The November 25, 1915 Albert Einstein presented the theory of General Relativityone of the pinnacles of the history of science that forever revolutionized our way of conceiving the Universe. General Relativity is a theory of gravity, and is based on the idea that this is not a force that manifests itself instantaneously between bodies, but rather a effect of the curvature of the fabric of space-time generated by all bodies with mass. The more massive a body is, the more space-time is “curved”, while in the absence of masses it is “flat”.
General Relativity was confirmed experimentally in 1919 thanks to Eddington’s experiment, and over time it has withstood ever new and more stringent validity tests. Thanks to General Relativity we have understood that theUniverse is expanding and which had a beginning with the big Bang. But not only that: thanks to this theory we now know that bodies with mass can emit gravity waveswhich is the curvature of space-time deflects the light and that a mass can be so large and dense as to bend space-time until not even light escapes, creating the so-called black holes. But General Relativity also has everyday applications, the best known of which are GPS navigation systems.
What we discovered thanks to Einstein’s General Relativity
The Big Bang and the expansion of the Universe
General Relativity is the foundation of modern cosmological theories. In the 1929 the astronomer Edwin Hubble he discovered that distant galaxies are standing together moving away from us to one speed proportional to their distance. Since gravity is an attractive force, in the context of a static universe this observation was inexplicable. General Relativity instead provides an explanation: it is not the galaxies that move, but the fabric of space-time of the Universe which is expanding and galaxies are dragged along with it.
Thanks to General Relativity and other experimental evidence – such as the cosmic background radiation – we have therefore also understood that the primordial Universe must have been extremely small, hot and dense. From here was born the idea that the Universe had a beginning: the big Bang (literally “Great Blast”), which occurred approximately 13.8 billion years ago and which gave rise to cosmic expansion.
Black holes
The prediction of the existence of black holes it follows the publication of the theory of General Relativity by just one year. In the 1916the mathematician Karl Schwarzschildduring his military service on the Russian Eastern Front in the First World War, found a particular solution to the equations of General Relativity which under certain conditions predicted the presence of static black holes.
Black holes are celestial objects whose mass, concentrated in a radius of a few tens of km for stellar black holes, is such that deform space-time to such an extent that don’t let anything slip away to its gravitational field, not even light, hence the name. This “one-way” region, which we call a black hole, is bounded by a “surface of no return” called event horizon.
Technically, any body with mass, if compressed enough, can turn into a black hole. The mass of our Sun, for example, would have to be compressed into a sphere of radius 2.95 km to transform into a black hole. The enormous compaction generates a gravitational field such that impede to any type of information to cross the event horizon from the inside out.
The Newtonian theory of gravity does not predict the existence of black holes and it is only thanks to the formulation of General Relativity and its concept of space-time deformation by a mass that we are aware of the existence of these mysterious cosmic objects.
Gravitational lenses
One of the most peculiar effects predicted by General Relativity is that of lensing or gravitational lens. Since bodies with mass deform space-time and light is forced to follow the curvature of space-time, then even the path of light can be diverted with massive bodies. The light will therefore no longer move in a straight line, but the trajectory will curve following the space-time deformation.
For the effect to be striking, however, something truly massive is needed, such as a galaxy or a cluster of galaxies that act like a lens“bending” the light rays and deforming their trajectory. The resulting effect is that objects that are behind the lens come distorted in the ed. form amplified in brightness.
Gravitational lensing was precisely the effect Sir Arthur Eddington exploited for try experimentally General Relativity in 1919, when during a solar eclipse the astronomer noticed how the position of the stars was changed by solar gravitational field. Today, through gravitational lensing, astronomers are able to reconstruct the distribution of matter in galaxy clusters, structures consisting of hundreds or thousands of galaxies orbiting under mutual gravitational attractions, and the distribution of dark matter in the cosmos.
Gravitational waves
In 2015, scientists detected for the first time in history the gravitational waves, ripples of space-time whose existence was one of the first predictions of the theory of General Relativity. Albert Einstein quickly realized that his theory implied the existence of gravity waves that move to speed of light generated by bodies with mass (not spherically symmetric) acceleratingjust as in electromagnetism an accelerated electric charge emits electromagnetic radiation.
In principle every body with mass, even ourselves, by accelerating we are able to emit gravitational waves. Unfortunately, however, the accelerations and masses involved are so small as to make detection impossible. Current astronomical technology only allows us to detect the waves gravitational generate give it events more extremesbut also more fascinating, that the cosmos has to offer us, namely the merger of two neutron stars, two black holes or a neutron star and a black hole.
Gravitational waves can be thought of as ripples of space-time which modify the space-time distances between the constituents of matter and cause them compressions and dilations. They interact weakly with matter, traveling undisturbed even for billions of light-years, allowing us to probe the most remote areas of the cosmos.
Gravitational waves are one of the most important predictions of General Relativity because they finally allow us to study inaccessible phenomena to a study based purely on electromagnetic radiation, for example how General Relativity behaves in strong field conditions, i.e. in the presence of objects that distort space-time in an incredible way, such as black holes.
GPS systems
GPS is the English acronym for Global Positioning System and it is the most famous global navigation satellite systemconsisting of approximately 31 satellites orbiting the Earth. Thanks to GPS it is possible locate and discover the exact location of a place, a person or an object through the data triangulationor the calculation of the distance to the receiver (for example our cell phone) by four or more satellites.
Although GPS itself takes advantage of the simple geometry to calculate the position of the receiver, in reality it would be completely useless if we did not know General Relativity. The satellites and the receiver actually experiment two gravitational fields of different intensityso the time marked by a clock on Earth and one on the satellite flows differently due to time dilation. In particular, the stronger the gravitational field the slower time passes if measured by an external observer, such as GPS satellites.
The clocks on the GPS satellites therefore run faster than those on Earth, by an amount of approximately 38 microseconds per day. These relativistic corrections are used when carrying out triangulations. Without them, GPS would calculate positions that would accumulate errors of about 10 km per daytherefore making them unusable for all purposes.
