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Collaborative physics: String theory finds a bench mate – The exotic theory of everything could shed light on the behaviour of real materials, thanks to an unexpected mathematical connection with condensed-matter physics – Zeeya Merali

“On one side,” says Jan Zaanen, “you have this refined, almost other-worldly intellectual — the perfectionist obsessed with detail, barely interested in earthly pleasures. On the other, you have the loud, boisterous, sometimes aggressive, business-savvy character who knows how to get his hands dirty.”

It might almost be a description of the misfit roommates known on stage, screen and television as The Odd Couple. But Zaanen, a condensed-matter physicist at the University of Leiden in the Netherlands, is actually describing the pairing of two groups of scientists: string theorists, who spend their days pursuing a rarefied, highly mathematical ‘theory of everything’, and his own colleagues — a considerably more grounded bunch who prefer to focus on how real-world materials behave in the laboratory.

The scientists trying to bridge these disciplines are motivated by the discovery of a startling coincidence: suitably interpreted, the equations of string theory can be a powerful tool for analysing some exotic states of matter, ranging from super-hot balls of quarks and gluons to ultracold atoms. The past year alone has seen at least four international workshops designed to stimulate collaborations across the disciplinary divide, including one hosted by Zaanen in Leiden.

Sceptics still question whether this strange alliance will actually lead to new insights, or whether it is just a marriage of convenience. String theory does hint at the existence of many new states of matter, for example. But those predictions will be difficult to verify, and decisive experimental tests are only now in the planning stages.

For the time being, the advantage to both partners is clear. String theory, long criticized for having lost touch with reality, gets experimental credibility. And condensed-matter physics, never the media darling that string theory has been, gets a new mathematical tool — and a chance to bask in new-found glamour.

The match-making began a dozen years ago with the reunion of Dam Thanh Son and Andrei Starinets, who had been undergraduates and dorm-mates at Moscow State University in the 1980s. The friends had lost touch with each other when they left Russia after the fall of communism in 1991. But in 1999, Son got a job at Columbia University in New York City, and heard that Starinets was doing a PhD in string theory just a few kilometres away at New York University. So Son went to pay Starinets a visit.

Collaboration was the farthest thing from his mind. String theory is mathematically rich and has an undeniable aesthetic appeal. But it is all about what physics might be like at scales of 10−35 metres — the idea being that seemingly point-like elementary particles such as quarks and electrons will actually turn out to be tiny, vibrating threads of energy when viewed at such scales. But these strings would be about 20 orders of magnitude smaller than a proton, putting the theory hopelessly beyond the reach of any direct experimental test. Son’s speciality, by contrast, was firmly rooted in experiment: he was trying to understand the properties of quark–gluon plasmas, the short-lived, super-hot fireballs that form when heavy nuclei such as gold are smashed together in accelerators. All this stringy stuff seemed utterly alien.

Except that, when Son saw the string-theory calculations that Starinets had been working on with fellow PhD student Giuseppe Policastro, he recognized the equations as the same ones he had been using to analyse the plasma.

Son immediately had to know what was going on, and Starinets began to explain. Starinets and Policastro had been working on an idea proposed in 1997 by Juan Maldacena, a physicist at Harvard University in Cambridge, Massachusetts. Maldacena, now at the Institute for Advanced Study in Princeton, New Jersey, had realized that string theory predicts a mathematical equivalence between two hypothetical universes, one of which would be similar to our own. It would have the same three dimensions of space and one dimension of time, for example, and be filled with much the same types of elementary particle, which would, in turn, obey familiar-looking (to physicists) quantum-field equations. But it would not contain strings — or gravity.

The other universe would be the opposite: it would contain both strings and gravity — indeed, the gravity could get strong enough to form black holes — but no elementary particles. It would also have an additional dimension of space.

Maldacena’s insight was simple, if audacious: take any process involving particles and fields in the first universe, he said, and it could equally well be described as a process involving gravity, black holes and strings in the second universe — and vice versa1. The equations might look very different. But the fundamental physics would be exactly the same.

That was why Son was seeing quark–gluon equations in a string-theory calculation, Starinets explained: they were the three-dimensional equivalent of the gravitational fields that he and Policastro had been studying in the four-dimensional universe.

Marriage of convenience

All this jumping back and forth between universes was weird even by string-theory standards (and even weirder for non-string theorists, as Maldacena had showed that the mapping worked not just between three and four dimensions of space, but also between four and five, five and six and so on). But as Son and Starinets talked, they began to see that Maldacena’s mapping might be a powerful problem-solving strategy. They could start with a messy set of quantum-field calculations in our real, three-dimensional world — the quark–gluon plasma equations, say — then map those into the four-dimensional world, in which the equations tend to be much easier to solve. Then they could map the results back to the three-dimensional world and read off the answer.

It worked. “We turned the calculation on its head to give us a prediction for the value of the shear viscosity of a plasma,” says Son, referring to a key parameter of the quark–gluon fireball2. “A friend of mine in nuclear physics joked that ours was the first useful paper to come out of string theory,” he says.

In 2008, the team’s predictions were confirmed3 at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York. “These were strong quantitative results, and they still stand today as the best results achieved by the programme to relate string theory to experiment,” says Steve Gubser, a string theorist at Princeton University, and one of the early champions of applying the principle to real-world problems.

The team’s success also caught the attention of Subir Sachdev, a condensed-matter theorist at Harvard. Just as Son had seen a plasma reflected back at him in Starinets’ equations, Sachdev saw quantum critical-phase transitions — the changes of state that occur in materials when they near absolute zero, when quantum-mechanical effects begin to dominate. “They were using different words,” he says, “but it was the same physics.”

Sachdev hoped that Maldacena’s idea could provide him and his fellow theorists with some much-needed help in exploring this chilly realm. Over the decades, experimentalists had discovered a long list of exotic, quantum-dominated states — including superconductors that allow current to flow without resistance; superfluids that have no viscosity and can creep up the walls of beakers; Bose–Einstein condensates made up of atoms moving in step like a single ‘super atom’; and ‘strange’ metals that behave in ways subtly different from ordinary metals. But physicists still have no way to predict what will turn up in the lab next. “We can’t even answer the fundamental question of how many phases of matter exist,” says Sean Hartnoll, a string theorist at Stanford University in California.

Sachdev’s first efforts to apply Maldacena’s idea to laboratory materials had resulted in two papers he co-authored in 2007, one with Son and his colleagues4, and another with a team that included Hartnoll5. Since then, Sachdev and his collaborators have built up a recipe for mapping the conductivity of strange metals into the properties of black holes in the string theorists’ four-dimensional universe — a strategy that string theorist John McGreevy at the Massachusetts Institute of Technology in Cambridge6 and others are also pursuing. These groups get answers that broadly reproduce the peculiar low-temperature behaviour of the metals. They have also mapped the behaviour of four-dimensional black holes in string theory to the conditions at which many materials will change phase into states other than the familiar solid, liquid and gas7. “We now have a whole new hammer for attacking the problems I have been working on for 20 years,” says Sachdev.

Sachdev’s involvement, in turn, has helped to ignite the interest of other condensed-matter physicists. “A lot of us got into this field because of the force of Subir’s personality and his reputation — we realized that if he was taking this seriously, maybe we should too,” says Andrew Green, a condensed-matter physicist at the University of St Andrews, UK, who co-organized a workshop on the correspondence at Imperial College London in January.

The condensed-matter results also got the string theorists excited — eventually. The field had been generally unenthusiastic about following up on the quark–gluon plasma calculations, says Clifford Johnson, a string theorist at the University of Southern California in Los Angeles. And at least part of the reason, he suspects, was a bias against sullying string theory’s purity. “There was a snobbery among some towards what was termed ‘mere applications’,” he says.

But in 2006, string theory took a public battering in two popular books: Not Even Wrong by Peter Woit, a mathematician at Columbia, and The Trouble With Physics by Lee Smolin, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. Both books excoriated the theory’s isolation from experiment.

“It’s hard to say whether the interest in condensed-matter applications is a direct response to those books because that’s really a psychological question,” says Joseph Polchinski, a string theorist at the Kavli Institute for Theoretical Physics in Santa Barbara. “But certainly string theorists started to long for some connection to reality.”

The condensed-matter partnership seemed perfect for that. If nothing else, it promised to make a virtue out of string theory’s embarrassment of riches — the roughly 10500 solutions to its basic equations, each of which describes a possible universe with its own size, shape, dimensionality and physical laws. Through Maldacena’s idea, says string theorist Jerome Gauntlett at Imperial College London, “each solution can be expressed in the countless materials yet to be discovered”.

The rewards are mutual, says Zaanen. “If I talk about superconductors and black holes in a colloquium, folk are attracted to it like bees to honey,” he says. “It’s now bringing young blood to condensed-matter physics, as their first choice.”

The flurry of workshops promoting the partnership have been very productive, agrees Polchinski. He co-organized a meeting at the Kavli Institute last year that sparked seven new collaborations, and he is running another from August through November of this year. “It is unique to try to actively make two groups of physicists collaborate so quickly — I haven’t personally seen any other similar drive over the course of my career,” says Gauntlett, who co-organized the London workshop.

Just as with the fictional odd couple, however, this partnership still has plenty of friction. Everyone agrees, for example, that condensed-matter physicists are much more hesitant about pairing up than their string-theory counterparts. “I have been remarkably unsuccessful at getting condensed-matter physicists to let string theorists speak at their big meetings,” says Zaanen. “They fear that they will need to learn string theory to talk to them. It’s as though I am asking them to have coffee with aliens.”

Polchinski admits that the condensed-matter sceptics have a point. “I don’t think that string theorists have yet come up with anything that condensed-matter theorists don’t already know,” he says. The quantitative results tend to be re-derivations of answers that condensed-matter theorists had already calculated using more mundane methods.

To make matters worse, some of the testable predictions from string theory look a tad bizarre from the condensed-matter viewpoint. For example, the calculations suggest that when some crystalline materials are cooled towards absolute zero, they will end up in one of many lowest-energy ground states. But that violates the third law of thermodynamics, which insists that these materials should have just one ground state. “That’s the gorilla in the room that should be keeping people awake at night,” says Gubser.

To win over sceptics, theorists are busily searching for testable predictions that will lead to killer evidence that the collaborations are worthwhile. Gauntlett’s group, and others, are hunting for black-hole configurations in the string-theory universe that map to undiscovered phase transitions8. The trick is to figure out which materials might exhibit those transitions. “Right now that involves going round asking, ‘have you seen something like this?’,” says Gauntlett. “But the hope is that as techniques advance, experimenters will be able to engineer materials with the properties we predict.”

Sachdev is applying string theory to an existing challenge: calculating how conductance should change with temperature as ultracold atoms transition from a superfluid state to an insulating one7. He thinks that it should be possible to test his predictions in the next couple of years.

Even if the programme is successful, there are limits to how much the relationship can benefit either partner. String theory can offer a handbook of properties to look for, and predictions for how they should change in experiments, says Green. But it will never be able to provide a theory of how these properties emerge from the behaviour of electrons. Similarly, experimental verification of string theory’s predictions about condensed matter will not prove that strings themselves are an accurate description of reality.

But perhaps, Green argues, the connection to materials will show that people have fundamentally misunderstood what string theory is. “Maybe string theory is not a unique theory of reality, but something deeper — a set of mathematical principles that can be used to relate all physical theories,” says Green. “Maybe string theory is the new calculus.”

Zeeya Merali is a freelance writer based in London.

String Theory Co-Founder: Sub-Atomic Particles Are Evidence the Universe Was Created – By Barbara Hollingsworth – June 17, 2016 – 3:56 PM EDT

( — Dr. Michio Kaku, a theoretical physicist at the City College of New York (CUNY) and co-founder of String Field Theory, says theoretical particles known as “primitive semi-radius tachyons” are physical evidence that the universe was created by a higher intelligence.

After analyzing the behavior of these sub-atomic particles – which can move faster than the speed of light and have the ability to “unstick” space and matter – using technology created in 2005, Kaku concluded that the universe is a “Matrix” governed by laws and principles that could only have been designed by an intelligent being.

“I have concluded that we are in a world made by rules created by an intelligence. Believe me, everything that we call chance today won’t make sense anymore,” Kaku said, according to an article published in the Geophilosophical Association of Anthropological and Cultural Studies.

“To me it is clear that we exist in a plan which is governed by rules that were created, shaped by a universal intelligence and not by chance.”

“The final solution resolution could be that God is a mathematician,” Kaku, author of The Future of the Mind: The Scientific Quest to Understand, Enhance, and Empower the Mind, said in a 2013 Big Think video posted on YouTube.

“The mind of God, we believe, is cosmic music, the music of strings resonating through 11-dimensional hyperspace.”

String Theory “revolutionized” mathematics and physics by demonstrating a “super symmetry” in the universe. Kaku said it also explains gaps in the Big Bang theory.

“First of all, the Big Bang wasn’t very big. Second of all, there was no bang. Third, Big Bang Theory doesn’t tell you what banged, when it banged, how it banged. It just said it did bang. So the Big Bang theory in some sense is a total misnomer,” the well-known physicist said in 2015.

“We need a theory that goes before the Big Bang, and that’s String Theory. String Theory says that perhaps two universes collided to create our universe, or maybe our universe is butted from another universe leaving an umbilical cord….

“Some people believe that maybe, just maybe, we have detected evidence of that umbilical cord.”

Scientists find a practical test for string theory – January 6, 2014

( —Scientists at Towson University in Towson, Maryland, have identified a practical, yet overlooked, test of string theory based on the motions of planets, moons and asteroids, reminiscent of Galileo’s famed test of gravity by dropping balls from the Tower of Pisa.

String theory is infamous as an eloquent theoretical framework to understand all forces in the universe —- a so-called “theory of everything” —- that can’t be tested with current instrumentation because the energy level and size scale to see the effects of string theory are too extreme.

Yet inspired by Galileo Galilei and Isaac Newton, Towson University scientists say that precise measurements of the positions of solar-system bodies could reveal very slight discrepancies in what is predicted by the theory of general relativity and the equivalence principle, or establish new upper limits for measuring the effects of string theory.

The Towson-based team presents its finding today, January 6, 2014, between 10 a.m. and 11:30 a.m., at the 223rd meeting of the American Astronomical Society, in Washington, D.C. The work also appears in the journal Classical and Quantum Gravity.

String theory hopes to provide a bridge between two well-tested yet incompatible theories that describe all known physics: Einstein’s general relativity, our reigning theory of gravity; and the standard model of particle physics, or quantum field theory, which explains all the forces other than gravity.

String theory posits that all matter and energy in the universe is composed of one-dimensional strings. These strings are thought to be a quintillion times smaller than the already infinitesimal hydrogen atom and thus too minute to detect indirectly. Similarly, finding signs of strings in a particle accelerator would require millions of times more energy than what has been needed to identify the famous Higgs boson.

“Scientists have joked about how string theory is promising…and always will be promising, for the lack of being able to test it,” said Dr. James Overduin of the Department of Physics, Astronomy and Geosciences at Towson University, first author on the paper. “What we have identified is a straightforward method to detect cracks in general relativity that could be explained by string theory, with almost no strings attached.”

Overduin and his group —- including Towson University undergraduate research students Jack Mitcham and Zoey Warecki —- expanded on a concept proposed by Galileo and Newton to explain gravity.

Fable has it that Galileo dropped two balls of different weights from the Tower of Pisa to demonstrate how they would hit the ground at the same time. Years later Newton realized that the same experiment is being performed by Mother Nature all the time in space, where the moons and planets of the solar system fall continuously toward each other as they orbit around their common centers of mass. Newton used telescope observations to conclude that Jupiter and its Galilean moons fall with the same acceleration toward the Sun.

The same test could be used for string theory, Overduin said. The gravitational field couples to all forms of matter and energy with precisely the same strength, an observation that led Einstein to his theory of general relativity and is now enshrined in physics as the equivalence principle. String theory predicts violations of the equivalence principle because it involves new fields which couple differently to objects of different composition, causing them to accelerate differently, even in the same gravitational field.

Building on work done by Kenneth Nordtvedt and others beginning in the 1970s, Overduin and his collaborators consider three possible signatures of equivalence principle violation in the solar system: departures from Kepler’s Third Law of planetary motion; drift of the stable Lagrange points; and orbital polarization (also known as the Nordtvedt effect), whereby the distance between two bodies like the Earth and Moon oscillates due to differences in acceleration toward a third body like the Sun.

To date, there is no evidence for any of these effects. Indeed, the standard astronomical ephemeris assumes the validity of Kepler’s Third Law in deriving such fundamental quantities as the length of the Astronomical Unit. But all observations in science involve some degree of experimental uncertainty. The approach of Overduin’s team is to use these experimental uncertainties themselves to obtain upper limits on possible violations of the equivalence principle by the planets, moons and Trojan asteroids in the solar system.

“The Saturnian satellites Tethys and Dione make a particularly fascinating test case,” said Warecki, who is presenting this work at Session 109 at the AAS meeting today. “Tethys is made almost entirely of ice, while Dione possesses a significantly rocky core. And both have Trojan companions.”

“The limits obtained in this way are not as sensitive as those from dedicated torsion-balance or laser-ranging tests,” said Mitcham. “But they are uniquely valuable as potential tests of string theory nonetheless because they cover a much wider range of test-body materials.”

Moreover, in an era of increasingly big-budget science, they come at comparatively little cost, said Overduin.

Energy, Matter, Inflation, Eternal Inflation and Multiverses

(Andrei Linde) eternal inflation and the multiverse.

Multiverse Controversy Heats Up over Gravitational Waves – By Clara Moskowitz on March 31, 2014

Some physicists think they can explain why the universe first formed. If they are right, our entire cosmos may have sprung out of nothing at all – By Robert Adler –
6 November 2014

The Case for Parallel Universes –
Why the multiverse, crazy as it sounds, is a solid scientific idea
By Alexander Vilenkin, Max Tegmark on July 19, 2011

What Came Before the Big Bang?
Cosmologist Alexander Vilenkin believes the Big Bang wasn’t a one-off event, but merely one of a series of big bangs creating an endless number of bubble universes – By Steve Nadis – Thursday, October 10, 2013

A Conversation with Andrei Linde [8.24.12]

Scientists find first evidence that many universes exist
December 17, 2010 by Lisa Zyga

Cause And Effect, The Perfect Vacuum & Absolute Nothing

In the Beginning before the Big Bang | Michio Kaku | Dr Param Singh

The Physics of Nothing, The Philosophy of Everything – Ethan Siegel (August 16th 2011)

Something from Nothing? A Vacuum Can Yield Flashes of Light

Physicists Confirm Power of Nothing, Measuring Force of Universal Flux
Published: January 21, 1997

The Bridge From Nowhere
How is it possible to get something from nothing?

Posted on January 4, 2013 by James Keen

“Gravity Doesn’t Exist” – Is This Fundamental Phenomenon Of The Universe An Illusion

Saint Augustine, Albert Einstein & The Augustinian Era

New York Times – Dennis Overbye
“Before the Big Bang, There Was . . . What?”

Scientific American – Gabriele Veneziano
“The Myth of the Beginning of Time”

Discover Magazine – Adam Frank
“3 Theories That Might Blow Up the Big Bang”

Arguing for the Existence of God in the Age of Quantum Indeterminacy
Author: Evan Cockshaw
Quodlibet Journal: Volume 3 Number 1, Winter 2001

What Came Before The Big Bang?
By Paul Davies

Wonder and Knowledge

Has Physics Made Philosophy and Religion Obsolete?
“A Universe From Nothing: Why There Is Something Rather Than Nothing” [Book]

String theory: From Newton to Einstein and beyond By David Berman December 1, 2007

This article is part of the Researching the unknown project, a collaboration with researchers from Queen Mary University of London, bringing you the latest research on the forefront of physics.

Unifying forces

To understand the ideas and aims of string theory, it’s useful to look back and see how physics has developed from Newton’s time to the present day. One crucial idea that has driven physics since Newton’s time is that of unification: the attempt to explain seemingly different phenomena by a single overarching concept. Perhaps the first example of this came from Newton himself, who in his 1687 work Principia Mathematicae explained that the motion of the planets in the solar system, the motion of the Moon around the Earth, and the force that holds us to the Earth are all part of the same thing: the force of gravity. We take this for granted today, but pre-Newton the connection between a falling apple and the orbit of the Moon would have been far from obvious and quite amazing.

The next key unifying discovery was made around 180 years after Newton by the Scottish mathematician James Clerk Maxwell. Maxwell showed that electrostatics and magnetism, by no means similar phenomena at first sight, are just different aspects of a single thing called electromagnetism. In the process Maxwell discovered electromagnetic waves, which are in fact light — Maxwell had inadvertently explained a further seemingly different aspect of nature.

Another two hundred years on, in 1984, the Pakistani Abdus Salam and the American Steven Weinberg showed that the electromagnetic force and the weak nuclear force, which causes radioactive decay, are both just different aspects of a single force called the electroweak force.

This leaves us with three fundamental forces of nature: gravity, the electroweak force and the strong nuclear force which holds protons together.

Unifying matter

That deals with the forces, but what about matter? Many ancient belief systems have postulated that matter — and reality itself — is made from a finite number of elements. Modern physics confirms this idea. Experiments performed with the particle accelerator at CERN in Geneva have shown that there are just twelve basic building blocks of matter. These are known as the elementary particles. Everything we’ve ever seen in any experiment, here or in distant stars, is made of just these twelve elementary particles.

All this is truly impressive: the entire Universe, its matter and dynamics explained by just three forces and twelve elementary objects. It’s good, but we’d like to do better, and this is where string theory first enters: it is an attempt to unify further. To understand this, we have to tell another story.

Quantum gravity

There have been two great breakthroughs in the 20th century physics. Perhaps the most famous is Einstein’s theory of general relativity. The other equally impressive theory is quantum mechanics.

Massive bodies warp spacetime. Image coutesy NASA.
General relativity is itself a unification. Einstein realised that space and time are just different aspects of a single object he called spacetime. Massive bodies like planets can warp and distort spacetime, and gravity, which we experience as an attractive force, is in fact a consequence of this warping. Just as a pool ball placed on a trampoline will create a dip that a nearby marble will roll into, so does a massive body like a planet distort space, causing nearby objects to be attracted to it.

The predictions made by general relativity are remarkably accurate. In fact, most of us will have inadvertently taken part in an experiment that tests general relativity: if it were false, then global positioning systems would be wrong by about 50 metres per day. The fact that GPSs work to within five metres in ten years shows just how accurate general relativity is.

The other great breakthrough of the 20th century was quantum mechanics. One of the key ideas here is that the smaller the scale at which you look at the world, the more random things become. Heisenberg’s uncertainty principle is perhaps the most famous example of this. The principle states that when you consider a moving particle, for example an electron orbiting the nucleus of an atom, you can never ever measure both its position and its momentum as accurately as you like. Looking at space at a minuscule scale may allow you to measure position with a lot of accuracy, but there won’t be much you can say about momentum. This isn’t because your measuring instruments are imprecise. There simply isn’t a “true” value of momentum, but a whole range of values that the momentum can take, each with a certain probability. In short, there is randomness. This randomness appears when we look at particles at a small enough scale. The smaller one looks, the more random things become!

The idea that randomness is part of the very fabric of nature was revolutionary: it had previously been taken for granted that the laws of physics didn’t depend on the size of things. But in quantum mechanics they do. The scale of things does matter, and the smaller the scale at which you look at nature, the more different from our everyday view of the world it becomes: randomness dominates the small scale world.

What happens to spacetime at small scales?

Again, this theory has performed very well in experiments. Technological gadgets that have emerged from quantum theory include the laser and the microchip that populate every computer, mobile phone and MP3 player.

But what happens if we combine quantum mechanics and relativity? According to relativity, spacetime is something that can stretch and bend. Quantum mechanics says that on small scales things get random. Putting these two ideas together implies that on very small scales spacetime itself becomes random, pulling and stretching, until it eventually pulls itself apart.

Evidently, since spacetime is here and this hasn’t happened, there must be something wrong with combining relativity and quantum mechanics. But what? Both these theories are well-tested and believed to be true.

Perhaps we have made a hidden assumption?

It turns out that indeed we have. The assumption is that it’s possible to consider smaller and smaller distances and get to the point where spacetime pulls itself apart. What has rested in the back of our minds is that the basic indivisible building blocks of nature are point-like — but this may not necessarily be true.

Strings to the rescue

This is where string theory comes to the rescue. It suggests that there is a smallest scale at which we can look at the world: we can go that small but no smaller. String theory asserts that the fundamental building blocks of nature are not like points, but like strings: they have extension, in other words they have length. And that length dictates the smallest scale at which we can see the world.

What possible advantage could this have? The answer is that strings can vibrate. In fact they can vibrate in an infinite number of different ways. This is a natural idea in music. We don’t think that every single sound in a piece of music is produced by a different instrument; we know that a rich and varied set of sounds can be produced by even just a single violin. String theory is based on the same idea. The different particles and forces are just the fundamental strings vibrating in a multitude of different ways.

The mathematics behind string theory is long and complicated, but it has been worked out in detail. But has anyone ever seen such strings? The honest answer is “no”. The current estimate of the size of these strings is about 10-34m, far smaller than we can see today, even at CERN. Still, string theory is so far the only known way to combine gravity and quantum mechanics, and its mathematical elegance is for many scientists sufficient reason to keep pursuing it.

The theory’s predictions

If string theory is indeed an accurate model of spacetime, then what else does it tell us about the world?

One of its more startling and most significant predictions is that spacetime is not four, but ten-dimensional. It is only in ten dimensions of spacetime that string theory works. So where are those six extra dimensions? The idea of hidden dimensions was in fact put forward many years before the advent of string theory by the German Theodor Kaluza and the Swede Oskar Klein.

Shortly after Einstein described the bending of space in general relativity, Kaluza and Klein considered what would happen if a spatial dimension would bend round and rejoin itself to form a circle. The size of that circle could be very small, perhaps so small that it couldn’t be seen. Those dimensions could then be hidden from view. Kaluza and Klein did show that in spite of this, these dimensions could still have an effect on the world we perceive. Electromagnetism becomes a consequence of the hidden circle with motion in the hidden dimension being electric charge. Hidden dimensions are possible and they in fact can give rise to forces in the dimensions that we can see.

String theory has embraced the Kaluza-Klein idea and currently various experiments are being devised to try and observe the hidden dimensions. One hope is that the extra dimensions may have left an imprint on the cosmic microwave background, the left-over radiation from the Big Bang, and that a detailed study of this radiation may reveal them. Other experiments are more direct. The force of gravity depends crucially on the number of dimensions, so by studying gravitational forces at short distances one can hope to detect deviations from Newton’s law and again see the presence of extra dimensions.

Mathematics and physics have always influenced each other, with new mathematics being invented to describe nature, and old mathematics turning out to lend perfect descriptions for newly-discovered physical phenomena. String theory is no different and many mathematicians work on ideas inspired by it. These include the possible geometries of the hidden dimensions, the basic ideas of geometry when there is a minimum distance, the ways in which strings can split and come together, and the question of how we can relate strings to the particles in the world that we see.

String theory gives us an exciting vision of nature as miniscule bits of vibrating string in a space with hidden curled-up dimensions. All the implications of these ideas are yet to be understood. String theory is an active area of research with hundreds of people working to see how the theory fits together and produces the world we see around us.

About the author

David Berman is a lecturer in theoretical physics at Queen Mary University of London. He previously spent time at the universities of Manchester, Brussels, Durham, Utrecht, Groningen, Jerusalem and Cambridge as well as a year at CERN in Geneva.

His interests outside of physics include football, music and theatre and the arts.

Physics Titan Still Thinks String Theory Is “On the Right Track” – By John Horgan on September 22, 2014

At a 1990 conference on cosmology, I asked attendees, who included folks like Stephen Hawking, Michael Turner, James Peebles, Alan Guth and Andrei Linde, to nominate the smartest living physicist.

Edward Witten got the most votes (with Steven Weinberg the runner-up). Some considered Witten to be in the same league as Einstein and Newton. Witten was and is famous for his work on string theory, which unifies quantum mechanics and relativity and holds that all of nature’s forces—including gravity–stem from infinitesimal particles wriggling in a hyperspace consisting of many extra dimensions.

Even then, string theory—which some enthusiasts (not including Witten) called a “theory of everything”–was extremely controversial, because there seemed to be no way to confirm experimentally the existence of strings or the extra dimensions they supposedly inhabit. I was thus thrilled a year later when Witten agreed, albeit reluctantly, to an interview at the Institute for Advanced Study in Princeton. After our encounter, I wrote a short profile of Witten, “The Pied Piper of Superstrings,” published in Scientific American in 1991; and a longer profile for my 1996 book The End of Science. Here is an excerpt from the latter:

I asked Witten how he responded to the claims of critics that superstring theory is not testable and therefore is not really physics at all. Witten replied that the theory had predicted gravity. “Even though it is, properly speaking, a post-prediction, in the sense that the experiment was made before the theory, the fact that gravity is a consequence of string theory, to me, is one of the greatest theoretical insights ever.”

He acknowledged, even emphasized, that no one has truly fathomed the theory, and that it might be decades before it yielded a precise description of nature. He would not predict, as others had, that string theory might bring about the end of physics. Nevertheless, he was serenely confident that it would eventually yield a profound new understanding of reality. “Good wrong ideas are extremely scarce,” he said, “and good wrong ideas that even remotely rival the majesty of string theory have never been seen.” When I continued to press Witten on the issue of testability, he grew exasperated. “I don’t think I’ve succeeded in conveying to you its wonder, its incredible consistency, remarkable elegance and beauty.” In other words, superstring theory is too beautiful to be wrong.

Recently, Witten won the prestigious Kyoto Prize, created by Japan’s Inamori Foundation to honor “those who have contributed significantly to the scientific, cultural and spiritual betterment of mankind.” Witten won 50 million yen, or about $500,000, in the category of basic sciences. The Kyoto Prize website states: “Dr. Witten has made significant contributions to theoretical physics for more than 30 years as a leader in the dramatic evolution of superstring theory. Moreover, by applying his physical intuition and mathematical skills, he has advanced mathematics, and prompted the cutting−edge research of many mathematicians.”

My profiles of Witten were pretty snarky. So when a publicist for the Kyoto Prize asked recently if I would like to interview Witten, I was pleasantly surprised. I submitted some questions, and Witten graciously answered them by email.

Horgan: When I interviewed you in 1991, you said that “good wrong ideas that even remotely rival the majesty of string theory have never been seen.” Are you still confident that string theory (or its descendant, M theory) will turn out to be “right”?

Witten: I think I will stick with what I said in 1991. Since then, we have lived through the second superstring evolution and many surprising developments in which string theory has been used to get a better understanding of conventional theories in physics (and math). All this makes most sense if one assumes that what we are doing is on the right track. (My 2005 article “Unravelling string theory,” gives somewhat more detail.)

Horgan: Do you see any other rivals for a unified theory of physics?

Witten: There are not any interesting competing suggestions. One reason, as remarked in “Unravelling,” is that interesting competing ideas (twistor theory, noncommutative geometry, …) tend to be absorbed as part of a larger picture in string theory. The competing interesting ideas have been very fragmentary and have tended to gain power when absorbed in string theory.

Horgan: There are an estimated 10500 versions of string theory, each of which “predicts” a different universe. Leonard Susskind and others propose that these hypothetical universes actually exist, forming a multiverse “landscape.” But physicists such as Peter Woit, Paul Steinhardt and George Ellis complain that string and multiverse theories may be untestable and hence not truly scientific. How do you respond to these criticisms?

Witten: Personally, I hope the landscape interpretation of the universe would turn out to be wrong, as I would like to be able to eventually calculate from first principles the ratio of the masses of the electron and muon (among other things). However, the universe wasn’t made for our convenience. Plenty of leading physicists — prominent examples being Steve Weinberg and Martin Rees — have taken the acceleration of the cosmic expansion seriously as a hint that a landscape interpretation of the universe may be correct. This was a monumental discovery, which was announced in 1998 (two years after publication of your book The End of Science — unfortunate timing!). It was so shocking that it was several years, and a lot more data, before I personally was convinced of that finding.

If the landscape interpretation is correct, can we get additional clues that would make this more believable? One obvious possibility involves the outcome of Large Hadron Collider experiments. I explain why in my 2004 article, “When symmetry breaks down.” What I wrote there wasn’t original but provides a succinct explanation. The literature is filled with other suggestions about how we might conceivably get more clues about a landscape interpretation if that is correct (for example seeing a signature of a prior phase transition in the cosmic microwave radiation). It is hard to summarize these suggestions for you as it is hard to know which proposals are most worth describing.

Another possibility is that a theory that predicts a landscape would become well-established because of other predictions it made. The trouble with criticizing string theory because it plausibly predicts a landscape of vacua is that the landscape interpretation of the universe might be correct. 200 years from now, if more clues have emerged, possibly including some that are unforeseeable now, it might seem obvious that the landscape of string vacua was necessary to make string theory viable.

Horgan: Do you agree with Sean Carroll that falsifiability is overrated as a criterion for distinguishing science from pseudo-science?

Witten: Scientists aim to get as reliable and precise an understanding of nature as we can. The gold standard is a precise prediction that can be tested in a precise way in a laboratory experiment. Experiments that disprove theories are an important part of the scientific process.
With that said, it is a little too narrow to claim that science consists of trying to falsify theories because a lot of science consists of trying to discover things. (Chemists who attempt a new synthesis could say they are trying to falsify the hypothesis that this new synthesis won’t work. But that isn’t what they usually say. People who search for life on Mars could say they are trying to falsify the hypothesis that there is no life on Mars. Again, people don’t usually talk that way.)

Horgan: Are multiverse theories—whether inspired by string theory or other ideas—falsifiable? If not, should they be taken seriously?

Witten: If the landscape interpretation of the universe is correct, it certainly might be possible to get additional clues supporting this, as I indicated in an answer above.

Horgan: Some proponents of multiverse theories have promoted the anthropic principle as an answer to the question of why we live in this universe and not an entirely different one. Do you think the anthropic principle is a legitimate scientific proposition?

Witten: As I also wrote above, I would prefer an explanation of the universe from first principles, with no anthropic considerations. But once again, the universe wasn’t created for our convenience and it is useless to have preconceptions about what the answer is.

Horgan: Do you think science will ever explain exactly how the universe came into being? Why there is something rather than nothing?

Witten: Science has had an amazing amount of success in understanding the early universe, and for example, as you probably know, the evidence for cosmic “inflation” is vastly greater than when we last talked (or when your book appeared). However, when you ask if we will understand “exactly” how the universe came into being, that is a very high bar.

I think I would prefer to just say what I can see that might give a better understanding. Obviously, there are forthcoming observations that may help. The most immediate will be more data on the polarization of the cosmic microwave radiation. We should know more there within a few years. On a time scale of a few decades, fundamentally new insight might come from observations of the cosmic 21-centimeter background radiation and probably from other techniques.

However, as a theorist, I would like to also comment on theoretical loose ends that might offer hope of further progress. One involves cosmic inflation. As I’ve remarked, there is by now quite a lot of evidence for cosmic inflation. But there also isn’t a completely consistent, logically sound mathematical framework for the theory of cosmic inflation. If inflation is correct, then I think there is a good hope this problem might eventually be solved and that might lead to a better understanding of the early universe. Likewise, since we last talked, there has been tremendous progress in understanding the behavior of string theory when quantum effects are strong. But there has been very little progress in understanding the behavior when time-dependent effects are large (which would be important in the very early universe).
This is a relatively well-posed mathematical problem and I imagine there is a good chance for eventually getting insight, which again might lead to a better understanding about the early universe.

Horgan: Are there any mysteries that you think science will never explain?

Witten: As in any endeavor, all we can do is to do our best, without knowing at the beginning of the journey how far we will be able to get. The modern scientific endeavor has been going on for hundreds of years by now, and we’ve gotten way farther than our predecessors probably imagined.

Horgan: Some physicists, such as Stephen Hawking and Lawrence Krauss, have denigrated philosophy as obsolete. Do you agree? Are there any philosophers, living or dead, whose work you find interesting or useful to your work?

Witten: Personally I am most influenced by some of the “natural philosophers” of the past, such as James Clerk Maxwell.
Horgan: Are you religious? Do you think science and religion are compatible?

Witten: I consider scientific explanations to be more interesting and illuminating.

Addendum: Like Witten, I was shocked by—and initially skeptical of–the discovery in the late 1990s that the universe is expanding at a growing rate. Now, I view it as “by far the most exciting advance in physics and cosmology in the last decade,” as I put it in 2006. If I wanted to criticize The End of Science, I’d lead with cosmic acceleration. But so far the discovery remains just an interesting twist in the big bang theory, which has not precipitated the kinds of revolutionary theoretical breakthroughs and paradigm shifts that took place in the early 20th century.

Clarification from Witten: I was not alluding to the BICEP findings when I said that the evidence for inflation today is vastly greater than it was when your book appeared. I was referring to the totality of data on cosmic microwave fluctuations which emerged in the decade after your book was published. The evidence for something like inflation in the early universe is indeed vastly stronger. Virtually any physicist or astronomer will tell you that, although there are exceptions. The only implicit allusion to BICEP in my remarks was actually when I wrote (later in the interview, in answer to the question about whether we would ever have a complete knowledge about the origin of the universe) that within a few years we would know more about the polarization of the CMB. What I meant (and I think my phrasing, which reflects questions that have been raised about BICEP, would be clear to colleagues) was that within a few years we will know whether the effect claimed by BICEP exists at a level that can be detected in the CMB.

A Theory of Everything? By Brian GreenePosted 10.28.03 NOVA

The fundamental particles of the universe that physicists have identified—electrons, neutrinos, quarks, and so on—are the “letters” of all matter. Just like their linguistic counterparts, they appear to have no further internal substructure. String theory proclaims otherwise. According to string theory, if we could examine these particles with even greater precision—a precision many orders of magnitude beyond our present technological capacity—we would find that each is not pointlike but instead consists of a tiny, one-dimensional loop. Like an infinitely thin rubber band, each particle contains a vibrating, oscillating, dancing filament that physicists have named a string.

Although it is by no means obvious, this simple replacement of point-particle material constituents with strings resolves the incompatibility between quantum mechanics and general relativity (which, as currently formulated, cannot both be right). String theory thereby unravels the central Gordian knot of contemporary theoretical physics. This is a tremendous achievement, but it is only part of the reason string theory has generated such excitement.


In Einstein’s day, the strong and weak forces had not yet been discovered, but he found the existence of even two distinct forces—gravity and electromagnetism—deeply troubling. Einstein did not accept that nature is founded on such an extravagant design. This launched his 30-year voyage in search of the so-called unified field theory that he hoped would show that these two forces are really manifestations of one grand underlying principle. This quixotic quest isolated Einstein from the mainstream of physics, which, understandably, was far more excited about delving into the newly emerging framework of quantum mechanics. He wrote to a friend in the early 1940s, “I have become a lonely old chap who is mainly known because he doesn’t wear socks and who is exhibited as a curiosity on special occasions.”

Einstein was simply ahead of his time. More than half a century later, his dream of a unified theory has become the Holy Grail of modern physics. And a sizeable part of the physics and mathematics community is becoming increasingly convinced that string theory may provide the answer. From one principle—that everything at its most microscopic level consists of combinations of vibrating strands—string theory provides a single explanatory framework capable of encompassing all forces and all matter.

String theory proclaims, for instance, that the observed particle properties—that is, the different masses and other properties of both the fundamental particles and the force particles associated with the four forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity)—are a reflection of the various ways in which a string can vibrate. Just as the strings on a violin or on a piano have resonant frequencies at which they prefer to vibrate—patterns that our ears sense as various musical notes and their higher harmonics—the same holds true for the loops of string theory. But rather than producing musical notes, each of the preferred mass and force charges are determined by the string’s oscillatory pattern. The electron is a string vibrating one way, the up-quark is a string vibrating another way, and so on.


For the first time in the history of physics we therefore have a framework with the capacity to explain every fundamental feature upon which the universe is constructed. For this reason string theory is sometimes described as possibly being the “theory of everything” (T.O.E.) or the “ultimate” or “final” theory. These grandiose descriptive terms are meant to signify the deepest possible theory of physics—a theory that underlies all others, one that does not require or even allow for a deeper explanatory base.

In practice, many string theorists take a more down-to-earth approach and think of a T.O.E. in the more limited sense of a theory that can explain the properties of the fundamental particles and the properties of the forces by which they interact and influence one another. A staunch reductionist would claim that this is no limitation at all, and that in principle absolutely everything, from the Big Bang to daydreams, can be described in terms of underlying microscopic physical processes involving the fundamental constituents of matter. If you understand everything about the ingredients, the reductionist argues, you understand everything.

The reductionist philosophy easily ignites heated debate. Many find it fatuous and downright repugnant to claim that the wonders of life and the universe are mere reflections of microscopic particles engaged in a pointless dance fully choreographed by the laws of physics. Is it really the case that feelings of joy, sorrow, or boredom are nothing but chemical reactions in the brain—reactions between molecules and atoms that, even more microscopically, are reactions between some of the fundamental particles, which are really just vibrating strings?

In response to this line of criticism, Nobel laureate Steven Weinberg cautions in Dreams of a Final Theory:
At the other end of the spectrum are the opponents of reductionism who are appalled by what they feel to be the bleakness of modern science. To whatever extent they and their world can be reduced to a matter of particles or fields and their interactions, they feel diminished by that knowledge….I would not try to answer these critics with a pep talk about the beauties of modern science. The reductionist worldview is chilling and impersonal. It has to be accepted as it is, not because we like it, but because that is the way the world works.

Some agree with this stark view, some don’t.

Others have tried to argue that developments such as chaos theory tell us that new kinds of laws come into play when the level of complexity of a system increases. Understanding the behavior of an electron or quark is one thing; using this knowledge to understand the behavior of a tornado is quite another. On this point, most agree. But opinions diverge on whether the diverse and often unexpected phenomena that can occur in systems more complex than individual particles truly represent new physical principles at work, or whether the principles involved are derivative, relying, albeit in a terribly complicated way, on the physical principles governing the enormously large number of elementary constituents.

My own feeling is that they do not represent new and independent laws of physics. Although it would be hard to explain the properties of a tornado in terms of the physics of electrons and quarks, I see this as a matter of calculational impasse, not an indicator of the need for new physical laws. But again, there are some who disagree with this view.


What is largely beyond question, and is of primary importance to the journey described in my book The Elegant Universe, is that even if one accepts the debatable reasoning of the staunch reductionist, principle is one thing and practice quite another. Almost everyone agrees that finding the T.O.E. would in no way mean that psychology, biology, geology, chemistry, or even physics had been solved or in some sense subsumed. The universe is such a wonderfully rich and complex place that the discovery of the final theory, in the sense we are describing here, would not spell the end of science.

Quite the contrary: The discovery of the T.O.E.—the ultimate explanation of the universe at its most microscopic level, a theory that does not rely on any deeper explanation—would provide the firmest foundation on which to build our understanding of the world. Its discovery would mark a beginning, not an end. The ultimate theory would provide an unshakable pillar of coherence forever assuring us that the universe is a comprehensible place.

This feature originally appeared on the site for the NOVA program The Elegant Universe.

Brian Greene is a professor of physics and mathematics at Columbia University and a leading string theorist. He is author of The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (Norton, 1999).

Watch: The best explanation of string theory we’ve seen We get it now. FIONA MACDONALD 5 NOV 2015

Brian Greene:
Making sense of string theory
TED2005 · 19:06 · Filmed Feb 2005

When it comes to physics, there are some pretty complicated ideas out there, but one of the most confusing is string theory, which involves the existence of extra dimensions, tiny strings within atoms and, well, a whole lot else we don’t really understand. But in this 2005 TED talk, theoretical physicist Brian Greene, author of New York Times bestsellers The Fabric of the Cosmos and The Hidden Reality, explains the basics. And to date it’s still the best explanation we’ve ever seen.

In addition to his work as a science communicator, Greene is also a researcher at Cornell University in the US, where he works on superstring theory, which is an attempt to come up with a theory for everything to explain how all the particles and forces in the Universe work. Importantly, superstring theory tries to explain why all the conditions in the Universe are finely tuned to support life.

So what does it all mean? As you might expect, Greene does a much better job of breaking it down than we can, so you should really watch the video above. But if you’re looking or the ELI5 version, we can help you out with that.

Basically, superstring theory is an attempt to figure out what the smallest, most indivisible, fundamental constituents of the Universe are. Take an object like a candle, for example. We all know that if you look closely enough, that candle is made up of atoms. And the atoms are made up of electrons, protons, and neutrons. If you get even further down, protons and neutrons are made up of quarks. And that’s where conventional ideas end.

But, according to string theory, there’s actually something even smaller inside the quarks, and these are dancing filaments of energy that look a lot like beautiful vibrating strings – hence the name string theory. According to the idea of string theory, these cosmic strings vibrate in different patterns to make up all the different types of particles that form the world around us.

And that’s where the theory of everything comes into it, because if string theory is correct, then everything in the Universe is controlled and composed of these tiny cosmic strings, and if we can understand them, then we could understand the fundamental rules that govern nature.

Unfortunately it’s not that simple, because for this theory to pan out mathematically, we can’t just have three dimensions of space in the Universe. We actually need 10 dimensions of space, and one dimension of time.

Watch the video above to find out why that makes more sense than you might think and how physicists are now actively seeking out these extra dimensions. We promise that by the end of it you’ll actually be able to explain string theory to your friends at your next dinner party.

But there is one quick update we need to make – obviously this was filmed while the Large Hadron Collider was still being built at CERN in Switzerland, and as yet, the machine hasn’t been able to confirm the existence of these extra dimensions.

However, now that it’s running at record-breaking new energy levels, and with China announcing they’re going to be buliding a particle accelerator twice the size in the coming years, we’re closer than ever to being able to actually test string theory. And that’s really, really exciting.