In terms of light, objects can be defined as luminous – those that emit their own light – and illuminated – all others. The Sun, and stars in general, are instances of the first kind. The Moon, which we sometimes describe as shining, does not actually shine: it simply reflects the light from the Sun. In other words, it is illuminated by it and thus an instance of the second.
The precise explanation of why the Sun shines remained a mystery until recently, and frustrated the efforts of many brilliant minds. Lord Kelvin, a justly famous 19th-century British physicist, vehemently claimed that the light of the Sun was due to its being constantly bombarded by meteorites (or similar elements). It was Arthur Eddington, another illustrious British scientist, who first suggested, in 1920, that the light of the Sun was the result of the fusion of hydrogen nuclei (protons) in its core. This notion was the germ of the theory of stellar nucleosynthesis, which gave Hans Bethe (doubtlessly one of the most remarkable physicists of the 20th century: born in Germany, he had to leave the country to escape the horrors of Nazism, later becoming an American citizen) the 1967 Nobel Prize in Physics. More specifically, today we know that the fusion of protons at the Sun’s core generates gamma rays, that is to say, particularly energetic photons (the basic units of light).
The Sun’s light is thus born deep within the dry, dense plasma that constitutes its nucleus (concretely, the Sun’s nucleus has a temperature of about 16 million degrees Celsius and a density of about 150 g/cm^3, conditions that are particularly conducive to fusion reactions). The photons born there must take a tortuous journey to the surface of the Sun. During that process, they will be absorbed and re-emitted countless times, in random directions. Their identity vanishes – those photons that eventually manage to escape the Sun are not the same that were generated in its nucleus. What is relevant, in terms of the Sun’s luminosity, is that there exists a causal relationship between the former and the latter. The average duration of this process of escape is estimated in tens of thousands of years.
However, once they reach the surface, things seem to become easier: unimpeded, the photons will take 8 minutes to reach the Earth (the average distance from the Earth to the Sun is 1.5×10^11 m; the speed of light across a vacuum is 3×10^8 m/s; if we divide the first by the second and convert the result into minutes we get the duration of the light’s journey, about 8 minutes). It is interesting to realise that we can be sure that the Sun still existed 8 minutes ago, but not in this exact moment; similarly, since the speed of light is finite, whenever we look at something we are only seeing it as it was in the past. Strictly speaking, the present moment is invisible to our eyes.
It is those photons that reach the Earth that allow us to see (in settings with natural lighting). They hit objects and are partially reflected by them (and partially absorbed, depending on the object’s colour; if it is black, for instance, no reflection will take place: the light is fully absorbed). Some of these reflected photons will then travel to our eyes – superb detectors fine-tuned by millions of years of evolution. The light’s refraction on the cornea, and then on the crystalline lens, carries these chosen photons to their final destination: the retina, where photoreceptor cells convert them into electric impulses, which will stimulate the visual cortex, thus allowing our brain to form the image of the object at which we are looking.
Nuno F. G. Loureiro*
* Assistant Professor, Department of Nuclear Science and Engineering, MIT
09.09.2016 - 29.10.2016