Literature impressions

I have often been frustrated by the inefficiency of reading through the physics literature. One problem is that physicists are sometimes bad teachers and are usually bad writers, and so it can take a long time of reading a paper before you even figure out what the author is trying to say. This gets worse when you look at papers that aren’t in your immediate physics niche, because then the author will probably use assumptions, mathematical techniques, and terminology you aren’t familiar with. If you had infinite time, you could spend days reading every paper that looks reasonably interesting, but you don’t. A preferred technique is to ask your colleagues to explain it to you, because they are more likely to speak your language and (unlike a paper) can answer your questions when you come up against a confusion. But generally your colleagues haven’t read it; they want you to read it so you can explain it to them. I spend a lot of time reading papers that end up being uninteresting, but it’s worth it for the occasional gems. And it seems clear that there is a lot of duplicated work being done sorting through the chaff.

So on the one hand we have a lengthy, fixed document from a single, often unfamiliar perspective (i.e.… [continue reading]

Hanson-ism: Travel isn’t about intellectual exposure

I often hear very smart and impressive people say that others (especially Americans) who don’t travel much have too narrow a view of the world. They haven’t been exposed to different perspectives because they haven’t traveled much. They focus on small difference of opinion within their own sphere while remaining ignorant of larger differences abroad.

Now, I think that there is a grain of truth to this, maybe even with the direction of causality pointing in the correct way. And I think it’s plausible that it really does affect Americans more than folks of similar means in Europe.Of course, here I would say the root cause is mostly economic rather than cultural; America’s size gives it a greater degree of self sufficiency in a way that means its citizens have fewer reasons to travel. This is similar to the fact that its much less profitable for the average American to become fluent in a second language than for a typical European (even a British). I think it’s obvious that if you could magically break up the American states into 15 separate nations, each with a different language, you’d get a complete reversal of these effects almost immediately. a   But it’s vastly overstated because of the status boost to people saying it.… [continue reading]

Impact discrepancies persist under uncertainty

[Tomasik has updated his essay to address some of these issues]

Brian Tomasik’s website,, contains many thoughtful pieces he has written over the years from the perspective of a utilitarian who is concerned deeply with wild animal suffering. His work has been a great resource of what is now called the effective altrusim community, and I have a lot of respect for his unflinching acceptance and exploration of our large obligations conditional on the moral importance of all animals.

I want to briefly take issue with a small but important part of Brain’s recent essay “Charity cost effectiveness in an uncertain world“. He discusses the difficult problem facing consequentialists who care about the future, especially the far future, on account of how difficult it is predict the many varied flow-through effects of our actions. In several places, he suggests that this uncertainty will tend to wash out the enormous differences in effectiveness attributed to various charities (and highlighted by effective altruists) when measured by direct impact (e.g. lives saved per dollar).

…When we consider flow-through effects of our actions, the seemingly vast gaps in cost-effectiveness among charities are humbled to more modest differences, and we begin to find more worth in the diversity of activities that different people are pursuing…

…For example, insofar as a charity encourages cooperation, philosophical reflection, and meta-thinking about how to best reduce suffering in the future — even if only by accident — it has valuable flow-through effects, and it’s unlikely these can be beaten by many orders of magnitude by something else…

…I don’t expect some charities to be astronomically better than others…

Although I agree on the importance of the uncertain implications of flow-through effects, I disagree with the suggestion that this should generally be expected to even out differences in effectiveness.… [continue reading]

Citation indices do not avoid subjectivity

Peter Higgs used his recent celebrity to criticize the current academic job system: “Today I wouldn’t get an academic job. It’s as simple as that. I don’t think I would be regarded as productive enough.” In this context, it was argued to me that using citation count, publication count, or some other related index during the hiring process for academics is a necessary evil. In particular, single academic job openings are often deluded with dozens or hundreds of applications, and there needs to be some method of narrowing down the search to a manageable number of applicants. Furthermore, it has been said, it’s important that this method is more objective rather than subjective.

I don’t think it makes sense at all to describe citation indices as less subjective measures than individual judgement calls. They just push the subjectivity from a small group (the hiring committee) to a larger group (the physics community); the decision to publish and cite is always held by human beings. Contrast this to an objective measure of how fast someone is: their 100m dash time. The subjectivity of asking a judge to guess how fast a runner appears to be going as he runs by, and the possible sources of error due to varying height or gait, are not much fixed by asking many judges and taking an “objective” vote tally.… [continue reading]

Cosmology meets philanthropy

[This was originally posted at the Quantum Pontiff.]

People sometimes ask me what how my research will help society.  This question is familiar to physicists, especially those of us whose research is connected to every-day life only… shall we say…tenuously.  And of course, this is a fair question from the layman; tax dollars support most of our work.

I generally take the attitude of former Fermilab director Robert R. Wilson.  During his testimony before the Joint Committee on Atomic Energy in the US Congress, he was asked how discoveries from the proposed accelerator would contribute to national security during a time of intense Cold War competition with the USSR.  He famously replied “this new knowledge has all to do with honor and country but it has nothing to do directly with defending our country except to help make it worth defending.”

Still, it turns out there are philosophers of practical ethics who think a few of the academic questions physicists study could have tremendous moral implications, and in fact might drive key decisions we all make each day. Oxford philosopher Nick Bostrom has in particular written about the idea of “astronomical waste“.  As is well known to physicists, the universe has a finite, ever-dwindling supply of negentropy, i.e.… [continue reading]

Decoherence Detection FAQ—Part 1: Dark matter

[Updated 2016-7-2]

I’ve submitted my papers (long and short arXiv versions) on detecting classically undetectable new particles through decoherence. The short version introduces the basic idea and states the main implications for dark matter and gravitons. The long version covers the dark matter case in depth. Abstract for the short version:

Detecting Classically Undetectable Particles through Quantum Decoherence

Some hypothetical particles are considered essentially undetectable because they are far too light and slow-moving to transfer appreciable energy or momentum to the normal matter that composes a detector. I propose instead directly detecting such feeble particles, like sub-MeV dark matter or even gravitons, through their uniquely distinguishable decoherent effects on quantum devices like matter interferometers. More generally, decoherence can reveal phenomena that have arbitrarily little classical influence on normal matter, giving new motivation for the pursuit of macroscopic superpositions.

This is figure 1:

Decoherence detection with a Mach-Zehnder interferometer. System \mathcal{N} is placed in a coherent superposition of spatially displaced wavepackets \vert N_{L} \rangle and \vert N_{R} \rangle that each travel a separate path and then are recombined. In the absence of system \mathcal{E}, the interferometer is tuned so that \mathcal{N} will be detected at the bright port with near unit probability, and at the dim port with near vanishing probability.
[continue reading]

Happier livestock through genetic bundling

Carl Shulman posted on OvercomingBias about an easier way to produce animals that suffer less: selective breeding.  In contrast to a lot of the pie-in-the-sky talk about genetically engineering animals to feel less pain, selective breeding is a proven and relatively cheap method that can produce animals with traits that increase a complicated weighted sum of many parameters.  As Shulman points out, the improvements are impressive, and breeding methods are likely to work just as well for reducing suffering as increasing milk output (although these goals may conflict in the same animal).

So suppose an animal-welfare organization is able to raise the resources necessary to run such a breeding program.  They immediately run up against the problem of how to induce large-scale farming operations to use their new breed of less-suffering chickens.  Indeed, in the comments to Shulman’s post, Gaverick Matheny pointed out that an example of a welfare-enhanced breed exists but is rarely used because it is less productive.

It’s true that there should be some low-hanging welfare fruit that has negligible effect on farm profits.  But even these are unlikely to be adopted due to economic frictions.  (Why would most farmers risk changing to an untested breed produced by an organization generally antagonistic toward them?)  So how can an animal-welfare organization induce adoption of their preferred breed?  … [continue reading]

PRISM and the exclusionary rule

I distinctly remember thinking in my high school Gov’t class that the exclusionary rule was weird.  Basically, the idea is that the primary mechanism for enforcing the 4th Amendment’s protection against unjustified searches is that evidence collected in violation of this Amendment cannot be used to convict someone.  But this is weird because (a) it can lead to setting free people guilty of egregious crimes because of minute privacy violations and (b) it offers zero protection against privacy violations by the government for other purposes, such as convicting third parties. I always thought it was kind of missing the point.

(There doesn’t seem to be a good pure check againt privacy violations to be found in the court system.  Right now, you can apparently sue the federal government through the Federal Tort Claims Act for privacy violations, but only if the government agrees.  Similar situations exist with the states.)

Now, as it turns out, problem (b) is front-and-center in the debate over FISC‘s powers.  It’s true that normal criminal courts grant warrants in a non-adversarial setting, just like FISC does.  But tptacek and dragonwriter point out on HackerNews that this is defensible because there is an adversary when this warrant is actually executed, and exclusionary rule can be used to rebuff unjustified warrants.… [continue reading]

Discriminating smartness

It seems to me that I can accurately determine which of two people is smarter by just listening to them talk if at least one person is less smart than I am, but that this is very difficult or impossible if both people are much smarter than me. When both people are smarter than me, I fall back on crude heuristics for inferring intelligence. I look for which person seems more confident, answers more quickly, and corrects the other person more often. This, of course, is a very flawed method because I can be fooled into thinking that people who project unjustified confidence are smarter than timid but brilliant people.

In the intermediate case, when I am only slightly dumber than at least one party, the problem is reduced. I am better able to detect over-confidence, often because I can understand what’s going on when the timid smart person catches the over-confident person making mistakes (even if I couldn’t have caught them myself).

(To an extent, this may all be true when you replace “smarter” with “more skilled in domain X”.)

This suggests that candidate voting systems (whether for governments or otherwise) should have more “levels”. If we all want to elect the best person, where “bestness” is hard to identify by most of us mediocre participants, we would do better by identifying which of our neighbors are smarter than us, and then electing them to make decisions for us (possibly continuing into a hierarchy of committees).… [continue reading]

Follow-up questions on the set-selection problem

Physics StackExchange user QuestionAnswers asked the question “Is the preferred basis problem solved?“, and I reproduced my “answer” (read: discussion) in a post last week.  He had some thoughtful follow-up questions, and (with his permission) I am going to answer them here. His questions are in bold, with minor punctuation changes.

How serious would you consider what you call the “Kent set-selection” problem?

If a set of CHs could be shown to be impossible to find, then this would break QM without necessarily telling us how to correct it. (Similar problems exist with the breakdown of gravity at the Planck scale.) Although I worry about this, I think it’s unlikely and most people think it’s very unlikely. If a set can be found, but no principle can be found to prefer it, I would consider QM to be correct but incomplete. It would kinda be like if big bang neucleosynthesis had not been discovered to explain the primordial frequency of elements.

And what did Zurek think of it, did he agree that it’s a substantial problem?

I think Wojciech believes a set of consistent histories (CHs) corresponding to the branch structure could be found, but that no one will find a satisfying beautiful principle within the CH framework which singles out the preferred set from the many, many other sets.… [continue reading]

Macro superpostions of the metric

Now I would like to apply the reasoning of the last post to the case of verifying macroscopic superpositions of the metric.  It’s been 4 years since I’ve touched GR, so I’m going to rely heavily on E&M concepts and pray I don’t miss any key changes in the translation to gravity.

In the two-slit experiment with light, we don’t take the visibility of interference fringes as evidence of quantum mechanics when there are many photons.  This is because the observations are compatible with a classical field description. We could interfere gravitational waves in a two-slit set up, and this would also have a purely classical explanation.

But in this post I’m not concentrating on evidence for pure quantum mechanics (i.e. a Bell-like argument grounded in locality), or evidence of the discrete nature of gravitons. Rather, I am interested in superpositions of two macroscopically distinct states of the metric as might be produced by a superposition of a large mass in two widely-separated positions.  Now, we can only call a quantum state a (proper) superposition by first identifying a preferred basis that it can be a superposition with respect to.  For now, I will wave my hands and say that the preferred states of the metric are just those metric states produced by the preferred states of matter, where the preferred states of matter are wavepackets of macroscopic amounts of mass localized in phase space (e.g.… [continue reading]

Verifying superpositions

Suppose we are given an ensemble of systems which are believed to contain coherent superposition of the metric. How would we confirm this?

Well, in order to verify that an arbitrary system is in a coherent superposition, which is always relative to a preferred basis, it’s well known that we need to make measurements with respect to (at least?) two non-commuting bases. If we can make measurement M we expect it to be possible to make measurement M` = RM for some symmetry R.

I consider essentially two types of Hilbert spaces: the infinite-dimensional space associated with position, and the finite-dimensional space associated with spin. They have a very different relationship with the fundamental symmetries of spacetime.

For spin, an arbitrary rotation in space is represented by a unitary which can produce proper superpositions. Rotating 90 degrees about the y axis takes a z-up eigenstate to an equal superposition of z-up and z-down. The rotation takes one basis to another with which it does not commute.

In contrast, for position, the unitary representing spatial translation is essentially just a permutation on the space of position eigenstates. It does not produce superpositions from non-superpositions with respect to this basis.

You might think things are different when you consider more realistic measurements with respect to the over-complete basis of wavepackets.… [continue reading]

Kent’s set-selection problem

Unfortunately, physicists and philosophers disagree on what exactly the preferred basis problem is, what would constitute a solution, and how this relates (or subsumes) “the measurement problem” more generally. In my opinion, the most general version of the preferred basis problem was best articulated by Adrian Kent and Fey Dowker near the end their 1996 article “On the Consistent Histories Approach to Quantum Mechanics” in the Journal of Statistical Physics. Unfortunately, this article is long so I will try to quickly summarize the idea.

Kent and Dowker analyzed the question of whether the consistent histories formalism provided a satisfactory and complete account of quantum mechanics (QM). Contrary to what is often said, consistent histories and many-worlds need not be opposing interpretations of quantum mechanics Of course, some consistent historians make ontological claims about how the histories are “real”, where as the many-world’ers might say that the wavefunction is more “real”. In this sense they are contradictory. Personally, I think this is purely a matter of taste. a  . Instead, consistent histories is a good mathematical framework for rigorously identifying the branch structure of the wavefunction of the universe Note that although many-worlders may not consider the consistent histories formalism the only way possible to mathematically identify branch structure, I believe most would agree that if, in the future, some branch structure was identified using a completely different formalism, it could be described at least approximately by the consistent histories formalism.  [continue reading]