THURSDAY, 20 FEBRUARY 2020
The little side street, Free School Lane, where the Physics Department’s Cavendish Laboratory used to be situated, is rather unassuming aside from a large, oval plaque that boasts the significance of the sand-coloured building on which it hangs:“Here in 1897 at the old Cavendish Laboratory J.J. Thomson discovered the electron subsequently recognised as the first fundamental particle of physics and the basis of chemical bonding, electronics and computing.”
We were all taught the popular tale about the genius of the young Mancunian experimentalist who balanced the electric and magnetic fields surrounding mysterious ‘cathode rays’ to derive the coveted e/m (charge to mass) ratio of the electron. However, I suggest that the claim that this experiment marks the ‘discovery’ of the electron is misleading.
In fact, our protagonist did not even call the object of his investigations ‘electrons’, despite the fact that the term had been coined by George Johnstone Stoney and used to describe a unit of charge since 1891. Thomson even actively refused to use ‘electron’ to describe what he had found! He opted instead for the term ‘corpuscle’ in order to emphasise his theory that a cathode ray was a continuous stream of discrete particles whose mass was tiny compared to their charge. Thomson had indeed presented the important finding that the carriers of charge were inconceivably tiny. However, this was merely a piece of the much bigger puzzle being assembled in the early 20th century on the nature of mass, charge and the constituents of matter. Whilst Thomson’s experiment in the Cavendish had been instrumental, had he really stumbled onto and discovered a genuinely new entity?
There were a plethora of crucial ‘electron’ experiments taking place across Europe at the same time as Thomson’s. Another one worth mentioning was conducted by the Dutch physicist Pieter Zeeman. Unlike Thomson, Zeeman demonstrated that a magnetic field could manipulate the spectrographic properties of atoms. From this insight, it was possible to directly calculate the ratio between the angular momentum and the magnetic moment of an electron in order to measure the e/m ratio, which Zeeman achieved just under a year before Thomson’s landmark experiment. This of course challenges the suggestion that the establishment of the e/m ratio was Thomson’s alone. Furthermore, despite what is written on the plaque described above, there was no immediate realisation that the ‘electron’ was a fundamental constituent of matter. The relationship between electromagnetism, mass and matter was being investigated with great zeal across Europe at this time, with theorists such as Lorentz and Larmor working on the significance of electromagnetism for newfound waves and rays, and by Niels Bohr, who contributed to the foundations for what we now call quantum mechanics. The significance of the electron thus arose out of its importance to these theorists, whose models all needed it to exist to some extent. Thomson was not a lone genius in a dark room who got lucky and stumbled on an entirely new entity, but rather he was a member of an emerging breed of experimentalists who were forging an exciting new theoretical landscape on which physics could rest, one entrenched in the physical observations that could be made with new technologies. While these crucial practitioners put us on the road to contemporary fundamental physics, their conceptions of the world were radically different to what ours are now. Thomson’s corpuscle was not our electron with its complicated fuzzy quantum properties, its significance in Feynman’s quantum electrodynamics, nor any of the other attributes we now give to e.
Of course, the reason Thomson is credited with being the hero in the electron story is because he was awarded the Nobel Prize in 1906 for his work with the cathode tube. The significance of prize-giving in shaping the popular narratives in the history of science has been noted by philosophers of science in depth. Science offers no second prizes when it comes to establishing new phenomena or research paths; awards are first-saw-first-credited. Whilst this seems unfair, since perhaps a practitioner who has made the most contributions in a field, as opposed to being its first member, is more deserving, there are important justifications for why science is constructed in this manner. The philosopher Philip Kitcher (1990) recognised the benefit of awarding prizes to the first person in an area as an incentive for scientists to expand their range of possible hypotheses, methods and research paths. If awards in science were distributed according to who has made the most significant progress in a certain field, it would be rational for scientists to follow behind a few trail blazers and invest resources on already well established areas, leaving who-knows-what unexplored. Hence, first-place prizes are important for the progress and expansion of science, Kitcher claims.
However, the emphasis on who-did-what-first leaves us with popular scientific narratives, oriented around ‘discovery’, that are blind to the important behind-the-scenes work that takes place in order to enable and validate the creation of new scientific knowledge. Thomson was working in a lab surrounded by brilliant and driven experimentalists all working on electrodynamics, he just happened to do something new. Furthermore, there is a deeper problem with emphasising a who-did-it-first scientific discourse – discovery is only ever recognised retrospectively. As the plaque details, the significance of the electron was “subsequently” understood by the new quantum mechanical theorists. The association between our electrons as fuzzy clouds of charge that orbit a heavy, atomic nucleus and Thomson’s ‘corpuscles’ as a spot of fluorescence on a screen was a long journey that did not stop in 1897, and yet this is not recognised in the simple, seemingly unproblematic statement, “Thomson discovered the electron”. Science’s emphasis on discovery makes scientists very bad historians and hence they fail to recognise the fact that their own work is deeply situated within their own space and time, just like how Thomson’s work was as has just been shown.
Is there a way to preserve the benefits of newness-oriented science historiography whilst simultaneously allowing better historical understanding to be forged? I think diverting the language of scientific discourse away from the passive and romanticised notion of ‘discovery’ and towards the historical context in which it happened is a first step. JJ. Thomson did not stumble on the electron, he calculated and derived the e/m ratios for entities he labeled ‘corpuscles’ within a certain experimental setup. He also built upon the masses of background knowledge on electromagnetism that he possessed in order to justify this new corpuscular theory of his. This permits an understanding of how and why his calculations were done and hence what the state of physics was like at the time he was experimenting. Appreciating this allows for a better and more accurate history of the electron to be accessed while still acknowledging that he did something novel. Suddenly, science is not encapsulated merely in the careers of a handful of lucky, trailblazing discoverers, but consists of key events orchestrated by active practitioners within, and thanks to, their surroundings.
The story of the electron and the role played by the events on Free School Lane is much more complicated than a single ‘discovery’ in one time and place. The emphasis in science on the new has created the establishment of false statements, such as “Thomson discovered the electron”, which go unquestioned by so many. Thomson actively investigated corpuscles that were later recognised to be what we now call electrons, and his e/m ratio is one of many attributes we use to characterise this entity. Instead of focusing our scientific language on discovery, we should use different and more specific active verbs to describe what those who came before us did. In this way, we will be able to understand not only that Thomson’s corpuscles are indeed our electrons, but how the processes that led to that realization took place.
Charlotte Zemmel is a 3rd year Natural Sciences undergraduate at Newnham College and is currently the physics subject editor of Bluesci magazine.
