Why Is the Sky Blue? The Polish Scientist Who Found the Answer
It’s a simple question and one of the first often asked by inquisitive children: Why is the sky blue? The complex answer to this seemingly straightforward inquiry was explained by one of the most distinguished scientists of the early 20th century, Polish physicist and contemporary of Albert Einstein, Marian Smoluchowski.
A scientific pioneer, Marian Smoluchowski’s work and theories contributed to the development of a new scientific field and served as the basis for the Nobel Prizes of three different scientists in the 1920s. Over 100 years after his last original work was published, his research remains at the top of most cited works of Polish scientists around the world.
Held in high regard by Einstein himself during his lifetime, upon Smoluchowski’s untimely death during a dysentery epidemic in 1917, Einstein wrote him an obituary in which he said:
Fate has cut off his fruitful work as researcher and teacher too early; we shall, however, continue to esteem his example and his work.
Smoluchowski’s research interests often overlapped with Einstein’s, with both reaching the same conclusion and, independently of each other, publishing an equation that is often referred to today as the Einstein-Smoluchowski equation.
Not just an outstanding scientist, Smoluchowski was also an avid mountaineer and alpinist, and though his scientific achievements prevail in descriptions of his legacy, it would be fair to describe him today as a pioneer of both disciplines.
Scientific discovery & explaining why the sky is blue
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A watercolour of the alps painted by Marian Smoluchowski, photo: uj.edu.pl
Smoluchowski’s most notable area of research was on particle theory, particularly on the kinetic theory of matter, which helps to explain the changes in the states of matter by heating and cooling, such as why ice melts when it is heated.
In 1904, Smoluchowski was the first to discover density fluctuations in gases and the first to introduce the concept into scientific use. In 1906, shortly after Albert Einstein, but independently of him, and using a different method, he published an explanation of Brownian motion (i.e., the chaotic movement of particles in a liquid).
It was this theory and equation that earned Smoluchowski the greatest renown among his contemporaries and which gave rise to the equation bearing his name. More importantly, it significantly contributed to the acceptance of the existence of atoms, at the time not yet widely acknowledged. It also marked the starting point for the study of stochastic processes in mathematics, making Smoluchowski the pioneer of a new scientific field: statistical physics.
It was Smoluchowski’s subsequent work on the refraction of light in gases, which he worked on between 1907 and 1911, that also would help to explain why the sky looks blue.
A theory for the blue colour of the sky was first proposed by Lord Rayleigh in 1871. Rayleigh calculated that the refraction of light on particles suspended in the air is inversely proportional to the fourth power of the light wave, which meant that blue light refracted more strongly than red light, making the sky look blue.
Smoluchowski went one step further in his theory of critical opalescence which helped to explain that when there are fluctuations of density in a gas and light refracts through this gas, Rayleigh’s equation will apply. This proved that the phenomenon Rayleigh observed also occurs in completely clean air and thus helped confirm his explanation as to why the sky is blue.
Smoluchowski’s research interest in this area once again overlapped with Einstein’s and the two wrote letters to each other, but never met in person.
Smoluchowski himself wrote of his theory in relation to the colour of the sky in the following way:
Text
In normal conditions, gases also show the phenomenon of opalescence, though to a much weaker degree, so that it only becomes visible at thicker layers. However, each of us can observe it an infinite number of times when we admire the blue of the sky or the flaming colour of the rising sun.
A life devoted to science
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A letter written by Smoluchowski in 1915, two years before his death, shows how much science dominated his life and how strongly he dedicated himself to it:
I renounced a long time ago all kind of social, political, and national activities. It was just after graduation when I decided that science would be a leading star of my life. This is the basic rule of my life, to which I will be faithful. and which is above any other aspect of my life.
Trans. BK
Born near Vienna to an Austrian father and Polish mother, Smoluchowski graduated, with distinction, with a doctorate in physics from the University of Vienna in 1895. He was one of just two students awarded the prestigious sub auspiciis imperatoris, a title only given to the very brightest of students. That same year, he would embark on a journey to the most prominent scientific centres for studies in physics around Europe, completing work placements in the laboratories of the most distinguished scientists of his day.
The first of these work placements, lasting 8 months between November 1895 and June 1896, was in the Paris laboratory of Gabriel Lippmann, a future Nobel Prize winner in physics in 1908. The second, from September 1896 to April 1897, was spent at the most prestigious centre of physics in the 19th century, the University of Glasgow and the laboratory of Lord Kelvin, where Smoluchowski undertook research on X-rays and radium radiation.
Smoluchowski’s third placement was perhaps the most significant in the evolution of his future career. Spending four months in the laboratory of Emil Warburg, a friend of Albert Einstein, in Berlin, he was allocated work on Warburg’s observations in thermodynamics. After conducting experiments, Smoluchowski developed a theoretical model for Warburg’s initial observations, which became the topic of his habilitation thesis. In 1898, he presented his thesis at the University of Vienna and earned the right to lecture.
A year later he began to lecture at the University of Lwów and took over the department of theoretical physics in 1901 after the death of its previous academic head. Well recognised by this point, that same year he was invited back to the University of Glasgow to be awarded an honorary doctorate for his work on Brownian motion, one of many awards he received for his work on the subject.
After becoming a full professor of physics at the University of Lwów in 1903, he would continue at the university for the next 10 years. In 1905, he travelled to Cambridge to spend a semester at the Cavendish Laboratory. He returned to Cambridge in 1912, when he was invited to lecture at the International Congress of Mathematicians.
During his stay in Lwów, he was elected dean of the department of philosophy for one term and took an active role in Polish academic circles. He was elected the president of the Copernicus Society of Natural Scientists for a two-year term between 1906 and 1908 and later a member of its board between 1910 and 1912.
Smoluchowski would continue at the University of Lwów until 1913, before moving to Kraków to take up a post as the chair of experimental physics at Jagiellonian University, an institution to which he would remain loyal despite attempts to recruit him by the University of Warsaw and University of Vienna.
If not for his death in 1917, Smoluchowski would have taken up the role of rector of the university, to which he was elected shortly before he succumbed to the dysentery epidemic.
By the time of his death, he was recipient of many awards and honours for his scientific achievements, particularly his work on Brownian motion.
One of many testaments to the global significance of Smoluchowski’s scientific discoveries is a statement once given by accomplished theoretical physicist Richard Feynman, who, when asked what one sentence from the history of scientific knowledge he would save in the face of a cataclysmic event, said he would save the sentence that all matter is made up of particles that are in constant motion, considering it the most fundamental scientific discovery.
His legacy is also present in three pieces of work awarded the Nobel Prize in the 1920s. When awarded the prize in 1925 in the field of chemistry, Richard Zsigmondy mentioned Smoluchowski’s work as the basis for his own winning work in his Nobel lecture. Similarly, in 1926, Nobel Laureates Jean Baptise Perrin and Theodor Svedberg, who received the Nobel Prize in physics and chemistry respectively, also directly referred to Smoluchowski’s theories. The influence of his work on numerous Nobel laureates led many to believe that, had he lived, Smoluchowski also would have been a strong candidate for the Nobel Prize.
An alpine pioneer
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Marian Smoluchowski skiing in the Czarnohora mountain range in Ukraine, photo: uj.edu.pl
Though science was his priority, Smoluchowski’s second great passion was for mountain climbing, an activity he was introduced to by his brother Tadeusz while still at school, and which, alongside skiing, he would help to popularise.
After experiencing his first climb in the Tatra Mountains at the age of 13, Smoluchowski and his brother would go on to become regular climbers of the Austrian mountains and, during his university days, the Dolomites. He would also undertake many climbs in the alps, including the ascent of Matterhorn and Ben Nevis during his stay in Glasgow when working with Lord Kelvin. When he moved to Kraków, he would often go for walks in the Tatras.
Belonging to an alpine club from early on, Smoluchowski and his brother quickly became known as expert climbers and were considered among the world’s top alpinists of the time. Together they successfully completed and forged 24 new mountain trails, many of which were summits that had been conquered for the first time, making them pioneers paving the way in previously uncharted territory and receiving many awards in the process.
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Marian Smoluchowski and his brother Tadeusz in the Tatra Mountains, photo: uj.edu.pl
In a letter he wrote to his alpine club in 1917, Smoluchowski explained what the mountains gave him:
The three most valuable things which I get from the mountains are:
- getting used to undertaking difficult tasks
- happiness from overcoming challenges
- the ability to make everyday life more beautiful through the loftiest poetry: the poetry of the mountains.
Trans. BK
Another pastime starting to emerge at the start of the 20th century was skiing, and Marian Smoluchowski and his brother were among the 22 mountain enthusiasts to come together to found the Karpackie Towarzystwo Narciarzy (The Carpathian Association of Skiers), Poland’s first skiing association, of which his brother would later become president.
Smoluchowski served as chairman of the Tourist Section of the Tatra Society and wrote many articles about mountain climbing for numerous magazines.
Perhaps surprisingly for a scientist, Smoluchowski also enjoyed painting and often combined this hobby with his love of the mountains by painting several dozen panoramas of the mountains during his mountain trips. He carried paints and a canvas on his mountain expeditions in an effort to capture and immortalize the beauty of the landscapes he walked through.
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Watercolour by Marian Smoluchowski of Ankogl in Austria, photo: uj.edu.pl
Sensitive to arts and culture in general, Smoluchowski was also a talented pianist, having inherited a love and aptitude for playing from his mother.
Therefore, though best remembered as a scientist and physicist, Smoluchowski is perhaps best described as a polymath, due to his talent in so many disciplines.
Written by Blanka Konopka, Dec 2021
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