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Home Edition Age of the Universe: Cosmic Times Size of the Universe: Infinite 1919 300,000 Light Years ’S GRAVITY BENDS STARLIGHT Einstein’s Theory Triumphs

ccording to Sir Joseph passes by the edge of a massive close to the Sun and later in the Thompson, “the greatest object, the ray should be bent year when its light is not passing Aachievement in the history by the gravity of that object. The close to the Sun. The photographs of human thought” was a prediction most massive object near Earth is must be taken with great care and made by Dr. Albert Einstein. Dr. the Sun. According to Newton’s precision. Einstein’s prediction was proven principles, a light ray from a distant true during the total eclipse of the star would be bent only a very Newton thought of gravity as a force Sun on May 29, 1919. small amount by the Sun’s gravity, that pulls things toward an object; but that change in position can be the more massive the object, the Sir Joseph made this measured by taking photographs stronger the pull. pronouncement at a joint meeting of the star when its light is passing of the Royal Society and the In his General Theory of Royal Astronomical Society in Relativity, Dr. Einstein London on November 6, 1919. explains that gravity and In the audience were a large inertia are the same. The number of astronomers and “force” of gravity pressing physicists. The excitement in you down in a chair is the the meeting was quite obvious, same force you feel when and everyone agreed that the the automobile you are in observations made during the quickly slows down, and you eclipse did prove Dr. Einstein’s continue to move forward. prediction. Dr. Einstein is Dr. Einstein says gravity, like a Professor of Physics at inertia, doesn’t pull. Instead, the University of Berlin and anything in space that has Director of the Kaiser Wilhelm mass will warp or curve Physical Institute. space and time around it. Think of a pillow as space. If The Prediction you place a heavy (massive) object on the pillow, the Two centuries ago Sir Isaac pillow will curve around the Newton explained how gravity object. Dr. Einstein says works in his book Optics. the amount of the curvature According to Newton, if a ray relates directly to the mass of of light from a distant star Image credit: Ferdinand Schmutzer, via Wikimedia Commons Albert Einstein “Sun’s Gravity” continued on page 2 2 “Sun’s Gravity” continued from page 1 the object. This curvature of space Brazil to view the eclipse. is what curves the path of the ray of Other astronomers had read earlier light from a distant star. versions of Einstein’s paper and If the weather was clear at either tried to test his prediction during of these locations on the day of the Dr. Einstein’s theory, which is total solar eclipses in 1912 and eclipse, it would be possible to take highly mathematical, predicts that 1914. They were not successful a set of photographs of the totally the curvature of space caused in their attempts due to cloudy eclipsed Sun along with a number by the Sun’s mass should bend weather and the start of the Great of bright stars that appeared close starlight twice as much as Newton’s War. When astronomers studied to the Sun. principles predicted it should. the conditions of the 1919 eclipse, it appeared the Sun would be well Results Discussed The amount by which starlight is bent placed in a group of bright stars. by the Sun’s mass is considered Also, the Sun’s light would be totally At the joint meeting, Sir Frank by astronomers and physicists to blocked by the for over five described the work of these two be the most important test of Dr. minutes which would allow enough groups. The purpose of the Einstein’s Theory of Relativity. time for the Sun and the stars to be meeting was to decide if the light photographed at the same time. coming from these stars was bent May’s Solar Eclipse (deflected) by the Sun as it passed Professor Eddington decided by. According to Sir Frank, if the Dr. Einstein published his to lead a group to the island of Sun did affect the light, it would prediction in Germany in 1915 Principe near the western coast of make the stars appear farther from during the Great War between Africa where the eclipse could be the Sun than they actually were. England and Germany. A Dutch photographed. He also convinced The group would also need to astronomer smuggled a copy of Sir Frank Dyson, Director of the determine how far out of position Dr. Einstein’s paper out of war-torn Royal Observatory, to send another the stars would appear on the Europe into England where it was group to a different location to photographic plates. read by Arthur Stanley Eddington, reduce the chance that clouds might Plumian Professor of block the eclipse at both locations The Royal Observatory party and Experimental Philosophy at and prevent photographing the arrived in Brazil in time to prepare Cambridge University. This was Sun. This other group, led by Dr. for the eclipse and to photograph the same position Newton held Andrew Crommelin from the Royal other star fields. It was cloudy when he developed his theory of Observatory, traveled to northern early on the day of the eclipse but gravity. cleared later, and the observations were carried out with almost complete success. The observers remained in Brazil until July so they could photograph the same set of stars during the night sky, when their light did not pass by the Sun. Between May and July the Earth had moved in its orbit, which changed the position of these stars compared to the positions of the Earth and Sun. Waiting for the Earth to move in its orbit positioned these stars in the night sky once again. Once this second set of photographs was obtained, the

Image credit: Royal Observatory Greenwich observers immediately returned to The path of totality for the eclipse of May 29, 1919 spanned the Atlantic (dark line); the Greenwich where each plate was eclipse was very long — nearly seven minutes at its maximum duration. The two dots carefully measured two times. indicate the positions of the expeditions led by Dr. Crommelin for the Royal Greenwich Observatory (left) and Prof. Eddington for Cambridge University (right). “Sun’s Gravity” continued on page 3 “Sun’s Gravity” continued from page 2 3 Why a TOTAL Solar Eclipse? The Cambridge University party ccording to predictions by both Sir Isaac Newton and Dr. arrived on Principe on April 23. “We Albert Einstein, a ray of light from a star very close to the Sun soon realised that the prospects of a A(as seen from Earth) will be bent (deflected) as it passes by clear sky at the end of May were not the edge of the Sun. Such a deflection would make the star look very good,” recalled Prof. Eddington. slightly farther away from the edge of the Sun than it really is. The sky was completely cloudy at the beginning of the eclipse, but about half an hour before totality Dr. Einstein’s Theory of Relativity predicts that the amount of they caught glimpses of the Sun the deflection should be twice as great as the deflection Newton through the clouds. They took predicted. photographs exactly as arranged, but only two out of 16 plates taken Astronomers would get the best results the more stars around the showed enough stars to measure. Sun during a solar eclipse and the more photographs they took. Prof. Eddington was also not able This would provide greater accuracy in comparing Dr. Einstein’s to stay on Principe for several more predictions to Newton’s. Each year on May 29 as the Earth revolves months to take a second set of around the Sun, the Sun appears to pass in front of a cluster of stars photographs for comparison. in the constellation of Taurus. This cluster of stars, called Hyades, is so bright that it is clearly visible in the night sky even without a Sir Frank explained in detail how telescope months later during the winter. the observations were made, the equipment used, and the way the The only way this could be viewed on May 29 would be if photographic plates were measured the Sun were totally eclipsed and its light was blocked. On May 29, back at the Greenwich Observatory. 1919 such an eclipse did occur, and this bit of good fortune allowed He also discussed how the observed star positions during the eclipse Dr. Einstein’s prediction to be accurately tested. ♦ were compared to their positions two months later when their light did not pass near the Sun. He convinced the audience at the joint meeting that the results of the experiment were definite and conclusive, and that the Sun’s gravity did deflect the light. He also stated that the amount of deflection was very close to what Dr. Einstein predicted and almost twice as much as Newton’s principles predicted.

“After a careful study of the plates I am prepared to say that there can be no doubt that they confirm Einstein’s prediction,” Sir Frank declared. “A very definite result has been obtained that light is deflected

Image credit: NASA’s Cosmic Times in accordance with Einstein’s law of Top: The apparent positions of the stars during the eclipse when their light is bent gravitation.” Prof. Eddington added, around the Sun. Middle: The positions of the stars when their light is not bent around “we must assume that gravity obeys the Sun. Bottom: The actual path of the light as it bends near the Sun and reaches the new law proposed by Einstein.” Earth, and the apparent position of that star as a result of its light bending around the ♦ Sun. 4 Mt. Wilson Astronomer Estimates 10 10 times Bigger than Thought But Disputes Suggestions the sky, he also analyzed their light that Spiral Nebulae are using a spectroscope to determine other “Island Universes” if they were approaching the Sun or moving away from it. From he Milky Way is a “discoidal” this analysis he calculated the (disk-shaped) of gravitational forces acting on the Tstars 10 times bigger than clusters. He also tried to determine astronomers have previously if they were revolving around any thought, according to Mt. Wilson common center and where that astronomer Dr. Harlow Shapley. common center was located. He also claims the Sun is located Furthermore, he calculated the

closer to the edge of the disk than distances of the globular clusters Image credit: AIP Emilio Segre Visual Archives, Shapley Collection the center. However, he disagrees from the Sun using Cepheid Harlow Shapley with the hypotheses of other variable stars. This new method of astronomers that claim dozens measuring distance was discovered of other spiral nebulae observed by Miss Henrietta Leavitt of Harvard Shapley, the positions of globular in the skies are other or Observatory. clusters in the galaxy indicate that “island universes” that resemble the the actual diameter of the galactic Milky Way. After years of diligent study, system is about 300,000 light-years often assisted by his wife Martha across. This is more than 10 times Dr. Shapley astonished B. Shapley, Dr. Shapley has larger than any other astronomer astronomers last year and this published a number of astonishing had hypothesized. year when he published a total conclusions. of 26 scientific papers in several Dr. Shapley said that this newer astronomical journals. In these Dr. Shapley concluded that our concept rules out the possibility papers, Dr. Shapley examined galactic universe is a single, that other spiral nebulae would be other recent astronomical work in enormous unit, the size and form groups of stars of a size comparable amazing detail. He also published of which is shown by the widely to the Milky Way. Such a huge size the results of his own astronomical scattered globular clusters. Dr. would mean these spirals were photography using the 60-inch Shapley added that the center inconceivable distances away in reflector of the Mount Wilson of our discoidal galaxy is more space. As an example, he pointed Observatory in southern California. than 60,000 light- years away in out that if a bright spiral of 10’ (0.17° His favorite subject of study the direction of the constellation in angular measure) diameter has was globular star clusters which Sagittarius. an actual diameter comparable to are nearly spherical clusters of that of the Milky Way, its distance hundreds of stars. These clusters His conclusions contradict generally must be greater than 100 million have puzzled astronomers for accepted astronomical wisdom. light-years, and its measured years, because they are located in Until last year, most astronomers rotational speed would exceed the peculiar positions in only certain believed that the Sun was near the speed of light. parts of the sky. In his study, center of our galaxy, and that the Dr. Shapley discovered 17 new radius of the galaxy was about 3000 In short, Dr. Shapley concludes, globular star clusters. light years. Some astronomers many observations “all seem thought the galactic system might definitely to oppose the ‘island In addition to pinpointing the exact be as large as 10,000 to 20,000 universe’ hypothesis of the spiral position of each in light-years across. According to Dr. nebulae.” ♦ 5

In Their Own Words Expanding or Contracting? Periods of 25 Variable Stars in Einstein’s Theory Predicts Universe Must the Small Magellanic Cloud - be Doing One or the Other Miss Henrietta Leavitt Einstein says Neither

A remarkable relation between the n 1917, Albert Einstein and the Dutch astronomer Willem de Sitter brightness of the studied variables and showed that Einstein’s General Theory of Relativity could describe a the length of their periods has been Ihighly simplified universe. noticed. There is a simple relation between the brightness of the variables But when it was applied to the real universe full of stars, there was a and their periods: the brighter variables problem. have the longer periods. Dr. Einstein’s model predicted that either space would be expanding Spectroscopic Observations of or contracting. If space were expanding, then all the stars would move apart from one another, as if from a gigantic explosion. If space were Spiral Nebulae - V. M. Slipher contracting, all the stars would ultimately collapse upon one another.

The average radial velocity of spiral Dr. Einstein however, believed strongly in the book Ethics by the Dutch nebulae is +400 km/sec. Radial velocity philosopher Spinoza. A passage from this book states, “It follows that is the speed along an observer’s line of God is immutable [not capable of being changed] or, which is the same sight; positive radial velocity means an thing, all His attributes are immutable.” Dr. Einstein was troubled by the object is receding, while negative radial concept of a universe that could change. velocity means an object is approaching. As well may be inferred, the average “For now, we can add on the left side of the field equation...an unknown universal constant,” Dr. Einstein said. He explained that the quantity velocity of the spirals is about 25 times he designates by the average stellar velocity. the Greek letter lambda is called The Relation of the System of the “cosmological Stars to the Spiral Nebulae - G. constant.” He goes on to explain, “Not much F. Paddock harm is done thereby …the proposed new Endeavors have recently been made to universal constant present a comparative list of average determines the radial or line-of-sight velocities of average density of the several different kinds of objects the universe that can in the sky, and to discuss the relation remain in equilibrium.” of the spiral nebulae to other objects. The average radial velocities of all According to Dr. Einstein’s beautiful except the spirals range in increasing unchangeable magnitude from zero to fifty kilometers universe, the per second. But a considerable jump presumably spherical is noticed from the fifty kilometers to universe would be 400 kilometers for the average of the neither expanding or spirals. This suggests the question: contracting. ♦ Are the spirals dissociated from the star system? The average velocity is age credit: University of Photographic Archive, apf6-04311, Special Collections Research decisively positive, which means that nter, University of Chicago Library. they are receding not only from the Im Ce Willem de Sitter observer or star system but from one another. ♦