The universe is simple.
Simulation of the sky as viewed by the Bell Labs microwave receiver. Credit: NASA / WMAP Science Team
This is the cosmic background radiation as detected with a Bell Labs radio telescope in 1964. The band across the middle is the center of our galaxy. The rest is the humming echo of the Big Bang, uniform in every direction—just as theorists had been predicting.
“Which is an amazing thing,” P. James E. Peebles—one of the very same cosmologists who helped predict it—recalls thinking. “But there it is: The universe is simple.”
As Einstein once famously said, “The most incomprehensible thing about the universe is that it is comprehensible.” But why should it be? Why would something so vast and complex and old be within the comprehension of a species that spent millennia believing it occupied the center of existence? Yet century after century cosmologists have operated under the assumption that the universe is simple, and it appears to have worked—at least so far.
That assumption goes back to Copernicus. The picture of the heavens he inherited from the ancients was crowded with invisible spheres that carried the moon, sun, planets, and stars. The geometry to explain those motions was embroidered with epicycles and deferents—circles, and circles within circles, and circles adjacent to circles, all fabricated by astronomers over the course of a couple of millennia in an attempt to make sense of the motions of the celestial bodies around a stationary Earth. The problem with this picture, Copernicus realized, was that it divided the universe into two realms, the terrestrial and the celestial. What if the universe instead was one big happy realm? Once Copernicus removed the Earth from its place of privilege and set it in orbit around the sun, he arrived at equations that predicted the motions of the heavens with far greater accuracy. A century and a half later, Isaac Newton used the sun-centered model to create his law of universal gravitation—emphasis on “universal.” By uniting the physics of the terrestrial with the physics of the celestial, he showed that Copernicus was right: The universe is simple.
For the next three centuries, the discoveries of moons and planets and comets corroborated Newton’s idea, with one exception: an aberration in the orbit of Mercury. In 1915 Einstein fixed that problem, via the general theory of relativity, by reconceiving gravity not as a force that acts across space but as a property of space itself. Two years later, he published a paper exploring the “cosmological considerations” of this new view of gravity. What might this tweaked law of universal gravitation have to say about the history and structure of the universe? To keep the math simple, Einstein and then other theorists had to assume the universe was simple, too. So they returned to Copernicus’s assumption: The Earth doesn’t have a privileged position in the universe. On the largest scale, the cosmos would look the same in every direction no matter where you are in it.
Which was what the 1964 vision of the cosmic background radiation revealed. This picture of the universe, however, was almost too simple. Where were the subtle fluctuations in temperature that would represent the seeds of the galaxies, clusters of galaxies, and superclusters of galaxies—everything that would grow into the universe as we know it?
To answer that question, NASA set to work designing a satellite to look for those fluctuations. In 1991 and 1992, that satellite, the Cosmic Background Explorer (COBE), found them—differences in the temperature at a level of one part in 100,000:
The sky as seen by COBE. Credit: NASA Legacy Archive for Microwave Background Data Analysis (LAMBDA)
I met the the co-principal investigator of that project, George Smoot, in his office at the University of California, Berkeley, just days after he won the Nobel Prize in physics. Never a particularly calm presence, he was even more animated on this occasion. Underslept and overadrenalized, he shouted, “Time and time again the universe has turned out to be really simple!”
Sitting across from him, nodding emphatically, was fellow physicist Saul Perlmutter of Lawrence Berkeley National Laboratory. “It’s like, why are we able to understand the universe at our level?” he said, echoing Einstein.
Yet Perlmutter himself is among the scientists whose work has most threatened the notion that the universe will be ultimately comprehensible. In 1998, he was the leader of one of the two teams that found the expansion of the universe is not slowing down, as you might naively expect, but speeding up. (He would share the Nobel for that discovery in 2011.) At first physicists considered the discovery of “dark energy” difficult to accept—a force more powerful than gravity on a cosmic scale?—but in 2003 came the first results from the successor to COBE, the Wilkinson Microwave Anistropy Probe (WMAP):
The sky as seen by WMAP. Credit: WMAP Science Team, NASA
By reading the patterns in those even finer fluctuations, cosmologists could calculate the portion of the universe that takes the form of dark energy: 72.8 percent. So what is it?
Yet before theorists can begin to answer that question, they need to know how dark energy behaves. Does it vary across space and over time, or is it constant? The successor to WMAP, the Planck satellite, should provide a strong clue when its results are released early next year. So far, though, all the data from less precise experiments are pointing toward dark energy being constant. In that case, theorists agree, the answer to “What is dark energy?” will require them to unite the physics of the very big (relativity) with the physics of the very small (quantum mechanics), just as Newton had united the physics of the terrestrial with the physics of the celestial.
“We shouldn’t be shocked that we’re finding a few surprises,” Perlmutter later told me. “Based on just some fragment of information, and a very interesting theory of Einstein’s, people were able to try out the simplest possible model of the universe. ‘We don’t know anything but let’s imagine that it’s as simple as it could possibly be, because we have no other information to go on.’ And then they said, ‘Let’s take a few more pieces of information,’ and those pieces of information fit, and they fit well into this ridiculously simple, intentionally cartoonish picture.”
But now? We don’t know what the vast majority of the universe is. And, physicists acknowledge, we might never know. The universe just might be incomprehensible after all. But assuming the solution exists, Perlmutter at least has faith as to what it will look like: Copernicus’ solution, and Newton’s, and Einstein’s.
“Something,” he said, “equally elegantly simple.”
Antimatter sounds like science fiction, and it has certainly powered its fair share of imaginary space ships and interplanetary blasters. But antimatter itself is fact, not fantasy. Antimatter is the opposite of matter: Bring the two of them into contact and they annihilate each other, generating energy according to Einstein’s famous equation E = mc2. For every kind of matter particle we know about, there is an antimatter "opposite." Physicists have observed antimatter versions of electrons, protons and neutrons. We’ve even made a few antimatter atoms, including anti-hydrogen and anti-helium. In fact, the only thing stopping us from making an entire anti-universe—with an anti-you, anti-me, anti-everything—is that it’s very hard to make enough antimatter. And that’s a mystery too. Scientists think that, when the universe was young, antimatter and matter were made in equal quantities, yet in the universe we see only matter. Why is that? Nobody knows the answer, but it is one of the most pressing questions of modern physics.
Editor's picks for further reading
BigThink: The Search for Antimatter
In this video, Michio Kaku answers questions about antimatter.
CERN: Antimatter: Mirror of the Universe
Everything you want to know about antimatter.
Fermilab: What Is Antimatter?
In this video, Fermilab scientist Don Lincoln describes antimatter and its properties.
NOVA scienceNOW: Antimatter Engines
In this video, Neil deGrasse Tyson answers questions about antimatter propulsion.
Of the four fundamental forces of physics, gravity is the one you know in your bones. Gravity owns you. Try to cross a downhill street slick with ice and you slide helplessly; whatever is in control, it's not you. First you appreciate friction, then you understand the full omnipotence and omnipresence of gravity.
Gravity pulled the first matter in the earliest universe into the largest structures and inside them, spun up galaxies in which stars coalesced. Gravity regulates stars' lives and when some die, it compacts them into neutron stars or black holes. Gravity sets the orbits of planets around their stars. On Earth, it drags down mountains, moves glaciers, creates the tides, and drives all convecting systems, from the earth's fluid mantle to the weather to a pot of soup. Physicists understand gravity in great detail and with great accuracy, but they suspect they’re missing something—something big enough to change or even unify our most comprehensive theories of the universe.
Gravity was the first force to be studied quantitatively, says David Kaiser, historian of science at MIT, most notably by Isaac Newton, who was trying to understand motion. The force that drives a moving body equals the body's mass times its acceleration, Newton said: The larger it is and the faster it accelerates, the bigger the force. Newton put that equation together with the laws proposed by Johannes Kepler—in particular, the law that says the planets' distances from the sun are related to the time they take to orbit it. And then, astoundingly—"It's famously difficult to figure out what led Newton from A to B in his head," says Kaiser—Newton proposed a law that describes the force between his putative, famously falling apple and the Earth. He then extended that law to the force between any two celestial bodies. Newton called this universal gravitation, the revolutionary idea that, whether you’re an apple or a planet, whether you’re falling from a tree or orbiting in space, you obey the same rules.
Using Newton's equations, scientists could for the first time measure masses that are otherwise immeasurable. "What amazes me," says Gabriela Gonzalez, physicist at Louisiana State University, "is that we can weigh the sun that way." Universal gravitation allows us to “weigh” planets, binary stars, black holes, and even the invisible dark matter which floats otherwise undetectably through the universe and about whose nature, no one has a clue.
Newton's law had a flaw: It did not explain how one thing could act on another instantly, across any distance, with nothing in between. Nobody liked this “action at a distance,” including Newton. "It sounded occult," says Kaiser, "like alchemy."
Einstein didn't like it either, and found an alternative. In his general theory of relativity, he proposed that gravity is the result of the nature of space-time. Space-time can be thought of as a continuous three-dimensional fabric which a body warps according to its mass; the more massive the body, the deeper the warp. A smaller body is not attracted to a larger one; it's just rolling into the deeper valley.
But Einstein's theory of gravity contains a flaw, or maybe just a puzzle. Gravity doesn't fit in with the universe's other three fundamental forces: the electromagnetic, the weak, and the strong. The other three can all be described by quantum mechanics, which explains the three forces as fields created and carried by waves which are also particles. To date, gravitational waves remain undetected and gravitational particles called gravitons are probably undetectable. So at bottom this force that's so familiar, whose quantification you read every day on your bathroom scales, is—what?
This is where gravity becomes odd. If gravity is, as physicists say, mass telling space how to curve and space telling mass how to move, then space is inextricably related to mass. And mass, says Einstein's E = mc2, always implies energy. So space must have energy too. And it does: In quantum theory, even empty space—a vacuum—has energy. The amount of energy in the vacuum, say quantum theorists, is so enormous that space should be curved so tightly that the universe would fit into a proton.
You could be forgiven for thinking this last is the ravings of theoretical physicists. But vacuum energy also crops up in another problem. For the last 14 years, astronomers measuring the universe's expansion have found that the universe is not, as they'd expected, being slowed by the pull of its own gravity. Instead, the expansion is accelerating, speeding up; some push is countering gravity's pull. A physicist with a sense of poetry, Michael Turner at the University of Chicago, called the push "dark energy." And Turner and other physicists say that the simplest, most elegant explanation for dark energy is the energy in the vacuum.
Except the universe isn't curled up inside a proton: Simple and elegant or not, vacuum energy doesn't make the dark energy problem go away. "The mystery of dark energy," says Leonard Susskind, physicist at Stanford University, is that compared to the calculated amount of vacuum energy, "there's so little of it." Maybe the calculations are wrong. Maybe dark energy won't be understood until the excruciatingly complex supersymmetric and/or string theories get worked out. By this point, however, the reasoning is so mathematical—"Oh boy, is it mathematical!" says Susskind—that it's hard even for physicists to follow.
An ordinary human hardly knows what to make of it. First, you can believe that Newton and Einstein between them described gravity exquisitely. Second, theorists don't have the last word, and experimentalists are looking for gravitational waves. Gravitational waves are created when accelerating bodies distort space-time, says Gonzalez, "though the distortions are very, very small." But something like two neutron stars coalescing into a black hole should create intense waves that should be detectable.
The most sensitive experiment is the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which can measure distortions smaller than 10-18 meters. LIGO hasn't yet found gravitational waves, but it's being upgraded to be able to detect waves over a larger volume of sky.
In fact, a number of different experiments, proposed and operating, stationed all over the world and out in space, are trying in differing ways to find gravitational waves of differing wavelengths coming from astronomical objects that range from binary white dwarf stars in our own galaxy, to the echo of the big bang itself. None of these experiments have found gravitational waves either. But if they do, the waves will carry new kinds of information from the hearts of the universe's most turbulent creatures.
So what's the matter with gravity? It may or may not be related to dark energy and it doesn't fit in with the other forces. If it's not a particle or a wave, then what else it might be is unclear. "And that's where it sits now," Turner says. "But wouldn't you rather have no answer than the wrong answer?"
Editor's picks for further reading
About.com: Introduction to Newton’s Law of Gravity
Astronomical Review: Why Gravity is So Weak
In this essay, Martin Rees compares the strength of gravity to the strength of the other fundamental physical forces.
FQXi: The Myth of Gravity
An article on a new model in which gravity is not a fundamental force.
NOVA: Relativity and the Cosmos
In this essay, Alan Lightman explores the history and meaning of general relativity.
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
Last week, we asked whether astronomers could be wrong about dark matter, the invisible stuff that seems to help hold galaxies together. Is it possible that dark matter doesn’t really exist?
This week, we’ll investigate whether there are viable alternatives to the idea of dark energy, the mysterious stuff that astrophysicists believe is pushing our universe apart.
In every direction we look, galaxies are hurtling away from us. That isn’t surprising in itself—after all, the Big Bang sent space and everything in it flying apart. One would expect that the gravitational pull of all the “stuff” in the cosmos would gradually slow down this expansion, bringing it to a dead stop or even collapsing everything back together in a "Big Crunch." Yet instead, astronomers see that the galaxies in our universe are rushing apart faster and faster.
What could be causing this acceleration? Physicists call it dark energy, and it could make up more than 70 percent of the cosmos. But so much remains unknown about dark energy that some scientists are asking whether it exists at all.
What if, instead of a mysterious unseen energy, "there is something wrong with gravity?" asks Sean Carroll, a theoretical physicist at the California Institute of Technology.
Einstein's theory of general relativity represents gravity as the curvature of space and time. Perhaps this idea "is still right, but we're not solving the equations correctly," suggests Carroll. "We're used to thinking of the universe expanding perfectly smoothly, and we know it isn't, and maybe these deviations are important." If we accounted for how the universe is clumpy instead of smooth, it might turn out that the gravitational pull of clusters of galaxies and other large agglomerations of matter alter spacetime more than previously appreciated. Distant objects would thus appear to be farther away than they actually are, leading to the false conclusion that the universe's expansion is accelerating.
The problem with this kind of model, Carroll says, is that while it suggests that these clumped-up astronomical bodies might distort our view of the universe more than suspected, gravity still remains the weakest of the known fundamental forces of nature. Also, these astronomical clumps would evolve in size and gravitational strength over time. In contrast, the mysteries that dark energy was invoked to solve require something with a lot of energy that changes less over time.
Another approach is to modify the laws of gravity to do away with dark energy. This tack suggests that "the laws of gravity as we know them work better on relatively small scales such as our solar system,” says Carroll, but perhaps they need “tweaks” to work on cosmic scales. Carroll and other theorists have developed alternative descriptions of gravity that could explain why the universe evolved as it did. One set of scenarios suggests that the strength of gravity increases over time and has different values depending on the distances involved. But critics argue that, to avoid contradicting well-established features of general relativity, these models are unacceptably contrived.
Another family of alternative gravity models analyzes how gravity behaves if there are extra dimensions of reality, as suggested by string theory. But this approach has problems of its own: It leads to empty space "decaying" into particles in potentially detectable ways, Carroll says.
To avoid modifying gravity, some theorists have suggested that our galaxy and its neighborhood might lie within a giant void, an emptier-than-average region of space roughly 8 billion light years across. With so little matter to slow down its expansion, the void would expand faster than the rest of the universe. If we lived near the heart of this void, our observations of accelerating cosmic expansion would be an illusion.
"The advantage of giant void models is that they don't require any new physics to explain the apparent acceleration of the universe, like the existence of some weird dark energy or a modified theory of gravity," says theoretical cosmologist Phil Bull at the University of Oxford.
Still, "there are lots and lots of problems with void scenarios," says theoretical physicist Malcolm Fairbairn at King's College London. "It's very difficult to get them to fit existing data—for instance, the cosmic microwave background (CMB) radiation usually gets distorted in these models compared to what we actually see." For the void model to match observations of CMB radiation, we would need to be very close to the center of the void, to within one part in 100 million. That "seems like an unacceptable 'fine-tuning' to some people,” says Bull. “Why should we find ourselves so close to the center?"
In addition, astronomers using NASA's Hubble Space Telescope recently found evidence against the existence of such a void. After refining their measurements of the rate at which the universe is expanding, they all but ruled out the possibility that the accelerating expansion is an illusion created by a void. In addition, if we are living inside a void, Bull and his colleagues argue, we should see very strong fluctuations of cosmic microwave background radiation reflected off hot gas in the clusters of galaxies surrounding the void. Yet we do not see any reflections that strong. "This was pretty much the final nail in the coffin for void models," Bull says.
To support the existence of dark energy—or vindicate one of these alternatives—we need giant sky surveys which will clock the speeds of even more galaxies, Fairbairn says. The colorful scenarios that theorists are dreaming up "ultimately show what an interesting and weird universe we live in," Carroll says. "It's one where we must keep an open mind as to what the answers may be."
Editor's picks for further reading
COSMOS: Doubts Over Dark Energy
Reexamining the evidence for dark energy.
New York Times Magazine: Out There
In this article, Richard Panek explores the evidence for dark energy.
NPR’s 13.7: Dark Energy and the Joy of Being Wrong
In this blog post, Adam Frank recounts the history of the discovery of dark energy.
What is dark matter?
An invisible substance thought to make up a quarter of all the “stuff” in the universe, dark matter leaves its gravitational fingerprints all over the cosmos. But despite decades of trying, scientists have failed to capture a single speck of dark matter, in part because they don't have a clear idea of what it actually is.
But what if the solution to the mystery of dark matter is that dark matter doesn't actually exist? What if this ghostly stuff is just a phantom of astronomers’ imaginations? Could there be another answer to the puzzles dark matter was invoked to solve?
Since the 1930s, astronomers have suspected that galaxies contain more mass that we can account for. That’s because, when astronomers clock the speed of stars circling around the center of the Milky Way and of galaxies moving in distant clusters, they all seem to be going too fast. They are going so fast that they should overtake the force of gravity tugging them inward and fly out into the void beyond. Yet something holds them back.
That “something,” most astronomers believe, is dark matter: matter we can’t see yet which has enough mass to keep those speeding stars in stable galactic orbits. But what is dark matter? Scientists have largely ruled out all known materials. The consensus is that dark matter must be a new species of particle, one that interacts only very weakly with all the known forces of the universe except gravity, with which it interacts as strongly as ordinary matter does. Dark matter is invisible and intangible, its presence detectable only via the gravitational pull it exerts.
But not every astronomer is satisfied with this interpretation. Some, like Stacy McGaugh at the University of Maryland, College Park, believe that the definition of dark matter is so slippery that it is impossible to prove or disprove. Researchers might be able rule out the existence of any specific conjectured form of dark matter particles, but "we cannot falsify the concept, so if one fails, we are free to make up another," says McGaugh. "This cycle can be endless — as long as we're convinced as a community that it has to be dark matter, we won't take alternatives seriously, but we can never be disabused of the concept of dark matter."
Instead of relying on mystery particles, a small community of researchers suggests an intriguing alternative: What if the answer lies in changing what we know about the laws of gravity? The leading alternative to dark matter is known as Modified Newtonian Dynamics (MOND). The assumption is that at large scales, the laws of gravity are different from Einstein's theory of general relativity. "MOND merely tweaks the way a known force, gravity, works—we don't have to accept that the universe is filled with invisible mass," McGaugh said.
In general, by tweaking Newton's laws of gravity when it comes to orbits at large scales, MOND predicts the velocities of stars within galaxies even better than dark matter does. "It works so well it seems there must be something to it," McGaugh said. MOND works especially well on a class of galaxies known as low surface brightness galaxies, very faint galaxies without bright centers, explains theoretical cosmologist Priyamvada Natarajan at Yale University. "It's better than dark matter at explaining the rotation curves of these galaxies, the speeds at which stars in a galaxy orbit the center."
However, critics point out that dark matter beats out MOND on other astronomical puzzles. "The biggest problem is perhaps clusters of galaxies—though MOND works well in individual galaxies, it doesn't fit clusters terribly well," McGaugh said.
In fact, even with MOND, there is still a need for dark matter. "The need for dark matter in such a theory is horrible," McGaugh said. "On the other hand, it is a fairly limited problem in scope—we believe there is more than enough ordinary matter in the universe that is yet undetected that would easily suffice to make up the difference."
Skeptics of MOND, however, point at the Bullet Cluster, two colliding clusters of galaxies. There is a clear separation of luminous and unseen matter seen there exactly matches what one would expect with the dark matter model—dark matter, being largely intangible even to itself, would "feel" the forces of the collision very differently than ordinary matter. MOND advocates say that although unseen matter could be involved, it might again be unseen forms of ordinary matter.
Maps of the cosmic microwave background—radiation left over from the Big Bang—also provide strong support for dark matter. Temperature aberrations seen in the cosmic microwave background seem to reflect the presence of both ordinary matter, which interacts with both matter and radiation, and dark matter, which influences matter but is essentially invisible to radiation.
So MOND advocates have a difficult task: Their theory must explain all the puzzles that dark matter has already solved, and it must present a new way of accounting for everything Einstein's theory of general relativity currently explains. For instance, general relativity proposes that matter and energy curve spacetime, creating the effect we know of as gravity. Massive bodies curve spacetime enough to visibly bend light, an effect known as gravitational lensing that astronomers have witnessed for decades. "We cannot explain the phenomenon of gravitational lensing without general relativity, and this is where MOND spectacularly fails," Natarajan said.
"It has proven hard to construct a relativistic version of MOND,” acknowledges McGaugh. “If one is going to introduce a new theory, it has to encompass existing, successful theories."
Meanwhile, physicists continue the quest to directly detect dark matter particles. "There are no significant results yet, but I am optimistic," says Natarajan. "In any case, I'm quite comfortable as it is with the evidence for the existence of dark matter."
But until physicists actually “see” a dark matter particle, researchers will continue to investigate alternatives to the dark matter model. "It could be wrong," McGaugh says. "We do not understand all there is to understand yet—there do remain fundamental mysteries to explore."
This is the first part of a two-part series on critics of dark matter and dark energy. Return next week for a look at alternatives to dark energy.
Editor's picks for further reading
FQXi: Out of the Darkness
Physicist Glenn Starkman is evaluating alternatives to general relativity.
NOVA scienceNOW: The Dark Matter Mystery
In this video, explore the evidence for dark matter.
Scientific American: What if There is no Dark Matter?
Could modifications to the theory of gravity eliminate the need for dark matter?
If you thought that physicists couldn’t take a joke, a web site called arXiv begs to differ.
Arxiv is a preprint server, meaning that it’s where you can get an advance look at papers that haven’t yet been published in scientific journals. Of course, not every paper that appears on the arXiv is bound for The Astrophysical Journal. And every year, just around April Fools' Day, a crop of unusual papers tends to appear on the site.
After all, April Fools' Day brings out the geeky best in us all. So let’s celebrate the week of pranks and pratfalls with some highlights from this year’s April Fools' day haul:
Non-detection of the Tooth Fairy at Optical Wavelengths: "A wisdom tooth, freshly removed from the author’s lower left jaw, was placed under a pillow, upon which the author subsequently laid her head and fell asleep. The telescope was programmed to obtain an eight-hour time series of a six-meter-radius circle centered on the author’s sleeping bag. For a distance of 17 meters, the limiting absolute magnitude M is 99.7."
On the influence of the Illuminati in astronomical adaptive optics: "It is clear that the Illuminati are alive and well in modern times (Brown 2000). For instance, it is well known that pop stars Britney Spears and Lady Gaga have been aided in their astronomical rise to the top by the Illuminati (YouTube 2012). The secret to success in ground-based diffraction-limited astronomical imaging is less well known."
Gods as Topological Invariants: "We show that the number of gods in a universe must equal the Euler characteristics of its underlying manifold. By incorporating the classical cosmological argument for creation, this result builds a bridge between theology and physics and makes theism a testable hypothesis. Theological implications are profound since the theorem gives us new insights in the topological structure of heavens and hells. Recent astronomical observations can not reject theism, but data are slightly in favor of atheism."
And one from last year:
Schrödinger's Cat is not Alone: "Cat interferometry will inevitably lead to the 'Many Cats' interpretation of Quantum Mechanics, allowing to shed new light on old mysteries and paradoxes. For example, according to this interpretation, conservative estimates show that decision making of a single domestic cat will create about 550 billion whole universes every day, with as many replicas of itself."
Of course, arXiv isn't the only place to find physics pranks. The Museum of Hoaxes maintains a colorful list of the top scientific April Fools' Day pranks, including:
The bigon: On April 1, 1996 Discover magazine announced the discovery of a new subatomic particle, the bigon. According to scientist responsible for the breakthrough, Discover reported, "Bigons could be responsible for ball lightning, migraines, the unexplained failures of equipment and soufflés, the spontaneous human combustion."
The lost day: In 2004, Nature got in on the fun with a report the trade winds had blown an entire day—specifically, April 1—off the calendar.
Have you been a part of a great science prank? Tell us about it—or tell us about the prank you only wish you could pull off.
Physicists are on the brink of a breakthrough discovery: They may have finally cornered the Higgs boson, the subatomic particle hypothesized to give mass to all the stuff in the universe. But should we really be calling this particle the “Higgs”?
A computer simulation of a detection of the Higgs boson. Or is that the ABEGHHK’tH boson? Credit: David Parker/Photo Researchers, Inc.
Peter Higgs, it turns out, wasn’t the only one to come up with the idea of a new field (the Higgs field) that endows particles with mass. In fact, he wasn’t even the first to publish the theory. That distinction goes to Robert Brout and Francois Englert at the Free University in Brussels, who wrote up the idea in August 1964. Higgs was close on their heels with his own paper in October of the same year. Just a few weeks later, Dick Hagen, Gerald Guralnik, and Tom Kibble published their take on what would come to be known as the Higgs field and Higgs boson.
This wasn’t plagiarism: It was a kind of synchronicity that is the norm in science, says MIT science historian David Kaiser. In fact, independent research groups simultaneously arrive at similar breakthroughs so often that Robert Merton, a sociologist of science, put a name to the phenomenon: multiples. One famous multiple is calculus, which was simultaneously “discovered” by both Isaac Newton and Gottfried Leibniz in the late 17th century. More recently, the accelerating expansion of the universe was observed at nearly the same time by two competing groups of astronomers, both of which were honored with the Nobel Prize in physics in 2011.
Higgs, Brout, Englert and the rest were continuing a tradition that is as old as physics itself. But why is “Higgs” the name that stuck? “Higgs expressed the challenge”—how do we get particles that have mass and still obey the rules of symmetry?—“and the expected solution especially sharply,” says Kaiser. Another recounting pins the name on Ben Lee, a physicist who used “Higgs” as shorthand in a 1972 Fermilab conference program after having had a productive lunch chat with Higgs.
Higgs himself has always been uncomfortable seeing his name ride solo. He prefers to call the particle the “scalar boson” or the “so-called Higgs,” Ian Sample writes in his book “Massive.” Higgs has also advanced the uncommonly inclusive acronym ABEGHHK’tH—that’s the Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble and ‘t Hooft—to honor all the scientists who played a part in originating the theory.
Frank Wilczek, a Nobel prize-winning physicist who has named a few particles of his own (anyons and axions—the latter inspired by a laundry detergent), thinks that the alphabet soup solution would be “especially absurd.” Says Wilczek: “History is complicated, and wherever you draw the line there will be somebody just below it!”
If the Higgs discovery is confirmed, though, someone will have to draw that line—and that someone will be the Nobel Prize committee. The discovery is seen as a shoo-in for the physics honor, but the prize can be divided among no more than three laureates. There are at least six scientists with reasonable claims on the Higgs—not to mention the cast-of-thousands teams whose instruments are responsible for the experimental evidence that the Higgs actually exists.
Complicating matters is physicists’ anarchic naming methodology. When astronomers have planets, moons, and asteroids in need of naming, they turn to the International Astronomical Union. Elements get their formal names from the International Union of Pure and Applied Chemistry. Physicists, who have no such official naming body, have historically opted for descriptive names, like “neutrino” (“little neutral one”), or names devoid of any physical meaning at all, like “up,” “down,” and “charm.” As a particle named after a person, the Higgs is essentially alone among the fundamental elementary particles.
So what should we be calling the Higgs? “By now it's so deeply embedded in the literature that changing to another name would be jarring, and might introduce a gratuitous complication in literature searches or eventually even a hurdle to parsing older papers,” says Wilczek. If he had to choose? “A possibly better choice might be ‘zeron,’ to connote that the particle has zero quantum numbers, and in some sense is an ingredient of what we call nothingness.”
“I’d find a fancy-sounding word in ancient Greek, to give it gravitas, and then add ‘on,’” says Kaiser. In the absence of a Greek dictionary, Kaiser nominates “lardon”—a particle that makes things heavy.
Ultimately, it may come down to branding. “In business, it would be considered destructive to take a well-known name and replace it with a long-winded, technical-sounding alternative that no one has heard of,” wrote the editors of Nature in a recent editorial. Indeed, “Higgs” seems to have captured the public imagination—and it makes a much better Twitter hashtag than #ABEGHHK’tH.
Now it’s your turn: If you could rename the Higgs, what would you call it?
Editor's picks for further reading
Facebook: Peter Higgs
No, you can't "friend" him, but you can "like" him.
FQXi: Higgs Almighty
Whatever you call it, please stop calling it the “God particle,” says blogger William Orem.
PHD Comics: Higgs Boson Explained
In this video, particle physicist Daniel Whiteson at CERN explains how the LHC is searching for the Higgs boson.
Imagine describing our universe to an alien from an alternate dimension. Where would you start?
You might reasonably begin by explaining that we live in three dimensions of space and one dimension of time. Space and time are so fundamental to our understanding of the universe that they are woven into nearly every equation in physics. They are the words in which we speak the language of nature—so tried, tested, and true that we don’t even know how to talk about the cosmos without engaging space and time in the conversation.
But what if it turns out that space and time are not the fundamental infrastructure of our cosmos—what if they are themselves products of some deeper physics?
This idea is called emergence. We see it in nature, as when fish school or birds flock. If you were only to study an individual fish or bird, you would never predict how they would come together as a group. Yet each one “knows” simple rules that, when combined, create a wide range of agile and elegant behaviors. Could it be that physicists have been studying flocks all along, not realizing that it’s the birds that are truly fundamental?
“There aren’t many things in quantum gravity that everyone agrees on,” says Eleanor Knox, a philosopher at King’s College London who specializes in the philosophy of physics. “Yet the one thing many people seemed to agree on in quantum gravity was that we were going to have to cope with space and time not being fundamental.”
It sounds radical, but physics has a long and proud history of spearheading exactly this kind of coup. “Historically, whenever we thought something was fundamental, it turns out that it is not,” says Nathan Seiberg, a theoretical physicist at the Institute for Advanced Study. Kepler, for instance, believed that the Platonic solids were the fundamental constituents of the universe. Today we know better. In the 17th century, scientists thought that cold was a substance that could flow from one place to another, chilling your doorstep or tip of your nose. Now we understand that heat and cold are just another way of talking about the statistical properties of a collection of molecules. Of course, that doesn’t mean that it feels any less real when you burn your tongue on your hot cocoa.
So why are physicists picking on space? Relativity delivered the first strike. “In relativity, space and time are not rigid. They are dynamic,” says Seiberg. Building all of physics on such a malleable infrastructure is akin to constructing your house on a foundation of Jello.
More alarmingly to theorists, our ability to measure features in space is intrinsically limited. A ruler can’t measure distances smaller than the width of its painted markings; the resolution of a microscope is constrained by the wavelength of the light in which it makes images; even scanning tunneling microscopes are limited by the physical size of their probe tips.
Can’t we just build a better microscope? “It’s not because we don’t have the budget to build a powerful enough machine,” explains Seiberg. If we somehow tried to make an infinitely small measuring device, that device would become so dense that it would warp the fabric of space. The conclusion: “Space itself is ambiguous,” says Seiberg. Strike two.
Space also took a hit from an unlikely foe: the hologram. We think of holograms as the dazzling, silvery images on postcards and credit cards: two-dimensional objects that project three-dimensional pictures. More generally, though, a hologram is anything—even an equation—that encodes an extra dimension’s worth of information. It turns out that you can write equations that describe our universe perfectly well using different combinations of spatial dimensions, creating mathematical holograms that are indistinguishable from reality. Like a book that can be translated into many disparate languages without losing a syllable of meaning, our universe seems to tell a story that is independent of the words in which we have always chosen to express it.
Finally, physicists have known for some time that their descriptions of space start to break down when they’re applied to the strange-but-true environments inside black holes and close to the time of big bang. In such cases, the familiar equations start popping out infinities—nonsense answers that suggest that the equations are missing some essential machinery. “Something else should kick in,” says Seiberg.
But what is that something else? “I don’t think I have an answer to that,” says Seiberg. Knox also leaves the door open to as-yet-unknown possibilities: “Whatever it is that’s fundamental, it’s not the stuff we have a handle on right now.” Morever, Seiberg adds that though theorists have assembled a strong case that space is emergent, time presents a more difficult problem. “In order to understand emergent time, we need a complete revolution in the way we think about physics.”
Letting go of space and time without ready replacements may seem like a surefire way to plunge into the abyss of abstraction. But it may be only by loosening our grip that we can come to grasp what is truly fundamental.
Editor's picks for further reading
Discover Magazine: Newsflash: Space and Time May Not Exist
If time isn't fundamental, what is it?
FQXi: Breaking the Universe's Speed Limit
John Donoghue investigates the possibility that the speed of light is not a constant.
FQXi: Melting Spacetime
Joanna Karczemarek investigates how space and time could emerge from deeper physics.
Are we living in someone else’s fantasy?
The Chinese philosopher Zhuangzi posed this question more than two thousand years ago when he recalled waking from a dream unsure whether he was a man who dreamed he was a butterfly or a butterfly dreaming that he was a man. Today, with the advent of computers that can simulate cells, cities, and even solar systems, philosophers and scientists are asking this ancient question in a new way: Are we living in a computer simulation?
This question is more than just the premise of "The Matrix." It's a conjecture that lives at the intersection of humanity and technology—and though it might seem like philosophy, it spurs ambitious new questions about what computers are capable of and about the nature of reality itself. As theorists begin to think of our universe as nothing more than a vast collection of information, can we ever truly know whether our reality is as “real” as we think it is?
The philosopher Nick Bostrom, director of the Future of Humanity Institute at the University of Oxford, posed the latest iteration of this ancient question in a 2003 paper. His "simulation argument" begins with the observation that modern computers have improved at an exponential rate since their invention. If computing power continues to grow at this pace, advanced civilizations will one day be able to build titanic, densely-packed supercomputers capable of doing everything from beating the stock market to predicting the weather months or years in advance. “Post-human” programmers might even use these machines to simulate entire civilizations, vast electronic worlds that would put today’s computer games to shame.
What would it take to create this kind of simulation?
When it comes to simulating a person, scientists estimate it might take 1017 operations per second—that's one followed by 17 zeroes—to simulate a human brain, based on the number of neurons in the brain and rate of which those neurons “talk” to each other. Assuming that simulating the sensory events a person experiences—every taste, sound, smell, touch and sight that is coded in our neurons—takes about 100 million bits per second, and that approximately 100 billion humans have lived on Earth to date, Bostrom estimates it might take 1036 calculations in total to create a simulation of the whole of human history that is indistinguishable from reality.
That’s just to simulate the parts of the universe that humans can sense. What about the microscopic structure of the Earth's interior or the subtle features of distant stars? These little details could be safely omitted until a simulated person needed to observe them. In addition, to save computing power, maybe not every person in a simulation would be fully simulated. Perhaps some of the characters in the simulation would be "zombies or 'shadow-people'—humans simulated at a level sufficient for the fully simulated people to not notice anything suspicious," Bostrom writes in his paper.
So how close are we to achieving this dream (or nightmare)? Today’s most powerful supercomputers are capable of operating at roughly 10 petaflops per second—that is, 1016 calculations per second. A planet-sized computer based on current electronics might carry out 1042 operations per second. Bostrom also notes that quantum physicist Seth Lloyd of MIT has calculated that a 1-kilogram "ultimate laptop" that operates at the known limits of physics might be capable of 5 × 1050 operations per second. So, the planet-sized computer might be able to simulate all of human history in a millionth of a second; the ultimate laptop, a hundredth of a billionth of a second.
Given that fully simulating every person who has ever lived might only take a tiny fraction of an advanced civilization's resources, Bostrom reasons that the number of computer-generated minds buzzing away inside simulations could vastly outnumber the total sum of real minds that have ever lived. If that is true, the odds are that we are simulated, not real. It may even be possible that our simulators are themselves simulated, and their simulators are simulated, and so on. "Reality may thus contain many levels," Bostrom says.
This does not prove that we live in a simulation, Bostrom emphasizes. There are a number of caveats that could stop this bizarre future before it starts. One glum possibility is that civilizations might very well go extinct or collapse—say, by annihilating themselves in a nuclear war—before they can develop supercomputers of such immense power. Another thought is that civilizations simply have no desire to commit the vast resources needed to create supercomputers. Or perhaps advanced civilizations might not indulge in such simulations—maybe they would be ethically opposed to simulating minds and their suffering, or they might prefer to entertain themselves with machines that directly stimulate their brain's pleasure centers. "Personally, I assign less than 50 percent probability to the simulation hypothesis—rather something like in the 20 percent region, perhaps, maybe," Bostrom writes, although he describes this as a gut feeling rather than part of his logical argument.
Unless the simulators decide to make themselves known, there may be no way to prove or disprove the simulation argument. Some have suggested looking for "glitches" in the simulation, but such glitches would be more plausibly explained as hallucinations, visual illusions, fraud or self-deception. Even if errors did pop up, a smart simulator could simply wipe any memory of the anomaly from our simulated brains.
If we are living in a computer simulation, how should we live our lives? "The simulation hypothesis currently does not seem to have any radical implications for how one should live," Bostrom said. Still, "it helps to shed light on, among other things, the prospects of our species."
Also, thinking of the universe as a computer may actually be a helpful approach in science. "You can start thinking about what kind of computer it is, what kind of operations can it do, what kinds of problems can it solve," said theoretical computer scientist Scott Aaronson at MIT. "That's an extraordinarily fruitful way of thinking about the universe that has led to the whole field of quantum computers—devices based on the quantum physics that explains how the fundamental building blocks of the universe behave."
We may never know whether we are living in someone else's fantasy; whether we’re the man or the butterfly. But if we do one day develop supercomputers capable of simulating minds and universes, perhaps our creations will be able to answer the question for us.