WAVES RULE?

Dick Pountain /Idealog 348/ 05 Jul 2023 04:19 There appears to be a widespread opinion that biology and maths don’t mix. Perhaps it’s behind the current panic over AI, and the depiction of robots as villains in superhero movies (bring back the more sociable Robbie?) It was already prevalent in the 1960s at school when I had to choose which A-levels to take: to do chemistry I was told firmly I must take maths and physics but not biology or art, both of which I loved. In those days, and at university, and long afterwards, the only real connection between maths and biology was via statistics which were and remain necessary for designing and interpreting experiments and stuff like population studies. My interest in biology didn’t go away (I ended up in biochemistry) but it combined with interests in philosophy and computation to lead me down ‘eccentric’ paths. I discovered D’Arcy Thompson’s magnificent ‘On Growth And Form’ which showed how geometry was expressed in the shapes of living things. I encountered the Belousov–Zhabotinsky reaction where a certain chemical mixture oscillates between different states rather than proceeding smoothly to a final product: the Belgian chemist Ilya Prigogine (no relation) received the 1977 Nobel prize for extending this insight to explain what he called ‘dissipative systems’ which can organise themselves to exhibit complex moving structures.Last week in the Imperial College alumni magazine I read an article about professors Robert Endres and Mark Isalan whose work with synthetic embryos is revisiting the patterns described by Alan Turing in his seminal 1951 paper ‘The Chemical Basis of Morphogenesis’. Turing had suggested that biological shapes and structures might arise when two different chemical compounds diffuse into one another, and he proposed a mathematical description using partial differential equations, whereby waves of chemical composition flow through the growing organism catalysing reactions that create structures and control their placement in space. Though Turing pioneered modern computing, he didn’t yet possess one powerful enough to solve these equations, which he did by hand using approximations. Turing was also working before Watson and Crick discovered the structure of DNA, and the revolution in molecular biology and genetics they triggered now lets us understand the mind-boggling chemical systems that support his proposal. The embryos of multicellular creatures like ourselves start out as sheets of undifferentiated cells, but genes within some of them start to exude proteins called transcription factors which travel across the sheet in synchronised waves, turning other genes in far cells on or off and causing them to divide, to die, or to migrate, and to release further waves. This fantastically choreographed cellular ballet must happen in time and space very precisely to sculpt the shape of each particular creature: similar small sets of so-called homeobox genes generate the wings of a fly, the leaves of a tree, the segments of a worm or your arms and legs, via waves of proteins intersecting and interfering at the right places and times. If waves intersecting and interfering to make stuff happen sounds familiar, that might be because particle physics, which of course underlies the whole of modern chemistry, is currently based on waves too. The Standard Model, which has so far survived all attempts to surpass it, proposes that the universe consists of nothing but a set of ‘quantum fields’, one for each kind of particle, currently 17. All matter and energy and things and people, everything that happens (except for gravity at the moment, to the great chagrin of physicists) happens via ‘perturbations’ of these fields which spread like waves through space and time. And now for the punchline. Last week NANOGrav (North American Nanohertz Observatory for Gravitational Waves) and various international collaborating observatories released results that suggest there’s a very low frequency background ‘hum’ of gravitational waves permeating the whole universe. This hum might be the result of supermassive black holes merging, or it might be left-over ripples in space-time from shortly after the big bang, and it’s only now become detectable by new more precise LIGO (Laser Interferometer Gravitational-Wave Observatory) arrays that exploit ultra-tiny changes in the frequency of pulsars. But already there’s speculation that at some time shortly after the big bang, before there was light, just a soup of newly forming particles, it could have been interference between waves in this all-permeating gravity wave background that caused them to clump together into the first stars and then the first galaxies. There’s an apocryphal story about Bertrand Russell giving an astronomy lecture where a woman in the audience challenged him by saying the earth is actually supported on the back of a giant turtle. When Russell asked what that turtle was standing on, she replied that it’s “turtles all the way down.” Perhaps she was almost right but it’s the waves rather than the turtles… [Dick Pountain would rather waive the rules than rule the waves]