Around the world (science and Einstein)
Gravitational Waves Discovered: Einstein Was Right
Theory of relativity confirmed. A new era in the study of the universe, black holes and the fabric of spacetime. Important Italian contribution
di Paolo Virtuani
At 10.50 and 45 seconds (Italian time) on 14 September 2015 the two LIGO detectors in the US (in Washington State and Louisiana) recorded something unusual. Then they started testing. And on Thursday, at 4.30 pm in Pisa (a few minutes before a similar press conference in Washington) the official announcement of the discovery of gravitational waves was finally made. For physics and science in general, 11 February 2016 will go down as an historic date. The existence of gravitational waves was predicted almost exactly a century ago, in November 1915, when Albert Einstein illustrated his general theory of relativity, of which they are a cornerstone. However, until now their existence has never been demonstrated. The news can rightly aspire to the title of “discovery of the century”, and the researchers who participated in the experiments to a future Nobel Prize in Physics. This collective enterprise brought together 1004 researchers from 133 scientific institutions around the world.
What was observed?
The gravitational waves detected were produced in the last split second of the fusion of two black holes, whose masses were equivalent to about 29 and 36 times that of the sun, into a single rotating black hole whose mass was 62 times greater than the sun’s. The mass missing from the total is represented by the energy emitted during the fusion of the two black holes, in the form of gravitational waves. Before merging, the two black holes spiralled towards each other and then collided at a speed of around 150,000 km per second, half the speed of light. The observation also confirms the existence of binary blacks holes of stellar mass, in particular with a mass over 25 times that of the sun. The fusion of the two black holes responsible for the gravitational waves detected took place 410 megaparsecs away, and thus dates back to nearly 1.5 billion years ago.
“A milestone”
“This discovery is a milestone in the history of physics, but above all is the beginning of a new era in astrophysics,” said Fulvio Ricci, professor at La Sapienza University in Rome, a researcher for the National Institute for Nuclear Physics (INFN) and Italian head of the Virgo project. “Being able to observe the cosmos through gravitational waves radically changes our ability to study it: it is as if we had been looking at in x-rays, but can now perform an ultrasound scan.” “At last we can see the universe in a different way,” added INFN researcher Pia Astone, who oversaw the preparation of the scientific paper on the discovery with five other colleagues from LIGO and Virgo, published in Physics Reviews Letters. “It is no coincidence that the first direct measurement of the amplitude and phase of gravitational waves was accompanied by another important discovery, that of the merger of a binary black hole.”
What are they?
Gravitational waves are like tiny ripples in the fabric of the spacetime that permeates the entire universe. According to Einstein, gravity itself is the result of the curvature of spacetime caused by mass. Gravitational waves are produced by the movement of bodies with mass in spacetime. The larger the events, for example those emitting extraordinary amounts of energy, such as the Big Bang itself, the collision of two black holes, or two “dancing” pulsars, the greater the magnitude of the gravitational waves, and – at least in theory – the easier it is to detect them. However, with the instruments previously at our disposal this proved virtually impossible, because we ourselves and our instruments are also located in spacetime, and subject to its fluctuations.
Why are they important?
The discovery of gravitational waves is not just another experimental confirmation of the validity of Einstein’s theory, but revolutionizes and extends the scope of physics and cosmology. For example, until now the cosmos has been studied only by using the signals emitted by stars and galaxies in the electromagnetic spectrum (visible light, x-rays, gamma-rays, the infrared and ultraviolet, and radio waves of various wavelengths). The discovery of gravitational waves changes everything, and makes it possible to study the universe and black holes in a completely different way. In addition to “seeing”, we will also be able to “hear”, in their most basic essence, space and time, two phenomena that for Einstein are one and the same. We will also be in a position to understand how and why the universe is not only expanding, but is expanding at an ever-faster rate. There are those who predict phenomena bordering on science fiction, such as the existence of wormholes in the vicinity of black holes, which could connect to distant parts of the universe or even universes other than our own. Lastly, the discovery will allow us to unify the basic components of spacetime according to the theory of quantum mechanics, currently divided into “strings”, “branes” and “loops”. “This result is a perfect way to mark the one hundredth anniversary of general relativity,” concluded Fernando Ferroni, president of the INFN. “It’s the last piece in the puzzle of that wonderful theory produced by the genius of Einstein, and also a just reward for the group of scientists who for decades have worked on this project, to which Italy has made a great contribution.”
The announcement
The LIGO observatories recorded incoming gravitational waves on 14 September last year with an interference lasting only 10 milliseconds. As said, Italy also played a part in the discovery. Researchers at the INFN took part in the experiment, and the INFN organised a press conference in Pisa at the laboratories of the European Gravitational Observatory (EGO), which is also home to Virgo, the Italian project that worked on the study with its American counterpart (LIGO). In Washington, the announcement at the National Press Club was coordinated by the National Science Foundation, the Massachusetts Institute of Technology (MIT) and the Laser Interferometer Gravitational Wave Observatory (LIGO). In addition, the magazine Physics Reviews Letters has published a scientific study on the subject.
How were gravitational waves detected?
As mentioned above, we are immersed in spacetime, so how was it possible to detect gravitational waves? In practice, the two experiments (LIGO and Virgo) are based on two tubes, 4 km and 3 km long respectively, arranged in an L shape, in other words perpendicular to one other. Each of these tubes houses a laser beam that is reflected 50 times by mirrors in order to increase the distance it travels. If a gravitational wave passes, it expands space in one direction (one of the tubes) and contracts it in the direction perpendicular to the other (by billionths of billionths of billionths of a metre). As space is stretched, the laser light takes longer to cross one of the two arms of Virgo or LIGO, while it employs less in the perpendicular arm, where space is contracted. By analysing the reduced or delayed time of passage with extreme precision (and eliminating any type of noise), it is possible to detect the gravitational wave. It sounds simple enough, but if you manage to pull it off, you’re looking at a Nobel prize.
English translation by Simon Tanner
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