Links of Randomness

♦  My wife has a new blog about quilting.

♦  This Nature article describes the subfield of quantum gravity which I've been working in—I know most of these people.  This is a lot more accurate of a description than your average pop journalism article, so check it out.  (Of course many of the ideas here are speculative and may turn out to be false.)

♦  What if you discovered one day that everyone except you has a magical superpower, and you never knew about it?  A true story, flippantly described yet also deeply moving.

(If you're curious to find out more, here's another decent article on the same subject.  [Warning: includes gratuitous disturbing art involving surreal faces])

♦  This is the best article about lichen I have ever read.  Well, maybe that isn't all that competitive an award, but it's still a pretty good article.  The runner-up lichen article is also pretty good.

♦  You already know that dolphins are really smart animals—but that doesn't mean you won't enjoy reading more about it.

♦  One way to go to college for free.  But not suitable for dolphins...

♦  Once upon a time, people thought that Jews were naturally the best at basketball, because of their short stature and scheming minds!

♦  How we know that the robots didn't take our jobs.

The scary chart (the one that shows how, as a result of poor structuring of government programs, poor people can actually be worse off as a result of getting a job or a pay raise) is from this article.  Honestly, how hard is it to phase out programs gradually with income so as to avoid truly stupid incentives?

♦  This critique of our current primary nomination process, may have changed the way I think about politics.  This article bothers me because, on principle, I dislike pretending to have a democracy when actually the important things are settled in the cliched "smoke-filled rooms" (I've disliked the Democratic superdelegates since I first heard of them) but it seems obvious in this election that that those methods have worked better.

Also one could question whether "democracy" should really mean majority (or plurality!) rule when we are talking about the plurality of a minority (those who vote in a given party).  The main way that the party establishment would like to modify raw democracy is to make the candidates more electable, which means in a way they represent the rest of the nation and make the results more democratic.

♦  How do you warn people thousands of years later about sites where radioactive waste is stored?

♦  Or for a more short term prediction about future developments: some predictions in the year 1900 about the year 2000.  About as accurate as these things ever are, i.e. not terribly but a few of them score some palpable hits.   (Here's a plaintext version if you find the first one hard to read.)

♦  "A Mathematician's Apology" by G. H. Hardy.  Still contains a lot of truth today, although when he lumps Quantum Mechanics and Relativity in with pure mathematics, and says that at least these things can never have any use in war... well, I think we have to count that as another failed prediction.

♦  Interesting article in the NY Times about a two sets of identical twins (2 x 2 = 4) where one twin from each set was swapped at birth, and what happened after they found out.

♦  An interesting series by St. Jason of Triablogue on some of the less well-known evidence in favor of the traditional authorship of the Four Gospels.

♦  Does wishing to believe in religion put one in a better or worse position, for learning whether it is true?  An interesting fictional dialogue [google books] on the subject by a Catholic author.  I read this on the strength of a quotation excepted at Siris.

♦  An actual dialogue about religion at First Things, with a Catholic and Muslim, both authors, about religion and their friendship with each other.

♦  Nobody Expects the Spanish Inquisition … to Be Explained Fairly, a review of a book addressing anti-Catholic history.

♦  Speaking of agencies that use the methodology of inquisition (the accused must prove themselves to be innocent)... please don't call Child Protective Services on parents for trivial issues unless you hate both them and their offspring.  Followup posts here, here, and [added later] hereSpoiler: happy ending.

About Aron Wall

I am a Lecturer in Theoretical Physics at the University of Cambridge. Before that, I read Great Books at St. John's College (Santa Fe), got my physics Ph.D. from U Maryland, and did my postdocs at UC Santa Barbara, the Institute for Advanced Study in Princeton, and Stanford. The views expressed on this blog are my own, and should not be attributed to any of these fine institutions.
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16 Responses to Links of Randomness

  1. TY says:

    Aron, the quilts are astounding, and as one who does oil painting, I admire the one on the far right and the expression of the lady’s eyes in the other. What a talent!

  2. Mactoul says:

    "Our most pressing political problem today is that the country abandoned the establishment, not the other way around."
    Strongly disagree. Both parties have been collaborating on the destruction of the norms and deserve what they are getting. Politics is means to an end, the end being flourishing of the community over generations.
    Politics is not an end in itself.

    "a democracy when actually the important things are settled in the cliched "smoke-filled rooms"
    Is your ideal of democracy a direct democracy with plentiful plebiscites and referendums?
    What is supposed to exist is representative democracy where the electors elect representatives who are supposed to deliberate on their behalf. Deliberation is all-important.
    It may be that a key problem is a decline in the quality of deliberation.

  3. TY says:

    Thanks for including in the Links of Randomness: “How we know that the robots didn't take our jobs” and “This critique of our current primary nomination process….”

    On the former, you say:
    “The scary chart (the one that shows how, as a result of poor structuring of government programs, poor people can actually be worse off as a result of getting a job or a pay raise) is from this article. Honestly, how hard is it to phase out programs gradually with income so as to avoid truly stupid incentives?”

    Very hard and the USA is a good example.

    In Public Finance, the working assumption is that in both direct and representative democracies, elected politicians represent the will of the people. The textbooks assume a benign government maximising social welfare (another thorny issue that made Kenneth Arrow the 1972 Nobel Laureate in Economics). Public Choice theorist like James Buchanan (who won the 1986 Nobel Prize for Economics), Gordon Tullock, and William Niskanen explicitly argue that governments often do not behave in an ideal manner.

    In Niskanen’s model of the budget-maximising bureaucrat, the goal of the bureaucrat is to maximise the size of the agency he or she controls and thus maximise its budget, rather than aim for efficiency in production.

    Can’t the government (the politicians) enforce efficient production? You would think so because they have the power; but they do not because the bureaucrats know more about the true costs of the mean-test programmes. Add to this the fact that politicians want to be re-elected, and you get a situation where nothing gets done to make government lean and efficient.

    As the resource constraint tightens around the many conflicting objectives, the calculus ought to force government to make the hard choices between economic growth, social welfare, national security, and so forth, to avert impending collapse.

    I say ought and not will because if we look at Rome and other major ancient civilisations that have declined, we see left and right incompetence in the politics and in the government. What as dismal view for a conclusion.

  4. Scott Church says:

    Aron, thanks for these links, especially the Nature article! I never realized that AdS/CFT could lead to emergent space-time as a consequence of entanglement... what a fascinating idea! But as interesting as that possibility is, and as fruitful an area of research as AdS/CFT has been, I've always wondered how it can be applied to the universe we actually live in, which is decidedly not AdS. As I was reading, I thought of something that hadn't occurred to me before... is there a duality that can map an asymptotic CFT boundary to a regular de-Sitter bulk rather than an AdS one, either at infinity, or conversely, at a boundary arbitrarily close to a singularity or the planck region? I poked around a little and sure enough, Strominger proposed such a duality back in 2001 and it's been a subject of investigation ever since. Malcadena has even given lectures on the subject at Columbia.

    Speaking as a quantum gravity newb, at first blush this strikes me as a much bigger carrot than AdS/CFT--if for no other reason because it would seem to result in a framework that can be made to resemble the real non-AdS universe. But compared to AdS/CFT it seems to have gotten very little attention, and I have to presume there's a reason why. What, if anything is wrong with this approach that makes it less attractive than AdS/CFT? And if it is a viable possibility, can we derive ER = EPR or tie emergent space-time to entanglement this way as well?

  5. Aron Wall says:

    Your instincts are good, but as you guessed there are reasons people don't talk about dS/CFT as much. It is less well-constructed than AdS/CFT, and there are a number of issues with it. To the best of my current knowledge, the problems are these:

    1. The AdS/CFT dictionary relates a whole Hilbert space of states in the bulk theory to a Hilbert space of states of the CFT. But dS/CFT relates the amplitude of the "Hartle-Hawking state" to the CFT partition function, so first of all it inherits all the issues with defining the Hartle-Hawking proposal, and second of all it is unclear how to generalize to other states that presumably exist in the theory.

    2. There are some unresolved conceptual issues, like what happens if part of the spacetime crunches into a singularity so it doesn't look like asymptotic de Sitter everywhere in the future? Even the meaning of the formula used to define the duality is a little more obscure than in the AdS/CFT case. Since matter can perturb de Sitter space and affect the geometry at future times, you are have to consider the CFT coupled to an arbitrary curved spacetime, which leads to additional complications.

    3. There are lots of known examples of AdS/CFT dualities involving string theory and various CFT's. But there is basically only one known example of dS/CFT, and it's weird! The gravity theory isn't string theory (whose spin-2 excitations look like Einstein's General Relativity in an appropriate limit), instead it's something more complicated called Vasiliev gravity, which involves a tower of massless gauge fields with arbitrarily high spins all interacting with each other, and which people claim is more nonlocal than GR. Meanwhile the CFT has a bunch of noninteracting fields (which makes it easy to calculate stuff) but it is nonunitary which means that probabilities wouldn't add up to 1 if it were treated as an ordinary field theory in spacetime. (Maybe this isn't a problem because we only care about this theory on the future boundary of dS, which has space but no time.)

    4. There is no known analogue of the Ryu-Takayanagi entanglement entropy formula since it doesn't make geometric sense to have a minimum area surface that is attached to a spacelike "boundary of time" (instead of a timelike "boundary to space", as in the case of AdS/CFT).

    But, maybe people can make it work somehow. I feel that there must be a way of understanding the holographic principle for every possible kind of spacetime allowed by general relativity, but it's hard!

  6. Mactoul says:

    Without a parable modern physics speaks not to the multitudes. An expression such as 'the curvature of the space' is strictly comparable to the old definition of God as 'a circle whose centre is everywhere and whose circumference is nowhere'. Both succeed in suggesting: each does so by offering what is, on the level of ordinary thinking, nonsense. By accepting the 'curvature of space' we are not 'knowing' or enjoying 'truth' in the fashion that was once thought to be possible.
    Epilogue to the Discarded Image (CS Lewis)

  7. kashyap vasavada says:

    Hi Aron,
    Thanks for the link to the nice nature article. What puzzles me most is why tensor networks and error correcting codes in condensed matter physics, which are presumably low energy objects and have nothing to do with gravity, would agree with ADS/CFT. I suppose ADS/CFT does not look like the universe we live in. Do you have any insight into this correspondence?
    BTW, I saw your name in the invited speakers list for strings 2016 in Beijing, but did not find your talk. Did you go that conference?

  8. Aron Wall says:

    I was there. Try looking at the parallel sessions, or if you want a recording look here. (Although some my work was mentioned in a somewhat more accessible way in St. Juan Maldacena's review talk.)

    Since quantum gravity is an huge unknown, we don't know what structures might or might not be relevant. It often happens that the same mathematical structure pops up in different areas of physics. Tensor networks include elements related to "holography" and "information theory", which we think are relevant to quantum gravity, but whether the analogy to AdS/CFT can be made precise or not is still a very open question.

    There are however also CFT's (models where the physics is the same at all distance scales) that are relevant in condensed matter physics, so that's part of the connection.

    As for AdS/CFT itself, it may not look much like the universe we live in, but it is a great source of toy models for exploring particular issues. For example, it seems unlikely that whether or not black holes preserve information can depend on the sign of the cosmological constant, since that only changes the physics at really big distance scales...

    Anyway, if everything were already understood, there would be nothing left for me to do!

  9. Scott Church says:

    Thanks Aron! I suspected as much... If someone had come up with a duality that truly mapped the full standard model onto the universe we actually live in there'd have been multiple Nobel's handed out by now. :-)

    I'm not nearly as up on all this as you of course, but beautiful and fruitful as AdS/CFT is, the fact that we live in a non-AdS universe still strikes me as a huge elephant in the living room that people aren't paying enough attention to. For the most part, I would imagine that's because it's viewed more as a treasure map of ideas than anything definitive, and everyone is hoping the problem will have resolved itself by the time we find the treasure. But even so, with some folks at least I can't help but wonder how much decidedly unscientific worldview pollution is involved...

    Case in point, Sean Carroll. As you know, he played the Quantum Eternity card in his debate with St. Craig, and in his post-debate thoughts. In his combox you responded to that by confronting him with an even bigger elephant--namely, the absence of a global absolute time in GR, which gives us a global energy that is zero or undefined, not conserved, and a Hamiltonian constraint rather than a Hamiltonian. And what does he do...? He refers to AdS/CFT as "[our] best-understood example of quantum gravity" ... and uses it to sweep the entire elephant under the rug. Perhaps it's just me, but fruitful or otherwise, a duality that lets us build lower-dimensional toy models of universes that don't look anything like the real one doesn't strike me as "our best-understood example of quantum gravity," much less an excuse to dismiss the difference between the Schrodinger and Wheeler-DeWitt equations as nothing more than a minor technicality. And to make matters worse, I know for fact that he's aware of the issues with unambiguously defining a non-zero energy for the universe because he's written about it elsewhere. Now I doubt Carroll's attitude is typical of most physicists, much less most quantum gravity researchers. And I certainly don't want to minimize the progress AdS/CFT has made possible either. But in his case at least, I suspect that none of this would have the gravitas that it does if he couldn't prop up his own "poetic naturalism" with it.

    So... as someone who works in the field, how much do you think the sort of worldview pollution I suspect Carroll of contributes to the larger popularity of AdS/CFT, and M-theory in general? Is it widespread, or isolated to a handful folks like him? I ask because it seems to come up a lot in the popular writings of atheist physicists and bloggers, and I'm curious how representative they are of the larger physics community.

  10. Scott Church says:

    One other quick question Aron... To what extent (if any) does the proposed link between emergent space-time and "entanglement" discussed in the Nature article depend on MWI vs. other interpretations, and if it doesn't, how does it deal with wave function collapse?

  11. Mactoul says:

    The physics along with its equations are written in space-time. The physics is physics of things moving about in space-time. That's why I am unable to digest how physics could give rise to space-time itself. There must be a philosophical muddle somewhere. As CS Lewis remarks, no amount of book-keeping could generate a single coin.

  12. kashyap vasavada says:

    Thanks for your reply. These days the blogosphere is full of disappointments about not finding SUSY at LHC! What is your opinion? It will be also nice if you could write a blog about major highlights (achievements?) of the Strings 2016 conference.

  13. Aron Wall says:

    Just because our current laws of physics are written in terms of spacetime, doesn't mean that all future models must be. It could be that spacetime emerges dynamically from some other kind of structure. Of course that would not remove the philosophical difference between the actuality of Nature (the "coin") and our description of Nature ("bookkeeping").

    Even before the LHC started, my prediction was that they would not find supersymmetry.

    Writing up conferences isn't really the sort of thing I do on this blog, but you can find some descriptions at the Reference Frame here and here, and (from a diametrically opposed anti-string viewpoint) a little bit here and here.

    Lenny Susskind has written some papers trying to relate this stuff to interpretational issues, but mostly people present it in a "neutral" way (just using the usual formalism of QM without making claims about which interpretation is right).

    While the "worldview pollution" (but a naturalist might just call it Bayesian inference) of wanting a coherent naturalistic framework might plausibly make a few people like Carroll or Hawing more certain about things like M-theory than they otherwise would be... I'm quite certiain that it plays little or no role in the vast majority of physics talk on this subject.

    There are already plenty of legitimate reasons to be excited by the subject, and humans don't really need a philosophical excuse to be overconfident of the value of what they do, it's sort of the default operating mode for many people...

  14. Andrew Wells says:

    Aron, this is a a minor addendum to one of your comments but your "saints" sections recommends point this out. I think it's "st Juan Maldacena".

    Interviewer: "Are you a Practising Catholic?"

    Maldacena: "Yes, I am"

    Interviewer: "And does not the evidence of science on the origin of the Universe and the account of religion live as a contradiction?"

    Maldacena: "As you learn about science, you change about how you see the role of religion. But one has to find how to make them compatible."


  15. Aron Wall says:

    Fixed, thanks. I actually knew that already, it was just an oversight.

  16. Andrew Wells says:

    Sure no worries Aron :)

    "...there is rejoicing in the presence of the angels of God over one sinner who repents.” (Luke 15:10)

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