How can we update data for a machine learning model after it has already trained on that data? In this paper, we introduce DART, a variant of random forests that supports adding and removing training data with minimal retraining. Data updates in DART are exact, meaning that adding or removing examples from a DART model yields exactly the same model as retraining from scratch on updated data. DART uses two techniques to make updates efficient. The first is to cache data statistics at each node and training data at each leaf, so that only the necessary subtrees are retrained. The second is to choose the split variable randomly at the upper levels of each tree, so that the choice is completely independent of the data and never needs to change. At the lower levels, split variables are chosen to greedily maximize a split criterion such as Gini index or mutual information. By adjusting the number of random-split levels, DART can trade off between more accurate predictions and more efficient updates. In experiments on ten real-world datasets and one synthetic dataset, we find that DART is orders of magnitude faster than retraining from scratch while sacrificing very little in terms of predictive performance.

Current deep learning research is dominated by benchmark evaluation. A method is regarded as favorable if it empirically performs well on the dedicated test set. This mentality is seamlessly reflected in the resurfacing area of continual learning, where consecutively arriving sets of benchmark data are investigated. The core challenge is framed as protecting previously acquired representations from being catastrophically forgotten due to the iterative parameter updates. However, comparison of individual methods is nevertheless treated in isolation from real world application and typically judged by monitoring accumulated test set performance. The closed world assumption remains predominant. It is assumed that during deployment a model is guaranteed to encounter data that stems from the same distribution as used for training. This poses a massive challenge as neural networks are well known to provide overconfident false predictions on unknown instances and break down in the face of corrupted data. In this work we argue that notable lessons from open set recognition, the identification of statistically deviating data outside of the observed dataset, and the adjacent field of active learning, where data is incrementally queried such that the expected performance gain is maximized, are frequently overlooked in the deep learning era. Based on these forgotten lessons, we propose a consolidated view to bridge continual learning, active learning and open set recognition in deep neural networks. Our results show that this not only benefits each individual paradigm, but highlights the natural synergies in a common framework. We empirically demonstrate improvements when alleviating catastrophic forgetting, querying data in active learning, selecting task orders, while exhibiting robust open world application where previously proposed methods fail.

Decision forests are popular tools for classification and regression. These forests naturally produce proximity matrices measuring how often each pair of observations lies in the same leaf node. It has been demonstrated that these proximity matrices can be thought of as kernels, connecting the decision forest literature to the extensive kernel machine literature. While other kernels are known to have strong theoretical properties such as being characteristic, no similar result is available for any decision forest based kernel. In this manuscript, we prove that the decision forest induced proximity can be made characteristic, which can be used to yield a universally consistent statistic for testing independence. We demonstrate the performance of the induced kernel on a suite of 20 high-dimensional independence test settings. We also show how this learning kernel offers insights into relative feature importance. The decision forest induced kernel typically achieves substantially higher testing power than existing popular methods in statistical tests.

The United Kingdom is already one of the leading countries for AI and is home to some of the world's most advanced AI companies, academics, and research centres. As being said, the nation has the opportunity to be a global leader in AI, but navigating Brexit might be a challenge. On January 31, 2020, the UK formally called off its EU membership in Brussels, with a deal called the withdrawal agreement. However, the deal only set out the process of how the UK would leave the EU. It covers areas, including citizens' rights, info about ways to stop checks along the Irish border, and the UK's financial settlement.

An artificial intelligence will pilot a fighter jet in a real life aerial dual with a Top Gun aviator after an AI won a simulated dogfight last month, Defense Secretary revealsThe Pentagon is set to pit an AI-controlled Jet against a pilot in a real-life aerial battle in 2024, Defense Secretary Mark Esper announced News of the slated battle comes just weeks after an artificial intelligence program defeated an F-16 fighter pilot in a virtual dogfightEsper made the announcement during a speech at the Defense Department's AI Symposium 2020 on Wednesday Esper hailed the'tectonic impact of machine learning on the future of warfighting' after the AI algorithm annihilated the human pilot on August 20While Esper provided no additional details on the face-off, he assured troops that AI will integrated help to enhance US warfighting, not replace pilots By Luke Kenton For Dailymail.com Published: 17:15 EDT, 10 September 2020 Updated: 17:23 EDT, 10 September 2020 Just weeks after an artificial intelligence program defeated an F-16 fighter pilot in a virtual dogfight, the Pentagon is set to raise the stakes by pitting an AI-controlled jet against a pilot in a real-life aerial battle, Defense Secretary Mark Esper announced.Esper unveiled the plans for the battle, slated for 2024, …

At a Friday event, Elon Musk revealed more details about his mysterious neuroscience company Neuralink and its plans to connect computers to human brains. While the development of this futuristic-sounding tech is still in its early stages, the presentation was expected to demonstrate the second version of a small, robotic device that inserts tiny electrode threads through the skull and into the brain. Musk said ahead of the event he would "show neurons firing in real-time. And he did just that. At the event, Musk showed off several pigs that had prototypes of the neural links implanted in their head, and machinery that was tracking those pigs' brain activity in real time. The billionaire also announced the Food and Drug Administration had awarded the company a breakthrough device authorization, which can help expedite research on a medical device. Like building underground car tunnels and sending private rockets to Mars, this Musk-backed endeavor is incredibly ambitious, but Neuralink ...

Artificial intelligence (AI) has the power to dramatically enhance the way public-sector agencies serve their constituents, tackle their most vexing issues, and get the most out of their budgets. Several converging factors are pressuring governments to embrace AI's potential. As citizens become more familiar with the power of AI through digital banking, virtual assistants, and smart e-commerce, they are demanding better outcomes from their governments. Similarly, public servants are pushing for private sector–like solutions to boost on-the-job effectiveness. At the same time, AI technology is maturing rapidly and being incorporated into many offerings, making it increasingly accessible to all organizations.

There are hundreds of trillions of microbes within the human body, which have a profound impact on modulating host function. Many of these microbes reside in the gastrointestinal tract and have been shown to influence normal physiology across all body systems ([ 1 ][1]). Disruptions in the delicate balance of microbes within the gut and other niches are associated with numerous disease states—including neurologic disorders, cardiovascular disease, gastrointestinal disorders, and even cancer ([ 2 ][2]). Accordingly, there is intense interest in targeting these microbes to promote overall health and to abrogate disease, with considerable advances made recently. Strategies to modulate gut microbes include fecal microbiota transplant (FMT), which involves the transfer of fecal material from one individual to another for a desired physiologic effect. This approach, among other gut microbiota modulation strategies, has shown promise in treating several disease conditions, although opportunities exist to iterate and build on these approaches. The idea that disruptions in the gastrointestinal tract could contribute to systemic disease was championed centuries ago by Hippocrates, a physician in ancient Greece. Strategies to modulate the composition of the gut have also been around for centuries, with the first reports of the use of FMT dating back to the fourth century BCE in China where fecal preparations were used to treat gastrointestinal disorders ([ 3 ][3]). Parallels have also been observed in the animal kingdom, where coprophagia (ingesting fecal material) is common and may confer an increase in gut microbial diversity and associated enhancements in host function for digestion and other physiologic processes. However, the first successful clinical application of FMT was not published until 1958 with the report of FMT from healthy donors used for patients with pseudomembranous enterocolitis from Clostridioides difficile infection (CDI) ([ 4 ][4]). Numerous clinical trials have since been undertaken, using FMT and other gut microbiota modulation strategies to treat diseases of the gut (such as CDI, and inflammatory bowel disease, IBD) as well as other systemic diseases—including metabolic syndrome, autism, multiple sclerosis, Parkinson's disease, and even cancer ([ 2 ][2]). ![Figure][5] Strategies to alter gut microbiota Fecal microbiota transplant (FMT) involves transfer of fecal microbiota from a donor to another individual . Alternatively, microbial consortia (targeted formulations used to augment host microbiota) are being developed. Diet, prebiotics, and postbiotics can also influence the microbial community. GRAPHIC: N. CARYI/ SCIENCE To date, many of the strategies to target gut microbes have involved the two extremes: either transfer of entire microbial communities (by using FMT) or transfer of a single microbial taxon. However, a growing number of approaches are now being developed as more is learned about the functional aspects and physiologic impact of microbes throughout the body. These iterative approaches transcend efforts that focus on taxonomic characterization of microbial niches through next-generation genomic sequencing, incorporating interrogation of functional characteristics of gut microbes (by metabolomic profiling and studies in preclinical models) to mediate the desired physiologic response. This has led to a host of therapeutic strategies from microbial consortia to pre-, pro-, and postbiotic interventions. Nonetheless, much still needs to be learned to implement true “precision” modulation of the gut microbiota. When considering strategies to modulate the gut microbiota, the indication for intervention in the intended population must be considered. Gut dysbiosis, an imbalance in the composition of commensal microbial communities, has been linked to numerous disease states, substantiating the use of FMT and other gut microbiota modulation strategies ([ 5 ][6]). This link is fortified by data demonstrating that although there has been a decrease in infectious diseases over the past several decades with the widespread use of antibiotics, there has been a concurrent increase in allergy and autoimmune diseases ([ 6 ][7]) presumably at least partially due to disruption of the gut microbiota. Notably, some of the diseases being treated by gut microbiota modulation have a profound dysbiosis (such as CDI), whereas others have a more subtle disruption of gut microbes, which has implications for choosing the appropriate strategy for gut microbiota modulation. Numerous other factors should be taken into account when contemplating modulation of the gut microbiota. These include the means of gut microbiota modulation, preparative regimen, measurement of engraftment of gut microbes and of the desired physiologic effect, and concurrent dietary intake ([ 7 ][8]). In general, the approach aims to restore a more “healthy” gut microbial community—although the definition of a “healthy” gut microbiota is not clearly established. However, data suggest that a diverse microbial community with a high degree of functional redundancy is associated with better overall health ([ 2 ][2]) and better outcomes in several disease states ([ 8 ][9], [ 9 ][10]). The most successful application of FMT thus far is in the treatment of refractory CDI, where treatment with FMT has been shown to be generally safe and highly effective ([ 2 ][2]). Nonetheless, guidelines for proper treatment and screening of donor stool are critical for safety and include screening for infectious diseases and disorders that are associated with perturbations of the gut microbiota, as well as the use of medications that can affect gut microbes such as antibiotics and proton pump inhibitors ([ 10 ][11]). Notably, these guidelines are iterative, as new recommendations are made to expand screening and testing of donors based on insights gained from ongoing trials. For example, screening of donors for multidrug-resistant organisms and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is now recommended. This follows reports of several patients with CDI who developed systemic infection with antibiotic-resistant bacterial infections following FMT ([ 11 ][12]), as well as concerns about possible infections with SARS-CoV-2. The use of FMT is being investigated across numerous other disease conditions, although most of these are associated with a less profound dysbiosis and greater heterogeneity in assessed endpoints and outcomes. However, there is clear evidence of success in some trials across a number of indications, including IBD, after hematopoietic stem cell transplant and in autism spectrum disorders ([ 5 ][6]). Limitations in measuring efficacy in FMT trials may arise from “true negative” results, or from numerous other confounding factors, including features not dependent on gut microbes that contribute to the development and persistence of disease in the recipient, as well as variability in trial design and outcome measures. Additionally, there may be factors inherent to the FMT donor that may affect efficacy (such as composition and functional aspects of the transplanted microbiota); however, such “donor effects” may be less prominent for indications in which a more profound dysbiosis is present, such as in CDI and even IBD ([ 12 ][13]). Optimal dosing and route of delivery for FMT are also incompletely understood and may be context dependent. Additional studies are critically needed to interrogate the success (or failure) of this approach for these indications and to develop optimal strategies for use of FMT. One attribute of FMT not possessed by other strategies to modulate the gut microbiota is the diversity of microbes that may be administered (including not only bacteria, but also viruses, fungi, and archaea) (see the figure). This provides potential functional redundancy for favorable impact on host physiology. In a profoundly dysbiotic state, this diversity represents a potential advantage over strategies that administer minimal-complexity microbial consortia, which may not engraft and may not be sufficient in reestablishing a “favorable” gut microbiota. However, the same attribute of increased diversity and complexity of FMT is also a limitation that creates issues with reproducibility and scalability. There are also concerted efforts under way to develop consortia of microbes that can be reliably and consistently manufactured and administered to favorably modulate the gut microbiota to address gastrointestinal and systemic disease, offering improved scalability over FMT. This includes commercially available probiotics, which are live microorganism preparations with presumed health benefits. The impact of administration of many of these formulations across disease indications has been studied in clinical trials with mixed results, and to date none of these commercially available formulations are approved for use by major regulatory bodies such as the U.S. Food and Drug Administration ([ 13 ][14]). However, next-generation live biotherapeutics (live microorganisms developed as therapeutic agents with defined clinical benefit claims) are now being developed based on insights gained from sequencing data in human cohorts and from studies in preclinical models ([ 13 ][14])—with many now in clinical trials. The first wave of these next-generation live biotherapeutics focused mainly on taxonomy—incorporating single or several bacterial taxa within a consortia based on insights gained from profiling gut microbial species in human cohort studies and in preclinical models. An example of this is in cancer immunotherapy: Clinical trials are now under way using modulation of the gut microbiota through administration of microbial consortia ([ 7 ][8]). These formulations range from simple (monoclonal microbial formulations) to complex (involving consortia of 50 or more bacterial taxa and strains). However, there is a growing appreciation that focusing on the functional aspects of these microbes may be far more important than simply focusing on taxonomy, and genetically modified organisms are now being developed with a wide range of functional attributes ([ 13 ][14]). Although overall these formulations are generally well-tolerated, safety still needs to be taken into account because there are reports of bacterial translocation of these organisms from the gut into the bloodstream in critically ill patients receiving gut microbiota modulation through administration of commercially available probiotics ([ 14 ][15]). Another strong consideration in gut microbiota modulation is the role of diet and prebiotics, as these can profoundly influence existing commensal gut microbes and those administered for therapeutic intent. These may ultimately serve as a stand-alone intervention in appropriate individuals with more subtle gut dysbiosis. Short-term studies have shown that large changes in diet can have a marked impact on gut microbes and associated physiology in the short term ([ 15 ][16]). However, this reliably reverts to a preintervention state if the instituted change in diet is not sustained. Nonetheless, numerous dietary intervention studies are currently under way ([ 7 ][8]), ranging from a somewhat simple intervention of adding one cup of canned beans per day to existing diets (NCT02843425) to extended (or longer-term) dietary interventions, where meals are prepared for (and shipped to) participants (NCT03950635). Such dietary modifications have potential relevance even if recipients are also treated with other gut microbiota modulation strategies such as FMT or live biotherapeutics, as they may sustain and promote optimal function of the transferred gut microbes, although optimal approaches of dietary intervention in these scenarios has yet to be defined. The use of prebiotic supplementation (such as resistant starches, polyphenols, and polyunsaturated fatty acids) is also being studied, because these compounds may provide optimal substrate to beneficial commensal (or administered) microbes. It is becoming evident that modulation of gut microbes will be increasingly employed to promote overall health and to help treat disease, although optimal strategies for “precision” gut microbiota modulation remain incompletely understood. It is probable that a personalized approach will be needed, incorporating strategies such as FMT, administration of live biotherapeutics, dietary strategies, and prebiotics—although it is not inconceivable that an ideal “one-size-fits-all” approach could be identified. Through additional research and collaborative efforts, the true definition of dysbiosis in the gut microbiota as it relates to disease states can be better understood, as well as what constitutes an optimal gut microbiota to promote overall health, which could have broad impact for public health. 1. [↵][17]1. I. Cho, 2. M. J. Blaser , Nat. Rev. Genet. 13, 260 (2012). [OpenUrl][18][CrossRef][19][PubMed][20] 2. [↵][21]1. J. R. Allegretti, 2. B. H. Mullish, 3. C. Kelly, 4. M. Fischer , Lancet 394, 420 (2019). [OpenUrl][22][CrossRef][23][PubMed][24] 3. [↵][25]1. F. Zhang et al ., Am. J. Gastroenterol. 107, 1755 (2012). [OpenUrl][26][CrossRef][27][PubMed][28] 4. [↵][29]1. B. Eiseman, 2. W. Silen, 3. G. S. Bascom, 4. A. J. 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SCI COMMUN### Astronomy Talk about a sharper image: A recently constructed imaging sensor array (above) that will be used when the Vera C. Rubin Observatory in Chile opens in 2021 has captured a world-record 3200 megapixels in a single shot. It recorded a variety of objects, including a Romanesco broccoli, at that resolution, which is detailed enough to show a golf ball clearly from 24 kilometers away. The sensor array's focal plane is more than 60 centimeters wide, much larger than the 3.5-centimeter sensors on high-end consumer digital cameras, says the SLAC National Accelerator Laboratory, which built the array. When the telescope, funded by the U.S. National Science Foundation, begins operating next year, it will image the entire southern sky every few nights for 10 years, cataloguing billions of galaxies each time. The surveys will shed light on mysterious dark energy and dark matter, which make up most of the universe's mass. With its repeat coverage, the telescope will make the equivalent of an astronomical movie in order to discover objects that suddenly appear, move, or go bang. ### Biomedicine Corticosteroids given orally or intravenously should be the standard therapy for people with “severe and critical” COVID-19, the World Health Organization (WHO) said in new guidelines issued last week—but they should not be given to patients with mild cases. In June, a large U.K. trial named Recovery first showed that the steroid dexamethasone cut deaths among ventilated COVID-19 patients by 35% after 28 days of treatment. That result was confirmed by a WHO-sponsored metaanalysis published in JAMA on 2 September that included Recovery and six other studies testing dexamethasone, as well as two other corticosteroids—hydrocortisone and methylprednisolone. Many countries, including the United States, had already included corticosteroids in their national treatment guidelines. But WHO's recommendations will be important as a signal to low- and middle-income countries, says Martin Landray, one of Recovery's principal investigators. ### Public health COVID-19 virus particles drifting through a Chinese apartment building's plumbing may have infected some residents, a study has found, raising fears of yet another way that the disease could spread. The case echoes a 2003 outbreak of severe acute respiratory syndrome (SARS) that spread through the pipes of a Hong Kong apartment building. Such transmission is difficult to prove. But scientists suspect that aerosolized coronavirus may have spread from the bathroom of a Guangzhou family of five through a floor drain and into the building's wastewater pipes. Two middle-aged couples living in apartments above the family later contracted COVID-19. The study appeared last week in Annals of Internal Medicine . ### Conservation A plan to reforest a cross-continental strip of Africa to hold back expansion of the Sahara Desert and the semi-arid Sahel has made little progress—even though the project is halfway toward its planned completion date in 2030, a report says. Participating countries have planted only 4 million hectares of trees and other vegetation for the Great Green Wall, well short of the 100 million planned to stretch 7000 kilometers from Senegal to Djibouti, says the report by the Climatekos consulting firm, presented on 7 September at a meeting of the countries' ministers. Supporters predicted the project would also create jobs and capture carbon dioxide. Scientists have said creating grasslands may be more effective than planting trees to resist desertification, The Guardian reported. ### Philanthropy Rice University last week received a $100 million gift for materials science. It is the largest to date in that discipline recorded in a database of gifts for engineering maintained by The Chronicle of Philanthropy . The funding will be used to pair materials science with artificial intelligence to advance the design and manufacturing of new materials, for applications that include sustainable water systems, energy, and telecommunications. The donor was the Robert A. Welch Foundation, which supports chemistry research in Texas. ### Conservation Scientists hailed a move last week by the European Union to ban the use of lead ammunition near wetlands and waterways. The European Chemicals Agency has estimated that as many as 1.5 million aquatic birds die annually from lead poisoning because they swallow some of the 5000 tons of lead shot that land in European wetlands each year. Its persistence in the environment is also considered a human health hazard. The EU Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) committee approved the ban after years of controversy. The German delegation, which had abstained in a July vote on the issue, changed its stance to support the measure after a letter from 75 scientists and petitions signed by more than 50,000 people called for it to do so. The European Commission and the European Parliament are expected to formally approve the ban, allowing it to go into effect in 2022. REACH may debate a complete ban on lead ammunition and fishing weights later this year. ### Chemical weapons Alexei Navalny, a Russian opposition politician, was poisoned with a nerve agent “identified unequivocally in tests” as a Novichok, an exotic Sovietera chemical weapon, German Chancellor Angela Merkel said on 2 September. Navalny fell ill on 20 August after drinking a cup of tea at a Siberian airport. He was flown to Berlin and this week emerged from a coma. German military scientists at the Bundeswehr Institute of Pharmacology and Toxicology in Munich haven't released details of their tests, but they had clear targets to hunt for: Like other nerve agents, Novichoks bind to the enzymes acetylcholinesterase and butyrylcholinesterase, creating a telltale conjugate compound. Novichok agents came to wide public notice in 2018 after one was used in an assassination attempt against former Russian spy Sergei Skripal in the United Kingdom. The attack prompted nations to push for a crackdown on Novichok agents, and last year they were added to the list of toxic chemicals regulated under the Chemical Weapons Convention. ### COVID-19 In one of the largest surveys of Americans since COVID-19 lockdowns began, a majority reported having some symptoms of depression, up from one-quarter in a prepandemic survey. The prevalence of symptoms graded as moderate to severe tripled, to 27.8% of respondents. A research team compared results from two surveys used to screen for depression: one administered to more than 5000 people in 2017 and 2018 by the U.S. Centers for Disease Control and Prevention, the other given to 1400 people in early April by NORC at the University of Chicago. Prevalence of depression symptoms rose in all demographic groups and especially among individuals facing financial problems, job loss, or family deaths. The increases in self-reported symptoms are larger than those recorded in previous surveys after large-scale traumatic events in other countries, including outbreaks of the severe acute respiratory syndrome, H1N1, and Ebola, the authors write in the 2 September issue of JAMA Network Open . ### A U.S. vaccine leader's vow: Politics stays out “I would immediately resign if there is undue interference in this process.” So said Moncef Slaoui, scientific director of Operation Warp Speed, the U.S. effort to quickly develop a vaccine for COVID-19, in an interview with Science . To date, Warp Speed has invested more than $10 billion in eight vaccine candidates. Three are now in large-scale efficacy trials, and interim reviews of their data by independent safety and monitoring boards could reveal evidence of protection as early as October. Slaoui, an immunologist who formerly headed vaccine development at GlaxoSmithKline, answered questions from Science last week about how Warp Speed operates and addressed concerns that political pressure before the 3 November U.S. presidential election may lead to an emergency use authorization of a COVID-19 vaccine before it is proven safe and effective. (On 8 September, nine companies developing vaccines for the pandemic coronavirus pledged not to seek a premature authorization.) “It needs to be absolutely shielded from the politics,” Slaoui says. “Trust me, there will be no [authorization request] filed if it's not right. … The science is what is going to guide us. … And at the end of the day, the facts and the data will be made available to everyone who wants to look at them and will be transparent.” Slaoui defended Warp Speed's decision to not consider vaccines made of whole, inactivated viruses, a time-tested approach. China has three such vaccines in efficacy trials, but he worries they could cause serious side effects in people who receive them. Slaoui also said if it had been his choice, the United States would have participated in COVAX, a mechanism for countries to invest collectively in vaccines and share them; the Trump administration declined to join. The full interview—one of Slaoui's most detailed since taking the job in May—is at .

Flush with money and a hard-won respectability, alien hunters are deploying new telescopes and tactics. In 2015, Sofia Sheikh was at loose ends. Her adviser at the University of California (UC), Berkeley, with whom she studied hot, giant exoplanets, had left for a new job. Browsing reddit, she saw a post about a lavishly funded new search for extraterrestrial intelligence (SETI) and noticed that its leader was also at UC Berkeley: astrophysicist Andrew Siemion. She asked her former adviser for an introduction and met with Siemion when he was still unpacking boxes in a new office. “Everything's kind of history from there,” says Sheikh, who became the team's first undergraduate student. Sheikh is now a Ph.D. student at Pennsylvania State University (Penn State), University Park, where she led a radio survey of 20 nearby star systems aligned with Earth's orbital plane. If an intelligent civilization inhabited one of these systems and pointed a powerful telescope our way, they would see Earth passing in front of the Sun, and they might detect signs of life in our atmosphere. They might even decide to send us a message. The results, published in February in The Astrophysical Journal , were unsurprising. “Spoiler alert: no aliens,” Sheikh jokes. SETI researchers are used to negative results, but they are trying harder than ever to turn that record around. Breakthrough Listen, the $100 million, 10-year, privately funded SETI effort Siemion leads, is lifting a field that has for decades relied on sporadic philanthropic handouts. Prior to Breakthrough Listen, SETI was “creeping along” with a few dozen hours of telescope time a year, Siemion says; now it gets thousands. It's like “sitting in a Formula 1 racing car,” he says. The new funds have also been “a huge catalyst” for training scientists in SETI, says Jason Wright, director of the Penn State Extraterrestrial Intelligence Center, which opened this year. “They really are nurturing a community.” Breakthrough Listen is bolstering radio surveys, which are the mainstay of SETI. But the money is also spurring other searches, in case aliens opt for other kinds of messages—laser flashes, for example—or none at all, revealing themselves only through passive “technosignatures.” And because the data gathered by Breakthrough Listen are posted in a public archive, astronomers are combing through it for nonliving phenomena: mysterious deep-space pulses called fast radio bursts and proposed dark matter particles called axions. “There are untapped possibilities here,” says axion searcher Matthew Lawson of Stockholm University. Perhaps the most important consequence of Breakthrough Listen is that it has nudged SETI, once considered fringe science, toward the mainstream. “Journals are relaxing and letting good technosignature papers be published,” says astrobiologist Jacob Haqq-Misra of the Blue Marble Space Institute of Science. “The giggle factor is reducing.” After nearly 3 decades of eschewing SETI, NASA organized a technosignature workshop in 2018. In June, it awarded a grant to model the detectability of possible technosignatures in the atmospheres of exoplanets, its first ever SETI-related grant not involving radio searches. But some astronomers worry the funding boon is distorting science. Fernando Camilo, chief scientist of the South African Radio Astronomy Observatory, says Breakthrough Listen's voracious appetite for time on large telescopes leaves him uncomfortable. “It leaves less time to do astronomy.” Others say SETI's high-risk, rush-for-the-prize approach could distract funders from a more rational, stepwise search for extraterrestrial life. “We do have a really thoughtful process on what gets funded and what doesn't,” says Harvard University astronomer David Charbonneau. “That doesn't happen with rich individuals.” But SETI proponents don't see themselves as separatists. They are increasingly working hand in hand with those searching for exoplanets and studying astrobiology. “Looking for intelligence is the logical conclusion of this search for life,” says astronomer David Kipping of Columbia University. SETI STARTED SMALL. In 1960, astronomer Frank Drake pointed a 26-meter radio telescope in Green Bank, West Virginia, at two nearby Sun-like stars. He scanned frequencies around 1.42 gigahertz, which correspond to wavelengths of about 21 centimeters—the part of the spectrum where clouds of interstellar hydrogen emit photons. This 21-centimeter glow is ubiquitous, and Drake supposed it might be a universal channel on the cosmic dashboard, a natural place for a clarion “We are here!” But his targets, Tau Ceti and Epsilon Eridani, were expressionless. The survey, called Project Ozma, saw no sign of artifice, such as an intense spike squeezed into a narrow frequency band. With funding from NASA and the National Science Foundation (NSF), however, searches continued, with bigger telescopes to listen for fainter signals and hardware that could scan thousands and eventually millions of narrow frequency channels at once. Drake devised his now famous, eponymous equation that estimates how many communicative extraterrestrial civilizations may exist in the Milky Way. It depends on seven variables, from the rate of star formation to the average lifetime of a civilization. Even though only one of the seven factors—star-formation rate—was known with any certainty, alien hunters were on the prowl. In 1992, NASA decided to look harder, only to quickly reverse course. It embarked on the Microwave Observing Project, a 10-year, $100 million SETI search using several large telescopes. But the following year, the project was ridiculed and cut by lawmakers focused on reducing the federal budget deficit. Ever since, NASA has mostly shied away from SETI. ![Figure][1] CREDITS: (GRAPHIC) N. DESAI/ SCIENCE (DATA) JASON WRIGHT/PENN STATE Even as federal funding shriveled, the 1990s gave SETI an unexpected gift. Until then no one had detected an exoplanet, much less a potentially hospitable one, but that decade brought a host of discoveries. Since then, missions such as NASA's Kepler telescope have suggested that planetless stars are rare, and that about one in five Sun-like stars has potentially habitable Earth-size planets—two more factors in the Drake equation that have fueled optimism among SETI advocates. The turn-of-the-century tech boom offered another boost: newly minted billionaires with a taste for space. A high point came in 2007 with the inauguration of the Allen Telescope Array, a SETI observatory in California kick-started with $11.5 million from Microsoft cofounder Paul Allen. Then the field took another plunge. The 2008 financial crisis struck and within a few years, with federal and state funding tight, UC Berkeley withdrew from the project. The array was put into hibernation for 8 months. A planned expansion from 42 to 350 dishes never materialized. “SETI was entirely decimated,” Siemion says. “I was one of maybe two or three in the whole world working on SETI.” That was when Yuri Milner called. BORN AND EDUCATED in Moscow, Milner worked as a particle physicist at the Lebedev Physical Institute. In 1990, as the Soviet Union collapsed, he left to study business at the University of Pennsylvania, and in 1999 he founded an internet investment fund. The fund was an early backer of Facebook and Twitter, and later Spotify and Airbnb. Forbes magazine puts Milner's net worth at $3.8 billion. “I made some lucky investments,” he tells Science . Milner says he's always felt a connection with space and SETI. He was born in 1961, days after Drake convened the first SETI conference. He is named after Yuri Gagarin, the first cosmonaut. Once he had built up a fortune, “I discovered that now I can give back to science,” he says. He knew of SETI's dire financial straits, and he believed his money and knowledge of the tech industry could help speed up the search. Siemion's UC Berkeley center, across the San Francisco Bay from Milner's home in Silicon Valley, became the beneficiary. Breakthrough Listen set out ambitious goals ( Science , 24 July 2015, p. [357][2]). It would survey 1 million of the closest stars to Earth and 100 nearby galaxies using two of the world's most sensitive steerable telescopes, the 100-meter Green Bank Telescope in West Virginia and the 64-meter Parkes radio telescope in Australia. Buying up about 20% and 25% of the time on those telescopes, Breakthrough Listen promised to cover 10 times more sky than previous surveys and five times more of the radio spectrum, and gather data 100 times faster. Achieving these goals required new hardware. The key electronic component is a digital backend, which chops telescope data into ultrathin frequency slices and records it. Siemion says Breakthrough Listen's backends are “orders of magnitude more powerful than anything else on site.” The instruments are available for 100 hours every year to other astronomers interested in such fine frequency resolution. That allocation is often oversubscribed at Green Bank, Siemion says, ever since the backend helped characterize the first repeating fast radio burst. The project is adding a major new telescope to its mix of collaborations: MeerKAT, a South African array of 64 dishes each 13.5 meters across ( Science , 22 June 2018, p. [1285][3]). Instead of buying time on the array, Breakthrough Listen is tapping into the data stream while the telescope observes its regular targets—a procedure known as commensal observing. “You take what you can get,” Camilo says. “When it works, it's fantastic.” Commensal observing will also be added to the Karl G. Jansky Very Large Array in New Mexico, the workhorse of U.S. radio astronomy, in a project led by the privately funded SETI Institute. Gathering data sets is one thing; scouring heaps of them for alien messages is another. SETI researchers have long looked for energy packed into narrow frequency signals—something that is hard for nature to replicate, although astronomers need to exclude humanmade signals. One test is to see whether the signal's frequency drifts over time: An alien transmitter would be on a moving planet, causing a Doppler shift. If the frequency is rock steady, it's likely to be earthly interference. Similarly, if the signal persists when the telescope moves from its target, it's noise from Earth. But aliens might send something more complex than a single loud note. How do you scan SETI data for something that just seems anomalous or weird? Researchers have been trying to enlist artificial intelligence (AI), but it hasn't been easy. One species of AI, natural language algorithms, can recognize key words in the flow of human speech—think of Amazon's Alexa, or eavesdroppers at the National Security Agency—after being trained on vast speech data sets. But the huge number of narrow frequency channels in SETI data overwhelms these algorithms. Converting the data stream into 2D diagrams that resemble images works better, at least in tests, in which machine vision algorithms picked out strange pictures from a torrent of similar ones. “We have to guess what an anomaly might look like and train the algorithm to look for this, or look for things that look similar,” says Steve Croft of UC Berkeley's SETI Research Center. THE FOCUS OF SETI searches tends to reflect the technology of the times. Radio was in its heyday when Drake started out. But as lasers have grown in power and sophistication, so have efforts to spot alien laser signals with so-called optical SETI. Astronomers have carried out optical searches with modest telescopes since the 1990s. Breakthrough Listen is doing its own, with time on the 2.4-meter Automated Planet Finder (APF) telescope at the Lick Observatory in California. APF has been scanning a sample of stars to distances up to 160 light-years but will now work through a new list: stars with potentially habitable planets identified by NASA's Transiting Exoplanet Survey Satellite ( Science , 30 March 2018, p. [1453][4]). Others are developing telescopes that wouldn't need to target individual stars. The LaserSETI project, funded by the SETI Institute, is a collection of $30,000 mini-observatories, made up of an off-the-shelf fisheye lens, two cameras, and electronics that would gather light from the entire sky. The first was installed last year on an observatory roof north of San Francisco. Eventually, the institute wants to install 60 instruments around the world for 24/7 coverage. LaserSETI's small telescopes would only pick up an especially bright flash from a nearby source. Shelley Wright of UC San Diego hopes to see much farther with the Pulsed All-sky Near-infrared Optical SETI (PANOSETI), an all-sky telescope able to detect ultrashort laser pulses across all optical wavelengths. PANOSETI's design includes lightning-fast photon counters sensitive to pulses less than one-billionth of 1 second long. “It's hard for nature to make that,” Shelley Wright says. It relies on a Fresnel lens, a type used in lighthouses to focus light into a narrow beam. Flipped over, a Fresnel can gather light from a 10°-wide patch of sky onto the photon counters. The team is building two observatories, each an array of 80 telescopes with lenses 50 centimeters across, bunched together in a fly's eye arrangement. The plan is to site the pair 1 kilometer apart—to help root out false positives—at the Palomar Observatory in California. Funded by Qualcomm co-founder Franklin Antonio, the project has built five telescopes but has been stalled by the COVID-19 pandemic. THEN AGAIN, even intelligent aliens might be too busy or too shy to send messages to the stars. So SETI researchers also hope to detect passive signs of technology. People's ideas about what to look for often reflect their time: Consider the 19th century “discovery” of canals on Mars when canals were still a common form of transport on Earth. In 1960, amid rapid economic growth and concerns about energy shortages, physicist Freeman Dyson imagined an advanced society might build a megastructure surrounding a star to capture its energy ( Science , 3 June 1960, p. 1667). Such “Dyson spheres” continue to fascinate and were suggested as an explanation for the strange dimmings of the star KIC 8462852, known as Tabby's Star. In 2015, Jason Wright led a search for the glow of Dyson spheres in 100,000 nearby galaxies, using data from NASA's Wide-field Infrared Survey Explorer satellite. Technosignatures could be more subtle. In the not-too-distant future, ultrasensitive radio telescopes might be able to pick up the beams of a radar, like the ones used for air traffic control, from a distant exoplanet. Future optical telescopes might reveal the glow of a city's lights or its infrared warmth. Heavy industry or geoengineering might leave fingerprints in a planet's atmosphere. These efforts chime with searches for biosignatures, detectable marks that organic life might leave on an exoplanet ( Science , 3 November 2017, p. [578][5]). “The line between technosignatures and biosignatures is blurring,” Sheikh says. “It makes sense to observe both.” In deciding to fund the 2018 workshop on technosignatures, NASA felt that they could be discussed “on a firmer scientific foundation than before,” says Michael New, the agency's deputy associate administrator for research. After the workshop, the wording in NASA funding calls that had for some years excluded SETI-related proposals quietly disappeared. In June, Jason Wright and his colleagues benefited from the new openness when they were awarded a grant to model exoplanet atmospheres and put together a “library” of potential technosignatures, which astronomers can refer to when observing exoplanets. The team will first model chlorofluorocarbons—a pollutant that isn't produced naturally—and vast solar power arrays, because they would leave an obvious cutoff in the ultraviolet part of the spectrum. “What we should look for is things that can't be avoided, civilization's manifestations in the biosphere,” says Adam Frank, lead investigator on the grant at the University of Rochester. BUT EVEN AFTER the fanfare of Breakthrough Listen, SETI remains far from a central concern for most astronomers. In 2018, panels of researchers convened by the National Academies of Sciences, Engineering, and Medicine (NASEM) drew up strategies for NASA on astrobiology and exoplanets. They made scant mention of technosignatures and didn't advise NASA to spend any money on the topic, or, more generally, SETI. SETI enthusiasts say they are trying to avoid being shut out of an even bigger NASEM effort: its decadal survey of astrophysics, a once-a-decade priority setting exercise that is influential with funding agencies and legislators. The survey is due to report early next year. “We've made a big push to get the decadal survey … to explicitly say that NASA and the NSF need to nurture this field,” Jason Wright says. He and colleagues made nine submissions, known as white papers, to the survey, compared with a single white paper in the previous survey. Sheikh says: “There are signs the winds are starting to shift.” But many astronomers think the more important hunt is for alien life of a more basic kind, not the higher risk search for technological societies. “We have to invest in general questions,” says Charbonneau, who co-chaired the NASEM panel that developed the NASA exoplanet hunting strategy. “If we just go for the prize and don't find anything, what have we learned from that?” Mainstream astrobiologists hope the decadal survey will give a thumbs up to the Large UV/Optical/IR Surveyor, or LUVOIR, a proposed NASA space telescope as much as six times wider than the Hubble Space Telescope ( Science , 14 December 2018, p. [1230][6]). It would scrutinize habitable planets for biosignatures and estimate the fraction of them that support life—another term in the Drake equation. “The progress we've made as scientists follows the terms of the Drake equation in order,” says astrobiologist Shawn Domagal-Goldman of NASA Goddard Space Flight Center. “That progress could lead to a search for technosignatures. I could see LUVOIR being used to do that, even though it wasn't designed for such a search.” Jason Wright, however, thinks the potential payoff of SETI is just too tempting to put off the search. In July, he and his colleagues reported the “discovery space”—all the possible locations, frequencies, sensitivities, bandwidths, timings, polarizations, and modulations—that SETI radio surveys have so far explored. The result: If the entire discovery space is represented by the world's oceans, SETI has so far searched the volume of a hot tub. Milner seems ready to support at least a few more SETI hot tubs. He says he wants Breakthrough Listen to continue past 2025, when his initial funding runs out. “It's one of the most existential questions in our universe,” he says. “Just knowing we are not alone … is something that can bring us together here on Earth.” [1]: pending:yes [2]: http://www.sciencemag.org/content/349/6246/357 [3]: http://www.sciencemag.org/content/360/6395/1285 [4]: http://www.sciencemag.org/content/359/6383/1453 [5]: http://www.sciencemag.org/content/358/6363/578 [6]: http://www.sciencemag.org/content/362/6420/1230