It was born in the Late 1960s and early 1970s - the orphan theory found wandering the streets by Gabriele Veneziano, Yoichiro Nambu, Holger Nielsen and Leonard Susskind. Originally invented to explain some peculiarities of the behaviour of hadrons (subatomic particles such as the proton and neutron which experience the strong nuclear force) String Theory has long promised to be the elusive Theory of Everything (TOE).According to String Theory, matter is not made up of small dot-like entities such as neutrons or quarks but of incredibly small threads of energy that vibrate. A string that vibrates one way becomes an electron. Another, vibrating differently, becomes a neutron. And another becomes one of the carriers of the force of gravity.
“You can think of the universe as a symphony or a song - for both are made up of notes produced by strings vibrating in particular ways,” said Professor Michael Green of Cambridge University.
It sounds intriguing. Unfortunately, to make their equations work, scientists had to add another six dimensions to the universe: four were not enough, though we cannot see these extra dimensions because they are so tightly crumpled up that they are invisible, it was argued. To the general public, of course, all this is faintly baffling.
Nevertheless, string theory proved encouragingly effective - at a theoretical level - to explain both the very small and the incredibly large, and so it began to dominate the study of fundamental physics at universities through the world. According to protagonists, it would soon be possible to describe the cosmos in a few simple equations that could fit on a T-shirt.
But as the years have passed, scientists failed to produced a single practical observation to support the theory. One problem, they said, was that the energy needed to break open matter and study the strings inside it is so colossal that it would require machines big enough to cover the planet. The Large Hadron Collider, currently under construction at CERN, near Geneva, hopes to correct that, at least to some extent.
On top of these problems, recent calculations have produced a surprising prediction from string theory: that there may be an almost infinite number of different universes, some of which would be like our own, and others that would be very different.
And it is at this point that the rot set in. An unprovable theory that talks of unseeable parallel universes and 10-dimensional space has proved too much for some physicists. “Quasi-theology” and “post-modern” have been among the most polite terms used; “bogus” and “nonsense” among the less forgiving.
Some of the best minds in Physics and Mathematics have dedicated their lives to studying and furthering this confusing branch of, arguably, Science. Part of the problem, say critics, is that, in the Eighties, talented young physicists were encouraged by professors to take up string theory because f its immense promise. Now they are middle-aged department heads who have committed their lives to the subject and cannot see it is bogus. It is all they have ever done. It is all they can teach. Consequently, subsequent generations have grown up convinced that String Theory is the Holy Grail of Physics and there is nothing beyond it.
Strings are far from the only game in town. There are other, potentially equally promising approaches to unifying physics’ two seemingly incompatible visions of the cosmos: general relativity and quantum mechanics. And the attacks on String Theory continue unabated. In, what must be, a painful turn of events the freshest and most intense attacks have come from within.
This year, Columbia University mathematician Peter Woit has published a critique of string theory (Not Even Wrong: The Failure of String Theory), pointing out that in more than three decades, string theory still has yet to make a single prediction that can be verified in the lab or through the lens of a telescope. If all scientific disciplines maintained such fluffy and forgiving standards, Woit argues, science would devolve into little more than medieval disputations about angels and heads of pins. The title of his book borrows from Wolfgang Pauli’s ridicule of a Physics paper as “not even wrong” implying that if could not even be disproved it was not Science. This is where the problems with String Theory start.
Over the years, string theory has simultaneously become more frustrating and fabulous. On the one hand, the original theory has become mind-bogglingly complex, one that posits an 11-dimensional universe (far more than the four- dimensional universe of Einstein). The modified theory is so mathematically dense that many Ph.D.-bearing physicists haven’t a clue what their string-theorist colleagues are talking about.
On the other hand, new versions of the theory suggest our universe is just one of zillions of alternate, invisible — perhaps even inhabited - universes where the laws of physics are radically different. String buffs claim this bizarre hypothesis might help to explain various cosmic mysteries.
But sceptics suggest it’s the latest sign of how string theorists, sometimes called “superstringers,” try to colourfully camouflage the theory’s flaws, like “a 50-year-old woman wearing way too much lipstick,” jokes Robert B. Laughlin, a Nobel Prize-winning physicist at Stanford. “People have been changing string theory in wild ways because it has never worked.”
Lee Smolin of Canada’s Perimeter Institute has taken the next step in his new book, The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next, outlining the most promising non-stringy paths to reconciliation between Einstein and the quantum.
Oxford University mathematical physicist Roger Penrose, author of The Road to Reality, invented a mathematical tool called “twistors.”
Smolin and Penrose take a look at the diverging paths beyond string theory.
Twistor String Theory. This retooling of string theory uses Penrose’s twistors, which reduce the number of dimensions in the theory to the familiar four - three spatial dimensions plus time. Twistors are, by definition, four-dimensional objects that locate not a position in space and time but rather a network of possible causal relationships between space-time events. Depicting a particle such as an electron as occupying a definite x, y, z and t gives a false sense of definiteness: Space and time are fuzzy at quantum scales. But cause and effect are not, and cause and effect are effectively what twistor space maps.
Pros: The mathematical beauty of string theory remains mostly unassailed, while the universe gets its four dimensions. Actual predictions for future particle accelerator experiments may yet emerge.
Cons: It’s still unclear what this “theory” is — and it may just be a sidelight on the 10-dimensional theory that yields more solvable equations. The inventor of twistors himself said, “I need to see a clear theory which I might be able to use, but I didn’t get that.”
Loop Quantum Gravity. If string theory evaporated tomorrow, something called Loop Quantum Gravity (LQG) would probably be the odds-on favourite to take its place. LQG, and a related approach called Spin Foam theory, posits that Einstein’s theories of space and time break down at very small scales (called the Planck scale, one-billion-billionth the size of an atomic nucleus) and in its place are entities described by another mathematical tool Penrose invented, called spin networks.
These graphs represent loops of field lines that, like string theory, become the fundamental building blocks of the universe. But unlike strings, no extra hidden dimensions are needed. The end result is that LQG predicts specific, quantifiable ways in which classical Einsteinian relativity would break down — and could soon be observable in fine-tuned measurements of the Big Bang’s microwave background or in observations by GLAST, a gamma-ray telescope scheduled to launch next year.
Pros: Smolin, one been able to make bold new predictions. And it reduces to something resembling classical, Newtonian gravity at low-energy and long-distance limits.
Cons: No one has yet been able to get spacetime itself, the stuff Einstein made famous, to emerge from LQG’s spin networks.
Causal Dynamical Triangulations. CDT breaks down tiny units of volume and area — the crucial stuff that makes up any spacetime — into tiny tetrahedra, a little like a computer graphics chip renders complex surfaces by decomposing them into many itsy bitsy squares and triangles.
CDT can be seen, says Smolin, “as a very simplified form of Loop Quantum Gravity.” And even if it is not The Ultimate Theory, CDT’s practitioners have developed clever solutions and approximation methods that could be used for the real thing.
Pros: Classical spacetime, as described by Einstein, does emerge from CDT models.
Cons: It’s not clear yet if falsifiable predictions can be made that would distinguish CDT from LQG or other theories.
Non-Commutative Geometry. Developed by a French mathematician named Alain Connes, this approach recognizes that observable quantities of a particle such as position and momentum cannot both be precisely measured — a quintessential aspect of quantum systems. Connes and his colleagues have outlined the spatial geometry that would produce this kind of “non-commutative” algebra. (Technically, a non-commutative operation is one in which AB does not equal BA.)
Pros: An extremely useful mathematical toolkit that has turned up both in string theory and in LQG.
Cons: May just be another extremely useful mathematical tool — along the lines of twistors and spin networks - and not a physical theory unto itself.
Of course, as Smolin points out in The Trouble With Physics, science is littered with present-day commonplaces that were once radical and courageous acts of unification: Copernicus said the Earth and the other planets were not two separate things but one. Giordano Bruno said the sun and the stars were not two separate things but one. Isaac Newton said the force that makes an apple fall from a tree is the same force that moves the planets through the heavens.
Sceptics of previous scientific grand unification efforts are often, though certainly not always, proved to have been lacking only in imagination. We are secretly rooting for String Theory. It would be a shame to see the Large Hadron Collider go to waste.
Also by Fungus
- The Virus Explained - December 5th, 2006
- Fun with Maglevs - October 9th, 2006