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Dr. Albert Einstein: Theoretical Physics, Relativity and the Photoelectric Effect, 1935

Curved Space

Einstein's theory of general relativity describes space as curved, with the "curved space" being the four-dimensional space-time conceived of by Minowski. The curvature of space results in the effects of gravity. This notion of curved space becomes more tangible by thinking again about the astronaut and the space capsule, but this time introducing a beam of light into the capsule. If a beam of light is shone from the top of one capsule wall to the opposite wall while the capsule is accelerating upwards in space, the light will appear curved. This is because, in the time it takes for the light beam to move across the cabin to the opposite wall, the cabin will have accelerated upwards and the beam will appear to curve across the cabin and hit below the spot directly across from where it started. The light will also appear to curve across the top of the space capsule if the capsule is at rest in Cape Canaveral. In other words, the light beam acts as if it is being pulled down by gravity. The space-time through which it moves can be understood to be curved by the presence of a massive body: in this case, the earth. In space, the curvature of space itself causes all objects, such as light or planets or spaceships, to follow the curvature. In both cases, the gravitational effect occurs because of the curvature of space.