The values of the fundamental physical constants – seemingly fine-tuned for the emergence of nuclear matter and ultimately life – might not have been fixed at the universe’s outset but instead changed over time through a process akin to biological evolution. That is the hypothesis of a physicist in the UK, who has shown that life-friendly limits on fluid viscosity and diffusion impose constraints on the constants’ values. Having found that those constraints go beyond the requirements of stellar nucleosynthesis, he conjectures that the conditions needed for fluid motion in and among living cells could have emerged later on in cosmic history.
For decades, physicists have debated the possible explanation for a striking fact of our universe – that the values of many physical constants appear just right for the existence of the world we see around us. Star formation, for example, requires both hydrogen and helium. But this condition depends on a very specific value of the strong nuclear force – any weaker than it actually is and there would have been no helium; but any stronger and all the hydrogen would have converted (to helium).
Some scientists argue that this apparent fine-tuning provides evidence of design in the universe, perhaps even the existence of God. Others instead have mooted the possibility of a myriad of different universes – whether existing simultaneously or one after another – with physical conditions varying very slightly from one to the next. We would then necessarily exist in that universe suited to generating life. Still other researchers have postulated that the ultimate theory of everything – still to be worked out – would logically require the constants to have the values that they do.
But Kostya Trachenko at Queen Mary University of London reckons there could be an alternative explanation. He suggests that there is no need for a “grand design” for the cosmos, but that each of the universe’s physical “traits” could independently emerge, and become entrenched, through a gradual process of evolution – somewhat like the proliferation of certain survival-enhancing features in animals.
The spur for this idea comes, as Trachenko puts it, not by considering physical constants in the context of particle physics or cosmology but investigating them instead at the much lower and biologically-relevant energies of condensed-matter physics. This approach involves reducing complex physical or biophysical processes to their bare essentials and then expressing them in terms of one or more fundamental constants.
In 2020, Trachenko and Vadim Brazhkin published a paper establishing a universal lower limit for viscosity. As the pair pointed out, a fluid’s viscosity reaches a minimum at the temperature marking its transition from liquid to gas (in the latter case higher temperatures lead to more molecular collisions, which create greater friction between fluid layers). By modelling that transition, they were able to express the “kinematic viscosity” – the ratio of viscosity to density – in terms of Planck’s constant (ħ), molecular mass and electron mass (me).
Fluid flow is essential
Trachenko has now explored the implications of that work for the existence of life. As he notes, fluid flow is essential for many processes that take place within cells – such as molecular transport or the diffusion involved in cell proliferation. It is also vital in larger-scale, multi-cellular processes, such as blood circulation.
The idea was to work out the constraints that such processes place on the values of the fundamental constants. In addition to kinematic viscosity, which governs pulsed blood flow and other time-varying phenomena, Trachenko also considered the dynamic viscosity of steady flow and diffusion constants. Using the Navier–Stokes equation and other elements of classical fluid dynamics, he showed that all three parameters could be cast in terms of me, the proton mass (mp) and ħ (with the dynamic viscosity and diffusion constant also featuring the electron charge, e).
Trachenko found that the three parameters depend on the fundamental constants in different ways. As such, he says, combining the limiting expressions for life in each case – minima for the two viscosities and a maximum for diffusion – yields a limited range, or “bio-friendly window”, within which the constants have to exist. This, he claims, is an unexpected result given the complexity and variety of the biological processes involved (although he adds that biochemists and biologists will be needed to establish the three parameters’ numerical limits).
Fred Adams of the University of Michigan in the US praises Trachenko’s “novel” approach to imposing constraints on the fundamental constants. But he cautions that it may not yield unique limits, arguing that current biological theory is insufficient to work out the full range of allowed viscosities. “If we had a complete and comprehensive theory of biology and that theory showed that viscosity in any ‘living’ universe must lie within a certain range, then the argument would be strong,” he says.
Moving beyond the viscosity-derived limits themselves, Trachenko also looked at how these limits relate to those imposed by the need to produce heavy nuclei inside stars. Specifically, he considered the necessary tuning between the fine-structure constant (which features e and ħ) and the proton-to-electron mass ratio (mp/me). He realised that simultaneous changes of me and mp or of ħ and e could leave the stellar parameters fixed while altering the fluid parameters. In other words, a universe with different fundamental constants could in principle still contain heavy elements while its fluids are all at least as viscous as tar – so prohibiting life.
He describes the extra tuning needed for life-friendly viscosity as “overkill” in the early universe, pointing out that the precise values of the constants would need to be baked in at least 10 billion years ago – long before there were even any hints of what life might look like. “It’s a bit like asking a chef to get the right ingredients for an exquisite meal before you decide what the meal is,” he says.
It was this insight, he says, that prompted him to consider an evolutionary mechanism instead. He acknowledges that the details of any such mechanism are sketchy at this stage, both in terms of how the constants might change and what evolutionary pressure would bear so that certain values are favoured over others. He says only that a certain set of physical constants would start to favour the emergence of a new physical “structure”, which would endure if it had robust properties.
“I realise that what I am saying is quite crude but we just don’t know enough at the moment to be more specific,” he says.
The research is described in Science Advances.
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