The Einstein Theory of Relativity | Page 7

in which we were just imagining ourselves to make our
observations. It was noted that when the compartment is falling with
the acceleration of 981 the phenomena therein will occur just as if there
were no attraction of gravitation. We can then see an object, A, stand
still somewhere in open space. A projectile, B, can travel with constant
speed along a horizontal line, without varying from it in the slightest.
A ray of light can do the same; everybody will admit that in each case,
if there is no gravitation, light will certainly extend itself in a rectilinear
way. If we limit the light to a flicker of the slightest duration, so that
only a little bit, C, of a ray of light arises, or if we fix our attention
upon a single vibration of light, C, while we on the other hand give to
the projectile, B, a speed equal to that of light, then we can conclude

that B and C in their continued motion can always remain next to each
other. Now if we watch all this, not from the movable compartment,
but from a place on the earth, then we shall note the usual falling
movement of object A, which shows us that we have to deal with a
sphere of gravitation. The projectile B will, in a bent path, vary more
and more from a horizontal straight line, and the light will do the same,
because if we observe the movements from another standpoint this can
have no effect upon the remaining next to each other of B and C.

DEFLECTION OF LIGHT
The bending of a ray of light thus described is much too light on the
surface of the earth to be observed. But the attraction of gravitation
exercised by the sun on its surface is, because of its great mass, more
than twenty-seven times stronger, and a ray of light that goes close by
the superficies of the sun must surely be noticeably bent. The rays of a
star that are seen at a short distance from the edge of the sun will, going
along the sun, deviate so much from the original direction that they
strike the eye of an observer as if they came in a straight line from a
point somewhat further removed than the real position of the star from
the sun. It is at that point that we think we see the star; so here is a
seeming displacement from the sun, which increases in the measure in
which the star is observed closer to the sun. The Einstein theory teaches
that the displacement is in inverse proportion to the apparent distance
of the star from the centre of the sun, and that for a star just on its edge
it will amount to 1'.75 (1.75 seconds). This is approximately the
thousandth part of the apparent diameter of the sun.
Naturally, the phenomenon can only be observed when there is a total
eclipse of the sun; then one can take photographs of neighboring stars
and through comparing the plate with a picture of the same part of the
heavens taken at a time when the sun was far removed from that point
the sought-for movement to one side may become apparent.
Thus to put the Einstein theory to the test was the principal aim of the
English expeditions sent out to observe the eclipse of May 29, one to
Prince's Island, off the coast of Guinea, and the other to Sobral, Brazil.
The first-named expedition's observers were Eddington and
Cottingham, those of the second, Crommelin and Davidson. The
conditions were especially favorable, for a very large number of bright

stars were shown on the photographic plate; the observers at Sobral
being particularly lucky in having good weather.
The total eclipse lasted five minutes, during four of which it was
perfectly clear, so that good photographs could be taken. In the report
issued regarding the results the following figures, which are the average
of the measurements made from the seven plates, are given for the
displacements of seven stars:
1''.02, 0''.92, 0''.84, 0''.58, 0''.54, 0''.36, 0''.24, whereas, according to the
theory, the displacements should have amounted to: 0''.88, 0''.80, 0''.75,
0''.40, 0''.52, 0''.33, 0''.20.
If we consider that, according to the theory the displacements must be
in inverse ratio to the distance from the centre of the sun, then we may
deduce from each observed displacement how great the sideways
movement for a star at the edge of the sun should have been. As the
most probable result, therefore, the number 1''.98 was found from all
the observations together. As the last of the displacements given
above--i.e., 0''.24 is about one-eighth of this, we may say that the
influence of the attraction of the sun upon light made