have come to associate Nature papers with high quality, and now with all these Nature-branded second-rate journals, they are confusing the media. This issue is seen in this article and in Wired article.

  3. Domotor Palvolgyi Says:

    How about scientists play tetris on quantum computer?

  4. Shmi Says:

    The salient passage in the paper seems to be

    An entangled two-particle state with a non-separable phase function can naturally emerge as a result of scattering of two localized wave-packets. However, as we have seen, the generation of the time-reversed state, where a particle gets disentangled in the course of its forward time evolution, requires specific two-particle operations which, in general, cannot be reduced to a simple two-particle scattering.

    The above consideration enables us to formulate important conjectures about the origin of the arrow of time: (i) For the time reversal one needs a supersystem manipulating the system in question. In the most of the cases, such a supersystem cannot spontaneously emerge in nature. (ii) Even if such a supersystem would emerge for some specific situation, the corresponding spontaneous time reversal typically requires times exceeding the universe lifetime.

    Their claim to fame is that “such a supersystem cannot spontaneously emerge in nature”. What distinguishes their “time-reversal” system from a garden-variety “supersystem” that can “spontaneously emerge in nature”? What are examples of such “spontaneously emerging” systems that are in some ways very much like transforming a wave function into its conjugate, but are so much more ubiquitous? These are the kind of questions I could not find answers to in the paper.

  5. Anonymous Says:

    Wow. This post is just asking for a link to the “hacking time” scene from the hilarious 1980s-inspired short film Kung Fury:

  6. Gil Kalai Says:

    I beg to disagree with the opinion and sentiment of Kakaes’s article and this post. The paper by Lesovik, Sadovskyy, Suslov, Lebedev and Vinokur does seem interesting.

    Understanding the possibilities and limitations of small quantum circuits with 5-30 qubits of the kind IBM (and other companies and academic groups) build is a very important part of the experimental efforts regarding quantum computers. In this context, experimentation with the IBM machines of how various computational sequences can (or cannot) be reversed is a very natural direction. In my opinion, a problem of Konstantin’s article and this post, may lie in a certain blasé view of 20 qubit quantum circuits. (“Those, have already been built. Of course we can compute with them, forward and backward. Lets move to 72, 100, and more qubits…!”) This view seems far from the experimental reality. Thus, reversing computational sequences (or “the arrow of time”) is among the various interesting ways to test existing small quantum circuits. (Of course, it is also perfectly OK to build larger circuits.)

    More ambitiously, it is quite possible that NISQ devices exhibit some systematic bias regarding the arrow of time for certain quantum evolutions, and it is interesting to study this possibility.

    Of course, if 20 qubits circuits cannot be improved at all to the quality needed (for larger circuits) for quantum supremacy and quantum error correction, then understanding which evolutions on NISQ devices could be time-reversed is even more interesting. (And I discussed it in various places.) But the paper by Lesovik et al. seems interesting anyway, and this scientific interest does not rely on fantasies regarding time-machines etc.

  7. Craig Gidney Says:

    This particular species of article, where researchers simulate or execute a basic quantum circuit and then give a ridiculous interpretation of what they did, is unfortunately common. I find it particularly frustrating how the outcome of experiment is often framed as surprising and worldview-overturning, when the statistics were trivially predicted ahead of time.

    From what I can tell skimming the paper, what the researchers did in this case is pick operations H and G such that H*G*H^-1*G^-1 = I, then executed H*G*H^-1*G^-1 on some test state. You can do the same thing on a classical machine by setting H to ‘a += b’ and G to ‘b *= -1’.

    I wonder how long it will be until someone 1) executes the circuit from Hardy’s paradox (which has very few qubits and very few gates, so you could run it on a NISQ machine), 2) confirms that of course the results match the predictions of quantum mechanics, 3) frames the result in terms of Frauchiger et al’s variant, and 4) journalists telephone-game it into something like “quantum researchers prove decisions aren’t real”.

  8. Scott Says:

    Gil #6: My impression is that many, many experiments on small numbers of qubits had already demonstrated reversing a unitary transformation and getting back to the initial state long before this one—indeed, I believe such things were done in the 90s. (And one could observe “unitary transformations being reversed,” if not on systems of programmable qubits, for many decades prior to that.)

    But if experts explained to me that there was something of interest about this particular experiment, I’d have no problem immediately updating my view—and yet I don’t think I’d need to retract a word of this post!

    I’m interested in the broader point: you and I both understand perfectly well that, whether or not there was anything of actual scientific interest in this paper, the reason it was covered in Newsweek, Cosmopolitan, etc had nothing whatsoever to do with that, and was entirely about the desire to write headlines like “Scientists Use a Quantum Computer to Reverse Time.” Furthermore—and this is the part I find unforgivable—the text of the paper does nothing whatsoever to head off such wild and irresponsible misreadings, but seems actively to encourage them. The authors should be ashamed.

  9. Scott Says:

    Craig #7:

      I wonder how long it will be until someone 1) executes the circuit from Hardy’s paradox (which has very few qubits and very few gates, so you could run it on a NISQ machine), 2) confirms that of course the results match the predictions of quantum mechanics, 3) frames the result in terms of Frauchiger et al’s variant, and 4) journalists telephone-game it into something like “quantum researchers prove decisions aren’t real”.

    I hate to be the one to tell you this, but I believe this has already happened (though for better or worse, I can’t find the link right now).

  10. Craig Gidney Says:

    Scott #9:

    > I hate to be the one to tell you this, but I believe this has already happened (though for better or worse, I can’t find the link right now).

    Urgh, you’re right; it’s already happened. I found the preprint:

    “Experimental rejection of observer-independence in the quantum world”

    and also the results of the telephone game:

    “A quantum experiment suggests there’s no such thing as objective reality”

  11. Tamás V Says:

    I find it fascinating, never ever occurred to me that reversing reversible transformations could make me world-famous.

    Actually, what if the input is a whole human? Then we’d have a tool for anti-aging!!! Should quickly patent it.

  12. EGME Says:

    Craig #10, Scott … can you comment on Fedrizzi’s article in the site linked by Craig, which lends credence to the work by Frauchiger and Renner, on which you earlier cast doubt … this is an experimental paper claiming to provide evidence in the direction of the FR work … casting doubt on the existence of “objective reality” and your own doubts about the FR work … I am a mathematician, not a physicist, so would very much like to hear what you have to say

  13. Scott Says:

    EGME #12: You’ve got to be kidding me! Everyone, including me, knew perfectly well that the outcome of the experiment would be the one predicted by QM (as applied by an observer external to the whole system), and said so. That was never even a point of contention. So even if we set aside that really doing this particular experiment would require placing conscious observers in superposition, not just qubits (!!)—in any case, no experiment whatsoever is “evidence” in favor of Frauchiger and Renner’s philosophical argument. Every experiment, insofar as it simply confirms QM to us external observers, leaves the discussion exactly where it was before.

    In short, this is a perfect example of the phenomenon that Craig Gidney #7 described very clearly.

  14. William Gasarch Says:

    Cosmopolitan? I thought you were kidding but I looked it up and yes, its in A magazine called Cosmopolitan. I couldn’t tell if its the same magazine that, as Sheldon pointed out, every issue has a article telling women how to get over a break up.

  15. Stella Biderman Says:

    William #14: It almost certainly is the same magazine. Cosmo is basically Gentleman’s Quarterly with worse relationship and sex advice. Like GQ, Playboy, Vogue, and other similar magazines, it has news, politics, and popular interest articles.

  16. Scott Glancy Says:

    Craig #10, the experiment by Proietti and co. [“Experimental rejection of observer-independence in the quantum world” is a realization of Brukner’s “A No-Go Theorem for Observer-Independent Facts” []. As I it Brukner’s result is to the CHSH inequality as Frauchiger’s and Renner’s result is to Hardey’s paradox. Both show conflict between “local-realistic” probability distributions (which are constrained by classical principles) and distributions allowed by quantum mechanics. F&R’s result strengthens the conflict by finding a LR distribution that gives 0 probability for an event that quantum mechanics allows with probability greater than 0.

    In my opinion, Proietti and co.’s experiment is a very fun demonstration of a conflict between quantum and classical physics. Inserting “measuring agents”, which are realized as single photons and probabilistic CNOT gates, into a CHSH experiment is cool and shows impressive technical skill. However, the experiment’s results are not surprising. I lost my faith in observer-independence long ago.

  17. Gabriel Nivasch Says:

    The curve-shortening flow can in theory be run backwards in time just as it can be run forwards. But (at least with my crude simulation methods) when you try to run it backwards, the curve quickly explodes, even if you start with a perfect circle.

  18. James B Says:

    Sorry if this is too off-topic, but there’s something I’d been wondering about for a while that seems vaguely related. Time-reversal (or at least CPT) is a symmetry of real physical systems, but it seems… rather more difficult to construct the time reverse of something, compared to a translated or mirrored copy of something. Has anyone investigated something along the lines of “given an arbitrary isolated, bounded classical(/quantum) system, what’s the minimal complexity to produce the same system running backwards (up to some tolerance)?” ?

  19. Scott Says:

    James #18: Complexity is always relative to your model of computation. In general models, like circuits or RAM machines, it’s trivial to time-reverse something (or for that matter CPT it), as easy as moving it from one memory location to another (or, if the information is classical, copying it). By contrast, for certain weak models like finite-state transducers, reversing the input string is impossible, while for one-tape Turing machines it provably requires quadratic time.

  20. Bob Strauss Says:

    The Cosmopolitan science beat! “Okay, so you might have had images of science now allowing you to undo awkward drunken conversations with your ex, but we’ve still got a while to go yet before researchers work out how to navigate that scenario. Still, this is a pretty huge development nonetheless, and may open the gateway to further research and breakthroughs relating to time travel.”

    This is so ripe for parody that I am drafting the article in my head right now.

  21. David R Says:

    Supposedly, in the upcoming Avengers movie the Avengers will use “quantum energy” to go back in time. So I think all these “Scientists reversed time with a quantum computer” headlines are just a guerrilla marketing campaign by Marvel :P. So you can blame Kevin Feige.

  22. TimeReversed? Says:

    Did time get reversed locally or not? Locally for the particles engaged in experiment?

  23. Scott Says:

    #22: A little simulation was run backwards. Time did not get reversed in any interesting sense. My whole post was ridiculing the notion that it did. Did you read it?

  24. DavidM Says:

    Old hat – G.I. Taylor was reversing time back in the 1960s!

  25. Mateus Araújo Says:

    A friend of mine contacted me to ask if this would allow one to send messages back to the past. Of course, what else a layperson is to understand from “reversing time”? It at least they had hyped it as the ultimate anti-aging treatment…

    But on the technical side, I’d be impressed if they had at least inverted a blackbox unitary of high-ish dimension, using for example the Japanese protocol. Inverting a known unitary is just too easy.

  26. Miquel Ramirez Says:

    I think that blaming the authors for choosing a title that accepts many readings is a bit rich. They are ESL – I am myself – and I find very cheap to criticise papers on the basis of their use of English mainly (the paper would benefit from an edit as parts of the paper read like a bad case of intestinal obstruction). Especially crazy readings like this one, only highlights that these days there are a lot of media people working for major universities and research companies/institutes who aren’t working closely with their academic colleagues.

    Regarding what the paper says at the end:

    On a practical side the time-reversal procedure might be helpful for the quantum program testing. Having in hands a multi-qubit quantum computer it is hard to verify that it really has computed the desired result. Indeed, the full tomography of the computed state is an exponentially hard task. Alternatively, making the time-reversal of the anticipated computed state and running the same evolution drives the computer back to its initial state if and only if the computer really made a correct computation. The initial state is typically non-entangled and therefore its verification is an easy task.

    This doesn’t sound wrong or useless to me or screams at me “OMFG TIME MACHINES”. If we had to be so vigilant then every paper in AI – my field – should carry a subtitle saying “TOTALLY NOT BRINGING ABOUT ROBOT APOCALYPSE TOMORROW”.

  27. Dan Staley Says:

    Scott Aaronson on time-reversal experiment: “One of the purest examples I’ve ever seen of a scientific claim”

    “…there was something of interest about this particular experiment”
    “…exactly as true and as earth-shattering as a layperson might think it is.”

    I’ve got to go write this up on my totally-honest, not-at-all-sensationalist science blog!

  28. Scott Says:

    Miquel #26: No, I looked at the paper and it’s not an ESL issue (I’ve been reading papers by my majority ESL colleagues for 20+ years). The issue, rather, is that the paper never once says anything like: “yes, obviously running a simulation backwards doesn’t reverse the flow of time or turn back the Second Law, etc., but we still think this is interesting because…”

    Instead, the elephant in the room is left completely unacknowledged and unaddressed. And to me, in a way that’s even worse than acknowledging the elephant and saying something knowingly false about it. It’s the academic version of gaslighting.

  29. gasarch Says:

    Bob 20- Cosmo Satire- Great idea. I’ll give it a shot:

    So your latest beau dumped you. Or you dumped him. In any case you’re feeling the blues. You wish you had never met the bum. But you can’t go back in time and change it. Or can you? Scientists are working on a device to do just that. Using quantum energy you don’t even have to try to forget you ever met him, you can go back in time and change the past so you truly never met him.

  30. Miquel Ramirez Says:

    Scott #28: Thanks for the answer. I still think this is more an issue of that journal having weak editorial processes than any intention by the authors to mislead or drive humanity like lemmings off a cliff. The Nature journal on “Machine Intelligence” carried on its first issue a wonky paper titled “Learnability can be Undecidable”, unfortunately we didn’t get headlines like “Strong AI Can’t Happen, Singularity Averted”.

  31. fred Says:

    Ori #1
    “I think the issue is that “scientists use quantum computer to…” can be followed by pretty much anything, and the average person will accept it ”

    Well, what’s been striking to me after following this blog for a while is that many claims made by team of professional academics working in the field of QM/QC will be followed by claims of other professionals in the fields that the first team basically doesn’t understand shit about QM – it never seems to be some subtle argument going on (like some tough 230 pages long math proof), but really something about the very basics of applying QM.
    For example the threads in (Manolis Kellis), (‘hadamring’ a brain).

    You’d think that given the mathematical nature of QM, that’d be something you would only see in fields like applied psychology or economics… but no. Either scientists in the field of QM are so bored that they start trolling each other, or there’s something a bit off in QM.

  32. Scott Says:

    fred #31: The first thing to realize is that the people making the outlandish claims, and the people debunking the claims, are generally not the same people. 🙂 So it’s not that we’re all bored and trolling each other—rather, it’s more like the incentives are set up in such a way that some groups (not naming names…) want to get their experimental paper into high-impact journals by attaching whatever words to the experiment will most impress non-experts, and making the experiment’s entirely predictable outcome sound as surprising as possible, and that then provokes others to respond to the claims.

    Having said that, yes, there often are difficult arguments between experts even in fields like physics and theoretical computer science, not only in psychology and economics. Usually, the arguments are not about the validity of some particular piece of math; instead they’re about what words are or aren’t reasonable to attach to the math.

  33. Craig Says:

    If I rewind my clock, do I reverse time?

  34. fred Says:

    I don’t really get all the flack that the authors are getting, obviously “reversing time” doesn’t mean that time as a whole flew backwards, because no-one would notice anything if that were to happen (for all we know it’s happening a zillion times a second… or more simply, time doesn’t even flow, spacetime is really an eternally static block and the sense of time is an illusion born by the way we form memories in one direction or another, based on initial conditions).

    It’s clear from the article (I think) that what they mean is for a system to move back to a prior state in terms of its thermodynamics, like somehow breaking an egg and then trick it to reassemble itself. The conventional explanation is to use thermodynamics to define the “arrow of time” (Prigogine, etc).

  35. Job Says:

    According to the paper, the 3-qubit experiment was run on a 5-qubit IBM machine with a success rate of about 50%.

    The corresponding circuit – listed in (f)? – has 28 single-qubit gates, 6 CNOTs and 2 Toffoli gates.

    Depending on the CNOT and Toffoli decompositions that were used, that’s about 75-100 gates total right?

    What i take away from this is that an average 3×100 circuit running on a 5-qubit QC is working about half of the time.

    I’m interested to see how much this improves by the end of the year.

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