In this series of posts, we sit down with a few of the keynote speakers of the 241st AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!
Professor Norman Murray has lived his life by the simple philosophy that “as long as [he] was having fun, [he was] gonna pursue science, because [he] like[d] doing this.” From his early days studying fusion and nonlinear dynamics for his PhD to the present where he works on active galactic nuclei (AGN), galaxies, and planets, Murray has left an indelible mark across wide swathes of physics and astronomy. It is for this that he is the recipient of the 2022 Dannie Heineman Prize for Astrophysics. As the Heineman Foundation aptly describes, he is receiving this prize for his deep theoretical insight into an exceptionally broad range of astrophysical phenomena, including the dynamics of planetary systems, accretion disk winds in active galactic nuclei, and star formation and feedback in galaxies.
Though he is now recognized as a pioneer in a panoply of subfields of theoretical astronomy, Professor Murray didn’t start out studying astronomy. As a graduate student studying physics at UC Berkeley, Murray worked on nonlinear dynamics under the supervision of two plasma physicists in the Electrical Engineering department. Upon completing the graduate program, he continued in that field as postdoctoral scholar in London. As he reached the end stages of this postdoc position, however, he realized that the field of nonlinear dynamics was “becoming a branch of mathematics, and [he] was more interested in physics,” so he decided to return to his childhood interests and pivot to a postdoc position studying astronomy at Caltech.
Given that he didn’t have much of an astronomy background, Murray focused on “learning lots and lots of astronomy” when he first arrived at Caltech. Part of how he did this was by attending talks. As he puts it, the work of a theorist crucially depends on attending talks and hearing about what’s happening in the field — “you learn some physics, then you go to all the talks from all the observers and listen to what they’re looking at, and try to figure well, how could that work?”. In one such experience early on in his postdoc career, he listened to a talk on the then-puzzling problem of why we see broad absorption lines in 20-40% of quasar spectra. This got him thinking and shortly thereafter led to his development of a now-widespread model of active galactic nuclei (AGN)-driven winds that produce these characteristic features. In later years, this work led to other projects describing how AGN regulate feedback in galaxies and he is now a part of the Feedback In Realistic Environments (FIRE) collaboration, which is aimed at understanding galaxy formation and evolution.
These days, Professor Murray has shifted some of his energy to thinking about topics a bit closer to home. Indeed, he will be giving his plenary lecture on a topic “which not many astronomers will have heard of or know much about” — thermal tides in the Earth’s atmosphere. Put simply, these tides are produced by heating of the Earth’s atmosphere by incident radiation from the Sun — the pressure variation that results from this heating produces a bulge in the atmosphere that is in turn gravitationally attracted to the Sun (see this astrobite for a description of this process in a bit more detail). As a result, these tides have the effect of spinning up the Earth; i.e., they act in the opposite direction to the gravitational tide from the Moon on the Earth (which works to slow down the spin).
Observations of the Moon’s orbit around the Earth show that because of the gravitational tide, the Moon is currently receding from the Earth (at a rate of roughly 3 cm/yr) and days on Earth are incrementally getting longer. By looking at the historical timing of lunar and solar eclipses and analyzing the geological fossil record, for example, scientists have been able to measure the length of the day going back nearly 3 billion years. These observations initially presented a mystery: though the current trend of lunar recession and lengthening days is the predominant behavior over this time, there is a period of roughly a billion years (between 0.8-1.8 billion years ago) during which the length of the day was fixed but the Moon was still receding. For this to happen, “if the length of day was staying fixed, but the Moon was moving farther from the Earth, then something was putting angular momentum in the system” and spinning up the Earth. Scientists in the 1980s suggested that the culprit here was the thermal tide!
Using numerical models of circulation patterns in the Earth’s atmosphere and the latest measurements of this phenomenon, Murray has been able to demonstrate that this tide could in fact be the source of the fixed length of day. However, measurements show that the length of the day at that time was around 19 hours (compared to our 24 hour days today). With the circulation models of the atmosphere and that length of the day, he has shown that the temperature of the Earth’s atmosphere must have been around 40o C (compare that to our current temperature of roughly 15o C) at that time. However, during this time, we also know that the Sun must have been dimmer than it is today, presenting another conundrum — why was the Earth warmer if it was receiving less radiation from the Sun? Murray proposes that this is a side effect of there being much more carbon dioxide in our atmosphere in the past than we see today (which has also been measured independently from this calculation). This means that one can use alternate measurements of the carbon dioxide content of the Earth’s atmosphere to test the global circulation models they’re using and more precisely nail down how and why the Earth’s spin has evolved over time!
In considering what advice he might offer to young scientists interested in astronomy, Murray reiterated his core philosophy that one should focus on doing what they enjoy. Acknowledging the competitiveness of academia, he described how he never allowed himself to become overly attached to the prospect of a career in astronomy. He would keep with it as long as he was having fun and it was working out, but if it didn’t, he would pivot and find joy in other things. For students in the field, especially those interested in theory, he emphasized the importance of understanding fundamental physics. Once you know that, “that physics can be applied to a lot of different problems… which makes life a lot more interesting [and] makes it more fun to talk to a broader range of people.”
Tackling a variety of topics means that there will be no shortage of problems to keep thinking about and no shortage of fun to be had, and Professor Murray continues to do just that.
Come hear more about thermal tides at Prof. Murray’s AAS 241 talk, currently scheduled for Wednesday, January 11th, 11:40 am-12:30 pm PT.
Astrobite edited by: Pratik Gandhi
Featured image credit: American Astronomical Society