We all dream of finding the one: dependable, motivated, and beautiful, “the one” has it all. But can the search for perfection keep us from appreciating the good thing we already have?
I’m talking, of course, about the search for the one theory of everything, a theory of physics that works in all circumstances no matter how extreme, is motivated by observations, and can be expressed in a few elegant axioms. While some theorists devote their careers to finding the one, others believe that this ideal may be fundamentally unattainable. So we are left with the big question: Is the hope for the one theory of everything realistic, or should we be satisfied to settle down and grow alongside the theories we have?
From the vastness of the cosmos to the inside of an atom: Can one theory accurately describe every layer of the cosmic onion? Image credit: Adapted from Brian Westin, ProLithic 3D & NASA/JPL-Caltech/T. Pyle (SSC/Caltech) by Greg Kestin.
We are currently living with a beautiful theory, a theory that almost
has it all: the Standard Model. It is the most precise theory in human history. The Standard Model can make predictions that match experiments to one part in 10 billion. That is like measuring the width of the United States to the accuracy of a human hair. The Standard Model explains the Sun’s glow, the inner workings of computers, and every atom that makes up our bodies. This theory is in our hands, it’s reliable, and we’re pretty happy...but it isn’t perfect.
There is one huge, glaring omission in the Standard Model: It doesn't explain gravity. Of the four forces in the universe—electromagnetism, the strong force (holding nuclei together), the weak force (governing radioactive decay), and gravity—gravity is the black sheep. Not only is it the runt of the forces, with a strength around a trillion trillion trillion times weaker than the typical strength of the other forces, but our current theory of gravity is completely separate from, and at odds with, the Standard Model. Until we reconcile the two, humanity's understanding of the universe will be incomplete.
Some scientists believe that this reconciliation is just around the corner. Theoretical physicist Garrett Lisi, for one, thinks that extensions of his model, which aims to unify general relativity and the Standard Model within a single framework, can “reproduc[e] all known ﬁelds and dynamics through pure geometry."
“The theory currently evolving from this observation is wonderfully complex and gives me hope that we might be getting close to the full picture,” says Lisi.
Others, like Dartmouth physicist Marcelo Gleiser, argue that we are stuck with at least partial ignorance. “As long as we can't measure all there is in the natural world—and the point is that we simply can't—we can't have a theory of everything. As a consequence, any theory that we may have that purports to explain ‘all’ that we know of the world is also necessarily incomplete.”
Indeed, the more closely we examine the universe, the more levels of complexity we find. Will observing the world more deeply finally lead us to a theory of everything? Or will we be perpetually pulling layers from an infinite onion—a prospect that would make innumerable physicists cry?
If it is the latter, then we must be content with what physicists call an “effective field theory.” The idea is that one should describe the world with the same degree of complexity one wishes to understand. The smaller the details you aim to study, the greater the complexity you should expect from your theory. It is like looking at the sun. If we peer at it just for a moment, it seems to be a smooth, bright, glowing sphere (see on the left, below). This is an “effective theory” of the sun. But if you zoom in (on the right), there is more going on: solar flares, sunspots, and streams of hot plasma shooting into space.
Image credit: NASA/SDO
In the same way, if you peer at a collision between two particles (say, electrons), then the effective theory would describe this as a simple bounce off each other (see on the left, below), but when you zoom in (on the right), there is more complexity: the electrons exchange other particles, causing them to repel each other and "bounce."
By looking closer, you realize your perfect, simple picture of what you may have thought was "the one" correct description is just an approximation of something more complicated and complete.
Indeed, every time we find a theory that seems to “have it all,” a closer look reveals gaps and errors in the theory. Yet from the time of Archimedes, who fathered the idea that we could describe all of nature from just a few axioms, to the modern era of the Standard Model, physicists have kept searching for the one, refusing to settle for “good enough” when flaws and omissions in their theories were revealed.
It is natural to wonder, then, is the Standard Model just an effective theory, the latest in a long line of close-but-not-quite ideas? The consensus among physicist is a resounding “yes,” leaving us with questions: Is there an infinite number of layers, or if we look closely enough can we find that there is one final center onion-core? And how does gravity fit into the onion?
There is reason to believe there may be a “core” to the onion. While historically physicists have peeled back the layers of the onion by “zooming in” on ever-smaller size scales, Heisenberg’s uncertainty principle may limit the ultimate resolution at which we can observe the universe. If you go small enough, then particles don’t have a definite position, you can’t tell where they are, and looking closer could not improve the resolution. The size at which this occurs is called the “Planck scale.”
At the tiny size where we lose particle resolution, gravity, once the runt of the forces, may intensify to a strength similar to that of the forces in the standard model. Gravity would no longer be the weak outcast, giving hope that gravity may “fit in” with the standard model, producing a theory of everything that unites quantum and gravitational phenomena.
Unfortunately, current experiments are far from being able to confirm such a unification. Physicists’ most powerful experiment for examining tiny-distance physics is the Large Hadron Collider (LHC), with its incredible smashing ability that can peel away layers of the onion. But to explore the physics of the Planck scale, we would need a machine more than a million billion times more powerful.
Despite experimental limitations, some of the greatest scientists have searched for a theory of everything. Einstein spent last decades of his life looking for a theory of everything. Unfortunately he passed away before he was able to find it. Stephen Hawking also searched for the theory of everything, before having a change of heart. “Some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles. I used to belong to that camp, but I have changed my mind,” he has said. Richard Feynman, who is often considered “the best mind since Einstein” once said, “If it turns out there is a simple ultimate law which explains everything, so be it—that would be very nice to discover. If it turns out it's like an onion with millions of layers... then that's the way it is.”
So, while searching for a theory of everything is exciting, we may be well advised to take time to appreciate what we already have.
Author's suggestions for further reading
American Museum of Natural History: Isaac Asimov Memorial Debate: Theory of Everything
A panel of acclaimed physicists, including Lee Smolin, Brian Greene, and Janna Levin, debates whether it is possible to explain the universe with a single, unifying theory.
Godel and the End of Physics
In this lecture, Stephen Hawking asks whether it is possible to find a complete set of laws of nature.
The Island of Knowledge: The Limits of Science and the Search for Meaning
In his forthcoming book, Marcelo Gleiser asks if there are fundamental limits to how much science can explain.
NOVA: A Theory of Everything
In this essay, Brian Greene explores how string theory could unite quantum mechanics and general relativity.
Astronomers have a pretty sweet job. They are paid to stare at the heavens and wonder. Some of their observations are pretty ordinary, but some observations are revolutionary—like the measurements of galaxy rotation that convinced astronomers that our universe is studded with invisible mass called dark matter. In this pencast, I will explain how that apparently simple observation led astronomers to such an extraordinary conclusion.
When astronomers watch rotating galaxies and compare their observations with predictions based on Newton’s laws of gravity, they find something strange. Stars near the center of galaxies are well behaved and move as expected. However stars farther from the center are rebellious. They move far faster than the laws of physics predict they should; so fast, in fact, that these galaxies shouldn’t exist: They should be ripped apart. Since we know that galaxies have existed for billions of years, this is a glaring paradox.
This conundrum nagged at scientists for over half a century. Astronomers proposed many solutions, from suggestions that our understanding of inertia is wrong to new ideas of how gravity works. But the likeliest explanation is that galaxies contain more matter than we see.
When I say “see,” I don’t mean just “seeing” with our eyes or even with the familiar telescopes that are sensitive to visual light. I mean “seeing” with any and every kind of telescope in our arsenal, including the huge antennas that pick up radio emission from the vast clouds of hydrogen that typically make up most of the mass of galaxies.
To acknowledge the fact that this proposed extra matter is invisible to our ordinary methods of detection, we call it “dark matter.” We know it’s out there, but what is it? Come back next week for more about the quest to capture traces of dark matter here on Earth.
Editor's picks for further reading
American Museum of Natural History: Vera Rubin and Dark Matter
In this profile, learn how astronomer Vera Rubin's galaxy observations helped establish the presence of dark matter.
NOVA scienceNOW: The Dark Matter Mystery
In this video, explore the evidence for dark matter.
TED: Patricia Burchat sheds light on dark matter
In this talk, physicist Patricia Burchat explores dark matter and dark energy.
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.
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?