Report of Science at Williams College 2018-2019 A record of the professional and academic activites of faculty and students in the natural sciences Williamstown, Massachusetts Front Cover Image: In Computer Science it is often desirable to generate "all possibilities" of one kind or another. For example, when solving a scheduling problem, each permutation of {1,2,3,4,5} represents a different order in which five jobs can be scheduled for completion. Alternatively, when solving a traveling salesman problem, the same permutations repre- sent the orders in which five cities can be visited. When efficiency is a concern, the order in which the possibilities are generated can become important. Typically, we want each possibility to be similar to the previous possibility, so that any associated values can be updated from object to object, rather than being recomputed from scratch for every object. These types of orders are known as minimal-change orders, or Gray codes. The cover illustrates a minimal-change order the 120 different permutations of {1,2,3,4,5}. If superimposed on a clock, the minute hand would move from one permutation to the next every thirty seconds, starting with 12345 at the top each hour, and ending back at 54321. In this order consecutive permutations differ by a suffix-reversal, meaning that some number of the final (i.e. innermost) values have their order reversed, such as 12345 turning into 123_54_, and then 12354 turning into 12_453_, and so on. (This type of change is "minimal" in the sense that only one pair of adjacent symbols is added and removed, making it useful in the context of bonded structures like DNA, or paths in undirected graphs.) Interestingly, there is a simple rule that describes how to change the most recent permutation into the next: Reverse the shortest suffix that doesn't produce an earlier permutation. This rule keeps churning out one permutation after another until reaching 54321, which marks the end of the order. Assistant Professor Aaron Williams has shown that similar greedy algorithms can be used to reinterpret many previously discovered minimal-change orders. In particular, the order shown in the cover was originally discovered by Shmuel Zaks in the 1980s, but using a much different approach. In his most recent work in this area, Williams and his collaborators present a single unified algorithm for generating dozens of well-known patterns including the "original" Gray code for binary strings, and the bell-ringing sequence called plain change order from the 1600's, as well as new patterns for divisions of squares into rectangles known as mosaic floor plans, and many others. The resulting paper Combinatorial Generation via Permutation Languages was accepted at this year's ACM-SIAM Symposium on Discrete Algorithms (SODA 2020) and represents the culmination of a research done with Elizabeth Hartung (MCLA) while visiting Torsten Mutze at the Technical University of Berlin. Rear Cover Image: View of the South Science Center outdoor seating area which has been lanscaped with native plants. This is a peace- ful place to study or just hang out and has been used for the past 2 years for summer science pizza lunches. The Science Executive Committee wishes to express its gratitude to the many contributors to this document and especially to the extensive efforts of all of the administrative assistants in the various science departments. Editor: Norman Bell, Science Center Manager This document is printed on recycled paper. III Williams College admits men and women of any background to all the rights, privileges, programs and activities generally accorded or made available to students at the College. It does not discriminate on the basis of race, color, religion, creed, sexual orientation, or national ethnic origin in administration of its educational policies, scholarship and loan programs, and athletic and other college administered programs. The College does not discriminate on the basis of sex in violation of the Title IX of the Education Amendments of 1973, or the regulations thereunder, in the education programs or activities which it operates, including employment therein. The College does not discrim- inate on the basis of handicap in violation of Section 504 of the Rehabilitation Act 1973, or the regulations there- under, in admission or access to its programs and activities. Inquiries concerning the College’s non-discrimination policies may be referred to the Dean of the College, Hopkins Hall, Williamstown, MA 01267. IV Table of Contents The Sciences at Williams College 1 Major Science Center Funding 2 Major Programs in the Sciences 3 Winter Study 2019 Offerings 7 The Science Center 11 In Memorium 11 2018-19 Science Lunch Colloquia 12 Summer Science Research 13 Summer Science Research Colloquia 2019 13 2019 SSR Significant Funding Sources 14 Summer Science Students and their Faculty Advisors 2019 15 Summer Science Research Poster Session: August 9, 2019 18 Pre-First Year Summer Science Program 21 Summer Science Lab Program 22 Williams College Sigma Xi Chapter and 2019 Inductees 23 Department News - Astronomy 24 Biology 28 Chemistry 35 Computer Science 42 Geosciences 49 Mathematics and Statistics 58 Neuroscience 75 Physics 77 Psychology 84 Abstracts from Student Theses 92 Abstracts from Faculty and Student Publications 122 V Top: In the summer of 2019, the science center purchased this new FEI/Thermo Quattro S Field Em- mission SEM with the help of generous external foundational support. This instrument dramatically improves our capabilities in muliple research fields and is already being heavily used. Bottom: One of the first images taken with the new microscope. This image of a geranium petiole was captured by the students Calla Khilnani, Cristina Mancilla and Yang Lee in the Integrative Plant Biol- ogy (BIOL 308) course taught by Professor Claire Ting. VI The Sciences at Williams College Students learn science best by doing science; that is that might not be possible within an individual depart- by formulating and testing their own hypotheses using ment at a small institution. methods capable of producing convincing evidence. Approximately 265 students graduate with a major in This is true at the introductory level, where students be- a science or mathematics discipline each year, and we come interested in further study by encountering science continue to see about half of all students with at least as discovery rather than rote facts. It is even more im- one major in the sciences. The quality of the our science portant at advanced levels, where students are most like- programs has nurtured this interest and this year 56 stu- ly to become interested in science careers by working as dents were inducted into the Sigma Xi Honor Society as fully involved junior colleagues with professionally ac- associate members. Williams has become a leader in the tive faculty on research projects that explore new scien- training of future scientists with more than 50 former tific ideas. The ability to conduct cutting-edge research students entering Ph.D. programs in science each year. at Williams helps to attract talented scientists as facul- As a result of this commitment, Williams has ranked first ty and keeps them at the forefront of their disciplines, among predominantly undergraduate institutions in stu- which in turn allows faculty to bring the excitement of dents receiving NSF pre-doctoral fellowships, averaging their research work to their teaching and course devel- about 7 per year over the past ten years. We attribute opment at all levels. The College has invested deeply in this success to an energetic faculty and staff dedicated this ambitious program of research and teaching through to providing an excellent educational experience and to research funding, modern laboratory space, shared in- the many research opportunities available to Williams strumentation, and technical support. The relatively students at both advanced and introductory levels. large number of faculty in all the science departments A positive undergraduate research experience is the promotes breadth and depth in both research activities single most important inspiration for future scientists. and curricular scope. More than 250 students are engaged in science research In May of 2018 we completed phase I of a major expan- with Williams faculty each year. This year, 86 students sion of the science center complex with the addition of a completed theses and 182 were engaged in full-time re- new facility to house 27 faculty offices and research labs search with science faculty during the summer of 2019. in Biology, Chemistry and Physics. Geosciences will Dozens of Williams students participated in conferences also share this space until phase II is complete early in where they presented the results of their research, and 78 2021. The new building greatly expands our microsco- students and recent graduates co-authored publications py and shop facilities to support all of the sciences and in peer-reviewed journals in the past academic year. includes an updated teaching lab for biochemistry. Con- Concurrent with the increased student involvement struction of a second building to house the Geosciences, in science, Williams has attracted talented and vibrant Math/Stats, and Psychology departments is now well science faculty engaged in competitive research and underway. This addition is being constructed on the site dedicated to teaching undergraduates. As a result, the of the Bronfman Science Center which was demolished number of external grants awarded to support faculty in the summer of 2018 after 50 years of faithful service. research or curricular innovation places Williams near This new construction will complete the foundation for the top of all non-Ph.D granting institutions. Williams the sciences at Williams in the 21st century. science faculty currently have 22 active research grants Our model of the entire science division as a cohesive totalling over 3.8 million dollars.