The story of a mystery revealed

Of the four fundamental interactions, gravity is the weakest. Yet it is the only one we are directly aware of: no one expects to float in the air if they jump out of a third-floor window. This awareness has always accompanied humans. In fact, it is part of the experience of every sentient being. However, it has only been the subject of rigorous scientific investigation for four centuries.

To present the progress made by physics, which has been brief but rich in theoretical and experimental revolutions, Luca Molinari, associate professor in the Department of Physics at the University of Milan, will give a lecture on Friday 23 January at 6 p.m. The lecture, organised by the Istituto ricerche solari Aldo e Cele Daccò (IRSOL) in Locarno and the Ticino Astronomical Society, will be held at the Cantonal Library in Lugano.

For millennia, the origin of gravitational force was a mystery. When and how did the theory of universal gravitation develop, and how did it establish itself in the centuries that followed, until Einstein's revolution?

During the years when the plague raged in London (1665-66), Newton took refuge in the countryside, where he produced his masterpieces on optics, dynamics and gravitation. His friend and biographer Stukeley reports that Newton himself told him the story of the apple that inspired him to formulate the law of universal gravitation. The triumphs were the deduction of Kepler's laws, cometary orbits and, later, the discoveries of Ceres (1801), Neptune (1846) and Pluto (1930). The measurement of Newton's constant G, carried out in Cambridge in 1798 by Cavendish using a special torsion balance designed by Mitchell, made it possible to determine the mass of the Earth and the Sun. The evolution of the instrument led Eötvös in 1885 to improve the measurement and achieve sufficient sensitivity to locate oil deposits through gravity anomalies.

Then Einstein arrived. How did the identity between inertial mass and gravitational «charge» enable the classical view of gravity to be superseded in favour of a geometric interpretation of space-time?

The fundamental identity of inertial mass (which determines resistance to a change in motion) with gravitational «charge» (which determines the response to gravity) was the subject of Newton's experiments with pendulums filled with different substances, confirming Galileo's experiments. Even today, tests of the weak equivalence principle are still being carried out in space (Microscope experiment) and with antimatter at CERN. This identification deeply inspired Einstein in his long development of the theory, which was completed in 1916. The fact that gravity can be locally cancelled out in free-fall reference systems led him to interpret it as a geometric fact, inscribed in the transformation of space-time coordinates from the accelerated system to the stationary one. This is his recollection: «I was sitting in the patent office in Bern when all of a sudden a thought occurred to me: if a person falls freely, he won't feel his own weight. I was startled. This simple thought made a deep impression on me. It impelled me toward a theory of gravitation».

I will mention the progress made in non-Euclidean geometry in the 19th and early 20th centuries, starting with Gauss and Riemann, which led to Einstein's elegant geometric formulation of gravity. The theory was soon corroborated by the correct prediction of the precession of Mercury's perihelion and achieved worldwide triumph with the measurement of the deviation of light in the solar eclipse of 1919. An important confirmation was the spectroscopy experiment by Pound and Rebka, which verified the variation in the frequency of photons in the Earth's gravitational field.

Gravitational waves are fundamental to modern gravitational theory. From Einstein's prediction to the present day, what have been the main successes of interferometers and large telescopes in confirming general relativity?

Gravitational waves are an interesting story: predicted, denied and then reconsidered by Einstein, they were indirectly detected in the observation of the decrease in the orbital period of a pair of neutron stars (Taylor and Hulse, 1974) due to the emission of gravitational waves. Direct detection took place in 2015 with interferometers in the USA (which now operate together with Virgo in Pisa and KAGRA in Japan). I will mention the black hole at the centre of the Milky Way, highlighted by the orbits of nearby stars (2012) in over ten years of observations by the twin Keck telescopes in Hawaii.

Yet Einstein was not convinced of at least one theoretical prediction of his own theory. How did he go from obstructing theories of a dynamic universe to introducing a constant that would become the pillar of modern cosmology?

The most revolutionary aspect of Einstein's theory is certainly in the independent predictions of Friedmann and Lemaître of the expansion of the universe observed by Hubble in 1928 by measuring the redshift of galaxies. Einstein hindered the publication of Friedmann's work for some time and introduced the cosmological constant Lambda to prevent expansion. Today, the constant reappears as necessary to introduce the negative pressure that agrees with the acceleration observed thanks to distant supernovae. The standard model of cosmology, LambdaCDM, is based precisely on Friedmann and Lemaitre's equations, in which, in addition to the source of matter, the Lambda constant appears, along with the necessary ingredient of dark matter... but that deserves another presentation.