17 December 2007

The problem with physics

Cosmos Magazine
Physics has become obsessed with strings, branes and multiple dimensions, yet the big questions remain fundamentally unanswered. Has the time come to admit these wild conjectures have failed, and move on?
The problem with physics

Credit: SPL

I was recently talking with a colleague who was a fellow theoretical physics graduate student at Princeton University back in the early 1980s. He had been thinking about an obscure academic physics journal he would occasionally skim in the library during those years. This journal was filled with bizarre extra-dimensional models of particles and forces, esoteric ideas about cosmology, and a slew of highly speculative theorising, with little in common other than a lack of any solid evidence for a connection with reality.

“You know,” he said, “at the time I thought these things were a joke, but now when I look at mainstream physics papers, they remind me a lot of what was in that journal.”

Why is it that central parts of mainstream physics have started to take on aspects that used to characterise the outer fringes of the subject? At the very centre of the physics establishment, things have been getting more and more peculiar.

A QUARTER-CENTURY AGO, in the 1980s, it was clear to both of us what serious theoretical physics looked like. A hugely successful theory of elementary particles and the fundamental forces governing them had come to final form a few years earlier. It was referred to as the Standard Model, and evidence for it was pouring in from experiments around the world.

The Standard Model is a quantum theory of fields – of which the electromagnetic field was just one variety – and much of our time as students was spent trying to master the complex mathematical techniques needed to understand these quantum field theories. According to the Standard Model, there are three fundamental forces: electromagnetic, weak and strong. There are also a small number of fundamental particles carrying specified charges that determine which forces they experienced, such as photons for the electromagnetic force, and gluons for the strong nuclear force.

The mathematics of the theory is deep and highly sophisticated; the fields responsible for the forces are basic geometrical quantities that mathematicians call ‘connections’. The excitations and interactions of these fields were also responsible for the fundamental particles. The whole thing satisfied a beautiful equation as presented to the world by British physicist Paul Dirac in 1928.

At the time, no experimental evidence had been found that contradicted the Standard Model, but it was clearly not complete, since it didn’t address certain fundamental questions. The task for theorists was to find a better theory that could.

On of the key questions was regarding the origin and nature of mass. In the Standard Model, one conjectures the existence of something called a ‘Higgs field’ (named somewhat arbitrarily after Peter Higgs, one of several theorists responsible for the idea it implements). This field is responsible for giving particles their unique mass. Unfortunately, in many ways, the Higgs field just highlights our ignorance; the mass of a particle is determined by a number that characterises how strongly it interacts with the Higgs field, but we have no idea where these numbers come from.

Another crucial question was why we have this specific pattern of forces and fundamental particles. In particular we’d like to be able to explain the charges of the fundamental particles, as well as the three different numbers that determine the strengths of the three forces.

Then there’s the question of the mysterious fourth force: gravity. We have an excellent theory of this force – Einstein’s theory of general relativity – but this theory doesn’t mesh with quantum mechanics, and there appears to be a problem of inherent inconsistency in treating one of the forces differently than the other three.

What neither my fellow student nor I would ever have guessed during our graduate student days was that, in our middle age some 25 years later, we’d be no closer to answering any of these questions, and ever more speculative attempts to find such answers would have taken on some of what used to be the characteristics of the fringes of science.

HOW DID THIS SITUATION come about, and what are the prospects for it changing before my friend and I drift off into senility? By far the most important factor is that the Standard Model has turned out to be simply too successful. Clearly, having a beautiful, mathematically sophisticated theory that predicts exactly what every new experiment will see is something physicists should be proud of. However, had the Standard Model catastrophically failed somewhere along the line, at least it would have given physicists a starting point for a new approach.

Instead, as each new generation of accelerators has been turned on, with the ability to explore higher and higher energy ranges – or equivalently, shorter and shorter distances – experimentalists have found exactly what the Standard Model predicts. Every time. (There has been just one minor surprise: that neutrinos are massive. But this discovery didn’t contradict the model, and eventually did little more than add to the list of masses we don’t understand.) As a scientific field, fundamental particle physics has become very much a victim of its own success.

Although particle physics has been in the doldrums, during this same period the field of cosmology has moved forward at a brisk pace. The Standard Model tells us what the fundamental particles and forces are, while cosmology is the study of the large-scale structure of the universe. Wonderful advances in ground- and space-based astronomy have provided a wealth of dramatic evidence about the Big Bang and the early history of the universe. Just as particle physics converged on the Standard Model, Big Bang cosmology has recently been converging on something now called the Concordance Model. A bit like the Standard Model, it fits the data all too well, while leaving crucial questions open as a precise parameterisation of our ignorance.

The Concordance Model doesn’t address the most fundamental questions about the origin of our universe, questions about what happened in the earliest moments of the Big Bang. Instead it just quantifies and places parameters on the resulting structure we are able to observe, with our most precise observations coming from the details of the cosmic microwave background that fills space with radiation at a temperature of about three degrees Celsius above absolute zero (–273°C). Particle physicists have great hopes that cosmology will help solve some of the problems left open by the Standard Model, but so far this has not happened. Instead, the success of the Concordance Model has just provided two extra puzzles.

The first new puzzle goes under the name of ‘dark matter': there appears to be some sort of matter of completely unknown origin, which only interacts weakly with conventional matter particles (producing no electromagnetic radiation, like light, thus it’s invisible, or ‘dark’), but whose effects are detectable indirectly through the gravity.

This exotic matter has a dramatic effect on the structure of galaxies, as without it, stars in their outer reaches would be flung into deep space. It also affects the large-scale structure of the universe, but we know virtually nothing about it beyond observing its gravitational influence. One can come up with a wide array of compatible extensions of the Standard Model that include dark matter by doing little more than postulating a new stable particle that experiences appropriately weak interactions with known particles. In fact, it was once thought – and hoped – that neutrinos could be the culprit behind dark matter, as they effortlessly pass through most other forms of matter and seem to possess mass. However, it has since been discovered they’re just too lightweight and travel at too high a velocity to account for the observed dark matter phenomenon.

Experiments are underway to search for rare collisions of other postulated weakly interacting dark matter candidates, but so far nothing has been seen. Collisions in high-energy particle accelerators might, in principle, produce these exotic particles, but again, all searches for evidence of this so far have been in vain. One possibility is that the mass of such particles is just so large that experiments to date have had insufficient energy to produce them.

The second of the new puzzles has a similarly ominous and mysterious name, ‘dark energy’ (see “Dark forces”, p56), but is of even less help with the unresolved questions in particle physics. In the Concordance Model of cosmology, dark energy is little more than an additional constant term in Einstein’s equations describing space-time. In physical terms, it has the interpretation of an energy density carried by the vacuum pervading space.

According to the Standard Model, the energy of the vacuum is an undetermined coefficient that theorists have to enter by hand into their equations. For many years physicists running through their calculations had assumed this number was zero, but the new mystery is that it has now been measured to have a small positive, non-zero value, providing one more fundamental number characterising physics, the origin of which remains an enigma.

THE CONTINUING STREAM of new data, sometimes of an unexpected nature, has driven progress in many areas of cosmology. This progress now seems to be converging on a more stable and less exciting pattern of continued confirmation of the Concordance Model, although new surprises may yet appear. There is one part of cosmology, though, where there is no such convergence, with ever more highly speculative models under investigation.

This is the study of the very earliest history of the universe: the Big Bang and the moments immediately before and after. With little relevant observational data, theorists have only a few constraints on their models that need to be satisfied. This certainly isn’t a new situation, since speculation about the origin of the cosmos is among the deepest of human impulses and is older than human history.

What is different now is that there is a large and prolific industry devoted to the construction of complex models purporting to provide an answer to these questions of origins. Such models are often far removed from any possible or convincing sort of scientific test, prompting science writer John Horgan to describe such efforts as “science fiction
in mathematical form”.

Most of these models try to implement, in one way or another, the notion of ‘inflation’. This is the idea, first formulated in the early 1980s, that the very early universe underwent a period of exponential expansion, driven by a sort of phase transition from one vacuum to another vacuum of lower energy. Such a theory does successfully explain some puzzling features of cosmology, but unfortunately one can come up with an infinite number of complicated physical models, all sharing the same behaviour of the vacuum energy necessary, to make inflation work.

It’s not clear whether – even in principle – any possible observations can ever distinguish between these models. Equally speculative but very different ‘cyclic’ universe models now compete with inflationary models for the favours of cosmologists. Both sorts of models of the extremely early universe can readily be constructed so as to agree with the Concordance Model and all observations to date. But whether there will be any way to experimentally distinguish between them in the near or even distant future remains to be seen.

Particle physicists and early universe cosmologists have often expressed the hope the two disciplines might come together to help solve each other’s problems. In particular, cosmologists would like a fundamental model of particle physics to provide a specific implementation of the inflationary scenario, picking out one amongst the otherwise all-too-many possibilities. Unfortunately, quite the opposite has happened, with particle physics entering a period of increasingly florid speculation, multiplying possibilities to a degree that has reached spectacular proportions.

RECALL THAT THE FIRST of the three problems left open by the Standard Model is the origin of mass and the status of the Higgs field. This problem has been hugely frustrating to theorists over the years, as they have worked on a wide range of ideas about how to extend the Standard Model, without so far coming up with anything that solves the problem. No one has found a way of explaining convincingly the mass of any of the observed fundamental particles.

If the Higgs field really exists, there should be an observable Higgs particle – called the Higgs boson – but this has never once been seen. At the moment, we know fairly accurately all the twenty-odd parameters that characterise the Standard Model except for one, and it is this exact parameter that determines how massive the Higgs boson will be, and thus how much energy is required in order to produce it. Consistency with the rest of the model requires that the mass for the Higgs boson cannot be too large, and that it should already be in the range accessible to our current accelerators.

The highest energy accelerator currently running is the Tevatron at the Fermi National Accelerator Laboratory near Chicago. It collides protons and anti-protons at a total energy of two trillion electron volts, and this should be enough to allow the production of Higgs bosons. Unfortunately the production rate of these particles is very small at the best of times, and it is extremely hard to single them out from the overwhelmingly large number of other events taking place during these high-energy collisions (see “God’s doodles”, Cosmos 15, p22).

Experimentalists at the Tevatron are hard at work trying to accumulate as much data as possible in order to have some hope of finding evidence for the Higgs boson. They’re in a race against the clock with another accelerator, the soon to be operational Large Hadron Collider (LHC) at CERN near Geneva. When it goes live in 2008, the LHC will operate at seven times higher energy than the Tevatron, providing a much higher rate of production of Higgs particles. If all goes well, a few years from now experimentalists at the LHC should have the data necessary to find the Higgs – if it really exists. That’s if the Tevatron doesn’t get there first.

If either the LHC or the Tevatron find a Higgs particle with exactly the predicted properties, it would be yet another triumph of the Standard Model. However, in many ways a disaster for particle physics. Physicists will have learned little more than one number, the Higgs mass, and knowing this number will not help significantly to address the fundamental issue of why different particles have different masses. The LHC is a very expensive machine, costing an estimated $10 billion Swiss francs (around A$9.6 billion) to complete and requiring a tunnel 27 km in circumference. Building something that could explore substantially higher energies would require vastly more money or radically new technology, and neither seems likely to be available for quite some years.

Spaghetti soup: Is this really what the universe looks like at the sub-quantum level: a quagmire of vibrating extra-dimensional strings? (Image: Mehau Kulyk/PhotoLibrary)

THE QUESTION OF the origin of mass is an extremely difficult one, and the lack of any experimental hints hasn’t helped to make it any easier. But perhaps one reason for the lack of progress is that, for the last 23 years, most of the attention of the best theorists has been directed elsewhere, toward a very speculative research program known as string theory. String theory became a hot topic in 1984, the year that my friend and I received our doctoral degrees and left Princeton. Since then it has increasingly dominated the subject, so much so that for many years now many of the particle theorists in Princeton have been working on this topic.

What is string theory? In highly over-simplified form it is the idea of replacing fundamental point-like particles with new fundamental objects: vibrating loops called ‘strings’. This idea was first floated around 1970 in an attempt to explain some aspects of the strong nuclear force, but this was quickly abandoned after the advent of the Standard Model a few years later. Recently, this possible application of string theory has again drawn a lot of attention, but in 1984 it was a much more dramatic proposal that swept the field of particle theory.

The revolutionary new proposal was to replace the entire Standard Model with a string theory, one that would not necessarily solve the Higgs problem, but would provide an answer to the other two open problems: why we have the particles and forces we do, and the origins and nature of gravity. According to the string theory proposal, the Standard Model and gravitational fields were just the lowest energy excitations of a fundamental string, appearing as particles since the actual strings were so small. The Standard Model, with its mathematically beautiful and highly accurate predictions, would just be a low-energy approximation of a very different fundamental theory, one that unified everything into the degrees of freedom of a vibrating string. Way-out, but elegant.

From the beginning, this proposal suffered from a serious difficulty. The mathematical structure of string theory was vastly more complex and difficult to deal with than that of quantum field theory, already an extremely challenging subject. Mathematical consistency required that strings live in not three, but nine dimensions of space, so something had to be done to explain away the six unobserved dimensions. The hope was that these could be taken to be so small as to be unobservable. Then, all one had to do was to find a stable, consistent shape for the extra six dimensions, one whose properties would yield the Standard Model as we know it. Initially it appeared that there were only a small number of possible such shapes, so all we had to do was identify them and see if one worked.

After more than twenty years of tireless effort by thousands of theorists writing tens of thousands of papers, string theory remains a very popular – and still poorly understood – subject. However, our hopes it can provide a unified theory are rapidly vanishing. The mid-1990s saw the development of a conjectural extension of string theory involving not six but seven extra dimensions, known as ‘M-theory’. M-theory leads to a bewildering array of new speculative possibilities, including the use of ‘branes’, which are spaces of various dimensions carrying Standard-Model-like structures and moving around inside other spaces of yet higher dimensions.

The most serious blow to the subject occurred a few years ago, with the discovery for the first time of a successful mechanism for stabilising the size and shape of small extra dimensions, which is essential to make the theory complete. This was very much a Pyrrhic victory, since what was found was not a small number of possible shapes, but an enormously large one. With perhaps 101,000 possibilities, each giving different physics, there is no hope of ever examining them all to see if one looks like our universe, nor is there a plausible idea of how to use such a theory to make testable predictions.

This situation has split the string theory community and led to a bitter controversy. Some theorists, such as Max Tegmark from the Massachusetts Institute of Technology and David Deutsch at Oxford, are now promoting the idea that our universe is part of a vast ‘multiverse’, with the laws of physics nothing more than a random feature of our local environment. If so, no predictions about these laws are possible, other than through invoking the ‘anthropic principle’ that says they must be such as to allow our existence. Other theorists, such as Nobel Prize winner David Gross, vehemently dismiss this as ‘giving up’, an abandonment of conventional notions of what it means to do science.

FUNDAMENTAL PHYSICS now finds itself in a historically unprecedented situation. The multi-decade dominance of string theory, along with its extremely speculative research into the implications of exotic scenarios far removed from any hope of testability, has changed the subject in dramatic and fundamental ways.

What used to be considered part of the dubious fringes of science has now become institutionalised within the mainstream. In physicist Lee Smolin’s recent book, The Trouble With Physics, he characterises the current sociology of the field as dominated by ‘groupthink’, with too few physicists willing to admit how far off the tracks things have gone. The nearly infinite complexity of string theory, M-theory, branes, higher dimensions and the multiverse has led to a vast number of possible challenging calculations for people to do to keep themselves busy, all embedded in a mathematical structure far too poorly understood to ever lead to definitive, falsifiable predictions.

The problems of the Standard Model that faced my colleague and I a quarter of a century ago continue to inspire new generations of young theorists to devote their lives to work that might some day lead to real progress. But these problems remain extremely difficult ones, and we have little in the way of promising ideas, with far too much effort going into the evasion of difficulties and the pursuit of the chimera of unification through ever more complex higher dimensional constructions inspired by string theory.

The hopes of particle theorists now rest on the efforts of experimentalists hard at work at the Large Hadron Collider near Geneva. Perhaps within the next few years they will report new results that will finally provide the right hints about how to move forward in the right direction, leading most people to abandon unsuccessful ideas. If the Standard Model continues to hold, particle physicists will be in a difficult spot, one that will require them to find ways to both acknowledge the failure of some well-entrenched speculative research programs, and encourage ambitious young theorists to take chances and try to find new, more promising ones.

Peter Woit is a mathematical physicist at Columbia University in New York City and author of Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law.

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