Curved Spacetime

You have probably learned in school that gravity is a force. In fact, gravity is not a force at all, according to Albert Einstein. Without gravity, space would be flat. Gravity actually curves, or "warps," space. The more dense the matter is, the more the space around it is curved. Not only are objects themselves curved, but the space around them is curved as well. You may be wondering in what direction the space is curved. It is curved into the fourth dimension! (NOTE: This fourth dimension is not the fourth dimension in space-time, the way Einstein described it, where three dimensions are the normal space dimensions and the fourth is time. This fourth dimension is a fourth spatial dimension.)

There has been experimental evidence for the curvature of spacetime by a massive object since the early part of last century (1922), when observers set out to test the predictions of general relativity. During a solar eclipse, they realized, the light from stars in the same general area of the sky as the Sun are visible during the day. If light from these stars is affected by the curvature of spacetime due to the Sun's mass, then this would be measurable as a deflection (or a change in location) of the star's position on the sky. The stars closer to the position of the Sun in the sky would suffer a larger deflection; in general the deflection would be proportion to the stars distance from the Sun's location on the sky. This effect was observed for 15 stars during the solar eclipse of 1922 in Western Australia, and was interpreted as observational verification of the predictions of general relativity. General relativity predicts that spherical masses deform spacetime in much the same way a lead ball would deform the surface of a rubber sheet. It is this deformation that causes the planets to orbit the Sun, and the Moon to orbit the Earth. In fact, all orbital motion is the result of bodies being affected by the curvature of the spacetime in which they move.