## 2024-02-12

### 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.

## 2024-01-22

### Betz limit for sailboats?

In the study of sustainable energy, there is a nice result on windmills, called the Betz limit: There is a finite limit to the fraction of the kinetic energy of the wind that a windmill can absorb or exploit. The reason is often stated as: If the windmill took all of the power in the wind, the wind would stop, and then there would be no flow of energy over the windmill. I'm not sure I exactly agree with that explanation, but let's leave that here.

On my travel home today I worked on the possibility that there is an equivalent to the Betz limit for sailboats. Is there an energetic way of looking at sailing that is useful?

One paradox is that a sailboat is sailing steadily when the net force on the boat is zero (just like when a windmill is turning at constant angular velocity). In the Betz limit, the windmill is thought of as having two different torques on it, one from the wind, and one from the turbine. Sailing has no turbine. So this problem has a conceptual component to it.

## 2024-01-19

### 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.

## 2024-01-14

### divide by your selection function, or multiply by it?

With Kate Storey-Fisher (San Sebastián), Abby Williams (Caltech) is working on a paper about large-angular-scale power, or anisotropy, in the distribution of quasars. It is a great subject; we need to estimate this power in the context of a very non-trivial all-sky selection function. The tradition in cosmology is to divide the data by this selection function. But of course you shouldn't manipulate your data. Instead, you could multiply your model by the selection function. You can guess which one I prefer! In fact you can do either, as long as you weight the data in the right way in the fit. I promised to write up a few words and equations about this for Williams.

## 2024-01-11

### 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).

## 2024-01-09

### Galactic cartography

Neige Frankel (CITA) and I discussed measurements of the age and metallicity gradients in the Milky Way today. In my machine-learning world, I am working on biases that come in when you use the outputs of regressions (label transfer) to perform population inferences (like mean age as a function of actions or radius). We are gearing up to do a fake but end-to-end simulation of how the Milky Way gets observed, to see if the observed Galaxy looks anything like (what we know in this fake world to be) the truth.

## 2024-01-08

### auto-encoder for calibration data

Connor Hainje (NYU) is looking at whether we could build a hierarchical or generative model of SDSS-V BOSS spectrograph calibration data, such that we could reduce the survey's per-visit calibration overheads. He started by building an auto-encoder, which is a simple, self-supervised generative model. It works really well! We discussed how to judge performance (held-out data) and how performance should depend on the size of the latent space (I predict that it won't want a large latent space). We also decided that we should announce an SDSS-V project and send out a call for collaboration.

[Note added later: Contardo (SISSA) points out that an autoencoder is not a generative model. That's right, but there are multiple definitions of generative model; only one of which is that you can sample from it. Another is that it is a parameterized model that can predict the data. Another is that it is a likelihood function for the parameters. But she's right: We are going to punk parts of the auto-encoder into a generative model in the sense of a likelihood function.]

## 2024-01-05

### 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“.

## 2024-01-03

### wind power

I met up with Matt Kleban (NYU) to discuss our dormant project on the physics of sailing. Our conversation ranged around many different things related to sustainable power. In particular, we discussed whether it was possible to take a energy or power point of view on sailing, which has to do with the work that the sailboat is doing on the water and on the air. I feel like there will be some symmetries in play there. We also discussed power generation with wind farms, including the Betz limit (which is a limit on how much power you can get out of the wind). Is there an equivalent of the Betz limit for a sailboat? Finally, Kleban made a remark that is simultaneously obvious and deep: If you have a propeller turning in a fluid (like air), it might be a turbine (generating power from the wind) or a fan (using power to make wind). The question of turbine or fan has a frame-independent (relativistically scalar) answer.

## 2024-01-02

### 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?

## 2023-12-29

### partial differential equations

I am trying to write a proposal to fund the research I do on machine-learning theory. The proposal is to work on ocean dynamics. It's a great application for the things we have done! But it's hard to write a credible proposal in an area that's new to you. Interdisciplinarity and agility is not rewarded in the funding system at present! At least I am learning a ton as I write this.

## 2023-12-28

### 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.

## 2023-12-02

### 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?

## 2023-11-30

### Terra Hunting Fall Science Meeting, day 4

Today we delved into even more detail about how the HARPS3 instrument works, looking at engineering drawings and discussing how charge-coupled devices (CCDs) read out. We discussed the time stability of various parts of the instrument and electronics. We are all very excited about assembly, verification, and testing in Cambridge this summer.

## 2023-11-29

### Terra Hunting Fall Science Meeting, day 3

Today was a delight! In a working session, Clark Baker (Cambridge) gave a beautiful, conceptual and concrete description of how an echelle spectrograph works and the blaze and the resolution and etc. My favorite moment was the aha! moment I had when he described the Littrow condition. This was followed by Alicia Anderson (Cambridge) explaining how the data reduction proceeds. Then she and Federica Rescigno (Exeter) helped us install the data-reduction software for the ESO instruments (ESPRESSO, HARPS-N, etc) and we started reducing raw echelle data.

Before all this there was a wide-ranging discussion of measuring 3-point functions of radial-velocity time series data. This was inpired by the question: Is a Gaussian process a good model for these data? I hope this turns into a project or set of projects.