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In classical mechanics, a gravitational field is a physical quantity. [5] A gravitational field can be defined using Newton's law of universal gravitation.Determined in this way, the gravitational field g around a single particle of mass M is a vector field consisting at every point of a vector pointing directly towards the particle.
A gravitational lens is matter, such as a cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein 's general theory of relativity .
This formulation is dependent on the objects causing the field. The field has units of acceleration; in SI, this is m/s 2. Gravitational fields are also conservative; that is, the work done by gravity from one position to another is path-independent. This has the consequence that there exists a gravitational potential field V(r) such that
James Clerk Maxwell's 1865 prediction [46] that light was an electromagnetic wave – which was confirmed experimentally in 1888 by Heinrich Hertz's detection of radio waves [47] – seemed to be the final blow to particle models of light. In 1900, Maxwell's theoretical model of light as oscillating electric and magnetic fields seemed complete.
Gravitational redshift has been measured in the laboratory [65] and using astronomical observations. [66] Gravitational time dilation in the Earth's gravitational field has been measured numerous times using atomic clocks, [67] while ongoing validation is provided as a side effect of the operation of the Global Positioning System (GPS). [68]
Diagram of gravitational lensing by a compact body. In the limit as the particle mass m goes to zero (or, equivalently if the light is heading directly toward the central mass, as the length-scale a goes to infinity), the equation for the orbit becomes
The first direct evidence for gravitational radiation was measured on 14 September 2015 by the LIGO detectors. The gravitational waves emitted during the collision of two black holes 1.3 billion light years from Earth were measured. [83] [84] This observation confirms the theoretical predictions of Einstein and others that such waves exist.
A naive application of Newtonian gravity can yield exactly half this value, where the light ray is assumed as a massed particle and scattered by the gravitational potential well. This approximation is good when 4 G M / c 2 b {\displaystyle 4GM/c^{2}b} is small.