John Biddle (Ph.D. ’13, physics) is often inspired by a challenging problem at work: How do you locate, identify or inspect something that you can’t actually see?

“It’s something that’s very interesting to me—how do you learn something about a particular thing without necessarily seeing it? How can you look inside a box without ever opening it or figure out what's under the ground,” he said. “It’s very interesting exploring the technologies you can use to see things and testing the different types of sensors that are out there to see if you can get them to work.”John Biddle

Biddle is the senior scientist at Spectrohm, a McLean, Virginia tech startup where he’s developing smarter sensing technologies that can ‘see’ inside everything from shipping boxes to backpacks. The company aims to disrupt the screening and inspection industry by providing a safe, cost-effective way to screen the billions of cargo containers, e-commerce packages and personal items that travel worldwide every year but often go uninspected. 

Instead of X-rays or manual searches, Spectrohm’s unique approach uses radio frequency imaging technology paired with artificial intelligence to rapidly inspect packages and identify prohibited or dangerous contents—imagine a system that can scan a backpack crammed full of stuff and quickly determine whether the liquid in a plastic bottle inside is water, milk or gasoline. 

“We’re piloting a system called CheckStream, and that system would be used to look at, say, a backpack or a bag, identify what’s in it and decide whether there is a threat inside it or not,” Biddle explained. “Another system called CargoStream is for applications like mail and package screening, cargo and commercial inspection, and that’s our more advanced system that’s basically creating internal images of what’s inside.”

The goal is to create problem-solving automated sensing systems that are safe, cost-effective and user-friendly. 

“There are a lot of packages that are sent around the world that are allowed to go through without actually being checked, because there are so many and the cost of going through all of them would be pretty high with current technology,” Biddle said. “We’re trying to lower that barrier to make the screening process easier.”

Discovering the beauty of physics

Growing up in Little Rock, Arkansas, Biddle was an inquisitive kid who competed in math contests and tinkered with Radio Shack electronics kits. His fascination with science took him all the way to Harvard, where, as an undergraduate, he discovered the beauty of physics.  

“I think it was when I first took my electricity and magnetism courses, where I thought, wow, this is really cool stuff. Once you kind of get past that initial barrier of entry, you can see things that look kind of beautiful; you also see how so many things can be explained with just very few first principles,” he said. “That really intrigued me.”

In 2004, after graduating with a degree in physics and electrical engineering, Biddle landed a position as a research associate at the Institute for Defense Analyses (IDA), a nonprofit Washington, D.C. think tank, where he got his first introduction to sensing technology.

“I landed in the science and technology division, where they were consulting about acquiring new technologies,” Biddle said. “At the time, there was a big focus on landmines and improvised explosive devices, so there were plenty of new technologies claiming they could either detect or mitigate these threats. And the question for us was how could we test to see if these technologies work.”

In 2006, inspired by the colleagues with physics doctorates he worked with at IDA—including many who studied at the University of Maryland—Biddle decided to take his physics education to the next level. He began his graduate work in condensed matter physics at UMD, exploring Anderson localization and quasi-disordered systems and doing innovative research with his advisor, Distinguished University Professor Sankar Das Sarma.

“He was well-known in the field, and what was great was that he was on top of a lot of different topics, so he always had a good sense of the next big thing people were working on,” Biddle recalled. “I would typically do a lot of research on my own and then come back and do a gut check with him. And then, at some point, he’d say, ‘Okay, we're at the point where I think this is going to turn into a publication.’ So, I had three or four research publications during that time.”

‘It’s cutting-edge research, and it’s fun’

After earning his Ph.D., Biddle joined IDA’s research team full time, continuing his work with sensing systems.

“I looked at a wide range of different sensing technologies like ground-penetrating radar, metal detection and acoustics to see how accurately they could identify what’s underground or what’s inside a package,” he explained. “I enjoyed the challenge.”

Then one night in 2019, a decision to attend a D.C. Tech Meetup event opened the door to a whole new opportunity.

“The founder of Spectrohm and I happened to sit next to each other,” Biddle explained, “and it was one of those things where the person next to you turns to you and says, ‘What do you do?’ And I said, ‘I'm a physics Ph.D., and I've been working on sensing technology. And he was like, ‘Really? I happen to be working on sensor technology too.’ The rest is history.”

In 2021, Biddle made the move to Spectrohm, and since then, he’s been drawing from his physics background and sensing experience to work on the company’s high-speed inspection systems, applying his scientific skills in ways he would have never envisioned a decade ago.

“There's a lot of applied physics because I'm doing lots of electricity and magnetism modeling. We have to turn these signals from the sensing technology into something that’s human-interpretable. So that involves taking certain sensors, having a model for how we think the sensors will respond and then doing essentially an inverse problem—we have a sensor output, so what can we say about what's inside the portal,” Biddle explained. “They’re hard problems, but it's cutting-edge research, and it's fun.”

And Biddle believes they’re just scratching the surface of what these innovative sensing technologies can do.

“There could be plenty of applications,” Biddle said, “like security situations where you have people coming in with bags and backpacks, but you want to check to make sure their bags don't have a threat without using very cumbersome technology like X-rays. Or for logistics, if you have a lot of products going down the assembly line, and you want to check to make sure things are okay without manually inspecting every single item on the line. Our technology can do that.”

As Spectrohm’s products begin to enter the marketplace, Biddle still sees more testing and problem-solving on the road ahead. But it’s work he enjoys, science that’s challenging and especially meaningful because he’s making a difference. 

“There’s definitely something rewarding about this work, and it's not just seeing the research come to fruition,” he said. “Most of all, seeing your work turn into an actual product and being able to say, ‘We made that,’ that’s very cool. Having that kind of impact, making something that makes someone's life easier or solves a problem—it’s great.” 

Fifty years ago, long before he gained international recognition and “hero” status for his contributions to gravitational wave theory and the game-changing Laser Interferometer Gravitational-wave Observatory (LIGO), Richard Isaacson (Ph.D. ’67, physics) had one of his most memorable adventures as a Ph.D. student at the University of Maryland: traveling to his first scientific conference and “surviving the experience” of giving his first research talk to an audience of legendary scientists.

“I knew I was going to be talking to some noteworthy scientists. One of the people who was there was Peter Bergmann, a physicist who was an assistant to Albert Einstein, and he was very influential in the field. I knew all these people were going to ask me about my little calculations. Could I withstand the questions, the probing from every wild direction? It was my first big exposure,” Isaacson recalled. “I had gone and sat in the audience at conferences before, but that was very different from this. And, you know, it wasn't terrifying; it was an incredibly valuable experience.”

Now half a century later, Isaacson is supporting a new generation of graduate students taking on the challenges of gravitational physics while honoring the mentor whose “encouragement, enthusiasm, intellectual breadth, mathematical analytic facilities and vision [provided] the intellectual foundations and audacity” for his success. With a generous gift, Isaacson established the Richard Isaacson Graduate Student Travel Award in Gravitational Physics in memory of Physics Professor Charles Misner, the gravitational theorist who inspired Isaacson—and an entire generation of physics students—at UMD.

“Misner was awfully impressive. He had a joy of doing research and of probing the unknown and learning about it that he communicated to his students, and he was enormously helpful and influential,” Isaacson said. “My own reaction was that I held him in awe. I think he would be delighted that I’m remembering him this way, and I think he would certainly approve.”

Isaacson’s gift supports Forward: The University of Maryland Campaign for the Fearless, a $2.5 billion fundraising initiative that aims to expand access to UMD’s world-class education, accelerate groundbreaking research and build stronger communities. 

His philanthropy also leverages the college's Bequest Legacy Challenge, an incentive program that provides an immediate cash match for donors who document new or increased planned commitments to the College of Computer, Mathematical, and Natural Sciences.

“The department is very proud of our foundational role in the development of gravitational wave theory and its experimental confirmation.  I was delighted to learn of the role one of our alumni, Dr. Isaacson, played in that discovery,” said Kara Hoffman, professor and chair of the Department of Physics. “Charlie Misner was certainly a luminary, and I can’t think of a more appropriate way to honor his memory.  We are humbled by this gift.”

For IsaacsRichard Isaacsonon, it’s all about giving something back to the place that gave so much to him.

“I’m at a stage of life where I think things now have a different perspective and priority. So, I started thinking about that, and I realized that I'd like to pay back a bit, for the experience that changed my life at Maryland,” he explained. “I'm not in the multi-millionaire or billionaire class, but I thought that in today’s uncertain and rapidly changing environment for basic research I could do something, at least, that could help students out in a field which is now flourishing, and I think will do so for the next 50 years.”

Making academic dreams a reality

Isaacson came from a working-class family, and support from scholarships and graduate fellowships helped make his academic dreams a reality. His generous gift to UMD will support graduate students studying gravitational physics, funding travel awards that will allow them to attend scientific meetings and conferences, present their research and expand their graduate experience.

“It's not just to give them the enjoyment of going to a conference. It's to enable them to do something to help their career, to get out and meet people and spread the word about what they're doing, and so it gives them a little extra push,” he said. “That was something I appreciated as a graduate student, and I think I'd like to continue the tradition.”

When Isaacson began his Ph.D. research on gravitational waves, inspired by some of Einstein’s most fascinating theories, he had no idea where the work would take him. What he did know was that UMD was a widely recognized leader in the field, one of a very few U.S. institutions where gravitational work was being done both theoretically and experimentally. With Misner’s support and mentorship, Isaacson studied how gravitational waves behave and how they might be measured, answering some critical questions along the way.

“I think my thesis contributed to making it very simple and clear how these waves propagated and how in many ways they were like light,” he said. “So, it opened up a new experimental realm and made clear that this was going to be an interesting way to explore the universe.”

Advancing science ‘by a hundred years’

Isaacson went on to teach at the Illinois Institute of Technology. Then in 1973 he became the founding program director of the National Science Foundation’s (NSF) gravitational physics program, where he was soon reviewing the plan for the massive LIGO project, a bold experiment aimed at detecting gravitational waves using lasers. For decades, Isaacson worked tirelessly, securing funding and support to make sure LIGO became a reality. 

“You know, it was just this extraordinary opportunity, and I was crazy enough not to know what would be involved,” Isaacson reflected. “I'm one of an army of a thousand scientists who were involved in the project, and we had a totally different Congress that had a view of the future and could take risks and were willing to go for it,” Isaacson said. “I think Einstein said something like ‘You have to choose a problem outside of your reach—not outside of your grip, but not beyond your reach.’ This was definitely a stretch, but it was a miraculous time, and we could do that stretch.”

On September 14, 2015, two LIGO facilities did something that had never been done before—they succeeded in detecting gravitational waves resulting from the collision of two black holes. Since then, the project has yielded a host of other groundbreaking discoveries in the U.S. and around the world.Richard Isaacson's UMD ID card from the 1960s.

“I certainly knew that as soon as they got something working to detect these gravitational waves and even more spectacularly, prove the existence of black holes, it would change everything,” Isaacson said. “We advanced science by a hundred years.”

Isaacson was honored by the American Physical Society in 2018 with the establishment of the Richard A. Isaacson Award in Gravitational-Wave Science, recognizing outstanding contributions in gravitational-wave physics, gravitational-wave astrophysics and the technologies that enable this science. Meanwhile, UMD’s influence in gravitational research continues today, with alumni and faculty members engaged in the LIGO project and related work. Looking ahead, Isaacson hopes his gift can help take gravitational physics—and the students who study it—into the future.

“Supporting students in the gravitational physics group, that's the highest priority of this, to keep this field strong. Every brick that you can put in helps build the wall,” Isaacson said. “I hope I can help these young students, pay something back for the opportunities I’ve had and keep science alive.”

Written by Leslie Miller

Ryo Mori was intimidated by quantum physics when he took a quantum chemistry class as an undergraduate in Japan. He was studying applied physics, which he found more approachable, and thought he would stick to that. However, when he learned about quantum computing during his senior year, it gave him a taste of quantum physics that has drawn him into a career studying the intricate quantum behaviors underlying exotic material properties.

After graduating from Keio University in Japan, Mori went to the University of California, Berkeley, where he pursued a Ph.D. in applied science and technology. He initially worked in a lab studying how imperfections in diamonds could be used to manipulate quantum information.

“I was not a serious physics guy back then, and this quantum information group actually belonged to the chemistry department,” Mori said.Undergraduate student Raymond Qin (left), Ryo Mori (center) and postdoctoral researcher Kaishu Kawaguchi (right) after assembling a gantry crane system in Mori’s new lab.

After a couple of years, he wasn’t hooked on quantum information and began looking around for a different research focus. He was drawn to the vibrant, symmetric data being produced by a lab studying quantum materials using a technique called angle-resolved photoemission spectroscopy, or ARPES for short.

“ARPES data looks really beautiful,” Mori said. “It's really visual. I really couldn't believe that nature itself, especially in just a normal material, shows this type of beautiful, symmetric data.”

The data was produced by the group of Alassandra Lanzara, a physics professor at the University of California, Berkeley, who had pioneered a new way to extract extra data during ARPES measurements. Mori decided to join her group, which required taking additional classes and basically starting his graduate journey over from scratch.

In her group, he learned that the beautiful data reflected the beauty of the underlying physics, and the tools used to observe it gave him a grounded, practical way to engage with the world of quantum physics that he initially found so intimidating.

“It turned out this ‘data looks like art’ or ‘data looks so beautiful’ was not a bad story for the experiment technique,” Mori said. “My path was not linear, and I found ARPES itself beautiful before I truly understood it.”

In 2025, Mori joined UMD as the Alford L. Ward Assistant Professor of physics and a member of the Quantum Materials Center. At UMD, he is taking the expertise he learned in Lanzara’s lab and working to combine a variety of techniques into an experimental platform that can provide an expansive look at quantum materials. His research goes beyond interesting quantum effects, like superconductivity, that naturally arise in materials. It also explores ways that researchers can produce and control quantum states and properties by shining a light on a material or adjusting a material’s structure, such as by stretching it or sticking two layers together in different ways.

A Bright Idea

As a member of Lanzara’s group, Mori learned both the basics of ARPES and the range of flavors that it comes in. ARPES uses the natural way light interacts with the surface of materials. Light with enough energy can knock electrons out of the surface of a material, and researchers can collect them. After capturing the ejected electrons, ARPES becomes a game of energetic accounting.

The researchers know how much energy and momentum they injected into the material using light, and they measure how much comes out with the ejected electron. With a little balancing of the books, they can do the math and determine how much energy and momentum the electron was carrying around before they forced it out of the material. With enough data, researchers paint a picture of the momentum carried by electrons in the material—crucial information for describing its electrical properties.

“ARPES measures the electrons as a function of energy and momentum,” Mori said. “And electrons' natural language is actually that—the momentum and energy. So ARPES matches very well to the natural language of the electrons, and electrons are involved in some exotic phenomena in quantum materials. That's why I think an ARPES group is very important for a strong research university.”

ARPES has evolved over time into a few different forms that specialize in revealing additional details of what electrons are doing inside of materials. For example, researchers can observe the behaviors of electrons during transitions by triggering a change in a material using one pulse of light and then ejecting an electron a fixed amount of time later with a different light pulse. Piecing together several different delays between pulses produces a time-lapse-like video of how the electron behavior changes over time—a process dubbed time- and angle-resolved photoelectron spectroscopy (trARPES).

In another variation called spin-resolved ARPES, researchers add sensors that allow them to also measure the spin of ejected electrons. Spin indicates the magnetic orientation of ejected electrons and is crucial for understanding many quantum properties of materials, including superconductivity.

In Lanzara’s Lab, Mori refined his skills at using ARPES techniques and began to look for new behaviors in materials that had already been well-studied using other approaches. He went through many materials looking for interesting results. In multiple materials, he and his colleagues found interesting things happening with excitons—quasiparticles made from an electron partnered with a hole, the positive charge left behind when an electron abandons its spot in a material’s structure. In one experiment they used trARPES to observe the exciton formation process unfolding in the material MoS2. In another experiment, he and his colleagues used a combination of trARPES and spin-resolved ARPES to study the role spin played in excitons forming in the material of Bi2Te3.

Now that Mori is building his own lab, he plans to continue using ARPES, and he is designing his equipment to give him a more complete picture of each sample.

“What I'm trying to build at UMD is a combination of all these three techniques: ARPES, spin-resolved ARPES, and time-resolved ARPES,” Moris said.

The equipment he is assembling will take up a lot of space in his new lab. It not only needs to include equipment to perform the various ARPES measurements but also requires a bulky vacuum chamber to keep the samples isolated from the air. Often a reaction with oxygen or other molecules changes the properties of a sample’s surface and can ruin an experiment.

Mori is also designing his equipment with a host of other convenient tricks in addition to the trio of ARPES techniques. It will feature a laser system that can produce different wavelengths, or colors, of light, the ability to measure electrical currents through the sample and a window that will allow additional ways to study how a sample interacts with light, such as measuring how much light is reflected from a material. Additional tools will allow the team to apply magnetic fields to samples, to grow new samples or to add new layers of a material on top of a sample.

Combining his tools into an all-service experimental setup will help prepare certain samples without worrying about contamination from the atmosphere and will eliminate the chances of misalignment or the sample being damaged or altered as it is moved between devices. Each measurement will reflect the same sample, in the same position, within a fixed environment, which will let the data tell a clearer story. The new experiments will be able to provide an even richer picture than the initial data that originally caught Mori’s eye.

A Growing Lab

Mori is gradually recruiting students and post-docs to his group and is ordering all the equipment that is needed to construct his versatile ARPES setup. 

“We're going to be very unique once we finish building the lab,” Mori said. 

He expects that once the lab is set up, combining insights from the host of tools will reveal new features of materials—even ones that have already been studied intensively. He hopes that revealing how spins behave in materials will uncover new phenomenon related to magnetic properties or will lead to potential applications for manipulating quantum information.

Mori said that UMD is one of the top schools in his field of quantum materials and offers many professional and personal advantages, from multiple airports for when he needs to travel for his research and expert colleagues to collaborate with to convenient places to hike in nature and all the things for his family to do in DC.

“I think UMD is in a good location and a very good community,” Mori said. “I think this is for sure one of the best places in the world—not in the states—in the world.”

Written by Bailey Bedford

Shown is a quantum chip developed in the laboratory of University of Maryland physicist Alicia Kollár, a senior investigator with the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation (NSF RQS). The institute’s renewed award will support the next phase of research in robust quantum simulation. Photo by John T. Consoli / University of MarylandA University of Maryland-led institute focused on robust quantum simulation will launch a new phase of research with a five-year U.S. National Science Foundation award expected to total $37.5 million.

The renewal for the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation (NSF RQS), which takes effect Sept. 1, reflects UMD’s long-term commitment to advancing quantum computing to take on society’s grand challenges and to spark a new era of scientific, technological and economic development, UMD President Darryll J. Pines said.

“Our researchers in quantum simulation are an important part of the vibrant quantum ecosystem we’ve built,” Pines said, “helping establish our campus and the surrounding region as the Capital of Quantum and making our Discovery District a dynamic hub where research, industry and community combine for real-world impact on the public good.”

Established in 2021 with a $25 million federal award, NSF RQS has developed novel ways to verify quantum systems, reduce errors and demonstrate increasingly sophisticated quantum simulations while building a collaborative community of scientists, engineers, educators and students.

Quantum simulation is widely viewed as one of the first practical applications of quantum computing. Rather than trying to make every type of computation faster, quantum simulators are designed to model extraordinarily complex quantum systems that overwhelm even today’s most powerful conventional computers.

The new award expands the institute’s scientific agenda; sustains education and workforce development programs; strengthens partnerships across academia, federal laboratories and industry; and positions NSF RQS to pursue the next generation of challenges in quantum simulation. Harvard University also will join the NSF RQS consortium, expanding the current collaboration between UMD, Duke University, Princeton University, Yale University and researchers from the National Institute of Standards and Technology (NIST).

NSF RQS is one of eight NSF Quantum Leap Challenge Institutes (QLCI), a network of interdisciplinary research centers created to accelerate advances in quantum information science through collaborative research, education and workforce development.

“For more than four decades, NSF has been laying the foundational groundwork of research and discovery that is powering today’s modern quantum computing, sensing and communication,” said Brian Stone, performing the duties of the NSF director. “It’s time for focused activities to leverage that base of knowledge to drive us even farther forward to the benefit of all Americans. The NSF Quantum Leap Challenge Institutes are a next step for us in understanding the quantum world we live in.”

Mohammad Hafezi, a Minta Martin Professor of Physics and Electrical and Computer Engineering at the University of Maryland, will become director of the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation under its renewed five-year award. Image courtesy of NSF RQSDuring its first funding cycle, NSF RQS researchers achieved breakthroughs in fault-tolerant quantum simulation with neutral atoms, new methods for verifying quantum advantage on analog quantum simulators, erasure detection in Rydberg atoms and quantum simulations of gauge theories and quantum materials. Together, these advances brought robust quantum simulation closer to becoming a practical tool for scientific discovery.

That progress was matched by the institute’s growth as a national research enterprise. NSF RQS researchers collectively produced more than 600 papers, generating nearly 17,000 citations. Along the way, NSF RQS trained more than 400 graduate students and postdoctoral researchers while developing programs designed to strengthen the nation’s future quantum workforce.

“NSF’s investment in the QLCIs established hubs for research on critical topics in quantum information science at a time when the field is rapidly advancing,” said Andrew Childs, a UMD professor of computer science who directed NSF RQS during its first five years. “I’m grateful for all the collaborations it enabled and proud of the progress RQS researchers made in just five short years.”

Childs is stepping down as director after guiding the institute through its formative years. Mohammad Hafezi, a UMD Minta Martin Professor with joint appointments in physics and electrical and computer engineering, will become director. Michael Gullans, a physicist at NIST and an adjunct assistant professor in physics and the University of Maryland Institute for Advanced Computer Studies (UMIACS), will serve as deputy director.

“We are at the cusp of determining exactly what quantum computers can and cannot do in the near future,” Hafezi said. “Realizing that potential will require vibrant partnerships among universities, federal laboratories and private industry, and we’re grateful for this renewed support to help lead that effort.”

The new award funds a shift toward what institute leaders call “quantum simulation engineering”—developing quantum technologies that are increasingly scalable and capable of addressing important scientific problems. Researchers will focus on three research themes: interacting fermion simulation, which seeks to model the particles that make up matter; fermion-boson simulation, which examines how matter particles interact with force-carrying particles in complex quantum systems; and dissipative quantum simulation, which studies quantum systems interacting with their environments instead of treating environmental effects solely as unwanted noise.

NSF RQS benefits from technical and administrative support provided by UMIACS, which helps coordinate research activities across the institute’s university and federal partners, Hafezi said.

The institute also will expand its educational mission alongside its research portfolio.

During its first funding cycle, NSF RQS helped launch the University of Maryland’s quantum science and engineering minor, supported K-12 teacher professional development workshops that reached 75 educators, and developed innovative quantum activity toolkits used by 100 teachers to introduce more than 2,000 middle and high school students to concepts like superposition and measurement.

The institute also established an international QSim conference series that attracted more than 600 attendees and launched the Quantum Leap Career Nexus, which connects students with more than 50 companies, government laboratories and universities through career fairs and networking opportunities.

The renewed award will build on those efforts through additional teacher development workshops, public outreach, K-12 quantum education programs, research seminars and summer schools while creating new opportunities for students and postdoctoral researchers to prepare for careers throughout the quantum workforce.

Gretchen Campbell, associate vice president for quantum research and education at UMD, will lead the institute’s education and workforce development efforts moving forward.

“Preparing the future quantum workforce means creating opportunities at every stage—from K-12 classrooms to graduate education and professional careers,” Campbell said. “By connecting students, educators and researchers across that entire pipeline, we can help ensure the talent needed to advance quantum science and technology is ready when the next breakthroughs arrive.”

—Story by UMIACS communications group

Bob Ellsworth, a UMD Physics Visiting Professor and Professor Emeritus at George Mason University, died on Aug. 6, 2026 at the age of 89.  He earned his Ph.D.  at the University of Rochester Bob Ellsworth, Zoa Conner, Betty Alexander, Me-Li Chen, Jordan Goodman.and worked in experimental particle and cosmic-ray physics, including the SuperKamiokande neutrino experiment in Japan, Cygnus and Milagro in New Mexico, and the HAWC collaboration in Mexico. Professor Jordan Goodman met Dr. Ellsworth as a UMD freshman, and recalls, "Bob was a tremendous mentor to me. He taught me about doing careful experimental work. Always start by looking at the signal."

Claude Kacser, a faculty member from 1964 to 1997, died on Aug. 24, 2026 at the age of 92.  Kacser received his doctorate at Oxford University and accepted postions at Princeton and Columbia universities before joining UMD. He was the author of the 1967 textbook, Introduction to the Special Theory of Relativity.   Kacser was sent to America from Europe as a six-year old in 1940 to escape the Nazi threat. He told his story as part of the "One Thousand Children" project in 2012: Claude Kacser's One Thousand Children Story (American Kindertransport) - YouTube.

 Claude Kacser