Archive for the ‘Adventures in Meatspace’ Category

Interstellar’s dangling wormholes

Monday, November 10th, 2014

Update (Nov. 15): A third of my confusions addressed by reading Kip Thorne’s book! Details at the bottom of this post.


On Saturday Dana and I saw Interstellar, the sci-fi blockbuster co-produced by the famous theoretical physicist Kip Thorne (who told me about his work on this movie when I met him eight years ago).  We had the rare privilege of seeing the movie on the same day that we got to hang out with a real astronaut, Dan Barry, who flew three shuttle missions and did four spacewalks in the 1990s.  (As the end result of a project that Dan’s roboticist daughter, Jenny Barry, did for my graduate course on quantum complexity theory, I’m now the coauthor with both Barrys on a paper in Physical Review A, about uncomputability in quantum partially-observable Markov decision processes.)

Before talking about the movie, let me say a little about the astronaut.  Besides being an inspirational example of someone who’s achieved more dreams in life than most of us—seeing the curvature of the earth while floating in orbit around it, appearing on Survivor, and publishing a Phys. Rev. A paper—Dan is also a passionate advocate of humanity’s colonizing other worlds.  When I asked him whether there was any future for humans in space, he answered firmly that the only future for humans was in space, and then proceeded to tell me about the technical viability of getting humans to Mars with limited radiation exposure, the abundant water there, the romantic appeal that would inspire people to sign up for the one-way trip, and the extinction risk for any species confined to a single planet.  Hearing all this from someone who’d actually been to space gave Interstellar, with its theme of humans needing to leave Earth to survive (and its subsidiary theme of the death of NASA’s manned space program meaning the death of humanity), a special vividness for me.  Granted, I remain skeptical about several points: the feasibility of a human colony on Mars in the foreseeable future (a self-sufficient human colony on Antarctica, or under the ocean, strike me as plenty hard enough for the next few centuries); whether a space colony, even if feasible, cracks the list of the top twenty things we ought to be doing to mitigate the risk of human extinction; and whether there’s anything more to be learned, at this point in history, by sending humans to space that couldn’t be learned a hundred times more cheaply by sending robots.  On the other hand, if there is a case for continuing to send humans to space, then I’d say it’s certainly the case that Dan Barry makes.

OK, but enough about the real-life space traveler: what did I think about the movie?  Interstellar is a work of staggering ambition, grappling with some of the grandest themes of which sci-fi is capable: the deterioration of the earth’s climate; the future of life in the universe; the emotional consequences of extreme relativistic time dilation; whether “our” survival would be ensured by hatching human embryos in a faraway world, while sacrificing almost all the humans currently alive; to what extent humans can place the good of the species above family and self; the malleability of space and time; the paradoxes of time travel.  It’s also an imperfect movie, one with many “dangling wormholes” and unbalanced parentheses that are still generating compile-time errors in my brain.  And it’s full of stilted dialogue that made me giggle—particularly when the characters discussed jumping into a black hole to retrieve its “quantum data.”  Also, despite Kip Thorne’s involvement, I didn’t find the movie’s science spectacularly plausible or coherent (more about that below).  On the other hand, if you just wanted a movie that scrupulously obeyed the laws of physics, rather than intelligently probing their implications and limits, you could watch any romantic comedy.  So sure, Interstellar might make you cringe, but if you like science fiction at all, then it will also make you ponder, stare awestruck, and argue with friends for days afterward—and enough of the latter to make it more than worth your while.  Just one tip: if you’re prone to headaches, do not sit near the front of the theater, especially if you’re seeing it in IMAX.

For other science bloggers’ takes, see John Preskill (who was at a meeting with Steven Spielberg to brainstorm the movie in 2006), Sean Carroll, Clifford Johnson, and Peter Woit.

In the rest of this post, I’m going to list the questions about Interstellar that I still don’t understand the answers to (yes, the ones still not answered by the Interstellar FAQ).  No doubt some of these are answered by Thorne’s book The Science of Interstellar, which I’ve ordered (it hasn’t arrived yet), but since my confusions are more about plot than science, I’m guessing that others are not.

SPOILER ALERT: My questions give away basically the entire plot—so if you’re planning to see the movie, please don’t read any further.  After you’ve seen it, though, come back and see if you can help with any of my questions.


1. What’s causing the blight, and the poisoning of the earth’s atmosphere?  The movie is never clear about this.  Is it a freak occurrence, or is it human-caused climate change?  If the latter, then wouldn’t it be worth some effort to try to reverse the damage and salvage the earth, rather than escaping through a wormhole to another galaxy?

2. What’s with the drone?  Who sent it?  Why are Cooper and Murph able to control it with their laptop?  Most important of all, what does it have to do with the rest of the movie?

3. If NASA wanted Cooper that badly—if he was the best pilot they’d ever had and NASA knew it—then why couldn’t they just call him up?  Why did they have to wait for beings from the fifth dimension to send a coded message to his daughter revealing their coordinates?  Once he did show up, did they just kind of decide opportunistically that it would be a good idea to recruit him?

4. What was with Cooper’s crash in his previous NASA career?  If he was their best pilot, how and why did the crash happen?  If this was such a defining, traumatic incident in his life, why is it never brought up for the rest of the movie?

5. How is NASA funded in this dystopian future?  If official ideology holds that the Apollo missions were faked, and that growing crops is the only thing that matters, then why have the craven politicians been secretly funneling what must be trillions of dollars to a shadow-NASA, over a period of fifty years?

6. Why couldn’t NASA have reconnoitered the planets using robots—especially since this is a future where very impressive robots exist?  Yes, yes, I know, Matt Damon explains in the movie that humans remain more versatile than robots, because of their “survival instinct.”  But the crew arrives at the planets missing extremely basic information about them, like whether they’re inhospitable to human life because of freezing temperatures or mile-high tidal waves.  This is information that robotic probes, even of the sort we have today, could have easily provided.

7. Why are the people who scouted out the 12 planets so limited in the data they can send back?  If they can send anything, then why not data that would make Cooper’s mission completely redundant (excepting, of course, the case of the lying Dr. Mann)?  Does the wormhole limit their transmissions to 1 bit per decade or something?

8. Rather than wasting precious decades waiting for Cooper’s mission to return, while (presumably) billions of people die of starvation on a fading earth, wouldn’t it make more sense for NASA to start colonizing the planets now?  They could simply start trial colonies on all the planets, even if they think most of the colonies will fail.  Yes, this plan involves sacrificing individuals for the greater good of humanity, but NASA is already doing that anyway, with its slower, riskier, stupider reconnaissance plan.  The point becomes even stronger when we remember that, in Professor Brand’s mind, the only feasible plan is “Plan B” (the one involving the frozen human embryos).  Frozen embryos are (relatively) cheap: why not just spray them all over the place?  And why wait for “Plan A” to fail before starting that?

9. The movie involves a planet, Miller, that’s so close to the black hole Gargantua, that every hour spent there corresponds to seven years on earth.  There was an amusing exchange on Slate, where Phil Plait made the commonsense point that a planet that deep in a black hole’s gravity well would presumably get ripped apart by tidal forces.  Plait later had to issue an apology, since, in conceiving this movie, Kip Thorne had made sure that Gargantua was a rapidly rotating black hole—and it turns out that the physics of rotating black holes are sufficiently different from those of non-rotating ones to allow such a planet in principle.  Alas, this clever explanation still leaves me unsatisfied.  Physicists, please help: even if such a planet existed, wouldn’t safely landing a spacecraft on it, and getting it out again, require a staggering amount of energy—well beyond what the humans shown in the movie can produce?  (If they could produce that much acceleration and deceleration, then why couldn’t they have traveled from Earth to Saturn in days rather than years?)  If one could land on Miller and then get off of it using the relatively conventional spacecraft shown in the movie, then the amusing thought suggests itself that one could get factor-of-60,000 computational speedups, “free of charge,” by simply leaving one’s computer in space while one spent some time on the planet.  (And indeed, something like that happens in the movie: after Cooper and Anne Hathaway return from Miller, Romilly—the character who stayed behind—has had 23 years to think about physics.)

10. Why does Cooper decide to go into the black hole?  Surely he could jettison enough weight to escape the black hole’s gravity by sending his capsule into the hole, while he himself shared Anne Hathaway’s capsule?

11. Speaking of which, does Cooper go into the black hole?  I.e., is the “tesseract” something he encounters before or after he crosses the event horizon?  (Or maybe it should be thought of as at the event horizon—like a friendlier version of the AMPS firewall?)

12. Why is Cooper able to send messages back in time—but only by jostling books around, moving the hands of a watch, and creating patterns of dust in one particular room of one particular house?  (Does this have something to do with love and gravity being the only two forces in the universe that transcend space and time?)

13. Why does Cooper desperately send the message “STAY” to his former self?  By this point in the movie, isn’t it clear that staying on Earth means the death of all humans, including Murph?  If Cooper thought that a message could get through at all, then why not a message like: “go, and go directly to Edmunds’ planet, since that’s the best one”?  Also, given that Cooper now exists outside of time, why does he feel such desperate urgency?  Doesn’t he get, like, infinitely many chances?

14. Why is Cooper only able to send “quantum data” that saves the world to the older Murph—the one who lives when (presumably) billions of people are already dying of starvation?  Why can’t he send the “quantum data” back to the 10-year-old Murph, for example?  Even if she can’t yet understand it, surely she could hand it over to Professor Brand.  And even if this plan would be unlikely to succeed: again, Cooper now exists outside of time.  So can’t he just keep going back to the 10-year-old Murph, rattling those books over and over until the message gets through?

15. What exactly is the “quantum data” needed for, anyway?  I gather it has something to do with building a propulsion system that can get the entire human population out of the earth’s gravity well at a reasonable cost?  (Incidentally, what about all the animals?  If the writers of the Old Testament noticed that issue, surely the writers of Interstellar could.)

16. How does Cooper ever make it out of the black hole?  (Maybe it was explained and I missed it: once he entered the black hole, things got extremely confusing.)  Do the fifth-dimensional beings create a new copy of Cooper outside the black hole?  Do they postselect on a branch of the wavefunction where he never entered the black hole in the first place?  Does Murph use the “quantum data” to get him out?

17. At his tearful reunion with the elderly Murph, why is Cooper totally uninterested in meeting his grandchildren and great-grandchildren, who are in the same room?  And why are they uninterested in meeting him?  I mean, seeing Murph again has been Cooper’s overriding motivation during his journey across the universe, and has repeatedly been weighed against the survival of the entire human race, including Murph herself.  But seeing Murph’s kids—his grandkids—isn’t even worth five minutes?

18. Speaking of which, when did Murph ever find time to get married and have kids?  Since she’s such a major character, why don’t we learn anything about this?

19. Also, why is Murph an old woman by the time Cooper gets back?  Yes, Cooper lost a few decades because of the time dilation on Miller’s planet.  I guess he lost the additional decades while entering and leaving Gargantua?  If the five-dimensional beings were able to use their time-travel / causality-warping powers to get Cooper out of the black hole, couldn’t they have re-synced his clock with Murph’s while they were at it?

20. Why does Cooper need to steal a spaceship to get to Anne Hathaway’s planet?  Isn’t Murph, like, the one in charge?  Can’t she order that a spaceship be provided for Cooper?

21. Astute readers will note that I haven’t yet said anything about the movie’s central paradox, the one that dwarfs all the others.  Namely, if humans were going to go extinct without a “wormhole assist” from the humans of the far future, then how were there any humans in the far future to provide the wormhole assist?  And conversely, if the humans of the far future find themselves already existing, then why do they go to the trouble to put the wormhole in their past (which now seems superfluous, except maybe for tidying up the story of their own origins)?  The reason I didn’t ask about this is that I realize it’s supposed to be paradoxical; we’re supposed to feel vertigo thinking about it.  (And also, it’s not entirely unrelated to how PSPACE-complete problems get solved with polynomial resources, in my and John Watrous’s paper on computation with closed timelike curves.)  My problem is a different one: if the fifth-dimensional, far-future humans have the power to mold their own past to make sure everything turned out OK, then what they actually do seems pathetic compared to what they could do.  For example, why don’t they send a coded message to the 21st-century humans (similar to the coded messages that Cooper sends to Murph), telling them how to avoid the blight that destroys their crops?  Or just telling them that Edmunds’ planet is the right one to colonize?  Like the God of theodicy arguments, do the future humans want to use their superpowers only to give us a little boost here and there, while still leaving us a character-forming struggle?  Even if this reticence means that billions of innocent people—ones who had nothing to do with the character-forming struggle—will die horrible deaths?  If so, then I don’t understand these supposedly transcendently-evolved humans any better than I understand the theodical God.


Anyway, rather than ending on that note of cosmic pessimism, I guess I could rejoice that we’re living through what must be the single biggest month in the history of nerd cinema—what with a sci-fi film co-produced by a great theoretical physicist, a Stephen Hawking biopic, and the Alan Turing movie coming out in a few weeks.  I haven’t yet seen the latter two.  But it looks like the time might be ripe to pitch my own decades-old film ideas, like “Radical: The Story of Évariste Galois.”


Update (Nov. 15): I just finished reading Kip Thorne’s interesting book The Science of Interstellar.  I’d say that it addresses (doesn’t always clear up, but at least addresses) 7 of my 21 confusions: 1, 4, 9, 10, 11, 15, and 19.  Briefly:

1. Thorne correctly notes that the movie is vague about what’s causing the blight and the change to the earth’s atmosphere, but he discusses a bunch of possibilities, which are more in the “freak disaster” than the “manmade” category.

4. Cooper’s crash was supposed to have been caused by a gravitational anomaly, as the bulk beings of the far future were figuring out how to communicate with 21st-century humans.  It was another foreshadowing of those bulk beings.

9. Thorne notices the problem of the astronomical amount of energy needed to safely land on Miller’s planet and then get off of it—given that this planet is deep inside the gravity well of the black hole Gargantua, and orbiting Gargantua at a large fraction of the speed of light.  Thorne offers a solution that can only be called creative: namely, while nothing about this was said in the movie (since Christopher Nolan thought it would confuse people), it turns out that the crew accelerated to relativistic speed and then decelerated using a gravitational slingshot around a second, intermediate-mass black hole, which just happened to be in the vicinity of Gargantua at precisely the right times for this.  Thorne again appeals to slingshots around unmentioned but strategically-placed intermediate-mass black holes several more times in the book, to explain other implausible accelerations and decelerations that I hadn’t even noticed.

10. Thorne acknowledges that Cooper didn’t really need to jump into Gargantua in order to jettison the mass of his body (which is trivial compared to the mass of the spacecraft).  Cooper’s real reason for jumping, he says, was the desperate hope that he could somehow find the quantum data there needed to save the humans on Earth, and then somehow get it out of the black hole and back to the humans.  (This being a movie, it of course turns out that Cooper was right.)

11. Yes, Cooper encounters the tesseract while inside the black hole.  Indeed, he hits it while flying into a singularity that’s behind the event horizon, but that isn’t the black hole’s “main” singularity—it’s a different, milder singularity.

15. While this wasn’t made clear in the movie, the purpose of the quantum data was indeed to learn how to manipulate the gravitational anomalies in order to decrease Newton’s constant G in the vicinity of the earth—destroying the earth but also allowing all the humans to escape its gravity with the rocket fuel that’s available.  (Again, nothing said about the poor animals.)

19. Yes, Cooper lost the additional decades while entering Gargantua.  (Furthermore, while Thorne doesn’t discuss this, I guess he must have lost them only when he was still with Anne Hathaway, not after he separates from her.  For otherwise, Anne Hathaway would also be an old woman by the time Cooper reaches her on Edmunds’ planet, contrary to what’s shown in the movie.)

Speaking Truth to Parallelism at Cornell

Friday, October 3rd, 2014

This week I was at my alma mater, Cornell, to give a talk at the 50th anniversary celebration of its computer science department.  You can watch the streaming video here; my talk runs from roughly 1:17:30 to 1:56 (though if you’ve seen other complexity/physics/humor shows by me, this one is pretty similar, except for the riff about Cornell at the beginning).

The other two things in that video—a talk by Tom Henzinger about IST Austria, a bold new basic research institute that he leads, closely modeled after the Weizmann Institute in Israel; and a discussion panel about the future of programming languages—are also really interesting and worth watching.  There was lots of other good stuff at this workshop, including a talk about Google Glass and its applications to photography (by, not surprisingly, a guy wearing a Google Glass—Marc Levoy); a panel discussion with three Turing Award winners, Juris Hartmanis, John Hopcroft, and Ed Clarke, about the early days of Cornell’s CS department; a talk by Amit Singhal, Google’s director of search; a talk about differential privacy by Cynthia Dwork, one of the leading researchers at the recently-closed Microsoft SVC lab (with a poignant and emotional ending); and a talk by my own lab director at MIT, Daniela Rus, about her research in robotics.

Along with the 50th anniversary celebration, Bill Gates was also on campus to dedicate Bill and Melinda Gates Hall, the new home of Cornell’s CS department.  Click here for streaming video of a Q&A that Gates did with Cornell students, where I thought he acquitted himself quite well, saying many sensible things about education, the developing world, etc. that other smart people could also say, but that have extra gravitas coming from him.  Gates has also become extremely effective at wrapping barbs of fact inside a soft mesh of politically-unthreatening platitudes—but listen carefully and you’ll hear the barbs.  The amount of pomp and preparation around Gates’s visit reminded me of when President Obama visited MIT, befitting the two men’s approximately equal power.  (Obama has nuclear weapons, but then again, he also has Congress.)

And no, I didn’t get to meet Gates or shake his hand, though I did get to stand about ten feet from him at the Gates Hall dedication.  (He apparently spent most of his time at Cornell meeting with plant breeders, and other people doing things relevant to the Gates Foundation’s interests.)

Thanks so much to Bobby and Jon Kleinberg, and everyone else who invited me to this fantastic event and helped make it happen.  May Cornell’s CS department have a great next 50 years.

One last remark before I close this post.  Several readers have expressed disapproval and befuddlement over the proposed title of my next book, “Speaking Truth to Parallelism.”  In the words of commenter TonyK:

That has got to be the worst title in the history of publishing! “Speaking Truth to Parallelism”? It doesn’t even make sense! I count myself as one of your fans, Scott, but you’re going to have to do better than that if you want anybody else to buy your book. I know you can do better — witness “Quantum Computing Since Democritus”.

However, my experiences at Cornell this week helped to convince me that, not only does “Speaking Truth to Parallelism” make perfect sense, it’s an activity that’s needed now more than ever.  What it means, of course, is fighting a certain naïve, long-ago-debunked view of quantum computers—namely, that they would achieve exponential speedups by simply “trying every possible answer in parallel”—that’s become so entrenched in the minds of many journalists, laypeople, and even scientists from other fields that it feels like nothing you say can possibly dislodge it.  The words out of your mouth will literally be ignored, misheard, or even contorted to the opposite of what they mean, if that’s what it takes to preserve the listener’s misconception about quantum computers being able to solve NP-hard optimization problems by sheer magic.  (Much like in the Simpsons-visit-Australia episode, where Marge’s request for “coffee” is misheard over and over as “beer.”)  You probably think I’m exaggerating, and I’d agree with you—if I hadn’t experienced this phenomenon hundreds of times over the last decade.

So, to take one example: after my talk at Cornell, an audience member came up to me to say that it was a wonderful talk, but that what he really wanted to know was whether I thought quantum computers could solve problems in the “NP space” in linear time, by trying all the possible solutions at once.  He didn’t seem to realize that I’d spent the entire previous half hour answering that exact question, explaining why the answer was “no.”  Coincidentally, this week I also got an email from a longtime reader of this blog, saying that he read and loved Quantum Computing Since Democritus, and wanted my feedback on a popular article he’d written about quantum computing.  What was the gist of the article?  You guessed it: “quantum computing = generic exponential speedups for optimization, machine learning, and Big Data problems, by trying all the possible answers at once.”

These people’s enthusiasm for quantum computing tends to be so genuine, so sincere, that I find myself unable to blame them—even when they’ve done the equivalent of going up to Richard Dawkins and thanking him for having taught them that evolution works for the good of the entire species, just as its wise Designer intended.  I do blame the media and other careless or unscrupulous parties for misleading people about quantum computing, but most of all I blame myself, for not making my explanations clear enough.  In the end, then, meeting the “NP space” folks only makes me want to redouble my efforts to Speak Truth to Parallelism: eventually, I feel, the nerd world will get this point.


Update (Oct. 4): I had regarded this (perhaps wrongly) as too obvious to state, but particularly for non-native English speakers, I’d better clarify: “speaking truth to parallelism” is a deliberate pun on the left-wing protester phrase “speaking truth to power.”  So whatever linguistic oddness there is in my phrase, I’d say it simply inherits from the original.

Another Update (Oct. 7): See this comment for my short summary of what’s known about the actual technical question (can quantum computers solve NP-complete problems in polynomial time, or not?).

Another Update (Oct. 8): Many commenters wrote to point out that the video of my talk at Cornell is now password-protected, and no longer publicly available.  I wrote to my contacts at Cornell to ask about this, and they said they’re planning to release lightly-edited versions of the videos soon, but will look into the matter in the meantime.

BosonSampling Lecture Notes from Rio

Saturday, December 28th, 2013

Update (January 3): There’s now a long interview with me about quantum computing in the Washington Post (or at least, on their website).  The interview accompanies their lead article about quantum computing and the NSA, which also quotes me (among many others), and which reports—unsurprisingly—that the NSA is indeed interested in building scalable quantum computers but, based on the Snowden documents, appears to be quite far from that goal.

(Warning: The interview contains a large number of typos and other errors, which might have arisen from my infelicities in speaking or the poor quality of the phone connection.  Some were corrected but others remain.)


The week before last, I was in Rio de Janeiro to give a mini-course on “Complexity Theory and Quantum Optics” at the Instituto de Física of the Universidade Federal Fluminense.  Next week I’ll be giving a similar course at the Jerusalem Winter School on Quantum Information.

In the meantime, my host in Rio, Ernesto Galvão, and others were kind enough to make detailed, excellent notes for my five lectures in Rio.  You can click the link in the last sentence to get them, or here are links for the five lectures individually:

If you have questions or comments about the lectures, leave them here (since I might not check the quantumrio blog).

One other thing: I can heartily recommend a trip to Rio to anyone interested in quantum information—or, for that matter, to anyone interested in sunshine, giant Jesus statues, or (especially) fruit juices you’ve never tasted before.  My favorite from among the latter was acerola.  Also worth a try are caja, mangaba, guarana, umbu, seriguela, amora, and fruta do conde juices—as well as caju and cacao, even though they taste almost nothing like the more commercially exportable products from the same plants (cashews and chocolate respectively).  I didn’t like cupuaçu or graviola juices.  Thanks so much to Ernesto and everyone else for inviting me (not just because of the juice).

Update (January 2): You can now watch videos of my mini-course at the Jerusalem Winter School here.

Videos of the other talks at the Jerusalem Winter School are available from the same site (just scroll through them on the right).

Five announcements

Tuesday, October 1st, 2013

Update (Oct. 3): OK, a sixth announcement.  I just posted a question on CS Theory StackExchange, entitled Overarching reasons why problems are in P or BPP.  If you have suggested additions or improvements to my rough list of “overarching reasons,” please post them over there — thanks!


1. I’m in Oxford right now, for a Clay Institute workshop on New Insights into Computational Intractability.  The workshop is concurrent with three others, including one on Number Theory and Physics that includes an amplituhedron-related talk by Andrew Hodges.  (Speaking of which, see here for a small but non-parodic observation about expressing amplitudes as volumes of polytopes.)

2. I was hoping to stay in the UK one more week, to attend the Newton Institute’s special semester on Mathematical Challenges in Quantum Information over in Cambridge.  But alas I had to cancel, since my diaper-changing services are needed in the other Cambridge.  So, if anyone in Cambridge (or anywhere else in the United Kingdom) really wants to talk to me, come to Oxford this week!

3. Back in June, Jens Eisert and three others posted a preprint claiming that the output of a BosonSampling device would be “indistinguishable from the uniform distribution” in various senses.  Ever since then, people have emailing me, leaving comments on this blog, and cornering me at conferences to ask whether Alex Arkhipov and I had any response to these claims.  OK, so just this weekend, we posted our own 41-page preprint, entitled “BosonSampling Is Far From Uniform.”  I hope it suffices by way of reply!  (Incidentally, this is also the paper I hinted at in a previous post: the one where π2/6 and the Euler-Mascheroni constant make cameo appearances.)  To clarify, if we just wanted to answer the claims of the Eisert group, then I think a couple paragraphs would suffice for that (see, for example, these PowerPoint slides).  In our new paper, however, Alex and I take the opportunity to go further: we study lots of interesting questions about the statistical properties of Haar-random BosonSampling distributions, and about how one might test efficiently whether a claimed BosonSampling device worked, even with hundreds or thousands of photons.

4. Also on the arXiv last night, there was a phenomenal survey about the quantum PCP conjecture by Dorit Aharonov, Itai Arad, and my former postdoc Thomas Vidick (soon to be a professor at Caltech).  I recommend reading it in the strongest possible terms, if you’d like to see how far people have come with this problem (but also, how far they still have to go) since my “Quantum PCP Manifesto” seven years ago.

5. Christos Papadimitriou asked me to publicize that the deadline for early registration and hotel reservations for the upcoming FOCS in Berkeley is fast approaching!  Indeed, it’s October 4 (three days from now).  See here for details, and here for information about student travel support.  (The links were down when I just tried them, but hopefully the server will be back up soon.)

Firewalls

Tuesday, August 27th, 2013

Updates (Aug. 29): John Preskill now has a very nice post summarizing the different views on offer at the firewall workshop, thereby alleviating my guilt for giving you only the mess below.  Thanks, John!

And if you check out John’s Twitter feed (which you should), you’ll find another, unrelated gem: a phenomenal TEDx talk on quantum computing by my friend, coauthor, and hero, the Lowerboundsman of Latvia, Andris Ambainis.  (Once again, when offered a feast of insight to dispel their misconceptions and ennoble their souls, the YouTube commenters are distinguishing themselves by focusing on the speaker’s voice.  Been there, man, been there.)


So, last week I was at the Fuzzorfire workshop at the Kavli Institute for Theoretical Physics in Santa Barbara, devoted to the black hole firewall paradox.  (The workshop is still going on this week, but I needed to get back early.)  For some background:

I had fantasies of writing a long, witty blog post that would set out my thoughts about firewalls, full of detailed responses to everything I’d heard at the conference, as well as ruminations about Harlow and Hayden’s striking argument that computational complexity might provide a key to resolving the paradox.  But the truth is, I’m recovering from a nasty stomach virus, am feeling “firewalled out,” and wish to use my few remaining non-childcare hours before the semester starts to finish writing papers.  So I decided that better than nothing would be a hastily-assembled pastiche of links.

First and most important, you can watch all the talks online.  In no particular order:

Here’s my own attempt to summarize what’s at stake, adapted from a comment on Peter Woit’s blog (see also a rapid response by Lubos):

As I understand it, the issue is actually pretty simple. Do you agree that
(1) the Hawking evaporation process should be unitary, and
(2) the laws of physics should describe the experiences of an infalling observer, not just those of an observer who stays outside the horizon?
If so, then you seem forced to accept
(3) the interior degrees of freedom should just be some sort of scrambled re-encoding of the exterior degrees, rather than living in a separate subfactor of Hilbert space (since otherwise we’d violate unitarity).
But then we get
(4) by applying a suitable unitary transformation to the Hawking radiation of an old enough black hole before you jump into it, someone ought to be able, in principle, to completely modify what you experience when you do jump in.  Moreover, that person could be far away from you—an apparent gross violation of locality.

So, there are a few options: you could reject either (1) or (2). You could bite the bullet and accept (4). You could say that the “experience of an infalling observer” should just be to die immediately at the horizon (firewalls). You could argue that for some reason (e.g., gravitational backreaction, or computational complexity), the unitary transformations required in (4) are impossible to implement even in principle. Or you could go the “Lubosian route,” and simply assert that the lack of any real difficulty is so obvious that, if you admit to being confused, then that just proves you’re an idiot.  AdS/CFT is clearly relevant, but as Polchinski pointed out, it does surprisingly little to solve the problem.

Now, what Almheiri et al. (AMPS) added to the simple logical argument above was really to make the consequence (4) more “concrete” and “vivid”—by describing something that, in principle, someone could actually do to the Hawking radiation before jumping in, such that after you jumped in, if there wasn’t anything dramatic that happened—something violating local QFT and the equivalence principle—then you’d apparently observe a violation of the monogamy of entanglement, a basic principle of quantum mechanics.  I’m sure the bare logic (1)-(4) was known to many people before AMPS: I certainly knew it, but I didn’t call it a “paradox,” I just called it “I don’t understand black hole complementarity”!

At any rate, thinking about the “Hawking radiation decoding problem” already led me to some very nice questions in quantum computing theory, which remain interesting even if you remove the black hole motivation entirely. And that helped convince me that something new and worthwhile might indeed come out of this business, despite how much fun it is. (Hopefully whatever does come out won’t be as garbled as Hawking radiation.)

For continuing live updates from the workshop, check out John Preskill’s Twitter feed.

Or you can ask me to expand on various things in the comments, and I’ll do my best.  (As I said in my talk, while I’m not sure that the correct quantum description of the black hole interior is within anyone‘s professional expertise, it’s certainly outside of mine!  But I do find this sort of thing fun to think about—how could I not?)

Unrelated, but also of interest: check out an excellent article in Quanta by Erica Klarreich, about the recent breakthroughs by Reichardt-Unger-Vazirani, Vazirani-Vidick, and others on classical command of quantum systems.

The SuperScott and Morgan Freeman FAQ

Monday, August 5th, 2013

chessboard

Update (Sept. 3): When I said that “about 5000 steps” are needed for the evolutionary approach to color an 8×8 chessboard, I was counting as a step any examination of two random adjacent squares—regardless of whether or not you end up having to change one of the colors.  If you count only the changes, then the expected number goes down to about 1000 (which, of course, only makes the point about the power of the evolutionary approach “stronger”).  Thanks very much to Raymond Cuenen for bringing this clarification to my attention.


Last week I appeared on an episode of Through the Wormhole with Morgan Freeman, a show on the Science Channel.  (See also here for a post on Morgan Freeman’s Facebook page.)  The episode is called “Did God Create Evolution?”  The first person interviewed is the Intelligent Design advocate Michael Behe.  But not to worry!  After him, they have a parade of scientists who not only agree that Chuck Darwin basically had it right in 1859, but want to argue for that conclusion using ROBOTS!  and MATH!

So, uh, that’s where I come in.  My segment features me (or rather my animated doppelgänger, “SuperScott”) trying to color a chessboard two colors, so that no two neighboring squares are colored the same, using three different approaches: (1) an “intelligent design” approach (which computer scientists would call nondeterminism), (2) a brute-force, exhaustive enumeration approach, and (3) an “evolutionary local search” approach.

[Spoiler alert: SuperScott discovers that the local search approach, while not as efficient as intelligent design, is nevertheless much more efficient than brute-force search.  And thus, he concludes, the arguments of the ID folks to the effect of “I can’t see a cleverer way to do it, therefore it must be either brute-force search or else miraculous nondeterminism” are invalid.]

Since my appearance together with Morgan Freeman on cable TV raises a large number of questions, I’ve decided to field a few of them in the following FAQ.

Q: How can I watch?

Amazon Instant Video has the episode here for $1.99.  (No doubt you can also find it on various filesharing sites, but let it be known that I’d never condone such nefarious activity.)  My segment is roughly from 10:40 until 17:40.

Q: Given that you’re not a biologist, and that your research has basically nothing to do with evolution, why did they ask to interview you?

Apparently they wanted a mathematician or computer scientist who also had some experience spouting about Big Ideas.  So they first asked Greg Chaitin, but Chaitin couldn’t do it and suggested me instead.

Q: Given how little relevant expertise you have, why did you agree to be interviewed?

To be honest, I was extremely conflicted.  I kept saying, “Why don’t you interview a biologist?  Or at least a computational biologist, or someone who studies genetic algorithms?”  They replied that they did have more bio-oriented people on the show, but they also wanted me to provide a “mathematical” perspective.  So, I consulted with friends like Sean Carroll, who’s appeared on Through the Wormhole numerous times.  And after reflection, I decided that I do have a way to explain a central conceptual point about algorithms, complexity, and the amount of time needed for natural selection—a point that, while hardly “novel,” is something that many laypeople might not have seen before and that might interest them.  Also, as an additional argument in favor of appearing, MORGAN FREEMAN!

morganfreeman

So I agreed to do it, but only under two conditions:

(1) At least one person with a biology background would also appear on the show, to refute the arguments of intelligent design.
(2) I would talk only about stuff that I actually understood, like the ability of local search algorithms to avoid the need for brute-force search.

I’ll let you judge for yourself to what extent these conditions were fulfilled.

Q: Did you get to meet Morgan Freeman?

Alas, no.  But at least I got to hear him refer repeatedly to “SuperScott” on TV.

Q: What was the shooting like?

Extremely interesting.  I know more now about TV production than I did before!

It was a continuing negotiation: they kept wanting to say that I was “on a quest to mathematically prove evolution” (or something like that), and I kept telling them they weren’t allowed to say that, or anything else that would give the misleading impression that what I was saying was either original or directly related to my research.  I also had a long discussion about the P vs. NP problem, which got cut for lack of time (now P and NP are only shown on the whiteboard).  On the other hand, the crew was extremely accommodating: they really wanted to do a good job and to get things right.

The most amusing tidbit: I knew that local search would take O(n4) time to 2-color an nxn chessboard (2-coloring being a special case of 2SAT, to which Schöning’s algorithm applies), but I didn’t know the constant.  So I wrote a program to get the specific number of steps when n=8 (it’s about 5000).  I then repeatedly modified and reran the program during the taping, as we slightly changed what we were talking about.  It was the first coding I’d done in a while.

Q: How much of the segment was your idea, and how much was theirs?

The chessboard was my idea, but the “SuperScott” bit was theirs.  Luddite that I am, I was just going to get down on hands and knees and move apples and oranges around on the chessboard myself.

Also, they wanted me to speak in front of a church in Boston, to make a point about how many people believe that God created the universe.  I nixed that idea and said, why not just do the whole shoot in the Stata Center?  I mean, MIT spent $300 million just to make the building where I work as “visually arresting” as possible—at the expense of navigability, leakage-resilience, and all sorts of other criteria—so why not take advantage of it?  Plus, that way I’ll be able to crack a joke about how Stata actually looks like it was created by that favorite creationist strawman, a tornado passing through a junkyard.

Needless to say, all the stuff with me drawing complexity class inclusion diagrams on the whiteboard, reading my and Alex Arkhipov’s linear-optics paper, walking around outside with an umbrella, lifting the umbrella to face the camera dramatically—that was all just the crew telling me what to do.  (Well, OK, they didn’t tell me what to write on the whiteboard or view on my computer, just that it should be something sciencey.  And the umbrella thing wasn’t planned: it really just happened to be raining that day.)

Q: Don’t you realize that not a word of what you said was new—indeed, that all you did was to translate the logic of natural selection, which Darwin understood in 1859, into algorithms and complexity language?

Yes, of course, and I’m sorry if the show gave anyone the impression otherwise.  I repeatedly begged them not to claim newness or originality for anything I was saying.  On the other hand, one shouldn’t make the mistake of assuming that what’s obvious to nerds who read science blogs is obvious to everyone else: I know for a fact that it isn’t.

Q: Don’t you understand that you can’t “prove” mathematically that evolution by natural selection is really what happened in Nature?

Of course!  You can’t even prove mathematically that bears crap in the woods (unless crapping in the woods were taken as part of the definition of bears).  To the writers’ credit, they did have Morgan Freeman explain that I wasn’t claiming to have “proved” evolution.  Personally, I wish Freeman had gone even further—to say that, at present, we don’t even have mathematical theories that would explain from first principles why 4 billion years is a “reasonable” amount of time for natural selection to have gotten from the primordial soup to humans and other complex life, whereas (say) 40 million years is not a reasonable amount.  One could imagine such theories, but we don’t really have any.  What we do have is (a) the observed fact that evolution did happen in 4 billion years, and (b) the theory of natural selection, which explains in great detail why one’s initial intuition—that such evolution can’t possibly have happened by “blind, chance natural processes” alone—is devoid of force.

Q: Watching yourself presented in such a goony way—scribbling Complicated Math Stuff on a whiteboard, turning dramatically toward the camera, etc. etc.—didn’t you feel silly?

Some of it is silly, no two ways about it!  On the other hand, I feel satisfied that I got across at least one correct and important scientific point to hundreds of thousands of people.  And that, one might argue, is sufficiently worthwhile that it should outweigh any embarrassment about how goofy I look.

Three announcements

Saturday, August 3rd, 2013

1. As many of you probably know, this week my EECS colleague Hal Abelson released his 180-page report on MIT’s involvement in the Aaron Swartz case.  I read the whole thing, and I recommend it if you have any interest in the case.  My take is that, far from being the “whitewash” that some people described it as, the report (if you delve into it) clearly and eloquently explains how MIT failed to live up to its own standards, even as it formally followed the rules.  The central insight here is that the world expects MIT to behave, not like some other organization would behave if someone hid a laptop in its supply closet to download the whole JSTOR database, insulted and then tried to flee from its security officers when questioned, etc. etc., but rather with perspective and imagination—worrying less about the security of its facilities than about the future of the world.  People expect MIT, of all places, to realize that the sorts of people who pull these sorts of shenanigans in their twenties sometimes become Steve Jobs or Richard Feynman (or for that matter, MIT professor Robert Morris) later in their lives, and therefore to speak up in their defense.  In retrospect, I wish Swartz’s arrest had sparked a debate about the wider issues among MIT’s students, faculty, and staff.  I think it’s likely that such a debate would have led to pressure on the administration to issue a statement in Swartz’s support.  As it was (and as I pointed out in this interview), most people at MIT, even if they’d read about the arrest, weren’t even aware of the issue’s continued existence, let alone of MIT’s continued role in it, until after Swartz had already committed suicide.  For the MIT community—which includes some prominent supporters of open access—to have played such a passive role is one of the many tragedies that’s obvious with hindsight.

2. Shafi Goldwasser has asked me to announce that the fifth Innovations in Theoretical Computer Science (ITCS) conference will be held in Princeton, a town technically in New Jersey, on January 12-14, 2014.  Here’s the conference website; if you want to submit a paper, the deadline is coming up soon, on Thursday, August 22.

3. As the summer winds to a close, I’m proud to announce my main goals for the upcoming academic year.  Those goals are the following:

(a) Take care of Lily.

(b) Finish writing up old papers.

It feels liberating to have no higher aspirations for an entire year—and for the aspirations I have to seem so modest and so achievable.  On the other hand, it will be all the more embarrassing if I fail to achieve even these goals.

Microsoft: From QDOS to QMA in less than 35 years

Friday, July 19th, 2013

This past week I was in Redmond for the Microsoft Faculty Summit, which this year included a special session on quantum computing.  (Bill Gates was also there, I assume as our warmup act.)  I should explain that Microsoft Research now has not one but two quantum computing research groups: there’s Station Q in Santa Barbara, directed by Michael Freedman, which pursues topological quantum computing, but there’s also QuArC in Redmond, directed by Krysta Svore, which studies things like quantum circuit synthesis.

Anyway, I’ve got two videos for your viewing pleasure:

  • An interview about quantum computing with me, Krysta Svore, and Matthias Troyer, moderated by Chris Cashman, and filmed in a studio where they put makeup on your face.  Just covers the basics.
  • A session about quantum computing, with three speakers: me about “what quantum mechanics is good for” (quantum algorithms, money, crypto, and certified random numbers), then Charlie Marcus about physical implementations of quantum computing, and finally Matthias Troyer about his group’s experiments on the D-Wave machines.  (You can also download my slides here.)

This visit really drove home for me that MSR is the closest thing that exists today to the old Bell Labs: a corporate lab that does a huge amount of openly-published, high-quality fundamental research in math and CS, possibly more than all the big Silicon-Valley-based companies combined.  This research might or might not be good for Microsoft’s bottom line (Microsoft, of course, says that it is, and I’d like to believe them), but it’s definitely good for the world.  With the news of Microsoft’s reorganization in the background, I found myself hoping that MS will remain viable for a long time to come, if only because its decline would leave a pretty gaping hole in computer science research.

Unfortunately, last week I also bought a new laptop, and had the experience of PowerPoint 2013 first refusing to install (it mistakenly thought it was already installed), then crashing twice and losing my data, and just generally making everything (even saving a file) harder than it used to be for no apparent reason.  Yes, that’s correct: the preparations for my talk at the Microsoft Faculty Summit were repeatedly placed in jeopardy by the “new and improved” Microsoft Office.  So not just for its own sake, but for the sake of computer science as a whole, I implore Microsoft to build a better Office.  It shouldn’t be hard: it would suffice to re-release the 2003 or 2007 versions as “Office 2014″!  If Mr. Gates took a 2-minute break from curing malaria to call his former subordinates and tell them to do that, I’d really consider him a great humanitarian.

The Tenured Toll-Taker

Sunday, May 5th, 2013

Update (5/6): In “honor” of the news below, Boaz Barak has written a beautiful blog post on the reasons to care about the P vs. NP question, offering his responses to several of the most common misconceptions.  Thank you so much, Boaz — this is one of the best presents I’ve ever gotten from anyone!


On Friday afternoon—in the middle of a pizza social for my undergrad advisees—I found out that I’ve received tenure at MIT.

Am I happy about the news?  Of course!  Yet even on such a joyous occasion, I found myself reflecting on a weird juxtaposition.  I learned about MIT’s tenure decision at the tail end of a fierce, weeks-long comment war over on Luboš Motl’s blog, in which I assumed the task of defending theoretical computer science and quantum information science as a whole: explaining why these fields could have anything whatsoever to contribute to our understanding of the universe.  Indeed, I took the title of this post from a comment Luboš made to me in the middle of the melee: that compared to string theorists, quantum computing researchers have as much to say about the nature of reality as toll-takers on the Golden Gate Bridge.  (Even though the Golden Gate tolls are apparently all-electronic these days, I still found Luboš’s analogy striking.  I could imagine that staring all day at the breathtaking San Francisco Bay would lead to deep thoughts about the nature of reality.)

Now, some people will ask: why should I even waste my time this way—arguing with Luboš, a blogger infamous for describing the scientists he disagrees with as garbage, worms, fungi, etc., and even calling for their “elimination”?  If I find the limits of computation in the physical universe to be a rich, fascinating, worthwhile subject; if I have hundreds of wonderful colleagues with whom to share the thrill of surprising new discoveries; if a large, growing fraction of the wider scientific community follows this field with interest; if my employer seems to want me doing it for the long haul … then why should I lose sleep just because someone, somewhere, declared that the P vs. NP problem is a random puzzle, of no deeper significance than the question of whether chess is a draw?  Or because he characterized the entire fields of quantum computing and information as trivial footnotes to 1920s physics, fit only for mediocre students who couldn’t do string theory?  Or because, on the “other side,” a persistent minority calls quantum computers an absurd fantasy, and the quest to build them a taxpayer boondoggle bordering on fraud?  Or because some skeptics, going even further, dismiss quantum mechanics itself as nonsensical mumbo-jumbo that physicists made up to conceal their own failure to find a straightforward, mechanical description of Nature?  Likewise, why should it bother me if some anti-complexites dismiss the quest to prove P≠NP as a fashionable-but-irrelevant journey to formalize the obvious—even while others denounce the Soviet-style groupthink that leads the “CS establishment” to reject the possibility that P=NP?  After all, these various naysayers can’t all be right!  Doesn’t it comfort me that, of all the confidently-asserted reasons why everything my colleagues and I study is dead-end, cargo-cult science, so many of the reasons contradict each other?

Sure, but here’s the thing.  In seven years of teaching and blogging, I’ve learned something about my own psychology.  Namely, if I meet anyone—an undergrad, an anonymous blog commenter, anyone—who claims that the P vs. NP problem is beside the point, since it’s perfectly plausible that P=NP but the algorithm takes n10000 time—or that, while quantum mechanics works fine for small systems, there’s not the slightest reason to expect it to scale up to larger ones—or that the limits of computation are plainly no more relevant to fundamental physics than the fact that cucumbers are green—trying to reason with that person will always, till the end of my life, feel like the most pressing task in the world to me.

Why?  Because, I confess, a large part of me worries: what if this other person is right?  What if I really do have to jettison everything I thought I knew about physics, computation, and pretty much everything else since I was a teenager, toss all my results into the garbage can (or at least the “amusing recreations can”), and start over from kindergarten?  But then, as I fret about that possibility, counterarguments well up in my mind.  Like someone pinching himself to make sure he’s awake, I remember all the reasons why I was led to think what I think in the first place.  And I want the other person to go through that experience with me—the experience, if you like, of feeling the foundations of the universe smashed to pieces and then rebuilt, the infinite hierarchy of complexity classes collapsing and then springing back into place, decades’ worth of books set ablaze and then rewritten on blank pages.  I want to say: at least come stand here with me—in this place that I spent twenty years of late nights, false starts, and discarded preconceptions getting to—and tell me if you still don’t see what I see.

That’s how I am; I doubt I can change it any more than I can change my blood type.  So I feel profoundly grateful to have been born into a world where I can make a comfortable living just by being this strange, thin-skinned creature that I am—a world where there are countless others who do see what I see, indeed see it a thousand times more clearly in many cases, but who still appreciate what little I can do to explore this corner or that, or to describe the view to others.  I’d say I’m grateful to “fate,” but really I’m grateful to my friends and family, my students and teachers, my colleagues at MIT and around the world, and the readers of Shtetl-Optimized—yes, even John Sidles.  “Fate” either doesn’t exist or doesn’t need my gratitude if it does.

“Closer to Truth”

Wednesday, May 1st, 2013

Two years ago, when I attended the FQXi conference on a ship from Norway to Denmark, I (along with many other conference participants) was interviewed by Robert Lawrence Kuhn, who produces a late-night TV program called “Closer to Truth.”  I’m pleased to announce (hat tip: Sean Carroll) that four videos from my interview are finally available online:

  • Is the Universe a Computer?
  • (like a politician, I steer the question toward “what kind of computer is the universe?,” then start talking about P vs. NP, quantum computing, and the holographic principle)

  • What Does Quantum Theory Mean?
  • (here I mostly talk about the idea of computational intractability as a principle of physics)

  • Quantum Computing Mysteries
  • (basics of quantum mechanics and quantum computing)

  • Setting Time Aright (about the differences between time and space, the P vs. PSPACE problem, and computing with closed timelike curves)

(No, I didn’t choose the titles!)

For regular readers of this blog, there’s probably nothing new in these videos, but for those who are “just tuning in,” they provide an extremely simple and concise introduction to what I care about and why.  I’m pretty happy with how they came out.

Once you’re finished with me (or maybe even before then…), click here for the full list of interviewees, which includes David Albert, Raphael Bousso, Sean Carroll, David Deutsch, Rebecca Goldstein, Seth Lloyd, Marvin Minsky, Roger Penrose, Lenny Susskind, Steven Weinberg, and many, many others who might be of interest to Shtetl-Optimized readers.