Just after the turn of the century, scientists knew that their fundamental theories weren't quite right — they just didn't know what to do about it.
If we're talking about what happened a century ago, this is where Albert Einstein came to the rescue. During the "miracle year" of 1905, he published five groundbreaking scientific papers that are still sparking innovations 100 years later.
But we could as well be talking about what's happening right now. Over the past decade, physicists have come to appreciate, to an even greater degree than in Einstein's day, just how little they know about how the cosmos works. The latest observations indicate that 95 percent of the universe consists of stuff we don't understand:
- , which can only be detected through its gravitational effect, makes up about 25 percent.
- , a property of empty space that seems to be pushing galaxies farther apart at an increasing rate, accounts for the other 70 percent.
"In a sense, it's the ultimate Copernican revolution," says Sean Carroll, a physicist at the University of Chicago. "Not only are we not at the center of the universe — we're not even made of the same stuff as most of the universe is made of."
The current knowledge gap presents "an amazing parallel" between 1905 and 2005, says John Rigden, a physicist at the Washington University of St. Louis who wrote the book "Einstein 1905."
"In 1905, if a person put their hand on top of their desk, they had no idea what their hand was contacting," Rigden says. "In other words, the nature of matter was unknown. Atoms were speculated about, and many people believed in them in 1905, but other people did not.
"Now, today, if you put your hand on top of your desk, you know what your hand is contacting — but that type of matter makes up only about 5 percent of the universe," he says. "One hundred years have passed, and we're still 95 percent ignorant about the material world. I find that amazing."
Seeking a 'theory of everything'
As if that weren't enough to chew on, physicists are still trying to do what Einstein could not: bridge the two grand pillars of modern physics, relativity and quantum mechanics, with a unifying "theory of everything" that could explain phenomena ranging from the Big Bang to the ultimate fate of the universe.
Some theorists say it could be done, if we could just add a few extra dimensions to the ones we perceive. Others, however, suspect that the effort might be fruitless.
"There's a huge, almost unanimous view that this is a fundamental problem, to unify these two things," says physicist Freeman Dyson, who like Einstein gravitated to the Institute for Advanced Study in Princeton, N.J. "My belief is that it very well may be that God didn't intend that."
So, as the world marks the centennial of Einstein's miracle year — and the 50th anniversary of his death on April 18, 1955 — legions of physicists are trying to extend the path of Einstein's genius, and sometimes leaving that path to blaze their own trail.
If the coming decades follow the model set by Einstein's miracle year, the effects won't be felt only in abstruse physics textbooks: Looking back at the past century, you can see the results of Einstein's theories in innovations ranging from the atom bomb to the microwave oven.
And it all started with a 16-year-old's daydream.
What Einstein did
Back in 1896, when Einstein was 16 years old, he tried to imagine what it would be like to catch up with a light wave. Would he see the light wave frozen in place as he sped alongside? Or would he see nothing at all?
That musing on the nature of light stuck with him over the decade that followed, as he studied physics in Zurich, then took up a post in the Swiss patent office while he was finishing up his doctoral thesis.
One of the crowning achievements of 19th-century physics was the determination that light was an electromagnetic wave. But there were still some nagging questions: For example, if light was a wave, exactly what medium was it waving in? And why did the waves always seem to travel at the same speed, whether you were moving toward them or away from them?
At the time, most physicists saw such questions as esoteric matters they'd resolve someday by tinkering with the existing theories. But not Einstein.
"The reason he was such a revolutionary was that he was much more aware than other people that there were still a lot of deep mysteries," Dyson says. "He asked much more penetrating questions."
Einstein's 'big year'
Rigden said Einstein's "big idea" was that light behaved not only like a wave, but like particles as well. The first of his miracle-year papers, completed in March 1905, showed how the particle theory could explain phenomena that puzzled wave theorists. The big idea has had practical consequences to this day, ranging from the rise of digital cameras to the rumblings of NASA's solar-powered Mars rovers.
In April 1905, Einstein finally completed his thesis, laying out how the properties of solutions could be used to determine the dimensions of molecules. He followed up with yet another paper claiming that the mysterious jiggling of microscopic particles, known as Brownian motion, were actually caused by molecular collisions.
Not only did that research confirm that matter was composed of atoms of a particular size, but the statistical methods that Einstein used are still being applied to fields ranging from air-quality monitoring to stock market analysis.
In June 1905, Einstein published a paper answering his own teenage riddle: It would be impossible to catch up with a light beam. No matter how fast you were running, the beam would flash away at its usual speed. In observing the universe, scientists could disagree about their measurements of space and time, but the laws of physics would always hold true.
That was the heart of Einstein's special theory of relativity, and it planted the seeds for a follow-up paper that September, proposing an equivalence between mass and energy expressed by the formula E=mc2. Einstein's claim encouraged other researchers to plumb the secrets of the atom, eventually opening the way for nuclear power and atomic weapons.
"That was a big year," Rigden says. "It got bigger as time went on."
In 1907, Einstein had what he called the "happiest thought of my life," that a person falling freely would not sense any gravitational field. In 1916, Einstein turned that realization into his general theory of relativity, which revolutionized the concept of gravity. In 1917, he wrote yet another paper that laid the groundwork for the invention of the laser four decades later — the theoretical keystone for technologies ranging from space-based weapons programs and fiber-optic communications to DVD players and grocery-store scanners.
Einstein's final quest
By the 1920s, Einstein's theories won him international fame — but also resentment. As a Jew in Germany, he came up against the anti-Semitic sentiments that would eventually force him to find refuge in the United States.
There was a scientific falling-out as well: Other physicists focused more and more on the fuzzy, probabilistic aspects of quantum theory — a view that Einstein dismissed with the famous statement that "God does not play dice."'
Einstein took a different path, searching for a deeper theory that would unify gravity and electromagnetism. "Everyone else was doing quantum theory, and he was trying to do the unified field theory," said Nancy Thorndike Greenspan, the biographer of physicist Max Born, one of Einstein's contemporaries.
In 1947, Einstein poured out his frustrations to Born in a letter: "I am quite convinced that someone will eventually come up with a theory whose objects, connected by laws, are not probabilities but considered facts, as used to be taken for granted until quite recently. I cannot, however, base this conviction on logical reasons, but can only produce my little finger as witness."
Einstein died in 1955 without finding the theory on which he staked his reputation and his little finger. But strangely enough, the discoveries of the past decade have revived interest in his quest.
"Other physicists made fun of Einstein back in the '30s, '40s and '50, but now many of the world's outstanding physicists are trying to do what Einstein was trying to do," Rigden says. "It's no longer something to laugh at."
Today's dark mysteries
Just as in the early 1900s, physicists today know their theories don't add up. In fact, they have an even finer appreciation of how much they don't know than Einstein's contemporaries ever did. But physicists are also hoping that ultra-powerful telescopes and ultra-high-energy particle accelerators — tools that were unavailable to Einstein — could shed light on those mysteries.
One such mystery is the nature of dark matter, the invisible stuff that makes up about a quarter of the universe and helps keep galaxies gravitationally bound. Physicists are coming around to the view that at least some dark matter consists of exotic, weakly interacting particles that don't register on current detectors.
That poses an experimental problem rather than a theoretical problem, Dyson says: "You want to find out what these damn particles are, whatever they are. What we need are better underground detectors."
Dark energy is an entirely different matter: In the past decade, physicists have gotten a great deal of high-quality data about the expansion of the universe from observations of faraway supernovas and the "afterglow" of the Big Bang itself. But instead of showing that the expansion of the universe was slowing down, as they expected, the observations revealed that the universe has been expanding faster and faster.
This weird accelerating effect was initially referred to as a kind of "antigravity," but physicists settled on the term "dark energy" as a parallel to dark matter. When you try to balance the accounts of the universe's matter-energy content, it turns out that dark energy is responsible for 70 percent of the total.
"It's not stuff that collects into one region or another," Carroll explains. "It's an energy density that as far as we know is the same everywhere in space, and also the same everywhere in time."
Einstein's theories go back to the future
When physicists started casting around for an explanation, the first place they looked was Einstein's own theories. At one time, Einstein had built a "cosmological constant" into his equations for general relativity, to account for the obvious fact that gravity wasn't causing the universe to crash in on itself. The cosmological constant represented a repulsive quality of empty space that would counteract the attractive force of gravity.
When later observations indicated that the universe was expanding due to a primordial Big Bang, Einstein discarded the cosmological constant, calling it the biggest mistake of his career. But dark energy has brought the idea back into vogue — and has also reinvigorated the Einsteinian quest to develop theories that explain general relativity as well as quantum mechanics.
"The really good news about dark energy is that it almost inevitably involves both gravity and quantum mechanics," Carroll says. "It's a feature of empty space, a feature of the quantum vacuum, and it's causing a gravitational effect. So therefore, the people who want to explain quantum gravity need to face up to the existence of dark energy. And in fact they are."
A virtual infinity of answers
Currently, the leading candidates in the quest for the unification of physics are in the realm of string theory — which describes particles as vibrating strings or multidimensional surfaces dubbed "branes." Such theories can be spun to explain quantum mechanics as well as the general-relativity view of gravity, if the equations play out in 10 or 11 spatial dimensions instead of the three we can perceive.
In fact, the existence of dark energy appears to allow for a virtually infinite number of possible solutions to the equations, Carroll says. Some might see that as a virtually hopeless task, but in Carroll's view, having too many possible solutions is far better than having no solutions.
"Since there are 10500 possibilities, you could hope to search through them and find the one that fits our world," he said. "Now, maybe that's a little bit ambitious. The number of particles in the observable universe is only 1090 — but people have started."
And there are yet more possibilities: Perhaps dark energy isn't constant at all, but represents a variable "fifth force" or "quintessence" that has gone unnoticed until now. Perhaps another class of theories, known as loop quantum gravity, will trump string theory. Perhaps we'll have to throw out all of Einstein's ideas and build a new theory from the ground up. Or perhaps, as Dyson believes, the universe just wasn't made to fit within a single "theory of everything."
Narrowing down the possibilities
To find out which possibility is the right one, some scientists are sifting through high-resolution sky surveys to see how the universe's rate of expansion has changed over the course of billions of years. Others hope that particle accelerators yet to begin operations, such as Europe's Large Hadron Collider or the International Linear Collider, will turn up weird phenomena on the subatomic scale.
"If we discovered supersymmetric particles or extra dimensions, or ruled them out, that would have a lot to say about why the value of dark energy is what it is," Carroll says.
But, just as in 1904, no one can predict exactly where the solution will come from.
"As usual, the solution will probably come from an unexpected direction," Dyson says. "So I think it's very foolish to predict how it's going to go."
Why does it matter?
If it's impossible to say where the solution will come from, it's doubly impossible to predict what the real-world consequences of that solution might be. But if the next century plays out like the last one, advances in our understanding of physics could turn today's science fiction into tomorrow's everyday technologies.
In a science-fiction world, dark energy could be harnessed as a power source or for interstellar travel. Weird quantum effects could serve as the basis for ultra-secure communications, ultra-miniaturized nanocomputer chips or new ways of storing data as holograms.
But Carroll believes cosmological research is more important for answering cosmic questions — for example, whether the universe will someday spread out to near-nothingness in a "Big Chill" (currently the most widely accepted scenario); tear itself apart in a "Big Rip"; fall back on itself in a "Big Crunch"; begin yet another Big Bang cycle; or even spawn another generation of "daughter universes."
"It might be the case that investigations into quantum gravity or dark energy might end up with tangible benefits on technology in the next 100 years," Carroll admits, "but mostly I emphasize the fact that people just want to know the answer. ... It's part of innate human curiosity to figure out how the universe around us works."
Many things may have changed in the century since Einstein's miracle year, but that innate curiosity is constant.
"I want to know God's thoughts," Einstein once said. "The rest are details."