EINSTEIN'S THEORY OF GRAVITATION

Dominion, Volume 13, Issue 41, 12 November 1919, Page 6

EINSTEIN'S THEORY OF GRAVITATION

ASTRONOMICAL TESTS. (By Dr. C. E. Adorns, Government Astronomer.) It is well to remember that to Fitzgerald ("Nature," xlvi, p. IGS, June 16, 1892) is duo the first important discovery of what is now known as Einstein's theory of gravitation. He showed that the dimensions of material bodies are slightly altered when they are in motion relative to the ether, the linear dimensions of a body being contracted in tho direction of its motion. For a body moving with the speed of the earth tho contraction is only one two-hundred-millionth. The obvious difficulty has been to measure such small' effects. As tho theory was developed it was 6een that a crucial test could bo made at a solar eclipse, as an effect should then bo detected on the rays of light passing near tho sun from the stars to the earth, Tho Astronomer Royal in March, 1917, pointed out that a suitable opportunity of testing tho theory would arise at tho solar eclipse of 1919, May 29, and tho cables have announced the success of tho t«st6 then made.

It was found that there were no less than thirteen stare -close to the sun at the time of the eclipse, and arrangements were made to photograph them when the sun was! totally eclipsed. The oxpedition then waited a month or so until tho sun had moved away, and further photographs of the same stars wore taken. By careful measurements and comparisons it was found that the relative positions of tho stars during the eclipse were different from their later positions when the sun was <not there. Tho amount of displacement was small—about one and three-quarter seconds of arc—hut was largo enough to he accurately measured. TSS is the first direct test that has been made, and although-details are riot yet available it appears that astronomers are satisfied with the results. It was confidently expected that tho eclipse test would be satisfactory, for tho theory had already offered a srf.urion t» the outstanding problem of discordance in the motion of tho perihelion of the planet Mercury, and at the time this was tho only observational test of the new theory which bad been applied. It is natural to inquire what are tho chief points of difference between Einstein's and Newton's laws of gravitation. Is the force between two masses at rest exactly according to tho inverse-square, and for bodies in motion is it directed along the lines of centres? Unfortunately it is not possible to give a plain answer. Before it is. possible to pay how the 'force Yurie? with the d\«(ancc' it is necessary to define bow tho distance is to be measured. For example, if tho circular orbit of a planet is measured with a measuring rod and the diameter and circumference are compared, wo should find (if Einstein's theory is true) that the ratio is not the usual one, there would be <i discrepancy of some kilo, metres. It must, therefore. 1* supposed that the rod change* length systematically according as it is placed radially or trnverscly, so that one or other of the measures' needs correction-it is impossible to< say which. Jho theory is a farinching one. and tho definite results of (Tie tests made at the eclipse are awaited with interest.