If an alien civilization wanted to study planet Earth, how might they do it? They could use powerful telescopes to measure the physical characteristics of our planet, or they could listen for signals from our TV and radio broadcasts. These are things we are doing in our search for alien civilizations. But a really advanced alien civilization might try something a bit more ambitious, such as an actual mission to Earth. One way to do this would be to build a probe within an asteroid, and send it on a journey across the stars. The asteroid could shelter the probe during it’s long trip through interstellar space. Once it arrived in our solar system, the probe could gather detailed information about Earth and the solar system. It might even try to communicate with humans by beaming a radio signal in Earth’s direction. Such an alien probe would look a lot like the recently discovered asteroid Oumuamua, which is why the Breakthrough Listen project wants to study it.

Oumuamua was discovered in October by the Pan-STARRS 1 telescope. Unlike any other asteroid, Oumuamua has an interstellar orbit. It is moving through our solar system so quickly that could not have originated in our solar system. Based on its trajectory, it came to our solar system from the general direction of the star Vega. Coincidentally, Vega is star aliens first communicated with humans from in Carl Sagan’s novel Contact. In addition to being the first confirmed interstellar object to enter our solar system, it also has a highly unusual cigar shape, with a length about 10 times longer than its width. Add to this the fact that Oumuamua made a relatively close approach to Earth, within 15 million miles of our planet, and it begins to look a bit alien.

Odds are this asteroid is just a chance visitor to our system. We’ve known that some asteroids can escape the solar system through close flybys of large planets like Jupiter, so it makes sense that asteroids from other star systems could travel between the stars. Such interstellar visitors might be rare, but they don’t require aliens to send them on their way. But Breakthrough Listen is interested in finding evidence of alien civilizations, no matter how long the odds. So when Oumuamua was discovered, it made for a promising target.

Oumuamua is currently about 2 astronomical units away from Earth. About twice as far as the Earth is from the Sun. That’s still much closer than many of the probes we’ve sent into space, such as Cassini and New Horizons. If it is an alien probe sending radio transmissions we should be capable of detecting them. So Breakthrough Listen will use the Green Bank Telescope to look for any evidence of alien technology, searching across four radio bands, from 1 to 12 GHz, for a total time of about 10 hours. If an alien probe wants to be detected, that’s a good frequency range to search.

Just to be clear, the odds of Breakthrough Listen finding anything are really slim. Studies so far haven’t found anything that would imply an artificial origin. But even if Breakthrough Listen doesn’t find anything, their observations will add to those we already have, and help us better understand asteroids that are rare, but natural, alien visitors.

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Fast Radio Bursts (FRBs) are short, intense pulses of radio energy that originate billions of light years away. They have incredibly intense energies, but last for only milliseconds, so it isn’t clear what could possibly cause them. Ideas include a neutron star collapsing into a black hole, the collision of two neutron stars, and even an evaporating black hole. Another idea that makes the rounds is that they are produced by an advanced alien civilization. 

One idea is that perhaps FRBs are used as some kind of intergalactic navigation beacons, similar to the way we could use pulsars to navigate our galaxy. A more recent idea is that they could be created by aliens to send space probes to distant stars, similar to Breakthrough Starshot’s idea to use lasers to send a tiny probe to Proxima Centauri. Going directly from “we don’t know” to “therefore aliens” is the realm of science fiction not science, but team of astronomers recently did a bit more than wild speculation. They asked whether it was conceivably possible for such powerful signals to be created artificially.

In the recent paper, they noted that FRBs have characteristics similar to the types of energy beams that could be used to power large light sails. If FRBs are, in fact, being used to power starships, then they would likely be a long lasting beam of energy directed at the starship. We would see them as a short burst because beam would sweep past us as the transmitter and starship line up just the right way. Calculating the energy requirements for such a beam, the team found that a solar powered array about twice the diameter of Earth could collect enough power to create it, and a water-cooled system orbiting a star could transmit the beam without overheating. In principle, at least, alien FRBs appear to be simply a matter of powerful engineering and not exotic physics.

The team went further and estimated the size of a starship that such a beam could power. Rough calculations put the upper size at about a billion tons, or the mass of about 20 cruise ships. For humans that would mean about 40,000 passengers or so, which is plenty large enough to start a colony on another star system. Given that the alien civilization would be capable of making planet-sized power transmitters, you might figure they would have mastered other things like cryogenic freezing or the ability to clone new members of the species once their destination is reached.

This all sounds like wild science fiction, and it’s almost certainly not true. But the team does point some things worth exploring further. Given the number of FRBs we observe, they probably wouldn’t all be caused by alien civilizations. So there would likely be some key signature differences between natural and alien FRBs. In particular, we now know that some FRBs repeat, which means these particular ones can’t be caused by cataclysmic events such as neutron star mergers. Alien FRBs could repeat, since the orbit of the transmitter could bring it back into alignment with Earth periodically. By studying FRBs that repeat, we might be able to see some kind of pattern that points to an artificial source.

There’s a long history of strange astronomical phenomena that seem alien at first, but turn out to be natural after all. FRBs will likely turn out to be natural as well. But it can be worthwhile to cautiously speculate about alien signals. After all, there are a lot of planets out there, and the existence of alien civilizations isn’t beyond the realm of possibility.

Paper: Manasvi Lingam and Abraham Loeb. Fast Radio Bursts from Extragalactic Light Sails.  arXiv:1701.01109 [astro-ph.HE] (2017)

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Nearly 1,500 light years away is a strange and mysterious star. It’s behavior seems to defy explanation, leading some to conclude its appearance could be due to an advanced alien civilization. Is the alien conclusion just clickbait hype, or is it a legitimate answer? 

As scientists we really don’t like jumping on the alien bandwagon. Regular radio pulses in space! Aliens? No, just rapidly rotating neutron stars. Strong unexplained radio bursts! Aliens? No, just a microwave oven. The Wow! signal, radioactivity on Mars, it could be aliens, right? Probably not. That’s not to say that aliens could never be the answer to strange astronomical behavior, it’s just that it’s bad form to use aliens as a filler for “I don’t know.” That said, KIC 8462852, commonly called Tabby’s star after its discoverer Tabetha Boyejian, continues to defy clear explanation, so the aliens conclusion keeps lurking in the background.

Whenever we have a mystery like this, the best approach is to be cautious and follow where the evidence leads. Even if the evidence doesn’t support a clear solution, it can at least rule out some of the options. So what does the evidence say?

One point of evidence is what originally drew our attention to Tabby’s star in the first place. KIC 8462852 is an F-type star, about 45% more massive than our Sun. It is one of the stars the Kepler spacecraft has studied, looking for exoplanets by watching the star dim as a planet transits the star. A transiting planet typically only dims the star by a few percent at most, and that’s typically for planets orbiting small red dwarfs. For larger stars like our Sun and Tabby’s star, the dip in brightness should be even less. If a distant civilization were to see Jupiter pass in front of our Sun, the Sun’s brightness would only decrease by about 1%.

It turns out that while Jupiter is not the most massive a planet can be, it is about as large as a planet can be. With more mass comes more gravity, so planets more massive than Jupiter are about the same size, just more dense. Since Tabby’s star is a bit larger than our Sun, we would expect a transiting planet to cause a dimming of no more than about 1%. Kepler observed brightness dips that look like planetary transits, but were often much more than a few percent. An orbiting planet couldn’t create such large dips.

Kepler data for Tabby’s star.

It is possible for such large dips in brightness to be caused by an interstellar body to pass between us and the star. If a rogue planet passed much closer to us than the star, then it could cause a significant dimming while still being planet-sized. But we’ve observed is multiple large dips. One rogue planet transiting the star would be rare enough, but the odds of several in succession is basically zero. So that likely doesn’t explain things either. What’s more perplexing is that there are also dimming events that don’t agree with transits at all. Rather than a sharp dip, the star shows some longer term variations in brightness, as if it is being obscured by some type of dust cloud or protoplanetary disk. The problem with that idea is that KIC 8462852 is well into the main sequence stage, so a protoplanetary disk is unlikely. Infrared observations of the star also find no evidence of a protoplanetary disk.

So not a planet, not an odd interstellar transit, and not a protoplanetary dust cloud. What else could it be? The leading contender has been a large swarm of comets. The data could be explained by clusters of large comets with a total mass roughly equal to that of Ceres. So if an object similar to Ceres was tidally ripped apart by some close planetary encounter, and fragmented into lots of large comets, it might, just might, be able to explain the data. If you think that isn’t a very likely answer you’d be right. It’s really the least unlikely of a range of wild ideas such as a complex ring system, a catastrophic collision of planets, or even that the star is strangely variable.

The brightness of Tabby’s star found in the Harvard data.

Clearly what’s needed is more data. So off scientists went to get some. A SETI radio search of the star turned up nothing, infrared searches didn’t find anything strange, but an examination of historical records did. It turns out that photographs of KIC 8462852 have been captured since 1890. The Harvard College Observatory has photographic plates spanning more than a century, and these have been digitized and put online. Tabby’s star happens to be in this data, and when the brightness of the star is plotted over the years it shows a gradual dimming over time. This is really strange because main sequence stars should not show this kind of dimming effect. It’s also a very controversial result. While the data is clear, it isn’t clear whether it shows an actual dimming of the star, or if it’s an artifact of the way photographs were taken over the years. The brightness of some other stars in the historical data also show some dimming effect, so it could easily be due to poor calibration of the photographic plates. However, an analysis of the average Kepler data also shows a slight overall dimming consistent with the dimming trend.

So why not admit it could be aliens?

In all honesty it could be aliens, but even that seems to be contradicted by evidence. If the dimming were caused by some kind of alien superstructure, then the blocked light being captured would emit some of that energy as heat. Based on the data, the alien superstructure would be absorbing about 20% of the star’s light, and that would generate a great deal of waste heat. So there should be an excess of infrared light coming from the star, but observations have found no significant infrared excess.

That doesn’t mean aliens are ruled out, but they aren’t any more compelling than some strange natural phenomena. In short, it’s a mystery, and that’s definitely worth studying further.

Paper:  T. S. Boyajian, et al. Planet Hunters IX. KIC 8462852 – where’s the flux? MNRAS 457 (4): 3988-4004. (2016) doi: 10.1093/mnras/stw218

Paper: Eva H. L. Bodman, Alice Quillen. KIC 8462852: Transit of a Large Comet Family. The Astrophysical Journal Letters, Volume 819, Number 2 (2016) arXiv:1511.08821 [astro-ph.EP]

Paper: Bradley E. Schaefer. KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989. arXiv:1601.03256 [astro-ph.SR] (2016)

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The Wow! signal is one of the great mysteries of radio astronomy.  It was detected in 1977 by the Big Ear radio telescope, and is so named because of its powerful signal. It’s origin is unclear, but one possible explanation is that is was an intentional signal from an alien intelligence.

Big Ear was a drift telescope, and used the rotation of the Earth to scan the sky for radio signals. It was designed to run for long periods autonomously as a way to scan the heavens. The Wow! signal came from a specific region of the sky, and emitted a strong signal at 21 cm wavelengths, which is an emission light produced by atomic hydrogen. It was observed for 72 seconds, which is how long it would take a specific point in the sky to drift across the range of the telescope.

One interesting aspect of the signal is that it doesn’t clearly originate from a known object. The area of the sky where the signal originated doesn’t have anything that would produce a strong hydrogen line. But new work suggests that back in 1977 there was something there, possibly two somethings.

Between the end of July 27 and mid August of 1977, two comets known as 266P/Christensen and P/2008 Y2 (Gibbs) were in the vicinity of the Wow! signal location. Comets are known to emit gas and dust, including monotomic hydrogen. So there may have been a hydrogen cloud in the region during that time. The Wow! signal was detected on August 15, 1977.

While this is not the definitive answer, it would explain some of the strange aspects about the signal, such as why later observations of the region didn’t detect any signal. Since the comets had moved on, any hydrogen cloud would have dispersed and any signal from it would have faded.

So there’s no need for aliens after all.

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The figure below shows two maps of Mars. The top shows the distribution of radioactive potassium, and the bottom shows the distribution of thorium. The images were taken from data gathered by the gamma-ray spectrometer on the Mars Odyssey mission. You might notice that on both of these maps there is a concentration of radioactive materials at about latitude 50, longitude -30. What could be the cause of that concentration? According to a 2011 presentation at the Lunar and Planetary Science Conference (pdf), this concentration is evidence of a natural fission reactor on Mars…

Credit: NASA/JPL/University of Arizona

We know, for example that such a natural reactor has occurred on Earth. About 1.7 billion years ago in the Oklo region of what is now the Gabonese Republic. The reactor lasted for a few hundred thousand years, and left a distinctive signature of radioactive isotopes in the surrounding rocks. The original paper is behind a paywall, but you can see a summary of the findings in Scientific American.

According to the conference paper, the conditions for a similar reaction existed on Mars, and given the concentration of potassium and thorium it clearly occurred. Furthermore, since the highest concentration is located in a local depression, it is likely that the fission site actually exploded, which would explain the concentration of radioactive elements on the Martian surface.

It’s a cool idea if it were right. But it’s not.

There is no real evidence to support the idea that a natural fission reactor occurred on Mars.

None.

If you do a search for natural fission reactors on Mars, you’ll find lots of articles and blog postings presenting it as a real scientific discovery. It was reported on space.com, Fox News, NBC News, etc. It is the type of finding that makes great press. Of course you’ll also find more speculative websites that claim it is evidence of an ancient nuclear explosion caused by aliens.

There’s a meme where Giorgio Tsoukalos (the guy with interesting hair) is quoted as saying “I don’t know … therefore aliens.” It’s an example of how you can take bits of data and construct a wild (and wildly wrong) hypothesis. Aliens did not build the pyramids, nor did they have a Chernobyl incident on Mars. It is easy to mock Ancient Aliens, but it also serves as a cautionary tale for popular science.

On the face of it, the claim of natural Martian reactors seems reasonable. The image above is taken from real scientific data. There really is a concentration of potassium and thorium in that region of Mars. There really was a natural fission reactor on Earth a couple billion years ago. The evidence for it is quite solid. The conference where the paper was presented is a serious and respected conference. And the paper itself (at least on the surface) doesn’t seem to be making a wild claim. So where’s the problem?

Actually there are several issues. The first is that the paper itself is not particularly good. If you read it, you’ll notice it takes the basic evidence (isotope concentrations) and uses a lot of phrases such as “it is likely” “could explain” “there is no reason why this could not have happened”. There’s a lot of focus on conclusions, but not much on supporting evidence. The paper presents a claim, not a result.

You see this sometimes in conference papers, which is the second issue. Many conference papers are not peer reviewed (though some are). This doesn’t mean conference papers are bad science (most are quite good), nor does it mean that peer reviewed papers are always right. But passing peer review means that glaring flaws or weaknesses are likely to be caught. This conference paper, for example, would not pass peer review for the reasons above.

Of all the popular press articles written about the Mars reactor idea, none of them cite a peer reviewed paper. It’s a common problem, because most of the stories in the popular press come from press releases, with print-ready graphics and the results clearly summarized. Science writers are pressed for time and have deadlines to meet. You can make their job easier by pre-packaging things in a press release. And the easier you make it for the science writer, the more likely they are to write about your research.

Even if you don’t have the time or inclination to look at original peer reviewed research, a good way to filter sense from nonsense is to see whether the popular article gives a link or clear reference to the original peer reviewed article upon which it’s based. Not just a reference to “Reputable Journal” or “Prestigious Institution”, but the actual paper. Science writers who do this have taken the time to track down the actual paper (not just the press release), and it’s quite likely they’ve at least scanned the original article. When you start looking for such a reference you learn pretty quickly which writers and websites strive for accuracy, and which ones simply strive for pageviews. If you’re interested in science, you can ignore the latter ones.

Some of you may still be asking “But what is up with the concentration of radioactive isotopes on Mars?” If you look at the graph again, you might notice that not only is there a local high concentration, there are also regions with low concentrations. Of course the scale of these graphs is low to high. Nowhere does it say what low or high actually means. To find that out, you’d need to go to the source data. While I can’t track down the specific range, I do know it isn’t large. The isotope concentration on Mars might be interesting, but it isn’t a radiological hot spot.

No need for aliens after all.

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One of the big questions about the universe is whether there is intelligent life “out there”. We know that life evolved here on Earth, so it seems possible that similar life could evolve on other worlds. Whether they would survive and evolve what we would consider intelligence is another matter. There have been some estimates made on just how likely this might be, such as through the Drake equation. There is a lot about these estimations that are purely speculative, but we do know that Earth-like planets (at least in terms of size and temperature) are likely very common. We also know the type of chemical elements life on Earth relies upon are common, and that life appeared on Earth relativity early in its history.

It would seem that life should be common in the cosmos. If our planet is a typical example, it would also seem that intelligence in the universe is fairly common. But this raises an interesting question as to where all these intelligent life forms are. It has taken only a few million years for humans to evolve from Australopithecus to astronaut, which is a mere moment of cosmological time. In another few million years humans could be walking among the galaxies. If a similar species is a few million years ahead of us, then why don’t we see evidence of them? This is sometimes referred to as the Fermi paradox, and there are lots of proposed solutions. One idea is that the evidence is there, we just haven’t noticed it yet. If these star-walking aliens haven’t taken an interest in us, evidence of their presence might be hard to spot. Recently, a paper in the Astrophysical Journal has proposed a way to detect powerful civilizations.

The work is based upon an idea first proposed by astronomer Nikolai Kardashev in 1964. His idea was that as civilizations become more advanced, they require increasing amounts of energy. This means you can rank civilizations by their energy consumption, now known as the Kardashev scale. Type I are civilizations that harness the resources of their home planet, such as humans today. Type II harness almost the full energy of their home star, possibly through technology such as Dyson spheres. Species within the Star Trek universe would typically be Type II. Type III are civilizations that can harness the energy of an entire galaxy, such as the Asgard of the Stargate universe.  Carl Sagan generalized the Kardashev scale to a general function of energy, rather than discrete steps, and showed that Earth is roughly at level 0.7.

Naturally all this is very speculative, but if advanced civilizations do use star-levels of energy they should emit an infrared signature of waste heat, which is where this new paper comes in.  The authors looked at the infrared sky survey from WISE and compared it with modeled infrared signatures of advanced civilizations. The one thing they assumed is that the civilizations are emitting waste heat through physics as we currently understand it. What they showed is that WISE data excludes any clear presence of type III civilizations within our corner of the galaxy. So either there aren’t any Asgard-type aliens in our neighborhood, or they’ve learned to cover up their heat signatures.

It’s important to keep in mind that this work still makes a number of assumptions about alien life, and we currently have no evidence of life elsewhere in the universe. But what’s interesting about this kind of research is that it shifts a science-fiction idea out of the realm of pure speculation and toward the realm of real observation. We are reaching a point where claims about alien life can be tested scientifically, which is kind of cool.

Paper: J. T. Wright et al. The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result. ApJ 792 27 (2014).

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Aliens are probably the most common topic of science fiction. They are typically an extension of our hopes and fears. Wise parental figures, evil enemies, noble savages, fierce predators. They are often physically quite similar to us, with a bipedal gait, opposable thumbs,etc. We dream of life on other worlds. Reaching out to the stars and meeting an alien intelligence. But is that likely, or even possible.

The difficulty with that question is that we currently only have one example of life in the universe, and that is us, the commonly descended family of life on Earth. Beyond that, there is a quite a bit of guesswork. It is at this point that the Drake equation is often invoked.

The Drake equation is often interpreted as a way to calculate the number of intelligent civilizations in the galaxy. It was originally proposed at the first SETI conference by Frank Drake in 1961 as a way to stimulate discussion. Drake did not intend it as a prediction of the correct value, but more as a “what if” to consider.

The equation itself is basically a product of the rate at which stars form in our galaxy, how many stars have planets, how many planets they typically have, what fraction are habitable, what fraction of habitable planets form life, how many form intelligent life, then civilizations, and how long those civilizations last.

When it was first proposed there weren’t any known extrasolar planets. We now know that planets are quite common, and stars are more likely to have planets than not. Current estimates calculate that about 1 in 3 Sun-like stars have terrestrial planets in their habitable zone. That means there may be 100 billion potentially habitable planets in our galaxy alone.

Of course “potentially habitable” doesn’t mean “has life”, only that it has the right temperature and orbits a stable star. This is where we have to start speculating. For life similar to ours there needs to be liquid water on the planet. Earth has liquid water, and we know Mars had liquid water in its youth, but we don’t know how likely it is for a potentially habitable planet to have liquid water.

If there is liquid water on a planet of the right temperature, how likely is it that life will appear? On Earth we know that life appeared quite early in its history. This hints that life is fairly likely to appear, but with only an example of one, we can’t read too much into it. Early Mars had liquid water, and even if life does or did exist on Mars it is not robust, which could indicate early life is quite fragile. Again we’re faced with a lack of information.

When life appears on a planet, how often does intelligent, technological life appear? It did on Earth, but does that mean civilizations are a near certainty, or are we the product of extraordinary circumstances. And how long does a technological species survive? How long do you think human civilization will survive? Decades? Millenia? Eons?

So while habitable planets appear to be common, we don’t know anything about how common life might be. If we assume that Earth is a relatively average terrestrial planet, then it would imply that life exists on billions of worlds. One would expect at least some of them lead to technological civilizations, so there could easily be hundreds or thousands of civilizations in our galaxy alone.

This leads to a bit of a puzzle, since one would assume that a technological civilization would eventually start exploring the stars. Assuming humanity survives for a million years, it would seem likely that we will explore at least a portion of our galaxy. If not ourselves, then through our robotic proxies. If we are typical, then there are civilizations a million years behind us technologically, and ones a million years ahead. So if civilizations are common, then why haven’t they made contact with us? (Yes, there are those who think they have, and apparently are highly interested in our body cavities, but there’s no evidence for that.)

This is known as Fermi’s paradox. If intelligent life is common, then why don’t we see it? Several solutions have been proposed. Perhaps civilizations have a very short lifespan. Once they are capable of space travel they nuke themselves, or pollute their planet, or form an idiocracy and go extinct. Maybe there is a vast galactic civilization, but contact with Earth is forbidden until we’ve proved our worth. Maybe we’re in the cosmic equivalent of the outback, and no one has happened to stop by.

Or maybe we’re the only civilization in the universe. Perhaps Earth is extraordinarily rare. Perhaps the appearance of life, much less intelligent life, requires such an improbable chain of events that Earth may be the only example in the universe.

As it stands, we have only a single example of life in the universe. Only one planet in the universe with an extraordinary diversity of creatures. One example in a galaxy of billions of stars, in a universe of billions of galaxies.

If we were to find a distant planet like Earth we would be awed by its complexity and beauty. We would long to communicate with its intelligent species, and learn about its diverse culture.

Look around, make contact.

Miss the beginning of the series? You can find it here.

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While fantastical stories have been with us as long as we’ve been human, in the early 1800s a new type of story appeared. Often Mary Shelly’s Frankenstein is named as the first example of this genre. Also known as The Modern Prometheus, it gives us the tale of a mad scientist who creates a creature from alchemy and science. By the late 1800s H. G. Wells wrote tales of The Time Machine, and an alien invasion with The War of the Worlds, and Jules Verne gave us adventure stories of an atomic powered submarine in 20,000 Leagues Under the Sea, and the first astronauts in From the Earth to the Moon.

It’s not surprising that the earliest works of science fiction were about time travel, space aliens and starships. We love dreaming of new horizons, and science fiction can create entire tales from the the mere whiff of scientific possibility. This can be both a blessing and a curse. On the one hand science fiction lets us explore the possibilities, and can inspire an interest in science. On the other hand, the trappings of science fiction can make awesome feats of human engineering seem trivial. Who cares if we can land a rover on Mars when science fiction lets us travel to the stars. On the gripping hand, many of the concepts of science fiction are now deeply rooted in our culture. The once futuristic idea of touch screens and verbally asking your shipboard computer for information on a particular star system are now very real. Even some of science fiction’s wildest ideas like teleportation and quantum computers are at the cutting edge of real science.

But some common ideas from science fiction are based only on the most tenuous science. While we all know what time machines, warp drive and wormholes are in the context of science fiction, scientifically these ideas are speculative at best. They also happen to show up all the time in popular science articles. You’ve likely come across the “NASA working on warp drive” articles, or “scientist wants to build time machine”. Popular media often portrays them as merely engineering challenges rather than speculative science. They are also ideas I get asked about all the time.

It’s because I’m so often asked about these topics that I decided to do this particular series. I’ll focus on the ones with a connection to astronomy and astrophysics. So the topics for this week are

• Time Travel
• Warp Drive
• Wormholes
• Ansibles
• Parallel Universes
• Aliens

I’ll focus on what we know both experimentally and theoretically. We’ll go to the very edge of current science, but separate scientific possibility from wild fantasy.

Starting tomorrow: Time travel. Is it really possible to go back in time? If your Mom falls in love with Calvin Klein instead of your Dad, does that mean you’ll cease to exist? Allons-y!

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