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.

90 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” https://arxiv.org/abs/1902.05080

    and also the results of the telephone game:

    “A quantum experiment suggests there’s no such thing as objective reality” https://www.technologyreview.com/s/613092/a-quantum-experiment-suggests-theres-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” https://arxiv.org/abs/1902.05080%5D is a realization of Brukner’s “A No-Go Theorem for Observer-Independent Facts” [https://arxiv.org/abs/1804.00749]. 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!

    https://youtu.be/51-6QCJTAjU?t=833

  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 https://www.scottaaronson.com/blog/?p=4145 (Manolis Kellis), https://www.scottaaronson.com/blog/?p=3975 (‘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.

  36. Miquel Ramirez Says:

    Job #35: from the abstract for a invited talk at my School

    […]The recent emergence of novel computational devices, such as adiabatic quantum computers, CMOS annealers and optical parametric oscillators, present new opportunities for hybrid-optimization algorithms that benefit from hardware acceleration. In this work, we explore the challenges involved in using and benchmarking these devices for discrete optimization and conduct a detailed validation study of a D-Wave 2X adiabatic quantum computer. The computational results demonstrate that the D-Wave hardware consistently produces high-quality solutions and suggests that as this technology matures it could become a valuable co-processor in hybrid-optimization algorithms.[…]

    My reading of the sentence “consistently produces high-quality solutions” leads me to infer that at least adiabatic devices produce models to small MAXSAT instances with a probability higher than .5 and well before Boltzmann brains can emerge. Yet the bit “as this technology matures” perhaps either of the two previous constraints/expectations need to be relaxed for devices existing in the real world.

    DISCLAIMER: The above was written by an English native speaker.

  37. Miquel Ramirez Says:

    “perhaps either” -> “perhaps means either” 🙂

  38. Job Says:

    I’m sure the success rate is decent, but how’s the performance relative to a classical solver?

    Isn’t that really the question for adiabatic QCs?

    For a gate array QC, a success rate of 50% makes it pretty useless – and that’s with only 3 out of its 5 qubits.

    Even for something like Simon’s algorithm you need n successful runs, in a row. With 50% per run, the overall success rate is tiny.

    You expect more from a 5-qubit machine.

  39. Scott Says:

    Job #38: No, that’s not quite right about Simon’s algorithm. You’d “merely” need a large enough fraction of the runs to succeed, that the resulting system of noisy linear equations can be solved in polynomial time (and you could always generate more equations if that helps, up to polynomially many). I’m not sure what noise rate is compatible with that, but it’s surely more than 0 (and more than 1/n).

    Yes, it’s obvious that the goal is to solve an interesting problem in the external world faster than you could’ve solved it classically, and the success probability of your algorithm figures into that goal! You don’t just get to declare that it doesn’t matter.

  40. David Byrden Says:

    Scott [Glancy] #16 :

    You said
    “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.”

    So I tried to analyse the FR experiment under the assumption of “local realism”. I got nowhere.

    FR’s own analysis doesn’t help. They use standard Quantum Mechanics throughout, except for that one mistake where a “collapse” happens and they miss it.

    Could you please explain to this amateur, how to analyse a QM system with added “local realism” ?

    Thank you.

  41. Job Says:

    You’d “merely” need a large enough fraction of the runs to succeed, that the resulting system of noisy linear equations can be solved in polynomial time (and you could always generate more equations if that helps, up to polynomially many). I’m not sure what noise rate is compatible with that, but it’s surely more than 0 (and more than 1/n).

    Yeah, but it’s less than 1/2 and varying unfavorably with respect to n. A QC with 50% success rate literally offers no advantage over a random guess.

    But i shouldn’t say that a 50% error rate makes a QC useless, it depends on gate count. Like, 50% over 100 gates.

  42. Scott Glancy Says:

    David Bryden #40: A full explanation of how to analyze systems of random variables under the assumption of local realism would be more involved than I can do in a blog comment. I would recommend that you first read proofs of the CHSH inequality. This one looks accessible: https://physics.stackexchange.com/questions/237321/simplified-derivation-of-chsh-bell-inequalities . Then I would read Brukner’s arXiv:1804.00749. That paper discusses the relationship between local realism and the existence of “observer-independent facts”. Essentially, if you or your “friend” (note the scare quotes because this friend is trapped in a box and is about to be measured in a superposition basis) believes an observer-independent fact, then you or they must have a local hidden variable in mind. (Local hidden variables obey the principle of local realism.) Brukner’s paper also gives a helpful comparison to F&R’s argument.

    You write that F&R “use standard Quantum Mechanics throughout”, but I think that parts of their argument use quantum mechanics and other parts use principles very similar (maybe exactly similar) to local realism. For example, although seemingly intuitive, F&R’s assumption C and some rules of Boolean logic invoked in their proof are not in the quantum mathematical formalism. Their theorem shows that a conflict arises when quantum mechanics is fused with these other principles.

    You also write that F&R make “one mistake where a ‘collapse’ happens”. I’m not sure what step you are referring to, but I am not aware of any mistakes in their proof. One might interpret certain steps of their argument as invoking collapse of superpositions, but that is by design. It is an essential part of finding the contradiction between quantum and classical reasoning.

  43. Gil Kalai Says:

    Hi Scott , Another experiment with IBM quantum computers that I would regard interesting is to use teleportation protocols to teleport around interesting quantum states on one, two, or three qubits. Yet another experiment of this nature is to create a superposition of two interesting quantum states with 2,3,4 qubit each.

    I discuss “time-reversing,” “teleportation”, and “superposition of individually challenging states” as tasks that require effort which quickly explodes with the complexity of the situation (at least without quantum error-correction). See Section 7 of this 2016 paper.

    Just as “time reversing” three-qubit states, such experiments (as earlier ones) can lead scientists themselves to make some over-the-top speculations, (like living life backwards in time, teleporting people around; superposing the minds of two individuals etc.) and can capture misguided interest also by the media.

    But the point is that for all these examples, witnessing quick deterioration of quality when we increase the number of qubits (and this seems to be the situation here) may suggest a scientific explanation for *why* these science fiction scenarios are out of reach, and this is of some interest.

  44. David Byrden Says:

    >> ” [F&R] theorem shows that a conflict arises when quantum mechanics is fused with these other principles.”

    I’m afraid not. You see, there is a subtle error in their logic, and their apparent “paradox” is not real. It is merely a consequence of the error. The theorem is invalid.

    Also, there is truly no “classical” aspect to the F&R experiment. Their apparatus is treated, in every equation, as quantum equipment, with superpositions and changes of basis.
    They visualise human “agents” inside their “labs” but they don’t allow for the decoherence that would entail. For example, they tell us that these humans must be measured in a basis midway between their eigenstates! I wish they would have explained how!

    If you recalculate with decoherence (mixed states rather than superpositions), the results are different. Thus, the F&R apparatus is merely the Hardy apparatus: two partially entangled qubits.

    Anyway, returning to their nonexistent “paradox”: I wrote a note explaining the error, resolving the error, and showing by example how to apply QM to systems with nested observers. See:

    http://byrden.com/quantum/Renner-explained.html

    If you find an error in that, I will be glad to revise it. But I stand by my assumption that quantum states are observer-relative. Perhaps that’s what F&R are truly showing us.

  45. Michael MacDonald Says:

    I really liked the Gil Kalai’s paper linked in #43, where (if I’m not mistaken) he posits that the “pessimistic” view of QC based on per-qubit noise scaling with the number of qubits can naturally account for things like the arrow of time and the observed non-quantum nature of macroscopic objects. In the past, you’ve objected that this idea of noise required new physics without explanation or sufficient motivation. If this is something you can answer quickly, I was wondering what in your opinion is the extant realization of QC, or other physical experiment, that sets the lowest upper limit on noise that scales the way Gil describes. Is it something that has or could be done with IBM’s quantum computer? How would you expect this realization or experiment to be extended to force this limit lower?

  46. Scott Glancy Says:

    David Byrden #44: I read your web page, and I completely agree with nearly everything there including your statement that “Renner and Frauchiger have not discovered a problem in Quantum Mechanics.”. However, I can’t fully support the last sentence, which says that F&R “did not recognize that an observer can be a superposition of multiple observers, all seemingly identical, but all seeing different things.”. I guess that this is the aspect of F&R’s proof that you describe as a mistake. However, I believe that F&R knowingly assume that an observer cannot be a superposition of multiple observers. I’m not sure if this assumption is exactly spelled out, if it is entailed by one of their other assumptions, or if they intend it to be inherent in the idea of “agent” (the word that F&R use for observer), so you might reasonably complain that they should have been more clear about this point in their paper. Rather than classifying the singleness of observers as a mistake, I would say that this assumption is an intentional part of the reasoning that leads to the contradiction described by F&R.

    I agree with your conclusion that the contradiction is avoided if one allows observers to become multiple observers when they are in superposition. There are many other ways to avoid the contradiction by abandoning other assumptions used in their argument. That is the entire purpose of their paper: to force us to choose which assumption(s) must be abandoned.

  47. David Byrden Says:

    Scott Glancy #46 :

    It’s an interesting experiment, made possible by the partial entanglement. It was only by visualising the 4 dimensional Hilbert state space that I really understood it.

    Let me clarify why I think my resolution is the right one :

    1.
    There can be no human observers in the F&R apparatus. If the “labs” are macro objects, decoherence will ruin it. F&R’s equations assume pure states and superpositions. For example, they prove that FBAR=TAILS must lead to W=FAIL, and the proof assumes that F is in a superposition. And, of course, there are changes of basis.
    So, where F&R say “Agent F”, we must interpret that as “Qubit F” in order to make the thing work.

    2.
    [1] above does not invalidate the experiment. You don’t need conscious observers inside it. e.g. you can calculate the system state relative to “Qubit F” regardless of the fact that “Qubit F” has no awareness and no memory.

    3.
    However, BECAUSE it’s qubits instead of humans, there IS the possibility of the superposition that I used to resolve the problem:
    observer F who sees FBAR=TAILS is a superposition of two observer Fs, one of whom sees FBAR=OK.

    We’re all used to quantum objects having superpositions of their own possible states. But in this case, the two superposed observers are identical, and it’s another part of the apparatus that distinguishes them. It’s what they see, not what they are, that’s different.

    I believe this is a valid way to use QM and I don’t believe the entanglement (as in the F&R apparatus) is necessary for you to use QM this way.

  48. LK2 Says:

    Off topic: I find incredible that some physicists waste time in this stuff:

    https://arxiv.org/pdf/1903.08879.pdf

    or maybe working at LHC is becoming boring…
    What do you guys think?
    LK

  49. a Says:

    So what if entanglement breaks down beyond 1 light year. Can it ever be tested?

  50. Scott Says:

    a #49: Sure, in principle that’s testable, though it would take a year or more to test it. 🙂 I’m curious if anyone has ideas for any astrophysical phenomena that could be used as the basis for such a test.

    But also note that it’s not particularly interesting just to make up a speculation like “what if entanglement breaks down at 1 light year??” without at least some idea of why such a thing might happen, and—cruciallly—what new theory might take the place of quantum mechanics in that regime. And whatever the proposed new theory was, one would then face the challenge of explaining why it was internally consistent and not already ruled out by other experiments.

  51. David Byrden Says:

    a #49 :

    > “what if entanglement breaks down?”

    Entanglement of which property? Entanglement is not a thing in its own right, you know. Like the horizon or the vanishing point, it only appears to exist because of how we look at things.

    A breakdown would imply that the conservation laws don’t always hold. We would be in a very different universe.

  52. Quantum of solace Says:

    I disagree that the authors should be ashamed; I think the authors should be munching popcorn and laughing their little authorial butts off. Most researchers try to write papers that will survive mangling. That’s a fool’s errand: you yourself point out that all the scientific content gets discarded.

    Why not give up, and instead amuse ourselves by making journalists write the silliest absurdities we can? The social harm is nil: it’s not like anyone was learning anything from pop sci dreck. It may have the benefit of making some readers realise the article is wrong. It certainly has the benefit of being funny.

  53. AlexM Says:

    Can we reverse time to before Mueller’s investigation started? 🙂

  54. Nick Nolan Says:

    Maybe I’m in the minority, but I welcome this new backwards in time hype.

    Why? Because it’s a change for “faster than light” articles in popular science magazines like New Scientist, Scientific American or Popular Science.

    I have kept a tally for two decades and without a fail one of these magazines has once a year the mandatory “faster than light” article in many years all of them. If you read the article there one small paragraph in the end explaining how it was superluminal group velocity and not actually faster than light.

  55. ppnl Says:

    David Byrden,

    There can be no human observers in the F&R apparatus. If the “labs” are macro objects, decoherence will ruin it.

    But why? First, imagine the lab as a single qubit. No problem right? Ok as several interacting qubits. They can still be entangled and coherent right? Now a vast number of qubits inside a quantum computer. Still coherent and entangled in a vast complex superposition right? Otherwise, it wouldn’t be a quantum computer. Now lets program that quantum computer to simulate a person in a lab. As long as you don’t look at any of the states of that running program it must be a superposition of all the things the person might see or do right?

    A person in a lab that is perfectly isolated is for all practical purposes just a quantum computer running a program. It is not the complexity of the object that causes decoherence. It is your thermodynamic contact with the object that causes decoherence. Complexity simply makes it difficult to isolate. That’s what makes quantum computers hard to build.

  56. a Says:

    It is unclear if entanglement will hold up at such vast distances. We speculate it to be so.

  57. fred Says:

    AlexM #53

    No, but POTUS is apparently the new Schrodinger cat: a superposition of the most anti-semite president and the most pro-Israel president!

  58. Scott Says:

    a #56: No, once you understand that the universe is fundamentally quantum-mechanical—that it’s not just some extra tacked on to our picture of the world, with adjustable knobs if you don’t like it, but is nearly as basic as the laws of probability—the weird and speculative idea is that there’s some distance scale at which entanglement would break down. Why would there be?

  59. fred Says:

    Scott #56

    The thing is that “laws of probability” isn’t exactly something that’s easy to interpret (is it a limitation of our knowledge as observer, or actual values of states regardless of observer, etc).

    It’s not as much about what the laws of probability are, but what does it mean that QM is probability based, how to interpret it.
    E.g. what does “pure randomness” even mean? When you think about it, it makes very little sense that, somehow, there’s some process independent of the universe that’s throwing a dice (what sort of dice is this?) to decide what happens in our world, in a manner that’s free of the basic laws of causality, yet the probabilities are based on the wave function of the universe…

    With MW/PilotWave you pretty much get rid of probabilities (you just have branching or full classical determinism), so I can see why they’re so attractive.

  60. David Byrden Says:

    ppnl #55 :

    There are two possibilities here.

    [1].
    The person in the lab takes absolutely no notice of the quantum measurement that happened in the lab – no information from it can reach her. This is like, for example, watching a Young’s Slit experiment and not detecting the which-way information.
    In this case, the lab and the person contribute nothing to the experiment. She’s an observer who doesn’t observe. She’s a memory that is blank. We might as well ignore her. So, back to square one! It’s all just a couple of qubits.

    [2].
    The person in the lab is exposed to information about the quantum measurement that happened in the lab. In this case, she splits into two persons (using the MWI interpretation, which is quite useful here).
    The two persons rapidly become different (see Decoherence theory) because of the different information entangling into them. Very soon they are in a mixed state rather than a superposition – like Schrodinger’s Cat.
    The equations of this experiment depend on superpositions. The experiment will not work in case [2].

  61. Gerard Says:

    @ppnl #55

    > A person in a lab that is perfectly isolated is for all practical purposes just a quantum computer running a program.

    It appears to be very difficult to build a quantum computer that avoids decoherence.

    What makes you think human evolution has overcome that difficulty ?

  62. David Byrden Says:

    ppnl #55 :

    > “it must be a superposition of all the things the person might see or do right? ”

    I will address your suggestion of simulating the person with a quantum computer in order to avoid decoherence.

    This approach gains us nothing, because we can regard the real lab as a quantum computer also.

    The fundamental problem is that the person, OR the simulated person, called “F who measured UP” rapidly becomes very different to the person / simulation called “F who measured DOWN”.

    I’m not qualified to speak freely of the math behind this, but an example will suffice.
    You can have a superposition of states of a simple thing, e.g. a polarised photon, because its states are similar enough that there’s high correlation between them. One can *imagine* a photon with an intermediate polarity.

    But can you imagine a cat who’s intermediate between the two states “dead” and “alive”? Not in the colloquial sense of “half dead”, but in a superposition? It is clear to you how every one of its molecules would be positioned and configured?

    The complexity of the cat – its “degrees of freedom” – is so high as to make intermediate states vanishingly small in probability. We’re twisting an enormous Rubik’s Cube here, at random, hoping to hit the solution by chance.

  63. Tamás V Says:

    a #56: If you lived in the 19th century, you would ask whether the classical velocity addition formula breaks down at very high speeds, e.g. something comparable to that of light. Then a scholar would reply: look, once you understand that the universe is fundamentally classical, you realize that the weird speculation is that the formula does break down at some speed, not that it doesn’t.

    But seriously, history suggests that the safest bet you can make is that nature is fundamentally not quantum-mechanical. Besides death, the only sure thing seems to be that every theory breaks down in the end.

  64. ppnl Says:

    Gerard,

    By definition, you have to avoid decoherence in order to build a quantum computer. That’s how they work. To do that you have to isolate them from all outside contact. A person in a lab that is isolated fits that situation. Yes, it is very hard to do but human evolution has nothing to do with it. It would be just as hard to do with a rock in a lab.

  65. Scott Says:

    Tamás #63: Even if we assume that QM is only an approximation to some deeper theory, we still don’t get to presuppose that the deeper theory will restore the old picture of the world—so that (e.g.) there would be only classical correlation and no entanglement at large enough distances. Again, why should it do that, among all the possible things it could do? Why shouldn’t it do just the opposite, and allow (for example) super-quantum correlations?

    Indeed, I’m having trouble thinking of a single example in the history of physics where the “ratchet of counterintuitiveness” turned backwards in this sort of way … can anyone else suggest one?

  66. ppnl Says:

    David Byrden,

    [1].
    The person in the lab takes absolutely no notice of the quantum measurement that happened in the lab – no information from it can reach her. This is like, for example, watching a Young’s Slit experiment and not detecting the which-way information.
    In this case, the lab and the person contribute nothing to the experiment. She’s an observer who doesn’t observe. She’s a memory that is blank. We might as well ignore her. So, back to square one! It’s all just a couple of qubits.

    I think this is a bit confused. If the results of the quantum measurement is in thermodynamic contact with the lab at large then it is in thermodynamic contact with the person in the lab. It makes no difference if the person takes notice of the measurement or not. The state will collapse or decohere from their point of view. If the results of the quantum measurement are not in thermodynamic contact with the lab then no measurement has been made from their point of view. Thermodynamic contact is how we define measurement.

    [2].
    The person in the lab is exposed to information about the quantum measurement that happened in the lab. In this case, she splits into two persons (using the MWI interpretation, which is quite useful here).
    The two persons rapidly become different (see Decoherence theory) because of the different information entangling into them. Very soon they are in a mixed state rather than a superposition – like Schrodinger’s Cat.
    The equations of this experiment depend on superpositions. The experiment will not work in case [2].

    Again “exposed” does not mean seeing or knowing the results of the measurement. It may mean inhaling an oxygen molecule that was exposed to the results of the measurement. Any thermodynamic contact at all. And without that thermodynamic contact, no measurement was made.

    Forget about MWI. It adds nothing but confusion here. It can be useful for visualizing different branches. But it does not help in understanding Bell’s inequality or decoherence.

    You are describing a person in a superposition of states. This is not a decoherent phenomenon. This is a coherent phenomenon. You could in principle create entangled pairs of these people and do a Bell test on them and show that they violate Bell’s inequality.

    But if you come into thermodynamic contact with them then they decohere and you can not observe quantum coherent phenomena with them. The wave collapses.

    This approach gains us nothing, because we can regard the real lab as a quantum computer also.

    I said exactly that the lab can be considered a quantum computer. Those are my words. Ah, let me rephrase it… the person+the lab is a quantum computer running a program.

    The point is a quantum computer cannot decohere no matter how large and complex the program it is running is. An isolated cat, person, lab or anything else cannot decohere for the very same reason. They can all be seen as quantum computers which we know do not decohere due to complexity.

    But can you imagine a cat who’s intermediate between the two states “dead” and “alive”? Not in the colloquial sense of “half dead”, but in a superposition? It is clear to you how every one of its molecules would be positioned and configured?

    In principle yes. Each molecule would be in a superposition of states while also entangled with all the other molecules of the cat. When you look at the cat the superpositions and entanglements will be vastly reduced. State reduction.

    The complexity of the cat – its “degrees of freedom” – is so high as to make intermediate states vanishingly small in probability.

    I don’t know what you are saying here. In any case a quantum computer, no matter how complex and how many degrees of freedom must still be in a coherent state or it isn’t a quantum computer. Ditto cats in boxes and isolated labs.

  67. Tamás V Says:

    Scott #65: Oh, by “QM breaking down” I’d not mean it turning back into classical, but gradually tranforming into something else, e.g. something even more weird than QM, like it happens with the addition of velocities. So yes, if we push the limits of our experimental setup, entanglement may “break down” in the end, turning into something even more fascinating. My point was that I’d bet entanglement will break down, maybe not during my lifetime though.
    When I hear the magical phrase “nature is fundamentally quantum-mechanical”, it can really drive me up the wall 😃

  68. Ajit R. Jadhav Says:

    Tamás # 63:

    If nature is not quantum mechanical, then an implication is that QM must be un-natural. … So, let me ask you (just out of curiosity): what part (or aspect) of QM do think is most un-natural?

    Best,

    –Ajit
    V # 63:

    If nature is not quantum mechanical, then an implication is that QM must be un-natural. … So, let me ask you (just out of curiosity): what part (or aspect) of QM do think is most un-natural?

    Best,

    –Ajit

  69. Tracy Hall Says:

    Scott #63:

    Indeed, I’m having trouble thinking of a single example in the history of physics where the “ratchet of counterintuitiveness” turned backwards in this sort of way … can anyone else suggest one?

    One could argue that the existence of randomness in the world has always been intuitive. Classical statistical mechanics said, “Your intuition is wrong; everything is deterministic and what appears random was hidden all along in subtle details of the initial conditions.” Then the Born rule came along and said, “Huh, it turns out that your intuition was right; there is necessary randomness inherent to any measurement prediction.”

    (Of course intuitive is not the same thing as elegant, in Albert Einstein’s famous opinion.)

    I agree, though, that it is very rare for the ratchet to slip back in the direction of intuition. It is a safe bet that the things that we do not yet understand will be less easily understandable than the things that we already do understand—in contradiction to our natural bias to expect that what we currently struggle to understand must be due to some error on the part of those trying to explain it to us.

  70. David Byrden Says:

    ppnl #64 :

    >> “To [avoid decoherence] you have to isolate them from all outside contact.”

    I’m afraid you misunderstand.

    Think about it. What’s so important about isolating a superposed system from outside contact? Is the outside world infected with something that causes decoherence?

    That answer is of course “yes”, but what is that thing? It’s *complexity*. That entails an enormous number of differences between the superposed states, making it practically impossible to have intermediate states.

    We isolate superposed systems, not to keep them “indoors” but to keep them simple.

    Now, if you seal up a lab with a person inside, it will avail you nothing, because you have put enormous complexity *inside the lab*. You’re going to get decoherence inside there, despite the isolation.

  71. fred Says:

    Scott #65

    The idea that particles don’t really exist but are really vibrations in quantum fields, permeating through all space, sounds a lot like the old concept of (a)ether 😛

  72. Itai Bar-Natan Says:

    “Indeed, I’m having trouble thinking of a single example in the history of physics where the “ratchet of counterintuitiveness” turned backwards in this sort of way … can anyone else suggest one?”

    The failure of the S matrix approach for strong interaction. Due to the proliferation of observed particles in strong interaction experiments physicists speculated the distinction between a fundamental particle and a bound state of particles breaks down in some way. They distrusted quantum field theory, which describes reality in terms of localized fields changing through local interactions (that is, quantum-local, which is rather different from classical-local). Instead, the only thing that exists is the S matrix, which describes the entirety of the interactions that happen in a scattering experiment all at once without attributing them to stuff that happen at particular locations and times. Later it turned out that quantum field theory works fine for describing strong interaction as long as you add quarks, which are never observed directly but only in bound states, so for now we can still describe reality in terms of stuff happening in particular places and times, at least in some quantum sense so that entanglement and similar sorta-nonlocal stuff nonetheless exist.

    Then again, quantum field theory is not a particularly intuitive theory either, and S matrix theory is still studied and has lead to some physical insights, and it has inspired string theory.

  73. David Byrden Says:

    Scott : 65 :

    >> “I’m having trouble thinking of a single example in the history of physics where the “ratchet of counterintuitiveness” turned backwards”

    When the “epicycle” system of planetary motion, invented by Apollonius of Perga, was replaced by Newton’s refreshingly simple ellipses.

    https://en.wikipedia.org/wiki/Deferent_and_epicycle

  74. Scott Says:

    Tamás #67:

      When I hear the magical phrase “nature is fundamentally quantum-mechanical”, it can really drive me up the wall

    Friend-of-the-blog Greg Kuperberg, who was just visiting UT Austin, likes to use the analogy of someone saying, “scientists once believed that the earth was flat. Now they believe it’s a sphere. So who knows what they’ll believe a thousand years from now—that it’s a two-handled torus? a Klein bottle? in any case, we can safely assume that this ‘sphere’ hypothesis is only temporary…” 🙂

    Seriously, would you be happier with the following phrasing? Quantum mechanics is the most fundamental set of laws governing those regimes of the universe that human beings have observed.

  75. Tamás V Says:

    Ajit #68: I assume by “un-natural” you don’t mean counter-intuitive, that would be too easy for me. Ok, as I know very little about QM, I can only say what if a professor in 1830 had asked a student “In what sense do you think the (classical) velocity addition formula is un-natural? Any concrete issue you’re aware of?”, it would have been a great victory for the professor.

  76. David Byrden Says:

    ppnl #66 :

    I’m sorry that I didn’t use the correct terminology. My case [1] represents “no thermodynamic contact” and my case [2] represents “thermodynamic contact”.

    > without that thermodynamic contact, no measurement was made.

    Surely the lab could contain quantum equipment that makes the measurement but keeps it isolated from the rest of the lab? Passing the results on to the next guy via quantum circuitry?

    > You are describing a person in a superposition of states.

    That was not my intent. Renner and Frauchiger describe their agents seeing the measurement results, thinking about them, and even taking actions dependent on them. Surely that causes immediate decoherence?

    > An isolated cat, person, lab or anything else cannot decohere for the very same reason.

    Well, this is the crux of it. My understanding of decoherence differs from yours.

    My understanding is that the agents must decohere into mixed states. I didn’t think that a superposition of very different states of a complex object, e.g. a cat, was possible.

    Can somebody settle this one way or the other?

  77. Tamás V Says:

    Scott #74: Yes, that would help me find my inner peace, thanks 😃

    … and Greg must be a very smart guy, I like his comment, and I think he is right, even if he was only kidding. (Maybe a 5-dimensional torus? Or maybe they will believe the Earth as such has never ever existed?) Sure, today, for a 3-dimensional being the Earth is a sphere (ok, not for the flat-earthers, so it’s not that obvious ☹️), but who knows what tomorrow will bring?

  78. a Says:

    I have a rudimentary shadow of a reason and since I have not thought about it I am not giving it away. Anyways not related to my reason for a layman or a laybeing the *bigger* picture of the universe *appears* to be classical.

  79. Tamás V Says:

    Scott #74: Here is a quote from your friend, Terry Rudolph: „[…] space and time are like the taste of a banana […] but I don’t think anyone would argue that the taste of a banana is really a fundamental physical property“.

    So if space turns out not to be fundamental, isn’t it conceivable that the fundamental description of the Earth will be different from a sphere?

    (Ok, on the other hand I’ve also heard from a professor that friendship and truth are different things.)

  80. Scott Says:

    Tamás #79: Even if space and time end up not appearing in the fundamental description of the laws of physics—which is entirely possible—it still seems obvious that “earth is approximately spherical” would be a true statement, in the regime where such a statement even makes sense in the first place. By analogy, do you agree that “the sky is blue” is a true statement, even though neither “the sky” nor “blue” appears anywhere in the Standard Model Lagrangian, let alone quantum gravity? 🙂

  81. Tamás V Says:

    Scott #80: Of course I agree, I’m not an idiot. This is exactly how I see it too (except that I don’t believe there is such a thing as fundamental description). I thought you meant by Greg’s analogy, between the lines, that QM is the end of the story. I blame it on my bad English 🙂

  82. peak.singularity Says:

    Hi !

    Scott , Tamás V :

    I can’t help but think about this :
    http://hermiene.net/essays-trans/relativity_of_wrong.html

    ‘The young specialist in English Lit, having quoted me, went on to lecture me severely on the fact that in every century people have thought they understood the Universe at last, and in every century they were proven to be wrong. It follows that the one thing we can say about out modern “knowledge” is that it is wrong.”
    […]
    My answer to him was, “John, When people thought the Earth was flat, they were wrong. When people thought the Earth was spherical, they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together.”‘

  83. David Says:

    Scott #80

    I think that if you had said “the universe is approximately quantum mechanical, in the regime where such a statement even makes sense”, Tamás would not have been bothered so much. And I also think that this is why the analogy you mentioned is not completely fair.

  84. Gerard Says:

    @ppnl #64

    I don’t think isolation from the environment alone is a sufficient condition to define a quantum computer.

    Many other criteria are required among which is that you have to be able to prepare the system in a pure state (or at least an approximately pure state). I don’t see how you can do that with a complex system like a human, a cat or a rock. Also that system didn’t just pop into existence for your experiment, it has a history of interactions with the outside world and therefore is already entangled with the environment, even if you can keep it isolated from some point in time forward.

  85. Tamás V Says:

    Ajit #68: Sorry, just noticed you said “If nature is not quantum mechanical” in your question, i.e. without the word “fundamentally”. Then my answer is simpler: I do think nature is quantum mechanical, and I do think nature is classical too. It’s all about looking at the right “regime” (or circumstances, or domain of validity). I have the impression that this “regime” aspect is a bit under-communicated, although it would be very instructive for a laypeople I think. Great, we’re back to the original topic of this thread: communication.

  86. Tamás V Says:

    peak.singularity #82: Thanks for the link, very interesting read. I like the part about the (ancient) flat Earth theory. But I still don’t see why the author feels himself justified at the end of the essay:

    “If quantum theory and relativity can be combined, a true “unified field theory” may become possible.
    If all this is done, however, it would be a still finer refinement that would affect the edges of the known—the nature of the big bang and the creation of the Universe, the properties at the center of black holes, some subtle points about the evolution of galaxies and supernovas, and so on.
    Virtually all that we know today, however, would remain untouched and when I say I am glad that I live in a century when the Universe is essentially understood, I think I am justified.”

    My reservation is that superposition and entanglement don’t feel like small refinements to classical thinking, but a giant leap. And imagine what would happen if it turned out that space and time are not fundamental.

  87. ppnl Says:

    Gerard ,

    Many other criteria are required among which is that you have to be able to prepare the system in a pure state (or at least an approximately pure state). I don’t see how you can do that with a complex system like a human, a cat or a rock.

    I don’t see how to do that either. But I don’t have to. I just note that it is in principle possible. I don’t see how to isolate a human or rock either. In fact, I have no idea how to isolate a quantum computer with as many quantum states as a rock. But we are just arguing from principle here and there is no reason in principle that it cannot be done.

    Also that system didn’t just pop into existence for your experiment, it has a history of interactions with the outside world and therefore is already entangled with the environment, even if you can keep it isolated from some point in time forward.

    Well yes but all the components of a quantum computer have a history of interactions with the outside world as well. The program that the computer is running is itself a history of interacting with the outside world. You cannot program it without entangling it with the world. I’m not seeing any argument in principle here.

    In a sense, the state of entanglement with the world that the rock is in when it is isolated can be seen as the quantum program that the rock is running when it is isolated. Past entanglement with the world is essential for a quantum computer to do anything because that is its program.

  88. jonas Says:

    John Baez points out “http://math.ucr.edu/home/baez/diary/march_2019.html#march_26” a recent article by David Harvey and Joris Van Der Hoeven that proves that we can multiply very big integers with a slightly faster asymptotic runtime (in certain reasonable models) than the previous algorithm. I completely fail to see the significance of this, but I also think that you Scott might be able to comment on this. Is this something we should be enthusiastic about, and why?

  89. Jones Volonte Says:

    This is way worse than what you describe: “time reversible laws” were always a stupid name, what you reverse is the direction of motion in classical physics, and in statistical laws an analogous thing happens. to reverse time has not empirical meaning, maybe just for sloppy thinkers.

    pd: bonus: the second law doesn’t explain “the arrow of time”, it presupposes the difference between past and future, what explains is that certain classes of microstates are bigger and hence more probable.

    chau

  90. Animesh Datta Says:

    I recall being asked about this back in the day by CNN who did report my comments faithfully: https://www.cnn.com/2019/03/14/world/russia-scientists-reverse-time-scli-scn-intl/index.html

    The media buzz seems to passed me by!