Is cosmology a science? Karl Popper (1902-1994) argued that the study of the universe as a whole is a field at the frontier of science, because its theories are difficult to refute. Monday 17 December marked this month’s edition of the lecture series “Physics & Philosophy”, organised by the Physics department of the University of Paris Diderot [in French]. Among the themes covered in cosmological epistemology, a surprising question: Are cosmological models refutable? Most renowned among them, the Big Bang theory fits so well with all experimental observations that it’s been taken for granted by cosmologists since the 1970s. So are we putting the cart before the horse?
In research works in physics, and even more so in cosmology, references to famous philosophers are rather common. Surprising? Not so much when you consider that both sciences have a field of study in common: the fundamental nature of things. The great discoveries of physics are, therefore, often relevant in philosophy, as well. Take Isaac Newton and his apple, for instance. The realisation that one and the same force is responsible for the fall of an apple from a tree and the movement of the Moon around the Earth helped establish that identical principles linked the terrestrial and the celestial world.
Karl Popper is among the most quoted philosophers in science. Stephen Hawking himself takes inspiration from him when he writes in ‘A Brief History of Time’: “Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the results will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.” This approach is followed by a majority of scientists. It consists of starting by considering the most easily refutable theories and then proceeding to the harder ones, in order to approach the truth. In other words: eliminate options to narrow the field of possibilities. Get close to the truth as rigorously as possible and avoid getting lost along the way.
The Big Bang: a theory at the frontier of physical science
To Karl Popper and other philosophers of science, the scientific process is of fundamental importance in the search for truth. At the time when Karl Popper was developing his ideas (1930-1940), a major cosmological theory began to appear: the Big Bang theory. As early as the 40s, it was becoming accepted among physicists and mathematicians (the term cosmologist did not exist yet). During the same period, a rival theory was also considered: the Steady State theory. It argued that the Universe is immutable in time and that its energy is continuously supplied by one or more unidentified sources.
Only it does not fit very well with observable results. And yet, this is the theory that Karl Popper will prefer, for good reason. One of the three scientists behind this assumption, Hermann Bondi, was a great admirer of the philosopher. He actually developed the Steady State theory following the Popperian perspective that one must always start with a theory easy to refute. Thomas Lepeltier, a researcher in philosophy of science [in French], explains that Karl Popper favoured ideas that forbid more than they allow. “As much as his role model, Hermann Bondi blames the Big Bang theory for allowing too many possibilities and for being too compatible with astronomical observations.” To him, as to Karl Popper, it is absolutely necessary to start by eliminating other options before moving on to theories that, in short, seem too good to be true.
The philosopher goes even further. When the Steady State ‘ was abandoned in favour of the Big Bang theory in the 1950s, he continued to think that it could have been better exploited. Despite the demonstrated incompatibility of this model with observable results, Karl Popper insisted that it was given up too quickly. He did not believe in an irrefutable way to discredit a theory. To him, a better understanding could resolve the conflict with the results. Why not put aside some aspects and keep others? “In his view, conjecture and debate is what makes the ‘game’ of science,” explains Thomas Lepeltier.
“Science needs the validation of prior theories to move on”
But then how can science move on? According to Thomas Buchert, cosmology professor at the University of Lyon [in French], it is necessary to acquire foundations on which to build. “To construct a think tank and push the issue further, we need schools of thoughts.” In practice, Karl Popper’s method is very restrictive. If no theory can ever be considered acquired or abandoned, then all progression in research is compromised. For instance, in order to make astronomical calculations, mathematical theories are necessary, and they are already a preconception. For Thomas Buchert, this is where the dialogue with observations begins. “It is only by communicating that one can choose the working conditions and decide what is real or not. That is why we need to establish universal cosmological principals.”
Karl Popper in the 1980s
Some schools of thoughts even believe that a great step forward can come from an audacious, if not hazardous, choice. “The initial conditions are apparently free. There is, therefore, a need for choice and this choice can sometimes be made quite arbitrarily,” notes Thomas Buchert. A blatant example is the cosmological constant of Albert Einstein. What he called the “biggest mistake of his life” ended up being considered by some an ahead-of-its-time intuition of the existence of dark energy. In 1917, the theoretical physicist believed that the Universe is static, but his theory of general relativity didn’t permit this conclusion. To overcome the problem, he decided to add a parameter to his equation: a force that counterbalances gravity. In this manner, he started with an unverified idea and built his theory accordingly. The method is disputable, but wasn’t it worth it?
This does not mean that cosmology is an approximate science. The researcher’s restricted point of view that his position in the Universe induces brings him to demonstrate imagination. To Karl Popper, this is the reason why cosmological theories are at the frontier of science. Not to encourage rash or reckless initiatives, it is, however, delicate to establish a perfect method for reaching the truth. Cosmological researchers need to be both adventurous and extremely careful and rigorous. The scientific process is therefore a particularly crucial issue for philosophical study. For that matter, in early 2012, several prestigious universities including Yale, Columbia, Oxford and Cambridge decided to launch a new domain of study: the philosophy of cosmology.