Excerpt from an article of the same title by Peter Woit, which appeared in Issue 16 of Cosmos (August 2007). With comments by Yours Truly.
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, and evidence for it was pouring in from experiments around the world.
The Standard Model is a quantum theory of fields…
…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…
How can geometrical quantities be responsible for particles, silly?
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.
Let’s keep in mind that the experimental evidence consists in the agreement of probabilities of measurement outcomes (predicted on the basis of previous measurement outcomes) with the observed probabilities. There is no evidence whatever of any kind of ontology beyond the correlations between measurement outcomes.
One 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.
Again, keep in mind that mass is not weight or anything you can feel or otherwise perceive but a numerical parameter that contributes to determined the probabilities of measurement outcomes.
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.
Just as we have no idea why there is anything, rather than nothing at all. But given that there is anything, rather than nothing at all, the existence of that numerical parameter can be predicted (though not its value in dimensionless units).
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.
So translate the question of “why we have this specific pattern of forces and fundamental particles” into why do those numbers have the values that they do. Numbers are one thing, ontology is quite another.
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.
We’ll let this pass.
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.
This appears to be the difference between business and science. In science success is bad.
