The predictions of the German physicist Albert Einstein (1879-1955) still surprise the scientific community more than a century later, both those already confirmed and those that we are still exploring.
Einstein tops the list of the most famous and iconic scientists in history. His theories of Special Relativity in 1905 and General Relativity in 1915 revolutionized the universe of physics.
The German physicist went beyond Newton’s theory of gravity, which had existed since 1687. Einstein also introduced his famous thought experiments, which also put the first developments in quantum mechanics to the test. His contributions in this field won him the Nobel Prize in Physics, which he won in 1921 for the photoelectric effect.
Many people believe that the Nobel Prize for General Relativity, which was not given to him, is a large outstanding debt. By this theory, gravity is understood as a deformation or curvature of space-time, caused by the distribution of masses and energies.
In other words: the more mass is compressed into less volume, the more spacetime around it is warped or curved. Any other particles or objects that pass close to these objects feel this curvature, which causes their trajectory to change.
Confirmed prediction: the day spacetime curvature was observed
Some of the predictions or consequences of General Relativity were put to the test in a short time. In 1919, just four years after the theory was published, a total solar eclipse occurred, the ideal event for testing the curvature of space-time.
There were several scientific expeditions that traveled to Brazil and the West African coast to take the best pictures and data of that eclipse and, above all, of the stars that surrounded the Sun.
The most massive and compact object we have in our surroundings is the Sun. What was wanted to verify was if the light of distant stars was affected by the curvature of the space-time generated by the Sun when passing close to it.
In that case, its path would deviate slightly from a straight line, causing the star’s apparent position in the sky to change slightly. Confirmation of this effect, consistent with measurements of the 1919 eclipse, made Einstein world famous.
Einstein’s doubts: the vibrations of spacetime
But to experimentally demonstrate other predictions of General Relativity, we need to wait much longer. In 1916, Einstein began to analyze his equations in detail and, in particular, a series of terms that, in short, look like a wave equation: the same structure that appears in many physical systems in which we have a disturbance that propagates carrying energy. .
In this case, the equations say that what vibrates is spacetime itself, and we call these disturbances gravitational waves.
Could they be seen? Is there a way to “hear” the vibrations of spacetime?
During his lifetime, Einstein doubted the real existence of this phenomenon (perhaps it was a mathematical artifact, but without physical realization?). Einstein was neither the first nor the only physicist to doubt the mathematical consequences of his theory. He had his ups and downs with peers and prestigious scientific journals that gave rise to very interesting stories.
Be that as it may, and with the contribution of prominent personalities, it was finally understood that gravitational waves were indeed a real prediction of the theory.
Its properties were analyzed and it remained to be seen whether the technological race to experimentally verify its existence was bearing fruit.
Prediction confirmed: gravitational waves were finally “heard”
The amplitude of these waves is so, so extremely weak that Einstein himself was not very confident that their detection would ever be possible.
Each of the tests that General Relativity was subjected to was not able to find discrepancies, but not detecting gravitational waves or detecting them with properties different from those theorized would be a demonstration that this theory did not faithfully reproduce reality.
The success of technological development took decades, and of the usual failed attempts that are not always mentioned in science, such as physicist Joseph Weber’s pioneering experiments with resonant bars in the 1960s.
The instruments that have finally managed to overcome this challenge are laser interferometers.
The first detection of gravitational waves took place in 2015.
It was carried out by the American LIGO observatories and was a literally historic event.
The gravitational waves detected have also been linked to another of the consequences of General Relativity: they came from the merger of two black holes of about 36 and 29 times the mass of the Sun, and passed through the detectors after traveling about 1.3 billion light-years. .
The European Virgo Observatory joined the data collection in the summer of 2017 with a triple detection of a neutron star merger that included gravitational waves in multi-messenger astronomy (astronomy based on the simultaneous recording and interpretation of multiple signals from space). The KAGRA observatory will join the global network in the next observation period, scheduled for December this year.
We already have a total of 90 confirmed events, all of which have as an astrophysical scenario the merger of two compact objects: pairs of black holes, pairs of neutron stars or mixed pairs of a black hole and a neutron star.
The research door is open to compact objects of a different nature, and the gravitational waves they generate can give us clues about their structure and properties. We are impatient to see the new surprises to come.
The cosmological constant: Einstein’s biggest “mistake”?
In the chapter on Einstein’s predictions, we cannot forget the famous cosmological constant, which also generated contradictions. This constant, its properties and whether it is able to faithfully model the evolution and expansion of the universe in light of future data is the page of the book being written right now.
Einstein introduced this constant into his equations to force (due to personal beliefs) a static model of the universe, a kind of “repulsive energy” without which the universe would eventually collapse due to the effect of gravity itself.
However, after physicist Edwin Hubble’s (1889-1953) observations in 1931 of the expansion of the universe, Einstein considered his proposal “the biggest mistake” of his scientific work. Was he right?
Interest in the cosmological constant introduced by Einstein resurfaced again with quantum field theories, as these predict a vacuum energy that might behave, for all intents and purposes, like the cosmological constant he predicted.
So it looks like Einstein, once again, got it right again.
*Isabel Cordero Carrión is a professor and researcher at the Faculty of Mathematics at the University of Valencia, Spain.
This article was originally published on the academic news site The Conversation and republished here under a Creative Commons license. Read the original version (in Spanish) here.