14 Nov 2018 Robert P Crease
Taken from the November 2018 issue of Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.
Robert P Crease explains why philosophers are fascinated by the redefinition of SI units
For scientists, the story of metrology has a simple plot: to ensure that standards keep improving. The pound of Imperial France made way for the platinum Kilogram of the Archives in 1799. In 1889 it in turn was traded for the platinum–iridium International Prototype Kilogram (IPK), which this month is on the verge of being replaced by a definition involving Planck’s constant. Each step has made the kilogram standard more secure, serviceable and stable, allowing more precise and definitive research. The latest development, to tether the definitions of all SI base standards to natural constants, seems to bring a triumphant end to this quest.
Philosophers like me look at such stories with different interests. If scientists study the world, philosophers study how scientists study the world. Philosophy, though, is practised in many ways, in what are referred to as the “analytic”, “pragmatic” and “continental” approaches, each of which focuses on different aspects of science (though in practice philosophers may use more than one of these lenses). Not surprisingly, each approach sees different things in the development of scientific standards. There’s more to metrology, it turns out, than simply producing ever-better measuring tools.
Three traditions
As I describe in more detail in my Physics World Discovery ebook Philosophy of Physics, the analytic tradition tends to focus on the logical conditions for science to succeed. Analytic philosophers have, for instance, discussed “stipulative” definitions, in which a term is given meaning by linking it with something else. This is what happens when a unit (such as the kilogram) is linked to a specific artifact (a lump of metal) to create a standard. The process protects the independence of the artifact from the phenomena measured.
In his 1953 book Philosophical Investigations, Ludwig Wittgenstein dramatized the point in a discussion of the International Prototype Metre (IPM), which was then in use and, like the IPK, was stored in a vault outside Paris. The IPM, he said, is the sole object of which we can say, paradoxically yet truthfully, that it is neither a metre long nor not a metre long (a bit like how the definition of a triangle or tree cannot itself be a triangle or tree). That same decade, the German philosopher Hans Reichenbach analysed what would happen if an earthquake destroyed the vault and disfigured its artifacts. The answer, he decided, was “logically very complicated”.
The impending redefinition of the kilogram will, however, mean we lose the independence of measuring standards and measured phenomena. Phenomena tied to Planck’s constant are bound to end up measured by a unit defined in terms of Planck’s constant itself. Though the SI community is content with this, the logical circularity has generated apprehension among some analytic philosophers.
Such circularity will not surprise pragmatists – philosophers more concerned with the practice of science than its logic. The US scientist Charles Peirce, who was also the country’s most original philosopher, was the first to experimentally tie a unit (the metre) to a natural constant (the wavelength of light). His work set in motion events that in 1960, nearly half a century after his death, led to SI, one of whose features was the definition of the metre by wavelengths of a spectral line of krypton.
Peirce developed pragmatism as a theory of thinking in the wake of lessons learned from his metrological experiences. You encounter a problem; you find your inherited concepts and instruments don’t do the work you want; so you study and improve that inheritance. You then use the improved tools to do better research, and so on. All the while you work within a community whose members you have to convince, and who will eventually improve on your work.
Pragmatists focus their attention on the very practice of measuring that analysts seek to bypass, and how that practice affects scientific puzzle-solving. What are the practical dissatisfactions behind the urge to create new standards? How will they affect the community? Does the SI reform truly set its standards outside social and political institutions and democratize access, or reinscribe elitism by making access to standards depend on forefront technology?
As for the third philosophical perspective – the continental – it focuses more on the measurers than on the measuring system or the measuring. A continental approach sees measuring as a special way that humans engage their surroundings. Charles Dickens famously depicted an extreme case in Thomas Gradgrind, the character in Hard Times who in obsessively measuring all aspects of human life loses track of his own; our age has those who obsessively monitor their health biometrically.READ MORE
Usually, though, measuring involves a momentary objectification of part of the world to foster some broader activity in it – whether it’s homeowners determining what furniture will fit in a house, or physicists determining if a theory accurately characterizes a phenomenon. A continental lens describes this objectification process, how it springs from broader activities, and how it affects them. In the past, improved measurement definitions, while making certain practices more efficient, have sometimes come at the expense of common practices and public understanding. What impact, if any, will the new remoteness of the language of the foundations of measurement have on the everyday perception of scientific practice?
The critical point
The most important impact of the new SI on the philosophy of science, though, may be to inspire philosophers to re-examine these lenses. Why does logical circularity really matter if the important thing is what works for scientists? Is “how it works for the scientific community” the only important issue? Don’t philosophers have to get more up to speed on the practice of technologies involved to judge social and political impact? The new SI may end up doing more than redefining the kilogram: it may also inspire philosophers to reflect on their own assumptions.
14/11/2018 from physicsworld.com
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