O Universo é finito ou infinito?

Nós sabemos que o universo está se expandindo, pois, com algumas exceções próximas, quase todas as galáxias no Universo estão se afastando de nós e de umas as outras. Não só isso, mas galáxias muito distantes parecem estar se afastando ainda mais rapidamente, o que evidencial que o universo está se expandindo a uma taxa crescente.

Observações dos vários momentos do universo sugerem que, para os primeiros bilhões de anos, a expansão do universo desacelerou – mas, em seguida, aproximadamente 8 bilhões de anos atrás, a expansão começou a acelerar. Se a aceleração continua (o que parece provável), o universo nunca vai abrandar a sua expansão ou re-colapsar. Isso corresponde à ideia de um universo “plano”, que é atualmente o modelo mais aceito. Mas um universo espacialmente plano pode ser característico de qualquer um universo finito ou infinito.

Quando dizemos que o espaço é “plano”, que significa que ele obedece a geometria euclidiana: linhas paralelas nunca se cruzam, e os ângulos de um triângulo sempre somam 180 graus. Podemos imaginar o universo em duas dimensões como um avião, que é plano e infinito (como um pedaço de papel infinito). Mas também podemos imaginar que esse papel esteja sendo dobrado em forma de um cilindro, e, em seguida, dobrado novamente em forma toroidal (forma de rosca). Nesse caso, a superfície do toro é espacialmente plana, como o pedaço de papel, mas finita. No entanto, com a expansão, é possível que, mesmo se o universo tenha apenas um volume muito grande, ele irá atingir o volume infinito no futuro infinito.

O Tamanho do Universo ObserváveL

O espaço que se pode observar, por outro lado, tem um tamanho definido. Como o universo nasceu a 13,8 bilhões de anos atrás, só podemos observar objetos cuja luz tem viajado dentro desses 13,8 bilhões de anos para chegar à Terra. Esta parte do universo é chamada de “universo observável”, e é a única parte do universo que podemos saber algo.

Mas, devido à expansão do universo, o raio do universo observável não é 13,8 bilhões de anos-luz. As estimativas atuais, em vez definir o seu raio de cerca de 46 bilhões de anos-luz, uma estimativa feita em coordenadas co-móvel, que representam a expansão do universo. Como as idades do universo, o tamanho do universo observável continuará a se expandir.

Fonte: http://www.misteriosdouniverso.net/

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Voyage to Infinity

Five overlaid Hubble Space Telescope images show the motion of 2014 MU69, which the New Horizons mission will reach on New Year's Day, 2019. Credit: NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team. Public Domain

After its pioneering exploration of Pluto and its five moons in July 2015, NASA’s New Horizons spacecraft is outward bound from the solar system on a voyage to infinity. Directly ahead of it in the void, however, is an object in the Kuiper Belt called 2014 MU69, an ancient dark reddish aggregate of rock and ice only a few dozen kilometers in diameter, discovered just a year before the Pluto encounter. MU69 has orbited the far reaches of the solar system since it, our sun, our own world condensed out of a cloud of dust and gas nearly five billion years ago.

We think, hope—guess, maybe—that it’s a raw, leftover lump of the ingredients that formed the solar system, a fossil witness to the creation of the planets and their moons. Regardless, MU69 provides the first in situ sampling of the Kuiper Belt, a diffuse torus of thousands of icy planetoids that circle the outer solar system. The voyage to MU69 marks the most distant destination in the history of civilization. It is the last speck of land New Horizons will pass before sailing on to deep interstellar space.

New Horizons reaches this last port of call on New Year’s Day, 2019, a year that will mark the 50^th anniversary of the first steps of humans on another world. There’s so much to prepare for, investigations to be sketched out, studies to be drawn up, so many decisions to be made that even now the New Horizons team of scientists and engineers is working full time to get ready. It’s been my pleasure to have a tiny part in this.

An email comes in near the end of the day, asking how we might best use one of the cameras on the spacecraft for this or that. I start typing my thoughts into my laptop, day dreaming a little perhaps, looking out my office window at the jagged Catalina mountains that rise out of the desert north of Tucson. Above them a crystal clear cerulean sky extends forever in all directions. It’s a great arena for an astronomer. Watching a full moon clear the rocky skyline, it hangs just above the distant peaks as an amber, delicately shaded globe. You can feel the distance to the moon. There’s no question that it’s a real world to explore.

Looking over the sky, I know that New Horizons is out there somewhere. It’s hurtling off into the galaxy at 14 km/s, or more than 31,000 m.p.h., far, far, far out in the outer solar system on a course we set it on after its flight past Pluto. From Earth it would take only eight hours to get out to the moon at this clip, not the three days it took the old Apollo astronauts. Yet we have two years before the mast until we make landfall at MU69, and this on top of a total journey that started 11 years ago.

We won’t have much time there. We can’t slow down in the slightest, let alone go into orbit around it. The best data will come from the single tightly packed hour of closest approach. What should we measure? What’s the cleverest use of our instruments? Right now we’re busy using email exploders, conference calls, meetings to pool all our ideas and then slice them ever, ever finer into a precise second by second mission timeline.

Long before the MU69 encounter we will speak across the solar system with the NASA’s Deep Space Network to the robotic mind of New Horizons. We will send it endless packets of commands, a painstaking and demanding distillation of our ideas, thoughts, wishes, guesses, contingencies, rules—a complete philosophy—to tell it exactly how to explore an unknown world. It now takes over five hours for the microwave signals traveling at the speed of light to reach New Horizons, but the spacecraft listens still, a beautiful ship ever responsive to the right touch at the helm. We call ourselves scientists, but we are sailors, too.

In the background on my laptop I’m streaming KUAT, the local Tucson classical music station. Smetana’s “Ma Vlast” is playing, a deeply romantic epic with energy that seems to fill the immense space stretching out in front of me. I know that it comes across the net as a long string of numbers—data converted by software into sounds. Likewise, the treasures returned by New Horizons from distant lands are just other numbers—pixels to be recorded, processed, analyzed, modeled by cool, logical, mathematical algorithms. But like the music, their impact is profoundly emotional—strange visions of places never imagined—startling discoveries that will shine a bright light into the darkness. We don’t know what awaits us at 2014 MU69, but it will touch our hearts as much as our minds.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

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ABOUT THE AUTHOR(S)

Tod R. Lauer

Tod R. Lauer is a research astronomer on the staff of the National Optical Astronomy Observatory in Tucson, AZ, and is a member of the New Horizons Science Team. Lauer's research has largely been concerned with cosmology, black holes, and galaxies, but he works with the New Horizons team to support the processing and analysis of images obtained with the spacecraft.

New icy world with 20,000-year orbit could point to Planet Nine

By Adam Mann

PASADENA, CALIFORNIA—The solar system has gained a new extreme object: L91, a small, icy world with one of the longest known orbits, taking more than 20,000 years to go around the sun. Researchers have yet to pin down the object’s size or mass, but they can add it to the growing list of frozen bodies circling well beyond Neptune in strange orbits that imply gravitational disruptions from outside the sun and the known giant planets. In the case of L91, some astronomers say that external disrupter could be a ninth giant planet, as yet undiscovered. However, L91’s discovery team favors a scenario in which the disturbance is more mundane: a passing star, or the Milky Way’s gravity.

“It’s right at the limit of what we can detect,” said astrophysicist Michele Bannister of Queen’s University Belfast, who described the result today at the American Astronomical Society’s Division for Planetary Science meeting here.

L91 never comes closer to the sun than 50 astronomical units (AU), or 50 times the Earth-sun distance. From there, it slowly crawls all the way out to 1430 AU. This means it has a more elongated orbit than Sedna, another distant Pluto-sized object, whose closest approach is 76 AU and whose estimated far point reaches 937 AU. L91 was found using the Canada-France-Hawaii Telescope at Mauna Kea in Hawaii, as part of the Outer Solar System Origins Survey.

Astronomers once thought the solar system was relatively static, with the planets’ current configurations roughly unchanged since their birth in a gigantic cloud of dust and gas more than 4 billion years ago. But during the past decade or so, researchers have realized that planetary history is full of chaotic movements, with gas giants like Jupiter and Saturn drifting inward and outward from the sun. As these gargantuan masses moved, their gravitational influence sent other objects careening around, in some cases getting jettisoned entirely.

L91 is thought to be another wanderer, except the ice giant Neptune might be responsible for its movements. Bannister sketched a scenario in which the icy object was born with a more regular elliptical orbit. Back then, its closest and furthest points from the sun would have been roughly similar.

Over billions of years, Neptune’s gravitational influence might have given it little kicks that stretched out its orbital far point all the way to the inner part of the Oort cloud—a cluster of frozen bodies thought to start 2000 or more AU from the sun. Then a passing star or gravitational interactions with our Milky Way galaxy could have retracted L91’s orbit down to the less elongated but still extreme shape we see today.

“It’s a story that’s not implausible, but I also think it’s not needed,” said planetary scientist Konstantin Batygin of the California Institute of Technology (Caltech) here, who wasn’t part of the recent discovery.

His preferred explanation is gravitational tugging from Planet Nine, an as-yet-unseen Neptune-sized world that he and Mike Brown, another Caltech astronomer, came up with in January toexplain the strange stretched-out orbits of a half-dozen objects, including Sedna. Bannister and her team modeled scenarios in which a Planet Nine–mass world could have provided the gravitational kicks necessary to elongate L91’s orbit but found that that would have tilted L91 into a different orbit. But Batygin says that galactic gravitational tugging is an inefficient process and that the Planet Nine explanation remains a less convoluted way of achieving the same result.

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• Space

DOI: 10.1126/science.aal0270

Adam Mann

Comet Collision Could Have Caused Rapid Carbon Rise

Evidence collected along the New Jersey coastline suggests that the collision of a comet or other extraterrestrial body 55 million years ago coincided with an intense warming period that is the closest comparison to today’s climate change.

The period of warmth, called the Paleocene-Eocene Thermal Maximum (PETM), was preceded by a rapid release of carbon into the atmosphere, with effects lasting for about 200,000 years. The increase in carbon raised the Earth’s temperature by up to 16 degrees Fahrenheit. As a result, the planet was almost ice-free and sea levels rose significantly higher than they are now. It caused a mass extinction for single-cell creatures in the ocean, but animals on land, especially primates, thrived and rapidly evolved by moving toward the poles, where temperatures are lower.

There is still disagreement among scientists as to what triggered the carbon release during that era. Current theory posits that this mysterious release of carbon dioxide came from volcanoes and suggests that the warming may have been abetted by a sudden release of methane from the ocean floor.

But the new study, published yesterday in the journal Science, connects the warming to the impact of the extraterrestrial object.

In New Jersey, researchers found tiny spherical droplets of glass called microtektites, which form from vaporized debris as it flies through the air after an extraterrestrial object hits the Earth. The glass debris was found in samples from the towns of Millville, Wilson Lake and Medford, as well as from one collected from the deep seabed off the coast of Bermuda.

The discovery of the droplets, found in a clay deposit traced to the PETM period, could signal that the area was “ground zero” for an impact, said Dennis Kent, a researcher at Columbia University’s Lamont-Doherty Earth Observatory and a co-author of the study. He said it warmed in a hurry, and the new study suggests where it came from.

If a comet triggered the last period of significant warming as a result of carbon release, Kent said, it can provide information for today’s researchers on climate. If carbon warmed the planet in a matter of years or decades, rather than millennia, it could provide insight into what today’s climate change may bring, he said.

“This could be even more rapid than today, and studying that in more detail could say something about the limit of that climate change compared to today,” he said.

In 2003, Kent first proposed that magnetized clay particles he discovered in New Jersey could be evidence that a comet hitting the Earth led to the PETM. A 2013 study found that the PETM was triggered almost instantaneously, based on carbon isotopes found in the same clay sediment.

The new study does not claim to have located a crater or to prove that the collision of a comet or other extraterrestrial object triggered the PETM. But it does build the body of evidence suggesting a connection, said Morgan Schaller, an assistant professor of earth and environmental sciences at Rensselaer Polytechnic Institute.

“It’s got to be more than coincidental that there’s an impact right at the same time,” he said in a statement. “If the impact was related, it suggests the carbon release was fast.”

Many in the scientific community have rebutted that argument, however. The new study doesn’t prove exactly what triggered the PETM period, Rice University marine geologist Gerald Dickens said. He said there is plenty of evidence that the carbon release took thousands of years. Current theory on the cause of the PETM was that the carbon was released over 5,000 to 20,000 years, not suddenly.

“Finding a few spherules does not change this,” he said.

Other researchers have found evidence that the carbon release during the PETM period was instantaneous.

Regardless of what triggered the PETM period, which profoundly changed the planet, humans are now warming the Earth 10 times faster than it heated up during that era, a study published earlier this year in Nature found. The PETM period saw the highest rates of carbon released in 66 million years, according to the study by University of Hawaii researchers.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. E&E provides daily coverage of essential energy and environmental news at www.eenews.net. Click here for theoriginal story.

A incrível complexidade da Teia Cósmica

Cientistas descobriram a chamada “teia cósmica” há menos de uma década. Desde então, várias questões persistiram, sobretudo esta: qual a aparência dessa teia? Uma nova visualização desenvolvida por Kim Albrecht no Center for Complex Network Research ajuda a resolver esse mistério intrigante.

Mas primeiro, vamos voltar um pouco: o que é, exatamente, a teia cósmica? Em resumo, é a vasta rede formada por todas as galáxias do universo e os fios semelhantes a teias que as unem. Composta por filamentos invisíveis de gás hidrogênio, essas conexões intergalácticas compõem a maior parte da matéria ordinária do universo e também traça a distribuição de matéria escura. Observar esses linhas cósmicas diretamente é desafiador, para dizer o mínimo. Astrofísicos conseguiram compor imagens de peças suficientes da teia cósmica para prever modelos confiáveis de sua estrutura. No ano passado, os pesquisadores por trás do projeto Illustris utilizaram esses modelos para construir umasimulação 2D da teia cósmica, visualizando dados essenciais, como densidade gasosa, temperatura e velocidade.

Utilizando o Illustris como base, o designer de informações e pesquisador visual, Kim Albrecht, criou agora uma ambiciosa visualização em 3D da teia cósmica. Intitulada “Network Universe”, o projeto de Albrecht baseia-se na simulação do Ilustris e a leva a um outro nível, não apenas interativo, mas imersivo.

À medida em que eu dava zoom e girava através do Network Universe, eu fiquei imediatamente impressionada pelas disparidades visuais entre os três “modelos” da teia cósmica que estavam ali apresentados. De onde vinham essas variações intrigantes? A respostava está na ciência na qual a visualização foi baseada.

A visualização de Albrecht nasceu de um projeto de pesquisa colaborativo que visava avaliar vários algorítimos que podem ditar a estrutura da rede cósmica. Os três “tipos” representam os três principais modelos testados. O site oferece um breve resumo de uma frase sobre cada modelo e a pesquisa os prolonga em fórmulas matemáticas. Eu admito que fiquei encarando as fórmulas e, apesar das descrições no site serem mais amigáveis, eu ainda tive dificuldades inicialmente para entender a origem precisa daquilo que meus olhos viam. Depois de alguns pensamentos críticos e uma troca de e-mails com Albrecht, as respostas começaram a se solidificar em minha mente.

O Modelo de Comprimento Fixo (Fixed Length Model) é razoalvemente simples: todos os nós (isto é, galáxias) localizados a uma dada distância uns dos outros estão conectados. Os nós que não possuem vizinhos dentro desse raio estão sozinhos, enquanto aqueles que possuem grande número de vizinhos possuem também uma grande número de conexões. Como resultado, a visualização aparece muito densa em alguns pontos, onde os nós estão agrupados, e esparsos em outros.

No Modelo de Comprimento Variável (Varying Length Model), as coisas se complicam um pouco. Aqui, quanto maior o nó, maior o alcance de suas conexões. Nós menores também podem formar conexões, mas apenas com aqueles relativamente próximos a eles. O que temos como resultado é uma rica densidade visual, caracterizada por fios extensos e sobrepostos e uma brancura acentuada concentrada ao redor dos nós maiores.

Finalmente, de acordo com o Modelo de Vizinhos Mais Próximos (Nearest Neighbours Model), cada nó é limitado a n ligações e essas ligações são compartilhadas apenas com os nós mais próximos. Então vamos supor que n = 3. Para alguns nós, os três vizinhos mais próximos estarão por perto, mas, para outros, eles podem estar muito distantes. No entanto, mesmo para aqueles com muitos vizinhos, o número de ligações não poderá exceder 3. Assim, a visualização nunca se torna tão densa quanto a dos outros e sua distribuição de branco e preto é relativamente homogênea.

Embora todas as visualizações sejam belas e divertidas de se explorar, para mim existe algo a mais sobre o Modelo dos Vizinhos Mais Próximos. As linhas conectivas parecem excepcionalmente limpas e existe uma certa naturalidade agradável em sua composição geral; uma sensação de aleatoriedade temperada com uma ordem primordial. Pode-se, talvez, compará-lo com certas estruturas na natureza, como redes de neurônios no cérebro ou uma malha de filamentos de proteína.

Interessantemente, como ressaltado na pesquisa, o Modelo dos Vizinhos Mais Próximos é o mais forte em termos de compatibilidade com as reais características da teia cósmica observadas. Enquanto, obviamente, existe muito mais de ciência para se explorar sobre esses resultados, a artista visual dentro de mim está satisfeita com sua face poética e simples. Aquilo que parece mais belo para os olhos humanos - e que de perto e de fato lembram coisas das quais o próprio humano é composto - também é a verdade.

Where Everything Is in the Solar System, Right Now

NASA knows, and it maintains active archives of these data. Here are maps for the positions of known natural objects in the inner, outer and distant solar system in January 2016

• By Caleb A. Scharf

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Credit: P. Chodas, NASA/JPL

Among the many wonderful things that NASA does, it maintains open-access scientific data on the positions and motions of known natural objects in the solar system. The solar system dynamics website at the Jet Propulsion Laboratory (and Caltech) also posts regularly updated visual maps of the locations of planets, numbered asteroids (over some 450,000 at present), and comets. 

So, want to know what's out and about right now, in January 2016? Here are the maps - first showing the view looking 'down' onto the ecliptic plane from a vantage above over the northern solar pole, then looking 'sideways' from a vantage point in the ecliptic plane. 

In each set of 3 maps we zoom away from the inner solar system, then see just the outer solar system (without plotting the inner stuff), and finally the 'distant' solar system of Kuiper-belt, or trans-Neptunian objects.

Bear in mind that these are just the known objects. Also bear in mind that as cluttered as our solar system appears, the individual bodies are absolutely miniscule compared to the gulfs of empty space that separate them. Even in the inner asteroid belt there are, on average, millions of kilometers between the larger members, and you could fit all the major planets end-to-end in the distance between the Earth and the Moon (a scale not even resolved in these maps).

INNER SOLAR SYSTEM FROM ABOVE. YELLOW DOTS INDICATE ASTEROIDS, WHITE 'ARROWS' POINTING SUNWARD INDICATE COMETS (CREDIT: P. CHODAS/NASA/JPL)

OUTER SOLAR SYSTEM, SYMBOLS AS ABOVE (CREDIT: P. CHODAS NASA/JPL)

DISTANT SOLAR SYSTEM (CREDIT: P. CHODAS NASA/JPL)

And now the same 3 scales, but viewed 'edge on':

INNER SOLAR SYSTEM, EDGE-ON (CREDIT: P. CHODAS NASA/JPL)

OUTER SOLAR SYSTEM, EDGE-ON (CREDIT: P. CHODAS NASA/JPL)

DISTANT SOLAR SYSTEM, EDGE-ON (CREDIT: P. CHODAS NASA/JPL)

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

Comets May Not Explain "Alien Megastructure" Star's Strange Flickering after All

It's looking less likely that a swarm of comets or an "alien megastructure" can explain a faraway star's strange dimming.

The star (nicknamed "Tabby's Star," after its discoverer, Tabetha Boyajian) made major headlines last October when Jason Wright, an astronomer at Pennsylvania State University, suggested that it could be surrounded by some type of alien megastructure. A more likely idea — one that's far less exciting — is that the star is orbited by a swarm of comets. But scientists can't be sure either way.

Now, Bradley Schaefer, an astronomer at Louisiana State University, has probed the star's behavior over the past century by looking at old photographic plates. Not only does the star's random dipping date back more than a century, but it also has been gradually dimming over that period — a second constraint that makes it even harder to explain. [13 Ways to Hunt Intelligent Alien Life]

The first signs of the star's oddity came from NASA's planet-hunting Kepler space telescope, which continually monitored the star (as well as 100,000 others) between 2009 and 2013. Astronomers, citizen scientists and computers could then search for regular dips in a star's light — a sign that an exoplanet has passed in front of that star. The largest planets might block 1 percent of a star's light, but Tabby's star dropped by as much as 20 percent in brightness. That, in and of itself, would be weird. But the periodic dimmings didn't occur at regular time intervals, either — they were sporadic. The signature couldn't be caused by a planet, scientists said.

In September, a team led by Boyajian, a postdoctoral fellow at Yale University, tried to make sense of the unusual signal. First, the researchers looked into any angles that might mean there was something wrong with the data itself. They even checked in with Kepler mission scientists. But everything came out clean. "The data that we were observing with Kepler is, in fact, astrophysical," Boyajian told Space.com.

Still, nothing about the observations indicated what might be causing the extreme interference. After considering many possible scenarios, Boyajian determined that dust from a large cloud of comets was the best explanation. But she admits that "it's a bit of a stretch to have comets that are large enough to block that much of the light from the star." With her paper published, she hoped that other astronomers would jump in with alternative solutions.

And they did. A month later, the star exploded into the public's eye when Wright announced that an advanced extraterrestrial civilization could be responsible for the signal, assuming this civilization built a megastructure, like solar panels, around the star. And Boyajian thinks the theory is definitely worth a follow-up.

"We have to look at every angle that we can — and that's one angle, as wild and crazy as it seems," she said. Slate blogger and astronomer Phil Plait, too, admitsthat "while it's incredibly unlikely, it does kinda fit what we're seeing."

A follow-up looking for alien signals, however, turned up empty-handed.

So Schaefer turned to old photographic plates from the Harvard College Observatory. Lucky for him, the star has been photographed more than 1,200 times as part of a repeated all-sky survey between the years 1890 and 1989. That many data points revealed that Tabby's star is acting strangely in more than one way: It's flickering on short timescales, as the Kepler and Harvard data show, and it's dimming over the course of a century, as the Harvard data show.   

"Occam's razor [the simplest explanation is likely the best one] needs to be considered in a scenario like this," Boyajian said. A single phenomenon must be causing both behaviors, she added. But what is it?

Well, the results don't look good for a family of comets. It would take a vast number of comets to pass in front of the star for a century, astronomers say.

"It would be more mass than what we have in the whole Kuiper Belt" [the band of icy bodies in the vast region beyond Neptune], said Massimo Marengo, an associate professor of astronomy at Iowa State University who co-authored a paper supporting the comets theory in December.

"You can get out of that if you assume it's the same family of comets passing in front of the star over and over," Marengo told Space.com. But with the century-long dimming trend, too, that family of comets has to get bigger every time it passes the star. "It's a difficult thing to do," he said.

The results also change the requirements for the alien megastructure hypothesis. Plait pointed out that the general fading is actually what you'd expect to see if aliens were building a massive sphere around their star. But before you get your hopes up, consider this: Plait calculated that aliens would need to build a minimum of 750 billion square kilometers (290 billion square miles) of solar panels to account for the 20 percent drop in their star's brightness. "That's 1,500 times the area of the entire Earth," Plait wrote. "Yikes."

So astronomers now have to hope that future observations might shed light on this stellar oddity. "Nature can help us by creating another one of these events," Marengo said. "But sometimes, we don't get lucky."

The Fermi Paradox Is Not Fermi's, and It Is Not a Paradox

Two big ideas often come up in discussions about the search for extraterrestrial intelligence, or SETI. One is the Drake Equation, which estimates the number of civilizations in our Galaxy whose signals we might be able to detect—potentially thousands, according to plausible estimates. The other is the so-called Fermi paradox, which claims that we should see intelligent aliens here if they exist anywhere, because they would inevitably colonize the Galaxy by star travel—and since we don’t see any obvious signs of aliens here, searching for their signals is pointless.

The Drake Equation is perfectly genuine: it was created by astronomer and SETI pioneer Frank Drake. The Fermi paradox, however, is a myth. It is named for the physicist Enrico Fermi—but Fermi never made such a claim.

I’d like to explain why the so-called Fermi paradox is mistaken, based on my deep-dive research on the topic, because this mistake had inhibited the search for E.T., which I think is worthwhile. It was cited by Sen. William Proxmire (D-WI) as a reason for killing NASA’s SETI program in 1981; the program was restarted at the urging of Carl Sagan, but was killed dead in 1993 by Senator Richard Bryan (D-NV). Since then, no searches in the U.S. have received government funds, even though thousands of new planets have been discovered orbiting stars other than our sun.

Enrico Fermi, a Nobel prizewinner who built the first nuclear reactor, never published a word on the subject of extraterrestrials. We know something about his views because physicist Eric Jones collected written accounts from the three surviving people present at a 1950 lunch in Los Alamos where the so-called Fermi paradox had its roots: Emil Konopinski, Edward Teller, and Herbert York (Fermi died in 1954).

According to these eyewitnesses, they were chatting about a cartoon inThe New Yorker showing cheerful aliens emerging from a flying saucer carrying trash cans stolen from the streets of New York City, and Fermi asked “Where is everybody?” Everyone realized he was referring to the fact that we haven’t seen any alien spaceships, and the conversation turned to the feasibility of interstellar travel. York seemed to have had the clearest memory, recalling of Fermi:

“... he went on to conclude that the reason that we hadn’t been visited might be that interstellar flight is impossible, or, if it is possible, always judged to be not worth the effort, or technological civilization doesn’t last long enough for it to happen.”

Both York and Teller seemed to think Fermi was questioning the feasibility of interstellar travel—nobody thought he was questioning the possible existence of extraterrestrial civilizations. So the so-called Fermi paradox—which does question the existence of E.T.—misrepresents Fermi’s views. Fermi’s skepticism about interstellar travel is not surprising, because in 1950 rockets had not yet reached orbit, much less another planet or star.

If Fermi wasn’t the source of this pessimistic idea, where did it come from?

The notion “... they are not here; therefore they do not exist” first appeared in print in 1975, when astronomer Michael Hart claimed that if smart aliens existed, they would inevitably colonize the Milky Way. If they existed anywhere, they would be here. Since they aren’t, Hart concluded that humans are probably the only intelligent life in our galaxy, so that looking for intelligent life elsewhere is “probably a waste of time and money.” His argument has been challenged on many grounds—maybe star travel is not feasible, or maybe nobody chooses to colonize the galaxy, or maybe  we were visited long ago and the evidence is buried with the dinosaurs—but the idea has become entrenched in thinking about alien civilizations.

In 1980, the physicist Frank Tipler elaborated on Hart’s arguments by addressing one obvious question: where would anybody get the resources needed to colonize billions of stars? He suggested “a self-replicating universal constructor with intelligence comparable to the human level.” Just send one of these babies out to a neighboring star, tell it to build copies of itself using local materials, and send the copies on to other stars until the Galaxy is crawling with them. Tipler argued that absence of such gizmos on Earth proved that ours is the only intelligence anywhere in the entire Universe—not just the Milky Way galaxy—which seems like an awfully long leap from the absence of aliens on our one planet.

Hart and Tipler clearly deserve credit for the idea at the heart of the so-called Fermi paradox. Over the years, however, their idea has been confused with Fermi’s original question. The confusion evidently started in 1977 when the physicist David G. Stephenson used the phrase ‘Fermi paradox’ in a paper citing Hart’s idea as one possible answer to Fermi’s question. The Fermi paradox might be more accurately called the ‘Hart-Tipler argument against the existence of technological extraterrestrials’, which does not sound quite as authoritative as the old name, but seems fairer to everybody.

As for the paradox, there is none, even in Hart’s and Tipler’s arguments. There is no logical contradiction between the statement “E.T. might exist elsewhere” and the statement “E.T. is not here” because nobody knows that travel between the stars is possible in the first place.

The Hart-Tipler argument, cloaked in the authority of Fermi’s name, has made some people pessimistic about the chances for success in SETI. But the suggestion that we should not look for intelligent life elsewhere because we don’t see aliens here is simply silly. There are some signs that the pessimism is lifting, most notably Yuri Milner’s privately fundedBreakthrough Listen project, which promises to contribute $100 million in funding over ten years. But searching millions of stars for signals at unknown frequencies might take more resources. Our searches typically ’see’ a spot on the sky no bigger than the Moon at any moment, which is only a tiny fraction of the sky. If we want to find something interesting in our era, we might need to look harder.