The problem with physics
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?

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 (see "The whole shebang", p62), 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.