Links for November 2016

  • Somehow I had never heard of Georges Lemaître, Jesuit priest:

    [Lemaître] proposed the theory of the expansion of the universe, widely misattributed to Edwin Hubble. He was the first to derive what is now known as Hubble’s law and made the first estimation of what is now called the Hubble constant, which he published in 1927, two years before Hubble’s article. Lemaître also proposed what became known as the Big Bang theory of the origin of the universe, which he called his “hypothesis of the primeval atom” or the “Cosmic Egg”.

    (H/t Sean Carroll.)

  • Pangolins are weird.

    (H/t Will Riedel.)
  • An interview about the Merriam-Webster twitter account.
  • Jon Baez’s excellent coverage of Jarzynksi.
  • The presidential scandal out of South Korea is more bizarre than previously reported. (H/t Will Eden.)
  • An anonymous Physics.SE user, on the meaning of Haag’s theorem and attempts to make quantum field theory mathematically rigorous:

    This is a little bit like the development of calculus, which underlies Newtonian mechanics. It took a long time, and was clearly a very valuable exercise for both mathematics and physics. But, long before the subject was rigorously defined it was clear that Newtonian mechanics was correct, but the correct language for it does not exist yet. So, I think Haag’s theorem demonstrates that we are at the same stage of development of QFT.

  • Stimulating the vestibular system (inner ear balance) leads to neat fat-loss effects.
  • Steve Hsu links to and discusses the work of Ted Chiang (1, 2, 3), whose short work “Story of your life” has recently been made into the movie “Arrival”. (The PDF can be found with some light Googling.)
  • Inside the world of Australian opal miners who live underground“.
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How to think about Quantum Mechanics—Part 1: Measurements are about bases

[This post was originally “Part 0”, but it’s been moved. Other parts in this series: .]

In an ideal world, the formalism that you use to describe a physical system is in a one-to-one correspondence with the physically distinct configurations of the system. But sometimes it can be useful to introduce additional descriptions, in which case it is very important to understand the unphysical over-counting (e.g., gauge freedom). A scalar potential V(x) is a very convenient way of representing the vector force field, F(x) = \partial V(x), but any constant shift in the potential, V(x) \to V(x) + V_0, yields forces and dynamics that are indistinguishable, and hence the value of the potential on an absolute scale is unphysical.

One often hears that a quantum experiment measures an observable, but this is wrong, or very misleading, because it vastly over-counts the physically distinct sorts of measurements that are possible. It is much more precise to say that a given apparatus, with a given setting, simultaneously measures all observables with the same eigenvectors. More compactly, an apparatus measures an orthogonal basis – not an observable.We can also allow for the measured observable to be degenerate, in which case the apparatus simultaneously measures all observables with the same degenerate eigenspaces. To be abstract, you could say it measures a commuting subalgebra, with the nondegenerate case corresponding to the subalgebra having maximum dimensionality (i.e., the same number of dimensions as the Hilbert space). Commuting subalgebras with maximum dimension are in one-to-one correspondence with orthonormal bases, modulo multiplying the vectors by pure phases.a   You can probably start to see this by just noting that there’s no actual, physical difference between measuring X and X^3; the apparatus that would perform the two measurements are identical.… [continue reading]

PI accepting 2017 master’s student applications

The Perimeter Scholars International (PSI) program is now accepting applications for this Master’s program, to start next fall. The due date is Feb 1st. Me previously:

If you’re in your last year as an undergrad, I strongly advise you (seriously) to consider applying. Your choice of grad school is 80% of the selection power determining your thesis topic, and that topic places very strong constraints on your entire academic career. The more your choice is informed by actual physics knowledge (rather than the apparent impressiveness of professors and institutions), the better. An additional year at a new institution taking classes with new teachers can really help.


Here’s the poster and a brand new propaganda video:
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Sank argues for a SciRate issue tracker

SciRate is the best location I know of for public discussion and feedback on academic papers, and is an impressive open-source achievement by Adam Harrow and collaborators. Right now it has the most traction in the field of quantum informationQuantum info leading the way, as usual…a  , but it could stand to become more popular, and to expand into other fields.

My colleague and good friend Dan Sank proposes a small but important tweak for SciRate: issue tracking, à la GitHub.

Issues in Scirate?

Scirate enables us to express comments/opinions on published works. Another very useful kind of feedback for research papers is issues. By “issue” I mean exactly the kind of thing I’m writing right now: a description of

  1. a problem with the work which can be definitively fixed, or
  2. a possible improvement to that product.

This differs from comments which are just statements of opinion which don’t require any reaction from the author. We all know that issues are essential in developing software, and based on a recent experience where I used github to host development of a research paper with three coauthors and more than a dozen group members providing feedback, I think that issues should also be used for research papers.

It might be nice to attach an issue tracker to Scirate, or at least have Scirate give links to an external issue tracker attached to each paper.

Why not just use a public github repo and get the issue tracker for free?

Making a github repo public makes everything public, including any sensitive information including comments about particular works/people. Having written a paper using github, I can imagine the authors would not want to make that repo public before going through the entire issue history making sure nobody said anything embarrassing/demeaning/etc.

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Abstracts for October 2016

  • [arXiv:math-ph/0411058]

    One of the key subtleties about trying to study quantum information in a field theory is that you can’t formally decompose the Hilbert space into a tensor product of spatially local subsystems. The reasons are technical, and rarely explained well. This paper is an exception, giving an excellent introduction to the key ideas, in a manner accessible to a quantum (non-field) information theorist. (See related work by Yngvason this blogpost by Tobias Osborne and my previous discussion re: Reeh-Schielder theorem.)

  • [PDF.]

    This paper is Dieter Zeh’s in-line commentary on what might be Feynman’s most explicit exposition of his interpretation of quantum mechanics:

    As far as I know, Feynman never participated in the published debate about interpretational problems, such as quantum measurements. So I was surprised when I recently discovered a little known report about a conference regarding the role of gravity and the need for its quantization, held at the University of North Carolina
    in Chapel Hill in 1957, since it led at some point to a discussion of the measurement problem and of the question about the existence and meaning of macroscopic superpositions. This session was dominated by Feynman’s presentation of a version of Schrodinger’s cat, in which the cat with its states of being dead or alive is replaced by a macroscopic massive ball being centered at two different positions with their distinguishable gravitational fields. I found this part of the report so remarkable for historical reasons that I am here quoting it in detail for the purpose of discussing and commenting it from a modern point of view….The discussion to be quoted below certainly deserves to become better known and discussed because of the influence it seems to have had on several later developments.

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Links for October 2016

I will start writing actual blog posts again soon, I promise. But until then, more nerdy space stuff…

  • ExoMars is approaching the Red Planet. The lander enters the atmosphere tomorrow.
  • The United States only operated continuous airborne alert — the maintenance of multiple nuclear-armed bomber aircraft continuously in flight to avoid the possibility of a sneak attack neutralizing the bomber force — during the ’60s, because the accident rate was too high. However, Operation Looking Glass kept at least one emergency command platform in the air around-the-clock for almost 30 years.

    At DEFCON 2 or higher, the Looking Glass pilot and co-pilot were both required to wear an eye patch, retrieved from their Emergency War Order (EWO) kit. In the event of a surprise blinding flash from a nuclear detonation, the eye patch would prevent blindness in the covered eye, thus enabling them to see in at least one eye and continue flying. Later, the eye patch was replaced by goggles that would instantaneously turn opaque when exposed to a nuclear flash, then rapidly clear for normal vision.

    They also continuously maintained airplanes flying over the ocean, dangling antenna into the water, to ensure constant communication with submarines. This stopped in 1991.

  • Very relatedly, former Secretary of Defense William Perry is teaching a MOOC about the continuing modern risk of nuclear weapons.
  • A history of the Project Orion. Abtract:

    The race to the Moon dominated manned space flight during the 1960’s. and culminated in Project Apollo. which placed 12 humans on the Moon. Unbeknownst to the public at that time, several U.S. government agencies sponsored a project that could have conceivably placed 150 people on the Moon, and eventually sent crewed expeditions to Mars and the outer planets.

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Executive branch reasonable on AI

President Obama was directly asked in a Wired interview about the dangers Bostrom raises regarding AI. From the transcript:

DADICH: I want to center our conversation on artificial intelligence, which has gone from science fiction to a reality that’s changing our lives. When was the moment you knew that the age of real AI was upon us?

OBAMA: My general observation is that it has been seeping into our lives in all sorts of ways, and we just don’t notice; and part of the reason is because the way we think about AI is colored by popular culture. There’s a distinction, which is probably familiar to a lot of your readers, between generalized AI and specialized AI. In science fiction, what you hear about is generalized AI, right? Computers start getting smarter than we are and eventually conclude that we’re not all that useful, and then either they’re drugging us to keep us fat and happy or we’re in the Matrix. My impression, based on talking to my top science advisers, is that we’re still a reasonably long way away from that. It’s worth thinking about because it stretches our imaginations and gets us thinking about the issues of choice and free will that actually do have some significant applications for specialized AI, which is about using algorithms and computers to figure out increasingly complex tasks. We’ve been seeing specialized AI in every aspect of our lives, from medicine and transportation to how electricity is distributed, and it promises to create a vastly more productive and efficient economy. If properly harnessed, it can generate enormous prosperity and opportunity. But it also has some downsides that we’re gonna have to figure out in terms of not eliminating jobs.

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Links for August-September 2016

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