China to lead particle physics

China will build the successor to the LHC.

Note that the China Daily article above incorrectly suggests that they will build a 50-70km circular electron-positron accelerator at ~100 TeV CoM. In fact, the project comes in two phases inside the same tunnel: first a 250 GeV electron-positron ‘precision’ machineNote that the 250 GeV electron-positron collisions will produce only one Higgs, and the fact that the COM energy is double the Higgs mass is a coincidence. See slides 9-16 here for some of the processes that will be studied.a  , the Circular Electron-Positron Collider (CEPC), followed by an upgrade to a 70 TeV proton-proton ‘discovery’ machine, the Super Proton-Proton Collider (SPPC). The current timeline for operations, which will inevitably be pushed back, projects that data taking will start in 2028 and 2042, respectively. (H/t Graeme Smith.)

The existence of this accelerator has lots of interesting implications for accelerators in the Wester hemisphere. For instance, the International Linear Collider (ILC) was planning on using a ‘push-pull’ configuration where they would alternate beam time between two devices (by keeping them on huge rolling platforms!). The idea is that having two completely separate and competing detectors is critical for maintaining objectivity in world where you only have a single accelerator. Since ILC is linear, there is only one interaction region (unlike for the common circular accelerator). So to use two detectors, you need to be able to swap them in and out! But this becomes largely unnecessary if CEPC exists to keep ILC honest.

I think this is a bad development for physics because I am pessimistic about particle accelerators telling us something truly deep and novel about the universe, at least in the next century.… [continue reading]

Links for October 2015

  • More well-deserved praise for the Stanford Encyclopedia of Philosophy. Lots to be learned from how the SEP was created. A key chicken-or-egg problem:

    …several SEP authors and editors…said that the encyclopedia is used frequently both as a reference and as a teaching tool. This means that philosophers are some of the SEP’s core readers, and they can alert authors or subject editors to incorrect or insufficient entries.

    Stanford does pay most of the operating costs. But the SEP has a paid staff of only three—Zalta, Nodelman, and Allen—plus five other Stanford employees who spend 20% of their time on technical support. Neither the authors, nor the dozens of subject editors, get so much as a dime for their troubles.

    To pay running expenses not covered by Stanford, the team obtained nearly $2 million in grants over the first 15 years. But they wanted something more sustainable… The SEP asks academic libraries to make a one-time contribution [that now provides around a third of the budget]. That doesn’t get them access to the SEP, since it’s already freely accessible, but they enjoy some extra “member benefits,” like the ability to use their own branding on a version of the encyclopedia, and to save the full archives.

    Moreover, their money goes into an SEP endowment, managed by the same company that takes care of Stanford University’s endowment of over $20 billion. If the SEP ever shuts down, Stanford promises to give the libraries that contributed to SEP all their money back, with interest. “It became a no-risk investment for the libraries, and it’s a way for them to invest in open access,” says Zalta.

    Libraries were enthusiastic. The SEP was able to raise over $2 million from the long list of contributors, and Stanford added $1 million to the library endowment.

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How fast do macroscopic wavefunctions branch?

Over at PhysicsOverflow, Daniel Ranard asked a question that’s near and dear to my heart:

How deterministic are large open quantum systems (e.g. with humans)?

Consider some large system modeled as an open quantum system — say, a person in a room, where the walls of the room interact in a boring way with some environment. Begin with a pure initial state describing some comprehensible configuration. (Maybe the person is sitting down.) Generically, the system will be in a highly mixed state after some time. Both normal human experience and the study of decoherence suggest that this state will be a mixture of orthogonal pure states that describe classical-like configurations. Call these configurations branches.

How much does a pure state of the system branch over human time scales? There will soon be many (many) orthogonal branches with distinct microscopic details. But to what extent will probabilities be spread over macroscopically (and noticeably) different branches?

I answered the question over there as best I could. Below, I’ll reproduce my answer and indulge in slightly more detail and speculation.

This question is central to my research interests, in the sense that completing that research would necessarily let me give a precise, unambiguous answer. So I can only give an imprecise, hand-wavy one. I’ll write down the punchline, then work backwards.


The instantaneous rate of branching, as measured in entropy/time (e.g., bits/s), is given by the sum of all positive Lyapunov exponents for all non-thermalized degrees of freedom.

Most of the vagueness in this claim comes from defining/identifying degree of freedom that have thermalized, and dealing with cases of partial/incomplete thermalization; these problems exists classically.


The original question postulates that the macroscopic system starts in a quantum state corresponding to some comprehensible classical configuration, i.e., the system is initially in a quantum state whose Wigner function is localized around some classical point in phase space.… [continue reading]

Links for September 2015

  • Chris Blattman on the Center for Global Development’s endorsement of cash transfers. (Report.)
  • Here’s to several decades of grinding out a couple of decimal places to parameterize a charged Higgs:

    Three years ago the BaBar collaboration at SLAC measured the branching ratios for B-meson decay to produce either a muon or a tau. For two slightly different decays, they found 2σ or greater deviations from the democratic standard-model expectation. Now the LHCb collaboration at CERN has confirmed the BaBar result for one of the decays. In a preprint, the Belle group at KEK in Japan has also announced results that show a similar though less strong deviation from the standard model. The figure below (from the Heavy Flavor Averaging Group) shows the branching ratios (R) measured by the groups for the two decays, denoted D and D*, along with the standard-model prediction. Taken together, the groups’ measurements have struck a 3.9-σ blow to the principle of lepton democracy. If they hold up, the standard model will have to be modified—perhaps by the addition of a new charged Higgs boson, whose interactions would depend on mass.

    Importantly, this is a combination of several experiments rather than easily attributable to a systematic mistake in one.

  • Advanced LIGO turns on after completing upgrade. From now on, LIGO will be able to notify any number of 75 astronomical observatories around the world who have agreed to, at a moment’s notice, point their telescopes to the sky in search of light signals corresponding to possible gravitational wave detections.
  • New data on great filter from density of habitable planets.

    these new results offer little support for the scenario where we have a good chance of growing out into the universe and meeting other aliens before a billion of years have passed.

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Abstracts for September 2015

I’m trying out a new type of post: a selection of abstracts I thought were particularly interesting this month (though not necessarily released this month). Some papers I’ll have read in detail, some not. I would be particularly interested in hearing commentary on them.

  • This papers has some intuition for how to think about symplectic geometry, which surprisingly still isn’t understood in a transparent intuitive way.

  • Proof-of-concept inflationary model producing natural Bell experiments (in the CMB?), which would testify to the quantum origins of primordial fluctuations. First time I’ve ever felt the inclination to learn about multi-field inflationary models.

  • Previously announced, but included here for completeness. Also see blog post by Scott Aaronson. I’m told that several experimental groups are very close to this, which is probably why this group announced ASAP rather than embargoing the preprint before it appears in Nature. Expect to see several new loophole-free Bell experiments using different systems (photons, electrons, etc.) from other group announced in the coming year or two.

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PI accepting 2016 Postdoc applications

Perimeter Institute is now accepting applications for 3- and 5-year postdoc positions to start Fall 2016. After having been here a year, I can tell you that PI is amazing. This is the greatest place for fundamental physics research in the world. Stop working on problems that someone else would do anyway and come tackle the big questions with me!

Here is the poster, and here is the blurb:

Perimeter Institute for Theoretical Physics invites applications for postdoctoral positions from new and recent PhDs working in fundamental theoretical physics. Our areas of strength include classical gravity, condensed matter theory, cosmology, particle physics, mathematical physics, quantum fields and strings, quantum foundations, quantum information, and quantum gravity. We also encourage applications from scientists whose work falls in more than one of these categories. Our postdoctoral positions are normally for a period of three years. Outstanding candidates may also be considered for a senior postdoctoral position with a five-year term.

Perimeter Institute offers a dynamic, multi-disciplinary environment with maximum research freedom and opportunity to collaborate within and across fields. Our postdoctoral positions are intended for highly original and intellectually adventurous young theorists. Perimeter offers comprehensive support including a generous research and travel fund, opportunities to invite visiting collaborators, and help in organizing workshops and conferences. A unique mentoring system gives early-career scientists the feedback and support they need to flourish as independent researchers.

The Institute offers an exceptional research environment and is currently staffed with 40 full-time and part-time faculty members, 42 Distinguished Visiting Research Chairs, 55 Postdoctoral Researchers, 47 Graduate Students, and 28 exceptional master’s-level students participating in Perimeter Scholars International. Perimeter also hosts hundreds of visitors and conference participants throughout the academic year.

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Additional material on the arXiv

The arXiv admin board is considering adding more options for linking to material related to a submission. Some examples: blog posts, news items, video lectures, scientific video, software, lecture slides, simulations,
follow-up articles, author’s personal website. What else might be useful?

Here is a mockup of what things could look like (link to HTML):

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Links for August 2015

Didn’t quite make it on time this month…

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