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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Your spacecraft is speeding toward Alpha Centauri at nearly 5% the speed of light. At that speed, your 95 year-long journey from Earth would end in a flyby lasting only a day or so. You’d like to stay a while, but to do that you have to slow down. How do you get the job done? 

Traveling the great distances of interstellar space will always have a tension between time and speed. The faster you go, the less time it takes to get there, but greater speed demands more energy, and all that speed has to be gotten rid of if you want to visit a while.

To date, the fastest spacecraft we’ve ever launched was New Horizons, which had a launch speed of about 58,000 km/hr. Even after getting a gravitational boost during it’s flyby of Jupiter, New Horizons approached Pluto with a speed of only 48,000 km/hr.

Voyager 2’s heliocentric velocity vs it’s distance. Credit: Wikipedia user Cmglee (CC BY-SA 3.0)

The fastest spacecraft to leave our solar system is Voyager 2, which used gravitational flybys of the four outer planets to leave our solar system with a speed of about 60,000 km/hr. At that speed a trip to Alpha Centauri would take about 78,000 years. Even then, a spacecraft such as Voyager would need to make another series of gravitational flybys to slow down once it arrived. While we know at least one planet orbits the smallest member of the Alpha Centauri system, Proxima Centauri, we don’t know of any planets large enough to slow a spacecraft down.  Without a gravitational assist, the alternative would be to bring fuel on the long journey. Unfortunately the amount of fuel you would need to slow down at the end of your journey is about the same as the amount of fuel it took to speed up in the first place. Your original launch would then have to accelerate both your spacecraft and the fuel it carries, making the whole thing unfeasible.

Part of the problem with sending a Voyager-type spacecraft to another star is its mass. Voyager 2 had a mass of over 700 kg, for example. But an alternative approach favored by the Breakthrough Starshot project is to send extremely light spacecraft, with a mass on the order of grams. Such spacecraft would be accelerated to a few percent of light speed by an array of lasers. We can’t currently build such small starcraft, but if we could, we might be able to slow down the spacecraft by the light of Alpha Centauri itself.

A new paper published in the Astrophysical Journal outlines the idea. Since light exerts a tiny amount of force on anything it lands upon, a small craft with a large enough sail could use light to slow down. The idea has been around for a long time, but this new work demonstrates that a craft the size and mass proposed by Breakthrough Starshot could orient itself in such a way to use the light of Alpha Centauri A and B to both slow down and change its trajectory toward Proxima Centauri. Instead of taking 95 years to make a quick flyby of the Proxima system, it would reach Alpha Centauri A in 95 years, swing past Alpha Centauri B, and make a slow 46 year journey to Proxima Centauri. Sure, the idea would increase the journey time by about 50%, but it would allow the spacecraft to stay for a while, gathering data until its systems failed.

Ideas like this are still in the earliest stages, but they do demonstrate that interstellar missions aren’t simply science fiction. Just as we once made our first tentative explorations of the solar system, in a century or two we may have tiny spacecraft orbiting a planet around another star.

Paper: Heller, R., & Hippke, M. Deceleration of high-velocity interstellar sails into bound orbits at Alpha Centauri. The Astrophysical Journal Letters, Volume 835, L32, DOI: 10.3847/2041-8213/835/2/L32 (2017)

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You’re the captain of a Federation starship, ready to seek out new life and new adventures. As you travel through the galaxy, how do you know where you are? How do you find your way home? 

On Earth, your position is given by latitude and longitude. They are measured in angles about the center of the Earth, where latitude is the angle north or south of the equator, and longitude is the angle east or west of the Prime Meridian, which runs through Greenwich, England. It’s fairly easy to determine your latitude, particularly in the Northern Hemisphere. Since the north star Polaris is almost directly above the North Pole, you can simply measure the angle of Polaris above the horizon, and that’s your latitude. You can also use a sextant to measure the altitude of the Sun above the horizon at noon, and calculate your latitude from that.

Using a sextant to measure latitude. Credit: Joaquim Alves Gaspar (CC BY-SA 2.5)

Longitude is much more difficult. Since stars rise and set over the course of a night and the night sky shifts over the course of a year, there isn’t a fixed reference point against which you can measure longitude. Instead, early navigators either had to compare the shift of stars with measured distances between cities. It wasn’t particularly accurate, and you can see that in early maps of Europe. Things got easier when Galileo discovered the moons of Jupiter. Their clockwork motion could be used as a celestial clock, and by comparing their motion to the rotation of the Earth cartographers finally had an accurate tool for measuring longitude. Unfortunately, this method wasn’t useful at sea, so it took the development of accurate clocks to bring accurate longitude to sea-faring vessels. Nowadays we can simply use the global positioning system (GPS). The GPS consists of more than 30 satellites that continually transmit their location and time. By picking up the signal of at least four of these satellites, your phone can triangulate your position on Earth.

Defining your position in the Milky Way can be done with galactic latitude and longitude. Simply define a galactic equator and a prime meridian, and determine your position relative to them. For galactic coordinates astronomers define the galactic equator (0° longitude) as the plane of the Milky Way running through its center. The prime meridian (0° latitude) is defined by a line running from the Sun to galactic center. In astronomy the sky can be treated as a celestial sphere, so the apparent position of a star can be given by its galactic coordinates. Your position in the galaxy could thus be given by three numbers: your galactic latitude, galactic longitude, and your distance from the Sun.

A map of the Milky Way using galactic coordinates. Credit: T. Jarrett (IPAC/Caltech)

The Star Trek universe is a bit fuzzy on the subject of galactic navigation, and there isn’t a definitively canon version. The most popular version is based upon the galactic coordinates astronomers use, with some slight differences. The prime meridian is still a line running from the Sun to galactic center, but rather than simply using galactic latitude and longitude, the Milky Way is divided into quadrants. Under this definition the Sun would lie on the line dividing the Alpha and Beta quadrant, and the Earth would cross between these two quadrants as it orbits the Sun. In the Next Generation episode “Relics” it is stated that Earth is in the Alpha quadrant, and is less than 90 light years from the Beta quadrant. That probably means the Alpha quadrant is extended around the Sun to put our local cluster of stars within the Alpha quadrant. We’ve done a similar thing with the international date line on Earth to ensure that countries aren’t divided by it.

Star Trek galactic quadrants based upon astronomical coordinates. Credit: NASA, with annotations.

Of course a coordinate system is only useful if you can determine what your coordinates are. Star Trek is again fuzzy on just how starships find their way. It’s possible that they calculate their position based upon some kind of stellar map, but identifying particular stars seems a bit impractical. It turns out there is an effective way to determine your position in the galaxy, and it’s one we’ve actually used.

It’s basically a galactic GPS. Neutron stars are dense old stars with strong magnetic fields. As a result beams of radio energy stream from their magnetic poles. As a neutron star rotates, those beams sweep across the sky like a lighthouse, and if their beams happen to point in our direction we see them as pulsing radio objects known as pulsars.

The plaque carried by Pioneer 10 gave the position of the Sun as reckoned by 14 pulsars.

The radio pulse pattern of each pulsar is unique, and we know there positions in the galaxy quite well. If your starship can detect enough known pulsars, you can use that information to calculate your position in the galaxy. So if the Enterprise drops out of warp unexpectedly, or if Q sends it to a strange part of the galaxy, the navigators just need to look for pulsars to find their way home. What’s interesting is that a neutron star’s rotation slows down over time, so older pulsars pulse more slowly than younger ones. This means you can use pulsars not only to determine your position in the galaxy, but also your stardate. It’s a useful trick when your plot of the week involves time travel.

This method has actually been used. Not to determine the position of a starship, but to give alien civilizations the location of our solar system. When Pioneer 10 was launched in 1972 it carried with it a plaque indicating 14 pulsar signals. The idea was that if an alien civilization were to come across Pioneer 10 as it traveled beyond the solar system, they would be able to determine just when and from where it was launched.

When it comes to the Star Trek universe, we still have to develop warp technology and subspace communication, discover wormholes, travel back in time to save the whales, and make contact with alien civilizations, but we already have a map to find our way.

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The new Star Wars movie Rogue One won’t arrive until December, but hype for the movie is already at a fever pitch. A new teaser poster has been released showing the Death Star looming over the horizon of an alien world. It makes for a foreboding shot, but could a Death Star really appear so large in the sky?

In the original Star Wars movie, the Death Star has the appearance of a “small moon.” The size of this Imperial superweapon isn’t specifically mentioned, but the technical specifications list its diameter as about 120 kilometers. That’s larger than the moons of Mars, but tiny compared to our own Moon, which has a diameter of about 3,400 km. If the Death Star orbited Earth at the same distance as our Moon, it would have the same apparent size as Venus at it’s brightest. In other words, it would look like a bright planet rather than a moon.

The Death Star looms large in the Rogue One poster. Credit: Disney/Lucasfilm

In the words of Master Yoda, “size matters not.” Or more accurately, size is only one factor among many. The key is what’s known as apparent size, which depends upon both the actual size of an object and its distance from you.  A small but close object can appear bigger than a larger object far away. So what if it’s simply a matter of the Death Star being close to the planet? While that would help, it wouldn’t solve all the problem. In the Rogue One teaser poster it looks like the Death Star spans about 40 degrees across the sky. With a bit of basic trigonometry we find it would need to be about 180 km away to have such a large apparent size. That’s closer than the International Space Station, and so close that atmospheric drag would be a serious problem.

So the Death Star can’t be so close it spans half the sky, but it could be close enough to appear larger than our Moon. For example, if the superweapon were 1,000 kilometers above the Earth, its apparent size would be about 8 times that of the Moon, making it by far the largest object in the sky. We would be able to see surface features of the Death Star such as those depicted in the poster. To our minds it would appear huge, but its actual apparent size would still be pretty small. The Moon itself has an apparent size of only half a degree. If you held your pinky up at arms length it would easily cover the Moon. Even if the Death Star had an apparent diameter 8 times larger, you could still cover it with two fingers at arms length.

The Moon appears to loom over ESO’s Very Large Telescope. Credit:
Author G.Gillet/ESO

While the Death Star couldn’t appear so large in real life, there is still a way to give it a deceptively large appearance. Photographers do it with our Moon all the time. The trick is to use a telephoto lens to focus on a distant object near the horizon, such as a building or tree line. The apparent angle of a distant building is small, but zooming in makes it look big. This also makes the Moon look much larger than it actually is. Using this trick the Death Star could be made to loom over a battlefield, as depicted in the poster.

In the Star Wars universe a good photographer might be able to such a shot after all.

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In the discussion of hypothetical time travel, someone almost always brings up Hitler. After all, if you’ve got access to a nuclear DeLorean or blue police box, who better to eliminate from the annals of time? In internet conversations, Godwin’s law is the observation that in any online argument someone will eventually bring up Hitler to make a point. “Yeah? You know who else liked vanilla ice cream? Hitler!” So when the New York Times decided to poll readers on whether they would kill baby Hitler, they enacted the Godwin’s law of time travel. It was only a matter of time.

The idea of killing Hitler is such a common trope in science fiction that the phrase “Everyone kills Hitler on their first trip.” has become a meme. From a physics standpoint, the challenges of killing Hitler are almost as big as the challenge of time travel itself. As I wrote about in an earlier post, there are metaphysical problems with the physics of time travel, and those would affect whether you could or should kill Hitler to prevent WWII.

One of the problems is the grandfather paradox, where if you go back in time to kill your grandfather before he has children, you wouldn’t have been born and couldn’t have killed your grandfather. The same paradox occurs with killing Hitler. If you eliminate baby Führer and prevent the rise of the Nazis, then you create a world where WWII didn’t occur, and thus you have no reason to travel back in time. Thus, Hitler survives to adulthood and we’re back where we started. Acting as a time-traveling executioner simply creates a paradox.

If time travel is self-consistent, then you can’t change history. You could, however, travel back in time to cause Hitler’s rise to power. So that’s not a good idea.

What about the many-worlds idea, where traveling back in time creates a parallel universe? In that case eliminating Hitler would create a new timeline without Hitler, but the old timeline would also still exist. Your time tripping does nothing to eliminate the pain and suffering of the original timeline. It might also create a new timeline with even more pain and suffering. After all, if Hitler didn’t rise to power, who’s to say that someone worse wouldn’t replace him?

So overall it seems traveling through time to kill Hitler is inconsequential at best, and could be downright harmful. That whole period of history is probably best for novice time-travelers to avoid.

I hear 1985 is nice.

This post originally appeared on Forbes.

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According to Back To The Future Part II, October 21, 2015 was the day Doc and Marty arrive from the wonderful world of 1985. They’re greeted by a world of flying cars, hoverboards and self-lacing shoes.

As far as time travel is concerned, making the journey from 1985 to 2015 is easy. Many of us took the long road, traveling at the usual 1 second per second through time. But we know it’s possible to make the journey in less time. A central property of special relativity is time dilation, where an object moving at some speed relative to you will appear to experience time at a slower rate. We see this effect in particle accelerators, where unstable particles moving at high speed decay at a slower rate than ones at rest relative to us.

Using time dilation, you could in principle travel away from Earth at high speed and return 30 years later. If you traveled at speeds approaching light, then your journey might only take a month for you while decades pass on Earth. The power required for such a journey is far beyond the total energy production of Earth, but it’s possible within known laws of physics. Traveling forward in time is easy.

When folks talk of a time machine, however, they mean one that can travel backward in time. We want to be able to visit key moments in history, watch dinosaurs walk the Earth, or accidentally prevent our mother from dating our geek of a father. While it’s mostly just a fun plot device, time travel has been studied by physicists in great detail. Not necessarily with the goal of building such a machine, but rather to explore the theoretical possibilities of general relativity. What they’ve found is that backwards time travel is extremely difficult at the very least, but not necessarily impossible.

In physics a time machine is known as a closed timelike curve. Objects move through time and space, but can never travel through space faster than light. Any path through spacetime that obeys this rule is known as a timelike curve. If a timelike curve could somehow loop back on itself (if we could meet our younger self for example) it would be a closed timelike curve (CTC), hence a time machine.

It turns out that CTCs are possible within general relativity. In 1949 Kurt Gödel found a solution to Einstein’s equations with CTCs. It described a universe that rotated, and the rotation caused some timelines to loop back upon themselves. While Gödel’s universe is a mathematical solution to general relativity, it isn’t a physical one. The real universe doesn’t rotate the way Gödel’s model does. But Gödel showed that CTCs were at least theoretically possible within relativity, and so other solutions have been explored.

Wormholes and warp drive could be used to make a time machine. Credit: Wikipedia user Raude. CC BY-SA 3.0.

We now know that anything that allows travel faster than light could be used to create a time machine. If we had warp drive, then we could use time dilation and warp drive to create a CTC. The same is true for wormholes that allow us to create shortcuts across cosmic distances. As far as we know neither of these exist, and it seems the speed limit of light is also a rule against time travel.

But even if we assume time travel is somehow physically possible, there are also metaphysical problems with time travel. The most famous one is known as the grandfather paradox. It was first proposed by science fiction author Nathaniel Schachner in 1933, and is alluded to in the Back To The Future series. Shachner’s paradox had a time traveler journey to the past and kill his grandfather before he married and had children. Since this prevented the time traveler from being born, he couldn’t have killed his grandfather, hence the paradox. In Back To The Future, Marty accidentally prevents his parents from going to the school dance, thus putting his very existence in danger.

Marty introduces Rock and Roll to Chuck Berry, but Marty first heard it from Berry. Credit: Universal Pictures

Interestingly, the grandfather paradox is easily resolved in general relativity through what is known as the Novikov self-consistency principle. If we assume that (somehow) CTCs are possible, then the Novikov principle requires that such time loops be self consistent. So you could travel back in time in an attempt to assassinate your grandfather, but only end up wounding him. He’s rushed to the hospital, where the attending doctor is none other than your grandmother. Your time-trip caused them to meet, and hence you were born, which is perfectly consistent. According to the Novikov principle, it would be physically impossible to create a paradox. This self consistent approach was used in that other 80’s time travel movie, Bill & Ted’s Excellent Adventure.

In the many worlds model any quantum outcome leads to both results. Christian Schirm.

Back To The Future uses a different take on time travel, specifically that traveling back in time would spawn a new timeline. Even though Marty reunites his parents by the end of the first film, there are small differences when he returns to 1985. As the trilogy progresses a wide range of timelines are formed (up to 21 depending on how you count them). This draws upon what is known as the many worlds model.

In quantum theory, the measurement of a quantum object leads to an outcome that is probabilistic. Rather than a deterministic cause and effect, as seen in Newtonian physics, quantum theory can only give us the likelihood of a certain result. In an effort to bring determinism to quantum measurements, Hugh Everett proposed a “relative state” interpretation of quantum theory in 1957. In this interpretation, all outcomes of a quantum measurement occur, but the act of measurement causes the outcomes to separate from each other. Basically, a quantum measurement causes the universe to split into separate “universes,” each with a different outcome. While Everett’s model isn’t that simple, the idea that events or choices can split the universe has become a popular trope in science fiction. The idea of time traveling between parallel worlds isn’t plagued with the paradoxes of single-universe time travel, but it relies upon a loose interpretation of an unproven model.

Because of the challenges and paradoxes of backwards time travel, most physicists don’t think time travel is possible. All the metaphysical problems go away if a time machine is simply unphysical, and in physics the simplest answer is usually the right one. So perhaps we should focus on things that, though highly unlikely, are in principle physically possible. Such as the Cubs winning the World Series.

Paper: Gödel, K. (1949). “An example of a new type of cosmological solution of Einstein’s field equations of gravitation”. Rev. Mod. Phys. 21 (3): 447–450.

Paper: Friedman, John; Michael Morris; Igor Novikov; Fernando Echeverria; Gunnar Klinkhammer; Kip Thorne; Ulvi Yurtsever (1990). “Cauchy problem in spacetimes with closed timelike curves”. Physical Review D 42 (6): 1915

Paper: Everett, Hugh (1957). “Relative State Formulation of Quantum Mechanics”. Reviews of Modern Physics 29: 454–462.

This post originally appeared on Forbes.

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In The Martian, journeys to Mars are made possible through a large spacecraft known as the Hermes. Unlike the Apollo program, where each trip to the Moon required a separate spacecraft, the fictional Ares program uses the Hermes as a taxi to between Earth and Mars. Individual missions dock with the Hermes, but the Hermes simply makes the rounds between Earth and Mars over and over. While the Hermes is a work of fiction, it’s based in well established science.

The first ideas a spacecraft traveling between Earth and Mars are nearly a century old. In 1925 Walter Hohmann proposed an elliptical orbit between the two worlds. The Hohmann transfer orbit, as it came to be known, relies on Earth and Mars to be in the right positions relative to each other so that a spacecraft in a Hohmann orbit. This occurs about every 26 months, and a low delta-v trajectory. While it has its advantages, the one big disadvantage is that each Hohmann orbit has a different orientation each time. Another problem is that the orbits of Earth and Mars are not quite in the same plane, so things aren’t quite as simple as Hohmann proposed.

To have a large spacecraft that passes Earth and Mars with each orbit, you need some kind of thrust to adjust your orbit. In principle, chemical rockets could do the job, but they aren’t well suited for it. Chemical rockets are great for producing a large thrust in a short time, but a craft like the Hermes would need gradual thrust over longer periods. This can be done with ion drives, which accelerate charged particles at high speeds. In the novel, ion drives accelerate the Hermes at a constant 2 mm/s², which is enough to continually adjust the orbit to match Earth and Mars. While we don’t yet have drives powerful enough for a craft like Hermes, ion drives are being used in missions such as the Dawn spacecraft currently at Ceres.

The only real disadvantage of ion drives is calculating their trajectories. If a spacecraft is continuously accelerating, its trajectory has to be determined computationally. This posed a real challenge for Andy Weir as he was writing the book. To get realistic trajectories for the Hermes he had to write a program to calculate them, and fiddle with parameters until he got a set of trajectories that worked. You can see the resulting trajectories here.

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The movie Interstellar features a fast rotating, supermassive black hole known as Gargantua. The Hollywood imagery of Gargantua, with its immense gravity warping light around it in a realistic way, has been praised for its integration of real science into cinematic storytelling. But despite it’s realism, Hollywood enhancement still played a role. Now a new paper in Classical and Quantum Gravity peels back the illusion and shows the Hollywood star without makeup or Photoshop.

Using ray bundles to determine the view.

The paper itself is freely available, so it’s worth checking out. Much of it details the challenges of rendering the black hole at IMAX resolutions. Since the film required Gargantua to be rotating at nearly the maximum theoretical rate, the bending of light is effected by the rotation through what is known as frame dragging. The black hole is also surrounded by a disk of gas and dust, which orbits the black hole. The hot material gives a source of light so that the black hole is visible, but its motion means that the authors had to calculate both how the material moved and how its light was distorted dynamically. Because of the complexity, the team couldn’t use “ray tracing,” where the path of a single beam of light is calculated. Instead they used ray bundles to approximate the effect. Throw the location and motion of the “camera” into the mix and even the approximation is extremely complex.

The paper also discusses the tension between the scientific desire for realism and the Hollywood desire for visual appeal. Because the black hole is rotating so quickly, and the gravity near the black hole is so strong, they greatly affect the visible light around it. The rotational motion of the material creates a Doppler effect, causing the material rotating toward the camera to be shifted toward the blue end of the spectrum, while the material rotating away from the camera appears deep red. This is combined with the fact that light from the approaching side appears much brighter than the receding side. As a result, the fairly accurate rendering shown above lacks visual appeal. So Interstellar‘s director Christopher Nolan opted for a version where the Doppler and brightness effects were minimized. It was also decided that blurring and lens flare effects were added to make it more in line with the overall movie.

Easy, breezy, beautiful. Credit: Interstellar.

It’s hard to be too critical of the cinematic version. Just as the actors are enhanced through makeup and lighting to make them more attractive to audiences, the black hole imagery was enhanced for visual appeal. But the enhancement was done on top of a real science. That’s a big change from the usual approach of simply making things up regardless of what science tells us. Hopefully Interstellar will demonstrate that science fiction movies can aspire for scientific accuracy while still creating interesting tales.

Paper: Oliver James et al. Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar. Class. Quantum Grav. 32 065001 (2015)

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A cosmic string is a very long (possibly as long as the diameter of the visible universe), very thin (less than the width of a proton) high-density object formed during the early moments of the big bang. You can see a rendering of such a string in the image above. There is a reason why it appears next to the starship Enterprise. Both of them are science fiction.

Like many science fiction ideas, cosmic strings are rooted in real theoretical work. Although they both contain the word “strings”, cosmic strings are not the strings of string theory. Instead they are topological defects in the fabric of space and time. To use a very rough analogy, as water freezes to ice, cracks can form at the intersection between two regions of the water that started to crystallize. In a similar way, during the inflationary period of the early universe, different sections of spacetime “freeze” into their current state, leaving cosmic strings like cracks in the fabric of spacetime.

The idea was first introduced by Tom Kibble in the 1970s, and it has risen and fallen in popularity over the years. Part of the reason for their popularity is that they can be introduced into cosmological models relatively easily. Another reason is that they have lots of interesting properties that could lead to observable consequences.

For example, as a very dense object extended over cosmic distances, cosmic strings would cause a type of galactic clumping similar to what we observe in the universe. We now know that this clumping matches dark matter extremely well. Based on current observation, no more than 10% of the clumping could be the result of cosmic strings (assuming they exist).

Cosmic strings also lead to more exotic phenomena. One of the most popular ideas is to use them as a time machine. If two cosmic strings passed by each other at high speeds, you could travel between them in such a way that you could travel back in time. In physics terms, we say they would create closed timelike curves. This type of exotic physics is possible because they are basically one-dimensional singularities.

Currently there is no observational evidence to support cosmic strings. Observational limits from galaxy distribution have found no signature of their existence. Searches for gravitational waves and the fact that no gravitational waves have yet been detected eliminates the existence of cosmic strings with high tension. If cosmic strings were relatively common, then we should detect them through the gravitational lensing of distant galaxies as the strings pass in front of them. We have thus far seen no such effect.

Where cosmic strings do appear from time to time is in science fiction works such as Star Trek. Like wormholes, tachyons, and warp drive, it is an interesting idea with fun properties to explore.

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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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The idea of parallel worlds is widely used in science fiction. Most often this alternate world is either populated by our evil doppelgangers, or the alternate universe is just slightly different from ours, such as having zeppelins in a modern city. Then there is the “alternate history” fiction, where their world is identical except for a key moment in history. Lincoln survived, Harold Godwinson won the battle of Hastings, etc.

Perhaps the earliest example of alternate universes in physics comes from the 1957 paper by Hugh Everett titled “Relative State Formulation of Quantum Mechanics.” Everett’s paper was an attempt to eliminate the need for an “observer” in quantum mechanics.

Although it is a well tested theory, quantum mechanics can be hard to wrap your head around. As the theory was developed, a standard interpretation was adopted known as the Copenhagen interpretation. The basic idea of this interpretation is that a quantum system is described by a wavefunction, which then collapses into a particular state when “observed”. For example, suppose had a quantum “coin”. Before we measure the coin, we only know the result will either be heads or tails. We have no way of knowing what the result will be, only that we have a 50/50 chance of either outcome. In the Copenhagen interpretation the coin is described by a probability wavefunction. Once we measure the coin (and, say, observe that it is “heads”) then the coin is in a definite state of “heads”. The wavefunction is said to have collapsed into a single state.

This seems rather simple, but the “observer” is not clearly defined. In early quantum theory it was simply assumed to be some larger system. But the world is not divided between “quantum” and “classical” things. If our coin is in a quantum state, then we as the observer should also be in a quantum state. We can’t simply declare a classical observer. So the Copenhagen interpretation isn’t really an accurate way to describe quantum mechanics, though it is still the most commonly taught approach.

Everett’s approach was to remove the “classical” observer. Instead, he proposed that observation is an interaction that decouples the outcome states. If we go back to the coin idea, initially the coin is described by a wavefunction where both heads and tails are possible. The observer is also described by a wavefunction in which it can observe either heads or tails as the outcome. When the observer and coin interact, their wavefunctions don’t collapse, but instead the quantum states separate into one quantum state where the coin is “heads” and the observer measures “heads”, and another quantum state where the result is “tails”.

This eliminates the need for a “classical” observer, but it means the system has split into two systems. On the cosmic scale it means the universe has split into one where the result is heads and one where the result is tails. Each measurement separates (decoheres) the quantum state of the universe into two states. Presumably this would happen with every possible outcome. So there is a universe where you asked that cute brunette out to dinner, and one where you didn’t. In the universe where you asked them out, there is a version where they said yes and another where they said no. So the universe splits at every possible outcome, and (in one interpretation) all outcomes are real. All of these parallel universes splitting and splitting at every quantum choice.

Of course in Everett’s formulation, once quantum states split they no longer influence each other. They have a common past, but no future interactions. So there is no way to travel to or communicate with the version of you that is a billionaire and ask for a loan. This also means that Everett’s many-worlds approach is completely untestable. It may have some philosophical appeal, and may even be real, but it is not a testable hypothesis.

Another appearance of parallel universes comes by way of string theory, and its generalization M-theory. In M-theory, there are 11 dimensions, and our 4-dimensional universe is a membrane (or brane) within this higher-dimensional space. Very roughly, you can visualize our universe as a sheet of paper in a large room. But this means there could be other universes in this 11-dimensional “multiverse”. These could be parallel to our universe. In M-theory, they might interact with our universe gravitationally. There are some models that propose this as a solution to dark matter, for example. But of course all of this is very speculative, and at present there is no experimental evidence for string theory or M-theory. And these other universes would not necessarily be parallels of our universe.

So it seems that Everett’s many-worlds hypothesis is the closest physics comes to a parallel universe in the way often presented in science fiction. Whether or not these parallel universes are real, we have no way to reach them.

For us, choices have consequences, and we always have to face ourselves when we look in the mirror.

Tomorrow: Aliens! As we learn more of the universe it seems increasingly likely that there is life on other worlds. But if that’s the case, where are all the intelligent aliens?

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