What's going on? by David Fletcher H ave you ever turned on a light switch and then tried to get out of the room before the light reaches you? Although this is an experiment you can do at home, it is perhaps not a very illuminating one (bad pun alert)! Nowadays, we take for granted that light has a finite speed. But it doesn’t take much imagination to notice that this was not always such an obvious fact. In our everyday experience, observations of light and its effects appear to us as an instantaneous phenomenon. The brilliant Russian physicist, Lev Landau, once claimed that cosmologists are “people who are frequently wrong, but never in doubt.” So let us examine how the speed of light was discovered.  Antiquity The history of so many celestial topics begin in Ancient Greece. Around the 5th century BC, the philosopher Empedocles asserted what is thought to have been the first theory of light. He imagined that light was composed of particles travelling from our eyes and then proposed that these particles should have some sort of speed with which they travel. Having no evidence to back up such a claim, other important philosophers such as Aristotle disputed the idea and the question of light travelling remained a mystery for a further 1000 years.  Ninety-Nine Years that Revolutionized Astronomy 1543-1642 It was the German mathematician, Johannes Kepler, who gave us what are now known as Kepler’s Three Laws of Planetary Motion. Towards the end of the sixteenth century, the positions and movements of the planets were observed and recorded by Danish astronomer, Tycho Brahe, with astonishing accuracy and detail. To help Tycho interpret his own data he employed 29-year-old Kepler in the year 1600. With brilliant intuition and mathematical skill, Kepler deduced that the planets orbited the Sun not in perfect circles, as previously thought, but in elliptical trajectories.  22 S EN I O R S O N T H E C OAS T Jupiter and Io, December 2000 NASA Cassini Spacecraft Image https://solarsystem.nasa.gov/resources/111/io-in-front-of-jupiter/ With the invention of the telescope, hitherto unobserved objects in our solar system were revealed. Galileo discovered that Jupiter had moons. Using Kepler’s laws, astronomers were now able to make accurate positional predictions about the planets and when their moons would appear and disappear. Evidence for Light Speed And so it was that in 1676 another Danish astronomer, Ole Rømer, noticed a discrepancy in his observations of Jupiter’s Moon, Io. Noting that these anomalies changed slightly at different times of the year, Rømer had an insight that if the light travelling from Io to the Earth was not instantaneous, but actually had a certain speed, it would put Io back onto its expected path around Jupiter. We now know that Rømer’s calculation of the speed of light had an error of 26%. Even so, when you’re trying to measure something that’s moving at 299,700,000 meters per second, I don’t mind giving a nod of respect to the ingenuity of 17th century scientists.  Light has a speed. A few hundred years after Rømer’s calculation, it was noticed that the speed of light never changed no matter who observed it. Our understanding of time and space was about to get weird! REFERENCES erkins 2003, Particle Astrophysics, Oxford P University Press  Weiner & Nunes 2013, Light-Matter Interaction, Oxford University Press  Seeds & Backman 2011, Foundations of Astronomy, Cengage Learning  Doberck 1877, Nature vol17, Nature Publishing Group  David Fletcher is a 45 year old undergraduate of Physics at Macquarie University, from Erina.