Physics

Caught in a fatal inward spiral, a neutron star met its end when a black hole swallowed it whole. Gravitational ripples from that collision spread outward through the cosmos, eventually reaching Earth. The detection of those waves marks the first reported sighting of a black hole engulfing the dense remnant of dead star. And in a surprise twist, scientists spotted a second such merger just days after the first.

Until now, all identified sources of gravitational waves were twos of a kind: either two black holes or two neutron stars, spiraling around one another before colliding and coalescing (SN: 1/21/21). The violent cosmic collisions create waves that stretch and squeeze the fabric of spacetime, undulations that can be sussed out by sensitive detectors.

The mismatched pairing of a black hole and neutron star was the final type of merger that scientists expected to find with current gravitational wave observatories. By pure coincidence, researchers spotted two of these events within 10 days of one another, the LIGO, Virgo and KAGRA collaborations report in the July 1 Astrophysical Journal Letters.

Not only have unions between black holes and neutron stars not been seen before via gravitational waves, the smashups have also never been spotted at all by any other means.

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“This is an absolute first look,” says theoretical physicist Susan Scott of the Australian National University in Canberra, a member of the LIGO collaboration.

The result adds another tick mark to the tally of new discoveries made with gravitational waves. “That’s worth celebration,” says astrophysicist Cole Miller of the University of Maryland in College Park, who was not involved with the research. Since the first gravitational waves were detected in 2015, the observatories keep revealing new secrets. “It’s fantastic new things; it’s not just the same old, same old,” he says.

Signs of the black hole-neutron star collisions registered in the LIGO and Virgo gravitational wave observatories in 2020, on January 5 and January 15. The first merger consisted of a black hole about 8.9 times the mass of the sun and a neutron star about 1.9 times the sun’s mass. The second merger had a 5.7 solar mass black hole and a 1.5 solar mass neutron star. Both collisions occurred more than 900 million light-years from Earth, the scientists estimate.

To form detectable gravitational waves, the objects that coalesce must be extremely dense, with identities that can be pinned down by their masses. Anything with a mass above five solar masses could only be a black hole, scientists think. Anything less than about three solar masses must be a neutron star.

One earlier gravitational wave detection involved a black hole merging with an object that couldn’t be identified, as its mass seemed to fall in between the cutoffs that separate black holes and neutron stars (SN: 6/23/20). Another previous merger may have resulted from a black hole melding with a neutron star, but the signal from that event wasn’t strong enough for scientists to be certain that the detection was the real deal. The two new detections clinch the case for black hole and neutron star meetups.

One of the new events is more convincing than the other. The Jan. 5 merger was seen in just one of LIGO’s two gravitational wave detectors, and the signal has a relatively high probability of being a false alarm, Miller says. “If this were the only event, then you would not be as confident.” The Jan. 15 event, however, “seems pretty solid,” he says.

Epic rendezvous between neutron stars and black holes happen regularly throughout the cosmos, the detections suggest. Based on the pace of detections, the researchers estimate that these events take place about once a month within 1 billion light-years of Earth.

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In a newly reported class of cosmic smashup, a neutron star (apparent in orange in this computer simulation, after the video zooms in) and black hole (dark gray) spiral inward, producing gravitational waves (blue) in a dance that ends when the black hole swallows the neutron star.

Scientists don’t yet know how neutron stars and black holes come to meet up. They might form together, as two stars that orbit one another until both run out of fuel and die, with one collapsing into a black hole and the other forming a neutron star. Or the two objects might have formed separately and met up in a crowded region packed with many neutron stars and black holes.

As a black hole and neutron star spiral inward and merge, scientists expect that the black hole could rip the neutron star to shreds, producing a light show that could be observed with telescopes. But astronomers found no fireworks in the aftermath of the two newly reported encounters, nor any evidence that the black holes deformed the neutron stars.

That could be because in both cases the black hole was significantly larger than the neutron star, suggesting that the black hole gulped down the neutron star whole in a meal worthy of Pac-Man, Scott says.

If scientists could spot a black hole shredding a neutron star in the future, that could help researchers pin down the properties of the ultradense, neutron-rich material that makes up the dead stars (SN: 4/20/21).

In past detections of gravitational waves, the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, based in the United States, has teamed up with Virgo, in Italy. The new observations are the first to include members of a third observatory, KAGRA, in Japan (SN: 1/18/19). But the KAGRA detector itself didn’t contribute to the results, as scientists were still preparing it to detect gravitational waves at the time. LIGO, Virgo and KAGRA are all currently offline while scientists tinker with the detectors, and will resume their communal search for cosmic collisions in 2022. More

Dark matter can be a real drag. The pull of that unidentified, invisible matter in the Milky Way may be slowing down the rotating bar of stars at the galaxy’s heart.

Based on a technique that re-creates the history of the slowdown in a manner akin to analyzing a tree’s rings, the bar’s speed has decreased by at least 24 percent since it formed billions of years ago, researchers report in the August Monthly Notices of the Royal Astronomical Society.

That slowdown is “another indirect but important piece of evidence that dark matter is a thing, not just a conjecture, because this can’t happen without it,” says astrophysicist Martin Weinberg of the University of Massachusetts Amherst, who was not involved with the study.

Many spiral galaxies, including the Milky Way, contain a central bar-shaped region densely packed with stars and surrounded by the galaxy’s pinwheeling arms. The bar also has some groupies: a crew of stars trapped by the bar’s gravitational influence. Those stars orbit a gravitationally stable point located alongside the bar and farther from the galaxy’s center, known as a Lagrange point (SN: 2/26/21). 

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If the bar’s rotation slows, it will grow in length, and the bar’s tagalongs will also move outward. As that happens, that cohort of hangers-on will gather additional stars. According to computer simulations of the process, those additional stars should arrange themselves in layers on the outside of the group, says astrophysicist Ralph Schönrich of University College London. The layers of stars imprint a record of the group’s growth. “It’s actually like a tree that you can cut up in your own galaxy,” he says.

Schönrich and astrophysicist Rimpei Chiba of the University of Oxford studied how the composition of stars in the group changed from its outer edge to its deeper layers. Data from the European Space Agency’s Gaia spacecraft revealed that stars in the outer layers of the bar tended to be less enriched in elements heavier than helium than were stars in the inner layers. That’s evidence for the group of stars moving outward, as a result of the bar slowing, the researchers say. That’s because stars in the center of the galaxy — which would have glommed on to the group in the more distant past — tend to be more enriched in heavier elements than those farther out.

The bar’s slowdown hints that a gravitational force is acting on it, namely, the pull of dark matter in the galaxy. Normal matter alone wouldn’t be enough to reduce the bar’s speed. “If there is no dark matter, the bar will not slow down,” Chiba says.

But the results have drawn some skepticism. “Unfortunately, this is not yet convincing to me,” says astrophysicist Isaac Shlosman of the University of Kentucky in Lexington. For example, he doubts that the tree ring layering would really occur. It is “hard to believe that this is the case in a realistic system” as opposed to in a simplified computer simulation, he says.

Weinberg, on the other hand, says that although the study relies on a variety of assumptions, he suspects it’s correct. “It’s got the right smell.” More

Despite their mysterious nature, black holes are thought to follow certain simple rules. Now, one of the most famous black hole laws, predicted by physicist Stephen Hawking, has been confirmed with gravitational waves.

According to the black hole area theorem, developed by Hawking in the early 1970s, black holes can’t decrease in surface area over time. The area theorem fascinates physicists because it mirrors a well-known physics rule that disorder, or entropy, can’t decrease over time. Instead, entropy consistently increases (SN: 7/10/15).

That’s “an exciting hint that black hole areas are something fundamental and important,” says astrophysicist Will Farr of Stony Brook University in New York and the Flatiron Institute in New York City.

The surface area of a lone black hole won’t change — after all, nothing can escape from within. However, if you throw something into a black hole, it will gain more mass, increasing its surface area. But the incoming object could also make the black hole spin, which decreases the surface area. The area law says that the increase in surface area due to additional mass will always outweigh the decrease in surface area due to added spin.

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To test this area rule, MIT astrophysicist Maximiliano Isi, Farr and others used ripples in spacetime stirred up by two black holes that spiraled inward and merged into one bigger black hole. A black hole’s surface area is defined by its event horizon — the boundary from within which it’s impossible to escape. According to the area theorem, the area of the newly formed black hole’s event horizon should be at least as big as the areas of the event horizons of the two original black holes combined.

The team analyzed data from the first gravitational waves ever spotted, which were detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in 2015 (SN: 2/11/16). The researchers split the gravitational wave data into two time segments, before and after the merger, and calculated the surface areas of the black holes in each period. The surface area of the newly formed black hole was greater than that of the two initial black holes combined, upholding the area law with a 95 percent confidence level, the team reports in a paper to appear in Physical Review Letters.

“It’s the first time that we can put a number on this,” Isi says.

The area theorem is a result of the general theory of relativity, which describes the physics of black holes and gravitational waves. Previous analyses of gravitational waves have agreed with predictions of general relativity, and thus already hinted that the area law can’t be wildly off. But the new study “is a more explicit confirmation,” of the area law, says physicist Cecilia Chirenti of the University of Maryland in College Park, who was not involved with the research.

So far, general relativity describes black holes well. But scientists don’t fully understand what happens where general relativity — which typically applies to large objects like black holes — meets quantum mechanics, which describes small stuff like atoms and subatomic particles. In that quantum realm, strange things can happen.

For example, black holes can release a faint mist of particles called Hawking radiation, another idea developed by Hawking in the 1970s. That effect could allow black holes to shrink, violating the area law, but only over extremely long periods of time, so it wouldn’t have affected the relatively quick merger of black holes that LIGO saw.

Physicists are looking for an improved theory that will combine the two disciplines into one new, improved theory of quantum gravity. Any failure of black holes to abide by the rules of general relativity could point physicists in the right direction to find that new theory.

So physicists tend to be grumpy about the enduring success of general relativity, Farr says. “We’re like, ‘aw, it was right again.’” More

Tiny crystals of uranium could set off massive explosions within a dead star, physicists propose, making for a cosmic version of a thermonuclear bomb.

Expired stars called white dwarfs slowly cool as they age. In the process, heavy elements such as uranium begin to crystalize, forming “snowflakes” in the stars’ cores. If enough uranium clumps together — about the mass of a grain of sand — it could initiate a chain of nuclear fission reactions, or the splitting of atomic nuclei.

Those reactions could raise temperatures within the star, setting off nuclear fusion — the merging of atomic nuclei — and generating an enormous explosion that destroys the star, two physicists calculate in a paper published March 29 in Physical Review Letters. The effect is akin to a hydrogen bomb, a powerful thermonuclear weapon in which fission reactions trigger fusion, says Matt Caplan of Illinois State University in Normal. The scenario is still hypothetical, Caplan admits — more research is needed to determine if uranium snowflakes could really spur a stellar detonation.

White dwarfs are already known to be explosion-prone: They’re the source of blasts called type 1a supernovas. Typically, these explosions happen when a white dwarf pulls matter off a companion star (SN: 3/23/16). The researchers’ uranium snowflake proposal is an entirely new mechanism that might explain a small fraction of type 1a supernovas, without the need for another star. More

Black holes sit on the cusp of the unknowable. Anything that crosses a black hole’s threshold is lost forever, trapped by an extreme gravitational pull. That enigmatic quality makes the behemoths an enticing subject, scientists explain in the new documentary Black Holes: The Edge of All We Know.
The film follows two teams working over the last several years to unveil the mystery-shrouded monstrosities. Scientists with the Event Horizon Telescope attempt to make the first image of a black hole’s shadow using a global network of telescopes. Meanwhile, a small group of theoretical physicists, anchored by Stephen Hawking — who was still alive when filming began — aim to solve a theoretical quandary called the black hole information paradox (SN: 5/16/14).
When big discoveries happen, the camera is right there — allowing us to thrill in the moment when Event Horizon Telescope scientists first lay eyes on a black hole’s visage. And we triumph as the team unveils the result in 2019, a now-familiar orange, ring-shaped image depicting the supermassive black hole in the center of galaxy M87 (SN: 4/10/19). Likewise, scenes where Hawking questions his collaborators as they explain chalkboards full of equations prove mesmerizing. Viewers witness brilliant minds playing off one another, struggling with mistakes and dead ends in their calculations, punctuated by occasional, groundbreaking progress.
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Watch the trailer for Black Holes: The Edge of All We Know.
Stunning cinematography and skillful editing lend energy to Black Holes, directed by Harvard physicist and historian Peter Galison and available on Apple TV, Amazon Prime Video and other on-demand platforms on March 2. When the Event Horizon Telescope team begins taking data, we’re treated to a crisp montage of telescopes around the world, all swiveling to catch a glimpse of the black hole. Later, bright sunbeams slice across an office floor while scientists muddle through calculations regarding the darkest objects of the cosmos. Such scenes are punctuated by delightfully strange black-and-white animations that evoke a pensiveness appropriate for contemplating cosmic oddities.

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There’s drama too: Event Horizon Telescope’s scientists wrestle with misbehaving equipment and curse uncooperative weather. The theoretical physicists grapple with the immense complexity of the cosmos on slow, distracted walks in the forest.
Other research topics garner brief mentions, such as the study of gravitational waves from colliding black holes (SN: 1/21/21) and black hole analogs made using water vortices (SN 6/12/17). The film treats these varied efforts to study black holes independently; some viewers may wish the dots were better connected.
The film Black Holes: The Edge of All We Know features this water vortex, lit by green light. Scientists used such vortices along with other techniques to re-create the physics of black holes.Giant Pictures
Still, Black Holes successfully leads viewers through a fascinating, understandable trek across the varied frontiers of black hole knowledge. As Harvard physicist Shep Doeleman of the Event Horizon Telescope team describes it in the film, “we are chasing down something that struggles with all of its might to be unseen.” Pulling us to the very rim of this fathomless abyss, Black Holes invites us to stand with scientists peering over the edge. More

The Milky Way glows with a gamma ray haze, with energies vastly exceeding anything physicists can produce on Earth, according to a new paper. Gamma rays detected in the study, to be published in Physical Review Letters, came from throughout the galaxy’s disk, and reached nearly a quadrillion (1015) electron volts, known as a petaelectron volt or PeV.
These diffuse gamma rays hint at the existence of powerful cosmic particle accelerators within the Milky Way. Physicists believe such accelerators are the source of mysterious, highly energetic cosmic rays, charged particles that careen through the galaxy, sometimes crash-landing on Earth. When cosmic rays — which mainly consist of protons — slam into interstellar debris, they can produce gamma rays, a form of high-energy light.  
Certain galactic environments could rev up cosmic ray particles to more than a PeV, scientists suspect. In comparison, the Large Hadron Collider, the premier particle accelerator crafted by humans, accelerates protons to 6.5 trillion electron volts. But physicists haven’t definitively identified any natural cosmic accelerators capable of reaching a PeV, known as PeVatrons. One possibility is that supernova remnants, the remains of exploded stars, host shock waves that can accelerate cosmic rays to such energies (SN: 11/12/20).
If PeVatrons exist, the cosmic rays they emit would permeate the galaxy, producing a diffuse glow of gamma rays of extreme energies. That’s just what researchers with the Tibet AS-gamma experiment have found. “It’s nice to see things fitting together,” says physicist David Hanna of McGill University in Montreal, who was not involved with the study.

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After cosmic rays are spewed out from their birthplaces, scientists believe, they roam the galaxy, twisted about by its magnetic fields. “We live in a bubble of cosmic rays,” says astrophysicist Paolo Lipari of the National Institute for Nuclear Physics in Rome, who was not involved with the research. Because they are not deflected by magnetic fields, gamma rays point back to their sources, revealing the whereabouts of the itinerant cosmic rays. The new study “gives you information about how these particles fill the galaxy.”
Lower-energy gamma rays also permeate the galaxy. But it takes higher-energy gamma rays to understand the highest-energy cosmic rays. “In general, the higher the energy of the gamma rays, the higher the energy of the cosmic rays,” says astrophysicist Elena Orlando of Stanford University, who was not involved with the research. “Hence, the detection … tells us that PeV cosmic rays originate and propagate in the galactic disk.”
Scientists with the Tibet AS-gamma experiment in China observed gamma rays with energies between about 100 trillion and a quadrillion electron volts coming from the region of the sky covered by the disk of the Milky Way. A search for possible sources of the 38 highest-energy gamma rays, above 398 trillion electron volts, came up empty, supporting the idea that the gamma rays came from cosmic rays that had wandered about the galaxy. The highest-energy gamma ray carried about 957 trillion electron volts.
Tibet AS-gamma researchers declined to comment on the study.
Scientists have previously seen extremely energetic gamma rays from individual sources within the Milky Way, such as the Crab Nebula, a supernova remnant (SN: 6/24/19). Those gamma rays are probably produced in a different manner, by electrons radiating gamma rays while circulating within the cosmic accelerator. More

When one of Hye-Sook Park’s experiments goes well, everyone nearby knows. “We can hear Hye-Sook screaming,” she’s heard colleagues say.
It’s no surprise that she can’t contain her excitement. She’s getting a closeup look at the physics of exploding stars, or supernovas, a phenomenon so immense that its power is difficult to put into words.
Rather than studying these explosions from a distance through telescopes, Park, a physicist at Lawrence Livermore National Laboratory in California, creates something akin to these paroxysmal blasts using the world’s highest-energy lasers.
About 10 years ago, Park and colleagues embarked on a quest to understand a fascinating and poorly understood feature of supernovas: Shock waves that form in the wake of the explosions can boost particles, such as protons and electrons, to extreme energies.

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“Supernova shocks are considered to be some of the most powerful particle accelerators in the universe,” says plasma physicist Frederico Fiuza of SLAC National Accelerator Laboratory in Menlo Park, Calif., one of Park’s collaborators.
Some of those particles eventually slam into Earth, after a fast-paced marathon across cosmic distances. Scientists have long puzzled over how such waves give energetic particles their massive speed boosts. Now, Park and colleagues have finally created a supernova-style shock wave in the lab and watched it send particles hurtling, revealing possible new hints about how that happens in the cosmos.
Bringing supernova physics down to Earth could help resolve other mysteries of the universe, such as the origins of cosmic magnetic fields. And there’s a more existential reason physicists are fascinated by supernovas. These blasts provide some of the basic building blocks necessary for our existence. “The iron in our blood comes from supernovae,” says plasma physicist Carolyn Kuranz of the University of Michigan in Ann Arbor, who also studies supernovas in the laboratory. “We’re literally created from stars.”
Lucky star
As a graduate student in the 1980s, Park worked on an experiment 600 meters underground in a working salt mine beneath Lake Erie in Ohio. Called IMB for Irvine-Michigan-Brookhaven, the experiment wasn’t designed to study supernovas. But the researchers had a stroke of luck. A star exploded in a satellite galaxy of the Milky Way, and IMB captured particles catapulted from that eruption. Those messengers from the cosmic explosion, lightweight subatomic particles called neutrinos, revealed a wealth of new information about supernovas.
But supernovas in our cosmic vicinity are rare. So decades later, Park isn’t waiting around for a second lucky event.
Physicist Hye-Sook Park, shown as a graduate student in the 1980s (left) and in a recent photo (right), uses powerful lasers to study astrophysics.from left: John Van der Velde; Lanie L. Rivera/Lawrence Livermore National Laboratory
Instead, her team and others are using extremely powerful lasers to re-create the physics seen in the aftermath of supernova blasts. The lasers vaporize a small target, which can be made of various materials, such as plastic. The blow produces an explosion of fast-moving plasma, a mixture of charged particles, that mimics the behavior of plasma erupting from supernovas.
The stellar explosions are triggered when a massive star exhausts its fuel and its core collapses and rebounds. Outer layers of the star blast outward in an explosion that can unleash more energy than will be released by the sun over its entire 10-billion-year lifetime. The outflow has an unfathomable 100 quintillion yottajoules of kinetic energy (SN: 2/8/17, p. 24).
Supernovas can also occur when a dead star called a white dwarf is reignited, for example after slurping up gas from a companion star, causing a burst of nuclear reactions that spiral out of control (SN: 4/30/16, p. 20).
Supernova remnants like W49B (shown in X-ray, radio and infrared light) accelerate electrons and protons to high energies in shock waves.NASA, CXC, MIT L. Lopez et al (X-ray), Palomar (Infrared), VLA/NRAO/NSF (Radio)
In both cases, things really get cooking when the explosion sends a blast of plasma careening out of the star and into its environs, the interstellar medium — essentially, another ocean of plasma particles. Over time, a turbulent, expanding structure called a supernova remnant forms, begetting a beautiful light show, tens of light-years across, that can persist in the sky for many thousands of years after the initial explosion. It’s that roiling remnant that Park and colleagues are exploring.
Studying supernova physics in the lab isn’t quite the same thing as the real deal, for obvious reasons. “We cannot really create a supernova in the laboratory, otherwise we would be all exploded,” Park says.
In lieu of self-annihilation, Park and others focus on versions of supernovas that are scaled down, both in size and in time. And rather than reproducing the entirety of a supernova all at once, physicists try in each experiment to isolate interesting components of the physics taking place. Out of the immense complexity of a supernova, “we are studying just a tiny bit of that, really,” Park says.
For explosions in space, scientists are at the mercy of nature. But in the laboratory, “you can change parameters and see how shocks react,” says astrophysicist Anatoly Spitkovsky of Princeton University, who collaborates with Park.
The laboratory explosions happen in an instant and are tiny, just centimeters across. For example, in Kuranz’s experiments, the equivalent of 15 minutes in the life of a real supernova can take just 10 billionths of a second. And a section of a stellar explosion larger than the diameter of Earth can be shrunk down to 100 micrometers. “The processes that occur in both of those are very similar,” Kuranz says. “It blows my mind.”
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Powerful, mysterious stellar explosions are difficult to understand from afar, so researchers have figured out how to re-create supernovas’ extreme physics in the lab and study how outbursts seed the cosmos with elements and energetic particles.
Laser focus
To replicate the physics of a supernova, laboratory explosions must create an extreme environment. For that, you need a really big laser, which can be found in only a few places in the world, such as NIF, the National Ignition Facility at Lawrence Livermore, and the OMEGA Laser Facility at the University of Rochester in New York.
At both places, one laser is split into many beams. The biggest laser in the world, at NIF, has 192 beams. Each of those beams is amplified to increase its energy exponentially. Then, some or all of those beams are trained on a small, carefully designed target. NIF’s laser can deliver more than 500 trillion watts of power for a brief instant, momentarily outstripping the total power usage in the United States by a factor of a thousand.
A single experiment at NIF or OMEGA, called a shot, is one blast from the laser. And each shot is a big production. Opportunities to use such advanced facilities are scarce, and researchers want to have all the details ironed out to be confident the experiment will be a success.
When a shot is about to happen, there’s a space-launch vibe. Operators monitor the facility from a control room filled with screens. When the time of the laser blast nears, a voice begins counting down: “Ten, nine, eight …”
“When they count down for your shot, your heart is pounding,” says plasma physicist Jena Meinecke of the University of Oxford, who has worked on experiments at NIF and other laser facilities.
At the moment of the shot, “you kind of want the Earth to shake,” Kuranz says. But instead, you might just hear a snap — the sound of the discharge from capacitors that store up huge amounts of energy for each shot.
Then comes a mad dash to review the results and determine if the experiment has been successful. “It’s a lot of adrenaline,” Kuranz says.
At the National Ignition Facility’s target chamber (shown during maintenance), 192 laser beams converge. The blasts produce plumes of plasma that can mimic some aspects of supernova remnants.Lawrence Livermore National Laboratory
Lasers aren’t the only way to investigate supernova physics in the lab. Some researchers use intense bursts of electricity, called pulsed power. Others use small amounts of explosives to set off blasts. The various techniques can be used to understand different stages in supernovas’ lives.
A real shocker
Park brims with cosmic levels of enthusiasm, ready to erupt in response to a new nugget of data or a new success in her experiments. Re-creating some of the physics of a supernova in the lab really is as remarkable as it sounds, she says. “Other­wise I wouldn’t be working on it.” Along with Spitkovsky and Fiuza, Park is among more than a dozen scientists involved in the Astrophysical Collisionless Shock Experiments with Lasers collaboration, or ACSEL, the quest Park embarked upon a decade ago. Their focus is shock waves.
The result of a violent input of energy, shock waves are marked by an abrupt increase in temperature, density and pressure. On Earth, shock waves cause the sonic boom of a supersonic jet, the clap of thunder in a storm and the damaging pressure wave that can shatter windows in the aftermath of a massive explosion. These shock waves form as air molecules slam into each other, piling up molecules into a high-density, high-pressure and high-temperature wave.
In cosmic environments, shock waves occur not in air, but in plasma, a mixture of protons, electrons and ions, electrically charged atoms. There, particles may be diffuse enough that they don’t directly collide as they do in air. In such a plasma, the pileup of particles happens indirectly, the result of electromagnetic forces pushing and pulling the particles. “If a particle changes trajectory, it’s because it feels a magnetic field or an electric field,” says Gianluca Gregori, a physicist at the University of Oxford who is part of ACSEL.
But exactly how those fields form and grow, and how such a shock wave results, has been hard to decipher. Researchers have no way to see the process in real supernovas; the details are too small to observe with telescopes.
These shock waves, which are known as collisionless shock waves, fascinate physicists. “Particles in these shocks can reach amazing energies,” Spitkovsky says. In supernova remnants, particles can gain up to 1,000 trillion electron volts, vastly outstripping the several trillion electron volts reached in the biggest human-made particle accelerator, the Large Hadron Collider near Geneva. But how particles might surf supernova shock waves to attain their astounding energies has remained mysterious.

Magnetic field origins
To understand how supernova shock waves boost particles, you have to understand how shock waves form in supernova remnants. To get there, you have to understand how strong magnetic fields arise. Without them, the shock wave can’t form.
Electric and magnetic fields are closely intertwined. When electrically charged particles move, they form tiny electric currents, which generate small magnetic fields. And magnetic fields themselves send charged particles corkscrewing, curving their trajectories. Moving magnetic fields also create electric fields.
The result is a complex feedback process of jostling particles and fields, eventually producing a shock wave. “This is why it’s so fascinating. It’s a self-modulating, self-controlling, self-reproducing structure,” Spitkovsky says. “It’s like it’s almost alive.”
All this complexity can develop only after a magnetic field forms. But the haphazard motions of individual particles generate only small, transient magnetic fields. To create a significant field, some process within a supernova remnant must reinforce and amplify the magnetic fields. A theoretical process called the Weibel instability, first thought up in 1959, has long been expected to do just that.
In a supernova, the plasma streaming outward in the explosion meets the plasma of the interstellar medium. According to the theory behind the Weibel instability, the two sets of plasma break into filaments as they stream by one another, like two hands with fingers interlaced. Those filaments act like current-­carrying wires. And where there’s current, there’s a magnetic field. The filaments’ magnetic fields strengthen the currents, further enhancing the magnetic fields. Scientists suspected that the electromagnetic fields could then become strong enough to reroute and slow down particles, causing them to pile up into a shock wave.
In 2015 in Nature Physics, the ACSEL team reported a glimpse of the Weibel instability in an experiment at OMEGA. The researchers spotted magnetic fields, but didn’t directly detect the filaments of current. Finally, this year, in the May 29 Physical Review Letters, the team reported that a new experiment had produced the first direct measurements of the currents that form as a result of the Weibel instability, confirming scientists’ ideas about how strong magnetic fields could form in supernova remnants.
For that new experiment, also at OMEGA, ACSEL researchers blasted seven lasers each at two targets facing each other. That resulted in two streams of plasma flowing toward each other at up to 1,500 kilometers per second — a speed fast enough to circle the Earth twice in less than a minute. When the two streams met, they separated into filaments of current, just as expected, producing magnetic fields of 30 tesla, about 20 times the strength of the magnetic fields in many MRI machines.
“What we found was basically this textbook picture that has been out there for 60 years, and now we finally were able to see it experimentally,” Fiuza says.
Surfing a shock wave
Once the researchers had seen magnetic fields, the next step was to create a shock wave and to observe it accelerating particles. But, Park says, “no matter how much we tried on OMEGA, we couldn’t create the shock.”
They needed the National Ignition Facility and its bigger laser.
There, the researchers hit two disk-shaped targets with 84 laser beams each, or nearly half a million joules of energy, about the same as the kinetic energy of a car careening down a highway at 60 miles per hour.
Two streams of plasma surged toward each other. The density and temperature of the plasma rose where the two collided, the researchers reported in the September Nature Physics. “No doubt about it,” Park says. The group had seen a shock wave, specifically the collisionless type found in supernovas. In fact there were two shock waves, each moving away from the other.

Learning the results sparked a moment of joyous celebration, Park says: high fives to everyone.
“This is some of the first experimental evidence of the formation of these collisionless shocks,” says plasma physicist Francisco Suzuki-Vidal of Imperial College London, who was not involved in the study. “This is something that has been really hard to reproduce in the laboratory.”
The team also discovered that electrons had been accelerated by the shock waves, reaching energies more than 100 times as high as those of particles in the ambient plasma. For the first time, scientists had watched particles surfing shock waves like the ones found in supernova remnants.
But the group still didn’t understand how that was happening.
In a supernova remnant and in the experiment, a small number of particles are accelerated when they cross over the shock wave, going back and forth repeatedly to build up energy. But to cross the shock wave, the electrons need some energy to start with. It’s like a big-wave surfer attempting to catch a massive swell, Fiuza says. There’s no way to catch such a big wave by simply paddling. But with the energy provided by a Jet Ski towing surfers into place, they can take advantage of the wave’s energy and ride the swell.
A computer simulation of a shock wave (structure shown in blue) illustrates how electrons gain energy (red tracks are higher energy, yellow and green are lower).F. Fiuza/SLAC National Accelerator Laboratory
“What we are trying to understand is: What is our Jet Ski? What happens in this environment that allows these tiny electrons to become energetic enough that they can then ride this wave and be accelerated in the process?” Fiuza says.
The researchers performed computer simulations that suggested the shock wave has a transition region in which magnetic fields become turbulent and messy. That hints that the turbulent field is the Jet Ski: Some of the particles scatter in it, giving them enough energy to cross the shock wave.
Wake-up call
Enormous laser facilities such as NIF and OMEGA are typically built to study nuclear fusion — the same source of energy that powers the sun. Using lasers to compress and heat a target can cause nuclei to fuse with one another, releasing energy in the process. The hope is that such research could lead to fusion power plants, which could provide energy without emitting greenhouse gases or dangerous nuclear waste (SN: 4/20/13, p. 26). But so far, scientists have yet to get more energy out of the fusion than they put in — a necessity for practical power generation.
So these laser facilities dedicate many of their experiments to chasing fusion power. But sometimes, researchers like Park get the chance to study questions based not on solving the world’s energy crisis, but on curiosity — wondering what happens when a star explodes, for example. Still, in a roundabout way, understanding supernovas could help make fusion power a reality as well, as that celestial plasma exhibits some of the same behaviors as the plasma in fusion reactors.
At NIF, Park has also worked on fusion experiments. She has studied a wide variety of topics since her grad school days, from working on the U.S. “Star Wars” missile defense program, to designing a camera for a satellite sent to the moon, to looking for the sources of high-energy cosmic light flares called gamma-ray bursts. Although she is passionate about each topic, “out of all those projects,” she says, “this particular collisionless shock project happens to be my love.”
Early in her career, back on that experiment in the salt mine, Park got a first taste of the thrill of discovery. Even before IMB captured neutrinos from a supernova, a different unexpected neutrino popped up in the detector. The particle had passed through the entire Earth to reach the experiment from the bottom. Park found the neutrino while analyzing data at 4 a.m., and woke up all her collaborators to tell them about it. It was the first time anyone working on the experiment had seen a particle coming up from below. “I still clearly remember the time when I was seeing something nobody’s seen,” Park recalls.
Now, she says, she still gets the same feeling. Screams of joy erupt when she sees something new that describes the physics of unimaginably vast explosions.

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