is the simulation hypothesis a physics question?

Against my better judgement, I am writing a paper on the question of whether we live inside a computer simulation. Today I was discussing this with Paula Seraphim (NYU), who has been doing research with me on this subject. We decided to re-scope the paper around the question “Is the simulation hypothesis a physics question?” instead of the direct question “Do we live in a simulation?”, which can't be answered very satisfactorily. But I think when you flow it down, you conclude that this question is, indeed, a physics question! And the simulation hypothesis motivates searches for new physics in much the same way that the dark matter and inflation do: The predictions are not specific, but there are general signatures to look for.

free energy and life

One amusing conversation today was between Ben Pope (NYU) and myself about whether hot stars are more or less likely to host planets with live. We believe (it's not extremely well established yet) that there are more habitable planets around M-type stars than G-type (and there is probably a relatively smooth function of temperature). So why do we live around a G star? Is it because there is more free energy per photon? I have assumed that this is why. But we realized that we can make this argument quantitative. One question that I have is this: Is this argument anthropic? Or is it just the simple observation that Earth hosts life? I think it is anthropic, because it has something to do with whether our place is special.

ABC, IMF, and habitability

While Foreman-Mackey and Barclay set off on a tangent to measure (or limit) exoplanet masses using n-body models of exoplanet systems observed by Kepler, I had a great phone call with Schaefer (CMU), Cisewski (CMU), Weller (CMU), and Lang about using Approximate Bayesian Computation (ABC) to ask questions about the universality of the high-mass initial mass function (IMF) in stellar clusters observed in the PHAT survey. The idea behind ABC is to do a kind of rejection sampling from the prior to make an approximation to posterior sampling in problems where it is possible to generate data sets from the model (and priors) but impractical or impossible to write down a likelihood function.

The reason we got this conversation started is that way back when we were writing Weisz et al on IMF inference, we realized that some of the ideas about how high-mass stars might form in molecular clouds (and thereby affect the formation of other less-massive stars) could be written down as a data-generating process but not as a computable likelihood function. That is, we had a perfect example for ABC. We didn't do anything about it from there, but maybe a project will start up on this. I think there might be quite a few places in astrophysics where we can generate data with a mechanistic model (a simulation or a semi-analytic model) but we don't have an explicit likelihood anywhere.

At the end of the day, Sarah Ballard (UW) gave a great Physics Colloquium on habitable exoplanets and asteroseismology, and how these two fields are related. They are related because you only know the properties of the exoplanet as well as you can understand the properties of the star, and asteroseismology rocks the latter. She mentioned anthropics momentarily, which reminded me that we should be thinking about this: The anthropic argument in exoplanet research is easier to formulate and think about than it is in cosmology, but figuring it out on the easier problem might help with the harder one.

probability

I spent day two of jury duty reading about probability, first this note by Andrew Gelman about prejudices regarding statistical philosophy and then this (long PDF) piece by Radford Neal about anthropic arguments. Gelman's piece emphasizes the oft-ignored point that it is the likelihood not the prior that is usually the most suspect and challenging thing about a statistical analysis. Neal's confirms my view that Susskind-like approaches to anthropic arguments are just plain wrong. I think I have complained about this before.

measuring bright galaxies, foundations of math

A few weeks ago I reported on my safari to Philosophy. Today, in the Astro seminar at NYU, Tim Maudlin (NYU Philosophy) went on safari to Physics. He works on the question of why mathematics is useful in describing the physical world, and this has led him to the basis of mathematics, or really the basis of the mathematics that is used in physics. He finds (remarkably to me) that if he builds topology (which is really the continuity structure of space or spacetime) on the properties of one-dimensional fundamental objects rather than open sets or neighborhoods, he gets some aspects of the causal structure of spacetime for free. We think (often, informally) of the causal structure as coming from the metric, but Maudlin finds that it can come in far earlier than that if we replace or transpose (in some sense) the foundations of topology. Crazy stuff, and a very lively seminar. My kind of Friday afternoon. Lunch was pretty hilarious too; Maudlin has at his fingertips many paradoxes that get at controversies about probability and information, related to the anthropic principle and the like.

In the morning, Mykytyn showed me that he can fit the intensity images of big galaxies, even in the presence of bright stars, even when those galaxies span different fields taken on different nights, and subsets of the images come from different photometric bandpasses. We are very close to having a system that can re-measure all the (very, very) bright galaxies in SDSS. That could have big impact.

beyond the virial radius, anthropics

There was productive conversation by email today about RR Lyrae stars in deep, multi-epoch visible data among Willman, Zolotov, and myself. Their models (by Governato et al) suggest that there should be a few RR Lyrae detectable in the 100 to 400 kpc range, even in SDSS Stripe 82, if only we can do robust detection below the individual-epoch detection limit. That kind of thing is my bread and butter. Do I feel a collaborative NSF proposal coming on?

In the afternoon, the high-energy physics seminar was by Adam Brown (Princeton) on bubble nucleation in the eternal-inflation (string-theory-inspired) metaverse. It seems that in many natural situations, the most likely bubble to nucleate in our neighborhood (future light cone) could be to a disastrously different vacuum, perhaps even one in which there would be no volume at all (it seems you would just get mashed against the expanding bubble wall). This has implications for our fate, though the standard anthropics (about which we are pretty skeptical at Camp Hogg) protect this theory from making any falsifiable predictions about our past. I used to say that astronomy was only about the past light cone, so perhaps I should be ignoring these subjects?

sabbatical plans, anthropics

Raphael Bousso (Berkeley) surprised me in the Physics Colloquium by saying some things about the anthropic principle that were not wrong. This is surprisingly rare. My principal objection to the principle is that there is no functional we can apply to a theory and determine the existence of or density of observers. I have secondary objections relating to whether it is observers we mean at all! But most talks we have had at NYU make an even more basic mistake, which is that of not clearly distinguising the need for observers from the observational matter that there are observers of the human type in our Universe. If you use the anthropic principle, ask yourself this: How does your anthropic constraint on your theory differ from an observational constraint that the observed Universe contains, say, galaxies or structure or carbon or metals or stars or people?

The fact that the Universe contains observers of our type is an observation, not a principle. The fact that any observed Universe must contain observers may qualify as a principle.

In related news, Bousso also gave a very nice argument about the cosmological constant problem, which I had not appreciated. You might think that lambda (plus vacuum energy corrections) gets set to exactly zero at the big bang by some requirement on cosmological initial conditions. That's a good idea! But then at the electroweak phase transition, the vacuum energy density drops by an amount that is some 50 or 60 orders of magnitude larger than the currently observed value for lambda. So if it is a fine-tuning that is performed by the Universe, it must be fine-tuned before electroweak to a value that makes it very close to zero after electroweak. Nice demolition, that! He had various other good arguments for the problem being a very serious problem, and proceeded to use these issues to motivate an anthropic selection from the string landscape.

I spent the afternoon in-between meetings trying to plan my sabbatical next semester. It is important that the entire time go to research.