Standing The Test Of Spacetime





    Nature and Nature’s Laws lay hid in night:



This universal property of the speed of light, verified by experiment, had profound implications, notably providing the basis for Einstein’s Special Theory of Relativity, published in 1905. Here space and time were inextricably linked, and two years later Einstein’s erstwhile tutor Hermann Minkowski formulated a four-dimensional geometry embracing both. In this new geometry — spacetime — everything moves from past to future, and things such as spatial distance and time difference that were measured differently by different observers were jettisoned. Instead, Minkowski endowed spacetime with a new “distance” between any two events, invariant under changes of motion and measured as equal by any two observers, whatever their relative speeds.

Speeds less than that of light were of no consequence, so the previous notions of kinetic energy and momentum — vital in classical three-dimensional physics — had to be abandoned. They were replaced by something called energy-momentum in four dimensions, which for a body apparently at rest led to the famous equation E = mc², E being energy, m the mass at rest, and c the speed of light. The destruction of even tiny amounts of mass could potentially release huge energy, as the atomic bombs in World War Two made horribly clear.

Ten years after the Special Theory came the General Theory of Relativity, inspired by a famous thought experiment. Imagine yourself in a stationary elevator on earth, separated from the world outside. A force holds you to the floor, just as it would if the same elevator were accelerated upwards outside the earth’s gravitational field. Is there any difference? Einstein decided not, and by thinking in terms of the new spacetime this led him to an incredible conclusion.

To get the idea, forget four dimensions and picture just two: one for space, one for time. In this case spacetime looks like a sheet of graph paper with a time-axis and a space-axis. The graph for constant velocity is a straight line, and for accelerated motion it is a curve. So the person in the elevator accelerating in outer space follows a curved path, and the same must be true for a person standing still in a gravitational field. This implied that gravity must turn straight lines into curves, so a gravitational field literally bends spacetime.

To appreciate the idea, think of two ships at the equator sailing due north. They gradually get closer. No force draws them together, just the curvature of the earth. This is what Einstein said was happening with spacetime. The curvature produced by a massive body would cause other things travelling from past to future, including light, to veer towards it. In November 1915 Einstein finally had the mathematics to produce numbers explaining an anomaly in the orbit of Mercury, the planet closest to the sun, and observations during a solar eclipse in 1919 verified that the sun’s gravity bent light rays passing close to it, just as the General Theory predicted.

Yet although Einstein was awarded the 1921 Nobel Prize, it was not for one of the greatest intellectual advances ever in physics but as a backhanded compliment for other work, in 1905. Scientists can exhibit a marked preference for more practical work, such as Aristotle’s zoology, and be quite sniffy about theoretical physics.

Underrated: Abroad

The ravenous longing for the infinite possibilities of “otherwhere”

The king of cakes
