Physics Awards: September 2023

Friday, September 29, 2023

Scientists Crack How Gravity Affects Antimatter: What That Means for Our Understanding of the Universe

A substance called antimatter is at the heart of one of the greatest mysteries of the universe. We know that every particle has an antimatter companion that is virtually identical to itself, but with the opposite charge. When a particle and its antiparticle meet, they annihilate each other—disappearing in a burst of light. Our current understanding of physics predicts that equal quantities of matter and antimatter should have been created during the formation of the universe. But this doesn’t seem to have happened as it would have resulted in all particles annihilating right away. Instead, there’s plenty of matter around us, yet very little antimatter—even deep in space. This enigma has led to a grand search to find flaws in the theory or otherwise explain the missing antimatter. One such approach has focused on gravity. Perhaps antimatter behaves differently under gravity, being pulled in the opposite direction to matter? If so, we might simply be in a part of the universe from which it is impossible to observe the antimatter. A new study, published by my team in Nature, reveals how antimatter actually behaves under the influence of gravity. Other approaches to the question of why we observe more matter than antimatter span numerous sub-fields in physics. These range from astrophysics—aiming to observe and predict the behavior of antimatter in the cosmos with experiments—to high-energy particle physics, investigating the processes and fundamental particles that form antimatter and govern their lifetime. While slight differences have been observed in the lifetime of some antimatter particles compared to their matter counterparts, these results are still far from a sufficient explanation of the asymmetry. The physical properties of antihydrogen—an atom composed of an antimatter electron (the positron) bound to an antimatter proton (antiproton)—are expected to be exactly the same as those of hydrogen. In addition to possessing the same chemical properties as hydrogen, such as color and energy, we also expect that antihydrogen should behave the same in a gravitational field. The so-called “weak equivalence principle” in the theory of general relativity states that the motion of bodies in a gravitational field is independent of their composition. This essentially says that what something is made of doesn’t affect how gravity influences its movements. This prediction has been tested to extremely high accuracy for gravitational forces with a variety of matter particles, but never directly on the motion of antimatter. Even with matter particles, gravity stands apart from other physical theories, in that is has yet to be unified with the theories that describe antimatter. Any observed difference with antimatter gravitation may help shed light on both issues. To date, there have been no direct measurements on the gravitational motion of antimatter. It is quite challenging to study because gravity is the weakest force. That means it is difficult to distinguish the effects of gravity from other external influences. It has only been with recent advances in techniques to produce stable (long-lived), neutral, and cold antimatter that measurements have become feasible.

Trapped Antimatter Our work took place at the ALPHA-g experiment at Cern, the world’s largest particle physics lab, based in Switzerland, which was designed to test the effects of gravity by containing antihydrogen in a vertical, two-meter-tall trap. Antihydrogen is created in the trap by combining its antimatter constituents: the positron and the antiproton.

Positrons are readily produced by some radioactive materials—we used radioactive table salt. To create cold antiprotons, however, we had to use immense particle accelerators and a unique decelerating facility that operates at Cern. Both ingredients are electrically charged and can be trapped and stored independently as antimatter in special devices called Penning traps, which consist of electric and magnetic fields. Anti-atoms, however, are not confined by the Penning traps, and so we had an additional device called a “magnet bottle trap,” which confined the anti-atoms. This trap was created by magnetic fields generated by numerous superconducting magnets. These were operated to control the relative strengths of the different sides of the bottle. Notably, if we weakened the top and bottom of the bottle, the atoms would be able to leave the trap under the influence of gravity. We counted how many anti-atoms escaped upwards and downwards by detecting the antimatter annihilations created as the anti-atoms collided with surrounding matter particles in the trap. By comparing these results against detailed computer models of this process in normal hydrogen atoms, we were able to infer the effect of gravity on the anti-hydrogen atoms. Our results are the first from the ALPHA-g experiment and the first direct measurement of antimatter’s motion in a gravitational field. They show that antihydrogen gravitation is the same as that of hydrogen, it falls downwards rather than rising, within the uncertainty limits of the experiment.

International Research Conference on High Energy Physics and Computational Science

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Wednesday, September 27, 2023

Breakthrough Prize for Quantum Field Theorists

The study of quantum fields is central to particle physics, but it has also led to breakthroughs in condensed-matter, statistical physics, and gravitational studies.

Many physicists hear the words “quantum field theory,” and their thoughts turn to electrons, quarks, and Higgs bosons. In fact, the mathematics of quantum fields has been used extensively in other domains outside of particle physics for the past 40 years. The 2024 Breakthrough Prize in Fundamental Physics has been awarded to two theorists who were instrumental in repurposing quantum field theory for condensed-matter, statistical physics, and gravitational studies.

“I really want to stress that quantum field theory is not the preserve of particle physics,” says John Cardy, a professor emeritus from the University of Oxford. He shares the Breakthrough Prize with Alexander Zamolodchikov from Stony Brook University, New York.

The Breakthrough Prize is perhaps the “blingiest” of science awards, with $3 million being given for each of the five main awards (three for life sciences, one for physics, and one for mathematics). Additional awards are given to early-career scientists. The founding sponsors of the Breakthrough Prize are entrepreneurs Sergey Brin, Priscilla Chan and Mark Zuckerberg, Julia and Yuri Milner, and Anne Wojcicki.

The fundamental physics prize going to Cardy and Zamolodchikov is “for profound contributions to statistical physics and quantum field theory, with diverse and far-reaching applications in different branches of physics and mathematics.” When notified about the award, Zamolodchikov expressed astonishment. “I never thought to find myself in this distinguished company,” he says. He was educated as a nuclear engineer in the former Soviet Union but became interested in particle physics. “I had to clarify for myself the basics.” The basics was quantum field theory, which describes the behaviors of elementary particles with equations that are often very difficult to solve. In the early 1980s, Zamolodchikov realized that he could make more progress in a specialized corner of mathematics called two-dimensional conformal field theory (2D CFT). “I was lucky to stumble on this interesting situation where I could find exact solutions,” Zamolodchikov says.

CFT describes “scale-invariant” mappings from one space to another. “If you take a part of the system and blow it up by the right factor, then that part looks like the whole in a statistical sense,” explains Cardy. More precisely, conformal mappings preserve the angles between lines as the lines stretch or contract in length. In certain situations, quantum fields obey this conformal symmetry. Zamolodchikov’s realization was that solving problems in CFT—especially in 2D where the mathematics is easiest—gives a starting point for studying generic quantum fields, Cardy says.

Cardy started out as a particle physicist, but he became interested in applying quantum fields to the world beyond elementary particles. When he heard about the work of Zamolodchikov and other scientists in the Soviet Union, he immediately saw the potential and versatility of 2D CFT. One of the first places he applied this mathematics was in phase transitions, which arise when, for example, the atomic spins of a material suddenly align to form a ferromagnet. Within the 2D CFT framework, Cardy showed that you could perform computations on small systems—with just ten spins, for example—and extract information that pertains to an infinitely large system. In particular, he was able to calculate the critical exponents that describe the behavior of various phase transitions.

Cardy found other uses of 2D CFT in, for example, percolation theory and quantum spin chains. “I would hope people consider my contributions as being quite broad, because that’s what I tried to be over the years,” he says. Zamolodchikov also explored the application of quantum field theory in diverse topics, such as critical phenomena and fluid turbulence. “I tried to develop it in many respects,” he says. The two theorists never collaborated, but they both confess to admiring the other’s work. “We’ve written papers on very similar things,” Cardy says. “I would say that we have a friendly rivalry.” He remembers first encountering Zamolodchikov in 1986 at a conference organized in Sweden as a “neutral” meeting point for Western and Soviet physicists. “It was wonderful to meet him and his colleagues for the first time,” Cardy says.

“Zamolodchikov and Cardy are the oracles of two dimensions,” says Pedro Vieira, a quantum field theorist from the Perimeter Institute in Canada. He says that one of the things that Zamolodchikov showed was the infinite number of symmetries that can exist in 2D CFT. Cardy was especially insightful in how to apply the mathematical insights of 2D to other dimensions. Vieira says of the pair, “They understood the power of 2D physics, in that it is very simple and elegant, and at the same time mathematically rich and complex.”

Vieira says that the work of Zamolodchikov and Cardy continues to be important for a wide range of researchers, including condensed-matter physicists who study 2D surfaces and string theorists who model the motion of 1D strings moving in time. One topic attracting a lot of attention these days is the so-called AdS/CFT correspondence, which connects CFT mathematics with gravitational theory (see Viewpoint: Are Black Holes Really Two Dimensional?). Cardy says that there’s also been a great deal of recent work on CFT in dimensions more than two. “I’m sure that [higher-dimensional CFT] will win lots of awards in the future,” he says. Zamolodchikov continues to work on extensions of quantum field theory, such as the “ $T \bar{T}$ deformation,” that may provide insights into fundamental physics, just as CFT has done.