General relativity support from the double pulsar

After the final publication of the Theory of General Relativity by Albert Einstein in 1916, experimental confirmation rested on three astronomical tests. These were the amount of bending of starlight at the edge of the Sun, the change in frequency of light emanating from the gravitational field of the Sun and an explanation in terms of the theory of a remnant quantity in the perihelion advance of Mercury which had been calculated previously. The field of activity then was sparse and Quantum Mechanics attracted many scientists to its realm. However, a proliferation of renewed interest emerged 50 years on from 1916 with new thinking, improved instrumentation, the advent of spacecraft and the discovery of a number of exotic objects. The previous tests had been within the solar system. Now, there could be a transition from a weak to strong gravitational field testing. Neutron stars and pulsars were proposed based on ideas inherent within Einstein’s conjecture as explanations for otherwise mysterious radio signals. In 2003, the advent of a two pulsars in mutual orbit allowed astrophysicists to delve into more precise tests of Einstein’s theory. One of the parameters measured with this double pulsar has agreed with General Relativity to the 0.05% level. Three others are different from predictions by 1.4, 0.68 and 5.5%. Testing of these other parameters over a longer period of time promises to distinguish the accuracy between Einstein’s ideas and concepts from other scientists

Astronomical tests of general relativity

This thesis is an in depth investigation of the history of the acceptance of Einstein’s Theory of General Relativity by scientists and by the public through the media. It emphasises the key role that Australia played in that acceptance and in the verification of General Relativity. This contribution came from the 1922 total solar eclipse across the continent as well as the discovery in 2003 at Parkes Radio Telescope of the first, and to this date only, pair of pulsars in mutual orbit. This system provides a unique opportunity to plumb the theory in a much stronger gravitational field regime than previously. This historical scrutiny provides an insight into scientific revolutions in general. The examination of this particular development may then act as a template for the study of other scientific revolutions. One of the key findings is that the Theory of General Relativity was prematurely accepted. The main argument of the thesis is for 1928 being the year when sufficient evidence existed for scientists to begin accepting the theory based on gravitational deflection of light instead of the commonly accepted date of 1919. Emphasis is given to the explorations of the 1922 eclipse parties in Australia and the activities of the eight groups measuring light deflection at this eclipse. This work is gathered together here for the first time. The upshot is that it was 1928 before the results were published in full and a conclusion could be drawn. It is also established that the situation for Mercury needed a much longer time period to near the end of the twentieth century before a decisive verdict could be made. Similar to the situation for light deflection, it is found that 1928 is also the year in which spectroscopic data from spectral line frequency shifts of the Sun and white dwarfs had accumulated sufficiently so that a strong conclusion on the third of the classical tests of General Relativity could be made. From the late 1960s the radio region of the spectrum was employed more frequently to investigate gravitational deflection. As a result of a subsequent extension of this application to interferometry, the increased precision of experiments provided a greater level of testing. No astronomical test yet has refuted General Relativity and agreement has been reached at the 0.05% level with one parameter involving the double pulsar. In line with the emphasis of the 1922 eclipse in Australia, the Australian newspapers were gleaned up to 1928 to see how the 1919 British total solar eclipse results were regarded by the media and the public and to ascertain how the media explained the purpose for those 1922 expeditions in Australia. It is found surprisingly that the newspapers performed admirably in explaining difficult concepts in simple terms for the public during this time. This work provides an historical account of the astronomical tests of General Relativity. More broadly, this thesis demonstrates how the acceptance of a scientific revolution depends on the constant accumulation of data by many scientists and the communication of those results to the wider community. A century after it began, Einstein’s revolution in thinking provides a suitable model for space and time in the Universe

Recent astronomical tests of general relativity

The history of experimentation relevant to general relativity covers the time post-1928. Classes of investigation are the weak equivalence principle (equivalence of inertial and gravitational mass and gravitational redshift), orbital precession of a body in gravitational fields (the relativistic perihelion advance of the planets, the relativistic periastron advance of binary pulsars, geodetic precession and Lense-Thirring effect), light propagation in gravitational fields (gravitational optical light deflection, gravitational radio deflection due to the Sun, gravitational lensing, time dilation and atomic clocks) and strong gravity implications (Nordtved effect and potential gravitational waves). The results of experiments are analysed to conclude to what extent they support general relativity. A number of questions are then answered: (a) how much evidence exists to support general relativity, (b) is it a reasonable way of thinking and (c) what is the niche it may occupy

Early astronomical tests of general relativity: the gravitational deflection of light

One of three astronomical tests of the general relativity theory of Einstein was the gravitational deflection of light. The British total solar eclipse of 1919 is lauded in history as having decided the case in favour of Einstein. This conclusion is questioned in the light of the philosophy of Science and the method employed to analyse the results. The case is put that more emphasis ought be placed on the outcome of the 1922 total solar eclipse in Australia where eight parties attempted measurements of light deflection in the vicinity of the Sun. Importance is attached to Campbell of the Lick Observatory, camped at Western Australia. His results were not completed until 1928. Other leaders, their affiliation and place of observation were Spencer Jones of the Royal Greenwich Observatory on Christmas Island, Freundlich for a German-Dutch expedition to Christmas Island, Evershed of the Kodaikanal Observatory in India also set up in Western Australia, Chant of the University of Toronto measuring at Western Australia, Dodwell of the Adelaide Observatory in a remote part of South Australia and Cooke from the Sydney Observatory and Baldwin of the Melbourne Observatory both in Queensland

Early astronomical tests of general relativity: the anomalous advance in the perihelion of Mercury and gravitational redshift

There were three astronomical tests of general relativity. Besides the gravitational bending of light, there were the anomalous advance of the perihelion of Mercury and gravitational redshift. The early history of these latter two tests is addressed here. For Mercury, data for its position were obtained principally from transit phenomena. Le Verrier was the first to account for the known perturbation effects on the elliptical orbit of Mercury and calculated an unexplained discrepancy. This was supported by Newcomb who revised the figure. With the use of his general theory of relativity, Einstein appeared to calculate this disagreement from Newtonian principles. Yet, other avenues needed to be explored before an acceptance of general relativity as a reasonable paradigm. This is part of a more general query of when should scientists endorse a theory. For the test of the redshift of radiation in the presence of a gravitational field, support for this phenomenon followed a winding route. Many factors, which could contribute to the redshift of spectral lines needed to be nominated, and their individual contribution, if any, had to be teased from the rest. Very small measurements had to be effected. This situation received some respite when measurements moved from the Sun to large mass objects such as white dwarfs which theory suggested should have a much larger redshift. 1928 was taken as the year in which the results could be interpreted as supporting general relativity. However, developments opened up subsequently and further confirmation has continued to the present day. The story is threaded with a theme that new ideas in science follow anything but a straightforward course and that real history is much more interesting