The LLMs, and why do we do astrophysics?

Today my rant on LLMs and the practices of our field hit the arXiv. I was scared to post it, because it is such a weird contribution, and it is so revealing about myself and my own political positions and hangups. But I have to say: I got great and supportive feedback all day.

I got two comments on saying ACAB in the literature. The Astronomer Royal of Scotland quoted (on BlueSky) the last sentence, which I put there because Andy Casey (Monash, Flatiron) insisted. Many people sent me appreciation and thank-yous, and many people sent me comments and objections. Always constructive. The whole experience made me feel very happy about the state of our field and the way we all interact. I think maybe there will be critical mass to write some kind of collection of essays on the subject. That's a plan for 2026.

getting the absolutely rawest APOGEE data

I spent time today (at the bar!) understanding the data model and directory structure for the raw, uncalibrated APOGEE data. The idea is that I want to do a real-data example for my paper with Casey (Monash) on combining spectra, and I want to get back to the raw inputs. I also might use these spectra for a problem set in my machine-learning class. The code I wrote is all urllib and request and re, because I think it is necessary to read directories to understand the data dependencies in the survey. Is that bad?

Putting aside my concerns: The coolest thing about this project is that the SDSS family of projects (currently SDSS-V) puts absolutely every bit of its data on the web, in raw and reduced form, for re-analysis at any level or stage. That's truly, really, open science. If you don't believe me, check out this this code that spelunks the raw data. It's all just URL requests with no authentication!

combining spectral exposures

I wrote words! I got back to actually doing research this week, in part inspired by a conversation with my very good friend Greg McDonald (Rum & Code). I worked on the words in the paper I am finishing with Andy Casey (Monash) about how to combine individual-visit exposures into a mean spectrum. The biggest writing job I did today was the part of the paper called “implementation notes”, which talks about how to actually implement the math on a finite computer.

the transparency of the Universe and the transparency of the university

The highlight of my day was a wide-ranging conversation with Suroor Gandhi (NYU) about cosmology, career, and the world. She made a beautiful connection between a part of our conversation in which we were discussing the transparency of the Universe, and new ways to study that, and a part in which we were discussing the transparency with which the University speaks about disciplinary and rules cases, which (at NYU anyway) is not very good. Hence the title of this post. On transparency of the Universe, we discussed the fact that distant objects (quasars, say) do not appear blurry must put some limit on cosmic transparency. On transparency of the University, we discussed the question of how much do we care about the behavior of our institutions, and changing those behaviors. I'm a big believer in open science, open government, and open institutions.

I've been privileged these years to have some very thoughtful scientists in my world. Gandhi is one of them.

Happy birthday, Rix

Today was an all-day event at MPIA to celebrate the 60th birthday (and 25th year as Director) of Hans-Walter Rix (MPIA). There were many remarkable presentations and stories; he has left a trail of goodwill wherever he has gone! I decided to use the opportunity to talk about measurement, which is something that Rix and I have discussed for the last 18 years. My slides are here.

I've been very lucky with the opportunities I've had to work with wonderful people.

why study astrophysics?

I spent the day with Neige Frankel (CITA), working on various projects. One of the things we discussed was her slides for an upcoming talk. I made the following blanket statement; is it true? There are only two ways to ultimately justify a subject of study in astrophysics. Either it will tell us something important about fundamental physics (think: dark matter, initial conditions of the Universe, or nucleosynthesis, say), or else it will tell us something about our origins (formation of our Galaxy, occurrence of rocky, habitable planets, origin of life, say). I am not entirely sure this is right, but I can't currently think of much in the way of counter-examples. I guess one other justification might be that we are developing technologies that will help people in other areas (CCDs, spacecraft attitude management, or machine learning, say).

what book am I going to write?

One possible new year's resolution this year is for me to decide which book am I going to write? I don't love this, because it is the hallmark of a scientist at the end of the career that they switch to writing books! I guess maybe I'm at the end of my career? But that said, I have (maybe like many scientists at the end of their careers?) a lot to say. Okay anyway, I had a long conversation this morning with Greg McDonald (Rum&Code) about all this, and he strongly encouraged me to make some content for the project code-named ”The Practice of Astrophysics“.

informal scientific communication

I have been sending out my draft manuscript on machine learning in the natural sciences to various people I know who have opinions on this. I've been getting great feedback, and it reminds me that there is a lot of important scientific communication that is on informal channels. One thing that interests me: Is there a way to make such conversation more public and viewable and research-able?

philosophy

I've been working on two philosophical projects this month. The first has been an interaction with Jim Peebles (Princeton) around a paper he has been writing, setting down his philosophy of physics. I am pretty aligned with his position, which I expect to hit the arXiv soon. I'm not a co-author of that. But one of the interesting things about science is how much of our work in in anonymous (or quasi-anonymous) support of others.

The second philosophical project is a paper about machine learning and science: I am trying to set down my thoughts about how ML can and can't help the sciences. This is fundamentally a philosophy-of-science question, not a science question.

try bigger writing

I have been buried in job season and other people's projects. That's good! Hiring and advising are the main things we do in this job. But I decided today that I need to actually start a longer writing project that is my own baby. So I started to turn the set of talks I have been giving about machine learning and astrophysics into a paper. Maybe for the new ICML Position Paper call?

conjectures about pre-training

On Monday of this week, Shirley Ho (Flatiron) gave a talk at NYU in which she mentioned the unreasonable effectiveness of pre-training a neural network: If, before you train your network on your real (expensive, small) training data, you train it on a lot of (cheap, approximate) pre-training data, you get better overall performance. Why? Ho discussed this in the context of PDE emulation: She pre-trains with cheap PDEs and then trains on expensive PDEs and she gets way better performance than she does if she just trains on the expsensive stuff.

Why does this work? One interesting observation is that even pre-training on cat videos helps with the final training! Ho's belief is that the pre-training gets the network understanding time continuity and other smoothness kinds of things. My conjecture is that the pre-training teaches the network about (approximate) diffeomorphism invariance (coordinate freedom). The cool thing is that these conjectures could be tested with interventions!

data augmentation

A highlight of my day was a colloquium by Renée Hložek (Toronto) about cosmology and event detection with the LSST/Rubin. Importantly (from my perspective), she has run a set of challenges for classifying transients, based on simulations of the output of the very very loud LSST event-detection systems. The results are a bit depressing, I think (sorry Renée!), because (as she emphasized), all the successful methods (and none were exceedingly successful) made heavy use of data augmentation: They noisified things, artificially redshifted things, dropped data points from things, and so on. That's a good idea, but it shows that machine-learning methods at the present day can't easily (or ever?) be told what to expect as an event redshifts or gets fainter or happens on a different night. I'd love to fix those problems. You can almost think of all of these things as group operations. They are groups acting in a latent space though, not in the data space. Hard problems! But worthwhile.

who owns a research project?

My day ended today with a great conversation about the ownership of research projects with a postdoc. When you make the transition from graduate student to postdoc, whose projects are whose? Are they the projects of your supervisors, or are they the projects of you? And should you keep doing them, or should you move to new things? I don't think there are easy answers, and I think that there are many subtle ways in which people have unresolved differences about these things. Since much of my work these days is postdoctoral mentoring, I've thought about this a lot. My only recommendation, which is hard to implement, is that clear communication about expectations is really, really important. And not just the expectations of the supervisors; the expectations of the (former) student are way more important!

information theory for spectroscopy

I had a meeting this morning with Megan Bedell (Flatiron) about our dormant paper about information theory and extreme-precision radial-velocity measurements. We see the paper a bit differently (is it about methods or is it about concepts?), but we were able to re-state a scope with which we are both happy. We assigned tasks (Bedell writing and me coding, mainly), and promised to make progress before next week. It is very, very, very hard to finish a paper! Especially when all authors are above some seniority, where they spend most of their time with others. I would love to get a lot more personal coding time!

Florida, day two

Today was day two of my visit to University of Florida. I had many interesting discussions. One highlight was with Dhruv Zimmerman, who wants to infer big labels (non-parametric functions of time) from small features (a few bands of photometry). That's my kind of problem! We discussed different approaches, and we discussed possible featurizations (or dimensionality reductions) of the labels. I also pitched an information-theoretic analysis. If there's one thing I've learned in the last few years, it is that you shouldn't be afraid to solve problems where there are fewer data than parameters! You just have to structure the problem with eyes wide open.

After many more (equally interesting) discussions, the day ended with Sarah Ballard's group out at a lovely beer garden. We discussed the question: Should students be involved in, and privy to, all the bad things with which we faculty interact as academics, or should we protect students from the bad things? You can imagine my position, since I am all about transparency. But the positions were interesting. Ballard pointed out that in an advisor–student relationship, the student might not feel that they can refuse when the advisor wants to unload their feelings! That power asymmetry is very real. But Ballard's students (Chance, Guerrero, Lam, Seagear) said that they want to understand the bad things too; they aren't in graduate school just to write papers (that comment is for you, Quadry!).

Florida, day one

I spent today with Sarah Ballard's group, plus others, at the University of Florida. I gave a talk, to a large, lively, and delightful audience. At the end of this talk I was very impressed by the following thing: Ballard had everyone in the room discuss with their neighbors (turn and talk) for about 3 minutes, after the seminar but before the question period began! This is a technique I use in class sometimes; it increases participation. After those 3 minutes, audience members had myriad questions, as one might imagine.

I spoke with many people in the Department about their projects. One highlight was Jason Dittman, who showed me gorgeous evidence that a particular warm exoplanet on an eccentric orbit has an atmosphere that undergoes some kind of phase change at some critical insolation, as it moves away from its host star on its orbit. Crazy!

Late in the day I discussed n-point functions and other cosmological statistics with Zach Slepian and Jiamin Hou. We discussed the plausibility of getting tractable likelihoods for any n-point functions. We also discussed the oddity that n-point functions involve sums over n-star configurations among N stars (N choose n), but there are mathematical results that show that any permutation-invariant function of any point cloud can be expressed with only a sum over stars (N). That sounds like a research problem!

biases from machine learning

Today I gave a talk (with these slides) at a meeting in Denver for the NSF initiative Harnessing the Data Revolution. I spoke about the necessity and also the dangers of using machine-learning methods in scientific projects. I brought up two very serious possible biases. The first is that if emulators are used to replace simulations, and they can't be easily checked (because the simulation requirements are too expensive), the emulators will lead to a confirmation-bias problem: We will only carefully check the emulations if they lead to results that we don't like! The second bias I raised is that if we perform joint analyses on objects (stars, say) that have been labeled (with ages, say) by a machine-learning regression, there will in general be strong biases in those joint analyses. For example, the average value of 1000 age labels for stars labeled by a standard ML regression will not be anything like an unbiased estimate of the true average age of those stars. These biases are very strong and bad! That said, I also gave many example locations where using machine learning methods is not just okay but actually intellectually correct, in areas of instrument calibration, foregrounds, and other confounders.

The question period was great! We had 25 minutes of questions and answers, which ranged across a very wide set of topics, including statistics, experimental design, and epistemology.

Bayesian evidence?

Kate Storey-Fisher, Abby Williams, and I spent some time discussing unpublished work that relies heavily on calculations of the Bayesian evidence. Bayesian evidence—what I call the “fully marginalized likelihood”—relates to the volume of the posterior in parameter space. It is generally extremely sensitive to the width of the prior pdf, since if you are comparing two models with different parameterizations, the numbers you get depend on how you normalize or scale out the units of those parameter-space volumes. Indeed, you can get any evidence ratios you want by tuning prior pdf widths. That's bad if you are trying to conclude something, scientifically! Bayesian inference is only principled, imho, when you can quantitatively state the prior pdf that correctly describes your beliefs, prior to seeing the new data. And even then, your evidence is special to you; any other scientist has to recompute from scratch.

machine-learning theory and practice

Today I got invited to be on a panel discussion (hosted by Soledad Villar of JHU) with Alberto Bietti (Flatiron) about the theory and practice of machine learning. It was great! We talked about why ML works for scientific applications, and Bietti said something (obvious maybe) that I loved: Maybe ML only works because of properties of the data. That is, maybe when we are analyzing ML methods we are looking in the wrong place, and we should be analyizing the data to which they are successfully applied? I made fun of interpretation in ML, and that led to interesting comments from both Bietti and the audience. Several audience members suggested taking something more like a causal approach to interpretation: How does the method work under interventions or in conditional situations? That's interesting; it isn't what a physicist would consider interpetation, but it might be sufficient in many cases.

lost

I got really lost with respect to research today. In almost all of my projects I am supposed to be mentoring postdocs and students. Today various blocks came up that interfered with that mentoring. And then I found that I had nothing sensible to work on! Of course that isn't true: I have literally a dozen projects in a mature state waiting on final work from me. But I couldn't figure out how to work on any of them. Research is hard. At the end of the day, Andy Casey (Monash) helped me out by giving me some very specific jobs to do.