There is a place where reality vanishes, like the fictional Alice stepping through the looking glass into unseen dimensions, according to a leading theory of the universe. Maria Spiropulu, a University of Chicago experimental physicist, aims to find that place and perhaps open the door to bizarre things even Alice never encountered.
But unlike Alice and her mirror, Spiropulu is looking for her extra dimensions among hidden, otherworldly inhabitants: elementary particles that are kicked screaming into brief existence by the giant atom smasher at Fermilab known as the Tevatron. She and her colleagues are looking for traces of the fleeting particles, like the smile left behind by Alice’s disappearing Cheshire cat.
“Our view of things has changed tremendously in the last five years,” she says. “We are being totally surprised by what we observe in nature.”
The search for extra dimensions, considered preposterous less than a decade ago, has become one of the highest priorities in physics. They hold the key to such unsolved mysteries of the universe as the nature of gravity, physicists believe, and may finally simplify our understanding of the multitude of particles and forces already discovered.
“If we had a couple of extra dimensions we would make sense of gravity and why gravity is so weak, perhaps because it’s spread into extra dimensions,” Spiropulu says. “And we would understand the physics of the infinite high energies–the beginning of the universe. It would all make sense. The way it is now, it doesn’t.”
Our view of the universe, Spiropulu suggests, is like that of the prisoners in Plato’s “Dialogues” who have been chained in a cave since birth. Seeing only shadows of people and things moving past the entrance, they conclude that the outside world is made of shadows. Similarly, our perception of the cosmos is limited to the things in front of our eyes. Extra dimensions, she says, may be our way out of the cave.
“After 2,400 years of human civilization and thinking about nature, we figured out the biggest scales of the cosmos and the smallest scales of particles and the physics in between them,” says Spiropulu. “Yet everything we know and measure makes up only 5 percent of the universe. We’re pursuing the theory of extra dimensions not because it’s extravagant, but because if we put in the extra dimensions, it gives us solutions to many of these issues.”
The search also represents Big Science’s most ambitious race, a high-stakes scramble between Fermilab and a multibillion-dollar accelerator called CERN under construction in Europe. Fermilab recently completed a $350 million upgrade of its Tevatron particle accelerator to try to answer some of science’s greatest questions before the five-times-more-powerful CERN accelerator goes online in five years. If the Tevatron comes up empty-handed, CERN will become the new mecca for physicists and Fermilab will face the prospect of dwindling funding and an uncertain future.
With more power than ever before to smash matter and antimatter together, the Tevatron fires up pure balls of energy a million times more intense then the center of the sun. The high temperatures re-create conditions that existed billions of years ago, just minutes after the Big Bang. According to the prevailing theory, it was a time filled with intense energy, many different particles and twisted dimensions, most of which have disappeared as the universe cooled. In addition to extra dimensions, scientists hope to find black holes, gravitons, supersymmetric particles, dark matter, superstrings and other exotic structures.
(But not to worry, the researchers say. If black holes are created, a long shot at this point, they will not suck in Batavia, where Fermilab is located. Nor will they swallow Illinois and eventually the world. They would be minuscule and vanish almost as soon as they are made.)
For Spiropulu, it all seems more exciting than Alice’s encounter with the Mad Hatter. “My dad says I have a bacteria in my brain that makes me ask how things work,” says Spiropulu, 33, who remembers hounding teachers all through her schooling until they came up with answers to her non-stop questions. At 9, Spiropulu wanted to become an astronaut to explore the universe and thought about enlisting in the Greek Air Force at 17 to get ready. But the air force didn’t accept women.
Her focus changed to physics: Understanding the universe requires understanding particles and forces, because the biggest things are made up of the smallest things. “A lot of people have a great deal of attraction for physics–every human does. It’s just that this is my life; it’s hard for me to imagine it otherwise.”
Many theorists believe there are at least six dimensions in addition to the three spatial dimensions–length, width, depth–and a fourth, space-time, that we know and can measure. The new dimensions might range from the irreducibly small to the infinitely large.
Already the giant accelerator is giving hints that some particles are disappearing into unseen dimensions. According to the laws of physics, energy is conserved; that is, it is neither created nor destroyed. But Spiropulu is seeing an apparent violation of the law: Some of the energy produced in the collisions that should be there in the form of gravitons–particles of gravity–isn’t. “So where did it go? Down a hole?” she asks. “With the theory that we have, there is a probability that this graviton is vanishing into an extra dimension.”
Would that mean that time travel might be possible? No one knows, but great discoveries have produced unimagined consequences that dramatically changed the world. Relativity not only predicted black holes and gave us a deeper understanding of the visible universe, it also resulted in the controlled release of nuclear energy. The strange science of quantum mechanics gave us transistors and paved the way for the great electronic revolution of the 20th Century.
“Finding extra dimensions would mean that the rules for doing things like time travel become a lot looser than we thought they were, because of the fact that extra dimensions have weird properties and shapes,” says Fermilab theoretical physicist Joseph Lykken.
“It’s hard to talk about it in a concrete way until you have a better idea of how string theory works to produce extra dimensions,” he says, referring to the idea that, at the most fundamental level, all matter, energy and the forces of nature are made of tiny vibrating strings. “By seeing extra dimensions and getting a better idea of what string theory is, you should be able to at least give a definite answer to the question: Is time travel possible and if so, how do you do it?”
If Fermilab scientists are really lucky, they might discover physics’ grand prize–superstrings, as yet unseen partners of the known particles: The partner of a quark, for example, is a squark and an electron’s mate is a selectron. Finding evidence of these theoretical structures could reveal for the first time the very building blocks of space, time, matter and energy, putting all of the confusing pieces of the universe together into one tidy package. “It has the potential to explain the whole microscopic world and the whole universe,” Lykken says.
As it is, the everyday world looks like a magic show to particle physicists. It seems real, and we think we understand it: It is made of solid objects that exist in four dimensions. But when scientists look beneath the surface they increasingly find that our reality is based on an eerie substructure that defies common sense. Ever since Einstein’s relativity theories nearly 100 years ago, physicists have been proving that a lot of what we think we know about nature is not true. “Reality is merely an illusion, albeit a persistent one,” Einstein quipped.
There have been two of these truth-revealing revolutions. Einstein showed that clocks are an illusion, that time and space are the same: Time does not measure change, but change does measure time.
And the floor you walk on is an illusion, according to quantum mechanics. On the smallest scale, all solid objects dissolve into a kind of subatomic fuzziness, consisting mostly of empty space where unimaginably tiny particles interact as both waves and particles and can be in two places at the same time.
“It’s a world in which common sense doesn’t work,” says Edward Witten of the Institute of Advanced Study in Princeton, where Einstein spent much of his academic life. “When you probe deeply, you find that the concepts you need [to understand what you find] and the ingredients you work with aren’t at all obvious in terms of our everyday experience.”
Or, as Nobelist Niels Bohr famously observed, “Anyone who is not shocked by quantum theory has not understood it.”
Now a third revolution is in the offing: Fermilab is using the Tevatron to lead the way in pursuit of supersymmetry, a theoretical model that promises to reveal the presence of superstrings and tie together the loose ends that now make the universe seem so complex and chaotic. Scientists hope the three-mile circular accelerator will generate enough energy to create particles like squarks and selectrons–manifestations of superstrings–which either disappeared after the Big Bang or vanished into extra dimensions.
“If there are extra dimensions of space, one of the things [the theory] predicts for sure is that there are new particles that exist in these extra dimensions,” Lykken says.
For most of human existence, probably no one imagined anything smaller than dust. But as civilization progressed, some scholars began to wonder about what things were made of. The ancient Greeks conducted thought experiments: If you divide an object in half and then divide one of the halves in half and so on, they reasoned, eventually you would arrive at a particle that couldn’t be divided again. In 430 B.C., Democritus called these particles atoms.
“I read about the early Greeks as a child,” says Spiropulu, who was born in a mountain town in northwestern Greece. “But I thought those people were very lazy. They were thinking a lot, but they were not doing any experiments.”
Spiropulu, who has the same inquisitiveness as her ancient Greek ancestors, began her search for gravity in her home town when she was 9. She tried to measure variations in gravitational pull by having kids jump off a wall, first alone and then flapping Styrofoam wings she had strapped to their arms. “I thought I would get a spectacular result, with the Styrofoam making a big difference. But it didn’t.”
With increasing technological prowess, scientists in the late 19th and early 20th Centuries began conducting experiments that broke up particles to find what they were made of.
First came the molecule, which was then found to be made of atoms. Atoms were found to be made of protons, neutrons and electrons. When physicists began using particle accelerators to divide protons and neutrons in the mid-20th Century, they found a teeming zoo of even smaller particles. Confusion reigned until physicist Murray Gell-Mann proposed a brilliant theory: What if there are only two basic types of particles, quarks that give substance to matter and leptons that endow matter with electrical potential. Just these two could be rearranged to make all the particles in the zoo.
At last the indivisible particle, scientists thought: Matter was made of quarks and leptons, such as the electron. Two quarks, called “up” and “down,” and the electron are what we, everything around us and the visible universe are made of. It was a wonderful time. Out of these basic particles plus the four forces–electromagnetism, the strong and weak nuclear forces and gravity–scientists thought they could account for just about everything. It was called the Standard Model and it predicted the existence of a family of quarks, all of which have been found.
But the blush of success soon faded. Why, theorists asked, are there 36 variations of quarks when only two are used to make ordinary matter, and why six leptons when only one is needed? And what about gravity? The three other forces seem to be manifestations of a single force, but gravity does not fit.
In other words, the universe looks unnecessarily complicated and nobody could figure out how to simplify it. Einstein spent the last three decades of his life in a fruitless attempt to devise a Grand Unified Theory that would bring all the particles and forces together in one elegant solution.
Physicists remained stumped for nearly half a century until the idea of superstrings and extra dimensions began to coalesce in the mid-’90s. According to this line of thought, elementary particles aren’t the point-like, dimensionless objects they were long thought to be. Instead they are wiggling strings that have substance and size, that take up space, though very small. Just as we can’t see atoms but only know of their existence in desks, doors and other massive clumps, strings are unseen until enough of them are gathered together in one place to exert an influence.
“If string theory is right, it means we’re not made out of particles, we’re made out of [vibrating] strings” that have different properties according to their shapes. Lykken said. “That’s a big change in our view of ourselves and everything else.
“Another thing: If string theory is right, there are extra dimensions. The universe is a lot bigger that we thought. We’ve only been looking in three dimensions and there are more directions to look in. We don’t know what’s out there in the extra dimensions. We don’t know what their shape is. They could be very large.”
Large extra dimensions are not all that unusual once you consider that our three spatial dimensions–forward and backward, sideways and up and down–are themselves infinite in size. And though the idea of strings seems improbable to most people, including some scientists, so has every other candidate for the final indivisible object of matter.
When Gell-Mann first came up with the notion of quarks, he didn’t believe they were real things. He thought they were neat mathematical tricks that could bring order to the chaotic zoo of elementary particles, if only on the blackboard. He was as surprised as everyone else when the quarks predicted by his theory were actually found in particle accelerators.
Extra dimensions may seem as unlikely as quarks, but they are not far-fetched to scientists. Basically, they could be something that affects us but that we can’t see. For example, people used to think the Earth was the center of the solar system, regally fixed and unmoving. Copernicus showed that the Earth actually circled the sun and is moving though space at a tremendous speed, yet we don’t notice it.
Not too long ago, astronomers were convinced that ours was the only galaxy, and the number of stars it contained ran into the paltry thousands. Edwin Hubble showed that many of the starlike objects actually were other galaxies. Suddenly the universe went from a small dimension to an extremely large one with billions of galaxies, each containing billions of stars, and all of them speeding away from us.
Scientists recently encountered two more phenomena that affect the universe on a grand scale. It used to be thought that ordinary matter–the stuff that makes up stars, planets and us–was all there was. But if that were true, spiraling galaxies would have been flung apart eons ago from their internal centrifugal force. Something unknown that exerted a great gravitational force was keeping galaxies bound together.
Scientists call it dark matter. We can measure its presence but we can’t see or feel it. Astronomers estimate there are five times more dark-matter particles than those that make up ordinary matter, and physicists believe they might exist in an extra dimension. These particles, which Fermilab hopes to uncover, do not react with other things in the universe like light does, so they are invisible. Gravity is perhaps the only thing that sneaks out of the extra dimensions.
More recently, scientists discovered something even stranger. It’s called dark energy, and it’s causing the universe to expand at an ever-accelerating speed. Making up 70 percent of the universe, dark energy also may be hiding in a yet undiscovered dimension.
Although quarks, dark matter and dark energy seem confusing, physicists believe they have a way to pull them all together in a simple formulation that will make the universe understandable. That’s where superstrings come in. The final indivisible building block of all known and unknown matter in the universe as well as all the forces, including gravity and dark energy, may be strings.
There is only one kind of string–created in the Big Bang–but it can take different shapes such as loops, ovals, spirals and lines. The shapes they take represent the extra dimensions they occupy that we haven’t been able to see so far because they are so small. But they fill space.
Just as a violin string can produce many different tones depending on where it is plucked, a string, depending on its shape, can produce a symphony of different notes, which are the elementary particles and forces of nature. In one shape, the string becomes a quark; another way and it’s an electron; still another way and it’s a graviton, the still-undiscovered gravity-carrying particle.
“Each harmonic of the string corresponds to a different particle, so you can unify all the particles as different modes of the same string,” said David Gross, director of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. “That was one of the miracles of string theory right from the beginning, that it could explain all the particles and forces we see as vibrations of the same object. So, instead of having dozens of particles, you have one string.
“We don’t know whether it really works yet. But it has this wonderful feature of unifying everything.”
Once released in uncountable numbers by the Big Bang, they could take different shapes and vibrate in different ways to produce a richly divergent universe. It’s as if the strings created in the Big Bang were used to build a grand mansion, which we think of as the universe. At first the mansion is brightly lighted, everything is visible. But as the universe expands and cools, things start to disappear from view, as though someone turned off the lights in some of the rooms.
The things in the dark rooms are still there, but we can’t see them. They are invisible to us because they don’t emit light, X-rays or any of the other wavelengths we can detect. But we know the objects in the dark rooms are there because we bump into them, like dark matter and dark energy.
They are the extra dimensions. Called membranes, they may be very big, possibly infinite in size, and they could be where most of the particles that convey gravity are hidden.
That’s why it’s so important to find out if gravitons created in the Tevatron are vanishing into an extra dimension. It could explain why gravity is so weak compared to the other forces: Perhaps most of the gravitons reside in an extra-dimensional membrane and we only feel a small part of their pull.
“I’ve been thinking about this for 15 years,” says Harvard theoretical physicist Lisa Randall, a leading membrane theorist. “The fact that there could be extra dimensions and [membranes] really does open up new doors, new ways of thinking about things. What’s different now is that we’re seeing that maybe these extra dimensions could have some impact on the things we actually see.”
