Deep in the ice, two kilometres under the heart of Antarctica, electronic pulses in the IceCube detector, the result of particles called neutrinos hitting the ice, have initiated a chain of reasoning which has recently led to their interpretation in terms of limits on supersymmetry.
Supersymmetry is a speculative theory about the basic physical principles of the universe. It has also been the focus of attention at the Large Hadron Collider, as well as at smaller precision experiments (for example of the magnetic moment of muons). There is no direct evidence that supersymmetry is correct, as a description of nature, but the fact that it can connect such disparate observations shows its usefulness as an aid to exploration. Being able display data from CERN and IceCube on the same graph, to measure them against a common scale, is important in gauging their relative sensitivity to the unknown, and in hunting for inconsistencies - or, in the case of a discovery, consistencies.
Jigsaw puzzles have been a big thing around our house since the holidays. They featured on a Christmas list, and Santa was keen on the idea. Mostly, we look at the picture on the box while we do them. When we tried without looking, the job was enormously more difficult, to the extent that we gave up and peeked. A theoretical framework, such as is provided by supersymmetry, or the Standard Model of particle physics, or the Lambda Cold Dark Matter model of cosmology, plays the role of the picture on the box. When looking at a jigsaw piece, the picture gives you an idea of where it might fit, and how it might connect to the others. Of course, you still have to try it to see if it really does fit. And in the science version, we also have to bear in mind that our picture is almost certainly incomplete (e.g. the Standard Model) and possibly completely wrong (e.g. supersymmetry). To some extent any picture is better than none, and anyway we don’t have much choice. At some point we might fit enough pieces together to realise it was wrong, throw it out and try a new picture. A sort of Kuhnian paradigm¹ shift that would see a jigsaw manufacturer go out of business under the weight of returned Christmas presents.
The framework provided by a good theory, or collection of theories, gives focus to research and makes it harder for a new theory to gain acceptance. A maverick new theory (I get examples sent to me practically every week) must either fit with the existing picture, or replace it completely. In the latter case, it has to accommodate all the jigsaw pieces which are already snugly interlocked. This, not a conspiracy of lizards or illuminati (or even hide-bound conservatives), is why a dramatic “Einstein was wrong” type of idea is unlikely to be taken very seriously without a lot of supporting evidence, and the ability to accommodate previous evidence that he was pretty much right.
In the sense that it emphasises the importance of the whole, and the interdependencies of the parts, this is a holistic² view of science. In his book “More and Different”, the great theoretical physicist Philip W. Anderson describes this as a “seamless web” –
a body of firmly established theory, now extending from physics through molecular biology, which in many situations, traps dubious observations. Already known laws, like conservation of energy, quantum³ mechanics, relativity, and the laws of genetics, constrain the explanation of any given result in a fashion which can be unique, or nearly so, and makes errors easy to spot. Much of science is “overdetermined” in this sense.
It is by no means infallible (Anderson’s essay is called “When Scientists Go Astray”) but it is the best we have.
Which brings me finally to what seems to me to be a borderline case.
A new measurement of the electric charge of antihydrogen was published in Nature this week by the Alpha experiment at CERN. The charge is expected to be zero, and the measurement confirms that to high precision. That is an important step in the main goal of the experiments, which is to measure for the first time whether antimatter experiences the same gravitational force as matter. According to the “seamless web” of theory, it should. But the force has never been measured, so from a purely observational point of view, we don’t know. It could even be that antimatter is repelled from matter – that is, antimatter may experience antigravity!
In his article on the new result (paywall), Thomas Phillips discusses a so-called “Dirac-Milne” universe, in which matter and antimatter gravitationally repel each other. This would be a very different picture on the jigsaw box, and it is sort of hard to believe that it could describe the observations we already have. These include measurements of particles in colliders, of the distribution of matter in the galaxy, of the cosmic microwave background, and more. All these observations have driven us to our current picture of physics and cosmology, often called the “Concordance” model.
On the other hand, our current picture has substantial gaps. Most of the matter in the universe is supposed to be unknown “Dark Matter”, not understood within the Standard Model. 69% of the energy in the universe is “Dark Energy” (even weirder) and in any case particle physics doesn’t know how to derive a universe with so much matter and so little antimatter. The Dirac-Milne model claims to address all these problems, and to agree with the data (and the Concordance model) on some important features such as the abundance of light elements and the main feature of the cosmic microwave background. Whether it can be accommodated with the seamless web of current knowledge – and thus profoundly change it – may depend upon whether enough people take it seriously enough to do the calculations and follow through the consequences. Or, as Dirk Gently might put it
Detecting and triangulating vectors of the interconnectedness of all things
One things is certain – if the Alpha experiment really measures antigravity for antimatter, someone will definitely have to try.
¹,²,³ Bingo!
Jon Butterworth’s book Smashing Physics, about his involvement in the discovery of the Higgs boson, is available as “Most Wanted Particle” in Canada & the US and was shortlisted for the Royal Society Winton Prize for Science Books.
