Archive for the ‘Announcements’ Category

Microsoft SVC

Tuesday, September 23rd, 2014

By now, the news that Microsoft abruptly closed its Silicon Valley research lab—leaving dozens of stellar computer scientists jobless—has already been all over the theoretical computer science blogosphere: see, e.g., Lance, Luca, Omer Reingold, Michael Mitzenmacher.  I never made a real visit to Microsoft SVC (only went there once IIRC, for a workshop, while a grad student at Berkeley); now of course I won’t have the chance.

The theoretical computer science community, in the Bay Area and elsewhere, is now mobilizing to offer visiting positions to the “refugees” from Microsoft SVC, until they’re able to find more permanent employment.  I was happy to learn, this week, that MIT’s theory group will likely play a small part in that effort.

Like many others, I confess to bafflement about Microsoft’s reasons for doing this.  Won’t the severe damage to MSR’s painstakingly-built reputation, to its hiring and retention of the best people, outweigh the comparatively small amount of money Microsoft will save?  Did they at least ask Mr. Gates, to see whether he’d chip in the proverbial change under his couch cushions to keep the lab open?  Most of all, why the suddenness?  Why not wind the lab down over a year, giving the scientists time to apply for new jobs in the academic hiring cycle?  It’s not like Microsoft is in a financial crisis, lacking the cash to keep the lights on.

Yet one could also view this announcement as a lesson in why academia exists and is necessary.  Yes, one should applaud those companies that choose to invest a portion of their revenue in basic research—like IBM, the old AT&T, or Microsoft itself (which continues to operate great research outfits in Redmond, Santa Barbara, both Cambridges, Beijing, Bangalore, Munich, Cairo, and Herzliya).  And yes, one should acknowledge the countless times when academia falls short of its ideals, when it too places the short term above the long.  All the same, it seems essential that our civilization maintain institutions for which the pursuit and dissemination of knowledge are not just accoutrements for when financial times are good and the Board of Directors is sympathetic, but are the institution’s entire reasons for being: those activities that the institution has explicitly committed to support for as long as it exists.

Speaking Truth to Parallelism: The Book

Monday, September 22nd, 2014

A few months ago, I signed a contract with MIT Press to publish a new book: an edited anthology of selected posts from this blog, along with all-new updates and commentary.  The book’s tentative title (open to better suggestions) is Speaking Truth to Parallelism: Dispatches from the Frontier of Quantum Computing Theory.  The new book should be more broadly accessible than Quantum Computing Since Democritus, although still far from your typical pop-science book.  My goal is to have STTP out by next fall, to coincide with Shtetl-Optimized‘s tenth anniversary.

If you’ve been a regular reader, then this book is my way of thanking you for … oops, that doesn’t sound right.  If it were a gift, I should give it away for free, shouldn’t I?  So let me rephrase: buying this reasonably-priced book can be your way of thanking me, if you’ve enjoyed my blog all these years.  But it will also (I hope) be a value-added proposition: not only will you be able to put the book on your coffee table to impress an extremely nerdy subset of your friends, you’ll also get “exclusive content” unavailable on the blog.

To be clear, the posts that make it into the book will be ruthlessly selected: nothing that’s pure procrastination, politics, current events, venting, or travelogue, only the choice fillets that could plausibly be claimed to advance the public understanding of science.  Even for those, I’ll add additional background material, and take out digs unworthy of a book (making exceptions for anything that really cracks me up on a second reading).

If I had to pick a unifying theme for the book, I’d sigh and then say: it’s about a certain attitude toward the so-called “deepest questions,” like the nature of quantum mechanics or the ultimate limits of computation or the mind/body problem or the objectivity of mathematics or whether our universe is a computer simulation.  It’s an attitude that I wish more popular articles managed to get across, and at any rate, that people ought to adopt when reading those articles.  The attitude combines an openness to extraordinary claims, with an unceasing demand for clarity about the nature of those claims, and an impatience whenever that demand is met with evasion, obfuscation, or a “let’s not get into technicalities right now.”  It’s an attitude that constantly asks questions like:

“OK, so what can you actually do that’s different?”
“Why doesn’t that produce an absurd result when applied to simple cases?”
“Why isn’t that just a fancy way of saying what I could’ve said in simpler language?”
“Why couldn’t you have achieved the same thing without your ‘magic ingredient’?”
“So what’s your alternative account for how that happens?”
“Why isn’t that obvious?”
“What’s really at stake here?”
“What’s the catch?”

It’s an attitude that accepts the possibility that such questions might have satisfying answers—in which case, a change in worldview will be in order.  But not before answers are offered, openly debated, and understood by the community of interested people.

Of all the phrases I use on this blog, I felt “Speaking Truth to Parallelism” best captured the attitude in question.  I coined the phrase back in 2007, when D-Wave’s claims to be solving Sudoku puzzles with a quantum computer unleashed a tsunami of journalism about QCs—what they are, how they would work, what they could do—that (in my opinion) perfectly illustrated how not to approach a metaphysically-confusing new technology.  Having said that, the endless debate around D-Wave won’t by any means be the focus of this book: it will surface, of course, but only when it helps to illustrate some broader point.

In planning this book, the trickiest issue was what to do with comments.  Ultimately, I decided that the comments make Shtetl-Optimized what it is—so for each post I include, I’ll include a brief selection of the most interesting comments, together with my responses to them.  My policy will be this: by default, I’ll consider any comments on this blog to be fair game for quoting in the book, in whole or in part, and attributed to whatever handle the commenter used.  However, if you’d like to “opt out” of having your comments quoted, I now offer you a three-month window in which to do so: just email me, or leave a comment (!) on this thread.  You can also request that certain specific comments of yours not be quoted, or that your handle be removed from your comments, or your full name added to them—whatever you want.

Update (9/24): After hearing from several of you, I’ve decided on the following modified policy.  In all cases where I have an email address, I will contact the commenters about any of their comments that I’m thinking of using, to request explicit permission to use them.  In the hopefully-rare cases where I can’t reach a given commenter, but where their comment raised what seems to like a crucial point requiring a response in the book, I might quote from the comment anyway—but in those cases, I’ll be careful not to reproduce very long passages, in a way that might run afoul of the fair use exception.

Steven Pinker’s inflammatory proposal: universities should prioritize academics

Thursday, September 11th, 2014

If you haven’t yet, I urge you to read Steven Pinker’s brilliant piece in The New Republic about what’s broken with America’s “elite” colleges and how to fix it.  The piece starts out as an evisceration of an earlier New Republic article on the same subject by William Deresiewicz.  Pinker agrees with Deresiewicz that something is wrong, but finds Deresiewicz’s diagnosis of what to be lacking.  The rest of Pinker’s article sets out his own vision, which involves America’s top universities taking the radical step of focusing on academics, and returning extracurricular activities like sports to their rightful place as extras: ways for students to unwind, rather than a university’s primary reason for existing, or a central criterion for undergraduate admissions.  Most controversially, this would mean that the admissions process at US universities would become more like that in virtually every other advanced country: a relatively-straightforward matter of academic performance, rather than an exercise in peering into the applicants’ souls to find out whether they have a special je ne sais quoi, and the students (and their parents) desperately gaming the intentionally-opaque system, by paying consultants tens of thousands of dollars to develop souls for them.

(Incidentally, readers who haven’t experienced it firsthand might not be able to understand, or believe, just how strange the undergraduate admissions process in the US has become, although Pinker’s anecdotes give some idea.  I imagine anthropologists centuries from now studying American elite university admissions, and the parenting practices that have grown up around them, alongside cannibalism, kamikaze piloting, and other historical extremes of the human condition.)

Pinker points out that a way to assess students’ ability to do college coursework—much more quickly and accurately than by relying on the soul-detecting skills of admissions officers—has existed for a century.  It’s called the standardized test.  But unlike in the rest of the world (even in ultraliberal Western Europe), standardized tests are politically toxic in the US, seen as instruments of racism, classism, and oppression.  Pinker reminds us of the immense irony here: standardized tests were invented as a radical democratizing tool, as a way to give kids from poor and immigrant families the chance to attend colleges that had previously only been open to the children of the elite.  They succeeded at that goal—too well for some people’s comfort.

We now know that the Ivies’ current emphasis on sports, “character,” “well-roundedness,” and geographic diversity in undergraduate admissions was consciously designed (read that again) in the 1920s, by the presidents of Harvard, Princeton, and Yale, as a tactic to limit the enrollment of Jews.  Nowadays, of course, the Ivies’ “holistic” admissions process no longer fulfills that original purpose, in part because American Jews learned to play the “well-roundedness” game as well as anyone, shuttling their teenage kids between sports, band practice, and faux charity work, while hiring professionals to ghostwrite application essays that speak searingly from the heart.  Today, a major effect of “holistic” admissions is instead to limit the enrollment of Asian-Americans (especially recent immigrants), who tend disproportionately to have superb SAT scores, but to be deficient in life’s more meaningful dimensions, such as lacrosse, student government, and marching band.  More generally—again, pause to wallow in the irony—our “progressive” admissions process works strongly in favor of the upper-middle-class families who know how to navigate it, and against the poor and working-class families who don’t.

Defenders of the status quo have missed this reality on the ground, it seems to me, because they’re obsessed with the notion that standardized tests are “reductive”: that is, that they reduce a human being to a number.  Aren’t there geniuses who bomb standardized tests, they ask, as well as unimaginative grinds who ace them?  And if you make test scores a major factor in admissions, then won’t students and teachers train for the tests, and won’t that pervert open-ended intellectual curiosity?  The answer to both questions, I think, is clearly “yes.”  But the status-quo-defenders never seem to take the next step, of examining the alternatives to standardized testing, to see whether they’re even worse.

I’d say the truth is this: spots at the top universities are so coveted, and so much rarer than the demand, that no matter what you use as your admissions criterion, that thing will instantly get fetishized and turned into a commodity by students, parents, and companies eager to profit from their anxiety.  If it’s grades, you’ll get a grades fetish; if sports, you’ll get a sports fetish; if community involvement, you’ll get soup kitchens sprouting up for the sole purpose of giving ambitious 17-year-olds something to write about in their application essays.  If Harvard and Princeton announced that from now on, they only wanted the most laid-back, unambitious kids, the ones who spent their summers lazily skipping stones in a lake, rather than organizing their whole lives around getting in to Harvard and Princeton, tens of thousands of parents in the New York metropolitan area would immediately enroll their kids in relaxation and stone-skipping prep courses.  So, given that reality, why not at least make the fetishized criterion one that’s uniform, explicit, predictively valid, relatively hard to game, and relevant to universities’ core intellectual mission?

(Here, I’m ignoring criticisms specific to the SAT: for example, that it fails to differentiate students at the extreme right end of the bell curve, thereby forcing the top schools to use other criteria.  Even if those criticisms are true, they could easily be fixed by switching to other tests.)

I admit that my views on this matter might be colored by my strange (though as I’ve learned, not at all unique) experience, of getting rejected from almost every “top” college in the United States, and then, ten years later, getting recruited for faculty jobs by the very same institutions that had rejected me as a teenager.  Once you understand how undergraduate admissions work, the rejections were unsurprising: I was a 15-year-old with perfect SATs and a published research paper, but not only was I young and immature, with spotty grades and a weird academic trajectory, I had no sports, no music, no diverse leadership experiences.  I was a narrow, linear, A-to-B thinker who lacked depth and emotional intelligence: the exact opposite of what Harvard and Princeton were looking for in every way.  The real miracle is that despite these massive strikes against me, two schools—Cornell and Carnegie Mellon—were nice enough to give me a chance.  (I ended up going to Cornell, where I got a great education.)

Some people would say: so then what’s the big deal?  If Harvard or MIT reject some students that maybe they should have admitted, those students will simply go elsewhere, where—if they’re really that good—they’ll do every bit as well as they would’ve done at the so-called “top” schools.  But to me, that’s uncomfortably close to saying: there are millions of people who go on to succeed in life despite childhoods of neglect and poverty.  Indeed, some of those people succeed partly because of their rough childhoods, which served as the crucibles of their character and resolve.  Ergo, let’s neglect our own children, so that they too can have the privilege of learning from the school of hard knocks just like we did.  The fact that many people turn out fine despite unfairness and adversity doesn’t mean that we should inflict unfairness if we can avoid it.

Let me end with an important clarification.  Am I saying that, if I had dictatorial control over a university (ha!), I would base undergraduate admissions solely on standardized test scores?  Actually, no.  Here’s what I would do: I would admit the majority of students mostly based on test scores.  A minority, I would admit because of something special about them that wasn’t captured by test scores, whether that something was musical or artistic talent, volunteer work in Africa, a bestselling smartphone app they’d written, a childhood as an orphaned war refugee, or membership in an underrepresented minority.  Crucially, though, the special something would need to be special.  What I wouldn’t do is what’s done today: namely, to turn “specialness” and “well-roundedness” into commodities that the great mass of applicants have to manufacture before they can even be considered.

Other than that, I would barely look at high-school grades, regarding them as too variable from one school to another.  And, while conceding it might be impossible, I would try hard to keep my university in good enough financial shape that it didn’t need any legacy or development admits at all.


Update (Sep. 14): For those who feel I’m exaggerating the situation, please read the story of commenter Jon, about a homeschooled 15-year-old doing graduate-level work in math who, three years ago, was refused undergraduate admission to both Berkeley and Caltech, with the math faculty powerless to influence the admissions officers. See also my response.

Raise a martini glass for Google and Martinis!

Saturday, September 6th, 2014

We’ve already been discussing this in the comments section of my previous post, but a few people emailed me to ask when I’d devote a separate blog post to the news.

OK, so for those who haven’t yet heard: this week Google’s Quantum AI Lab announced that it’s teaming up with John Martinis, of the University of California, Santa Barbara, to accelerate the Martinis group‘s already-amazing efforts in superconducting quantum computing.  (See here for the MIT Tech‘s article, here for Wired‘s, and here for the WSJ‘s.)  Besides building some of the best (if not the best) superconducting qubits in the world, Martinis, along with Matthias Troyer, was also one of the coauthors of two important papers that found no evidence for any speedup in the D-Wave machines.  (However, in addition to working with the Martinis group, Google says it will also continue its partnership with D-Wave, in an apparent effort to keep reality more soap-operatically interesting than any hypothetical scenario one could make up on a blog.)

I have the great honor of knowing John Martinis, even once sharing the stage with him at a “Physics Cafe” in Aspen.  Like everyone else in our field, I profoundly admire the accomplishments of his group: they’ve achieved coherence times in the tens of microseconds, demonstrated some of the building blocks of quantum error-correction, and gotten tantalizingly close to the fault-tolerance threshold for the surface code.  (When, in D-Wave threads, people have challenged me: “OK Scott, so then which experimental quantum computing groups should be supported more?,” my answer has always been some variant of: “groups like John Martinis’s.”)

So I’m excited about this partnership, and I wish it the very best.

But I know people will ask: apart from the support and well-wishes, do I have any predictions?  Alright, here’s one.  I predict that, regardless of what happens, commenters here will somehow make it out that I was wrong.  So for example, if the Martinis group, supported by Google, ultimately succeeds in building a useful, scalable quantum computer—by emphasizing error-correction, long coherence times (measured in the conventional way), “gate-model” quantum algorithms, universality, and all the other things that D-Wave founder Geordie Rose has pooh-poohed from the beginning—commenters will claim that still most of the credit belongs to D-Wave, for whetting Google’s appetite, and for getting it involved in superconducting QC in the first place.  (The unstated implication being that, even if there were little or no evidence that D-Wave’s approach would ever lead to a genuine speedup, we skeptics still would’ve been wrong to state that truth in public.)  Conversely, if this venture doesn’t live up to the initial hopes, commenters will claim that that just proves Google’s mistake: rather than “selling out to appease the ivory-tower skeptics,” they should’ve doubled down on D-Wave.  Even if something completely different happens—let’s say, Google, UCSB, and D-Wave jointly abandon their quantum computing ambitions, and instead partner with ISIS to establish the world’s first “Qualiphate,” ruling with a niobium fist over California and parts of Oregon—I would’ve been wrong for having failed to foresee that.  (Even if I did sort of foresee it in the last sentence…)

Yet, while I’ll never live to see the blog-commentariat acknowledge the fundamental reasonableness of my views, I might live to see scalable quantum computers become a reality, and that would surely be some consolation.  For that reason, even if for no others, I once again wish the Martinis group and Google’s Quantum AI Lab the best in their new partnership.


Unrelated Announcement: Check out a lovely (very basic) introductory video on quantum computing and information, narrated by John Preskill and Spiros Michalakis, and illustrated by Jorge Cham of PhD Comics.

Subhash Khot’s prizewinning research

Saturday, August 16th, 2014

I already congratulated Subhash Khot in my last post for winning the Nevanlinna Award, but this really deserves a separate post.  Khot won theoretical computer science’s highest award largely for introducing and exploring the Unique Games Conjecture (UGC), which says (in one sentence) that a large number of the approximation problems that no one has been able to prove NP-hard, really are NP-hard.  In particular, if the UGC is true, then for MAX-CUT and dozens of other important optimization problems, no polynomial-time algorithm can always get you closer to the optimal solution than some semidefinite-programming-based algorithm gets you, unless P=NP.  The UGC might or might not be true—unlike with (say) P≠NP itself, there’s no firm consensus around it—but even if it’s false, the effort to prove or disprove it has by now had a huge impact on theoretical computer science research, leading to connections with geometry, tiling, analysis of Boolean functions, quantum entanglement, and more.

There are a few features that make the UGC interesting, compared to most other questions considered in complexity theory.  Firstly, the problem that the UGC asserts is NP-hard—basically, given a list of linear equations in 2 variables each, to satisfy as many of the equations as you can—is a problem with “imperfect completeness.”  This means that, if you just wanted to know whether all the linear equations were simultaneously satisfiable, the question would be trivial to answer, using Gaussian elimination.  So the problem only becomes interesting once you’re told that the equations are not simultaneously satisfiable, but you’d like to know (say) whether it’s possible to satisfy 99% of the equations or only 1%.  A second feature is that, because of the 2010 work of Arora, Barak, and Steurer, we know that there is an algorithm that solves the unique games problem in “subexponential time”: specifically, in time exp(npoly(δ)), where δ is the completeness error (that is, the fraction of linear equations that are unsatisfiable, in the case that most of them are satisfiable).  This doesn’t mean that the unique games problem can’t be NP-hard: it just means that, if there is an NP-hardness proof, then the reduction will need to blow up the instance sizes by an npoly(1/δ) factor.

To be clear, neither of the above features is unique (har, har) to unique games: we’ve long known NP-complete problems, like MAX-2SAT, that have the imperfect completeness feature, and we also know NP-hardness reductions that blow up the instance size by an npoly(1/δ) factor for inherent reasons (for example, for the Set Cover problem).  But perhaps nothing points as clearly as UGC at the directions that researchers in hardness of approximation and probabilistically checkable proofs (PCP) would like to be able to go.  A proof of the Unique Games Conjecture would basically be a PCP theorem on steroids.  (Or, since we already have “PCP theorems on steroids,” maybe a PCP theorem on PCP?)

It’s important to understand that, between the UGC being true and the unique games problem being solvable in polynomial time, there’s a wide range of intermediate possibilities, many of which are being actively investigated.  For example, the unique games problem could be “NP-hard,” but via a reduction that itself takes subexponential time (i.e., it could be hard assuming the Exponential-Time Hypothesis).  It could be solvable much faster than Arora-Barak-Steurer but still not in P.  Or, even if the problem weren’t solvable any faster than is currently known, it could be “hard without being NP-hard,” having a similar status to factoring or graph isomorphism.  Much current research into the UGC is focused on a particular algorithm called the Sum-of-Squares algorithm (i.e., the Laserre hierarchy).  Some researchers suspect that, if any algorithm will solve the unique games problem in polynomial time (or close to that), it will be Sum-of-Squares; conversely, if one could show that Sum-of-Squares failed, one would’ve taken a major step toward proving the UGC.

For more, I recommend this Quanta magazine article, or Luca Trevisan’s survey, or Subhash’s own survey.  Or those pressed for time can simply check out this video interview with Subhash.  If you’d like to try my wife Dana’s puzzle games inspired by PCP, which Subhash uses 2 minutes into the video to explain what he works on, see here.  Online, interactive versions of these puzzle games are currently under development.  Also, if you have questions about the UGC or Subhash’s work, go ahead and ask: I’ll answer if I can, and otherwise rely on in-house expertise.

Congratulations again to Subhash!

Seth Teller (1964-2014)

Friday, July 11th, 2014

Seth Teller

Seth Teller was a colleague of mine in CSAIL and the EECS department, and was one of my favorite people in all of MIT.  He was a brilliant roboticist, who (among many other things) spearheaded MIT’s participation in the DARPA Grand Challenge for self-driving cars, and who just recently returned from a fact-finding trip to Fukushima, Japan, to see how robots could help in investigating the damaged reactor cores there.  I saw Seth twice a week at lab and department lunches, and he often struck up conversations with me about quantum computing, cosmology, and other things.  His curiosity was immense, wide-ranging, and almost childlike (in the best way).  One small indication of his character is that, in the DARPA challenge, Seth opted not to preload MIT’s car with detailed data about the course, because he thought doing so made the challenge scientifically less interesting—even though DARPA’s rules allowed such preloading, the other teams did it, and it almost certainly would have improved MIT’s standing in the competition.

Seth was a phenomenal speaker, whose passion and clarity always won me over even though my research interests were far from his.  I made it a point to show up for lab lunch whenever I knew he’d be speaking.  Seth was also, from what I’ve heard, a superb mentor and teacher, who won an award earlier this year for his undergraduate advising.

Seth died ten days ago, on July 1st.  (See here for MIT News’s detailed obituary, and here for an article in Cambridge Day.)  While no cause of death was given at the time, according to an update yesterday in the MIT Tech, the death has been ruled a suicide.  Seth is survived by his wife, Rachel, and by two daughters.

With his cheerful, can-do disposition, Seth is one of the last people on earth I’d imagine doing this: whatever he was going through, he did an unbelievable job of hiding it.  I’m certain he wouldn’t abandon his family unless he was suffering unimaginable pain.  If there’s a tiny atom of good to come out of this, I hope that at least one other person contemplating suicide will reflect on how much Seth had to live for, and that doing so will inspire that person to get the medical help they need.

Incidentally, outside of his research and teaching, Seth was also an activist for protecting the physical environment and open spaces of East Cambridge.  At the “Wild and Crazy Ideas Session” of one CSAIL retreat, Seth floated a truly wild idea: to replace Memorial Drive, or at least the part of it that separates the MIT campus from the Charles River, by an underground tunnel, so that the land above the tunnel could be turned into a beautiful riverfront park.  In his characteristic fashion, Seth had already done a pretty detailed engineering analysis, estimating the cost at “merely” a few hundred million dollars: a lot, but a worthy investment in MIT’s future.  In any case, I can’t imagine a better way to memorialize Seth than to name some green space in East Cambridge after him, and I hope that happens.

Seth will be sorely missed.  My thoughts go out to his family at this difficult time.

CCC’s Declaration of Independence

Friday, June 6th, 2014

Recently, the participants of the Conference on Computational Complexity (CCC)—the latest iteration of which I’ll be speaking at next week in Vancouver—voted to declare their independence from the IEEE, and to become a solo, researcher-organized conference.  See this open letter for the reasons why (basically, IEEE charged a huge overhead, didn’t allow open access to the proceedings, and increased rather than decreased the administrative burden on the organizers).  As a former member of the CCC Steering Committee, I’m in violent agreement with this move, and only wish we’d managed to do it sooner.

Now, Dieter van Melkebeek (the current Steering Committee chair) is asking complexity theorists to sign a public Letter of Support, to make it crystal-clear that the community is behind the move to independence.  And Jeff Kinne has asked me to advertise the letter on my blog.  So, if you’re a complexity theorist who agrees with the move, please go there and sign (it already has 111 signatures, but could use more).

Meanwhile, I wish to express my profound gratitude to Dieter, Jeff, and the other Steering Committee members for their efforts toward independence.  The only thing I might’ve done differently would be to add a little more … I dunno, pizzazz to the documents explaining the reasons for the move.  Like:

When in the Course of human events, it becomes necessary for a conference to dissolve the organizational bands that have connected it with the IEEE, and to assume among the powers of the earth, the separate and equal station to which the Laws of Mathematics and the CCC Charter entitle it, a decent respect to the opinions of theorist-kind requires that the participants should declare the causes which impel them to the separation.

We hold these truths to be self-evident (but still in need of proof), that P and NP are created unequal, that one-way functions exist, that the polynomial hierarchy is infinite…

Quantifying the Rise and Fall of Complexity in Closed Systems: The Coffee Automaton

Tuesday, May 27th, 2014

Update (June 3): A few days after we posted this paper online, Brent Werness, a postdoc in probability theory at the University of Washington, discovered a serious error in the “experimental” part of the paper.  Happily, Brent is now collaborating with us on producing a new version of the paper that fixes the error, which we hope to have available within a few months (and which will replace the version currently on the arXiv).

To make a long story short: while the overall idea, of measuring “apparent complexity” by the compressed file size of a coarse-grained image, is fine, the “interacting coffee automaton” that we study in the paper is not an example where the apparent complexity becomes large at intermediate times.  That fact can be deduced as a corollary of a result of Liggett from 2009 about the “symmetric exclusion process,” and can be seen as a far-reaching generalization of a result that we prove in our paper’s appendix: namely, that in the non-interacting coffee automaton (our “control case”), the apparent complexity after t time steps is upper-bounded by O(log(nt)).  As it turns out, we were more right than we knew to worry about large-deviation bounds giving complete mathematical control over what happens when the cream spills into the coffee, thereby preventing the apparent complexity from ever becoming large!

But what about our numerical results, which showed a small but unmistakable complexity bump for the interacting automaton (figure 10(a) in the paper)?  It now appears that the complexity bump we saw in our data is likely to be explainable by an incomplete removal of what we called “border pixel artifacts”: that is, “spurious” complexity that arises merely from the fact that, at the border between cream and coffee, we need to round the fraction of cream up or down to the nearest integer to produce a grayscale.  In the paper, we devoted a whole section (Section 6) to border pixel artifacts and the need to deal with them: something sufficiently non-obvious that in the comments of this post, you can find people arguing with me that it’s a non-issue.  Well, it now appears that we erred by underestimating the severity of border pixel artifacts, and that a better procedure to get rid of them would also eliminate the complexity bump for the interacting automaton.

Once again, this error has no effect on either the general idea of complexity rising and then falling in closed thermodynamic systems, or our proposal for how to quantify that rise and fall—the two aspects of the paper that have generated the most interest.  But we made a bad choice of model system with which to illustrate those ideas.  Had I looked more carefully at the data, I could’ve noticed the problem before we posted, and I take responsibility for my failure to do so.

The good news is that ultimately, I think the truth only makes our story more interesting.  For it turns out that apparent complexity, as we define it, is not something that’s trivial to achieve by just setting loose a bunch of randomly-walking particles, which bump into each other but are otherwise completely independent.  If you want “complexity” along the approach to thermal equilibrium, you need to work a bit harder for it.  One promising idea, which we’re now exploring, is to consider a cream tendril whose tip takes a random walk through the coffee, leaving a trail of cream in its wake.  Using results in probability theory—closely related, or so I’m told, to the results for which Wendelin Werner won his Fields Medal!—it may even be possible to prove analytically that the apparent complexity becomes large in thermodynamic systems with this sort of behavior, much as one can prove that the complexity doesn’t become large in our original coffee automaton.

So, if you’re interested in this topic, stay tuned for the updated version of our paper.  In the meantime, I wish to express our deepest imaginable gratitude to Brent Werness for telling us all this.


Good news!  After nearly three years of procrastination, fellow blogger Sean Carroll, former MIT undergraduate Lauren Ouellette, and yours truly finally finished a paper with the above title (coming soon to an arXiv near you).  PowerPoint slides are also available (as usual, you’re on your own if you can’t open them—sorry!).

For the background and context of this paper, please see my old post “The First Law of Complexodynamics,” which discussed Sean’s problem of defining a “complextropy” measure that first increases and then decreases in closed thermodynamic systems, in contrast to entropy (which increases monotonically).  In this exploratory paper, we basically do five things:

  1. We survey several candidate “complextropy” measures: their strengths, weaknesses, and relations to one another.
  2. We propose a model system for studying such measures: a probabilistic cellular automaton that models a cup of coffee into which cream has just been poured.
  3. We report the results of numerical experiments with one of the measures, which we call “apparent complexity” (basically, the gzip file size of a smeared-out image of the coffee cup).  The results confirm that the apparent complexity does indeed increase, reach a maximum, then turn around and decrease as the coffee and cream mix.
  4. We discuss a technical issue that one needs to overcome (the so-called “border pixels” problem) before one can do meaningful experiments in this area, and offer a solution.
  5. We raise the open problem of proving analytically that the apparent complexity ever becomes large for the coffee automaton.  To underscore this problem’s difficulty, we prove that the apparent complexity doesn’t become large in a simplified version of the coffee automaton.

Anyway, here’s the abstract:

In contrast to entropy, which increases monotonically, the “complexity” or “interestingness” of closed systems seems intuitively to increase at first and then decrease as equilibrium is approached. For example, our universe lacked complex structures at the Big Bang and will also lack them after black holes evaporate and particles are dispersed. This paper makes an initial attempt to quantify this pattern. As a model system, we use a simple, two-dimensional cellular automaton that simulates the mixing of two liquids (“coffee” and “cream”). A plausible complexity measure is then the Kolmogorov complexity of a coarse-grained approximation of the automaton’s state, which we dub the “apparent complexity.” We study this complexity measure, and show analytically that it never becomes large when the liquid particles are non-interacting. By contrast, when the particles do interact, we give numerical evidence that the complexity reaches a maximum comparable to the “coffee cup’s” horizontal dimension. We raise the problem of proving this behavior analytically.

Questions and comments more than welcome.


In unrelated news, Shafi Goldwasser has asked me to announce that the Call for Papers for the 2015 Innovations in Theoretical Computer Science (ITCS) conference is now available.

Is There Anything Beyond Quantum Computing?

Friday, April 11th, 2014

So I’ve written an article about the above question for PBS’s website—a sort of tl;dr version of my 2005 survey paper NP-Complete Problems and Physical Reality, but updated with new material about the simulation of quantum field theories and about AdS/CFT.  Go over there, read the article (it’s free), then come back here to talk about it if you like.  Thanks so much to Kate Becker for commissioning the article.

In other news, there’s a profile of me at MIT News (called “The Complexonaut”) that some people might find amusing.

Oh, and anyone who thinks the main reason to care about quantum computing is that, if our civilization ever manages to surmount the profound scientific and technological obstacles to building a scalable quantum computer, then that little padlock icon on your web browser would no longer represent ironclad security?  Ha ha.  Yeah, it turns out that, besides factoring integers, you can also break OpenSSL by (for example) exploiting a memory bug in C.  The main reason to care about quantum computing is, and has always been, science.

Umesh Vazirani responds to Geordie Rose

Thursday, February 6th, 2014

You might recall that Shin, Smith, Smolin, and Vazirani posted a widely-discussed preprint a week ago, questioning the evidence for large-scale quantum behavior in the D-Wave machine.  Geordie Rose responded here.   Tonight, in a Shtetl-Optimized exclusive scoop, I bring you Umesh Vazirani’s response to Geordie’s comments. Without further ado:


Even a cursory reading of our paper will reveal that Geordie Rose is attacking a straw man. Let me quickly outline the main point of our paper and the irrelevance of Rose’s comments:

To date the Boixo et al paper was the only serious evidence in favor of large scale quantum behavior by the D-Wave machine. We investigated their claims and showed that there are serious problems with their conclusions. Their conclusions were based on the close agreement between the input-output data from D-Wave and quantum simulated annealing, and their inability despite considerable effort to find any classical model that agreed with the input-output data. In our paper, we gave a very simple classical model of interacting magnets that closely agreed with the input-output data. We stated that our results implied that “it is premature to conclude that D-Wave machine exhibits large scale quantum behavior”.

Rose attacks our paper for claiming that “D-Wave processors are inherently classical, and can be described by a classical model with no need to invoke quantum mechanics.”  A reading of our paper will make it perfectly clear that this is not a claim that we make.  We state explicitly “It is worth emphasizing that the goal of this paper is not to provide a classical model for the D-Wave machine, … The classical model introduced here is useful for the purposes of studying the large-scale algorithmic features of the D-Wave machine. The task of finding an accurate model for the D-Wave machine (classical, quantum or otherwise), would be better pursued with direct access, not only to programming the D-Wave machine, but also to its actual hardware.”

Rose goes on to point to a large number of experiments conducted by D-Wave to prove small scale entanglement over 2-8 qubits and criticizes our paper for not trying to model those aspects of D-Wave. But such small scale entanglement properties are not directly relevant to prospects for a quantum speedup. Therefore we were specifically interested in claims about the large scale quantum behavior of D-Wave. There was exactly one such claim, which we duly investigated, and it did not stand up to scrutiny.