Science is an enterprise that has grown and flourished as a result of man’s continuing efforts to unravel nature’s secrets. The diverse natural events observed by naked eyes or through sophisticated instruments in the laboratory pose a challenge to human intellect through questions like “why”, “what” and “how”.

The human response today consists of a search for a minimal set of basic laws of science which account for all that is observed. The key to its success lies in the word “minimal”; for one then is arguing for one basic law explaining a very wide range of phenomena. The scientist, in fact, is working towards one law explaining all, ideally.
Facts and speculations play a useful role in this quest. Facts are what actual observations tell us, and what we should try to understand in terms of basic laws. Having gained confidence in a law of science because of its success in explaining the facts in question, we may like to go beyond the observed realm and make predictions about what may be found if we extended our observations to this unexplored region. For example, early experiments showed that like electric charges repel each other and so do magnetic charges. But further experiments showed that moving electric charges exert forces on magnetic charges, thus hinting that the electric and magnetic forces are related. It was in the 1860s that James Clerk Maxwell finally proposed an electromagnetic theory, detailing how this relationship works.
Great scientists have progressed in their work because of their use of speculation. Speculation takes the scientist’s theory to a wider domain. If speculation turns out to be correct, the scientist has the satisfaction of contributing to the overall progress of science. Indeed fertile imaginations of such scientists have led to the advance of science that we observe today.
Having said this, it is essential to remind ourselves that howsoever attractive the speculation may be, it is the facts that have the last word. Many theories with attractive credentials have fallen by the wayside because they failed to account for all the facts. Indeed it is dangerous to get carried away by the attractiveness of a theory and believing its veracity without factual evidence in favour of it.
Astronomy as a branch of science is vulnerable on this count. Unlike other branches of science, astronomy has its main laboratory in the form of the entire cosmos, thus extending well beyond our Earth. It is not possible to perform controlled experiments on remote celestial objects, unlike a laboratory scientist who can alter the parameters of his experiments. Nevertheless, the subject has progressed remarkably as can be seen from the example of stellar evolution.
Let us ask: Why does the Sun shine? Evidently, the answer to this question should lead us to the energy reservoir in the Sun. The work by the Indian astrophysicist Meghnad Saha opened the doors for stellar investigations. For Saha’s equation tells us how to calculate the surface temperature of the Sun, from its spectroscopic studies. Using this as input data, the Cambridge astronomer Arthur Stanley Eddington set up an elaborate system of equations describing the interior of the Sun. The mathematical solution of these equations led him to predict a central temperature of the order of 10 million degrees. From this result, Eddington was bold enough to deduce that the core of the Sun has thermonuclear reactions going on, in which atomic nuclei of hydrogen get converted to those of helium. In this process energy is generated which the Sun radiates. In the 1920s, this idea was considered highly speculative and most atomic physicists did not believe it.
However, as the scientific understanding of the interior of an atom improved, Eddington’s idea began to appear more and more credible. By the late 1930s, the Cornell physicist Hans Bethe used this idea to construct a viable model of the Sun. The model can predict the amount of radiation emitted per second by a star with a given mass. The formula agrees with what we know about the Sun. In other words, given the Sun’s mass as two million million million million million kilograms, we can predict its luminosity and then check it against the observed value, which is four hundred million million million million watts. The answer checks correctly, thus establishing the credibility of the entire approach.
Science insists on repeated confirmation by independent experiments. How can one enforce this condition in this example? The obvious way to do so is to treat each star as an independent experiment. Not all stars have the same mass as the Sun. Stars with lower and higher masses exist. So we can apply the Eddington-Bethe model to these stars. When this is done, the results are fully consistent with theory.
A further check on this work became possible when scientists set up neutrino detectors. Neutrinos are particles created in the thermonuclear reactions like those going on in the Sun. The neutrinos can go through solid masses of great thickness undisturbed, since they react with ordinary matter very weakly. Clearly, detecting neutrinos is a difficult job. But after some initial problems, the neutrino detection technology settled down. And the results again check out.
Examples like these show how astronomical observations allow the physicist to extrapolate his theories. And a successful exercise of this kind brings a bonus. For we find that the laws of science that we learnt on the Earth also apply on the much larger cosmic scale. Without astronomical evidence such extrapolations would be no more than idle speculation. That we have such evidence and it corroborates what we learn from terrestrial studies remains a mystery.
Albert Einstein summarised this situation admirably when he said: “The most incomprehensible thing about the universe is that it is comprehensible.”

The writer, a renowned astrophysicist, is professor emeritus at Inter-University Centre for Astronomy and Astrophysics, Pune University Campus