Dr Tomer Yavetz

Today Tomer Yavetz (Columbia) defended his PhD, which was in part about the dynamics of stellar streams, and in part about macroscopically quantum-mechanical dark matter. The dissertation was great. The stellar-stream part was about stream morphologies induced by dynamical separatrices in phase space: If the stars on a stream are on orbits that span a separatrix, all heck breaks loose. The part of the thesis on this was very pedagogical and insightful about theoretical dynamics. The dark-matter part was about fast computation of steady-states using orbitals and the WKB approximation. Beautiful physics and math! But my favorite part of the thesis was the introduction, in which Yavetz discusses the point that dynamics—even though we can't see stellar orbits—does have directly observable consequences, like the aforementioned streams and their morphologies (and also Saturn's rings and the gaps in the asteroid belt and the velocity substructure in the Milky Way disk). After the defense we talked about re-framing dynamics around this idea of observability. Congratulations, and it has been a pleasure!

topological gravity; time domain

Much excellent science today. I am creating a Monday-morning check-in and parallel working time session for the undergraduates I work with. We spoke about box-least-squares for exoplanet transit finding, about FM-radio demodulators and what they have to do with timing approaches to timing-based planet finding, scientific visualization and its value in communication, and software development for science.

At lunch, the Brown-Bag talk (my favorite hour of the week) was by two CCPP PhD students. Cedric Yu (NYU) spoke about the topological form of general relativity. As my loyal reader could possibly know, I love the reformulation of GR in which you take the square-root of the metric (the tetrad, in the business). Yu showed that if you augment this with some spin fields, you can reformulate GR entirely in terms of topological invariants! That's amazing and beautiful. It relates to some cool things relating geometry and topology in old-school math. Oliver Janssen (NYU) spoke about the wave function of the Universe, and what it might mean for the initial conditions. There is a sign ambiguity, apparently, in the argument of an exponential in the action! That's a big deal. But the ideas are interesting because they force thinking about how quantum mechanics relates to the entire Universe (and hence gravity).

In addition to all this, today was the first-ever meeting of the NYU Time Domain Astrophysics group meeting, which brings together a set of people at NYU working in the time domain. It is super diverse, because we have people working on exoplanets, asteroseismology, stellar explosions, stellar mergers, black-hole binaries, tidal disruption events, and more. We are hoping to use our collective wisdom and power to help each other and also influence the time-domain observing projects in which many of us are involved.

six-volume, Fools, TOIs

I spent my science time today commenting on the first draft of a nice paper on phase-space volume by Matt Buckley (Rutgers). He shows that it is possible, in some cases, to measure the phase-space volume (six-volume) of structures in the ESA Gaia data. He wants to use Liouville's Theorem (that 6-volume is conserved) to measure the former bound masses of structures in the Milky Way halo that are now disrupted.

At Stars & Exoplanets Meeting at Flatiron, we discussed the Luger et al and Burns et al April-Fools papers. They both represent very impressive results, and are also a bit silly. On the Burns paper, we learned how to continue a spherical spectral representation down to zero radius without introducing a singularity. Reminded me of undergraduate quantum mechanics!

In addition, Bedell (Flatiron) spoke a bit about cool things that happened at #TessNinja2 last week in Chicago. Among other things, she showed a system that Foreman-Mackey (Flatiron) and collaborators set up to automatically fit the light curves of every announced TESS Object of Interest. It's hilarious: It produces a complete executable (and modifiable) Jupyter notebook for every TOI.

Hawking radiation, Gaussians

At lunch time, Matt Kleban (NYU) gave a nice overview of the simplest arguments that black holes must radiate. It was a memorial, of sorts, for Stephen Hawking. Fundamentally, the argument is that if GR is to be consistent with quantum mechanics, then you must have black-hole radiation. That argument is good and sensible, but it is certainly theoretically prejudiced, since it confidently predicts something that will never be observed, and by privileging one part of theory over another. In the discussion afterwards, we learned that GR people tend to think that black holes obviously destroy information, whereas particle physicists tend to think that information will be preserved by some heretofore unknown mechanism. That's interesting, and highlights how socially constructed some aspects of theory might be. But I learned a lot and loved the talk and the discussion. Kleban is a very deep person and a great colleague.

Earlier in the day, I got challenged on a claim that the prior prediction for a snapshot of the amplitude of one mode of a Gaussian-driven damped, harmonic oscillator would be zero-mean and Gaussian. Not the squared amplitude but the straight linear amplitude of the sinusoid with a particular phase. That rattled around in my head all day. Late in the day, I think I have a good argument: Every linear projection of a Gaussian process onto any basis function or anything else (so long as it is a linear function of the Gaussian-process data) will be Gaussian-distributed.

black holes and quantum neural networks

It seems like a low-research month! But at lunch time, Gia Dvali (NYU) gave us a very surprising black-board talk in which he compared a black-hole horizon (which contains an enormous number of microstates, implied by the black-hole entropy argument) to a quantum neural network with a particular kind of hamiltonian term on each edge. In the network, there is an occupation number for the states in which there is an exponential increase in the number of microstates, which he was arguing is similar to the huge increase in entropy when a black hole forms. That's interesting! But there was plenty of skepticism in the room about its significance. Discussion was heated, especially afterwards.

looking at the Sun, through the freakin' walls

In the CCPP Brown-Bag talk, Duccio Pappadopulo (NYU) gave a very nice and intuitive introduction to the strong CP problem (although he really presented it as the strong T problem!). He discussed the motivation for the QCD axion and then experimental bounds on it. He mentioned at the end his own work that permits the QCD axion to have much stronger couplings to photons, and therefore be much more readily detected in the laboratory. He discussed an important kind of experiment that I had not heard about previously: The helioscope, which is an x-ray telescope in a strong magnetic field, looking at the Sun, but inside a shielded building (search "axion helioscope"). That is, the experiment asks the question: Can we see through the walls? This tests the coupling of the QCD sector and the photon to the axion, because (QCD) axions are created in the Sun, and some will convert (using the magnetic field to obtain a free photon) into x-ray photons at the helioscope. Crazy, but seriously these are real experiments! I love my job.

stellar masses without models; light scalar dark matter

I discussed with Lauren Anderson (Flatiron) our project to use photometry and parallax to transfer spectroscopic labels to stars without spectroscopy (and, first, to de-noise the spectroscopic labels). This got me confused about how to explain the project to spectroscopists and non-spectroscopists alike: We have a way to use Gaia parallaxes to put logg values onto stars, but making no use whatsoever of stellar structure or evolution models, nor even scalings. Not even in the training set of labels! Indeed, I think we have a way to measure stellar masses with no use of physical models of stellar structure. I called Hans-Walter Rix (MPIA) to discuss further.

At lunch time there was an excellent brown-bag talk on light scalar dark matter by Ken Van Tilburg (NYU). He made beautiful, simple arguments about computing the properties of light scalar dark matter, and also very simple arguments about limiting the mass scale. When the dark matter gets very light, it becomes like a field of radio waves, but with a strange dispersion relation (because the particle rest mass isn't zero). This leads to highly observable effects. Huge interesting regions of parameter space are unexplored, experimentally, but there are prospects for both astrophysical and laboratory tests. There is an interesting regime at the massive end, where occupation numbers get small and the dark matter could even show macroscopic wave–particle duality effects. Overall it was highly educational, and a perfect example of the interdisciplinarity of the CCPP.

Simons Symposium on Evidence

I spent the day at the Simons Foundation, where there were talks on evidence all day. Highlights for me were the following. Thomas Hales (Pitt) spoke about his proof of the Kepler Conjecture (about packings of spheres) and the problem of verifying it. A dozen (yes, 12) referees were assigned, a conference was held and the refereeing work proceeded over a three-year seminar process. And yet they could neither verify nor refute the proof! The proof relied heavily on computers. Hales's response: To eschew the refereeing process and work with formal computational verification, which is a system for replacing mathematics referees with computer referees. The project may complete soon, 25 years after his original proof. The proof runs to 300 pages and contains thousands of inequalities, each of which is numbered with a seven-digit random integer hash. And so on!

David Donoho (Stanford) and Bill Press (Texas) spoke about the problem of reproducibility in medical studies and trials: There is now solid evidence that most biomedical results do not hold up under replication, that most effect sizes decrease as samples get larger, and that independent sets of studies come to different conclusions. There are many possible and probable reasons for this, including perverse incentives, investigator freedoms, and editorial decisions at journals. Interestingly, irreproducibility increases with study citation rate and impact. Donoho argued for moving to method pre-registration for confirmatory trials. Press argued for changing incentive structures. They both also argued for changes to educational practices, which relates to things we are thinking about in the Moore–Sloan Data Science Environments.

Tim Maudlin (NYU) talked about the foundations of quantum mechanics, building heavily on old work by J. S. Bell, who he argued is one of the deepest thinkers ever on the foundations of physics. He asked the key question of whether an effective theory might also be a true theory, and what that would mean. He argued that the foundational issues that plague quantum mechanics undermine its claim to be predictive in a principled way: Sure you can predict g−2 of the electron to 11 decimals, but if you don't know what the fundamental objects of the theory are or mean (you don't have a proper ontology), you are making these predictions with heuristic decisions or distinctions. For example, the idea of "measurement" that has to be invoked in most descriptions of quantum mechanics is not well defined nor non-arbitrary.

MaxEnt2013, day 3

I spoke at MaxEnt2013 today, in a short astronomy session that included also Ensslin (MPA), Frean (Wellington), and Brewer. Brewer spoke about our project to fully marginalize out catalogs, and Frean showed some exceedingly general methods for source discovery in data streams, applied (among other things) to astronomical data. I pitched a project to him at lunch about predicting or automatically inspecting survey data as it comes off of telescopes, which would be a beautiful extension of his work. Ensslin showed awesome reconstructions of astrophysical fields (especially the magnetic field in the Galaxy) from sparse data samples (rotation measures, in this case). He uses ideas from field theory to go beyond the Gaussian Process.

There were many valuable talks; too many to mention. Stand-outs for me included a talk by Hutter (ANU) about things that overlap my crazy paper. He was arguing for a message-length approach to selecting theories, especially huge theories of everything. He made the good point that the message must include both the initial conditions and a description of the position of the observer. Hutter describes himself as a mathematical philosopher. Lineweaver (ANU) argued passionately that the Universe is not a fluctuation away from a high-entropy state (I agree) and Goyal (Albany) argued that exchangeability can be used to prove that the universe can only contain fermions and bosons (nothing else). On the latter, I would like to understand it better; I certainly grew up learning the opposite: I learned that this was an additional postulate. Wood (ANU) gave a nice overview of probabilistic topic models and their value and limitations.

After lunch, there were break-out sessions, and we guided the (very well attended) astronomical one to things where Brewer, Murray, and I overlap. We talked about combining information from images taken at different times, through different bandpasses, and with very different calibration properties. The issues are very different if you have the multiple images or if you just have catalogs. Many good ideas came up, including many that I had (nearly) forgotten from my Gaia paper. In the end, we didn't resolve anything but we specified a very sensible project, which is to figure out how one might construct catalog outputs such that the catalogs can be combined to produce inferences that are almost as good as the inferences you get from working with the images directly. Very sensible! And very related to abortive projects I have started with Marshall.

At the end of a long day, Huppenkothen (Amsterdam) was showing Murray and me bursts from Fermi observations of a magnetar, and discussing ways we might fit the data with some kind of process (Gaussian or dictionary). We accreted Brewer and Frean and then challenged ourselves to produce a result by midnight. After a monster hack session we succeeded; we hope to be able to use what we have to constrain rise times (or make some new discovery) in these kinds of bursts.

data-driven spectral models; Oort cloud

I spent a good chunk of the day at Stanford, chatting with Blandford and Strigari. Blandford had lots of good thoughts to contribute to my general ideas about how one might build empirical (data-driven) and yet physically interpretable models of stars from enormous amounts of high signal-to-noise, high resolution spectral data (like we have in APOGEE). In particular, he pointed out that we don't have to ignore what we know about atomic physics and quantum mechanics when we do it! Strigari is thinking about the Oort cloud and the comets that allegedly fill it: Do they really have to be in a fluffy cloud around every star, or could they instead be in a space-filling population not bound to any star? Or a mixture of the two? Radical! And, he hopes, testable.