In this video pencast, theorist Delia Schwartz-Perlov explains what physicists are really talking about when they talk about extra dimensions of space. Could our universe actually contain unseen dimensions, and could these extra dimensions help unify quantum theory and gravity?
Editor's picks for further reading
Cosmos: Carl Sagan: The 4th Dimension
In this scene from the classic "Cosmos" series, Carl Sagan imagines what happens when a three-dimensional character enters a two-dimensional world.
FQXi: Taking on String Theory’s 10-D Universe with 8-D Math
In this article, discover how theorists Tevian Dray and Corinne Manogue are using ten-dimensional math to describe subatomic particles.
NOVA: Imagining Other Dimensions
Journey from a two-dimensional "flatland" to the ten- (or more) dimensional world of superstring theory in this illustrated essay.
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.
Einstein’s special theory of relativity calls for radical renovation of common-sense ideas about time. Different observers, moving at constant velocity relative to one another, require different notions of time, since their clocks run differently. Yet each such observer can use his “time” to describe what he sees, and every description will give valid results, using the same laws of physics. In short: According to special relativity, there are many quite different but equally valid ways of assigning times to events.
Einstein himself understood the importance of breaking free from the idea that there is an objective, universal “now.” Yet, paradoxically, today’s standard formulation of quantum mechanics makes heavy use of that discredited “now.” Playing with paradoxes is part of a theoretical physicist’s vocation, as well as high-class recreation. Let’s play with this one.
First, some background. Despite special relativity’s freedom in assigning times, for each choice there is a definite ordering of events into earlier and later. In a classic metaphor, time flows like a river through all space, and the flow never reverses.1 Figures 1, 2, and 3 tell the central story.
To organize our thoughts, let us make a definite choice of time; in the jargon, let us fix a frame of reference. Then we can frame the history of the world as shown in Figure 1. Here time runs vertically, while space runs horizontally. Since we’re going to be considering several versions of time, we’ll name this one t1. For convenience in drawing, we are restricting attention to a one-dimensional slice of space—in other words, a line. One-dimensional “spaces” of events sharing the same value of time t1 would appear as horizontal lines (which I haven’t drawn). The meaning of the colored regions and their labels will be elucidated presently.
Observers moving at constant velocity with respect to our frame of reference will need to use their own physically appropriate, different versions of “time,” corresponding to how their clocks run. Figures 2 and 3 display the lines for which two different versions of time, t2 and t3, are constant. t2 is the appropriate measure of time for observers moving at a certain constant velocity toward the right, while t3 is the appropriate measure of time for observers moving at a certain velocity toward the left—that is, in our figures, in the horizontal, “spatial” direction—relative to our reference frame. For observers with higher speeds, the tilt of these lines will be steeper. But the tilt never exceeds 45 degrees, because 45 degrees corresponds to the limiting speed, namely the speed of light.
With this background, we are ready to appreciate the distinctions shown in Figure 1. In the center of the diagram is a blue point b representing a specific event. Some events—those that lie in the green future region of space-time—occur at a later time than b, whether we use t1, t2, t3, or any other allowed observer’s measure of time. We say that these events are in b’s causal future (or, if there is no danger of confusion, simply b’s future). What happens at b can affect events in b’s causal future, without upsetting any observer’s sense that a cause—b—must occur before its effect. Closely connected is the fact that signals from b can reach events in b’s future without ever exceeding the speed of light. We call such physically allowed signals “subluminal” signals.
Similarly, we can define b’s causal past, depicted in red. It consists of all events that can affect b. There is a nice symmetry here: If we draw cones emanating from an event a in b’s causal past, we will find b in the upper colored region. An event a is in b’s causal past, if and only if b is in a’s causal future.
But many events fall into neither of those regions; they are neither in b’s causal future, nor in b’s causal past. We say that such events are “space-like” with respect to b. The event a, which appears in Figures 2 and 3, is of that kind. According to t2, a occurs after b; but according to t3, a occurs before b. Neither a nor b can send subluminal signals to the other.
In a similar way, we can consider the regions that are future, past, or space-like with respect to a. This leads us to a more elaborate division of space-time, illustrated in Figure 4. The orange region contains events in the common (causal) past of both a and b, the purple region their common future, and so forth. This colorful diagram hints at a potentially rich subject, the geometry of causation, that could be developed much further. (Specifically, it could add some spice to high-school geometry and analytical geometry courses, and provide material for independent projects.)
As we’ve seen, if a and b are space-like separated, then either can come before the other, according to different moving observers. So it is natural to ask: If a third event, c, is space-like separated with respect to both a and b, can all possible time-orderings, or “chronologies,” of a, b, c be achieved? The answer, perhaps surprisingly, is No. We can see why in Figures 5 and 6. Right-moving observers, who use up-sloping lines of constant time, similar to the lines of constant t2 in Figure 2, will see b come before both a and c (Figure 5). But c may come either after or before a, depending on how steep the slope is. Similarly, according to left-moving observers (Figure 6), a will always come before b and c, but the order of b and c varies. The bottom line: c never comes first, but other than that all time-orderings are possible.
These exercises in special relativity are entertaining in themselves, but there are also serious issues in play. They arise when we combine special relativity with quantum mechanics.
Two distinct kinds of difficulties arise as we attempt to combine those two great theories. They are the difficulties of construction and the difficulties of interpretation.
The difficulties of construction dominated 20th century physics. (One measure of this: By my conservative count six separate Nobel Prizes, shared by 12 individuals, were awarded primarily for advances on this problem.) The tough issues that arose here, in the construction of relativistic quantum theories, are in some sense technical. Combining special relativity and quantum mechanics leads to quantum field theory, and the equations of quantum ﬁeld theory are dicey to solve. If you try to solve those equations in a straightforward way, you find nonsensical results—for example, inﬁnitely strong forces. In fact it emerged, after many adventures, that most quantum ﬁeld theories really don’t make sense! They are mathematically inconsistent. Those that do make sense can only be defined using tricky mathematical procedures. Passing in silence over that epic, we reach the bottom line: After heroic struggles, the difficulties of construction were eventually (mostly) overcome, and today quantum ﬁeld theory forms the foundation of our immensely successful Standard Model.
The difficulties of interpretation have a different flavor. Closely related to our issues with time-orderings, they arise because labeling events by time plays an absolutely central role in the conventional formulation of quantum mechanics.
The quantum state of the world is represented by its wave function, which is a mathematical object defined on surfaces of constant time. Furthermore, measurements “collapse” the wave function, introducing a drastic, discontinuous change. Suppose, for example, that we decide to use t1 as our time. Then a measurement at t1 = 0 changes the wave function everywhere at all times subsequent to t1 = 0.
But what if we had chosen t2 or t3? The occurrence of that sort of collapse implies that there is a drastic difference between the formal descriptions of quantum mechanics based on our choice of reference frame. If we work with t2, then measurements at b will collapse the wave function seen at a, since b comes before a. For the same reason, measurements at b do not collapse the wave function at a. But if we work with t3, since the time-ordering between a and b is reversed, the situation is just the opposite!
Yet special relativity demands that either t2 or t3 can be used in a valid description of nature. Have we discovered a contradiction?
The point is that quantum-mechanical wave functions are tools for describing nature, rather than nature herself. Mathematically, quantum-mechanical wave functions contain a lot of excess (unobservable) baggage and redundancy, so that wave functions that look drastically different can nevertheless give the same results for most, or possibly all feasible physical observations.
While it falls short of outright contradiction, there remains, it seems fair to say, considerable tension at the interface between quantum mechanics and special relativity. During the long struggle to construct quantum ﬁeld theories, several physicists speculated that the inﬁnitely strong forces they calculated were surface symptoms of a fundamentally rotten core, whose rottenness was indicated more directly by the difficulties with interpretation. It didn’t work out that way. We have been able to construct theories that are not only consistent but also immensely successful, despite their near-contradictions and excess baggage.
As new technologies for probing the nano-world render possible what were once purely thought experiments, we have wonderful new opportunity to ask creative questions, confronting the paradoxes of quantum mechanics head on. Maybe we’ll ﬁnd some surprising answers—that’s what makes paradoxes fun.
1 There are more speculative possibilities: that time exhibits cycles, or branches, or even has several dimensions of it own. In general relativity we let time bend together with space, and in describing the Big Bang and black holes we encounter singularities, where time begins or ends. This is fascinating stuff! But “flat, unidirectional” time is the basis for almost all practical physics, and it already provides rich food for thought, so that’s what I’ll be considering here.
Editor's picks for further reading
arXiv: Constraints on Chronologies
Read the author's technical paper on chronologies, written with theoretical particle physicist Alfred Shapere.
FQXi: Cheating the Causal Game
In this article, discover how researchers at the University of Vienna are deconstructing the physics of cause and effect.
Relativity for the Questioning Mind
Explore the fundamentals of relativity in this book by Oberlin College physics professor Dan Styer.
Quantum mechanics is one of the most devilishly confusing theories ever devised. Cats that are simultaneously alive and dead, objects that are both particles and waves, subatomic particles that know whether you are looking at them or not—and, most bafflingly, these quantum effects can be erased when individual atoms, electrons, and photons interact with their environment.
That is what makes Serge Haroche and David Wineland’s Nobel Prize-winning work in physics so remarkable: They have achieved mastery of the microrealm. Both of them have spent decades trying to generate systems in which a single atom or a single photon can be studied.
David Wineland, at the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, is an expert at trapping individual atoms using electric fields and by keeping them in an ultra-high vacuum. The mastery of individual atoms is possible by artfully employing laser beams and laser pulses. The laser beams can cool the motion of the atoms and even can transfer quantum information about the atom’s location to the location of electrons inside the atom. This is an extraordinary achievement.
Serge Haroche, of the Collége de France and the Ecole Normale Supérieure, Paris, essentially does the opposite. He uses atoms to study individual photons. Using superconducting niobium, he creates the two most reflective mirrors ever achieved. With the mirrors placed about an inch apart, he introduces a single photon, which bounces back and forth for over a tenth of a second until it eventually hits an imperfection in the mirror and is absorbed. While the photon is bouncing, it travels a distance equivalent to circling the entire globe.
In order to measure the photon, Haroche fires single rubidium atoms through his equipment. These atoms are of a special class called “Rydberg atoms,” in which the electrons “orbit” very far from the atomic nucleus. (Though we now know that atoms do not operate as mini solar systems, the analogy of orbits can still be a useful one.) By measuring the configuration of the Rydberg atom before and after it travels through his apparatus, Haroche can determine if there is a photon inside his equipment without absorbing or altering the photon.
These techniques have made it possible to probe quantum mechanics in more detail than ever before, with Haroche’s work making it possible to effectively make a movie of the transition of a photon from one state to another, a process that scientists call “the collapse of the wave function." But these two scientists’ work has more practical applications. For instance, if we are able to put equipment into a quantum state and read that state without destroying it, this opens the possibility of quantum computing. The potential power of quantum computing is enormous and if we are able to actually accomplish it, this will change computing in the same degree that ordinary computing has changed the world since the 1940s. Quantum computing is still a ways in the future, but Haroche and Wineland’s work has brought it closer to fruition.
Wineland's work has also made possible a new generation of clocks that are 100 times more accurate than the best timekeepers in the world. These new clocks are precise to one part in 1017. To give some context, if these clocks were started when the universe began 13.7 billion years ago, by now they would be off by a mere four seconds. Such accuracy is useful for communication and navigation, and could also enable even more stringent tests of Einstein’s theory of general relativity, which states that time runs slower in stronger gravitational fields. When people think about this effect, they usually invoke the mind-bending gravitational fields surrounding black holes, but these new clocks are so precise that the effect of time dilation due to gravity would be obvious if one of them were raised a mere foot off the surface of the Earth.
Haroche and Wineland’s work is of the highest caliber, with potential society-changing implications. In this year’s Nobel Ceremony on December 10, they will rightfully join their peers in the pantheon of great scientists.
Author's picks for further reading
Minute Physics: 2012 Nobel Prize: How Do We See Light?
In this video, learn more about how Serge Haroche uses atoms to study individual photos.
Nobel Prize: Particle control in a quantum world
Explore Haroche and Wineland's Nobel Prize-winning work in this popular-level article.
Nobel Prize: Measuring and manipulating individual quantum systems
Explore Haroche and Wineland's Nobel Prize-winning work in this technical-level article.
Can science fiction influence the course of real science?
By “science fiction,” I don’t mean fantasy—vampires, werewolves, elf princesses, that kind of thing. Science fiction may seem fantastical, but even its most fantastic elements are driven by real science.
The obvious predictions of science fiction are all around us, from iPads to cell phones and various other electronic wonders that we treat as disposable. My 2-year-old son entertains himself with toys that are more technologically sophisticated than the first computer I ever owned. The next phase in casually transforming us all into cyborgs may be fully-immersive augmented reality, at least if Google has anything to say about it.
Science fiction isn’t just a sneak preview of future gadgets, though. For scientists, it is an inspiration machine. The theoretical physicist and TV personality Michio Kaku recalls watching "Flash Gordon" in his youth and realizing that the real hero of the series wasn’t the handsome, athletic Flash: it was the brilliant scientist Dr. Zarkov. As Kaku recounts in his book "Physics of the Future," “[Dr. Zarkov] invented the rocket ship, the invisibility shield, the power source for the city in the sky, etc. Without the scientist, there is no future.”
Dr. Wernher von Braun (center), then Chief of the Guided Missile Development Division at Redstone Arsenal, Alabama, discusses a "bottle suit" model with Dr. Heinz Haber (left), an expert on aviation medicine, and Willy Ley, a science writer on rocketry and space exploration. Source: NASA, via the Wikimedia Commons
To Stephen Hawking
, science fiction offers a kind of exercise for the imagination. As he wrote in the forward to Lawrence Krauss
’ 1995 classic "The Physics of Star Trek":
“Science fiction [...] is not only good fun but it also serves a serious purpose, that of expanding the human imagination. We may not yet be able to boldly go where no man (or woman) has gone before, but at least we can do it in the mind.[…] There is a two-way trade between science fiction and science. Science fiction suggests ideas that scientists incorporate into their theories, but sometimes science turns up notions that are stranger than any science fiction.”
Yet even some of those strange notions, like Einstein’s theory of general relativity, were anticipated by science fiction. As Krauss pointed out in "Hiding in the Mirror," the very first page of H.G. Wells’ "The Time Machine," published in 1895, included an explanation from the unnamed time traveler about how objects require existence in time as well as space. To modern ears, his description sounds a lot like Einstein’s vision of space and time.
Yet at the same time that Wells was presaging Einstein, some physicists believed that science was turning the final pages in the book of nature. In 1900, the scientist Lord Kelvin famously declared that physics was nearly complete—that we only needed to solve two minor lingering problems to know all there was to know about the universe. As it turned out, resolving those two problems did not usher in the end of physics—it led directly to the theory of relativity and quantum theory, as well as all of the scientific discoveries and technology that’s come about from them: television, nuclear energy, computers, transistors, cell phones...you get the idea. So while physicists thought that we were nearing the end of a journey, science fiction writers, with their fantastical stories of time travel and robots, showed that we were just at the beginning. And the science fiction writers were right.
In the aftermath of this quantum revolution, science fiction doubled down. This was the era of pulp adventures like the "Flash Gordon" serials that inspired Michio Kaku. Science fiction authors like Isaac Asimov, Robert Heinlein, and Arthur Clarke were the vanguard of a generation of science fiction authors who also had strong scientific backgrounds.
And it wasn’t just science fiction authors writing about the future. Theoretical physicist S. James Gates, Jr. recounts how his father brought home four non-fiction books in the late 1950s, all written by the science writer Willy Ley. With titles like "Space Pilots," "Man-Made Satellites," "Space Stations," and "Space Travel," these books brought scientific credibility to dreams of mankind’s star-faring future and inspired Gates to pursue the sciences. (Lest we think the only benefits of science fiction are intellectual, in a recent interview for the radio program "On Being", Gates also relates how Isaac Asimov’s "Lucky Starr" books helped him cope with his mother’s death.) Today Gates serves as director for the Center for String and Particle Theory at University of Maryland.
This isn’t to say that science fiction gets everything right, of course. For one thing, the golden age of sci-fi was full of laser pistols and flying cars that never quite made it into the mainstream. (At least, not yet.) In "The Amazing Story of Quantum Physics," physicist James Kakalios explains that this pulp-era futurism went awry by over-estimating the amount of energy we’d have access to. Turns out it takes a lot of energy to build a laser pistol or a flying car!
But the deepest error in science fiction—and the one that most rankles physicists like Gates—is how easy it often makes scientific accomplishment look. “I know from a life in science that nothing could be farther from the truth. The effort to advance science is one of the most monumental struggles I have witnessed in my life. Progress is usually painfully slow.”
David Brin, a physicist who now has a successful career as a science fiction author, agrees:
“The most annoying thing is when sci-fi or fantasy stories get the process of science all wrong. When, for plot reasons and just to get the heroes in jeopardy, they show science and scientists behaving in ways that are paranoid, incurious, conniving, unscrupulous, and addicted to secrecy....But science is about doing things in the open. And that's when horrible mistakes get pointed out, in advance.”
Science fiction has its fair share of mad scientists slinking about in gloomy dystopias. But more often, it is an optimistic genre. When science fiction author Robert J. Sawyer was recently asked about his top five science fiction predictions, for instance, his top pick was that there was a future. During the Cold War decades, it was science fiction that offered a hopeful vision of the future. Gates actually attributes much of the success of science fiction to the zeitgeist of this post-World War II era. “The challenge of Sputnik only turbo-charged these conditions and the kind of science fiction produced in this climate was almost guaranteed to have caught my attention.”
The finest science fiction is inspired by the same thing that has inspired the greatest science discoveries throughout the ages: optimism for the future. As I read today’s science fiction, I worry that many modern authors do not seek to inspire the way they once did. Brin points out that “images of a can-do, problem-solving humanity seem to be offered less and less,” despite his own best efforts to buck this trend.
Like Gates, I was strongly influenced by Isaac Asimov, who first inspired my interest in science, fiction, and the future. Which authors have inspired you with their hopeful visions of the future?
Editor's picks for further reading
Bulletin of the Atomic Scientists: The Science Fiction Effect
In this essay, Laura Kahn explores the connection between science fiction and science fact.
Inside NOVA: Cinema Science: Time Travel
In this blog post, explore the real science behind time travel as seen in science fiction films.
Smithsonian’s Surprising Science: NASA Picks Best and Worst Sci-Fi Movies
Find out which films get the science right—and which ones get it very, very wrong.
Technology Review: The Best Hard Science Fiction Books of All Time
Conventional wisdom has it that putting the words “quantum gravity” and “experiment” in the same sentence is like bringing matter into contact with antimatter. All you get is a big explosion; the two just don’t go together. The distinctively quantum features of gravity only show up in extreme settings such as the belly of a black hole or the nascent universe, over distances too small and energies too large to reproduce in any laboratory. Even alien civilizations that command the energy resources of a whole galaxy probably couldn’t do it.
Physicists have never been much for conventional wisdom, though, and the dream of studying quantum gravity is too enthralling to give up. Right now, physicists don’t really know how gravity works—they have quantum theories for every force of nature except this one. And as Einstein showed, gravity is not just any old force, but a reflection of the structure of spacetime on which all else depends. In a quantum theory of gravity, all the principles that govern nature will come together. If physicists can observe some distinctively quantum feature of gravity, they will have glimpsed the underlying unity of the natural world.
Even if they can’t crank up their particle accelerators to the requisite energies, that hasn’t stopped them from devising indirect experiments—ones that don’t try to swallow the whole problem in one gulp, but nibble at it. My award-winning colleague Michael Moyer describes one in Scientific American's February cover story, and lots of others are burbling, too. Rather than matter and antimatter, “quantum gravity” and “experiment” are more like peanut butter and chocolate. They actually go together quite tastily.
An example came out at the American Astronomical Society meeting in Austin earlier this month. Robert Nemiroff of Michigan Technological University presented his team’s study of extremely high-energy, short-wavelength cosmic gamma rays. The idea, which goes back to the late 1990s, is that short-wavelength photons may be more sensitive than long-wavelength ones to the microscopic quantum structure of spacetime, just as a car with small tires rattles with road bumps that a monster truck doesn’t even feel. The effect might be slight, but if the photons travel for billions of years, even the minutest slowdown or speed-up can appreciably change their time of arrival. Nemiroff’s team focused on gamma-ray burst GRB 090510A, observed by the Fermi space telescope. It went off about 7 billion years ago, and photons of short and long wavelength arrived at almost the same time—no more than about 1 millisecond apart. Any speed difference was at most one part in 1020, implying that quantum gravity hardly waylaid these photons at all.
Theoretical physicists have long debated whether quantum gravity would alter photon speed, and most were not surprised by the negative result. But what’s important is the change of mindset. Experimenters and observers care less about what we should see than what we can see. These are people who love to build stuff. If they can build some gizmo that might bring gravity and quantum mechanics into contact, they’ll do it, whatever the theorists might say. They take an “if you build it, something will come” attitude. Historically, physics has been well-served by going out to look at nature with a minimum of prejudice.
The latest brainstorm is to apply techniques from quantum optics and related disciplines, which manipulate photons of light and other particles in order to build encrypted communications links, develop the components of a quantum computer, and study matter at extremely low temperatures. The tool of this trade is an interferometer, an apparatus that probes the wave nature of particles. It consists of a particle source, a particle detector, and two paths to get from one to the other. Being quantum, a particle goes both ways. That is to say, the wave corresponding to the particle splits in two, travels the distance, and fuses back together again. The relative length of the paths (or anything else that differentiates them) determines whether the waves will mutually reinforce or cancel and therefore what the detector will detect.
At first glance, these setups are the last place you’d go to look for quantum gravity. They are decidedly low-energy experiments, usually conducted on lab benches the size of dining-room tables. There is nary a gamma ray or accelerated particle to be found. But Moyer’s cover story describes how an interferometer can serve as an extremely precise ranging instrument. Any change in the paths’ relative length, as you might expect if spacetime is roiled by quantum fluctuations, will register at the detector.
Last spring, a team of physicists in Vienna led by Çaslav Brukner explored another use of interferometers: to see whether quantum particles truly obey gravity as Einstein conceived it. This isn’t quantum gravity, per se—the particles are quantum, but gravity behaves in a strictly classical way. Nonetheless, it is a fascinating case of how the two theories interact. You might think that the gravity on a single particle is way too feeble to measure, but an interferometer can manage it. You set it up so that the two paths are at different heights and therefore experience a different gravitational potential, which registers at the detector.
The Vienna team proposed sending not just any particle through the interferometer, but one that acts like a miniature clock—marking time by rotating or decaying. General relativity predicts that clocks run slower the deeper they get into a gravitational field, which, in this experiment, would act to wash away the wave nature of the particle altogether. The fading-away of the wave properties would be the unmistakable fingerprint of general relativity and a stepping-stone to quantum gravity. Current interferometers lack the necessary precision to look for this effect, but it is just a matter of time. (Sorry, couldn’t resist.) For more, see the authors’ own blog post and their paper in Nature Communications last fall.
It it also possible that quantum gravity could modify Heisenberg’s famous uncertainty principle. As Sabine Hossenfelder at Backreaction described last Wednesday, gravitational effects may set a minimum length that anything in nature could ever have, which means that no matter how much momentum imprecision you’re willing to accept, a position measurement could never be more precise than the minimum length. Experiments like this one could use tiny mirrors and springboards to pick up that effect.
Still another approach suggested by the ever-inventive Viennese is to define quantum gravitational ideas in concrete rather than abstract terms. Theorists think that quantum fluctuations in spacetime might make cause-effect sequences ambiguous, with the practical consequence of changing the types of correlations physicists observe in the lab. But the Viennese suggest thinking about it the other way round: Physicists observe certain types of correlations in the lab and, from these, draw conclusions about spacetime.
Some such correlations—those that muddle cause and effect—would be be inexplicable in ordinary physics. When quantum effects enter into play, “spacetime” loses some of the most basic features we associate with it, such as the notion that objects reside in certain places at certain times. In the Viennese scenario, you lose the ability to tell a story: One thing happened, then another, then another. It becomes a Dadaist jumble.
This approach hasn’t lent itself to a specific experiment yet, but is generally inspired by the experimentalist mindset. In this, it follows a trail blazed by Einstein himself, who developed his theories of relativity by thinking of abstract ideas in a concrete way. Even when experimenters can’t build actual experiments, their feet-on-the-ground mentality provides a fresh look at some of the hardest problems in modern science.
This post is adapted and reprinted from Scientific American; find the original here.
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