Van Loo Postdoc Research Fellow Spotlight – Nick Geiser

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Nick Geiser holds dual postdoc appointments at U-M as a Van Loo (MCAIM/Mathematics) and Leinweber Postdoctoral Research Fellow (Physics).  He came to U-M from UCLA in the Fall of 2023. In his first year as a postdoc in Ann Arbor, Nick finished a research manuscript, began a new research collaboration with U-M Physics Professor Henriette Elvang and several grad students, and presented his research at the University of Chinese Academy of Sciences, Beijing. 


Nick currently studies the “S-matrix bootstrap,” an approach to fundamental physics which attempts to map out the space of consistent physical theories using mathematical first principles and numerical techniques. His latest manuscript studies the properties of a particular scattering amplitude related to string theory within the context of the S-matrix bootstrap program. All of Nick’s research papers are available here.

Nick works on several S-matrix bootstrap problems along with U-M Physics Professor Henriette Elvang, graduate student Justin Berman, and graduate student Loki Lin. Together, their group hopes to explore the space of consistent physical theories using only pen, paper, and laptops. 

How would you describe the S-matrix bootstrap to a friend or relative who works in a non-scientific field?  

The S-matrix is a mathematical object which encodes the probabilities for quantum mechanical scattering processes. For instance, if two electrons collide in an experiment, the S-matrix describes the directions in which those electrons scatter and whether any other particles like photons (light) are created. Theoretical particle physicists—like Nick—study and calculate the S-matrix. Experimental particle physicists—like those working at the Large Hadron Collider at CERN in Geneva, Switzerland—directly measure the S-matrix of nature.

The S-matrix bootstrap is an old theoretical approach dating back to the 1960s which attempts to constrain the S-matrix using mathematical principles. In this way, theoretical physicists can answer questions such as “Are certain particles allowed in nature?” or “What is the minimum/maximum possible value of certain fundamental physical parameters?” In the last decade, the S-matrix bootstrap has been revitalized using modern computers. Certain bootstrap calculations which were unfeasible in the 1960s now take seconds on a standard laptop.”


Outside of research, Nick works with a variety of organizations to support under-represented minority groups in STEM fields. He is a member of the U-M Physics Department’s Diversity, Equity, and Inclusion (DEI) committee which supports under-represented minorities in physics and hosts various DEI events for the department’s students, faculty, and staff. Nick is also involved with the Research University Alliance (RUA), a network of universities including UM, UCLA, and Harvard that is funded by the National Science Foundation to support under-represented minorities in mathematics and the physical sciences. In November 2023, Nick attended a RUA Symposium at UCLA for early-career researchers supporting RUA’s DEI initiatives. 

Outside Interests

What are your favorite things to do in Ann Arbor?

My hobbies include exercise of all kinds, exploring the many great restaurants in Ann Arbor, and enjoying winter weather after six warm years in Los Angeles.”