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Titan is the best place for humans to colonize in our solar system. It won’t be easy and won’t happen for a long time, but we’ve shown that if propulsion systems get faster it could be practical, at least compared to the other options, and, if practical, it likely will happen eventually. But what about after that? Beyond our solar system’s outermost planets, the next stop is a very long way off. The closest star to Earth is Proxima Centauri, 4.24 light-years away, a distance that would take well over one hundred thousand years to reach at the fast- est speed any human being has ever traveled (Apollo 10). The closest Earthlike planet is probably twice as far.
Einstein’s thought experiments showed that matter cannot travel faster than the speed of light, a law well confirmed by experiment. Time slows down as objects speed up until, at the speed of light, time stops. GPS satellites have to account for the dilation of time to give you an accurate fix on your location. Even if we could build a spacecraft that could go near the speed of light (a huge if), it would still take too long to get to the nearest Earthlike planet for the survival of the passengers, unless we also solved the problem of space radiation, psychological stress, nutrition, and the other issues we’ve discussed.
Futurists have various solutions for this problem. Juan Enriquez imagines changing the human body to survive the trip, doing away with our flesh and replacing it with silica so we can live the thousand years it might take to get to another star.
“That’s the only scenario under which I could see somebody traveling between solar systems. I don’t see a fragile, carbon-based life-form leaving,” he said. But he added, “How you would engineer a human body, and maintain the semblance of humanity, that’s an interesting question.”
At this point in the book, we could choose to go with that. By the time humans settle the outer solar system, enough time will have passed that technology and society could be completely unrecognizable to us. Almost anything could be possible. But we’ve limited ourselves throughout the book to predictions supported by evidence. Dreaming of futures so far out that no one can argue the point one way or the other isn’t interesting.
Besides, pity the immortal, silica-based person bored silly on a thousand-year trip through empty space. It just doesn’t sound like fun. The best hope for exploring the rest of the universe lies in the impossible: traveling faster than the speed of light. It has happened. After the Big Bang, the universe expanded faster than the speed of light. But that didn’t violate Einstein’s law that nothing can go through space faster than light, because space itself was expanding; nothing moving within space went faster than light. If we could warp space artificially, we might be able to create a shortcut that would allow a spacecraft to get ahead of light without beating it in a fair race. Relativity and quantum mechanics may make this paradox possible. An exotic form of matter suggested by the math of advanced physics could warp space-time. Negative matter or energy would bunch up space like a rug, shortening the distance to the destination. A Mexican physicist, Miguel Alcubierre, then a student studying relativity, came up with the idea in 1994 after watching an old episode of Star Trek and wondering what it would take to make a warp drive. Alcubierre calculated that matter with negative mass, if it exists, could bend space into a bubble around a spacecraft, bunching space up in one direction and stretching it out in the other. Traveling in the bunched-up direction would allow the craft to exceed the speed of light from the perspective of someone standing outside the bubble. Inside the bubble, the craft would travel at a much slower speed in a patch of undistorted space that would move along with it. The effect would be something like walking on an airport conveyor belt.
If this all sounds like nonsense, follow us for a one-paragraph review of Einstein’s theory of general relativity. Einstein linked space, time, and gravity by conceiving of space and time as a universal fabric that is deformed by the presence of matter. Mass bends space-time into dips, or wells, producing the attraction of gravity and the slowing of time. Observations confirm these predictions. For example, the gravitational distortion of space around large objects like stars bends the light passing by. Alcubierre’s idea was to contract the fabric of space in front of a spacecraft and expand it behind the craft to drastically shorten the time it would take to travel between two points.
Matter with negative mass is not available on Craigslist, but quantum field theory suggests it could exist. Quantum fields give rise to the subatomic particles that make up matter and everything else. Quantum fields also occupy all of empty space. You can think of a quantum field as a collection of particles linked so they act together as waves. A quantum particle is never at rest and its energy can only change in discrete quantities or quanta—in other words, energy doesn’t change smoothly, but jumps in finite steps with a minimum size. (If you ask why, you get transferred to philosophy class.) One of the results of this aspect of reality is that empty space has energy, because its quantum state can never be zero. This is also why particles pop randomly into existence from empty space.
Many strange experiments demonstrate the bizarre consequences of quantum physics. For example, two metal plates held very close together in a vacuum will develop an attractive force between them for no reason other than the pressure created by the quantum vacuum energy. The narrow gap constrains the quantum field, reducing its energy compared to the field outside the plates. This Casimir force was demonstrated in the lab only in 1997 and it remains controversial, but some physicists take it as evidence of the production of negative vacuum energy between the plates. Negative vacuum energy satisfies the negative mass requirements in Alcubierre’s equations.
Could a spacecraft use this exotic negative mass to warp space and zip across the galaxy? Alcubierre said no and gave up his work. A later researcher, Richard Obousy, showed it could work with a ring of exotic matter around a craft, but the amount of exotic matter needed would be as large as Jupiter, obviously impossible.
That’s where the concept stood in 2011, when Sonny White was invited to give a talk to the 100 Year Starship Symposium, an annual meeting of a group dedicated to achieving interstellar travel within a century.
He said, “I didn’t really have any objective, I was just fiddling around. ‘Hey we want you to come give a talk.’ OK, well I don’t just want to say everything that I’ve said before, so I’m going to do something different.”
In the process of playing with the field equations, Sonny designed a spaceship that wouldn’t need as much exotic matter, as he explained to us while sketching on a whiteboard at the Johnson Space Center, where he heads an advanced propulsion group and a lab called the Eagleworks.
“The concept requires this doughnut that goes around this little central portion of the spacecraft. This might be where the instruments are. Scotty would be there. And this ring is where you’ve got this exotic matter. That matter is necessary to make the trick work. And what I found is, instead of making that ring very thin, like a wedding band—very thin aspect ratio—if you instead make it like a lifesaver, it will significantly reduce the amount of energy that is required for the concept.”
Besides a fatter ring, he would vary the strength of the field to reduce the stiffness of space-time (strange as that sounds). With his changes, the ring around the spacecraft would produce a warp bubble 10 meters (32 feet) across traveling ten times the speed of light. At the beginning of a trip, the spacecraft would get going in the right direction at a tenth of the speed of light. Turning on the space warp, the bubble would direct itself toward the destination, taking the bubble along with the spacecraft, but effectively a hundred times faster. Nearing the end of the journey, the warp drive would deactivate and the spacecraft would arrive under conventional power.
Within the bubble, the math indicates that space would remain flat. No gravity, no distorted time, and no sensation of acceleration. Space itself moves while the spacecraft sits calmly as if in the eye of a hurricane. Since the spacecraft doesn’t approach the speed of light within its own space, clocks run at the same rate as back on Earth. The astronauts age at the same pace as their siblings at home.
And with Sonny’s design, the amount of exotic matter needed would be reduced to less than a metric ton, more than 24 orders of magnitude less than the mass of Jupiter.
We met a lot of fascinating people while researching this book, but Sonny was one of our favorites. To go with his brilliance he doesn’t seem to have the self-importance of many successful scientists. Instead, the pure excitement persists in him that he discovered as a child making frequent trips to the National Air and Space Museum from his home in Washington, D.C. He’s a bit like a Star Trek fan who finds he has partly moved into that fictional universe in real life.
In his 2011 talk, he presented the new ideas for a warp drive and a diagram of a possible device to test the creation of a warp field. A handout said, “While this would be a very modest instantiation of the phenomenon, it would likely be a Chicago Pile moment for this area of research.” The Chicago Pile was the first nuclear reactor, built in a squash court at the University of Chicago in 1942.
Talk of this kind produced a swarm of exaggerated publicity saying that NASA had invented a warp drive. White’s actual device, in the Eagleworks Lab at JSC, is intended to create a weak warp effect in a small area and to test it with extremely precise optics. Sonny believed negative vacuum energy could be produced with lasers or powerful capacitors, but he’s cagey about how that would work. He said his device was cobbled together out of pieces in surplus and cost less than fifty thousand dollars. He has to fit the work in around other NASA priorities.
Several important physicist experts on negative energy say the warp drive won’t work, including the originator of the concept, Alcubierre. No one has published a model of how to accumulate a large amount of negative mass or energy. Larry Ford, of Tufts University, and colleagues demonstrated mathematically that negative energy is limited to either a tiny area or a very short period of time but can’t be both lasting and large scale. That fits with the narrow gap between plates that creates the Casimir force. Without this limitation, Ford wrote, negative energy acting at a distance could produce a perpetual motion machine, overcoming entropy and violating Newton’s second law of thermodynamics.
But, while Sonny is secretive about how he would produce negative energy in the lab, he does suggest an engineering work-around to make his lifesaver-shaped warp drive. In a typically charming e-mail responding to our question about Ford’s points, he shared a thought experiment about simply duplicating the narrow gaps that produce the Casimir force.
“What if I made many of these little cavities and arrayed them next to one another on a little substrate analogous to billions of transistors on a wafer?” Sonny wrote. “What if I then stacked a bunch of these wafers atop one another to the point I have a cube assembly the size of say a sugar cube? Then I have a cubic volume that has all the normal matter that went into making the cavities and substrates, but I also now have a bulk of negative vacuum energy distributed throughout the cube as a result of the presence of the billions of Casimir cavities. I can extend this thought process to stack things up in the shape and size of a mint-flavored lifesaver (my favorite flavor) instead of the sugar cube.”
“Further, the cube/lifesaver will do nothing to decrease entropy, so it does not violate the second law of thermodynamics. The Casimir force exists and has been measured, but it has never resulted in my coffee arbitrarily getting hot on its own.”
We’ve gone far enough into the edge of theoretical physics. But it’s worth mentioning that studies linking general relativity and quantum mechanics are white-hot at the moment, as new ideas seem to be nearing a breakthrough for a unifying physical theory of the forces of nature, a goal that has stood since Einstein puzzled over the problem a century ago. Sonny’s thinking is right on the edge of that movement, which may or may not prove successful.
Does that mean we will break the speed of light? To that big question—Will this work?—Sonny said we might know in twenty years, or two hundred, or never.
If it does work, however, he said anywhere in the Milky Way could be in reach.
Humility is required. NASA is working on a warp drive, but basic laws explaining how the universe works remain unknown. Physicists could soon open our eyes to a new view of reality, as they did in the early twentieth century. It’s reasonable to expect that this under- standing will have to mature before finding a way to build technol- ogy using the relationship of quantum mechanics and gravity. Ideas emerging from the mind of some brilliant young physicist right now may forever finish off ideas like the warp drive or may finally show a clear way across interstellar space.
The biggest unknowns may be our best hope for leaving the solar system.
* * * *
Planets outside our solar system, called expoplanets, were first discovered less than 25 years ago. Now we know that exoplanets are common—at least as numerous as stars and probably more so. The planet hunt rushes onward at a pace that amazes even the scientists in the thick of it. Discoveries are coming so fast that theories explaining them can barely keep up. Recently, astronomers measured the wind on an exoplanet blowing around 2 kilometers per second (4,500 miles per hour), and possibly much faster—a supersonic speed faster than any airplane. Those findings required new models to understand such fast winds.
But they’re working on figuring it out. Emily Rauscher, an astronomer at the University of Michigan, specializes in studying the atmospheres of gas-giant exoplanets. Since her 2010 PhD, her entire career has happened while NASA’s Kepler Space Telescope has been in orbit finding exoplanets.
Early in 2015, Kepler exceeded 1,000 verified exoplanet discoveries, mostly while looking at a patch of sky about 1,000 lightyears away. Studying planets so far away from the Earth requires interpreting tiny fragments of evidence. The Kepler satellite made its load of discoveries by measuring dips in the strength of starlight, dips that suggested planets were passing in front of the stars, known as transits. From the duration and intensity of the change in bright- ness during transits, astronomers could calculate the size of the planets and their orbits.
As to which star to look at, that’s just luck. “You stare at a bunch of stars and you hope to see something,” Emily said.
Kepler looked at a distant region of space to see many stars at once, scanning for planets the way a pollster surveys a random sample of voters. Astronomers working with Kepler’s survey and other studies can now calculate the probable frequency of planets throughout the galaxy, including the kind people care the most about: planets suitable for life of the kind we have on Earth. There are probably billions of habitable exoplanets. In 2015, Courtney Dressing and David Charbonneau, both then at Harvard, figured the closest habitable planet should be about 8.5 light-years away.
In the last few years, astronomers have found planets at distances from their stars that could produce temperatures suitable for liquid water. In 2015 alone NASA announced several planets with characteristics more like Earth than any other planet in our solar system. The announcements got plenty of media attention, with artist conceptions of what they might look like, and even imagined pictures of the surfaces. But that vastly exaggerates how much is known about exoplanets.
We’re not certain even of the habitable zone around our own star, the area of space that is neither too hot nor too cold for life. Recent calculations put Earth near the inner edge and Mars near the outer edge, but Mars doesn’t seem very habitable, while Earth is quite nice. Earlier estimates put searing hot Venus in the habitable zone.
Some climate scientists have taken a detour from studying global warming to enter the discussion of where the habitable zone lies. Using their computer models of our atmosphere, they tinker with Earth’s orbit, length of days, and other parameters to see how changes would affect our weather. Figuring out what makes Earth so pleasant could tell astronomers where to look for an exoplanet like Earth.
Meanwhile, Rauscher is studying the bizarre planets we already know about. Exoplanets come in an incredible variety of sizes, compositions, distances from their suns, and fates, from the very old and stable to a planet that is disintegrating around a neutron star. Information from these worlds comes in tiny slivers, but it has been enough to compile a menagerie of about 2,000 worlds.
To observe exoplanet winds, astronomers did a precise accounting of the changing color of light as a planet passed a star. The Doppler effect changes the wavelength of light from objects rapidly approaching or moving away from an observer, like the change in the pitch of sound heard from a passing vehicle. For some large exoplanets, the shift was too large to be explained only by their orbit. The atmosphere had to be moving fast—very fast, like 2 kilometers a second.
“It’s mind-blowing that the winds can be this fast,” Emily said.
“From the very beginning, discoveries of exoplanets have shown us that our own solar system is not just copied and reproduced all over the place. Exoplanets are weird and strange and make us revisit things we thought we understood about planets when there were only those in our solar system.”
Galileo demonstrated 400 years ago that we were not at the center of the universe, but we’re still getting over that comedown. We know intellectually that Earth isn’t privileged, but theories about other planets and life elsewhere in the universe have used Earth as a model, either assuming that planets are rare and we are unique, or that other solar systems are similar to our own. Our surprise at the Kepler findings says something about the human ego. In hindsight, it seems obvious that billions of stars would have billions of planets and that if they were all like the eight we have nearest us, the universe would be astonishingly boring.
We’re still looking for life on other planets that is like life on Earth. But even Earth’s own variety of life suggests that life elsewhere could be vastly different. Some qualities are shared by all known life, but we don’t know which are essential and which are accidents of early evolution.
NASA planetary scientist Chris McKay said, “We have one example. Life on Earth. So all we can do is guess.”
We met Chris in chapter 2 as a pioneer of the idea of terraforming Mars. His long career studying the possibilities of extraterrestrial life has taught him remarkable humility about our place in the universe. The work put him in the world’s driest deserts and on the frigid ice of Antarctica. The interesting species, with the greatest variety and tenacity, turned out to be microscopic. Bacteria and archaea on Earth live in solid rock, underneath ice sheets, and in volcanoes. In a South African mine, a species of bacteria gets energy from radioactivity instead of the Sun.
“Large life-forms are, in my accounting, of no importance to the story of life on Earth,” Chris said. “They’re latecomers, and they’re not really fundamental in maintaining the biogeochemistry of the planet. It’s not that I dislike large life-forms. All of my friends are large life-forms. But from the point of view of life, when we talk about it in this context of life on other planets, it’s not the large life- forms that matter.”
He broke down the apparent prerequisites for what we think of as life. Based on our experience, living organisms need a liquid medium where chemistry can happen, a source of energy, a way to transfer information for reproduction, and an ability to both isolate themselves from the environment and to exchange material with the environment.
But do we really know enough to make even these generalizations? Maybe life can evolve in a gaseous environment.
“Trying to generalize life does force you to look at how life on Earth works in a more critical way, and it’s discouraging, because you realize how little we understand about why life on Earth is the way it is,” Chris said. “We can very easily sample it and study it, and our knowledge of it is still very rudimentary. We can’t reproduce it in the laboratory from scratch. We don’t understand how or even where it got started. We assume that it started on Earth, but we have no direct evidence for that. We don’t know what variations on the fundamental biochemistry, even in a liquid water environment, would still serve it. So we have one example of life and we still don’t even understand it, so I think it is premature to draw cosmic conclusions about life. Instead, we need to go look so we get more data.”
Chris thinks Titan would be the most exciting place to find life in our solar system, because it would establish a data point so far from our starting point—the habitable zone would become enormous. But finding signs of life almost anywhere off the Earth, most easily in the plume of water vapor and particles squirting out of Saturn’s moon Enceladus, would show that it is ubiquitous. It would be too improbable for life to develop independently twice in our tiny corner of the universe and nowhere else.
We may find evidence of life outside our solar system before we find it here (if it is here beyond Earth). A SpaceX rocket scheduled to launch in August 2017 will carry a NASA mission that could bring a major advance. Developed at the NASA Ames Research Center, the TESS telescope, or Transiting Exoplanet Survey Satellite, will look for Earthlike exoplanets around the closest, brightest stars. Those targets will be much easier to study from the ground than the far-off Kepler planets, and the international James Webb Space Telescope will get a close look when it launches in October 2018.
The TESS exoplanets will be close enough for astronomers to examine them directly, not only by looking at them as they cross their stars. If those observations find a large amount of oxygen in the atmosphere of an exoplanet, McKay will be ready to pop the cham- pagne and send a probe to look for living creatures (although the craft probably wouldn’t report back in our lifetimes). Oxygen reacts so readily with other elements that Chris thinks it highly unlikely that large amounts of it could be free in a planet’s atmosphere with- out photosynthesis to replenish it.
Emily Rauscher is more conservative. Oxygen can be produced in other ways. But she thinks the chances are good that planetary scientists can find a chemical signature that would make us highly confident that life was thriving on an exoplanet. “It could be an answerable question,” she said. “There is good reason to be optimistic.”
This is exciting, but enthusiasts are hoping for something much more: contact with alien intelligence. That search has gone on since Carl Sagan’s early career. SETI, the Search for Extraterrestrial Intelligence Institute, is still in business, running a radio telescope array donated by Microsoft billionaire Paul Allen. Recently they’ve been aiming it at the habitable exoplanets found by Kepler. But after decades of searching, they’ve found nothing.
Seth Shostak, senior astronomer at the SETI Institute, said that the equipment is not sensitive enough to pick up any signal other than a very strong broadcast intended to reach us. We would not be able to pick up another civilization like ourselves, unintentionally sending out their version of Katy Perry recordings and The Bachelor reality shows.
But why would a civilization many light-years away in space send us a signal? Shostak deflects such questions. It is impossible to know how a civilization more advanced than our own might do things. But the entire SETI project is built on a tower of assumptions about aliens, not only that they want us to know about them but that they broadcast with radios, all adding up to Seth’s prediction that we will hear from them in a few decades. He further predicted, “If we hear a signal, it’s not coming from biological intelligence at all. It is coming from machine intelligence. And machine intelligence doesn’t have to be on any planet.”
The lack of contact has already worried Elon Musk and others. This concern is called the Fermi paradox, for Enrico Fermi, who first advanced the idea, in a conversation with friends, that if intelligent life is out there, it should be all around us. This reasoning holds that compared to the longevity of the universe, the time it would take for an advanced civilization to colonize many worlds throughout the galaxy is not very long. Even if it took millions of years, there is plenty of opportunity for them to have spread by now. So where are they?
Musk worries that the extraterrestrials are absent because civilizations die before they can become spacefaring, a concern that helps drive his desire to colonize Mars. But that piles more anthropocentrism upon itself, with the idea not only that all advanced intelligence is like us, but also that we’re better, thanks to Elon Musk, and can escape this universal fate.
Maybe the aliens found out that interstellar flight is impossible. Maybe they preferred to stay home. Or maybe some did colonize, but that was millions of years ago, and something else has happened since then. Predicting what an intelligence will do is difficult, even if you understand that intelligence well—Chris McKay said he often fails to predict what his spouse will do.
“Trying to anticipate what alien intelligence would be doing—would they really be coming to New Mexico and abducting cows, or would they really be sitting on their home planet, unable to travel? It’s really hard to do,” Chris said. “This is an example where our understanding has to be data driven, not theory driven.”
Eventually, if very fast space travel becomes possible, human beings or our surrogate robots may go to planets outside our solar system. We can confidently expect to find some good destinations in our galactic neighborhood, places similar to the Earth in terms of temperature and gravity. Out of potentially billions of Earthlike planets in the galaxy, some probably have nearly identical astronomi- cal attributes to Earth, or may be even better. “We should be able to find arbitrarily pleasant planets, if we look long enough,” Chris said.
Will someone already be there? Will we be able to go physically or only send machines? Will we all be robots by then, or aggressive colonists, or Zen masters happy to stay home and meditate?
We’ve reached the point in our scenario where one prediction is as good as another.
From Beyond Earth: A Path to Our New Home in the Planets. Used with permission of Pantheon. Copyright © 2016 by Charles Wohlforth and Amanda R. Hendrix, Ph.D.