Particle theory: how a humble finding in plant science transformed physics and chemistry

Disregard for plant research by scientists and prize committees means that it doesn’t get the same prestige as other disciplines. Plant science is vital for our food security and in our fight against climate change. But it’s equally important to generate new knowledge to advance research and innovation across all disciplines. Low recognition given to scientific contributions by plant science is highly detrimental as prospective young scientists often overlook plant science. Because of this, I never considered studying plants myself – it was entirely accidental that I studied plant science.

But the fact is that ever since the early days of science, plants have been central to breakthroughs that have enabled technological advances that we enjoy today. Therefore, I’m writing a series of blog posts to highlight a few significant findings from research in plants. This is the second blog of the series. In the first blog, I report how breakthrough studies by plant scientists using plant models gave birth to the new field of cell biology or cell science. Here, I explain how plant research gave rise to the area of particle theory, inspired one of the greatest scientists of all time, and bought recognition to esteemed scientists in different disciplines.

Brownian motion

In 1827, plant scientist Robert Brown – who was the first person to discover nucleus (first cellular compartment to be found) – reported his observations of molecule movement in liquid. Brown’s work in plants established that particles move randomly in fluids – a phenomenon known as the “Brownian motion”. He recorded the movement of pollen grains of the Clarkia pulchella plant in water. Brown reports that “particles, when immersed in water, are generally seen in vivid motion”. He found similar movement in several other plants including in mosses – making him one of the first bryologists. (Fun fact: the moss Tetrodontium brownianum is named after Brown, who discovered it in Scotland whilst still a student). Brown also observed this effect in dead plants and inorganic matter, concluding that these motions “arose neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself.”

For all of Brown’s intelligence, he could not explain why these particles were moving randomly. This question was outside his discipline of expertise, and it was the physicists that tried to find the answer. Several physicists attempted and failed to provide a credible explanation on why particles were undergoing the Brownian motion. This finding in plant science ensued nearly eight decades of scientific feuds, disagreements and arguments. But the secrets of Brownian motion remained precisely that, a secret. That is until a clerk in Swiss patent office took up the challenge to resolve the mystery identified through plant science.

Einstein explains the Brownian motion

In 1905, a young physicist called Albert Einstein announced his arrival at the world-stage by publishing three major papers that shook the scientific world to the core and revolutionised science forever. And one of these papers were based on the Brownian motion – observations made in plant science by plant scientist. Einstein’s 1905 paper provided a quantitative description of the Brownian motion based on the molecular-kinetic theory of heat. He predicted that moving molecules or atoms in the fluid collide with the larger particles causing them to move randomly.

Einstein’s work explains why Brown observed the random movement of pollen in water. Smaller water molecules move and knock against the bigger particles (pollens in this case). Upon collision, pollens move in random directions and disperse in the fluid over time. Einstein explained that Brownian motion occurs at a particular rate. Therefore, the diffusion of the pollen in the water occurs at a particular rate. And that the rate of the spread on these larger particles in the fluid depends on the number of atoms in a mole of the fluid. He determined the way to quantify the size of atoms, and how many atoms are present in a mole (Avogadro’s number).

Albert Einstein – one of the greatest minds of all time. In one of his most significant work, Einstein explained the Brownian motion that was initially described by plant scientist Robert Brown.

French scientist Jean Baptiste Perrin carried out a series of experiments in 1908 to confirm Einstein’s predictions. As Einstein predicted, Perrin showed the Brownian motion occurred due to molecules colliding with larger particles in the fluid. This convinced doubters that matters are indeed made up of atoms or molecules resolving a near century-old scientific dispute.  Einstein’s ground-breaking paper and experimental confirmation transformed the 20th-century physics.

Particle theory and scientific recognition

Einstein received the 1921 Nobel Prize in Physics mainly for his discovery of the law of the photoelectric effect – but also in part for his explanation of the Brownian motion. Perrin also received the 1926 Nobel Prize in Physics for his experiments confirming Einstein’s analysis on the Brownian motion. Their work provided evidence and laid the foundations for the establishment of particle theory (or the kinetic theory of matter).

It’s easy to ask the question, “who cares about how atoms move?” As it turns out, the particle theory helps us understand how matters behave. As all matter are made of atoms, the particle theory helps explain the physical property of the matter. It also explains the change in properties of the matter when their physical state changes. For example, the H2O is ice when it’s solid, water when it’s liquid and steam as a gas. And different physical forms of the water have different properties, which could be explained by the particle theory. Yes, this is high school science. Who thought that something that started as a plant science experiment ended up in high school physics/chemistry syllabus?

In summary, a fundamental study of plant science spiralled into a heated scientific debate for near a century. This finding in plant science inspired one of the greatest ever scientist to make one of his most significant discovery.  And in the end, follow up studies, that started in plant sciences, revolutionised physics and chemistry. We must not forget the humble beginnings of these remarkable discoveries. Findings of a plant scientist while studying plant science shape our understanding of the things that make up a big part of the universe.

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