Will Barker is part of a new generation of theoretical physicists reshaping how we think about gravity, black holes, and the fundamental structure of the universe. His recent research explores the strange frontier where Einstein’s general relativity may no longer hold — a region populated not by traditional black holes, but by their uncanny cousins: black hole mimickers. Since joining the Physics for Future fellowship in Prague, Barker’s work has taken on a broader scope. In this interview, he reflects on the role of uncertainty in physics, how cosmology can test bold new ideas, and why running to work through the hills of Prague has become part of his scientific routine.
Your work deals with solutions to Einstein's equations that mimic black holes without event horizons. Can you clarify the significance of these "black hole mimickers" in current astrophysics research?
This project was really precipitated by working with people at FZU. Back in 2020, two FZU researchers Constantinos Skordis and Tom Zlosnik proposed a new theory of gravity in which the effects we attribute to dark matter are instead explained by a modification of the gravitational force. To unpack this: dark matter is the name we give to the unknown substance that makes up most of the mass in the Universe. We can only infer its presence through its gravitational effects. This new FZU theory became very popular, and a lot of people are working on it. What we showed is that the theory allows the existence of so-called "black hole mimickers". The mimicker is a very strange object, best described as a wormhole that leads to a black hole on its other side, or a "wormhole that swallowed a black hole". From the outside, they look very similar to black holes, but you might be able to notice some differences. For example, if you could "hear" gravitational waves, the mimickers might ring like a bell after they collide and merge together. We can already "hear" black hole mergers to the required precision, so this leads to a possible observational test for the FZU theory.
Your research often highlights deviations from general relativity (GR). Why is it important for physicists to explore theories that go beyond Einstein’s theory?
It is important to understand that our confidence in Einstein's theory of gravity comes from profoundly different kinds of experiments, as compared to our confidence in the other forces of nature (i.e. electromagnetism, and the strong and weak nuclear forces). For these other forces, we can constrain them even at the quantum level using our particle colliders. For gravity, we study mostly its classical effects when we observe the Universe. Modern precision cosmology is every bit as sophisticated as collider physics, but it leaves open some surprisingly basic ambiguities. For example, we have to add these mystery substances called dark matter and dark energy to Einstein's gravity before it can explain cosmology. If you're running a collider, you can try to engineer an experiment to test something you're not sure about. In cosmology, one experiment has been unfolding for fourteen billion years: we can only think of different ways to watch it!
The lack of quantum tests is also a problem: most theoretical physicists would say that Einstein's theory is valid a low energies, and at higher energies is gets some quantum corrections. Changes to the classical theory affect how you compute these corrections, so there is a lot of potential uncertainty. I'll give you one example: you probably know at least from documentaries that Einstein described gravity as the curvature of spacetime, right? Well, that's not necessarily true! Spacetime can have two other geometric properties called torsion and non-metricity, and it was shown recently that there are pure-torsion and pure-non-metricity theories of gravity with precisely the same predictions as Einstein's theory, no curvature at all. This is called the "geometric trinity" of gravity. Thus, at the classical level, all of precision cosmology and astrophysics could be explained just as well without curved space. However, we don't yet know how these alternative geometries affect the quantum corrections. This is a big part of my current research.
You’ve also worked recently with wormhole solutions. How plausible are wormholes as physical realities, rather than purely theoretical constructs?
The black hole mimicker is technically a wormhole. However, it is nothing like the wormholes you see in science fiction. In science fiction, wormholes are pictured as useful tunnels through spacetime that connect two universes, or two distant points in the same universe. In the case of the black hole mimicker, the outside of the wormhole is indeed our familiar Universe, but the inside is an inhospitable and tortured patch of spacetime. On the outside, space becomes flat and boring when we move away from the wormhole throat. On the inside, space becomes more and more deformed, and you eventually end up at something like the horizon of a black hole. We think we've observed many black holes in nature, and Einstein's theory predicts them to be stable, however wormholes of the kind used in science fiction are not stable. What we need to calculate next is how the black hole mimicker is stable.
Your career has moved from foundational gravitational theory towards observational implications. Was this a deliberate choice, and how has this transition influenced your research approach?
You can have a lot of fun in theoretical physics when you're not constrained by observations. However, this leads to diminishing returns, and the chances that you'll make important contributions are very small. My choice was influenced by the network of collaborators which I developed after the end of my Ph.D. As for deliberate changes to my research approach, I now try to start all my papers with a candid discussion of the observational implications. If I can't think of any, then I have to think twice about whether the work should be published.
What do you consider the greatest challenges currently facing the field of gravitational physics?
In the 21st century, a lot of theorists are feeling somewhat starved of what we call "signals of new physics": some experimental or observational evidence that cannot be explained by commonly accepted theories. Actually, the gravity and cosmology community is currently split over whether we have or have not seen such signals. The biggest candidate is something called the "Hubble tension". Basically, this means that when we measure the expansion rate of the Universe using different methods, we get different answers. This discrepancy has persisted for years, and it is getting worse with time. I actually proposed a solution to this problem back in 2019, right at the start of my career. The idea is that Einstein's theory of gravity emerges dynamically out of a process called torsion condensation, which happens in the early Universe. One of my students has been stress-testing this theory against the latest precision cosmology data, with excellent results: keep an eye out for her paper in the next few weeks! Then, just last month, there was another candidate signal from the Dark Energy Spectroscopic Instrument: basically, the indication is that the cosmological constant (the thing we use to parameterise dark energy, and which accelerates the expansion of the Universe) is not constant at all. So these are the kinds of challenges we face: we have a huge amount of data in the form of precision cosmology surveys, data about the microwave afterglow of the Big Bang, the positions of galaxies, and so on. These datasets have some apparent discrepancies, but we can't all agree whether those discrepancies are real and what causes them if they are. A big part of the problem is that there are relatively few people who are fluent in both observational cosmology and theoretical physics.
Has your participation in the Physics for Future program impacted your research direction or methodology significantly?
My research direction has changed a lot. In particular I am much more interested now in observational implications than I was a year ago. As for methodology, by far the most substantial change is the use of artificial intelligence in my work. This was facilitated by P4F: shortly after starting my fellowship, I made an "emergency" decision to redirect a very large segment of my research budget into generative AI, to make sure that my collaborators and I would stay at the absolute forefront of available technology throughout the two-year tenure. So far, the rewards have been astonishing.
PHYSICS FOR FUTURE (P4F) a MSCA COFUND-supported fellowship programme with a goal of recruiting 60 postdoctoral fellows into the Institute of Physics of the Czech Academy of Sciences and ELI Beamlines, so as to pursue topics in physics relevant to society and the economy. The first call was highly successful, and the second one opens in August 2025.
You’ve collaborated internationally throughout your career. How has working across different countries and research groups shaped your perspective on physics?
It's true that I have collaborated internationally, but this is very common in academia. In fact, Czechia is technically only the second country in which I've officially worked (though I was a brief research visitor in the Netherlands as part of my Ph.D.). I spent many years in the UK, having done my undergraduate, Master's, Ph.D. and Junior Research Fellowship all in Cambridge. This was not by design (I got very lucky with local applications) and by the end of my fellowship I was getting a definite feeling of "stagnation". Because academia is inherently international, the way physics is done has actually homogenised to a surprising degree across the world. Especially in theoretical physics, the rules of conduct are pretty uniform. Researchers freely associate to engage in scientific commerce. This process always came very naturally to me, and I wouldn't want to work any other way. Almost without exception, my collaborations have led to lasting friendships. Sometimes, you do come across institutions that are more insular—tellingly, these are not the strongest places.
What are the prospects for practical applications of your research? Or do you primarily see your work as contributing foundational theoretical insights?
It's funny that you present these as a dichotomy. The truth is, you're right: fundamental theory and practical applications began to diverge from each other after the World War II ended. And given the part played by the physicists in that conflict, we may count ourselves lucky. Practical applications have grown scarce, but remember that nature does not actually owe us technology, or even progress. (A lot of people don't know this, but our best fundamental theories haven't changed much since 1964. Since then there were mostly lots of important experiments.) What you call "foundational theoretical insights" are the province of my branch of FZU: the Division of Elementary Particle Physics. Here, broadly speaking, we study the four forces of nature. So far, it was like a roulette game. The electromagnetic force gave us almost all of modern technology. The strong and weak nuclear forces gave us the atomic bomb. What will gravity do for us? Well, according to science fiction, gravity will yield wormholes and warp drives. These sound useful in an era of chemical rockets, when so many Earth-sized exoplanets lie beyond our reach, and I think about them a lot. In reality, we are not close to such technologies. To give a slightly disappointing answer to your question, I can tell you that gravitational theory has applications in revolutionising GPS, and that FZU is also spearheading this work.
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Do you engage in outreach or science communication? If so, what key message do you strive to communicate to students or the broader public about your research?
I do. Early in the spring I spent two weeks touring some of the best international high schools across China, delivering lectures and holding workshops. There were two objectives to each visit. The first (and more personal) objective was inspire the next generation of theoreticians. The second (more practical) objective was to guide students through the Western university admissions process. When I look back on my own experience, I feel deep gratitude to the mentors who helped me when I was in school. The process of providing that assistance from the other side—especially to students who face tougher cultural challenges than I did—was surprisingly meaningful. Thinking back to my own preconceptions in school, I think I had a lot more confidence in scientific authority than I do now. During my Ph.D. I began to understand that nobody was actually "running the show" in theoretical physics, and the front lines of research are dominated by uncertainty. In my outreach talks, I try to convey some of this uncertainty to students. Where there is uncertainty, there can be hope for progress.
As your fellowship involves aspects beyond research, like soft skills training or networking, have you found these components beneficial to your career progression?
I'll tell you after the next job application cycle, in this autumn. I do feel my networking skills have grown stronger, but the proof is in the pudding, so let's see.
Lastly, how has living and working in Prague influenced your research productivity or personal enjoyment of science? Do you have any favorite activities or hobbies that help balance your scientific work?
The commute has been a lot harder, but I managed to make it part of my running route: I don't allow myself to take the metro unless I'm late for a meeting, so this keeps me in shape. The route starts with a huge hill overlooking the Prague cityscape on one side, and the Vltava winding through the Bohemian countryside on the other: it makes a good start to the day. Nonetheless, there is a penalty to productivity because I work best shortly after waking, and getting to work is hard.