The galaxy cluster Abell 2218 imaged by Hubble with the WFPC2 instrument. The massive elliptical galaxies in the cluster create gravitational lenses, bending the path of light coming from very distant background galaxies [Andrew Fruchter et al., NASA, ESA]

Thought Experiments With Gravity

Even before completing general relativity, Einstein had devised ingenious thought experiments, which demonstrated the unexpected consequences of gravity on light and time. The predictions of those thought experiments were later all confirmed by observation

The most characteristic element of Einstein’s genius was probably his ability to devise thought experiments. Using imaginary trains and elevators, the German physicist managed to predict the existence of phenomena very far from common intuition. It often took many years for technologies to allow some of Einstein’s predictions to be verified by observation. Still, when those verifications finally arrived, the scientists could not help but observe the perfect agreement of the observational data with what the thought experiments had prefigured.

The equivalence principle says that, given a sufficiently small reference system such as an elevator cabin or a rocket, it is not possible to distinguish between the effects of gravity and those of any acceleration (upper half of the drawing). Furthermore, in the same limited reference system, it is not possible to distinguish a free fall in a gravitational field from a situation in which one is floating freely in space, far from any gravitational field (lower half of the drawing)

Exemplary cases of physical phenomena anticipated by pure thought experiments are two effects due to gravity: the bending of light and the time dilation under the action of a gravitational field. Both derive from the surprising…