ScienceWeek

HISTORY OF PHYSICS: EINSTEIN AGAINST PHYSICAL REVIEW JOURNAL

The following points are made by Daniel Kennefick (Physics Today 2005 September):

1) Albert Einstein had two careers as a professional physicist, the first spent through 1933 entirely at German-speaking universities in central Europe, the second at the Institute for Advanced Studies in Princeton, New Jersey, from 1933 until his death in 1955. During the first period he generally published in German physics journals, most famously the Annalen der Physik, where all five of his celebrated papers of 1905 appeared.

2) After relocating to the US, Einstein began to publish frequently in North American journals. Of those, the Physical Review, then under the editorship of John Tate, was rapidly assuming the mantle of the world's premier journal of physics.[1] Einstein first published there in 1931 on the first of three winter visits to Caltech. With Nathan Rosen, his first American assistant, Einstein published two more papers in the Physical Review: the famous 1935 paper by Einstein, Boris Podolsky, and Rosen (EPR) and a 1936 paper that introduced the concept of the Einstein-Rosen bridge, nowadays better known as a wormhole. But except for a letter to the journal's editor he wrote in 1952 --in response to a paper critical of his unified field theory work -- that 1936 paper was the last Einstein would ever publish there.

3) Einstein stopped submitting work to the Physical Review after receiving a negative critique from the journal in response to a paper he had written with Rosen on gravitational waves later in 1936.[2] That much has long been known, at least to the editors of Einstein's collected papers. But the story of Einstein's subsequent interaction with the referee in that case is not well known to physicists outside of the gravitational-wave community. Last March, the journal's current editor-in-chief, Martin Blume, and his colleagues uncovered the journal's logbook records from the era, a find that has confirmed the suspicions about that referee's identity.[3] Moreover, the story raises the possibility that Einstein's gravitational-wave paper with Rosen may have been his only genuine encounter with anonymous peer review. Einstein, who reacted angrily to the referee report, would have been well advised to pay more attention to its criticisms, which proved to be valid.

4) Einstein introduced gravitational waves into his theory of general relativity in 1916, within a few months of finding the correct form of the field equations for it. Although the concept of gravitational radiation was then relatively new and no experimental evidence existed to support it, the analogy with the case of the electromagnetic field was so compelling that by the 1930s most scientists thought that gravitational waves must exist in principle. Nevertheless, in 1936 Einstein wrote to his friend Max Born:

"Together with a young collaborator, I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation. This shows that the non-linear general relativistic field equations can tell us more or, rather, limit us more than we have believed up to now."[4]

Einstein submitted this research to the Physical Review under the title "Do Gravitational Waves Exist?" with Rosen as coauthor. Although the original version of the paper no longer exists, Einstein's answer to the title question, to judge from his letter to Born, was "No". It is remarkable that at this stage in his career Einstein was prepared to believe that gravitational waves did not exist, but he also managed to convince his new assistant, Leopold Infeld, who replaced Rosen in 1936, that his argument was valid.[5]

References (abridged):

1. A. Pais, in The Physical Review: The First Hundred Years, H. H. Stoke, ed., AIP Press, New York (1995), p. 1

2. A. Pais, "Subtle is the Lord": The Science and Life of Albert Einstein, Oxford U. Press, New York (1982), p. 494

3. D. Kennefick, in The Expanding Worlds of General Relativity, H. Goenner, J. Renn, J. Ritter, T. Sauer, eds., Birkhäuser-Verlag, Boston (1999), p. 207

4. A. Einstein, The Born-Einstein Letters: Friendship, Politics, and Physics in Uncertain Times, MacMillan, New York (2005), p. 122

5. L. Infeld, Quest: An Autobiography, Chelsea, New York (1980)

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ON GRAVITATIONAL WAVES

The following points are made by B.C. Barish and R. Weiss (Physics Today October 1999):

1) The idea of gravitational waves was already implicit in the 1905 special theory of relativity, with its finite limiting speed for information transfer. The explicit formulation for gravitational waves in general relativity was put forward by Einstein in 1916 and 1918. He showed that the acceleration of masses generates time-dependent gravitational fields that propagate away from their source at the speed of light as warpages of spacetime. Such a propagating warpage is called a "gravitational wave".

2) The best empirical evidence we have of the existence of gravitational radiation is indirect. It comes from the 1974 discovery and beautiful observations, by Russell Hulse and Joseph Taylor, of the first binary pulsar ever found. Exploiting the clockwork pulsar signal from the neutron star, they were able to monitor the orbital period of the binary star system with exquisite precision and confirm that it was indeed gradually speeding up at just the rate predicted for the general-relativistic emission of gravitational waves.

3) The direct detection of gravitational waves will mark the opening of a new window on the near and far reaches of the Cosmos. For physics, its most important promise is the direct observation of gravitation in highly relativistic settings, so that one can test general relativity in the strong-field limit, where it is not merely a small correction to Newtonian gravity. In that limit, the strong curvature of the spacetime geometry should show us fundamentally new physics.

Physics Today http://www.physicstoday.org

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ASTROPHYSICS: ON GRAVITATIONAL WAVES

The following points are made by Peter S. Shawhan (American Scientist 2004 92:350):

1) To understand how gravitational radiation arises requires at least a rudimentary understanding of Einstein's general theory of relativity. This theory posits that time is a dimension similar to the three dimensions of space and that the combined four-dimensional "spacetime" can be treated using the language of geometry.

2) The complete history of an object's position as a function of time is described by a "world line", which threads through the four-dimensional coordinate system, from past to present to future. If no force acts on the object, it will move with a constant velocity, and its world line will be a straight line at some fixed angle relative to the coordinate axes.

3) An object near a large mass feels the force of gravity accelerate it, so that its world line follows a curved path relative to the coordinate system. For example, if a ball is thrown straight up into the air, a graph of its height versus time traces out a parabola. At least, that is the conventional view, dating back to Isacce Newton (1642-1727). Albert Einstein (1879-1955) took the bold step of casting that notion aside and postulating that a massive body curves the coordinate system itself. Rather than following a curved path in a Cartesian coordinate system, the ball actually follows a "straight" path (a geodesic) in a curved coordinate system, returning to the thrower's hand at a later time because the geodesic leads it there. Gravity, therefore, is not really a force but is a manifestation of curvature in the geometry of spacetime.

4) The difference between these two points of view may sound like a matter of definition, but Einstein's theory made a few specific predictions that have since been experimentally verified. For example, the British astrophysicist Sir Arthur Eddington (1882-1944) took advantage of a 1919 solar eclipse to measure the deflection of starlight passing near the Sun, finding it to be in agreement with theory. His result was trumpeted on the front pages of newspapers around the world, instantly establishing Einstein's popular reputation.

5) General relativity says that the geometrical curvature induced by a massive object does not arise everywhere instantaneously. Rather, it travels outward from its source at the speed of light. Thus, if a massive object alters its shape or orientation, or if a collection of objects changes its spatial arrangement, the gravitational effect -- the curvature of spacetime -- propagates away as a gravitational wave.

6) A gravitational wave may be described as a time-varying distortion of the geometry of space, temporarily altering the effective distance between any given pair of points. If the causative shift in mass is abrupt, the wave will take the form of a short pulse, much like the ripple produced after dropping a rock into a still pond. In the case of a periodic change, the wave will be sustained, much like the carrier wave for a broadcast radio signal. In either case, the amplitude of the wave will be inversely proportional to the distance from the source. Unlike ordinary gravitational acceleration, which always points toward the source, a gravitational wave acts perpendicularly to the direction in which it is traveling, and thus is called a transverse wave. In this sense it is like light, rather than like sound, which propagates as longitudinal waves.(1-5)

References (abridged):

1. Barish, B. C., and R. Weiss. 1999. LIGO and the detection of gravitational waves. Physics Today 52(10):44-50

2. Lyne, A. G., M. Burgay, M. Kramer, A. Possenti, R. N. Manchester, F. Camilo, M. A. McLaughlin, D. R. Lorimer, N. D'Amico, B. C. Joshi, J. Reynolds and P. C. C. Freire. 2004. A double-pulsar system: A rare laboratory for relativistic gravity and plasma physics. Science 303:1153-1157

3. Saulson, P. R. 1994. Fundamentals of Interferometric Gravitational Wave Detectors. Singapore: World Scientific. Schutz, B. S. 2003. Gravity from the Ground Up. Cambridge, U.K.: Cambridge University Press

4. Taylor, J. H., and J. M. Weisberg. 1982. A new test of general relativity: Gravitational radiation and the binary pulsar PSR 1913+16. Astrophysical Journal 253:908-920

5. Will, C. M. 1993. Was Einstein Right? Putting General Relativity to the Test. New York: Basic Books

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