Graduate Students Present Everhart Lectures on Gravitational Memory and Genome Defense
At the end of the academic year, recent doctoral graduates Keefe Mitman (PhD '24) and Peiwei Chen (PhD '24) presented Everhart Lectures about their research to the Caltech community.
Hosted by Caltech's Graduate Student Council (GSC) in collaboration with the Graduate Studies Office, the Everhart Lectures highlight outstanding graduate students for their research accomplishments and ability to communicate their work. Ongoing for more than 25 years, the lectures are named for Caltech president emeritus Thomas Everhart.
This year, 60 graduate students were nominated by faculty, and a group of seven finalists was selected by the GSC Everhart Lecture selection committee. The finalists included Chen, a biologist; Mitman, a theoretical physicist; as well bioengineer Hristos Courellis, engineer and computational climate scientist Emily de Jong, quantum physicist Adam Shaw, molecular biologist Mackenzie Strehle, and materials scientist Prachi Thureja. Mitman and Chen were ultimately selected to deliver lectures.
"The committee found the task of narrowing it down to two candidates challenging," says grad student Sitanshu Gakkhar, a member of the Everhart Lecture selection committee. "Ultimately, we felt that Keefe and Peiwei gave a sense of narrative to difficult and technical subjects that could bring out the wonder hidden in details to a nonexpert audience, and, hopefully, start a conversation about the science."
Keefe Mitman—The Universe Never Forgets: Pushing Einstein's Theory to the Limits
Mitman presented an Everhart Lecture on May 28 discussing his research on black holes and gravitational waves in the group of Saul Teukolsky, the Robinson Professor of Theoretical Astrophysics.
In his lecture, Mitman explained how his work, based on Albert Einstein's general relativity equations, uses supercomputer simulations to study the gravitational waves emitted by two merging black holes. A key focus of his research is to improve the simulations so that they accurately reflect the math and physics predicted by Einstein's equations, especially the gravitational memory effect. The memory effect is a phenomenon in which gravitational waves permanently alter the spacetime they travel through. In the talk, Mitman described his theoretical investigation into the memory effect and what it might tell us about the universe.
According to physicists, the memory effect seems to be linked to so-called asymptotic symmetries, which are conserved properties or laws of physics at the "edges" of spacetime infinitely far away. The effect also seems to relate to "soft theorems," which describe the movement of particles in quantum systems. In fact, the memory effect, asymptotic symmetries, and soft theorems appear to be mathematically equivalent, suggesting that they all describe the same thing.
Although scientists have yet to observe the memory effect with current gravitational-wave detectors, Mitman says, "If we observe memory effects, not only do we learn something about the symmetries of the edge of our universe, but we might also learn something about quantum gravity." Quantum gravity refers to a theory that would reconcile general relativity with quantum physics to describe the universe at all scales.
"That's why this is really exciting," Mitman says. "Because the whole point of observing gravitational waves is to test Einstein's theory. We know Einstein's theory is wrong when you consider quantum effects. So maybe when we detect memory, it'll look completely different than what our numerical simulations produce."
Peiwei Chen—Taming the Diverse Beasts Within: Evolutionary Innovations in Genome Defense
On June 10, Chen presented an Everhart Lecture about his research in the group of Alexei Aravin, professor of biology. In his talk, Chen characterized the genome as a "battleground where every bit of DNA has to fight for inheritance and for evolutionary survival." He went on to describe the interplay between "good" genes and "selfish" genes, which act to outcompete other genes, often causing fertility defects or even harming the organism.
Chen discovered an example of this battle between genes on the sex chromosomes of the fruit fly species Drosophila melanogaster. On one side of the fight: a presumably selfish gene on the X chromosome that Chen named "pirate," which may sabotage sperm carrying the Y chromosome and impact fertility. On the other side: a segment on the Y chromosome that Chen named "petrel," which produces short stretches of RNAs called piRNAs that target and silence the pirate gene.
"The name petrel refers to a type of seabird whose appearance predicts bad weather on the ocean," Chen says. "Seeing petrel birds must be bad news for the pirates because it tells them that a storm is coming—just like the petrel piRNAs that will soon bring a set of protein machinery to destroy the pirate RNAs in the fruit fly."
In the lecture, Chen discussed his work investigating how pirate and petrel genes evolved, as well as his work on a protein he recently discovered called "Trailblazer," which illustrates a new type of evolutionary innovation that protects the genome against expanding selfish genes.
"I am humbled and honored to have this opportunity," Chen says of his selection for the Everhart Lecture. "I am glad that the selection committee resonated with my excitement for my work, especially because I feel that many of my fellow grad students are equally, if not more, qualified to deliver an Everhart Lecture. I hope I can take advantage of this unique chance to communicate my PhD work to a broader audience that I wouldn't otherwise reach."