Science classics that changed history

Author: Editor: Gao Xinghua, Wu Weiqiang, Xu Deming
Publisher:
Publish Date: 2006-04-27
Features: According to the equivalence principle, the situation of any object in an accelerating reference frame is the same as it is under the influence of gravity in an inertial reference frame. For example, an astronaut inside a sealed, weightless spacecraft cannot determine whether the sensation of regaining weight is caused by the spacecraft's acceleration or by returning to the ground. The equivalence principle naturally links the general principle of relativity with gravity, and Einstein took both the general principle of relativity and the equivalence principle as fundamental premises for establishing general relativity. In an accelerating reference frame, the concepts of spacetime in special relativity need further development, especially the geometric structure of spacetime must undergo fundamental changes. In special relativity, the geometric properties of spacetime are described by a flat, Euclidean four-dimensional spacetime (also known as Minkowski spacetime). Euclidean geometric theorems fully apply to four-dimensional spacetime, such as the sum of the angles in a triangle being π, the ratio of the circumference of a circle to its diameter being π, and so on. However, in an accelerating reference frame, Euclidean geometric theorems no longer hold. For example, when an open umbrella rotates in a circular, accelerated motion, the edge of the umbrella canopy undergoes length contraction due to its motion, but the diameter of the umbrella canopy, which is perpendicular to the motion, does not contract. As a result, the ratio of the circumference of the umbrella canopy to its diameter will be less than π. In fact, every point on the umbrella canopy is rotating, so the circumference at each point is undergoing contraction, but the edge moves faster, so it contracts more. This uneven contraction curls the flat umbrella canopy into a curved hemisphere, causing space to bend! The geometry of curved space is Riemannian geometry, not Euclidean geometry. Acceleration causes space to bend, and according to the equivalence principle, gravity must also cause space to bend. The stronger the gravity, the greater the curvature of space. Einstein believed that empty space without matter does not exist; the gravitational field distributed throughout space, which determines the structure of space, itself constitutes a form of matter in space, i.e., the gravitational field. Einstein also established the famous field equations of gravity, which state that the spacetime structure of the gravitational field is completely determined by matter. In this way, matter and time and space achieved true unification. Since Euclid, Newton, and Kant, Euclidean space has been considered the only correct and unchanging form of space in the universe. However, general relativity materializes spacetime, attributing its properties to observable and variable matter, thereby completely negating the a priori absolute spacetime view of Newton and Kant and completing the revolution in the spacetime view of relativity. The curvature of space can be demonstrated by the bending of light. In 1919, the Royal Society sent two scientific expeditions to observe whether the light from stars passing near the Sun during the solar eclipse on May 29 would bend due to the Sun's gravity. This was undoubtedly a major test of general relativity. The results showed that the starlight indeed bent! General relativity passed the test. This significant result was announced to the world on November 6, 1919, by the Royal Society and the Royal Astronomical Society through a joint meeting. President Thompson of the Royal Society declared to the world beneath the massive portrait of former President Newton, "Einstein's theory of relativity is one of the greatest achievements in the history of human thought—perhaps the greatest achievement."