# Man's physical universe

NEWTON'S LAWS UNIVERSAL IN APPLICATION 95

use spring scales, in which the weight is balanced by the pull of a

spring, whose elongation gives a measure of the force it exerts. If they

were transported to the surface of the moon, the balances would still

indicate a balance, for the masses are unchanged. The spring scale,

however, would read only one-sixth as much, for the weight would be

decreased in that proportion.

Einstein Has Shown That Newton's Law of Gravitation Applies Rigorously

Only to Matter at Rest.

Newton's law of gravitation holds for matter at rest but is slightly

inaccurate for matter in motion.

Einstein's theory of relativity holds that mass increases as matter

travels faster and that at the speed of light it would be infinitely great.

Experiments with very small particles moving at great velocities show

that Einstein is right — that they do increase in mass as they increase

in speed.

Einstein's theory of relativity considers time to be a fourth dimension

of the calculation concerning time and space; the entire space-time

system is considered to be curved into a spherical shape caused by the

fairly equal distribution of the masses in the universe. There are no

ways of testing many of the revolutionary ideas presented in Einstein's

theory, but some of the conclusions based upon this theory have been

tested and confirmed. For example, Einstein predicted that the mass

of the sun would curve the space near the sun to such an extent that

light passing through this space near the sun's surface would deviate

from its normal path by L75 seconds of an arc.

The British Royal Astronomical Society sent out two expeditions —

one to Sobral in Brazil and the other to Principe in West Africa — to

make observations of the eclipse of the sun on May 29, 1919. Photographs

of positions of the stars appearing close to the edge of the sun

were made, and the discrepancies observed in the positions of these

stars were found to be in good agreement with Einstein's predictions.

Later, in 1922, the Lick Observatory found the deviation to be 1.72

seconds, as compared with Einstein's predicted deviation of 1.75

seconds.

Because of its speed, the path of light is curved only slightly, but

the paths of less rapidly moving bodies would be curved to the extent

that they would travel around the sun in hyperbolic, parabolic, or

elliptical orbits, depending, respectively, upon their speeds. The paths

of certain rapidly moving comets are nearly in agreement with Einstein's

ideas. This theory has also explained some otherwise puzzling

details in the motion of the inmost planet, Mercury.

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