In 1916, a physicist who had been stealing hours from his job in a patent office to do imaginary experiments and develop a theory about the behavior of the universe first had the idea of the existence of gravitational waves. In the cosmos, he reasoned, there are phenomena so violent that they warp space and time. Gravitational waves would travel at the speed of light in all directions, curving everything in their path, as happens in the water in a pond where a stone has been thrown. The stars, the planets and everything they contain would move like buoys in the wake of these waves. That man was called Albert Einstein, father of the theory of general relativity. His conclusion on this matter was that gravitational waves occur at such far distances that when they reach our planet they are too faint to be captured by any human-made instrument. No one, he thought, would be able to prove its existence. That was the only thing he was wrong about.
A century and three years later, exactly two minutes after 5 a.m. on May 21, 2019, a signal appeared on the screens of the US LIGO laser interferometer.Almost at the same time, more than 8,000 kilometers away. , the signal appeared on the screens of Virgo, a similar detector located in a town not far from the Leaning Tower of Pisa. More than 2,000 scientists from 19 countries began to understand the meaning of that signal. On paper it was just an upward curve that ended abruptly. The scientists translated the frequency of that signal into sound and heard a very brief blow, or a buzz. The signal lasted only a tenth of a second, which made it very difficult to understand where it was coming from and how it had been generated. Last Wednesday, the members of this scientific collaboration announced that this was the most powerful gravitational wave in history.
This merger of black holes released the energy of 10 billion trillion trillion atomic bombs like the one at Hiroshima
José Antonio Font, Virgo
Einstein was right and he was not. Gravitational waves exist, but it is not impossible to detect them. Since the discovery of the first wave, announced in 2016, LIGO and Virgo laser interferometers have ushered in a new era in astronomy. It is no longer only possible to observe the cosmos using light at all wavelengths, but it is also possible to hear and understand the echoes produced by some of the most amazing and unknown objects that it houses. “It is the most important discovery so far this century”, according to the Nobel Prize in Physics George Smoot.
The signal announced this week occurred 7 billion years ago, about 2.5 billion before the Sun, Earth and the rest of the solar system formed. Two black holes with masses 85 and 66 times greater than the Sun, respectively, got too close. Black holes are the densest objects that exist and they generate such a force of attraction that everything that crosses their jaws – the event horizon – inevitably falls into and disappears, including light. That is why they are black: they cannot be seen with a conventional telescope. These two holes began to orbit one around the other until they were swallowed up by its enormous forces of gravity.
The collision of these two invisible colossi spit out as much energy as 10,000 million trillion trillion atomic bombs like the one at Hiroshima, according to the calculations of José Antonio Font, a Virgo collaborator professor of astrophysics at the University of Valencia. In other words, the mass of eight stars like the Sun was released following the equation coined by Einstein to the letter: energy is equal to mass times the speed of light squared (E = mc²). That explosion produced gravitational waves that began to travel through the universe in all directions, deforming space-time as if it were gelatin.
Since Einstein predicted the existence of these waves, capturing them became an obsession for many experimental physicists, who racked their brains trying to design a measurement method capable of recording them. Gravitational waves are so violent in origin that they cause storms in which it is literally possible to travel in time. But they are weakening and by the time they reach Earth after billions of years crossing space, they are so tiny that it was impossible to capture them with the first detectors that were built in the 1960s.
“These detectors are so sensitive that we can tell if it is a weekday or weekend by the noise of cars, we listen to the swell and we can distinguish if it is in the Mediterranean or the Atlantic
Julia Casanova, Virgo
Sitting at the controls of Virgo’s control center, Spanish physicist Julia Casanova explains how the problem was solved. The key was to measure minute amounts of space and time. Detectors like LIGO and Virgo have two arms through which laser light constantly runs from one end to the other 24 hours a day, seven days a week, all year round. Scientists know exactly how long it takes for a photon – the particle of light – to make this journey. When a gravitational wave hits Earth and warps time and space, the photon’s time of flight changes by one hundredth of a nanosecond. Expressed in space, after the arrival of the wave announced this week, the distance that these particles travel in the arms of Virgo changed one millionth of a millionth of a millionth of a meter. It is a distance about a thousand times smaller than the diameter of the nucleus of an atom; the smallest distance measurement ever made, according to those responsible for LIGO.
“When the signal came, we didn’t know it was something exceptional until a long time later,” confesses Casanova. Controllers like her are only in charge of detecting the small modification in the time of flight of photons and correcting it. The signal is recorded and then analyzed and filtered from the background noise, which is thunderous. “These detectors are so sensitive that we can tell if it is a weekday or weekend from the noise of cars; we listen to the airplanes, the air conditioners, the earth tremors, we listen to the swell and we can distinguish if it is in the Mediterranean or in the Atlantic ”, says Casanova.
LIGO has two identical detectors, one in Washington state, in the far west of the country, and another in Louisiana, to the southeast. Both must pick up the same signal if a gravitational wave passes. Virgo provides a third measurement that allows you to triangulate and know from which part of the sky, from which part of the universe, the wave has arrived. This week’s wave, produced by the largest merger of black holes captured to date, came from a region of the sky of 700 square degrees where there are three constellations: Coma Berenices – Berenice’s Hair – Canes Venatici – Los Perros Cazadores – and El Fénix, all of them in the northern hemisphere, explains Toni Font.
Together, these detectors have already captured 12 black hole mergers, but this week’s is unique. With the laws of stellar physics in hand, the origin of the two black holes with those masses cannot be explained by the conventional method. The theory may be incomplete and needs to be expanded to encompass these new types of monsters, whose masses place them outside what physicists know as the graveyard of stars. When stars die, they implode and their remains compress into a black hole. But this is so only for stars up to about 65 solar masses. In theory, above these masses and up to 120 solar masses, the stars that reach the end of their life explode without leaving any solid trace. The two black holes involved in this merger break out of this norm, and so does its end product: a 142 solar mass black hole, something that had never been observed.
“We are still far from understanding all the processes that end up producing black holes,” acknowledges the physicist from the European laboratory for particle physics CERN Luis Álvarez-Gaumé. “From the first observation, the results on their masses and abundances are quite surprising. There is much to learn, ”he says.
Einstein’s skepticism about the detection of gravitational waves does not detract from his worth and his clairvoyance when it comes to understanding the essence of the universe, he highlights Alicia sintes, Principal Investigator of the LIGO group at the Universitat de les Illes Balears. “When she made her prediction there was no certainty about the astronomical objects that we now know, almost nothing was known about the life cycle of a star or what fuel fuels it,” she says. The laser and its potential to make extremely high precision measurements had not been discovered either. And yet the general theory of relativity perfectly predicted the existence of black holes and other cosmic objects that would be discovered later. “There is nothing that we see that contradicts her,” summarizes Sintes.
This wave of gravity of a tenth of a second has brought us another data that makes our heads fly. The merger happened somewhere in the universe 7 billion years ago, which is how long it took to reach us. But the universe is not a static sphere, but since its birth it is constantly growing, expanding at an increasing speed. This is one of the greatest enigmas of the cosmos and to explain it physicists resort to dark energy: a force that would push the cosmos into its inflation and that is dark —or black— because we cannot see it and we have no idea what it is made of. What we do know is that due to the accelerated expansion of the cosmos, the point where this merger of black holes happened is already 17,000 million light years away, that is, we would have to travel at the speed of light for 17,000 million years – more than the age of the universe — to reach it.