Behind the mask of the LHC
[Here is my latest Muse for Nature News, which, bless them, they ran at its extravagant length and complexity.]
The physics that the Large Hadron Collider will explore has tentative philosophical foundations. But that’s a good thing.
Physicists, and indeed all scientists, should rejoice that the advent of the Large Hadron Collider (LHC) has become a significant cultural event. Dubbed the ‘Big Bang machine’, the new particle accelerator at CERN — the European centre for particle physics near Geneva — should answer some of the most profound questions in fundamental physics and may open up a new chapter in our exploration of why the world is the way it is. The breathless media coverage of the impending switch-on is a reassuring sign of the public thirst for enlightenment on matters that could easily seem recondite and remote.
But there are pitfalls with this kind of jamboree. The most obvious is the temptation for hype and false promises about what the LHC will achieve, as though all the secrets of creation are about to come tumbling out of its tunnels. And it is an uneasy spectacle to see media commentators duty-bound to wax lyrical about matters they understandably don’t really grasp. Most scientists are now reasonably alert to the dangers of overselling, even if they sometimes struggle to keep them in view.
It’s also worth reminding spectators that the LHC is no model of ‘normal’ science. The scale and cost of the enterprise are much vaster than those enjoyed by most researchers, and this very fact restricts the freedom of the scientists involved to let their imaginations and intuitions roam. The key experiments are necessarily preordained and decided by committee and consensus, a world away from a small lab following its nose. This is not intrinsically a bad thing, but it is different.
There is, however, a deeper reason to think carefully about what the prospect of the LHC offers. Triumphalism can mask the fact that there are some unresolved questions about the scientific and philosophical underpinnings of the enterprise, which will not necessarily be answered by statistical analyses of the debris of particle collisions. These issues are revealingly explored in a preprint by Alexei Grinbaum, a researcher at the French Atomic Energy Commission (CEA) in Gif-sur-Yvette .
Under the carpet
Let’s be clear that high-energy physics is by no means alone in preferring to sweep some foundational loose ends under the carpet so that it can get on with its day-to-day business. The same is true, for example, of condensed-matter physics (which, contrary to media impressions, is what most physicists do) and quantum theory. It is a time-honoured principle of science that a theory can be useful and valid even if its foundations have no rigorous justification.
But the best reason to tease apart the weak joints in the basement of fundamental physics is not in order to expose it as a precarious edifice — which it is not — but because these issues are so interesting in themselves.
Paramount among them, says Grinbaum, is the matter of symmetry. That’s a ubiquitous word in the lexicon of high-energy physics, but it is far from easy for a lay person to see what is meant by it. At root, the word retains its everyday meaning. But what this corresponds to becomes harder to discern when, for example, symmetry is proposed to unite classes of quantum particles or fields.
Controlling the masses
It is symmetry that anchors the notion of the Higgs particle, probably the one target of the LHC that anyone with any interest in the subject will have heard of. It is easy enough to explain that ‘the Higgs particle gives other particles their mass’ (an apocryphal quote has Lenin comparing it to the Communist Party: it controls the masses). And yes, we can offer catchy analogies about celebrities accreting hordes of hangers-on as they pass through a party. But what does this actually mean? Ultimately, the Higgs mechanism is motivated by a need to explain why a symmetry that seemed once to render equivalent two fundamental forces — the electromagnetic and weak nuclear forces — has been broken, so that the two forces now have different strengths and ranges.
This — the ‘symmetry breaking’ of a previously unified ‘electroweak’ force — is what the LHC will primarily probe. The Higgs explanation for this phenomenon fits nicely into the Standard Model of particle physics — the summation of all we currently know about this branch of reality. It is the only component of the Standard Model that remains to be verified (or not).
So far, this is pretty much the story that, if pressed beyond sound bites, the LHC’s spokespeople will tell. But here’s the thing: we don’t truly know what role symmetry does and should play in physical theory.
Practically speaking, symmetry has become the cornerstone of physics. But this now tends to pass as an unexamined truth. The German mathematician Hermann Weyl, who introduced the notion of gauge symmetry (in essence, a description of how symmetry acts on local points in space) in the 1920s, claimed that “all a priori statements in physics have their origin in symmetry”. For him and his contemporaries, laws of physics have to possess certain symmetry properties — Einstein surely had something of this sort in mind when he said that “the only physical theories that we are willing to accept are the beautiful ones”. For physicist Steven Weinberg, symmetry properties “dictate the very existence” of all physical forces — if they didn’t obey symmetry principles, the Universe would find a way to forbid them.
Breaking the pattern
But is the Universe indeed some gloriously symmetrical thing, like a cosmic diamond? Evidently not. It’s a mess, not just at the level of my desk or the arbitrary patchwork of galaxy clusters, but also at the level of fundamental physics, with its proliferation of particles and forces. That’s where symmetry-breaking comes in: when a cosmic symmetry breaks, things that previously looked identical become distinct. We get, among other things, two different forces from one electroweak force.
And the Higgs particle is generally believed to hold the key to how that happened. This ‘particle’ is just a convenient, potentially detectable signature of the broader hypothesis for explaining the symmetry breaking — the ‘Higgs mechanism’. If the mechanism works, there is a particle associated with it.
But the problem with the Higgs mechanism is that it does not and cannot specify how the symmetry is broken. As a result, it does not uniquely determine the mass of the Higgs particle. Several versions of the theory offer different estimates, which vary by a factor of around 100. That’s a crucial difference in terms of how readily the LHC might observe it, if at all. Now, accounts of this search may present this situation blandly as simply a test of competing theories; but the fact is that the situation arises because of ambiguities about what symmetry-breaking actually is.
The issue goes still deeper, however. Isn’t it curious that we should seek for an explanation of dissimilar entities in terms of a theory in which they are the same? Suppose you find that the world contains some red balls and some blue ones. Is it more natural to decide that there is a theory that explains red balls, and a different one that explains blue balls, or to assume that red and blue balls were once indistinguishable? As it happens, we already have very compelling reasons to believe that the electromagnetic and weak forces were once unified; but deciding to make unification a general aim of physical theories is quite another matter.
Physics Nobel laureate David Gross has pointed out the apparent paradox in that latter approach: “The search for new symmetries of nature is based on the possibility of finding mechanisms, such as spontaneous symmetry breaking, that hide the new symmetry” . Grinbaum is arguing that it’s worth pausing to think about that assumption. To rely on symmetry arguments is to accept that the resulting theory will not predict the particular outcome you observe, where the symmetry may be broken in an arbitrary way. Only experiments can tell you what the result of the symmetry-breaking is.
Should we trust in beauty?
Einstein’s statement is revealing because it exposes a strand of platonic thinking in modern physics: beauty matters, and it is a vision of beauty based on order and symmetry. Pragmatically speaking, arguments that use symmetry have proved to be fantastically fertile in fundamental physics. But as Weyl’s remark shows, they are motivated only by assumptions about how things ought to be.
A sense of aesthetic beauty is now not just something that physicists discover in the world; it is, in the words of Gian Francesco Giudice, a theoretical physicist at CERN, “a powerful guiding principle for physicists as they try to construct new theories” . They look for ways to build it in. This, as Grinbaum points out, “is logically unsound and heuristically doubtful”.
Grinbaum says that such aesthetic judgements give rise to ideas about the ‘naturalness’ of theories. This notion of naturalness figures in many areas of science, Giudice points out, but is generally dangerously subjective: it is ‘natural’ to us that the solar system is heliocentric, but it wasn’t at all to the ancient Greeks, or indeed to Tycho Brahe, the sixteenth-century Danish astrologer.
But Giudice explains that “a more precise form of naturalness criterion has been developed in particle physics and it is playing a fundamental role in the formulation of theoretical predictions for new phenomena to be observed at the LHC”. The details of this concept of naturalness are technical, but in essence it purports to explain why symmetry-breaking of the electroweak interaction left gravity so much weaker than the weak force (its name notwithstanding). The reasoning here leads to the prediction that production of the Higgs particle will be accompanied by a welter of other new particles not included in the Standard Model. The curious thing about this prediction is that it is motivated not to make any theory work out, but simply to remove the apparent ‘unnaturalness’ of the imbalance in the strengths of the two forces. It is basically a philosophical matter of what ‘seems right’.
There are also fundamental questions about why physics has managed to construct all manner of workable theories — of electromagnetism, say — without having to postulate the Higgs particle at all. The simple answer is that, so long as we are talking about energies well below the furious levels at which the Higgs particle becomes apparent, and which the LHC hopes to create, it is enough to subsume the whole Higgs mechanism within the concept of mass. This involves creating what physicists call an effective field theory, in which phenomena that become explicit above a certain energy threshold remain merely implicit in the parameters of the theory. Much the same principle permits us to use Newtonian mechanics when objects’ velocities are much less than the speed of light.
Effective field theories thus work only up to some limiting energy. But Grinbaum points out that this is no longer just a practical simplification but a methodology: “Today physicists tend to think of all physical theories, including the Standard Model, as effective field theories with respect to new physics at higher energies.” The result is an infinite regression of such theories, and thus a renunciation of the search for a ‘final theory’ — entirely the opposite of what you might think physics is trying to do, if you judge from popular accounts (or occasionally, from their own words).
Effective field theories are a way of not having to answer everything at once. But if they simply mount up into an infinite tower, it will be an ungainly edifice at best. As philosopher of science Stephan Hartmann at Tilburg University in the Netherlands has put it, the predictive power of such a composite theory would steadily diminish “just as the predictive power of the Ptolemaic system went down when more epicycles were added” .
Einstein seemed to have an intimation of this. He expressed discomfort that his theory of relativity was based not simply on known facts but on an a priori postulate about the speed of light. He seemed to sense that this made it less fundamental.
These and other foundational issues are not new to LHC physics, but by probing the limits of the Standard Model the new collider could bring them to the fore. All this suggests that it would be a shame if the results were presented simply as data points to be compared against theoretical predictions, as though to coolly assess the merits of various well-understood proposals. The really exciting fact is that the LHC should mark the end of one era — defined by the Standard Model — and the beginning of the next. And at this point, we do not even know the appropriate language to describe what will follow — whether, for example, it will be rooted in new symmetry principles (such as supersymmetry, which relates hitherto distinct particles), or extra dimensions, or something else. So let’s acknowledge and even celebrate our ignorance, which is after all the springboard of the most creative science.
1. Grinbaum, A. Preprint at http://www.arxiv.org/abs/0806.4268 (2008).
2. Gross, D. in Conceptual Foundations of Quantum Field Theory Cao, T. Y. (ed.) (Cambridge Univ. Press, 1999).
3. Giudice, G. F. Preprint at http://www.arxiv.org/abs/0801.2562 (2008).
4. Hartmann, S. Stud. Hist. Phil. Mod. Phys. 32, 267-304 (2001).