Einstein & the bending of light

Most scientists would place Albert Einstein next to Isaac Newton in their impact on our understanding of physics. Newton started the process of understanding natural phenomena with a few basic laws and the use of mathematics to calculate their consequences. Einstein added new twists to the process. On two counts Einstein led a revolution in thinking along Newtonian lines.

In 1905, Einstein, when working as a clerk in the Swiss patent office in Bern, wrote three important papers. The first, on Brownian motion, discussed the random drift of molecules in a fluid; the study led to a deeper understanding of matter at the molecular level. The second paper, on photoelectric effect, gave a glimpse of the microscopic world in which Newton’s laws of motion prove to be inadequate. But the third paper is the one with which Einstein is associated the most: the theory of relativity.

In 1905, Einstein introduced new concepts of space and time. Since Newton’s time, physicists were accustomed to working against a backdrop of space, which is ‘absolute’ in the sense that it is the same for all observers. Likewise, time was taken to flow at the same rate for all observers. Einstein changed all that. According to relativity each observer carries his own frame of reference for measuring space and time coordinates. The ‘special’ relativity of 1905 discussed how to connect the measurements made by two observers in uniform motion. That there is no absolute nature associated with space and time measurements came as a shock to most physicists and it took several years for the special theory to be accepted.

But ten years later Einstein followed up with another, even more, fantastic idea, known as the ‘general’ theory of relativity. He interpreted the Newtonian idea of force of gravity as a consequence of curved space and time. To get an idea of curved space, imagine the surface of the Earth. The school geometry of Euclid does not apply here. The three angles of a triangle drawn on a sphere will add up to more than two right angles, for example.

Einstein argued that the presence of gravity distorts space and we need new laws of geometry to interpret our measurements. For example, light is supposed to move in a straight line. But what if the gravity in the region has distorted the geometry? The straight line in the new regime would be different. To those accustomed to Euclid’s way, it would appear that a ray of light has been bent. Can this effect be observed?

This remarkable effect, minute though it is, has been observed. The direction to an astronomical source appears to change slightly if the path of light from it towards us passes through a strong gravitational field enroute. Thus a Euclidean observer would argue that light has changed its path because of gravity. In reality, light still continues to move in a straight line, only the definition of a straight line has changed in the curved spacetime.