“Young man, if I could remember the names of these particles, I would have been a botanist.” Enrico Fermi’s reply to the unfortunate student who asked him the name of a certain subatomic particle came in the mid-1950s, when only a dozen such particles were known to exist. Within a decade, the number had swelled to nearly 100 as ever more powerful accelerators and detectors came online. But as Peter Watson explains in his latest book Convergence, although this proliferation in the number of particles “seemed counterintuitive at first, [it] would also help unify our understanding of certain aspects of the universe that had been beyond comprehension beforehand”. Indeed, it was not long before Murray Gell-Mann managed to turn what had become a nightmarish particle jungle into an ordered particle zoo, by devising a classification scheme called the Eightfold Way that grouped certain particles (the hadrons) into families of eight. In the process, this reorganization allowed the existence of undiscovered particles to be predicted.

Watson, a former journalist and a prolific (but always thought-provoking) historian of ideas, calls Convergence “a history of modern science but with a distinctive twist”. This twist, he claims, “has been there for all to see but so far it has not been set out as clearly as it deserves”. His main argument is that the various disciplines – despite their very different beginnings and apparent areas of interest – have in fact been gradually coming together over the past 150 years. Like Gell-Mann’s particle zoo in the early 1960s, these disciplines are “Converging and coalescing to identify one extraordinary master narrative, one overwhelming interlocking coherent story: the history of the universe.”

The two great unifying topics of the 19th century, Watson argues persuasively, were the conservation of energy and Darwin’s theory of evolution by natural selection. Each was the “fruit” of the coming together of the sciences: of heat, optics, electricity, magnetism and blood chemistry in the case of energy conservation; of geology, palaeontology, anthropology, geography and biology in the case of evolution.

Given this impulse to unite and simplify, Watson points out that physics has advanced when seemingly diverse phenomena have turned out to be different aspects of the same thing. Newton’s great discovery was that the same force that pulled the apple to the ground also held the Moon in its orbit around the Earth, and the Earth in orbit around the Sun. Magnetism, electricity and light were thought to be completely disparate phenomena until James Clerk Maxwell and Michael Faraday found that all were just different manifestations of electromagnetism. Albert Einstein’s theory of relativity grew out of his efforts to reconcile electromagnetism with classical mechanics.

It is hard to argue with Watson’s choice of Einstein’s unification of mass and energy and space and time as “the first great convergence event after the 1850s”. Max Born described Einstein’s masterwork as “the greatest feat of human thinking about nature, the most amazing combination of philosophical penetration, physical intuition and mathematical skill”. However, I do wonder how many would have chosen, as Watson has, Linus Pauling and his work on the nature of the chemical bond as one of their top three unifiers of the 20th century.

Watson also makes much of the Nobel laureate Steven Weinberg’s claim that convergence is “the deepest thing about the universe”. However, I find it remarkable that the “convergence of the sciences” (or, as Watson puts it “their synthesis, symphysis and coherence”) should be accorded such an accolade in the face of what those sciences have revealed to us about the nature of the universe and our place within it. It is widely accepted that when the universe was born in the Big Bang there was only a single force that soon shattered into four. Far from converging, these four forces have played largely separate roles ever since. The strong force holds the quarks together in the atomic nucleus. The weak force transmutes matter and makes the different elements. The electromagnetic force binds atoms and controls their chemical reactions. And then there is gravity, which has so far defeated all attempts to make it converge with the other three.

Weinberg was one of those responsible, in the late 1960s, for unifying electromagnetism and the weak force into the electroweak. Even so, I am surprised by his claim about convergence, which allows Watson to assert that “the most exciting intellectual breakthrough of all time” is “the way one science supports and interconnects with another, the beginning of a form of understanding like no other in history”.

Wherever experimental evidence can be coaxed out of nature, it suffices to corroborate or refute a theory and serves as the sole arbiter of validity. But where evidence is sparse or absent, other criteria, including aesthetic ones, have been allowed to come into play – both in formulating a theory and evaluating it. Watson believes that because of this, in some ways “physics has become mathematics”, arguing that we are currently “living in an in-between time, and have no way of knowing whether many of the ideas current in physics will endure and be supported by experiment”.

This, Watson explains, deeply worries the likes of cosmologists Joseph Silk and George Ellis. At the end of 2014, Silk and Ellis argued in a Nature comment piece that some scientists appear to have “explicitly set aside” the need for experimental confirmation of our most ambitious theories, “so long as those theories are sufficiently elegant and explanatory”. They further complain that we are at the end of an era, “breaking with centuries of philosophical tradition” of defining scientific knowledge as empirical.

As Silk and Ellis point out, this situation has come about because particle physicists have struggled to go beyond the Standard Model. Their most prominent attempt has been the theory of supersymmetry, but the problem is that no supersymmetric particles have been found, and Silk and Ellis fear that its advocates will simply “retune” their models “to predict particles at masses beyond the reach of the LHC’s power of detection”.

The result, Watson writes, is “a discernible sense of crisis”. The problem with a process of convergence, as he acknowledges, is that it presupposes a final end point. So while it may be true that “convergence is happening all over the sciences”, he admits, “The problem in physics is that it may be just too expensive to build the equipment that might bring that final convergence about.”

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