It seems like atoms and molecules were always known to exist. But you would be surprised to know that until 1908, they were seen more as conceptual tools.
It was Einstein and Perrin’s work explaining a phenomenon discovered by a botanist in 1827 that laid the foundation for atoms and molecules as physical entities.
Physicist-mathematician Pierre-Simon, marquis de Laplace (1749-1827) said in 1812 that if we know “for a given point in time, ...all the forces acting upon the universe and the positions of the objects of which it is composed,” and if we are equipped “with facilities large enough to submit these data to numerical analysis”, then we can come up with a formula that includes “the movement of the largest bodies of the universe and those of the lightest atom”.
With such knowledge, “nothing would be uncertain ..., and the past and future would be known to it”.
Physicist Albert Abraham Michelson (1852-1931), whose name every high school student can recall from the popular Michelson-Morley experiment (1887) to measure the speed of light, was confident that as “most of the grand underlying principles have been firmly established”, future discoveries in physical sciences would be “looked for in the sixth place of decimals”.
The Michelson-Morley experiment would pave the way for Albert Einstein’s special theory of relativity (1905). In 1927, the physicist Werner Heisenberg introduced the uncertainty principle. Surely the ‘New Physics’ that the coming decades witnessed introduced fresh insights and unveiled a whole new universe of strange phenomena, and cannot be considered as the science of ‘the sixth place of decimals’. Today it is heartening to see how much physics has moved from the Laplacian determinism of the nineteenth century. Physicists now speak about parallel universes, hidden dimensions, and super strings.
One can contrast this way of thinking with the way biology – particularly genetics and molecular genetics – progressed. Charles Darwin and Alfred Russel Wallace, with their discovery of evolution by natural selection (1859), had made biology a systemic study of dynamic interactions between the environment and the living organisms. After the discovery of the genetic laws by Gregor Mendel (published in 1865 and rediscovered in 1900), the synthesis of Darwinian evolution and Mendelian genetics ushered in the era of Neo-Darwinian understanding of life. And after that, with the discovery of the physical basis of hereditary material and ultimately the structure of DNA itself by biologist James Watson and physicist Francis Crick (1953), biology seemed to have moved definitely towards a more mechanistic framework – similar to what Laplace contemplated for the physical sciences. Psychologist Harold Morowitz provides a vivid imagery for what was happening to the two disciplines, biology and physics, “as if the two disciplines were on fast-moving trains, going in opposite directions and not noticing what is happening across the tracks”.
While two trains headed in opposite directions on parallel rail lines cannot have a meeting point, in the case of biology and physics, there has been constant interaction and exchange of influences. An exploration of these facets provides an insight into how science progresses in a way that surpasses simplistic binaries. In fact, some of the crucial advancements in biological sciences come from the insights obtained by so-called New Physics. And so, if modern molecular biology and New Physics seem to be moving in two opposite directions, it may be an interesting optical illusion hiding a process that may be more profound and holistic.
The articles in this series aim to uncover that relationship between physics and biology and how insights obtained from one discipline advanced the knowledge in the other.
In 1827, Robert Brown, an eminent botanist from Scotland, was observing pollen particles under the microscope. He found the particles to be in constant motion. The next year, he reported “an account of microscopical observations ...on the particles contained in the pollen of plants; and on the general existence of active molecules in organic and inorganic bodies”. Brown thought initially that it was a phenomenon peculiar to life.
Later, he observed the same motion in a water drop trapped in a quartz piece and concluded that it occurs in both living and non-living fluids with particles suspended in it.
Today, this peculiar movement of particles is named after its discoverer and called ‘Brownian motion’. Brown’s most important insight was his conclusion that the chaotic rapid movement he observed arose “neither from currents in the fluid nor from its gradual evaporation, but belonged to particle itself”.
In 1863, Christian Wiener attributed Brownian motion to molecular collisions. French physicist Louis Georges Gouy proposed the same in 1888. However, molecules and atoms were then seen more as conceptual tools for understanding chemical reactions. “The question whether atoms exist or not has but little significance from a chemical point of view,” said August Kekule, the discoverer of Benzene structure. To him, such a question belonged to the realm of metaphysics, not science proper.
We now know atoms make up everything, as memes remind us time and again. But then, there was a debate at the time that a section of scientists were making up this thing called atoms for their convenience.
With the emergence of thermodynamics theory, the question of atoms and molecules, as to whether they are physical realities or conceptual tools, became increasingly acute. However, not many thought of Brownian motion as providing an insight into the question.
James Clerk Maxwell, the physicist whose theories on electromagnetism made him the progenitor of New Physics, along with Ludwig Boltzmann worked out a probability distribution for the speed of particles (atoms or molecules) in an ideal gas. This distribution forms the basis for the kinetic theory of gases. But even he did not view Brownian motion as related to molecules. Maxwell thought that Brownian movement was more because of some unobserved external disturbance and hence if “submitted to a more powerful microscope the bodies will demonstrate only more perfect repose”. Nevertheless, the kinetic theory of gases conceptualising gas particles (atoms or molecules) as small ball-like structures colliding with each other, opened up the possibility of explaining a phenomenon like Brownian motion for the first time through thermodynamics.
In 1905, Einstein, then a clerk at a patent office, published a paper applying the molecular theory of heat to liquids, to explain Brownian motion. The same year, he published his work on the special theory of relativity. This more recognised work, in a way, eclipsed the importance of his paper on Brownian movement. Through a statistical analysis, Einstein established the relationship between the rate of displacement in this peculiar movement and the particles (atoms/molecules) in the medium. With this analysis, one can determine the dimensions of the particles. Einstein pointed out that the molecular kinetic theory of heat actually necessitated Brownian movement, and its absence would have been an argument against the theory.
At that time, most of the influential scientists belonged to the positivist school, advocating the inductive method. You start with observations and, as atoms and molecules were not observable, they could not be considered as real but only convenient tools at best or metaphysical conjectures at worst. In such a situation, Einstein’s paper on Brownian movement provided a possibility of experimental confirmation of atoms and molecules.
In 1908, Einstein produced another paper on the same subject. The same year, physicist Jean Baptiste Perrin came out with work based on experiments he had devised and conducted on Brownian movement, which confirmed that Einstein was right. In 1909, he published an important book, Brownian Movement and Molecular Reality.
Perrin was also the first person to use the term which now every high school student of science knows – Avogadro’s number. Avogadro was a lawyer who got drawn to the physical sciences. He proposed the famous Avogadro’s hypothesis that equal volumes of different gases at the same temperature and pressure contain equal number of particles. Through his work on Brownian movement, Perrin made an experimental estimation of Avogadro's number – yet another milestone in the advancement of physical sciences.
In his textbook (1903), Perrin criticised the rejection of atomic theory by positivist-inductive scientists and argued in favour of the presence of atoms, drawing a parallel between molecules and microbes. (This is indeed a remarkably beautiful symmetry considering the fact that a few decades later, Niels Bohr would give a lecture – the 1932 ‘Light and Life’ talk – drawing parallels between the way New Physics looks at atomic phenomena and the way biologists look at life; we will see this in detail in the next part.)
One would certainly have been able, without the aid of the microscope, to arrive at the thought that contagious diseases were due to the multiplication of very small living beings. One, guided by these ideas a priori, would have been able to discover almost all of the Pasteurian technique. One would have thus followed deductive science and cured the contagious diseases, but following a way condemned by the supporters solely of the inductive method, until the very day in which the microscope had proved that the microbe hypothesis expressed several possible sensations. Here then is an indisputable example of a structure which could escape our senses and the knowledge of which allows anticipation of certain properties which are [to our senses] directly accessible.Niels Bohr, ‘Light and Life’ (1932)
What Einstein and Perrin achieved was that they clearly proved the existence of atoms. German physical chemist Friedrich Wilhelm Ostwald, who rejected atomic theory based on positivism (even in 1908, almost a decade after the discovery of electrons), came to accept it only because of the Einstein-Perrin work on Brownian movement. Einstein and Perrin also linked Newtonian physics with thermodynamics and chemistry. And this happened through a discovery by a biologist observing a biological system in 1827.
Perrin’s approach to the method of science was refusing to limit one’s search to only the knowable domain. He wrote:
To divine in this way the existence and properties of objects that still lie outside our ken, to explain the complications of the visible in terms of the invisible simplicity is the function of the intuitive intelligence which, thanks to men such as Dalton and Boltzmann, has given us the doctrine of Atoms.Jean Perrin
Einstein also believed in such ‘invisible simplicity’ and he expressed it metaphorically when he said that he would have felt sorry for the Lord if the observations had not vindicated his theory (with respect to his general theory of relativity).
From such Newtonian beginnings, we shall go into deeper influences in the coming weeks. For now, let us remember this, that the existence of atoms and molecules, which we take for granted today, was debated seriously until 1908 and it was a phenomenon discovered by a botanist in 1827 and explained by Einstein and Perrin in 1905 and 1908 respectively that put the existence of atoms and molecules on solid ground.