the black-body law

Today Miles Cranmer (Princeton) chatted with Weichi Yao (NYU), Soledad Villar (JHU), and me about things related to symbolic regression, dimensional analysis, and so on. He brought up a very interesting problem in the history of physics: The black-body radiation law, which is attributed to Planck. Planck knew about temperature, wavelength, the speed of light, and Boltzmann's constant k. Dimensionally, these can be combined into only one thing that has units of intensity, and that one thing is the long-wavelength black-body law. At short wavelengths, the behavior can't be explained without the introduction of a new constant, and that constant has to have non-trivial dimensions (units). He figured it out, and that constant ended up governing the hydrogen atom spectrum, quantum mechanics, and everything else. Indeed that constant, h, bears his hame. Could we have learned this ourselves just from the data directly with a machine? After all, that's what Planck did, right?

what is a bolometric correction?

Today Katie Breivik (Flatiron) asked me some technical questions about the bolometric correction. It's related to the difference between a relative magnitude in a bandpass and the relative magnitude you would get if you were using a very (infinitely) broad-band bolometer. Relative magnitudes are good things (AB magnitudes, in contrast, are bad things, but that's for another post): They are relative fluxes between the target and a standard (usually Vega). If your target is hotter than Vega, and you choose a very blue bandpass, the bandpass magnitude of the star will be smaller (relatively brighter) than the bolometric magnitude. If you choose a very red bandpass, the bandpass magnitude will be larger (relatively fainter) than the bolometric magnitude. That's all very confusing.

And bolometric is a horrible concept, since most contemporary detectors are photon-counting and not bolometric (and yes, that matters: the infinitely-wide filter on a photon-counting device gives a different relative magnitude than the infinitely-wide filter on a bolometer). I referred Breivik to this horrifying paper for unpleasant details.

astronomy in film

One of my jobs at NYU is as an advisor to student screenwriters who are writing movies that involve science and technology. I didn't get much research done today, but I had a really interesting and engaging conversation with film-writers Yuan Yuan (NYU) and Sharon Lee (NYU) who are writing a film that involves the Beijing observatory, the LAMOST project, and the Cultural Revolution. I learned a lot in this call!

quasar lifetimes

Today Christina Eilers (MPIA) gave a great colloquium talk at MPIA about the intergalactic medium, and how it can be used to understand the lifetime of quasars: Basically the idea is that quasars ionize bubbles around themselves, and the timescales are such that the size of the bubble tells you the age of the quasar. It's a nice and simple argument. Within this context, she finds some very young quasars; too young to have grown to their immense sizes. What explanation? There are ways to get around the simple argument, but they are all a bit uncomfortable. Of course one idea I love (but it sure is speculative) is the idea that maybe these very young quasars are primordial black holes!

In other research today (actually, I think this is not research according to the Rules), I finished a review of a book (a history of science book, no less) for Princeton University Press. I learned that reviewing a book for a publisher is a big job!

Aspen, day 4

Adrian Price-Whelan (Princeton) resolved some of our code differences today as unit or dimensions differences. That was good! But we still have the problem that different elements (in comparison with kinematics) lead to different inferences about orbits in the Milky Way disk. Don't know what to do about that! Either the data are wrong, or there is a big discovery here.

Ana Bonaca (Harvard), Price-Whelan, and I discussed how to build a pseudo-likelihood for comparing the models that Bonaca has for a stream perturbation to the real data. This is a bit of a hard problem, because we want objectives that improve as the agreement improves, but we don't want to build a fully generative model of the data. Why not? because we don't have a good generative model, and perturbations away from a bad generative model could lead to very wrong inferences. All we want, after all, is a rough sense of what kinds of events are consistent with the data.

In the afternoon, I gave the Aspen Center for Physics Colloquium. I spoke about Gaia and dark matter, but I also threw in my thinking about the inference of Solar System dynamics in the 17th Century: We would do it very differently now! I have much more to say but I am too tired to write it here.

John Bahcall (and etc)

I spent today at Tel Aviv University, where I gave the John Bahcall Astrophysics Lecture. I spoke about exoplanet detection and population inferences. I spent quite a bit of the day with Dovi Poznanski (TAU) and Dani Maoz (TAU). Poznanski and I discussed extensions and alternatives to his projects to use machine learning to find outliers in large astrophysical data sets. This continued conversations with him and Dalya Baron (TAU) from the previous evening.

Maoz and I discussed his conversions of cosmic star-formation history into metal enrichment histories. These involve the SNIa delay times, and they provide new interpretations of the alpha-to-Fe vs Fe-to-H ratio diagrams. The abundance ratios don't drop in alpha-to-Fe when the SNIa kick in (that's the standard story but it's wrong); they kick in when the SNIa contribution to the metal production rate exceeds the core-collapse rate. If the star-formation history is continuous, this can be far after the appearance of the first Ia SNe. Deep stuff.

The day gave me some time to reflect on my time with John Bahcall at the IAS. I have too much to say here, but I found myself in the evening reflecting on his remarkable and prescient scientific intuition. He was one of the few astronomers who understood, immediately on the early failure of HST, that it made more sense to try to repair it than try to replace it. This was a great realization, and transformed both astrophysics and NASA. He was also one of the few physicists who strongly believed that the Solar neutrino problem would lead to a discovery of new physics. Most particle physicists thought that the Solar model couldn't be that robust, and most astronomers didn't think about neutrinos. Boy was John right!

(I also snuck in a few minutes on my stellar twins document, which I gave to Poznanski for comments.

#JudyFest, day 2

Today was the second day of The Galactic Renaissance. Two scientific themes of the day were globular-cluster star abundance patterns, and stellar models that account for 3-d and non-thermal-equilibrium (NLTE) effects. On the former, it was even suggested by one speaker that the existence of chemical-abundance variations of certain kinds might be part of the definition of a globular cluster! There are some extreme cases, and various claims that the most extreme examples might be the stripped centers of ancient accreted galaxies!

On the stellar modeling front, there were impressive demonstrations from Frebel (MIT), Bergemann (MPIA), and Thygesen (Caltech) that improving the realism of the physical inputs to stellar models improves their precision and their accuracy. Thygesen did a very nice thing of using (relatively cheap) 1-D models to inform functional forms for interpolation across grid points of a (relatively expensive) 3-D model grid. That got me interested in thinking about physics-motivated or physics-constrained interpolation methods, which could have value in lots of domains.

In a session about Judy's scientific and intellectual life, Steve Shectman (OCIW) described what the world was like in 1967, when Judy Cohen (Caltech) started graduate school. It was a time of optimisim, disruption, and violence. This resonated with things I know about Cohen, because she and I used to discuss the historical context of her origins as an astronomer back when I was a graduate student.

Another highlight of the day was a discussion with Kim Venn (Victoria) and Matt Shetrone (Texas) about persistence effects that damage a significant fraction of spectra in a significant fraction of APOGEE exposures. We discussed the trade-offs between correction and avoidance, and what it might take to fix the problem.

Over dinner, I and others delivered tributes to Judy Cohen. She really has had an amazing scientific impact, and also been a wonderful person, and had a big influence on me. She also said nice things about me in her own speech!

#AstroHackWeek, day one

They say you shouldn't mess with the timeline! #AstroHackWeek was so busy and full, I ended up not blogging properly during the week, and am writing these blog posts after the fact, based on telegraphic notes taken day-of. This is not uncommon here at Hogg's Research, and, for that, I apologize: Even when I write a post after the fact, I (misleadingly) date it for the day to which it corresponds (and give it a time stamp of one minute before midnight). One of the many reasons that this blog should not be seen as a precise historical document is that these after-the-fact blog posts can certainly be contaminated by present knowledge.

Today was the kick-off day for #AstroHackWeek, our now annual meeting at which participants learn about computational data analysis, and also work on their own computational data analysis projects. This year we had the meeting at the Berkeley Institute for Data Science (and partially supported it with the Moore-Sloan Data Science Environment that spans UCB, UW, and NYU). It was organized (beautifully) by Kyle Barbary (UCB) and Phil Marshall (SLAC).

In the morning today, both I and Jake VanderPlas (UW) spoke, about the basics of probabilistic inference. Then we had what Phil Marshall calls a “stand-up”, at which every participant introduced her or himself, said what it was they wanted to learn, and said what it was they knew well and could help with. They also said what they wanted to do or produce, if there was a well-defined plan.

In the stand-up and early in the hack session, Adrian Price-Whelan (Princeton) talked about joining matched sequential colormaps into diverging colormaps, with one option (or, really, style) emphasizing values near zero, and one emphasizing values far from zero. He had immediate success, and showed some nice results. One amusing thing that might bear fruit later in the week is that the author of the (currently ascendent) Viridis colormap is apparently owner of one of the BIDS desks in our vicinity this week. The conditions on a colormap are many and in tension: There are b/w printer issues, colorblindness issues, there are no-saturate-to-white and black issues, there are small-scale resolution issues, and etc.

I started (perhaps foolishly) two hacks. The first, which I started with Dalya Baron (TAU) and Matt Mechtley (ASU), was to make the demonstration I have of low-photon-rate, direct molecular imaging much more realistic. My demo, which I mention here, is an extreme toy, and there are many directions to make it less toy-like, and improve the internal engineering. I spent time with Baron and Mechtley getting them up to speed on what works and what doesn't, and what needs to change. The easiest change to make first is to go from one-dimensional angle sampling to full three-dimensional sampling in Euler angles (or, equivalently, projection matrices).

My second hack is to somehow, some way, build an MCMC sampler that can successfully and believably sample from all the modes in the multimodal posterior pdfs that we get in standard radial-velocity fitting problems (think: finding exoplanets and binary stars by measuring precise radial velocities). When the observations are sparse, the number of qualitatively different orbital solutions is large, and no sampler that I know of convincingly samples them all. Very late in the day, over coffee, Adrian Price-Whelan, Dan Foreman-Mackey (UW), and I had a very good idea: Sample exactly in a linear problem (mixture of sinusoids) that can be sampled more-or-less analytically, transform those samples into samples at the nearest points in the parameter space of the orbit-fitting problem, and then use importance sampling to get a provably correct (in the limit) sampling from the true posterior pdf. We have a plan for tomorrow!

Kant was wrong?

Very early in the day, Rix and I talked about the future of stellar spectroscopy. With The Cannon we have shown that detailed abundances can be measured in lower signal-to-noise and lower resolution data than anyone imagined. Now we have to make this case in such a way that we influence future projects!

Late in the day, Juna Kollmeier (OCIW) gave a talk at the Simons Foundation. She gave a wide-ranging talk, about gravity from large scales to black holes. The questions were all about Einstein! She said some provocative things, for example: She said that Immanuel Kant was wrong about physics, which surprised me! I am going to look up the quotation she gave; my guess is that he was talking about materialism, not physics, and therefore was not wrong. But I will find out. I am a huge fan of Kant (in university I painted his face on the back of my leather jacket). She showed cave paintings of the sky, and it made me wonder if the time baseline trumps the precision for measuring proper motions? Probably not, but I bet there's a literature. She showed that Slipher was the first astronomer to get good evidence for a black hole. And etc.

#ArloFest, day 1

Today was the first day of Landolt Standards & 21st Century Photometry in Baton Rouge, organized by Pagnotta (AMNH) and Clayton (LSU). I came to speak about self-calibration. The day started with a historical overview by Bessel (MSSSO), who gave a lovely talk filled with profiles of the many people who contributed to the development of photometric calibration and magnitude systems. Many of the people he talked about (including himself) have filter systems or magnitude systems named after them! Among the many interesting things he touched on was this paper by Johnson, which I have yet to carefully read, but apparently contains some of the philosophy behind standard-star systems. He also discussed the filter choices for the Skymapper project, which seem very considered.

Suntzeff (TAMU) gave an excellent talk about the limitations of the supernova cosmology projects; his main point is that systematic issues with the photometric calibration system are the dominant term in the uncertainty budget. This is important in thinking about where to apportion new resources. He made a great case for understanding physically every part of the photometric measurement system (and that includes the stars, the atmosphere, the telescope, and the detector pixels, among other things). I couldn't agree more!

Grindlay (CfA) blew us away with the scale and content of the DASCH plate-scanning project at Harvard. It is just awesome, in time span, cadence, and sky coverage. Anyone not searching these data is making a mistake! And, as we were recovering from that, Kafka (AAVSO) blew us away again with the scale and scope of the APASS survey, which was designed, built, operated, reduced, and delivered to the public almost entirely by citizen scientists. It is dramatic; we are not worthy!

There were many other great contributions—too many to mention them all—but the day ended with a crawfish boil and then Josh Peek (STScI) and I at the bar discussing recent explosive conversations in the astronomical community around TMT and development in Hawaii.

One last thing I should say: Arlo Landolt (LSU) has had a huge impact on astronomy; his work has enabled countless projects and scientific measurements and discoveries. The development and stewardship of photometric standards and systems, and all the attention to detail it requires, is unglamorous and time-consuming work, ill-suited to most of the community, and yet absolutely essential to everything we do. I can't thank Landolt—and his collaborators and the whole community of photometrists—enough.