Vibrating Filaments of Imagination

Some science. Brian Greene defends string theory against charges that it is of little use because it fails to deliver testable implications:

The Universe on a String, by Brian Greene, Commentary, NY Times: ...Einstein's belief that he'd one day complete the unified theory rarely faltered. Even on his deathbed he scribbled equations in the desperate but fading hope that the theory would finally materialize. It didn't.

In the decades since, the urgency of finding a unified theory has only increased. Scientists have realized that without such a theory, critical questions can't be addressed, such as how the universe began or what lies at the heart of a black hole. These unresolved issues have inspired much progress, with the most recent advances coming from an approach called string theory. Lately, however, string theory has come in for considerable criticism. And so, this is an auspicious moment to reflect on the state of the art. ...

After Einstein's death, the torch of unification passed to other hands. In the 1960's, the Nobel Prize-winning works of Sheldon Glashow, Abdus Salam and Steven Weinbergwon 1979 revealed that at high energies, the electromagnetic and weak nuclear forces seamlessly combine, much as heating a cold vat of chicken soup causes the floating layer of fat to combine with the liquid below, yielding a homogeneous broth. Subsequent work argued that at even higher energies the strong nuclear force would also meld into the soup, a proposed consolidation that has yet to be confirmed experimentally, but which has convinced many physicists that there is no fundamental obstacle to unifying three of nature's four forces.

For decades, however, the force of gravity stubbornly resisted joining the fold. The problem was the very one that so troubled Einstein: the disjunction between his own general relativity, most relevant for extremely massive objects like stars and galaxies, and quantum mechanics, the framework invoked by physics to deal with exceptionally small objects like molecules and atoms and their constituents.

Time and again, attempts to merge the two theories resulted in ill-defined mathematics... The combined equations of general relativity and quantum mechanics yield similar problems. While the conflict rears its head only in environments that are both extremely massive and exceptionally tiny — black holes and the Big Bang being two primary examples — it tells of a fissure in the very foundations of physics.

Such was the case until the mid-1980's, when a new approach, string theory, burst onto the stage. Difficult and complex calculations by the physicists John Schwarz and Michael Green, who had been toiling for years in scientific obscurity, gave compelling evidence that this new approach not only unified gravity and quantum mechanics, as well as nature's other forces, but did so while sweeping aside previous mathematical problems. As word of the breakthrough spread, many physicists dropped what they were working on and joined a global effort to realize Einstein's unified vision of the cosmos.

String theory offers a new perspective on matter's fundamental constituents. Once viewed as point-like dots of virtually no size, particles in string theory are minuscule, vibrating, string-like filaments. And much as different vibrations of a violin string produce different musical notes, different vibrations of the theory's strings produce different kinds of particles. An electron is a tiny string vibrating in one pattern, a quark is a string vibrating in a different pattern. Particles like the photon that convey nature's forces in the quantum realm are strings vibrating in yet other patterns.

Crucially, the early pioneers of string theory realized that one such vibration would produce the gravitational force, demonstrating that string theory embraces both gravity and quantum mechanics. In sharp contrast to previous proposals that cobbled gravity and quantum mechanics uneasily together, their unity here emerges from the theory itself.

While accessibility demands that I describe these developments using familiar words, beneath them lies a bedrock of rigorous analysis. We now have more than 20 years of painstaking research, filling tens of thousands of published pages of calculations, which attest to string theory's deep mathematical coherence. These calculations have given the theory countless opportunities to suffer the fate of previous proposals, but the fact is that every calculation that has ever been completed within string theory is free from mathematical contradictions.

Moreover, these works have also shown that many of the prized breakthroughs in fundamental physics, discovered over the past two centuries through arduous research using a wide range of approaches, can be found within string theory. ... When you also consider that string theory has opened new areas of mathematical research, you can easily understand why it's captured the attention of so many leading scientists and mathematicians.

Nevertheless, mathematical rigor and elegance are not sufficient to demonstrate a theory's relevance. To be judged a correct description of the universe, a theory must make predictions that are confirmed by experiment. And as a small but vocal group of critics of string theory justly emphasize, string theory has yet to do so. This is a key point, so it's worth serious scrutiny.

We understand string theory much better now than we did 20 years ago. We've developed powerful techniques of mathematical analysis that have improved the accuracy of its calculations and provided invaluable insights into the theory's logical structure. Even so, researchers worldwide are still working toward an exact and tractable formulation of the theory's equations. And without that final formulation in hand, the kind of detailed, definitive predictions that would subject the theory to comprehensive experimental vetting remain beyond our reach.

There are, however, features of the theory that may be open to examination even with our incomplete understanding. ... Without the exact equations, our ability to describe these attributes with precision is limited, but the theory gives enough direction for the Large Hadron Collider, a gigantic particle accelerator now being built in Geneva and scheduled to begin full operation in 2008, to search for supporting evidence by the end of the decade.

Research has also revealed a possibility that signatures of string theory are imprinted in the radiation left over from the Big Bang, as well as in gravitational waves rippling through space-time's fabric. In the coming years, a variety of experiments will seek such evidence with unprecedented observational fidelity. And in a recent, particularly intriguing development, data now emerging from the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory appear to be more accurately described using string theory methods than with more traditional approaches. ...

Some critics have taken this lack of definitive predictions to mean that string theory is a protean concept whose advocates seek to step outside the established scientific method. Nothing could be further from the truth. Certainly, we are feeling our way through a complex mathematical terrain, and no doubt have much ground yet to cover. But we will hold string theory to the usual scientific standard: to be accepted, it must make predictions that are verified. ...

Finally, some have argued that if, after decades of research involving thousands of scientists, the theory is still a work in progress, it's time to give up. But to suggest dropping research on the most promising approach to unification because the work has failed to meet an arbitrary timetable for complete success is, well, silly.

I have worked on string theory for more than 20 years because I believe it provides the most powerful framework for constructing the long-sought unified theory. Nonetheless, should an inconsistency be found, or should future studies reveal an insuperable barrier to making contact with experimental data, or should new discoveries reveal a superior approach, I'd change my research focus, and I have little doubt that most string theorists would too.

But this hasn't happened.

String theory continues to offer profound breadth and enormous potential. It has the capacity to complete the Einsteinian revolution and could very well be the concluding chapter in our species' age-old quest to understand the deepest workings of the cosmos.

Will we ever reach that goal? I don't know. But that's both the wonder and the angst of a life in science. Exploring the unknown requires tolerating uncertainty.

Comments

[anne]

http://www.nytimes.com/2005/04/08/opinion/08greene.html?ex=1270612800&en=c87d6bab356df279&ei=5090&partner=rssuserland

April 8, 2005

One Hundred Years of Uncertainty
By BRIAN GREENE

JUST about a hundred years ago, Albert Einstein began writing a paper that secured his place in the pantheon of humankind's greatest thinkers. With his discovery of special relativity, Einstein upended the familiar, thousands-year-old conception of space and time. To be sure, even a century later, not everyone has fully embraced Einstein's discovery. Nevertheless, say "Einstein" and most everyone thinks "relativity."

What is less widely appreciated, however, is that physicists call 1905 Einstein's "miracle year" not because of the discovery of relativity alone, but because in that year Einstein achieved the unimaginable, writing four papers that each resulted in deep and formative changes to our understanding of the universe. One of these papers - not on relativity - garnered him the 1921 Nobel Prize in physics. It also began a transformation in physics that Einstein found so disquieting that he spent the last 30 years of his life in a determined effort to repudiate it.

Two of the four 1905 papers were indeed on relativity. The first, completed in June, laid out the foundations of his new view of space and time, showing that distances and durations are not absolute, as everyone since Newton had thought, but instead are affected by one's motion. Clocks moving relative to one another tick off time at different rates; yardsticks moving relative to one another measure different lengths. You don't perceive this because the speeds of everyday life are too slow for the effects to be noticeable. If you could move near the speed of light, the effects would be obvious.

The second relativity paper, completed in September, is a three-page addendum to the first, which derived his most famous result, E = mc2, an equation as short as it is powerful. It told the world that matter can be converted into energy - and a lot of it - since the speed of light squared (c2) is a huge number. We've witnessed this equation's consequences in the devastating might of nuclear weapons and the tantalizing promise of nuclear energy.

The third paper, completed in May, conclusively established the existence of atoms - an idea discussed in various forms for millenniums - by showing that the numerous microscopic collisions they'd generate would account for the observed, though previously unexplained, jittery motion of impurities suspended in liquids.

With these three papers, our view of space, time and matter was permanently changed.

Yet, it is the remaining 1905 paper, written in March, whose legacy is arguably the most profound. In this work, Einstein went against the grain of conventional wisdom and argued that light, at its most elementary level, is not a wave, as everyone had thought, but actually a stream of tiny packets or bundles of energy that have since come to be known as photons.

This might sound like a largely technical advance, updating one description of light to another. But through subsequent research that amplified and extended Einstein's argument, scientists revealed a mathematically precise and thoroughly startling picture of reality called quantum mechanics.

Before the discovery of quantum mechanics, the framework of physics was this: If you tell me how things are now, I can then use the laws of physics to calculate, and hence predict, how things will be later. You tell me the velocity of a baseball as it leaves Derek Jeter's bat, and I can use the laws of physics to calculate where it will land a handful of seconds later. You tell me the height of a building from which a flowerpot has fallen, and I can use the laws of physics to calculate the speed of impact when it hits the ground. You tell me the positions of the Earth and the Moon, and I can use the laws of physics to calculate the date of the first solar eclipse in the 25th century. What's important is that in these and all other examples, the accuracy of my predictions depends solely on the accuracy of the information you give me. Even laws that differ substantially in detail - from the classical laws of Newton to the relativistic laws of Einstein - fit squarely within this framework.

Quantum mechanics does not merely challenge the previous laws of physics. Quantum mechanics challenges this centuries-old framework of physics itself....

[anne]

http://www.nytimes.com/2005/09/30/opinion/30greene.html?ex=1285732800&en=75072acd6902749d&ei=5090&partner=rssuserland&emc=rss

September 30, 2005

That Famous Equation and You
By BRIAN GREENE

DURING the summer of 1905, while fulfilling his duties in the patent office in Bern, Switzerland, Albert Einstein was fiddling with a tantalizing outcome of the special theory of relativity he'd published in June. His new insight, at once simple and startling, led him to wonder whether "the Lord might be laughing ... and leading me around by the nose."

But by September, confident in the result, Einstein wrote a three-page supplement to the June paper, publishing perhaps the most profound afterthought in the history of science. A hundred years ago this month, the final equation of his short article gave the world E = mc².

In the century since, E = mc² has become the most recognized icon of the modern scientific era. Yet for all its symbolic worth, the equation's intimate presence in everyday life goes largely unnoticed. There is nothing you can do, not a move you can make, not a thought you can have, that doesn't tap directly into E = mc². Einstein's equation is constantly at work, providing an unseen hand that shapes the world into its familiar form. It's an equation that tells of matter, energy and a remarkable bridge between them.

Before E = mc², scientists described matter using two distinct attributes: how much the matter weighed (its mass) and how much change the matter could exert on its environment (its energy). A 19th century physicist would say that a baseball resting on the ground has the same mass as a baseball speeding along at 100 miles per hour. The key difference between the two balls, the physicist would emphasize, is that the fast-moving baseball has more energy: if sent ricocheting through a china shop, for example, it would surely break more dishes than the ball at rest. And once the moving ball has done its damage and stopped, the 19th-century physicist would say that it has exhausted its capacity for exerting change and hence contains no energy.

After E = mc², scientists realized that this reasoning, however sensible it once seemed, was deeply flawed. Mass and energy are not distinct. They are the same basic stuff packaged in forms that make them appear different. Just as solid ice can melt into liquid water, Einstein showed, mass is a frozen form of energy that can be converted into the more familiar energy of motion....

[ninjaplease]

Whats useless is the quantum elevator idea.

I wonder what would happen if we colonize mars. Would the Human Martians eventually rebel, knowing that any retribution is atleast a year and a half away?

I wonder if Martians would outsource their jobs...

[calmo]

I like this popularizer of String Theory as soon as he writesat high energies, the electromagnetic and weak nuclear forces seamlessly combine, much as heating a cold vat of chicken soup causes the floating layer of fat to combine with the liquid below
and less so for thisString theory continues to offer profound breadth and enormous potential. It has the capacity to complete the Einsteinian revolution and could very well be the concluding chapter in our species' age-old quest to understand the deepest workings of the cosmos.
Einstein (like Nature) did not work in a vacuum and Brian (not monty's Brian) needs to expand his fixation on Albert. And here too:I have worked on string theory for more than 20 years because I believe it provides the most powerful framework for constructing the long-sought unified theory. Unification may not be all that it is cracked up to be.

[Bruce Webb]

At one point I planned to be a physicist. Until I discovered that being "good at math" was not an absolute but instead a relative thing, and that compared to people who would excel in physics I was a sad shirt-tail relative indeed. But I respect them.

But the fact remains is that for most of the last century physicists maintained that the curve ball was impossible, it was just an illusion. Why? because their models couldn't explain it and so it couldn't exist. The fact that people could and did line up three poles in a row and from the first throw a pitch that went to the left of the second and to the right of the third or that millions of Americans had been left futilely swinging at a wicked curve didn't seem to matter.

This is not a joke, until 1959 the curve ball was a scientific impossibility. And then it wasn't: http://www.100.nist.gov/baseball.htm.

The math caught up with the reality. People who would dismiss string theory because it doesn't (yet) explain everything need to understand what Dizzy Dean knew. Just because you are not the sharpest pencil in the box (and they didn't call him 'Dizzy' for nothing) doesn't mean: "Ball can't curve?" countered Dean, leader of the Cards' famed Gashouse Gang. "Shucks, get behind a tree and I'll hit you with an optical illusion."

Thomas Kuhn would be spinning in his grave. If he were dead. Science at the cutting edge is always wrestling with unsolved problems that challenge accepted theory. That's why they give out Nobel prizes.

[Adrasteia]

Are you string cultists at some time going to stop pontificating about baseballs and golfballs and start making testable hypotheses? No? Didn't think so.

[STS]

Bruce:

Kuhn died in 1996.

http://en.wikipedia.org/wiki/Thomas_Kuhn

People do string theory because, a) other respected people work on it, b) it's a box of really cool mathematical toys, c) there are no experimental results at high enough energy to tell them to stop calling it "physics".

It will continue until at least one of those conditions fails. Meanwhile, it stimulates research in math which may eventually be applied to physics or somewhere else.

[realpc]

"... could very well be the concluding chapter in our species' age-old quest to understand the deepest workings of the cosmos."

Yeah, that "concluding chapter" has been announced many times before. It's fun trying to understand nature, but she always turns out to be so much smarter than we are.

If Intelligent Design theory were as ridiculous as the Democrats think it is, we would understand what matter is made of by now.

[anne]

There is not a biologist worth the name in America who would spend a moment thinking seriously about the self-evident lunacy of intelligent design, but, no matter, no matter, no matter.

[Bruce Webb]

Well that explains that whirring noise. Thomas Kuhn is in fact dead and spinning in his grave in response to realpc's nonsense. I was starting a new job in 1996 and must have missed the worldwide headlines.

I share Anne's bemusement. realpc suggests that the "End of Knowledge" has been presented all too many times before, yet puts forward that fact to validate accepting every bit of pseudo-science out there.

Dude until Einstein there were anomalies in Mercury's orbit that were not explainable under strict Newtonian rules. Which didn't mean that your batty Aunt Isabel the astrologist knew just as much about orbital mechanics as those hoity-toity astronomers out at the observatory.

The fact that I don't know everything (like TK being in fact dead) doesn't validate realpc's ignorance. Though I doubt he will get the difference.

[calmo]

hoity toity astronomers...wickedly good pen, Bruce, unlike this:Will we ever reach that goal? I don't know. But that's both the wonder and the angst of a life in science. Exploring the unknown requires tolerating uncertainty. which is solabored and over-written you (not only me!) want to go back to Mr String Theorist and point his nose into that vat of chicken soup...which was so good --not like this angst slop.

STS, are you saying that String Theory is an on-going (worthwhile) study because it will eventually be useful or because it is a cool box of toys?

[supersaurus]

from what I've read string theory has been around for over 30 years, but has not produced a single prediction that would falsify the theory, i.e. something different from already known behavior that can be tested. quantum electrodynamics predicted lots of unknown things that turned out to be correct and so did einstein's general theory, to give two examples of real theories. where are similar examples from string theory?

and please spare me the baseball and bumblebee stories. "a baseball can't curve" is easily falsified, and is every day...anyone who has actually played baseball, or pingpong for that matter, knows that any theory of the motion of balls is going to have to explain the observed fact that the paths of balls *do* curve, and not just vertically.

[KS]

Some here are missing the point of Brian Greene's article.

Perhaps an analogy works: Suppose I have some theory A which explains the macroeconomic behavior of countries rather well. Suppose I have a second theory B that explains rather well how two interested parties interact in economic transactions. In principle, scaling up the parties in theory B to countries should give me theory A. If it doesn't, you have a problem. You need to fix your theories. Now, if your lucky there is some intermediate data that can guide you to this solution. However, the fact that you need to solve the conflict between A and B is independent of that whether you have intermediate data. For strictly speaking you should believe neither A nor B until they agree in the scaling limit. Of course you can set ranges of applicability for each, saying A only applies to countries and B only applies to one-on-one parties. For all practical purposes these ranges of applicability will prevent you from running into trouble .... until you deal with a country that exists only of one interested party. Would you honestly argue that because single-inhabitant countries do not exist in our current world, and are therefore experimentally irrelevant, you are absolved from solving the conflict between theories A and B? I doubt it.

Translating back, this is what Brian Greene is trying to get across about string theory. The situation is physics is even more drastic, however. It is not just that theory B, quantum mechanics, fails to scale up to theory A, general relativity. The two theories are actually mathematically inconsistent. As in the analogy: their ranges of applicability only intersect in extreme situations: at the formation of black holes or our universe, and are therefore beyond the reach of our current experimental capabilities.

The one beacon of light is that there after many years of research there exists one ---- and only one --- known framework which appears to be able to resolve the conflict between A and B: string theory. Whether string theory itself a consistent framework is still an large open question, which is why research is still needed: the original question is still very much open. The point, however, is that any theory that resolves the conflict between general relativity and quantum mechanics will fail to give experimental predictions. This is not a failure of the theory, but of the experimental misfortune that we cannot test the situation where both theories are important at the same time. Very much in the same way that no (group of) single-inhabitant country exists and this situation can therefore not give experimental input into conflicts between country-size vs. individual-individual economic theories.

[anne]

KS:

"It is not just that theory B, quantum mechanics, fails to scale up to theory A, general relativity. The two theories are actually mathematically inconsistent. As in the analogy: their ranges of applicability only intersect in extreme situations: at the formation of black holes or our universe, and are therefore beyond the reach of our current experimental capabilities.

"The one beacon of light is that there after many years of research there exists one ---- and only one --- known framework which appears to be able to resolve the conflict between A and B: string theory. Whether string theory itself a consistent framework is still an large open question, which is why research is still needed: the original question is still very much open. The point, however, is that any theory that resolves the conflict between general relativity and quantum mechanics will fail to give experimental predictions. This is not a failure of the theory, but of the experimental misfortune that we cannot test the situation where both theories are important at the same time."

Wonderfully explained.

[supersaurus]

KS: ...any theory that resolves the conflict between general relativity and quantum mechanics will fail to give experimental predictions....

assuming you meant it will fail to give predictions that can be tested by experiment, and pardoning me for being simple minded, then what good is it? the theory that "god did it" explains any observation at all, but it doesn't predict anything; in effect the theory is adjusted every time a new observation comes up. the nice thing about "god did it" is the "theory" doesn't require recourse to convoluted mathematics to state it, but of course, not being falsifiable "god did it" is not really a theory at all. I think string theory sceptics are saying that string theory and "god did it" are similar because neither is falsifiable and neither offers new prediction.

to offer a different analogy, you cannot say anything about the next member in a sequence of numbers no matter how many are given by a finite subsequence. you may be able to fit a nice generator to a given subset of a sequence, but the fact that n^2 generates 1, 4, 9, 16 doesn't prove that 25 is next. I think what some string theory sceptics are saying is all that fancy math is just a generator that has been adjusted to fit known data, but that doesn't prove a thing by itself.

[anne]

Brian Greene:

"[R]esearchers worldwide are still working toward an exact and tractable formulation of the theory's equations. And without that final formulation in hand, the kind of detailed, definitive predictions that would subject the theory to comprehensive experimental vetting remain beyond our reach.

"There are, however, features of the theory that may be open to examination even with our incomplete understanding. ... Without the exact equations, our ability to describe these attributes with precision is limited, but the theory gives enough direction for the Large Hadron Collider, a gigantic particle accelerator now being built in Geneva and scheduled to begin full operation in 2008, to search for supporting evidence by the end of the decade.

"Research has also revealed a possibility that signatures of string theory are imprinted in the radiation left over from the Big Bang, as well as in gravitational waves rippling through space-time's fabric. In the coming years, a variety of experiments will seek such evidence with unprecedented observational fidelity. And in a recent, particularly intriguing development, data now emerging from the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory appear to be more accurately described using string theory methods than with more traditional approaches. ..."

[supersaurus]

someday my prince will come? sheldon glashow, in an interview about the significance of string theory:

The string theorists have a theory that appears to be consistent and is very beautiful, very complex, and I don't understand it. It gives a quantum theory of gravity that appears to be consistent but doesn't make any other predictions. That is to say, there ain't no experiment that could be done nor is there any observation that could be made that would say, "You guys are wrong." The theory is safe, permanently safe. I ask you, is that a theory of physics or a philosophy?

[calmo]

Thanks for that link supersaurus. Half way down we haveI [Sheldon, winner of 1979 Nobel Prize in Physics] can't understand the titles, and I can't understand the lectures, and it's not just me. I think most theoretical physicists who are not themselves string theorists could not possibly follow these lectures. And I wonder how familiar our readers here are with this branch of mathematics (which we allow to be...academic) that is not content to be merely academic but would like to ignore the traditional pattern/custom and be regarded as non-experimental physicists.
I'm not sayin we're stupid or ignorant...
[Or worse, ballbarians who know (just know) that the curve ball is a mighty good pitch.]...esp after the Nobel prize winner has shamlessly stated his position (dragging philosphy, not relgion) into it (where are you eva?). Is this a topic about the philosophy of science rather than science?

[Bruce Webb]

As a disciple of Karl Popper I bow down to Falsification. A theory that is not subject to test or one whose premises can be adjusted at will (hello Marxists and Freudians) is in my eyes not science at all. Which is not to say that a scientific theory has to explain everything in order to be valid.

Newton wasn't wrong. Newtonian physics was not complete. Einstein wasn't wrong. But he went to his grave trying to transcend Heisenberg. "God doesn't play dice." "There is no action at a distance". Unless there is.

I don't pretend to understand why changing the quantum state of a particle here would change the quantum state of a particle separated by light years, I don't even pretend to understanding whether what I just said was right or wrong. All I know is that some German guy shoved a cat in a box back in the thirties and to this day I don't know for certain whether that cat is alive or dead.

Schroedinger's Cat

Quantum mechanics is weird and spooky. The universe simply should not work like that. Yet engineers are working to exploit quantum effects in computing and me and Albert don't have to like it.

[ken melvin]

If one goes in search of the essence that is both energy and matter, and probably force, they come to look for the phases, if you will, of existence. How energy forms in to matter and vice versa. This too leads one to think along the lines of string theory.

[James Killus]

There is a human and social component to all science. At the very least, it shows up in scientists' notions of what is and is not interesting to study. I once did a literature search on the photochemistry of chlorine gas (my reasons had to do with urban air pollution and smog) and was enlightened to discover that there had been a flurry of activity right after WWI, obviously stimulated by the use of chlorine as a weapon of war. None of the papers I read said that, of course; it was all fundamental research as far as the writing of it was concerned.

The grand unification attempts don't have any obvious phenomena that they are trying to study or explain. As a largely disinterested observer for the past several decades, I have to say that it looks a lot like religious quest, attempting to extend cosmology back another fractional second after the big bang, for the satisfaction of whatever psychological impulse that makes such a quixotic quest necessary.

[supersaurus]

calmo: you don't need to understand the mathematics to understand that one of the cornerstones of the scientific method is falsifiablity. I don't understand quantum electrodynamics, but I do know it has been around a long time, has made many predictions verified by subsequent observation and was believed by people like richard feynman who knew a *lot* about such things, and thus I consider it to be a real theory. the key difference here is string theory has *not* made testable predictions according to what I've read. ever since I can recall reading about it, its adherents have said they could see light at the end of the tunnel, and they still are saying it. people who don't believe in it think maybe the string theorists are blinded by that light.

[pborder]

As I understand string theory (I have a Ph.D. in experimental particle physics, which is definitely not the same as theoretical, but I have read Dr Greene's book), what it does is give a way to get around the problems of particles being infinitely small and, consequently, infintely dense. Electrons, as far as anybody knows, have no size whatsoever, and thus must have a finite amount of mass packed into 0.0 volume, which obviously means they are to infinitely dense. General relativity gets very upset with infinite density- according to GR, whenever the density gets above a certain threshold, space collapses and you are left with an infintesimal black hole. Electrons are rather obviously not little black holes. If they were, they would eat up everything around them and that doesn't seem to be happening. So something is wrong.

String theory provides a way out by giving all particles a very small, but not exactly 0.0, size. Thus their density is not infinite, GR is OK and the universe is saved. Hurray!

Unfortunately, string theory has two problems: 1. Nobody really knows if it works and 2. Nobody knows how to test it.

What do I mean by "if it works?" Well, theories like this are very prone to making predictions that don't make any sense- some processes may turn out to be infinitely probable according to the theory (not 100% probable, but infinitely so), all objects may have infinite mass, etc. String theory is so fiendishly complicated that nobody knows if there any impossibilities left in it or not. There has been some remarkable progress in the last decade and a lot of what looked like impossibilities are now gone, but nobody knows if they're all gone yet. There may still be some lurking in places where they can't be removed, which would be a disaster.

The other problem is that string theory doesn't make any verifiable predictions (yet). This is sort of related to the first problem, of course- if string theory ever does turn out to predict infinite probabilities, that's something we could test pretty easily (the answer would be "no"). But, again, it's still early days for this thing and theories like this do sometimes have consequences at surprisingly low energies. It's still too soon to tell.

As far as I can tell, the whole field of GR and cosmology is in a period of tremendous change. When I was in grad school back in the '80s cosmology was kind of a joke- there was so little data that almost anything was OK. Nowadays that's changing quickly and things like Dark Matter and, even worse, Dark Energy are coming out of nowhere. Apparently we have absolutely no idea whatsoever 95% of the universe is made of. None! Dark Energy just makes no sense whatsoever and I dont think anybody has the faintest clue what the hell is going on with that sucker- it really smells like something fundamental is just grievously wrong somewhere.

It's an interesting time.

Don't give up on string theory.