In a U.S. government laboratory on Long Island, scientists have forged matter out of pure light using a sophisticated particle accelerator, while also demonstrating an elusive phenomenon for the first time ever on Earth.
The experimental breakthrough validated predictions made by influential physicists nearly a century ago and sheds new light on mysterious processes that occur on both quantum and cosmic scales.
This conversion of photons, which are massless light particles, into electrons, an elementary form of matter, was achieved by a team of researchers working with the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s Brookhaven National Laboratory. Though the theoretical groundwork of the new research has its origins in the early 20th century, it took special upgrades to an experiment called the Solenoidal Tracker at RHIC (STAR) detector to finally make it a reality.
“All the stars lined up for us to get this right,” said Zhangbu Xu, a member of the STAR collaboration and the lead author of a recent study about the experiment in Physical Review Letters, in a joint call with fellow STAR members Lijuan Ruan and Daniel Brandenburg.
Ruan, a physicist at Brookhaven and a co-spokesperson for STAR, added that the kinematics of the experiment sit “right in the sweet spot” for this type of ground-breaking transformation of energy into matter.
Achieving this star-aligned sweet spot is a dream that dates back to 1934, when physicists Gregory Breit and John Wheeler suggested that smashing photons together could produce a matter-antimatter pair composed of electrons, which are negatively charged particles of matter, and positrons, which are antimatter counterparts of electrons that carry a positive charge.
The idea, now known as the Breit-Wheeler process, was inspired in part by the dawn of quantum mechanics during this period, which revealed that photons could interact on quantum levels in ways that are not predicted by classical mechanics. The physicists were also building on Albert Einstein’s famous mass-energy equivalence, written as E=mc2, which demonstrates that mass and energy are two sides of the same coin.
That said, it is much trickier to transform energy into matter than it is to convert matter into energy. It would have seemed especially inconceivable back in the 1930s. As a credit to their foresight, Breit and Wheeler speculated that a device that could accelerate ions, which are atoms stripped of electrons, might be able to do the trick, even though no such machine existed at the time.
“It shows some of their brilliance because this was in the early 30s, before many of the modern experiments that we have,” said Brandenburg, who is a Goldhaber Fellow at Brookhaven. “But they already predicted, in the last paragraph of their paper, how you could actually achieve this really difficult process, and they discuss exactly the experiment that we finally have been able to do.”
“I find it very amazing that they had the insight to predict not only this theory calculation, but that they predicted experimentally how it would come about nearly 100 years before we had the technology to do it,” he added.
The experiment that Breit and Wheeler envisioned, and that the STAR collaboration has now successfully conducted, requires shooting heavy ions (in this case gold) past each other at 99.995 percent the speed of light within RHIC. The strong positive charge and extremely high speeds of the ions create a circular magnetic field and a cloud of photons that travel with the particles through the collider.
As the gold ions skim each other, their halos of light particles interact and produce the matter-antimatter pairs that were predicted so many decades ago. While RHIC was able to demonstrate the Breit-Wheeler process, the STAR detector was the instrument that actually observed, measured, and confirmed the achievement.
Though the milestone is the result of a century of theoretical groundwork, there was also an element of serendipity involved, as STAR researchers only recently realized their setup could experimentally prove this otherworldly conversion of energy into matter.
“It’s actually only a few years back, in 2018, that we started to see something interesting, but at that time we didn’t realize it was the Breit-Wheeler process,” said Ruan. “We saw something different from what we regularly expected from heavy ion collisions, but it was really when Daniel [Brandenburg] started to do the data analysis with STAR-caliber precision, with all the differential kinematics measurements, that we could say: ‘Oh, this is really the Breit-Wheeler process.’”
This landmark validation of a long-theorized process is exciting by itself, but the experiment also achieved another equally important breakthrough: the first Earth-based demonstration of a phenomenon known as vacuum birefringence, a concept that also dates back nearly a century.
In 1936, physicists Hans Heinrich Euler and Werner Heisenberg (of “Heisenberg uncertainty principle” fame) predicted that powerful magnetic fields could polarize a vacuum, an effect that would shape the path of light traveling through this empty space in bizarre ways. About 20 years later, physicist John Toll elaborated on this idea by describing vacuum birefringence, which describes how polarization affects the absorption of light by a magnetic field in a vacuum.
Birefringence can be observed in more familiar materials, such as crystals, resulting in a light splitting its waveform and producing a double image. This effect can also be observed in extreme environments in space, such as the region surrounding neutron stars, which are collapsed dead stars with extremely strong magnetic fields that can expose the polarization of light.
The STAR collaboration has now captured vacuum birefringence on Earth for the first time, which is a major experimental validation of a bedrock quantum mechanical principle.
“The reason that this is so interesting is because a photon has no charge, so it shouldn’t, in the classical sense, be affected by a magnetic field,” Brandenburg explained. “That’s why this is a clear proof of these very fundamental aspects of quantum mechanics. A photon can constantly fluctuate into this electron-positron pair that does interact with the magnetic field, and that’s exactly what we measured.”
“The real discovery here is that you can do this in the vacuum of space with a strong magnetic field, and the reason that’s so important is that it’s the first time ever that you can measure the wavefunction of the photon directly,” he added.
The dual demonstration of the Breit-Wheeler process and vacuum birefringence is what distinguishes the STAR breakthrough from previous experiments that have converted energy into matter.
During an influential experiment in 1997, the SLAC National Accelerator Laboratory used collisions between lasers and electron beams to create electron-positron pairs from photons. However, that process was not captured with the high-level precision achieved by the STAR team, which revealed never-before-seen details of the conversion that stemmed, in part, from the vacuum birefringence effect.
“This is the first measurement that can say, from an experimental standpoint, that we actually observe—even though it’s only just a blink of an eye—these ultrastrong electric and magnetic fields,” Brandenburg said. “That led to the ability for us, for the first time, to experimentally prove that we have these ultra strong-magnetic fields—the strongest in the universe. There’s nothing else in the universe that produces such strong fields.”
Likewise, a recent experiment at the Large Hadron Collider transformed energy into mass by smashing photons together to produce W bosons, which are short-lived forms of matter that mediate the weak nuclear force: one of the four fundamental forces of nature. However, compared to electrons, W bosons are an extremely exotic form of matter that decays within a tiny fraction of a second. While the achievement represents a unique breakthrough of its own, it is not a demonstration of the Breit-Wheeler process (nor does the LHC claim that it is).
“It is two photons colliding to create something which has a mass, but clearly it’s not what Breit and Wheeler calculated or predicted,” said Xu. “In their time, there was no concept of the weak interactions, or [quantum chromodynamics]. The laser was not even invented.”
In this way, the LHC, SLAC, and Brookhaven experiments serve as complementary proofs that Einstein’s famous formula works both ways, even though it is significantly harder to create mass out of energy than the reverse. The additional demonstration of vacuum birefringence from the STAR collaboration has added a new layer of innovation and insight that can shed light on exotic processes that range in scale from the tiny quantum interiors of atoms to enormous cosmic expanses.
For instance, the new measurements can help astrophysicists and cosmologists model the creation of electron-positive pairs from light around the most energetic objects and events in the universe, such as supernovae or the explosive environments near some black holes. The STAR collaboration also plans to follow up on this experiment by attempting to take the first 2D pictures of the nucleus of an atom, exposing unprecedented details about these fundamental structures of matter.
Beyond the scientific implications of the new experiment, the discovery also illustrates how federally funded research can bring people together to unravel some of the biggest mysteries in physics. After all, the STAR collaboration includes more than 700 scientists from 14 nations, each with their own unique path toward their current role as part of the detector team.
Growing up in China during the 1970s and 80s, Xu recalled that physics, math, and chemistry were treated as “golden subjects” by his peers and teachers, which sparked his early interest in science. But it was ultimately Xu’s PhD advisor at Yale University, Jack Sandweiss, who motivated him to become a leading researcher in his field.
“He was very passionate about science and was an interesting, inspiring character,” Xu said of Sandweiss, who died last year at the age of 90. “He was part of the original committee to actually approve the RHIC project” which was filled with “inspiring characters involved in the heavy ion program at Brookhaven.”
“When I graduated, I joined the RHIC program,” he added, “so I have a long connection to RHIC even before I started there.”
Brandenburg, who was raised on Florida’s Space Coast, was also shaped by a childhood immersed in science-centric culture. His father worked on the Apollo Moon missions and Space Shuttle flights, so it’s no wonder he dreamt of following his footsteps into the frontiers of science while watching rocket launches from his backyard.
“My dad is a naturally curious person and he was always talking to me about what they were working on,” Brandenburg said. “I’m not sure I can give you a direct route to how I got into high-energy nuclear physics, but I was really fascinated by the colliders, the huge amount of data that’s produced, and the fact that it takes really advanced computing techniques just to analyze all of it.”
Like Xu and Brandenburg, Ruan said she owed her journey to Brookhaven in part to an exceptional role model: in this case, a fourth grade math teacher. Her teacher’s talent and love for math instilled such an intense curiosity in Ruan that she began devouring high-school-level textbooks while she was still at elementary school.
Ruan’s career has since evolved alongside the STAR detector; she spent her PhD at the University of Science and Technology of China working on the machine, and was reunited with it at Brookhaven in 2007. Now, she and her colleagues are guiding a new generation of scientists to push the limits of what can be achieved with the detector.
“The STAR experiment is now about 20 years old, but it’s still a discovery machine,” Ruan said. “That’s because we have all these outstanding scientists and students who work really tirelessly to make these things happen. That’s the value of the STAR collaboration.”
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