![]() ![]() Now imagine that there is another observer in a spaceship, shown on the left: The rocket engine of that observer’s spaceship is firing and produces an acceleration of 9.8 metres (32 feet) per square second. We, the crew of the spaceship shown on the right, are floating freely in space, far away from all major sources of gravity. Michael Foale aboard the ISS, together with two floating grapefruits: As an example, the following picture shows the astronaut-scientist C. Those astronauts haven’t escaped the earth’s gravity – they’re experiencing a very special kind of free fall, a free-falling orbit around the earth. ![]() Most readers will have seen footage showing situations like this, involving, for instance, astronauts aboard the international space station ISS. In both situation, she would float, weightlessly, in the elevator, as would all objects around her. Imagine a scientist in a small elevator more precisely, in a small, windowless compartment that looks like an elevator cabin) That scientist has great difficulty to tell whether she is in free space, far from all sources of gravity, or in free fall in a gravitational field. This seemingly harmless property has far-reaching consequences. At least in a vacuum (where there’s no air resistance), objects you place at the same location fall with the same acceleration – the mouse or the elephant, the feather or the cannonball. One central feature of gravity is that it makes no distinctions. ![]() Overall, gravity is intimately connected with the geometry of space and time. In part, it is associated with a quantity called “curvature”. What is gravity? Einstein’s general theory of relativity has an unusual answer to that question which will be explored in this spotlight text. So what is gravity in Einstein’s theory? The answer: in part, an illusion in part, an aspect of geometry. ![]()
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