If You Want to Go Far, Go Together: Professor Ben Safdi

December 23, 2024
Avi Rosenzweig

Studying the physics of atomic particles takes a lot of room. The Large Hadron Collider at CERN, the biggest particle accelerator, is in a ring tunnel 27km (17 miles) long buried about two football fields deep underground. It serves as the factory, or artisanal manufacturer, of bespoke subatomic particles like quarks. But where is the design studio for these rare particle models? That would be the Berkeley Center for Theoretical Physics (BCTP), located in a building started by the atom smasher Ernest Lawrence himself. 

The BCTP is the best of its kind in the world. Here theorists and experimentalists, students and faculty, commune together in a central space as social as it is pedagogical, within a ring of offices where the dream team of contemporary particle physicists keep one another at the top of their game. Strong support from their bench of nimble postdocs and promising PhD students pushes the pace and adds to the intense atmosphere. 

Assistant Professor Benjamin Ryan Safdi joined the team in 2021. “CERN is great,” he says, “but you can’t be a student there. The closest you can come is our center at UC Berkeley.”

Safdi knew already when he was in high school that he wanted to work on high energy physics. An inspirational teacher would add thought experiments concerning relativity to time in class, and Safdi felt that this looked like a great way to make a living. He was aiming at physics, but he chose the College of Engineering at the University of Colorado for his bachelor’s degree. “I wanted to keep my options open,” he admits. “There were fewer elective requirements, too.” While there, he was fortunate to land an undergraduate research opportunity with quantum physicist Jun Ye. Professor Ye’s mentorship made a strong impression; Safdi also found, importantly, that he did not like building optical setups—his temperament is more suited to theory. On the other hand, he double majored in applied math, which goes well with physics, but felt there wasn’t enough of a connection with nature. He would eventually find an area of specialization that was just right: dark matter.

The road to dark matter, for Ben Safdi, went through the quirky territory of string theory. His PhD advisor at Princeton, Igor Klebanov, was a pioneer in multi-dimensional string mapping. Safdi emphasizes that Klebanov was a great mentor, and he tries to emulate his manner with his own students today. Klebanov won the Princeton student body’s choice award for advising. “He was smart, but also thoughtful,” says Safdi. You left a meeting with him feeling good about what you were doing and knowing that mistakes were part of growing better. “I want my students to feel the same.” Professor Safdi is mindful of the “careful balancing act” between supportiveness and rigor. The BCTP makes the pose easier to hold because of its particularly friendly environment: the particle physics group eats lunch together every day, spread out on the pleasant lawn of the sci/engin quad (Hearst Mining Circle) when the weather allows; they don’t follow a pecking order, and everyone is encouraged to speak up as the (theoretical) spirit moves them.

Lunching on the lawn is an important detail: during his postdoc at MIT, most of Safdi’s colleagues never climbed out of the underground hallways to see the light of day. It’s not only the social interaction that makes BCTP special; it’s the connection with nature, the embedding in the California setting. Don’t knock MIT too much, though. During the postdoc Safdi started picking at the phenomenology of dark matter. What is its nature? Are its key components particles or fields or what?

Professor’s Safdi’s lab at the BCTP is the right place to ask these questions because it “straddles the border between theory and experiment.” He still puts a lot of stock in thought experiments and mathematical legerdemain, only now he has access to data gathered by astrophysicists that can tell whether their thoughts are moving in the right direction. His office neighbors in the Center are using the LHC at CERN and getting results that provide the connections to nature that he seeks.

As much as Professor Safdi enjoys theoretical speculation, he warns “We can’t think our way out of the dark matter problems.” We depend on the instruments and the data collection that experimentalists provide. Dark matter is real and known laws can’t explain it. That’s been our situation for almost a hundred years. Recent progress, though, shows that our models are improving. When the understanding is reached, he is confident dark matter physics will “indicate how nature works at a deeper level.”

What are we missing? If our understanding is incomplete, how so? BCTP faculty are coming at it from several different angles, which is the best strategy to use. Professor Safdi is focusing on axion particles. Axions are still hypothetical – detecting them is even harder than finding neutrinos – but they could provide a unified explanation for both a fundamental problem in particle physics and the nature of dark matter. It is important that predictions involving axion models are testable. Performing the necessary experiments takes a lot of effort, but researchers will be able to determine if their models are right or wrong—the outcomes will not forever remain “up in the air.” They know what success will look like. 

These concrete guideposts point to another way that the BCTP balances the best of both theory and experiment. Professor Safdi acknowledges the investigators’ investment in their own favored perspective by celebrating what he calls their “well-motivated theoretical priors.” This is his way of praising his colleagues for pursuing their pet theories but doing so only insofar as their conclusions are backed up by hard results. The true scientist finds a balance between two seemingly incompatible attitudes—an openness to new ideas, and a skeptical scrutiny of all ideas including their own. As Carl Sagan famously quipped, “This is how deep truths are winnowed from deep nonsense.” 

There is one last but highly important circumstance that makes the Berkeley Center for Theoretical Physics unbeatable. Lawrence Berkeley National Lab (LBNL) is just up the hill. Professor Safdi spends two days a week up there on the hill, and LBNL theoretical scientists come down to The Center for two days as well. No other top university and pioneering research lab share the same hillside. Together BCTP and LBNL can “break down the boundaries between astro-particle physics, cosmology, and collider physics. Since all these areas will provide insight into fundamental physics, an inclusive view is crucial to fully interpret the plethora of experimental results in the coming years.” 

Ben Safdi was impressed by high energy physics back in high school, and now he picnics with colleagues in the hills of Northern California while observing nature by probing for the secrets of dark matter. Definitely a good way to make a living.

Topics

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