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Physics portal; Bentley's paradox – Cosmological paradox involving gravity; Gauss's law for gravity – Restatement of Newton's law of universal gravitation; Jordan and Einstein frames – different conventions for the metric tensor, in a theory of a dilaton coupled to gravity
The equivalence principle is the hypothesis that this numerical equality of inertial and gravitational mass is a consequence of their fundamental identity. [1]: 32 The equivalence principle can be considered an extension of the principle of relativity, the principle that the laws of physics are invariant under uniform motion
Based on the principle of relativity, Henri Poincaré (1905, 1906), Hermann Minkowski (1908), and Arnold Sommerfeld (1910) tried to modify Newton's theory and to establish a Lorentz invariant gravitational law, in which the speed of gravity is that of light. As in Lorentz's model, the value for the perihelion advance of Mercury was much too low.
In physics, gravity (from Latin gravitas 'weight' [1]) is a fundamental interaction primarily observed as a mutual attraction between all things that have mass.Gravity is, by far, the weakest of the four fundamental interactions, approximately 10 38 times weaker than the strong interaction, 10 36 times weaker than the electromagnetic force, and 10 29 times weaker than the weak interaction.
The generalization of this statement, namely that the laws of special relativity hold to good approximation in freely falling (and non-rotating) reference frames, is known as the Einstein equivalence principle, a crucial guiding principle for generalizing special-relativistic physics to include gravity. [38]
To be complete, a theory of gravity must be capable of analysing the outcome of every experiment of interest. It must therefore mesh with electromagnetism and all other physics. For instance, any theory that cannot predict from first principles the movement of planets or the behaviour of atomic clocks is incomplete.
In theoretical physics, particularly in discussions of gravitation theories, Mach's principle (or Mach's conjecture [1]) is the name given by Albert Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach.
The Einstein–Hilbert action in general relativity is the action that yields the Einstein field equations through the stationary-action principle.With the (− + + +) metric signature, the gravitational part of the action is given as [1]