These days it often seems that if a theory has loose ends, its dangling threads are surreptitiously tied together out of view within the hidden fabric of a parallel universe. While some researchers recoil from introducing unseen aspects to a theory, others find that the invisible knots create an irresistibly pretty package.
Depending on one’s taste, there are so many types of parallel universes to choose from—alternative cosmos galore. If extra dimensions are not your thing, maybe bifurcating timelines would work. If an endless array of gigantic bubble universes seems intimidating, then perhaps a nursery of baby universes is more endearing. While there is not yet a GPS device or app to navigate through the cartography of scientifically sanctioned parallel possibilities, perhaps this guide to all things alternative will help.
Detlev van Ravenswaay / Photo Researchers, Inc
Let’s start with the oldest, most basic idea and work our way toward newer, more complex models:
What if? Here is the simplest way to transport yourself to a parallel universe: Just imagine all the ways in which our universe might have turned out differently. Each of these might-have-been realities represents a parallel universe. The mathematician Gottfried Leibniz posited that we live in the “best of all possible worlds” (famously satirized by Voltaire in "Candide") and that all these other, unrealized, possibilities for creation would have been less desirable. His perspective has persisted for three centuries as a way of explaining why the cosmos is the way it is. Contemporary physicists who make use of the so-called Anthropic Principle argue that if the universe’s conditions were slightly different, it couldn’t have supported intelligent life, and we wouldn’t be here today to speculate about it. For example, if the inflationary era, a fleeting period of ultra-rapid growth in the very early universe, had continued for a long enough time, the stable structures we see in the cosmos today, such as stars and galaxies, couldn’t have formed. The super-quick expansion would have ripped them apart.
Alternative realities made possible by time travel: Science fiction writers relish the intricate plots woven by introducing time travellers into a story. Einstein’s general theory of relativity does not distinguish between space and time and hence hypothetically permits travels to the past, though the mechanics of such a journey are still largely beyond us. In recent decades, backward time travel ideas have been explored in serious articles published in reputable physics journals. If journeying back in time is possible, what would happen if someone changed history? Would they launch a new timeline, and hence a new universe, in which the chain of events was different? The answer won’t be known until backward time travel is either developed or ruled out.
Sum over histories: Physicist Richard Feynman had a practical, no-nonsense approach to physics, supporting notions that are potentially testable. Yet his approach to quantum field theory introduced the startling concept of reality as a weighted sum of alternative histories. For example, according to Feynman’s formulation, if two electrons approach each other, deflect and scatter, their overall behavior from start to finish must take into account every possible intermediate path—weighted according to each path’s likelihood. It is like assessing how tired someone will be after taking a walk in the woods by assuming that they somehow split up and took every possible route from entrance to exit—assigning more weight to the shortest (and therefore likeliest) paths, but still taking all of them into account.
Many-worlds interpretation of quantum mechanics: While Feynman did not assert that the ghostly alternative histories he described represented actual parallel universes, a young graduate student, Hugh Everett III (who shared the same research advisor as Feynman, John Wheeler), made the case that they are. Everett proposed a fundamental reinterpretation of quantum mechanics in which each time that particles interact, reality bifurcates into a set of parallel streams, each representing a different possible outcome. Researchers observing the outcome of such quantum experiments would similarly split up into multiple selves—each thinking that he or she is the only one. For example, suppose a physicist named Eve wants to measure the position of an electron and there are three possible outcomes. Upon taking the measurement, she would instantly divide into three distinct selves, each recording a different result. Each version of Eve would be convinced that she was the real one—wholly unaware of her near-doppelgangers.
Copycat regions of the universe: We now turn from the exceedingly small to the incomprehensibly large. If the universe is infinite, as many cosmologists surmise, then if you travel far enough you will eventually reach regions nearly identical to ours. That’s because if you take a finite number of elements and mix them into an infinite number of combinations, eventually chance will reproduce one of the previous arrangements. It is like playing tic-tac-toe—play enough times and you are bound to repeat yourself. Hence somewhere, by pure chance, there could be a near-parallel Earth where a nearly-identical version of you is reading this article on a parchment scroll illuminated by a glowworm.
Bubble Universes and Baby Universes: In general relativity, an energy field of the right variety can trigger space to grow explosively. Researchers use this phenomenon to explain how the universe expanded so rapidly during the inflationary era. However, they’ve come to realize that if explosive expansion took place in one part of space, it probably happened elsewhere, too. Hence, myriad bubble universes could have emerged from the primordial cosmic sea of energy. We would never have access to other bubble universes, though, because they would have since moved away from us well beyond the limits of observation. Baby universes represent a related idea, in which universes would be seeded in the extreme conditions of black holes. The embryonic regions of space would then grow into successor universes in their own right.
Higher Dimensions: For this type of parallel universe, we move beyond the three dimensions of space itself and consider the possibility of a higher, unseen dimension. While such a scenario sounds a bit like "The Twilight Zone," higher dimensions are a vital part of string theory and other attempts at unifying the natural forces. If a higher dimension exists beyond space and time, why can’t we travel through it? Theorists hypothesize that the particles of matter and light cling to our three-dimensional space, preventing us from entering or even observing the extra dimension.
While our bodies have remained in our own universe, our minds have completed an excursion through a weird assortment of parallel universe possibilities. Do any of these types of parallel universes exist? If so, how are they connected? Suggestions for testing these various hypotheses are too numerous to recount in this post. I refer the reader to several interesting proposals:
Testing Many-Worlds Quantum Theory By Measuring Pattern Convergence Rates
Testing for Large Extra Dimensions with Neutrino Oscillations
Is Our Universe Inside a Bubble? First Observational Test of the 'Multiverse'
Editor's picks for further reading
FQXi: Philosophy of the Multiverse
In this essay, discover why many theorists are drawn to the idea that our universe is just one among many.
NOVA: Parallel Worlds, Parallel Lives
Discover web resources associated with NOVA's "Parallel Worlds, Parallel Lives," a film about the life and work of Hugh Everett III.
Scientific American: Parallel Universes
In this article, physicist Max Tegmark explores four "levels" of multiverses.
If you were born on an isolated desert island in the middle of the ocean and had no communication with the outside world, your knowledge of geography would be limited. Peering through binoculars, gazing out in any direction, your view would be bounded by the sea’s horizon. Although you might speculate about what lies beyond the edge, you’d lack tangible evidence to support your hypothesis.
Confined to our planet and its environs, we face the same situation: We can see a portion of the universe, but we can only speculate about its full extent. We might surmise through its flat geometry that it continues indefinitely in all directions, like a prairie stretching out as far as the eye can see. (Flat in this context refers to a straight three-dimensional space, like an endless box.) However, our understanding of the actual universe is bounded by the edge of the observable universe. We cannot know for sure what lies beyond the enclave our instruments can detect.
Accordingly, we might wonder: How large is the part of the universe we’re potentially able to observe directly? At first glance, the answer might seem like a simple calculation. The speed of light is approximately 186,282 miles per second, or about 5.9 trillion miles per year. The time that has elapsed since the Big Bang is 13.75 billion years. Multiple the two figures and—voilà—we find that over the entire history of the universe, light could have travelled 13.75 billion light-years, or 81 billion trillion miles. But, in fact, that answer would be wrong.
Let’s think about when the light was produced. From the time of the Big Bang to the era of recombination (when neutral hydrogen atoms formed) some 380,000 years later, the universe was opaque to light. Photons bounced between charged particles and didn’t travel very far. The reason is that charged particles interact with photons—either absorbing or emitting them. Only after the era of recombination could light journey through space. That is because photons can pass through neutral hydrogen gas without being diverted. Therefore, any estimate of the size of the observable universe must assume that the farthest light we see was released after that pivotal era when space became transparent. (We may someday be able to detect neutrinos and other particles from before that era, pushing the timeline earlier and enlarging the realm of what is observable, but for now we are still limited.) The difference between the two times doesn’t change the calculation much, but is important to note.
Another adjustment is far more important. Since the primordial burst of creation, space has been stretching as the universe expands. A galaxy’s distance from us today is far greater than it was when it released the light. We can think, by analogy, of a relay race in which a girl tosses a ball to her teammate and then runs away from him. If the coach later asks the teammate what is the farthest throw he has caught he would give a very different answer than if he is asked where is the farthest player he has caught a ball from. Similarly, the distances traveled by the photons hurled by light sources do not reflect the much greater extent of the sources’ current positions. Thus, we could potentially observe light sources that are much farther out than 13.75 billion light-years, if their light was released when they were close enough to Earth.
Yet another factor that expands the limit of the observable universe is its acceleration. Not only is the universe expanding; its growth has been speeding up. Data from the Hubble Space Telescope, the WMAP (Wilkinson Microwave Anisotropy Probe) satellite and other instruments have been used to pin down the rate of acceleration, along with the current expansion rate, the age of the universe, and other important cosmological parameters.
Taking advantage of this wealth of information, in 2005 a team of astrophysicists led by J. Richard Gott of Princeton performed a detailed calculation of the radius of the observable universe. Their answer was 45.7 billion light-years—more than three times bigger than our first, naïve estimate! Within this sphere lie hundreds of billions of galaxies, each with hundreds of billions of stars.1
Image credit: Andrew Colvin
Gott’s team calculated this radius by figuring out how far away from us a source would be today if the light we now observe from it was emitted during the recombination era. In our relay race analogy, that’s determining where someone must have stood if she threw a ball and we caught it, and then using her running speed to figure out where she must be right now.
Interestingly, as the universe expands, the size of the observable portion will grow—but only up to a point. Gott and his colleagues showed that eventually there will be a limit to the observable universe’s radius: 62 billion light-years. Because of the accelerating expansion of the universe, galaxies are fleeing from us (and each other) at an ever-hastening pace. Consequently, over time, more and more galaxies will move beyond the observable horizon. Turning once again to our relay race analogy, we imagine that if the players get faster and faster as the race goes on, there will be more and more who were so far away when they first threw the ball that the light would never have had time to reach us.
Naturally not everything within the observable universe has been identified. It represents the spherical realm that contains all things that could potentially be known through their light signals. Or to draw from a famous comment by former Secretary of Defense Donald Rumsfeld, the observable universe contains “known unknowns,” such as dark matter, that could eventually be analyzed. Beyond the observable universe lie “unknown unknowns”: the subject of speculation rather than direct observation.
1The 45.7 billion light-year radius includes only light sources. If neutrinos and other particles that could penetrate the opaque conditions of the early universe are included the value becomes 46.6 billion light-years.
Editor's picks for further reading
arXiv: The Long-Term Future of Extragalactic Astronomy
In this article, astrophysicist Avi Loeb investigates how our view of the universe will change in the distant future.
Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond
In his latest book, Paul Halpern investigates what may lie beyond the boundaries of the observable universe.
James Webb Space Telescope: The End of the Dark Ages: First Light and Reionization
Learn more about the era of recombination and observations of the very early universe in this NASA resource.
We think of gravity as weighty—its omnipresent grasp pulling us down to the ground. Try to lift a piano up a flight of stairs and you can feel gravity’s resistance. (Laurel and Hardy showed this best!) Yet in a match with the other fundamental forces of nature—electromagnetism, the weak force and the strong force—gravity gets pummeled.
You can see gravity’s relative weakness simply by using an ordinary bar magnet to pick up paper clips from a desk. Battling the gravitational pull of all of Earth, the tiny magnet wins! In fact, gravity is a staggering 1040 times weaker than electromagnetism. But why, among the fundamental forces, is gravity the runt of the litter? Explaining gravity’s relative feebleness is a profound challenge for physics, and an essential milestone on the road to a unified theory of all four forces.
Uniting the four fundamental forces into a single unified theory is a longstanding scientific dream. On the face of it, these forces are very different. They each operate across different distances, with different strengths, determined by the properties of a special class of particles, called “exchange particles,” whose job it is to convey the forces.
Exchange particles are like Frisbees thrown between particle players; the process of tossing the Frisbee brings ordinary particles together—or in some cases pushes them apart. In electromagnetism, for example, electrons interact by exchanging photons. Because the photons have no rest mass, they travel through the vacuum at the speed of light, making electromagnetism a long-range force. Long-range means that it can operate over great distances. For example, terrestrial receivers can pick up radio signals (a type of electromagnetic radiation) transmitted by Voyager 1, situated at the edge of the solar system more than 11 billion miles away. The weak force, in contrast, is conveyed by massive exchange particles called the W+, W- and Z bosons. Because these exchange particles are so heavy, particles feel the weak force only on very short ranges—within atomic nuclei. The strong force, too, operates only on short ranges. But gravity, which theorists believe is carried by particles called gravitons traveling at the speed of light, is a long-range force.
Physicists have made great strides toward unifying electromagnetism with the weak interaction and, to some extent, with the strong interaction, by looking at how the forces behave at very high energies—energies that existed just moments after the Big Bang. Theoretical and experimental discoveries of the past few decades suggest that at these energies—above about 100 GeV (gigaelectronvolts)—the weak and electromagnetic forces behave as a single type of interaction, called electroweak, with identical range and strength. During this brief, hot period, the “Frisbees” that convey the weak force had no mass at all. But as the universe cooled, interactions with the Higgs field caused the W+, W- and Z bosons to acquire mass, differentiating them from massless photons and splitting what was once one force, the electroweak force, into two, electromagnetism and the weak force.
Although physicists have yet to develop a “grand unified” model that includes the strong force, they believe that, at even higher energies, it too may merge with the weak and electromagnetic forces.
But what about gravity? If you could draw the evolutionary family tree of the physical forces, you might see the split between electromagnetism and the weak force as something like the division of primates into various species. Gravity, though, represents a far more radical diversification. Reconciling gravity along with the other forces is something like showing how viruses and whales have common ancestry. Spanning the differences is possible, but tricky.
How far up the energy scale must we climb to find a point at which gravity could be unified with the other forces? The answer is a number called the Planck energy: around 1028 eV. That’s about 1017 times greater than the energy of electroweak unification—an enormous difference. One would need a machine almost a quintillion times more powerful than the Large Hadron Collider or Fermilab’s (retired) Tevatron to probe the Planck energy, putting experimental tests of this kind of unification well out of reach for the conceivable future! The sheer magnitude of the Planck-to-electroweak energy ratio, related to the stark weakness of gravity, is called the hierarchy problem.
String theorists have proposed an innovative solution to the hierarchy problem: Perhaps gravity is weakened through its ability to travel across an extra dimension. What sounds like science fiction has become an active branch of scientific inquiry called the “braneworld hypothesis.” The braneworld hypothesis suggests that the observable universe lives within a four-dimensional (three dimensions of space and one dimension of time) membrane, or “brane.” Beyond this brane, extended along a fifth dimension, is a region called the “bulk.” Unique among exchange particles, gravitons are free to wander the bulk; everything else is stuck on our brane. So while most particles are like ants confined to the surface of a wooden picnic table (the brane), gravitons are like termites that can bore within (and visit) the bulk. (In the language of string theory, the difference is that gravitons are represented by closed strings; other particles are open strings and have ends that stick to our brane like a curved handle attached to a door.) Because photons cannot enter the bulk, it remains invisible.
An advantage of explaining gravity’s weakness through its dilution into the bulk and making the brane the venue for the other forces is that unification can take place at energies not much higher than the electroweak scale, rather than at the Planck scale, rendering the search for a unified theory much easier.
There are a number of variations of the braneworld idea. One model, proposed in 1998 by physicists Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali envisions a “large extra dimension” of about one millimeter throughout which gravity is spread. (String theorists are typically concerned with such tiny size scales that one millimeter does indeed qualify as “large.”) A second brane, parallel to ours, would constitute the opposite boundary of the bulk, confining gravitons to the region between the two limits. You can imagine this second brane as something like the underside of the picnic table. The extra dimension would then comprise the thickness of the table—the distance between its top and bottom—limiting the graviton “termites” to travel through a finite amount of wood.
However, this proposal modifies the law of gravity in measurable ways, which have since been ruled out experimentally. Another version, proposed by physicists Lisa Randall and Raman Sundrum, includes only one brane and posits that the warping of the structure of the bulk, along the direction of the extra dimension, would be enough to confine gravity to a limited region and dilute its strength. The theory predicts a measurable leakage of gravitons from our brane into the warped bulk, which could potentially be detected in collisions at the Large Hadron Collider through unexpected energy loss that finds no other explanation. Researchers have sought such telltale clues to test the Randall-Sundrum idea and other braneworld approaches. The tricky part is using statistical models to rule out more mundane effects that could mimic the leakage of energy (for example, the release of neutrinos). While LHC results have not yet confirmed the braneworld idea, the jury is still out as to whether or not gravity’s weakness is a result of its slick ability to leave the visible universe and travel through a higher dimension.
Editor's picks for further reading
CERN Courier: The Nobel path to a unified electroweak theory
In this article, CERN marks the anniversary of the 1979 Nobel Prize in physics, which honored the foundations of electroweak unification.
The Particle Adventure: What holds it together?
Explore the physics of force-carrying particles on this interactive web site.
Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions
In this book, physicist Lisa Randall explores extra dimensions, braneworlds, and the bulk.
First, the good news: Despite earlier doomsday prognoses, the cosmos is not fated to implode. If you stay up late at night worrying that the entirety of creation will cave in on you in a universal Big Crunch, rest assured. Astronomical evidence suggests that the Big Bang expansion will never reverse itself; the universe will not collapse back down to a point. Rather, the universe, like ever-sprawling suburbs, will burgeon forever, its galaxies receding farther and farther away from each other.
Now, the bad news: Cosmic expansion, discovered in 1998 through supernova observations conducted by Nobel Prize-winning physicists Saul Perlmutter, Brian Schmidt, and Adam Riess and their research teams, is not only continuing, it is picking up its pace. Though we don’t know what is causing the acceleration, one leading idea, called “phantom energy,” has ominous implications. If phantom energy continues to drive the universe faster and faster outward, it could literally tear space into shreds—a doomsday scenario called the “Big Rip.”
Phantom energy, proposed by Dartmouth physicist Robert Caldwell in 1999, is one possible variety of the mysterious stuff researchers call dark energy. Dark energy is the catchall phrase for the hidden agent of cosmic acceleration, and encompasses many different approaches. The simplest proposal adds an extra “antigravity” term, called a “cosmological constant,” to Einstein’s equations of general relativity. Einstein himself had proposed the term to stabilize his equations, but then hastily removed it after Edwin Hubble showed in 1929 that the universe was expanding—and not stable at all. Adding the term back into the equations, while assuming that the geometry of the universe is flat, leads to a prediction that the cosmic growth rate will gradually and steadily increase.
While a cosmological constant seems to model the accelerating universe nicely, at least according to current astronomical observations, many researchers are seeking a more tangible explanation. Instead of an extra term arbitrarily added to the gravitational equations, they are looking for an actual physical substance with repulsive properties. This substance must have a bizarre property called negative pressure. Though we never encounter negative pressure in everyday life, you can picture how it works by imagining poking a soap bubble and seeing it inflate rather than pop.
Gravitational researchers model the behavior of substances using an equation, called an equation of state, that relates the material’s pressure to its density. For conventional substances such as fluids and gases, both pressure and density are positive, so their ratio, usually called “w,” is positive as well. However, if a substance were to mimic a cosmological constant, its w ratio would be negative one. That is, if you try to squeeze it, it will get bigger, not smaller.
In 1998, shortly after the discovery of cosmic acceleration, Caldwell, along with astrophysicists Rahul Dave, then at Penn, and Paul Steinhardt of Princeton, argued for the greatest amount of flexibility in trying to describe dark energy. Borrowing the ancient Greek term for the “fifth essence,” they dubbed it “quintessence” and asserted that its equation of state could vary over time and space. Rather than assuming that w is negative one, they urged that its actual value be pinned down through astronomical observation.
This flexibility spurred Caldwell to think about the worst-case scenario imaginable for dark energy. He wondered what would happen if the amount of negative pressure exceeded the density—that is, if w was less than negative one. In our soap bubble analogy, larger negative pressure would mean that pressing on its exterior would cause it to puff up even faster. In the far future, he realized, gravity and all other forces would eventually lose potency, overwhelmed by the hulking menace of what he dubbed phantom energy. The reason is that unlike the steady density of the energy associated with a cosmological constant, the density of phantom energy would grow greater and greater, rising along with its negative pressure, building to a lethal crescendo—a Big Rip.
In Caldwell’s end game scenario, our distant descendants (relocated to other planets, perhaps) will notice the first sign of trouble billions of years from now as other galaxies recede beyond detection and disappear from the sky. (That would happen under the cosmological constant picture too, but is exacerbated in the case of phantom energy.) Eventually our local group of galaxies, including Andromeda, will become cosmic hermits. Then, tens of millions of years before the ultimate doomsday, the local group and then the Milky Way itself will break up like a dropped box of peanut brittle.
In the cosmic twilight times, all planetary systems will be torn apart and the stars and planets will explode. About thirty minutes later, all atoms will burst like fireworks. Then, in a mere fraction of a second, space itself will tear to shreds in the all-destroying Big Rip.1
If the devouring of reality by a ravenous shredder sounds revolting, you can console yourself with an alternative developed by physicist Pedro Gonzalez-Diaz of the Institute of Mathematics and Fundamental Physics in Madrid. He conjectures that phantom energy could feed one of the many wormholes that could exist in the fabric of spacetime. Over billions of years, the wormhole would swell up faster than the cosmic expansion rate and eventually engulf the material content of whole universe—galaxies and such—sparing it from a Big Rip. Theoretically, then, everything could pass unscathed out the other end of the wormhole, emerging in another sector of reality, wherever that would be. Not exactly a joy ride, but at least it wouldn’t leave you in tatters!
The jury is still out on what causes cosmic acceleration. It could well be the case that the culprit is not phantom energy, but rather a gentler form of dark energy—such as a type of quintessence with a w value greater than or equal to negative one. If so, the expansion of space would increase more gradually, leaving the local group and the Milky Way gravitationally bound together as other galaxies eventually recede from view. While our enclave of space would be lonely, it would remain intact. In short, we’d experience a “Big Stretch” rather than a “Big Rip.”
1A recent calculation by a team of researchers led by Chinese physicist XiaoDong Li uses current data to estimate that the Big Rip will take place 16.7 billion years from now. The dissolution of atoms would take place a mere 30 quintillionth of a second before the Big Rip.
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NBCNews.com: Will universe end in a ‘big rip’?
Space writer Robert Roy Britt talks with cosmologists about the possibility of a Big Rip.
Open Yale Courses: Dark Energy, the Accelerating Universe, and the Big Rip
In this video, Yale astronomer Charles Bailyn reviews the evidence for dark matter and dark energy and presents the possibility of a Big Rip.
You’ve just started reading this post. The decision is done. Seconds have ticked by and you’ve chosen to use them clicking on this article, rather than pursuing hang gliding, water skiing, mountain climbing, chocolate sampling, or countless other options. Sure you could do those things later, but what was “now” is already gone. If only you had a wormhole time machine and could go back in time to undo your choice! But how to make a wormhole time machine? Read on if you’d like some suggestions from the world of theoretical physics.
Step in to my time machine. Credit: NASA/Les Bossinas (Cortez III Service Corp.), via Wikimedia Commons
Flash back to the late 1980s—with your imagination, not a time machine just yet. The extraordinary astronomer and science communicator Carl Sagan, fresh off his award-winning PBS series Cosmos, decided to write a science fiction novel about interstellar travel, "Contact." Needing a way for his protagonist to travel quickly to another planet, he asked his friend Caltech astrophysicist Kip Thorne for advice.
Thorne is an expert in general relativity, Einstein’s masterful theory of gravity. The equations of general relativity serve as a recipe for how nature kneads the dough of spacetime (space and time combined) into various shapes—from as flat as a pancake to as curvy as a croissant. These shapes determine how other things move. Just as an ant at a picnic would take a more winding route around an apple than across a napkin, objects in the universe (planets, comets, and so forth) veer along curved paths in warped regions. What distorts these sectors of spacetime is the amount and distribution of mass and energy. For example, the gravitational well of the solar system is carved out by the mass of the Sun.
In extreme cases, a glop of mass concentrated in a small enough region will tear the fabric of spacetime, causing what is called a singularity—a point of infinite density where spacetime seems to reach a dead end. Such is the case with what is called the Schwarzschild solution of Einstein’s equations of general relativity, used to describe the ultra-dense, collapsed stellar cores known as black holes. However, as Einstein and his assistant Nathan Rosen showed in 1935, one can mathematically extend the Schwarzschild solution across an “Einstein-Rosen bridge” and link it to another region of spacetime. In the 1960s, the creative Princeton physicist John Wheeler, who was Thorne’s PhD advisor, dubbed these connections “wormholes,” imagining a worm taking a shortcut by crossing an apple’s interior. (Wheeler also coined the term “black hole.”)
When Sagan contacted Thorne he was envisioning something like a Schwarzschild wormhole connecting two otherwise distant parts of space—an interstellar Chunnel, so to speak. But Thorne realized that a Schwarzschild wormhole wouldn’t do. For one thing, it was unstable to matter, meaning that the gravitational effect of even the slightest drop of mass would cause it to collapse. Therefore it would close off if a spaceship tried to enter—that is, if the space voyagers could make it that far. If the wormhole entrance lay in the bowels of a black hole, the travelers would encounter deadly radiation, bone-crushing gravitational forces, and enough stomach-churning acceleration to make even the Dangerous Sports Club give it a miss.
Thorne asked his then-student Michael Morris to help him come up with an alternative. They crafted a novel solution of Einstein’s equations of general relativity that would represent a wormhole that could be traversable by human voyagers, such as the fictional heroine of "Contact." The solution was custom-designed to eliminate the nasty aspects of navigating into a black hole and allow for a relatively quick, comfortable ride. After passing into the wormhole’s “mouth” (as its entrance was called) and journeying through its “throat” (as its passageway was called), a voyager would find herself emerging from another mouth somewhere in another part of space. Instead of traveling hundreds of years or more to reach another star, if all went well, she’d swiftly arrive in its vicinity.
Morris and Thorne realized that their scheme was extremely hypothetical—requiring a virtually inconceivable engineering feat. For one thing, the amount of mass needed to create the wormhole was comparable to that of a galaxy. Moreover, a new type of negative mass material, called “exotic matter,” would be necessary to prop open the wormhole’s throat and prevent it from collapsing. No known substance has negative mass.
Offering some cause for optimism, physicist Matt Visser of Victoria University of Wellington soon found a way to minimize the amount of exotic matter required. As he and others have pointed out, exotic matter has features in common with the energy of the quantum vacuum, the bedrock state of particle physics, which has a repulsive pressure. Perhaps a future civilization could mine enough of this energy to suffice for wormhole construction. A hypothetical energy called “phantom energy,” a type of dark energy with a considerable amount of negative pressure, used to explain the acceleration of the universe’s expansion, also holds promise as a potential way to stabilize wormholes.
Shortly after Morris and Thorne published their first paper they collaborated with Ulvi Yurtsever, another of Thorne’s PhD students at Caltech, on another remarkable article showing how a wormhole could be used as a time machine. The key would be to speed up one of the mouths of the wormhole to close to the speed of light while leaving the other one fixed. According to the phenomenon of time dilation, an aspect of Einstein’s special theory of relativity, time in the vicinity of a near-light-speed object will slow down significantly relative to a stationary observer. Therefore, while the fixed mouth ages 100 years, the high-speed mouth, if it is fast enough, might experience only one year. If the calendar reads 2112 for the former, it would read 2013 for the latter. Now suppose a space traveler sails into the fixed mouth in 2112. If passage through the throat is quick enough, she would emerge through the moving mouth in 2013.
If you are still thinking about all the things you could have done if you hadn’t clicked on this post, you now know the answer. Assuming you have an advanced spaceship and a CPS device (Cosmic Positioning System), simply find a wormhole, journey through it, go back to the time before you started reading this, and convince yourself to go surfing instead. You are cautioned however that your actions would create a paradox1, because if you never read the article you wouldn’t know how to go back in time (or at least wouldn’t have the need). Proceed to the past at your own risk!
1 To avoid paradoxes such as meeting yourself in the past and convincing yourself never to pursue time travel, or going back in time and accidently eliminating your ancestors, some physicists have asserted that backward time travel is impossible. Stephen Hawking, for example, postulated the Chronology Protection Conjecture to shield the past from tampering. Others such as Igor Novikov of Moscow State University and the Lebedev Physics Institute in Russia have argued, in what he called the Self-Consistency Principle, that past-directed temporal voyages are fine as long as the altered past is consistent with the present—that is, it was really supposed to happen. For example, if you go back in time and convince Carl Sagan that wormholes wouldn’t fit into his novel, maybe that’s just the incentive he needed to contact Kip Thorne and check if they would, leading to what actually happened. Finally, there are some who speculate that backward time travel could lead to a bifurcation of time into parallel realities.
In any case, the work of Thorne, Morris, Yurtsever, Novikov, Hawking, Visser and others has propelled the discussion of time travel and wormholes from fanciful science fiction into serious, peer-reviewed—albeit highly speculative—science. Who knows, perhaps someday our civilization will be advanced enough to test such far-reaching hypotheses and create or find actual wormholes. Only time will tell—and if wormholes exist, we have all the time in the world.
Editor's Picks for Further Reading
Daily Mail: Stephen Hawking: How to Build a Time Machine
Stephen Hawking on wormholes and the paradoxes of time travel.
Space Time Travel: Flight Through a Wormhole
Explore computer-generated images of a hypothetical trip through a wormhole.
Wikipedia: Wormholes in Fiction
From "A Wrinkle in Time" to "Fringe," discover how writers of books, television, and movies have used wormholes in their storytelling.
Modern astronomy offers a curious mixture of humility and bravado. Earth is not special, we say, as Copernicus asserted and Galileo confirmed. Rather it is, in the scheme of things, a tiny speck in an unremarkable location. Yet, from our modest perch, we make sweeping statements about the entire observable universe—from here to billions of light years away. Although we are the smallest of the small, we speak with authority about the largest of the large.
How can we be so bold? The key is to make use of our typicality to assume that the universe is homogenous—pretty much the same throughout. But now, the discovery of a phenomenon called dark flow is challenging our assumption that the universe doesn’t allow one place to be any more “special” than any other. Astronomers call this assumption the Copernican Principle. Thus when Edwin Hubble discovered in 1929 that all galaxies, except our nearest neighbors, seem to be moving away from us, astronomers used the Copernican Principle to infer that space is expanding in a uniform way—at the same rate in every cosmic locale. Big Bang growth marches at the same pace everywhere.
In my own calculations related to cosmology, I’ve generally followed the accepted method of using the assumption of homogeneity to greatly simplify the equations. Otherwise the procedure would be much trickier—like applying a recipe to a dish that requires different ingredients for every morsel. If it does turn out that the universe has local differences, cooking up cosmological solutions will be a tall order indeed!
Astonishingly, a newly identified phenomenon called dark flow could slash through cosmic uniformity, casting the Copernican Principle into doubt. Dark flow represents the movement of hundreds of galaxy clusters at about two million miles per hour in the direction of a patch of sky between the constellations Centaurus and Vela. Like the cloaked duo of dark matter and dark energy, dark flow is another masked marauder challenging long-held cosmological assumptions. It is “dark” in the sense of having mysterious origins—origins that may lie beyond our cosmic horizon, or perhaps even in another universe.
Galaxy clusters like the one shown in the inset seem to be drifting toward the patch of sky indicated in purple on this image of the cosmic microwave background radiation. Credit: NASA/WMAP/A. Kashlinsky et al.
Discovered in 2008 by Alexander “Sasha” Kashlinsky of NASA’s Goddard Space Flight Center, dark flow is a streak of irregularity in a universe that is otherwise as uniform as a perfect, rising loaf of bread. Kashlinsky and his research team discovered dark flow by cleverly analyzing data collected by the Wilkinson Microwave Anisotropy Probe
(WMAP) satellite during the 2000s. WMAP’s main purpose was to map out the cosmic microwave background (CMB) radiation released some 380,000 years after the Big Bang, when electrons and protons first cooled down enough to form hydrogen atoms. This “baby picture of the universe” has offered cosmologists a unique look back in time and given them new insight into the universe’s birth, growth, and structure formation.
Kashlinsky’s group put the detailed map to different use, though. when they examined the motion of galaxy clusters through the background radiation. The galaxy clusters are filled with hot gas that scatters light from the background radiation, shifting the spectral lines that define the “fingerprint” of that light. Because the amount of shifting depends on the clusters’ speeds relative to the CMB, this acts as a kind of speedometer, telling us how fast they are moving.
Soon after Kashlinsky and his collaborators published their results, they were confronted by a sharp challenge to their claims. In an article posted on his website, “Dark Flow Detected – Not!”, UCLA astronomer Edward (Ned) Wright pointed out several errors and inconsistencies in their paper’s statistical analysis and argued that these placed their conclusions in doubt. Undaunted by Wright’s allegations, Kashlinsky posted a detailed rebuttal and gathered further evidence for dark flow.
In 2010, Kashlinsky and his team published a follow-up paper with results that were even more startling. Not only did they confirm dark flow, they found its parade of clusters to be far more extensive that they had previously thought. Remarkably, from a survey of more than 1000 clusters, they provided evidence that dark flow extends out as far as 2.5 billion light years away. With such a large scale, it slashes through a significant chunk of the observable universe.
In recent decades, cosmology has become an increasingly rigorous science. Claims in the field, particularly ones of such a revolutionary nature as dark flow, must be sifted by sophisticated statistical tests and verified by independent analyses. Interestingly, while no other teams have found dark flow to the extent mapped out by Kashlinsky’s group, some groups have found a less potent, but still notable, movement in roughly the same direction. For example, researchers Richard Watkins of Willamette University, Hume Feldman of the University of Kansas, and Michael Hudson of the University of Waterloo have noted a significant flow of galaxies, but at much lower rate than Kashlinsky’s team found. Kashlinsky’s next goal is to analyze data from the European Space Agency’s Planck satellite, hoping it will offer proof positive of dark flow and reveal its extent.
If dark flow were conclusively established what would it mean? The Copernican Principle, at least for the observable universe, would be cast into doubt. There would be something special about a particular segment of space. Space would have a gaping irregularity—a fissure through its firmament. How could astronomy explain such a rift?
Kashlinsky and others have speculated that the inflationary universe model could provide the answer. According to many versions of inflation, the observable universe grew from a fluctuation in a primordial energy field. Beyond our “bubble” could be countless other universes that grew from other fluctuations in the great cosmic bath called the multiverse. Perhaps dark flow represents the result of a gravitational tug from mass housed in another universe—or at least a region beyond the observable universe. This interaction would have happened very early in cosmic history, long before the universe grew to its present-day size. Nevertheless it could have left the relic of irregularity, much like geological processes long ago produced today’s mountain chains.
In coming years, we’ll see if dark flow positions itself in the pantheon of bona fide cosmic mysteries, such as dark matter and dark energy, or if further analysis will reveal dark flow to have been an illusion. If dark flow does stand up to scrutiny, though, we may have to reevaluate the assumptions we’ve made about our universe. Perhaps the universe isn’t as uniform as we thought; perhaps what we see from our perch here on Earth isn’t necessarily what you get in distant corners of the cosmos.
Editor's picks for further reading
Ars Technica: Supernova Research Challenges Cosmic "Dark Flow" Mystery
The case against dark flow.
Science Friday: Does The Universe Have a "Dark Flow?"
Host Ira Flatow talks with theoretical physicist Michael Turner about dark flow.
Scientific American: In Our Expanding Universe, Earth Is Nothing Special
John Rennie asks: What do "Fight Club" and the Copernican principal have in common?