Steve Giddings comes into his office most mornings and sits down at his desk with a piece of paper and a pencil.
Then, he stares off into space. The 38-year-old physicist, who rides his bike to campus and likes rock climbing, is trying to solve some really difficult physics questions.
Such as why, for example, the pencil he is holding, which seems to be a perfectly normal pencil, actually might exist in 11 dimensions. But then, so do all other pencils, as well as Giddings himself, the room and the rest of the universe.
It’s a sticky problem.
“We’ve believed there are three spatial dimensions since our first inkling of what dimension is,” Giddings said. “Now this may change.”
This is the extraordinary claim of string theory, which says there may be as many as 11 dimensions, possibly including large sideways dimensions that we cannot point to or see.
The theory also claims that the elementary building blocks of the universe are tiny, one-dimensional, vibrating strings.
It’s weird. Very weird.
But more and more physicists agree that string theory could provide the answer – the one, single theory – that will unite everything in the universe under one mathematical framework, and explain all the matter and forces in it.
“If string theory describes the universe, this is certainly a revolution of the caliber of the discovery of relativity or quantum mechanics,” Giddings said. “If it really fulfills its promise of being a theory of all physics, it rises to an even higher level.”
The only trouble is the math is so complicated that they aren’t quite sure yet exactly what their theory is.
“We have neither the equations, nor the math,” Institute for Theoretical Physics Director David Gross said.
In other words, they’ve got the theory; they just don’t know what it is.
Experiments?
A direct test of string theory is entirely out of the question.
Although it is more than 20 years old, string theory is still in the realm of theoretical physics. But it’s getting a lot of theorists excited, and some big-name physicists have jumped onboard – like Stephen Hawking, who, in addition to being one of the world’s most visible astrophysicists, is a frequent visitor to UCSB. He has said he enjoys the view from the UCen.
Hawking was in Santa Barbara last quarter for a conference on string theory, along with dozens of the world’s top string theorists. The people here are widely regarded.
Twenty years ago, UCSB gambled on string theory, investing in an idea that was unproven and untested. The university had the foresight – and the luck – to choose wisely.
Now, as established physicists, both Gross and physics Professor Joseph Polchinski have permanent ocean-view offices in the Institute for Theoretical Physics. Giddings and physics Professor Gary Horowitz have temporary offices there. All four of them have prestige. They have serious research money.
And they still have most of the world, including professors in their own department, baffled as to what they actually do.
What is String Theory?
The basic idea of string theory is easy enough: The elementary particles of the universe are one-dimensional, vibrating loops. Different vibrations of the loop, or string, correspond to different elementary particles or forces. Their actions can be described mathematically, giving physicists a way to test strings in different situations.
“The whole idea seems to work quite well in reproducing the basic features of the world,” Horowitz said.
Most people are familiar with something else – the idea that matter and energy are made up of dimensionless, point-like particles, like quarks and photons. For most purposes, this standard model works perfectly for describing the world.
But it has one huge problem. In a universe made up of dimensionless particles, two of the pillars of modern physics – relativity and quantum mechanics – are incompatible.
Albert Einstein’s relativity, which claims that gravity is caused by the bending of space, relies on an assumption that space is smooth. The theory claims that an object slightly warps the space around it and that other objects in that space will change their movement through space because of the disturbance. Space needs to be smooth to accurately transmit the effects of gravity.
Quantum mechanics, which describes the universe only on an incredibly small scale, claims that the subatomic world is a seething, tumultuous mess. Because of a theory called the quantum mechanical uncertainty principle, a particle’s location and speed cannot be simultaneously determined. Instead, physicists rely on probabilities, meaning that particles, on a quantum mechanical scale, could be several different places other than the spot where they appear to be.
“There is a very high probability, of course, that I am sitting right here right now,” Gross said. “There is also, however, a very, very small probability that I am on the other side of that wall.”
Almost everything in the universe can be described using either relativity or quantum mechanics. But some extreme objects, such as black holes, require both quantum mechanics and relativity. And here, everything breaks down.
Physicists trying to combine quantum mechanics and relativity to describe black holes get nonsense math. They get probabilities that are greater than one, or probabilities that are infinite, or probabilities that are negative.
Probabilities cannot be infinite, negative or greater than one.
Enter string theory.
String theory gets around this problem because strings, unlike particles, have a dimension.
“Picture a rubber band floating through space, only with no thickness,” Giddings said. “It would also have to be around 10-33 centimeters in size.”
Because of this, strings are more constrained in their interactions than particles. All they can do is join to form a third string or split to form two strings. And, since strings are bigger than particles, the interactions spread out over space, and do not take place at one point.
“These two combined,” Giddings said, “ease the clash between quantum mechanics and general relativity.”
Good.
Put another way, string theory solves the clash because it eliminates the problem of jittery short distances. Because of the uncertainty principle, to observe shorter distances requires more energy. This, Giddings said, is why physicists have to build big expensive machines to study the fundamental nature of matter.
“In string theory, there comes a point where going to higher energies just makes bigger strings, so you never succeed in observing things at shorter distances,” Giddings said. “In a sense, shorter distances may as well not exist.”
This means, essentially, that space is smooth, because all the quantum foam can be ignored.
Although it can claim this success, string theorists still have a long way to go – and plenty of questions left to solve. Like, for example, what the theory is they are working on.
“In fact, the truth is that we don’t really know what string theory is,” Polchinski said. “A lot of scientific theories in the past have gone through periods – they take a long time to develop. Maybe you don’t have the key central principle for a while. And that’s really where we are in string theory.”
Why then, if the theory is incomplete, are so many theorists excited?
The History of String Theory
One explanation is the theory’s tremendous successes in the last 20 years.
It started originally as a theory to describe the strong force – the interaction between small particles called quarks that holds the nucleus of an atom together. But string theory had problems, like extra dimensions. In 1973, Gross and a group of researchers discovered quantum chromodynamics (QCD), which seemed to better explain the strong force. Most people gave up on string theory.
Still, a small group kept at it, and soon realized string theory was not a theory of the strong force, it was a theory of gravity. This means that one of the particular vibration patterns of strings has the exact properties of a graviton, a particle with no mass that, in theory, transmits the force of gravity. String theorists point out that everyone before, from Issac Newton to Albert Einstein, described gravity after observing its effects. String theory’s success, they say, is that it necessarily predicts the graviton.
The small group of researchers who had continued studying strings saw this and came up with three different theories, each using the same building blocks – strings – but with slightly different parameters. While the three were exciting because they provided the consistent theory of quantum gravity, they seemed impossible, Gross said, and once again, string theory seemed stuck.
But in 1984 there were several important developments. Michael Green, then of Queen Mary College, and John Schwarz, of Caltech, figured out a way to eliminate some mathematical inconsistencies in string theory, allowing the different vibration pattern of strings to more closely correspond to the particles seen in nature. Building on this, Gross made a new discovery that propelled string theory back into popularity and started the first “superstring revolution.”
That discovery was of two new kinds of string theory, called heterotic type E8 x E8 (“pronounced E eight times E eight”) and heterotic type O(32) (pronounced “oh thirty-two”). For some abstract mathematical reasons, these theories worked better.
In the same year, theorists including Horowitz and then-UCSB Professor Andrew Strominger discovered a way to explain the extra dimensions.
“Then,” Gross said, “you had what really looked like the real world.”
At this point, Polchinski said, theorists were willing to recognize that the fundamental building blocks of nature were vibrating loops. Even then the theory was not complete and extensive experimentation was too complex.
The answer was not discovered for another decade, but its discovery has been crucial to getting physicists to believe. In 1995, string theorists discovered something called dualities, setting off a second revolution.
The discovery showed that each of the five string theories was just a different version of one larger theory, and that each could be transformed into another. So, as the conditions for one theory are changed to make the math more and more difficult, another theory works in a much simpler fashion.
“The general idea is that just when things seem they are getting very complicated and maximally bad, there is some other simple description,” Polchinski said.
Now, string theorists have the one theory, which most call M-Theory, to unify the five string theories and explain the universe.
“It’s called the ultimate theory of everything, a unified theory,” Gross said. “It seems to have the ability to answer all the questions we might want to ask.”
David Gross
At 60, Gross is one of UCSB’s most prominent theorists. His contributions to theoretical physics, particularly the discovery of QCD, have led to speculation that he has been nominated for a Nobel Prize.
For his birthday in early March, Polchinski organized a conference. Hawking showed up and it was not just for the UCen view. Gross is an important man in theoretical physics.
He is a busy man, as well. He conveys a sense of detachment in his voice. And it seems like the people who ask questions below his level are wasting his time.
At the press conference for his birthday party, a reporter asked Gross to describe his contributions to the field. It was a stupid question. The reporter was obviously out of her league and Gross made sure she knew it.
He paused for a minute, while she turned red. “Profound,” he said. The room burst into laughter.
The reporter looked like she wanted to disappear. Gross had firmly established his right not to be bothered with inane questions.
From then on, only New York Times reporter James Glanz asked questions. He has a Ph.D. in plasma physics, and was able to ask Gross about possible deviations in the magnetic moment of a muon.
The New York Times has been good to Gross. Last year, he was the star of a full-page feature on the top 10 questions physicists face in the coming century. In the photo that dominates the center of the page, Gross poses in the Aspen Center for Physics, hands over a chair, a smile on his face.
Steve Giddings
Giddings has also been the subject of a full-page piece and a featured photograph.
“I hate that picture,” he said.
Actually, there are two pictures. One shows Giddings hanging off the side of an icy cliff. The other shows Giddings the professor – pointer slung across his back, standing in front of the chalkboard, a somewhat severe look on his face.
Giddings said the photographer never told him they were going to use that picture.
“What I don’t like about it is that I cooperated in posing for it without realizing how artificial it would look,” he said.
He posed for the Nexus pictures in his office, in front of his chalkboard in a sparsely furnished office in the ITP. The office is still new. Giddings only moved in recently, when he began organizing a string theory program for the institute.
The chalkboard is covered in physics scribbles. So is the pad of paper on his desk. The room is barely furnished. Giddings hangs his bike helmet on a hook behind the door and leaves his sunglasses on the desk.
Giddings has been pictured several times in Climbing magazine, including the cover of a special section on ice climbing, and he’s climbed in spots all over the world.
“I got started climbing as a high-school kid growing up in Utah; luckily I survived my mistakes,” he said. “Its challenges have enriched and inspired my life for many years.
“I’m not sure, but in part I think that climbing breaks some of the stereotypical images of scientists, which aren’t too accurate anyway. I think it’s good for physics to break these stereotypes and for people to perceive scientists as human beings with lives and other interests, and to perceive science as a profound but human endeavor.”
Giddings is busy these days. One of the ITP’s big draws is the five-month conferences it holds once a year. This year, the conference is on string theory and Giddings is one of the chief organizers.
He is hard to catch in the office, but he strides around the building, popping in and out of colleague’s offices with the air of a man who has many places to be.
Giddings does this in the middle of an interview with Polchinski, sticking his head in to remind his colleague to go to a conference on neutrinos before passing on to his next appointment.
Joe Polchinski
Polchinski has one of the nicest offices, probably, of any string theorist in the world. It’s on the second floor of the ITP, with a view of the beach and the coastline stretching down from Goleta Beach to Hope Ranch.
He speaks softly but confidently, and frequently jumps out of his chair to draw on the chalkboard. It’s much easier to explain string theory with the aid of pictures.
But it’s still hard to conceptualize. Polchinski said his 14-year-old son, Steven, asks him to point to the extra sideways dimension, which, of course, he can’t.
Steven is one of Polchinski’s two boys. The other is 11-year-old Daniel. The kids like sports, particularly roller hockey. Rather reluctantly, Polchinski took the two to see an XFL game several weekends ago.
“Not my idea,” he said.
Polchinski and his wife, Germanic Studies Associate Professor Dorothy Chun, are also on a coed roller hockey team with Steven.
At a conference Saturday, Polchinski’s family sat with a crowd of 140 high-school physics teachers to watch him speak. Then, long before the conference was over, they headed home to watch the NHL playoffs.
Gary Horowitz
Horowitz moved into the ITP recently, and temporarily, to organize that conference. The goal was to bring high-school physics teachers up to date on the latest in physics with a series of lectures by high-profile string theorists.
Although he directed the conference, herding teachers in and out for refreshment breaks and reminding them to pick up their ITP hats, Horowitz seemed to stay out of the way.
He seems, generally, to prefer life out of the limelight. He walks on the beach and enjoys gardening, a far cry from Giddings’ adventures on the mountain.
Horowitz looks and talks like a teacher. His voice is quiet but steady, and he explains concepts clearly. He usually teaches one course a quarter, although he took Spring Quarter off to organize the conference.
It seems odd that a man who took part in the explanation for extra dimensions and who has worked closely to study black holes has regular office hours.
Nonetheless, he’s there, in his new ITP office, in a button-down shirt and a sweater, just like the stereotypical physics teacher.
His new office is a significant step up from the old one on the third floor of Phelps Hall. It still looks new: The ethernet outlet on the wall has not been covered up, leaving an exposed yellow cable climbing around the room under the chalkboard.
But the view of the ocean and the lawn beneath the ITP is much better than the view of the airport from Phelps. And, although he said he hasn’t taken advantage of it yet, the walk along the top of the bluffs beats the walk past Campbell Hall.
UCSB
Giddings, Gross, Horowitz and Polchinski make up the core of one of the strongest string theory programs in the country.
“It’s pretty neat that a public school like this, and not even the biggest school in the UC system, can compete on this basis,” Polchinski said.
Part of that is luck. In the 1980s, when string theory was in doubt, skeptics questioned whether string theorists should be hired. UCSB was one of the few places that had the interest in recruiting some.
“They brought really good people here,” Polchinski said. “And it’s great. Berkeley has none. On most fields, Berkeley is on the top of the UC system. UCLA has a few people, but not on the caliber of here. But Santa Barbara … ”
The other, bigger part of UCSB’s strength is the ITP. The conventions, which usually draw the best minds in the field, are a big aid in recruiting. The constant exposure to visiting scholars helps to advance research and keeps the top people coming here.
How Does A Person Go About Working on String Theory?
String theory is not an individual process. All four of the researchers said they spend considerable time reading other people’s papers and conferring with other physicists.
Giddings said the campus string theorists have lunch together to talk physics, and usually go to two formal seminars per week.
The other activity Giddings engages in, popular among theorists, is to sit down at the desk – pencil in hand, paper on the desk – and stare off into space.
Polchinski said he spends his research hours reading papers to keep up on new developments and talking to colleagues. When he is fully prepared, he begins his “creative” work.
“The staring off into space bit – I usually do this when pacing, sometimes on the cliffs outside the ITP,” he said. “I used to play a lot of chess, and this is much the same: moving pieces around to see whether one can find a good pattern.”
With all the pacing done, Polchinski moves on to the math.
“A lot of it is just very nasty calculus. Derivative and integrals, just technique piled on technique,” he said. “A lot of it is geometry, the shape of things. Once you have six dimensions and things aren’t round, they’re kind of lumpy … it kept mathematicians busy for centuries trying to characterize different shapes.”
String theory has been very exciting for the mathematicians, who get to invent new math and new structures to help define the 11-dimensional universe.
Edward Witten, one of David Gross’s former students, has been a pioneer in this field. Witten is a math genius who looks like a math genius – shocks of hair reminiscent of Einstein and a nervous voice.
He’s been responsible for much of the math behind string theory, including, along with former UCSB professor Andrew Strominger, one explanation for the other six dimensions. Those dimensions are curled up in six-dimensional shapes, so tiny that they are beyond the reach of the most sophisticated detection equipment.
Called Calabi-Yau shapes, the structures look like a crumpled up piece of paper that is folded over and over upon itself. The theory suggests that these structures are hidden inside the three large dimensions that we see.
Craziness: M-Theory, D-Branes and Extended Dimensions
When researchers discovered dualities in 1995, it was a big deal.
“You can continuously – we believe – transform one of those theories into another one,” Gross said. “So there’s really only one theory, but we’re not sure what it is.”
Most string theorists call that theory M-Theory, although Gross said he prefers to “reserve the name for when it’s invented.”
One of the implications of M-Theory is that strings are not actually the fundamental objects of the universe.
Horowitz and Strominger had found earlier that in any of the five theories, as the experiment was twisted, the string became a two-dimensional membrane.
Polchinski discovered that there was no reason that there could not be more dimensions, and a new fundamental object, called a brane, was born. A string, which is the best known type of brane, is simply a one-dimensional brane, or “one-brane.”
The weirdest part is that branes can exist in any dimension up to nine.
“One might have thought that strings were fundamental, and everything else like these branes were made of them,” Polchinski said. “Or maybe it’s the other way around, these are the fundamental things and everything else is made of them. Or, most likely, it’s some third thing which has some of the properties of each.”
The discovery of branes has left string theorists “groping to discover what the theory really is,” Giddings said. Now, they’d like to know what, exactly, the fundamental objects in string theory are.
“Meanwhile,” he added, “we know enough to discover a lot of other interesting phenomena, like the possibility that the extra dimensions are much larger than previously thought. These are very interesting times.”
Interesting indeed. One of the speculations from the Gross birthday conference was that the extra dimensions in string theory may be larger than previously thought.
Big meaning either smaller than a millimeter – at that length the extra dimension could be detected by particle accelerators – or big meaning the extra dimensions could be infinite.
Either way, the extended dimensions can be kept hidden by impurities in smooth space. Essentially, people could be stuck in one of these impurities. And since everything scientists would use to probe the space, like light, is also stuck in the impurity, it is possible that the extra dimensions could stay hidden, beyond the reach of humans.
It is generally assumed that gravity is not stuck to the impurity, so experiments using gravity could detect any dimensions larger than a millimeter.
Still, Polchinski said, “The fact that they could be that large was a surprise.”
The other possibility, which was suggested only recently, is that gravity is also stuck in the impurities.
“In that case, you really could be sitting at a point in a much larger space, in these sideways directions that I can’t point in, and you would have no way to know directly,” Polchinski said. “Because all of the things you would be trying to use to see this direction, they don’t go that way.”
This seems, in part, to explain the staring off into space bit. The idea is still so speculative that the researchers who first suggested the large extended dimensions came up with the idea not through confirmation, but because experiment hadn’t ruled it out.
“The surprise,” Polchinski said, “was that it wasn’t obviously wrong.”
If the dimensions do turn out to be large and extended, they could conceivably be discovered with better particle accelerators. Sooner or later, when two particles are smashed together with enough force, one of them will fly off in the extra dimension.
“That’s speculative, and it could very well be that the old picture, where [the dimensions] are scrunched up into little balls, is correct,” Gross said. “There are possibilities, and we’ll have to wait for nature to tell us what the story is.”
