its active mass) was first conducted in 1797 by Henry Cavendish (Fig. 5‑9a).

In 1859, Urbain Le Verrier, in an analysis of available timed observations of transits of Mercury over the Sun's disk from 1697 to 1848, reported that known physics could not explain the orbit of Mercury, unless there possibly existed a planet or asteroid belt within the orbit of Mercury.

For example, the Fizeau experiment of 1851 demonstrated that the speed of light in flowing water was less than the sum of the speed of light in air plus the speed of the water by an amount dependent on the water's index of refraction.

In 1859, Urbain Le Verrier, in an analysis of available timed observations of transits of Mercury over the Sun's disk from 1697 to 1848, reported that known physics could not explain the orbit of Mercury, unless there possibly existed a planet or asteroid belt within the orbit of Mercury.

Einstein himself noted, that with so many people unraveling separate pieces of the puzzle, "the special theory of relativity, if we regard its development in retrospect, was ripe for discovery in 1905." An important example is Henri Poincaré, who in 1898 argued that the simultaneity of two events is a matter of convention.

In 1900, he recognized that Lorentz's "local time" is actually what is indicated by moving clocks by applying an explicitly operational definition of clock synchronization assuming constant light speed.

In 1900 and 1904, he suggested the inherent undetectability of the aether by emphasizing the validity of what he called the principle of relativity, and in 1905/1906 he mathematically perfected Lorentz's theory of electrons in order to bring it into accordance with the postulate of relativity.

His theory was an advance over Lorentz's 1904 theory of electromagnetic phenomena and Poincaré's electrodynamic theory.

(No length changes occur in directions transverse to the direction of motion.) By 1904, Lorentz had expanded his theory such that he had arrived at equations formally identical with those that Einstein was to derive later (i.e.

In 1900 and 1904, he suggested the inherent undetectability of the aether by emphasizing the validity of what he called the principle of relativity, and in 1905/1906 he mathematically perfected Lorentz's theory of electrons in order to bring it into accordance with the postulate of relativity.

However, in 1905, Albert Einstein based a work on special relativity on two postulates: The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference) The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source. The logical consequence of taking these postulates together is the inseparable joining together of the four dimensions—hitherto assumed as independent—of space and time.

Einstein himself noted, that with so many people unraveling separate pieces of the puzzle, "the special theory of relativity, if we regard its development in retrospect, was ripe for discovery in 1905." An important example is Henri Poincaré, who in 1898 argued that the simultaneity of two events is a matter of convention.

In 1900 and 1904, he suggested the inherent undetectability of the aether by emphasizing the validity of what he called the principle of relativity, and in 1905/1906 he mathematically perfected Lorentz's theory of electrons in order to bring it into accordance with the postulate of relativity.

For these and other reasons, most historians of science argue that Poincaré did not invent what is now called special relativity. In 1905, Einstein introduced special relativity (even though without using the techniques of the spacetime formalism) in its modern understanding as a theory of space and time.

While it would appear that he did not at first think geometrically about spacetime, in the further development of general relativity Einstein fully incorporated the spacetime formalism. When Einstein published in 1905, another of his competitors, his former mathematics professor Hermann Minkowski, had also arrived at most of the basic elements of special relativity.

Max Born recounted a meeting he had made with Minkowski, seeking to be Minkowski's student/collaborator: Minkowski had been concerned with the state of electrodynamics after Michelson's disruptive experiments at least since the summer of 1905, when Minkowski and David Hilbert led an advanced seminar attended by notable physicists of the time to study the papers of Lorentz, Poincaré et al.

But Stella observes the time on her ship chronometer to be as she passes the finish line, and she calculates the distance between the starting and finish lines, as measured in her frame, to be 259.81 light-seconds (about ). 1). ==== Deriving the Lorentz transformations ==== There have been many dozens of derivations of the Lorentz transformations since Einstein's original work in 1905, each with its particular focus.

Nor is it clear if he ever fully appreciated Einstein's critical contribution to the understanding of the Lorentz transformations, thinking of Einstein's work as being an extension of Lorentz's work. On 5 November 1907 (a little more than a year before his death), Minkowski introduced his geometric interpretation of spacetime in a lecture to the Göttingen Mathematical society with the title, The Relativity Principle (Das Relativitätsprinzip).

However, in order to complete his search for general relativity that started in 1907, the geometric interpretation of relativity proved to be vital, and in 1916, Einstein fully acknowledged his indebtedness to Minkowski, whose interpretation greatly facilitated the transition to general relativity.

On 21 September 1908, Minkowski presented his famous talk, Space and Time (Raum und Zeit), to the German Society of Scientists and Physicians.

Furthermore, even as late as 1909, Poincaré continued to believe in the dynamical interpretation of the Lorentz transform.

In 1915, Emmy Noether discovered that underlying each conservation law is a fundamental symmetry of nature.

However, in order to complete his search for general relativity that started in 1907, the geometric interpretation of relativity proved to be vital, and in 1916, Einstein fully acknowledged his indebtedness to Minkowski, whose interpretation greatly facilitated the transition to general relativity.

General relativity involves the systematic stitching together of these local frames into a more general picture of spacetime. Shortly after the publication of the general theory in 1916, a number of scientists pointed out that general relativity predicts the existence of gravitational redshift.

It was ultimately established that no such planet or asteroid belt existed. In 1916, Einstein was to show that this anomalous precession of Mercury is explained by the spatial terms in the curvature of spacetime.

17–18. == External links == Albert Einstein on space–time 13th edition Encyclopædia Britannica Historical: Albert Einstein's 1926 article Encyclopedia of Space–time and gravitation Scholarpedia Expert articles Stanford Encyclopedia of Philosophy: "Space and Time: Inertial Frames" by Robert DiSalle. Concepts in physics Theoretical physics Theory of relativity Time Time in physics Conceptual models

The naive expectation for asymptotically flat spacetime symmetries might be simply to extend and reproduce the symmetries of flat spacetime of special relativity, viz., the Poincaré group. In 1962 Hermann Bondi, M.

The puzzling surprise in 1962 was their discovery of a rich infinite-dimensional group (the so-called BMS group) as the asymptotic symmetry group, instead of the finite-dimensional Poincaré group, which is a subgroup of the BMS group.

They used their measurements to tighten the limits on any discrepancies between active and passive mass to about 10−12. ==== Gravitomagnetism ==== The existence of gravitomagnetism was proven by Gravity Probe B , a satellite-based mission which launched on 20 April 2004.

The spaceflight phase lasted until 2005.

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