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In another hiccup for AstraZeneca’s COVID-19 vaccine, data suggest it is in fact linked to blood clots that have formed in the brains of some vaccinated people, the European Medicines Agency announced April 7.

The blood clots are incredibly rare, EMA experts say. But because COVID-19 itself is deadly and can put people in the hospital, the benefits of the vaccine still outweigh the risks, they say. “We need to use the vaccines that we have to protect us from devastating effects” of COVID-19, Emer Cooke, executive director of the EMA, said in a news conference on April 7.

The EMA had previously concluded that the vaccine, developed by AstraZeneca and the University of Oxford, was not linked to blood clots overall (SN: 3/18/21). But experts were uncertain about 18 case reports of blood clots in the sinuses that drain blood from the brain, a rare condition called cerebral venous sinus thrombosis or CVST.

Enough data has now accrued to implicate the jab’s involvement in those rare blood clots, meaning CVST and similar conditions should be listed as a possible rare side effect of vaccination, the EMA’s Pharmacovigilance Risk Assessment Committee concluded after reviewing numerous cases reported in the United Kingdom and the European Union. As of March 22, countries had reported 62 cases of CVST out of around 25 million people who got the AstraZeneca vaccine. There were also 24 reported cases of clots in veins that drain blood from the digestive system, called splanchnic vein thrombosis or SVT. Eighteen of the people with CVST or SVT died. 

It’s still unknown how the vaccine could cause blood clots. One potential explanation is that some people develop an immune response that attacks platelets, making them clump together, Sabine Straus, chair of the EMA committee, said in the news conference. That would make the condition similar to low platelet levels and blood clots sparked by an immune response to the anticoagulant drug heparin. One preliminary study of four people who died from blood clots after vaccination had platelet-binding antibodies in their blood, researchers reported March 29 at Research Square, a preprint server. The study has not yet been reviewed by other scientists. 

If that is the way those clots form, there are ways to treat them, Beverley Hunt, a hematologist at King’s College London, said in a call with reporters. A therapy called IVIG, which contains portions of antibodies that interact with platelets, given with non-heparin anticoagulants might help break up clots.  

Why the vaccine might spark clotting is also unclear. It could be related to the technology AstraZeneca used in the vaccine or something about the coronavirus protein the shot uses, Adam Finn, a pediatrician and vaccine expert at the University of Bristol in England said in a call with reporters. “We just don’t know at this point.”

A few people given other COVID-19 vaccines have also developed rare clots; for example, three people in the European Union and United Kingdom developed them after getting Johnson & Johnson’s jab. But those shots have not been not linked with clotting issues, Peter Arlett, head of data analytics and methodology at the EMA, said at the news conference.

Determining how often people actually do develop blood clots after their shots is difficult, since the committee is relying on vaccinated people to report their symptoms rather than routine monitoring done by experts, Straus said. So far, the reported rate differs among EU countries. In Germany, for instance, the reported rate is around 1 case of blood clots per 100,000 people while it is around 1 case per 600,000 people in the United Kingdom, Straus said. Normally, there are 1 to 2 cases per 100,000 people.

While the numbers are hard to parse, the reported rate of blood clots post-vaccination is still lower than that of other common medications that carry risks of the same medical issue. For women taking oral contraceptives, for instance, the rate of blood clots is 4 cases per 10,000, Arlett said.

Although most of the rare blood clots have occurred in women younger than 60 years old, the factors that could put people at risk of forming clots remain unclear, Straus said. That’s in part because some of the records do not provide all the necessary information about the people who developed clots, such as age or gender. More women than men have received the shot overall in the European Union and United Kingdom, which could also skew the results.

Even without clear information on risk factors, many countries have placed restrictions on the use of AstraZeneca’s vaccine. Some countries, including Denmark and Norway, have stopped using the shot. Others, like Canada and Germany, are using it only to vaccinate older adults. On April 6, the United Kingdom paused an ongoing trial of AstraZeneca’s vaccine in children 6 to 17 years old. U.K. officials now recommend that health experts offer other COVID-19 vaccines to people younger than 30 years old.

In the United States, AstraZeneca plans to apply soon for emergency use authorization from the U.S. Food and Drug Administration.

As light shines steadily on a silver slip of a fish, minuscule dots on the fish start flashing: blue, yellow, blue, yellow.

The bodies “do not glow like luminous fish,” Masakazu Iwasaka, an interdisciplinary engineer at Hiroshima University in Japan has discovered. Instead of making their own light, it turns out that remarkable little photonic crystals in fish spots reflect certain wavelengths of light, alternating between blues and more greenish-yellows, he reports April 7 in Royal Society Open Science.

Lots of biological materials have evolved tricks manipulating light. The iconic morpho blue butterfly doesn’t have a flake of blue pigment. It creates its dream-perfect sky blue with stacks of microscopic light-manipulating plates. So do blue-leaved begonias (SN: 11/28/16).

Those fish reflectors are doing something similar in wide-banded hardyhead silversides (Atherinomorus lacunosus). “I found the flashing of a small spot by chance” while screening the dots no bigger than 7 to 10 micrometers across on fish backs, he says. Inside the reflective flash spots lie little platelets of the compound guanine that have grown in such a way that they can reflect colorful light depending on the angle.

Guanine may sound familiar. It’s one of the four major coding units that pair up in storing DNA’s genetic information. What gives the fish guanine platelets particular abilities though remains a puzzle. Iwasaka suspects that inside a spot, platelets move in ways that change their apparent color and dazzle power. The blue-yellow light pulses only in living silversides. Dead fish just reflect white-white.

Iwasaka hopes to create human-made counterparts to the fish reflectors. He proposes mimicking fishy structures for sensors far, far smaller than the period on a magazine page. Versions of little sparkling fish lights could fit into the world of micro-electromechanical systems (MEMS) to monitor conditions inside living tissues, responding to light or flashing themselves. In earlier work, he’s shown how guanine platelets can be manipulated in magnetic fields, suggesting that such sensors could be targeted and herded.

What the fish uses its lightshow for remains a mystery. Flashing spots aren’t unique to wide-banded hardyhead silversides, Iwasaka points out. At least two other papers reported flashing (blue to red) in other tropical fishes, probably for communication. Maybe silverside flashing communicates something too, Iwasaka says.

Or there could be safety benefits. Fish ecologist David Conover of the University of Oregon in Eugene has worked with a silvery Menidia species in the same fish family as the species Iwasaka studies. “For fish that live in bright light and near the surface, as do silversides, the reflectivity probably serves as a type of camouflage or distraction from predators lurking or striking from below,” Conover says.

Whatever drives the evolution of iridophores, those reflective spots where Iwasaka found inspiration, they’re common in the fish world. There could be plenty more places to look for flashes.

Watch a group of lions yawn, and it may seem like nothing more than big, lazy cats acting sleepy, but new research suggests that these yawns may be subtly communicating some important social cues. Yawning is not only contagious among lions, but it appears to help the predators synchronize their movements, researchers report March 16 in Animal Behaviour. 

The discovery was partially made by chance, says Elisabetta Palagi, an ethologist at the University of Pisa in Italy. While studying play behavior in spotted hyenas in South Africa, she and colleagues often had the opportunity to watch lions (Panthera leo) at the same time. And she quickly noticed that lions yawn quite frequently, concentrating these yawns in short time periods.

Yawning is ubiquitous among vertebrates, possibly boosting blood flow to the skull, cooling the brain and aiding alertness, especially when transitioning in and out of rest (SN: 9/8/15). Fish and reptiles will yawn, but more social vertebrates such as birds and mammals appear to have co-opted the behavior for purposes conducive to group living. In many species — like humans, monkeys, and even parakeets (SN: 6/1/15) — yawners can infect onlookers with their “yawn contagion,” leading onlookers to yawn shortly afterwards.

Seeing the lions yawn reminded Palagi of her own work on contagious yawning in primates. Curious if the lions’ prodigious yawning was socially linked, Palagi and her team started recording videos of the big cats, analyzing when they were yawning and any behaviors around those times. 

Over four months in 2019, the researchers closely monitored 19 lions at the Greater Makalali Private Game Reserve, just west of Kruger National Park. The team found that lions that saw another member of the pride yawn were about 139 times as likely to yawn themselves within the next three minutes. 

But the yawn contagion didn’t stop there. Lions that caught a yawn from another lion were 11 times as likely to mirror the movements of the original yawner than those that hadn’t. This “motor synchrony” involved one lion yawning, then another yawning, then the first getting up and walking around or laying back down and the other doing the same thing. 

In lions, contagious yawning might be important for maintaining social cohesion, Palagi says. Yawns that help lions harmonize their group movements could help get the pride all on the same page, crucial behavior for an animal that hunts and rears offspring cooperatively. 

“If yawn contagion has evolved to foster the creation of bonds,” says Palagi, “after a yawn contagion event, the two animals need to do something together [like getting up and walking] to increase their probability of interacting.”

Other researchers have hypothesized that yawning could help coordinate group behavior in some species, notes Andrew Gallup, a biopsychologist at State University of New York Polytechnic Institute in Utica. “But this is the first study that I’m aware of that’s actually attempted to quantify that,” he says.

“The spreading of [yawning] across the group via contagion could serve to enhance overall collective vigilance,” says Gallup. “I think as time continues, we’ll find that the contagious yawning is more common among some of these highly social species.”

Palagi notes that yawning often marks a shift between different physiological or emotional states. So, a yawn could be a good way for an individual in a social species to communicate to group mates that it is experiencing some kind of internal change. 

“Yawning is a widespread behavior, but I think it’s one of the most mysterious,” Palagi says, since it appears to have different functions from species to species. 

A new World Health Organization report investigating the origins of the coronavirus has raised more questions than answers for how — and where — the virus that exploded into a global pandemic emerged.

The report, released March 30, tallies where the evidence currently points: The virus, called SARS-CoV-2, probably jumped to people from bats through another animal; it likely did not come from a lab. But officials can’t yet prove — or rule out — any scenario. And questions about just how much access to potential evidence an international team of experts had on their 28-day trip to Wuhan, China, in January and February has cast a shadow on the findings.

On that trip, 17 experts with the WHO teamed up with 17 Chinese scientists to assess four potential scenarios for the origins of the coronavirus. The two leading scenarios, the team concluded, are transmission of the virus to people either directly from bats or, more likely, via an intermediate animal like a civet or raccoon dog.

A third possibility is the virus got to people through contaminated frozen food products, which the team considers less likely but says merits further investigation. The last scenario — that the virus began spreading among people following a lab accident — is “extremely unlikely,” the researchers wrote.

In a joint statement on March 30, 14 countries including the United States expressed concern that the WHO team was delayed and didn’t have access to original data and samples from people and animals. That reaction comes amid reports that the Chinese government had a hand in the mission, controlling the sites the team accessed during the visit and the report’s wording. “Scientific missions like these should be able to do their work under conditions that produce independent and objective recommendations and findings,” the countries wrote in the statement.

Some explanations may be more probable than others, but for now all possibilities remain on the table, says WHO Director-General Tedros Adhanom Ghebreyesus. The report raised questions that require further study, such as additional work to pinpoint the earliest cases of COVID-19, he noted in a March 30 meeting with WHO member states. He also said that when it came to the hypothesis that the virus came from a lab accident, “I do not believe that this assessment was extensive enough. Further data and studies will be needed to reach more robust conclusions.”   

“This report is a very important beginning, but it is not the end,” he added. “We have not yet found the source of the virus, and we must continue to follow the science and leave no stone unturned. Finding the origin of a virus takes time. … No single research trip can provide all the answers.”

For now though, here are four big takeaways from the 120-page report:

1. Markets are the most probable source of major transmission of the virus.

The focus is back on markets that sell animals.

COVID-19 made its global debut amid a cluster of cases linked to the Huanan Seafood Market in Wuhan in late December 2019. Researchers tested hundreds of animals in and around the market for the coronavirus — including animals for sale such as rabbits, hedgehogs, salamanders and birds — but none tested positive. Neither did thousands of domestic or wild animals in and around Wuhan. Additionally, some early COVID-19 cases that experts identified later, after the coronavirus had begun to spread in other countries, were not linked to the market.

Together, the findings hinted that the market may have helped the virus spread among people because of large crowds but that the Huanan Seafood Market was not the original source.    

Other markets may have also played a role in the virus’ spread, the investigation found. The earliest known case of COVID-19 was in a person who began showing symptoms on December 8, 2019. That person was not associated with the Huanan market but had recently visited another market.

Overall, of 174 people who were sick with COVID-19 in December, more than half had recently gone to a market, where they might have been exposed. An additional 26 percent were exposed to meat and fish or frozen food products.

The failure so far to find an animal that tests positive for SARS-CoV-2 highlights how difficult it is to identify particular species as a potential host, Peter Ben Embarek, lead investigator for the WHO mission and a food safety expert, said in a March 30 news conference. The hunt for where viruses came from takes time — sometimes years (SN: 3/18/21).

Future studies should expand the search for infected animals to wildlife farms that supplied products to the markets linked to COVID-19 cases. The people who work at the farms and those who handled the products should also be tested for antibodies to see if they once had coronavirus infections, the team suggests.

2. The coronavirus probably was not widely circulating before December 2019

There is not yet evidence that the virus was extensively spreading among people before the earliest documented case of COVID-19 in early December, the WHO team found.

Researchers combed through more than 76,000 clinical records from October to November 2019. Within those records, there were 92 possible cases of COVID-19. But 67 of those people did not have signs of an infection based on antibody tests done a year later. And all 92 were ultimately ruled out based on the clinical criteria for COVID-19. The records would not have included mild cases in people who never went to the hospital, however, so there are potential gaps in the evidence.

Additional evidence of isolated cases in countries outside China at the end of 2019 had hinted that the virus may have spread in those places before COVID-19 was first detected in Wuhan. But those reports have not yet been confirmed, the team wrote.

The timing for when the virus began spreading in China is in line with a recent study that analyzed genetic data and ran simulations of the early days of the pandemic to estimate when the virus may have emerged. The spillover from animals to humans may have happened between mid-October and mid-November 2019, Joel Wertheim and colleagues reported March 18 in Science.

After the virus transmitted from animals to humans, cases in people with mild symptoms may have helped the virus fly under the radar until December when some people fell severely ill, says Wertheim, an evolutionary biologist and molecular epidemiologist at the University of California, San Diego.

What’s more, the pandemic itself was far from inevitable, Wertheim says. In the simulations, more than two-thirds of SARS-CoV-2 transmissions from animals to humans went extinct, causing only a few infections in people before dying out. “Even a virus capable of causing a pandemic that brings the world to its knees wasn’t necessarily a foregone conclusion.”

3. The “lab-leak” hypothesis is unlikely, though hard to completely disprove

Based on a visit to the Wuhan Institute of Virology and interviews with scientists who work there, the report concludes that the virus most likely did not get its start in a lab. Though some experts have called for a full audit of the institute’s labs, the WHO mission was not designed to conduct a forensic investigation, WHO’s Ben Embarek said in the March 30 news conference.

Researchers at the institute considered the lab-leak hypothesis at the beginning of the pandemic and searched the institute’s records but did not find any evidence that anyone there was working with a SARS-CoV-2–like virus, Ben Embarek said. What’s more, antibody tests didn’t turn up any employees with signs of ever having had a coronavirus infection.

The lab leak “is possible, but there’s no evidence to support it,” says Massa Shoura, a biophysicist and genomics expert at Stanford University who was not involved with the report. Other coronaviruses that caused SARS and MERS made the jump to humans from animals, so it makes sense that it would be the most likely pathway for SARS-CoV-2 as well.

Yet accumulating the data to prove a negative may be extremely difficult. “I don’t think we’ll ever be able to provide enough evidence to convince people who are convinced that it escaped from a lab that it didn’t,” Wertheim says. “Even if you find a virus literally identical to SARS-CoV-2 [in animals] … they could still argue that that virus had previously been found and isolated and brought into a lab and it escaped just the way it was.”

4. Experts are far from knowing the coronavirus’s origins

Overall, the report offers few clear-cut conclusions regarding the start of the pandemic. Instead, it provides context for the possibilities and helps hone in on the studies researchers should tackle next.

Still, more than a year has passed since the virus made its jump to humans. That time lapse may hinder the investigation if SARS-CoV-2 no longer circulating in its reservoir, the animals that originally harbored it.

“We have to be prepared that we may never find the natural reservoir for this virus,” Wertheim says. But sometimes, all it takes is one good sample to give researchers important clues. Perhaps that’s a chance encounter with the right animal during a wildlife survey, or testing people from the right market.

Stepping even further back, researchers need to better understand the diversity of coronaviruses in bats and other wildlife in southeastern Asia, Shoura says. That means compiling a “dictionary” of the viruses found there to help researchers track viral evolutionary history, something the WHO team also recommends.

“We have only scratched the surface of these very complex studies that need to be conducted,” Ben Embarek said.

Invasive species can wreak havoc on local ecosystems. Cleaning up that biological wreckage comes at a big price.

These invaders, often thrust into new environments unintentionally (or intentionally, to combat pests) by humans, can transmit new diseases, devastate crops and eat away at crucial infrastructure. From 1970 to 2017, such invasions cost the global economy at least $1.28 trillion in damages and in efforts to control them, researchers report March 31 in Nature. As the globe becomes increasingly interconnected and invasive species take over new habitats, that cost grows.

“For decades, researchers have been evaluating the significant impacts of invasive species, but the problem isn’t well known by the public and policy makers,” says Boris Leroy, a biogeographer at the French National Museum of Natural History in Paris. “By estimating the global cost, we hoped to raise awareness of the issue and identify the most costly species.”

Leroy and his colleagues screened over 19,000 published papers, ultimately analyzing nearly 1,900 that detailed the costs of various invasions at particular times. The team then constructed a statistical model that estimated yearly costs, adjusting for factors like inflation, different currencies and timescales. Between 1970 and 2017, annual costs roughly doubled every six years, reaching a yearly bill of $162.7 billion in 2017.

Intensified global trade over that period gave invaders more opportunities to hitch rides on cargo ships or airplanes, the researchers say. And deforestation and agricultural expansion probably sped their spread by allowing easier access to pristine areas.

On the whole, cleaning up the damage caused by invasive species cost $892 billion, about 13 times higher than the $66 billion spent managing invasions, the researchers found.

“This is a really ambitious effort,” says Helen Roy, an ecologist at the UK Centre for Ecology and Hydrology in Wallingford, England. “There are major gaps in the data, which the authors are extremely transparent about,” she says. The analysis was heavily weighted towards North America, Europe and parts of Asia and Oceania. Agricultural pests, like insects, tended to be overrepresented in published literature compared with invasive plants.

“Still, getting a global look is very important,” Roy says. While this number is almost certainly an underestimate, she says, the study “shows us that this is a massive problem that’s getting worse.” Investing more in cargo inspections and other biosecurity measures or monitoring could help minimize these costs with comparatively small spending increases. “It’s much cheaper than waiting for the species to establish and spread widely before responding,” she says.

Here’s a closer look at the top five costliest invasive species.

1. Aedes mosquitoes (Aedes albopictus and Aedes aegypti): about $149 billion 

The Asian tiger mosquito (A. albopictus) arrived in the United States in the mid-1980s, by way of hitchhiking in used tires shipped from its native Asia. First detected in Houston, it rapidly spread to 40 states. It’s also invaded parts of Europe, South America, Africa and Australia. A. aegypti, or the yellow fever mosquito, is native to sub-Saharan Africa and spread around the world by similar methods.

Together, these two mosquitoes cause significant damage to public health by transmitting a range of diseases like Zika, chikungunya, yellow fever and dengue, which accounts for the bulk of their cost. As the mosquitoes spread, the toll of these diseases grows (SN: 11/20/19).

2. Rattus (rats): about $67 billion

These rodents’ worldwide occupation stems from about 3,000 years of hitchhiking on human boats. Once they arrive in a new location, rats often outcompete other small mammals, but can also harm birds and aquatic species. On islands around the world, rats have driven many species to extinction. For example, the Pacific rat, native to mainland southeast Asia, has snuffed out 1,000 species of island birds. Rats’ high cost stems from these biodiversity losses, but the rodents also can damage crops, destroy property and transmit disease (like the bubonic plague). 

3. Felis catus (cats): about $52 billion

Native to Europe and the Middle East, our feline friends have established themselves on all nonfrozen continents. Cats are excellent predators, and can make a quick meal from a variety of prey, from insects to birds. By some estimates, cats kill a billion birds each year in the United States alone (SN: 1/29/13). The bulk of the economic damage inflicted by cats cataloged in Leroy’s analysis comes from their impact on native biodiversity and resulting losses in spending on birdwatching and hunting birds like ducks, pheasants and grouse. 

4. Coptotermes formosanus (termites): about $19 billion

These subterranean termites native to East Asia have spread around the globe via trade. Termites can thrive wherever there is cellulose (like wood) and moisture, which has helped them quickly establish colonies upon being introduced to a new region. Their appetite for wood can wreak havoc on all kinds of structures, from homes to bridges. While they can also damage crops and tree farms, their high cost in this analysis boils down to their impact on infrastructure.

5. Solenopsis invicta (fire ants): about $17 billion

Fire ants usually become the dominant ant species when introduced to a new region, due to their aggressive foraging tactics, which include potent stings and bites. Native to South America, these ants arrived in the United States in the 1930s by boat, most likely carried in soil from the region. They’ve also spread to Australia, New Zealand, China and around the Caribbean. Fire ant colonies have wide-ranging impacts; they can feed on a variety of seedlings, from citrus to soybeans, reduce the size of grazing lands for livestock and bite and sting farm animals and humans.

Second of two parts

Physicists have a lot in common with Ponce de León and U2’s Bono. After decades of searching, they aren’t getting any younger. And they still haven’t found what they’re looking for.

In this case, the object of the physicists’ quest is SUSY. SUSY is not a real person or even a fountain relevant to aging in any way. It’s a mathematical framework based on principles of symmetry that could help physicists better explain the mysteries of the universe. Many experts believe that particles predicted by SUSY are the weakly interacting massive particles, or WIMPs, that supposedly make up the invisible “dark matter” lurking throughout the cosmos.

So far, though, SUSY has been something of a disappointment. Despite multiple heroic searches, SUSY has remained concealed from view. Maybe it is a mathematical mirage.

If SUSY does turn out to be a myth, it won’t be the first time that symmetry has led science on a wild WIMP chase. Reasoning from the symmetry of circular motion originally suggested the existence of a new form of matter out in space more than two millennia ago. Devotion to that symmetry blinded science to the true nature of the solar system and planetary motion for the next 19 centuries.

You can blame Plato and Aristotle. In their day, ordinary matter supposedly consisted of four elements: earth, air, fire and water. Aristotle built an elaborate theory of motion based on those elements. He insisted that they naturally moved in straight lines; earth and water moving straight down (toward the center of the world), air and fire moving straight up. In the heavens, though, Aristotle noticed that motion appeared to be circular, as the stars rotated around the nighttime sky. “Our eyes tell us that the heavens revolve in a circle,” he wrote in On the Heavens. Since the known four elements all moved in a straight line, Aristotle deduced that the heavens must consist of a fifth element, called aether — absent on Earth but predominant in space.

Plato, on theoretical rather than observational grounds, had already insisted that circularity’s symmetry signified perfection, and therefore circular motion should be required in the heavens. And so for centuries, the assumption that celestial motion must be circular held a stranglehold on natural philosophers attempting to understand of the universe. As late as the 16th century, Copernicus was willing to depose Aristotle’s Earth from the middle of everything but still believed that the Earth and other planets revolved around the sun with a combination of circular motions. Another half century passed before Kepler established that planetary orbits are elliptical, not circular.

Aristotle’s belief in an exotic form of matter in space is not so different from the picture scientists paint of the heavens today, albeit in a rather more rigorous and sophisticated theoretical way. Dark matter predominates in space, astronomers believe; it is inferred to exist from gravitational effects altering the motions of stars and galaxies. And physicists have determined that the dark matter cannot (for various noncircular reasons) be made of the same ordinary matter found on Earth.

SUSY particles have long been one of the most popular proposals for the identity of this cosmic dark matter, based on more complicated notions of symmetry than those available to Plato and Aristotle. And since the onset of the 20th century, symmetry math has generated an astounding string of scientific successes. From Einstein’s relativity to the theory of elementary particles and forces, symmetry considerations now form the core of science’s understanding of nature.  

These mathematical forms of symmetry are more elaborate examples of symmetry as commonly understood: a change that leaves things looking like they did before. A perfectly symmetric face looks the same when a mirror swaps left with right. A perfect sphere’s appearance is not altered when you rotate it to see the other side. Rotate a snowflake by any multiple of 60 degrees and you see the same snowflake.

In a similar way, more sophisticated mathematical frameworks, known as symmetry groups, describe aspects of the physical world, such as time and space or the families of subatomic particles that make up matter or transmit forces. Symmetries in the equations of such math can even predict previously unknown phenomena. Symmetry in the equations describing subatomic particles, for instance, revealed that for each particle nature allowed an antimatter particle, with opposite electric charge.

In fact, all the known ordinary matter and force particles fit neatly into the mathematical patterns described by symmetry groups. But none of those particles can explain the dark matter.

SUSY particles as a dark matter possibility emerged in the 1970s and 1980s, when theorists proposed an even more advanced symmetry system. That math, called supersymmetry (hence SUSY), suggested the existence of a “super” partner particle for each known particle: a force-particle partner for every matter particle, and a matter-particle partner for every force particle. It was an elegant concept mathematically, and it solved (or at least ameliorated) some other vexing theoretical problems. Plus, of the super partner particles it predicted, the lightest one (whichever one that was) seemed likely to be a perfect dark matter WIMP.

Alas, efforts to detect WIMPs (which should be hitting the Earth all the time) have almost all failed to find any. One experiment that did claim a WIMP detection seems to be on shaky ground — a new experiment, using the same method and materials, reports no such WIMP evidence. And attempts to produce SUSY particles in the world’s most powerful particle accelerator, the Large Hadron Collider, have also come up empty.

Some physicists have therefore given up on SUSY. And perhaps supersymmetry has been as misleading as the Greek infatuation with circular motion. But the truth is that SUSY is not a theory that can be slain by a single experiment. It is a more nebulous mathematical notion, a framework within which many specific theories can be constructed.   

“You can’t really kill SUSY because it’s not a thing,” physicist Patrick Stengel of the International Higher School of Advanced Studies in Trieste, Italy, said at a conference in Washington, D.C., in 2019. “It’s not an idea that you can kill. It’s basically just a framework for a bunch of ideas.”

At the same conference, University of Texas at Austin physicist Katherine Freese pointed out that there was never any guarantee that the Large Hadron Collider would discover SUSY. “Even before the LHC got built, there were a lot of people who said, well, it might not go to a high enough energy,” she said.

So SUSY may yet turn out to be an example of symmetry that leads physics to success. But just in case, physicists have pursued other dark matter possibilities. One old suggestion that has recently received renewed interest is a lightweight hypothetical particle called an axion (SN: 3/24/20).

Of course, if axions do exist, symmetry fans could still rejoice — the motivation for proposing the axion to begin with was resolving an issue with yet another form of symmetry.