Links for May 2017

  • Methane hydrates will be the new shale gas. There is perhaps an order of magnitude more methane worldwide in hydrates than in shale deposits, but it’s harder to extract. “…it’s thought that only by 2025 at the earliest we might be able to look at realistic commercial options.”
  • Sperm whales have no (external) teeth on their upper jaw, which instead features holes into which the teeth on their narrow lower jaw fit.


  • Surprising and heartening to me: GiveWell finds that distributing antiretroviral therapy drugs to HIV positive patients (presumably in developing countries) is potentially cost-effective compared to their top recommendations.
  • Relatedly: the general flow of genetic information is DNA-RNA-protein. At a crude level, viruses are classified as either RNA viruses or DNA viruses depending on what sort of genetic material they carry. Generally, as parasites dependent on the host cell machinery, this determines where in the protein construction process they inject their payload. However, retroviruses (like HIV) are RNA viruses that bring along their own reverse transcriptase enzyme that, once inside the cell, converts their payload back into DNA and then grafts it into the host’s genome (which is then copied as part of the host cell’s lifecycle). Once this happens, it is very difficult to tell which cells have been infected and very difficult to root out the infection.
  • Claims about what makes Amazon’s vertical integration different:

    I remember reading about the common pitfalls of vertically integrated companies when I was in school. While there are usually some compelling cost savings to be had from vertical integration (either through insourcing services or acquiring suppliers/customers), the increased margins typically evaporate over time as the “supplier” gets complacent with a captive, internal “customer.”

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Toward relativistic branches of the wavefunction

I prepared the following extended abstract for the Spacetime and Information Workshop as part of my continuing mission to corrupt physicists while they are still young and impressionable. I reproduce it here for your reading pleasure.


Finding a precise definition of branches in the wavefunction of closed many-body systems is crucial to conceptual clarity in the foundations of quantum mechanics. Toward this goal, we propose amplification, which can be quantified, as the key feature characterizing anthropocentric measurement; this immediately and naturally extends to non-anthropocentric amplification, such as the ubiquitous case of classically chaotic degrees of freedom decohering. Amplification can be formalized as the production of redundant records distributed over spatial disjoint regions, a certain form of multi-partite entanglement in the pure quantum state of a large closed system. If this definition can be made rigorous and shown to be unique, it is then possible to ask many compelling questions about how branches form and evolve.

A recent result shows that branch decompositions are highly constrained just by this requirement that they exhibit redundant local records. The set of all redundantly recorded observables induces a preferred decomposition into simultaneous eigenstates unless their records are highly extended and delicately overlapping, as exemplified by the Shor error-correcting code. A maximum length scale for records is enough to guarantee uniqueness. However, this result is grounded in a preferred tensor decomposition into independent microscopic subsystems associated with spatial locality. This structure breaks down in a relativistic setting on scales smaller than the Compton wavelength of the relevant field. Indeed, a key insight from algebraic quantum field theory is that finite-energy states are never exact eigenstates of local operators, and hence never have exact records that are spatially disjoint, although they can approximate this arbitrarily well on large scales.… [continue reading]

Links for April 2017

  • Why does a processor need billions of transistors if it’s only ever executing a few dozen instructions per clock cycle?
  • Nuclear submarines as refuges from global catastrophes.
  • Elite Law Firms Cash in on Market Knowledge“:

    …corporate transactions such as mergers and acquisitions or financings are characterized by several salient facts that lack a complete theoretical account. First, they are almost universally negotiated through agents. Transactional lawyers do not simply translate the parties’ bargain into legally enforceable language; rather, they are actively involved in proposing and bargaining over the transaction terms. Second, they are negotiated in stages, often with the price terms set first by the parties, followed by negotiations primarily among lawyers over the remaining non-price terms. Third, while the transaction terms tend to be tailored to the individual parties, in negotiations the parties frequently resort to claims that specific terms are (or are not) “market.” Fourth, the legal advisory market for such transactions is highly concentrated, with a half-dozen firms holding a majority of the market share.

    [Our] claim is that, for complex transactions experiencing either sustained innovation in terms or rapidly changing market conditions, (1) the parties will maximize their expected surplus by investing in market information about transaction terms, even under relatively competitive conditions, and (2) such market information can effectively be purchased by hiring law firms that hold a significant market share for a particular type of transaction.

    …The considerable complexity of corporate transaction terms creates an information problem: One or both parties may simply be unaware of the complete set of surplus-increasing terms for the transaction, and of their respective outside options should negotiations break down. This problem is distinct from the classic problem of valuation uncertainty.

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Branches and matrix-product states

I’m happy to use this bully pulpit to advertise that the following paper has been deemed “probably not terrible”, i.e., published.

When the wave function of a large quantum system unitarily evolves away from a low-entropy initial state, there is strong circumstantial evidence it develops “branches”: a decomposition into orthogonal components that is indistinguishable from the corresponding incoherent mixture with feasible observations. Is this decomposition unique? Must the number of branches increase with time? These questions are hard to answer because there is no formal definition of branches, and most intuition is based on toy models with arbitrarily preferred degrees of freedom. Here, assuming only the tensor structure associated with spatial locality, I show that branch decompositions are highly constrained just by the requirement that they exhibit redundant local records. The set of all redundantly recorded observables induces a preferred decomposition into simultaneous eigenstates unless their records are highly extended and delicately overlapping, as exemplified by the Shor error-correcting code. A maximum length scale for records is enough to guarantee uniqueness. Speculatively, objective branch decompositions may speed up numerical simulations of nonstationary many-body states, illuminate the thermalization of closed systems, and demote measurement from fundamental primitive in the quantum formalism.

Here’s the figureThe editor tried to convince me that this figure appeared on the cover for purely aesthetic reasons and this does not mean my letter is the best thing in the issue…but I know better!a   and caption:


Spatially disjoint regions with the same coloring (e.g., the solid blue regions \mathcal{F}, \mathcal{F}', \ldots) denote different records for the same observable (e.g., \Omega_a = \{\Omega_a^{\mathcal{F}},\Omega_a^{\mathcal{F}'},\ldots\}).
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Comments on Cotler, Penington, & Ranard

One way to think about the relevance of decoherence theory to measurement in quantum mechanics is that it reduces the preferred basis problem to the preferred subsystem problem; merely specifying the system of interest (by delineating it from its environment or measuring apparatus) is enough, in important special cases, to derive the measurement basis. But this immediately prompts the question: what are the preferred systems? I spent some time in grad school with my advisor trying to see if I could identify a preferred system just by looking at a large many-body Hamiltonian, but never got anything worth writing up.

I’m pleased to report that Cotler, Penington, and Ranard have tackled a closely related problem, and made a lot more progress:

Locality from the Spectrum
Jordan S. Cotler, Geoffrey R. Penington, Daniel H. Ranard
Essential to the description of a quantum system are its local degrees of freedom, which enable the interpretation of subsystems and dynamics in the Hilbert space. While a choice of local tensor factorization of the Hilbert space is often implicit in the writing of a Hamiltonian or Lagrangian, the identification of local tensor factors is not intrinsic to the Hilbert space itself. Instead, the only basis-invariant data of a Hamiltonian is its spectrum, which does not manifestly determine the local structure. This ambiguity is highlighted by the existence of dualities, in which the same energy spectrum may describe two systems with very different local degrees of freedom. We argue that in fact, the energy spectrum alone almost always encodes a unique description of local degrees of freedom when such a description exists, allowing one to explicitly identify local subsystems and how they interact.
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Links for March 2017

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Research debt

Chris Olah coins the term “research debt” to discuss a bundle of related destructive phenomena in research communities:

  • Poor Exposition – Often, there is no good explanation of important ideas and one has to struggle to understand them. This problem is so pervasive that we take it for granted and don’t appreciate how much better things could be.
  • Undigested Ideas – Most ideas start off rough and hard to understand. They become radically easier as we polish them, developing the right analogies, language, and ways of thinking.
  • Bad abstractions and notation – Abstractions and notation are the user interface of research, shaping how we think and communicate. Unfortunately, we often get stuck with the first formalisms to develop even when they’re bad. For example, an object with extra electrons is negative, and pi is wrong.
  • Noise – Being a researcher is like standing in the middle of a construction site. Countless papers scream for your attention and there’s no easy way to filter or summarize them. We think noise is the main way experts experience research debt.

Shout it from the rooftops (my emphasis):

It’s worth being clear that research debt isn’t just about ideas not being explained well. It’s a lack of digesting ideas – or, at least, a lack of the public version of ideas being digested. It’s a communal messiness of thought.

Developing good abstractions, notations, visualizations, and so forth, is improving the user interfaces for ideas. This helps both with understanding ideas for the first time and with thinking clearly about them. Conversely, if we can’t explain an idea well, that’s often a sign that we don’t understand it as well as we could…

Distillation is also hard.

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Abstracts for March 2017

  • Recent progress in synthetic chemistry and molecular quantum optics has enabled demonstrations of the quantum mechanical wave–particle duality for complex particles, with masses exceeding 10 kDa. Future experiments with even larger objects will require new optical preparation and manipulation methods that shall profit from the possibility to cleave a well-defined molecular tag from a larger parent molecule. Here we present the design and synthesis of two model compounds as well as evidence for the photoinduced beam depletion in high vacuum in one case.

    The technique of using “laser grating”, in place of physical grating (slits), for producing spatial interference of molecules relies on the laser’s ability to ionize the molecule. (Once ionized, standing electric fields can sweep it out of the way.) But for some molecules, especially large nanoparticles, this is ineffective. Solution: attach a molecular tag to the nanoparticle that reliably cleaves in the presence of a laser, allowing the nanoparticle to be vacuumed up. Rad.

  • This chapter discusses the asymptotics, singularities, and the reduction of theories. The reduction must involve the study of limits—asymptotics. The reduction is obstructed by the fact that the limit is highly singular. In addition, the type of singularity is important, and the singularities are directly connected to the existence of emergent phenomena and underlie some of the most difficult and intensively studied problems in physics today. The chapter provides six examples of singular limits and emergent phenomena such as special relativity and statistical mechanics. Reduction in its simplest form is well illustrated by special relativity.
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