Einstein (28 page)

Even before his new theory had been completely understood by the great majority of physicists, it was already evident to Einstein that it was unable to give a correct presentation of the universe as a whole.

During the nineteenth century the commonest conception of
the universe was that there are groups of material bodies like our Milky Way, and outside this region is “empty” space, which extends infinitely far. This view had, however, already aroused doubts among some scientists around the end of the century. For in this case the stars would behave like a cloud of vapor and there was nothing to prevent them from dispersing into the surrounding empty space. Since infinite time and space are available, the whole universe would eventually become completely empty.

From the standpoint of Einstein’s theory, this conception of the material universe as an island in empty space had additional difficulty. This is due to the equivalence principle, by which gravitational and inertial masses are considered identical. It will be remembered that Ernst Mach first pointed out as a defect of Newtonian mechanics that in it inertial motion, rectilinear motion in empty space, is a process uninfluenced by the presence of other masses. Mach proposed instead the assumption that the effect of inertia is due to motion relative to the fixed stars. Einstein had introduced this idea in his theory as “Mach’s postulate” when he assumed that gravitational field, and consequently inertial effects, are determined by the distribution of matter. If the material bodies formed an island in empty space, then, according to Einstein, only a finite part of space would be “curved.” This region, however, would be surrounded by a “flat” space extending to infinity in all directions. In this flat space, bodies not acted on by any force would move in straight lines in accordance with Newton’s law of inertia, and the inertial force would not be determined by the distribution of matter. For this reason, the idea of curved space being enclosed in an infinite flat space is inconsistent with Mach’s postulate.

The next possible assumption then was that matter does not form an island, but rather that all of space is filled more or less densely with matter. However, if we further assume that all these masses act upon each other according to Newton’s law, then we again run into a difficulty. For matter at large distances exerts individually small effects, but the total amount of matter at large distances increases in such a way that there is an infinite amount of matter at infinity which exerts an infinitely strong force. Observations show that stars are not acted on by such forces, for in this case they would reach high velocities, while all actually observed velocities of stars are small in comparison with the speed of light.

Einstein cleared up this difficulty by pointing out that in his
theory of curved space uniform distribution of matter does not necessarily mean that there is an infinite amount of matter. There is the possibility that, owing to the curvature, space does not extend to infinity. This does not mean, however, that there are boundaries in space beyond which is nothing, not even empty space. The situation may perhaps be illustrated by the same example with which I explained the curvature of space. The surface of the earth is a two-dimensional curved surface which has finite area but has no boundaries. Certain objects, say cities, may be distributed more or less uniformly on its surface, but the total number of cities is finite. Furthermore, if one travels in a given direction along any geodesic (a great circle in this case), one returns to the original point of departure. In the same way the space of our experiences may be curved in such a way that it is finite but unbounded. It becomes meaningful to ask how much matter is contained in the universe, what is the “radius of curvature” of our space, and consequently what is the average density of matter in space.

There is still another possibility, however. Matter may fill “infinite” space with approximate uniformity, but the whole universe may not be at rest, but expanding, so that the density of matter is decreasing. At present it is not yet possible to say with certainty which of the two hypotheses concerning the distribution of matter is correct. Later on, Einstein envisaged also the possibility that space might be “curved” without the presence of masses, contrary to Mach’s original assumption.

At any rate, the view that matter does
not
form an island in infinite empty space is supported by modern astronomy. The researches of Harlow Shapley and his collaborators have shown that space, as far as can be seen with present telescopes, seems to be similar everywhere to the region of our Milky Way. Thus it is plausible to assume with Einstein that on the average the entire universe is uniformly filled with matter. Also by counting the number of stars and measuring their distances from us, Shapley has been able to obtain a rough value for the average density of matter in the universe. Furthermore, from observations of the velocity of recession of the distant nebula and Einstein’s law of motion it has been possible to calculate such quantities as the radius of curvature and the volume of space, and the total amount of matter in it.

For the mathematician, Einstein’s new conception of gravitation was characterized by beauty and logical simplicity. For the observational astronomer there still remained the disquieting doubt that all this might be mere fantasy. Newton’s theory had served them well and it would require more than mathematical elegance to change their views. According to the astronomers, a solar eclipse was needed for the test.

New theories — to use a comparison that Einstein likes to employ — are comparable to beautiful dresses, which when displayed in a dressmaker’s salon attracts every feminine eye. A celebrated beauty orders this dress, but will it fit her? Will it add to or detract from her beauty? Not until she has worn it in the full glare of lights can she tell. Einstein’s theory was a kind of unworn dress that had been in a shop window. The solar eclipse was the first affair at which it was to be worn.

While the war was still in progress, Einstein’s papers on the general theory of relativity became known in England. The abstract discussion could be followed only with difficulty, and the new conceptions about motion in the universe could not yet be appreciated in all their logical implications. But their boldness was already admired. For the first time a well-founded proposal had been advanced to change the laws of the universe set up by Isaac Newton, England’s pride.

For the English, with their tendency toward experimental verification, one thing was clear. A number of definite experiments had been pointed out to the observer of nature whose results could give decisive evaluation to the merits of the theory. And among these it was pre-eminently Einstein’s prediction on the shift in the position of the stellar images during a total solar eclipse that made it possible to test his two theories, the Prague theory of 1911 and the Berlin theory of 1916. As early as March 1917 the Astronomer Royal had pointed out that on March 29, 1919 a total solar eclipse would take place that would offer unusually favorable conditions for testing Einstein’s theories, since the darkened sun would be situated in the midst of a group of particularly bright stars, the Hyades.

Although at that time no one knew whether it would be possible to send expeditions to those regions of the earth where the observation of the total eclipse would be possible, the Royal Society
and the Royal Astronomical Society of London appointed a committee to make preparations for an expedition. When the armistice was signed on November 11, 1918, the committee immediately set to work and announced the detailed plans for the expedition on March 27. The committee was headed by Sir Arthur Eddington, one of the few astronomers who were able at that time to delve deeply into the theoretical foundations of Einstein’s theories. Eddington, moreover, was a Quaker who had always attached great importance to the maintenance of a friendly feeling between the people of “enemy” nations, and both during and after the war he did not join in the customary feeling of hate for the enemy. He also regarded all new theories about the universe as a means of strengthening religious feeling and of directing the attention of people away from individual and national egoism.

When the sun is eclipsed by the moon, there is only a certain zone on the earth’s surface where the entire solar disk is darkened. Since there is the chance that the weather may be poor during the few minutes of darkness and thwart all plans of observation, the Royal Society sent two expeditions to widely separated points within the zone of total eclipse. One set out for Sobral in northern Brazil, while the second sailed for the isle of Principe in the Gulf of Guinea, West Africa. Eddington was in personal charge of the second group.

When the expedition arrived in Brazil, it aroused not a little astonishment and something of a sensation. The war with Germany was hardly over, and the newspapers were still full of propaganda and counter-propaganda. These had not spared scientific activities, but yet here was a costly expedition coming from England to test the theories of a German scientist. A newspaper in Pará, Brazil, wrote: “Instead of trying to establish a German theory, the members of the expedition, who are well acquainted with the heavens, should rather try to obtain rain for the country, which has suffered from a long drought.” The expedition was really in luck, since several days after its arrival it began to rain in Sobral. The savants had justified the public’s confidence in science.

I shall not describe the observations made in Brazil, but merely those made by the group on the isle of Principe. The astronomers arrived a month before the date of the eclipse in order to set up their instruments and to make the necessary preparations. And then came the few minutes of total eclipse, with the disquieting uncertainty whether it would be possible to photograph
the stars in the neighborhood of the darkened sun, or the clouds would hide the stars and nullify the months of preparation. Sir Arthur Eddington gave the following description of these moments:

“On the day of the eclipse the weather was unfavourable. When totality began, the dark disc of the moon surrounded by the corona was visible through cloud, much as the moon often appears through cloud on a night when no stars can be seen. There was nothing for it but to carry out the arranged programme and hope for the best. One observer was occupied changing the plates in rapid succession, whilst the other gave the exposures of the required length with a screen held in front of the object-glass to avoid shaking the telescope in any way.

“Our shadow box takes up all our attention. There is a marvellous spectacle above and as the photographs afterwards revealed, a wonderful prominence flame is poised a hundred thousand miles above the surface of the sun. We have no time to snatch a glance at it. We are conscious only of the weird half-light of the landscape and the hush of nature, broken by the calls of the observers and the beat of the metronome ticking out the 302 seconds of totality.

“Sixteen photographs were obtained, with exposures ranging from 2 to 20 seconds. The earlier photographs showed no stars … but apparently the cloud lightened somewhat towards the end of totality, and a few images appeared on the later plates. In many cases one or the other of the most essential stars was missing through cloud, and no use could be made of them; but one plate was found showing fairly good images of five stars, which were suitable for a determination.”

Tense with excitement, Eddington and his collaborators compared the best of the pictures that they had obtained with photographs of the same stars taken in London, where they were far removed from the sun and therefore not exposed to its direct gravitational effect. There actually was a shift of the stellar images away from the sun corresponding to a deflection of the light rays approximately as large as that expected on the basis of Einstein’s new theory of 1916 (
fig. 3
and
4
).

It was quite a few months, however, before the expeditions had returned to England and the photographic plates were carefully measured in the laboratory, taking into consideration all possible errors. These errors were what actually worried the experts.
Around them revolved the discussions in astronomical circles, while the lay public was interested, and could only be interested, in the question whether the observations had demonstrated the “weight of light” or the “curvature of space.” The latter was even more exciting since hardly anyone could imagine anything very definite under the phrase “curvature of space.”

On November 7, 1919 London was preparing to observe the first anniversary of the armistice. The headlines in the London
Times
were: “The Glorious Dead. Armistice Observance. All Trains in the Country Stop.” On the same day, however, the
Times
also contained another headline: “Revolution in Science. Newtonian Ideas Overthrown.” It referred to the session of the Royal Society on November 6, at which the results of the solar-eclipse expedition were officially announced.

The Royal Society and the Royal Astronomical Society of London had convened a combined session for November 6 to make the formal announcement that the expeditions that had been dispatched by these societies to Brazil and West Africa to observe the total solar eclipse had from their observations reached the conclusion that the rays of light are deflected in the sun’s gravitational field and with just the amount predicted by Einstein’s new theory of gravitation. This remarkable agreement between a creation of the human mind and the astronomical observations gave the session a wonderful and exciting atmosphere. We have an eyewitness account of this meeting by one of the most highly regarded philosophers of our time, Alfred North Whitehead. As a mathematician, logician, philosopher, and a man endowed with a fine historical and religious sense, he was better suited to experience the uniqueness of this hour than most scientists.