Viking Project tests general relativity, November 25, 1976

2.1. Einstein

Einstein, see Fig. 1, had a very important role in a number of areas in physics but the pinnacle is without a doubt the creation of general relativity. After great persistence from a work that started with the idea of the principle of equivalence in 1907 [1, 2] and continued through the collaboration with Grossmann in the entwurf theory of gravitation [3 –5], Einstein presented to the Prussian Academy of Sciences, in November 18 and 25, 1915 [6, 7], see also [8], a totally new theory, namely, a covariant, tensorial, and relativistic theory, that he immediately called the general theory of relativity, or simply, general relativity. Einstein equation that governs general relativity is

where ��� is a quantity that represents the geometry of spacetime called the Einstein tensor, ��� is a quantity that represents the matter content of the spacetime called the energy-momentum tensor, G is the constant of gravitation, and c is the speed of light, see [9] for the genesis of this equation. In a stroke, the theory confirms the Minkowskian spacetime notion, states that gravitation is geometry, spacetime is curved, and particles follow geodesics. For accounts of this period in Einstein's life see [10 –12].

In its more than one hundred years, general relativity has passed through very rigorous tests, it is accepted as the standard theory of gravitation, and is considered one of the great feats in history. Notwithstanding all these achievements, gravitation is the most intriguing of all the known interactions.

The tests and implications of general relativity are many and profound. Weak field classical tests within the solar system are the perihelion precession of Mercury, the light deflection in the gravitational field of the Sun, the gravitational redshift Doppler effect, and the Shapiro gravitational time delay in the radar echo. Technological applications of general relativity, are now current, as the global position system, or GPS, would not work at all without the general relativistic corrections related to the gravitational redshift Doppler effect, necessary to synchronize clocks in the satellites with clocks on the Earth's surface. Gravitational lensing is an abundant special case of light deflection and of great importance to understand the gravitational mass and gravitational structure of the Universe. Cosmology, the dynamical and physical study of the Universe, was started by Einstein in 1917 with a static finite universe, continued with the proposal by Friedmann, Lemaitre, and Hubble for an expanding universe, along with the establishment of the big bang scenario through the discovery of the cosmic microwave background radiation, up to the establishment of the acceleration of the Universe, and to the most recent astonishing developments, that converged in the awarding of the shared 2019 Nobel Prize in Physics to Peebles of Princeton University, one of the exponents in the field throughout the last six decades. Fundamental theories, theories that make the unification of gravitation and electromagnetism, were initiated by Weyl in 1918, and continued by Eddington and Einstein. Now they are called theories of everything and try to unify the four fundamental fields in a unique quantum scheme. Black holes, the geometric object par excellence in general relativity, were found by Oppenheimer and Snyder in 1939 as the endpoint of gravitational collapse and thus occurring necessarily in nature. Millions of solar mass black holes float through our galaxy, and all, or almost all, galaxies contain a central supermassive black hole in its center. Gravitational waves, spacetime waves predicted by Einstein in 1916, were detected indirectly in the binary pulsar discovered by Hulse and Taylor in 1976, which gave the Nobel prize in 1993, and detected directly in 2015 by the LIGO antennas, from the collision of two black holes, which in turn gave the Nobel prize in 2017. General relativity has left an immense and amazing legacy and we are still in the middle of many of its developments.