LISTEN TO THE LAPPING OF GRAVITATIONAL WAVES!
Dancing black holes, neutron stars clashes, supernovae exposions,
are just some of the monstrous events that took place in the distant past, in deep space. Today,
surprisingly, we are able to hear the echo of those events thanks to a very precious new
messenger of the cosmos: gravitational waves. In order to understand what they are,
when they were discovered, and what their importance is for cosmology of the future,
stay with us, we will tell you everything during this video!
Let's do a dip into the past. We are in 1915. Albert Einstein has recently given birth to the
theory of general relativity in which he paves the way for modern cosmology. Relativity, in addition
to deeply revising the concepts of space and time, completes the law of universal gravitation
proposed by Isaac Newton in 1687. Newton, in fact, had described gravity as a force at a distance,
without however clarifying what was actually causing and transmitting it. How was it
possible that two objects far by millions of kilometers could feel the presence of each other?
Newton himself wondered; "It is inconceivable that brute and inanimate matter (without the mediation
of something immaterial), can act and influence other matter without mutual contact", he said.
In general relativity, space and time are brought together within a four-dimensional entity,
formed by the 3 spatial dimensions plus time: space-time. Einstein reformulates and
integrates Newton's gravity into a coherent framework, and clarifies that it is space-time
that determines gravity. It is the presence of matter that "deforms" the space-time
which, in return, in accordance with Newton's action-reaction principle,
acts on the matter itself, constraining it to move on very precise trajectories.
We can imagine space-time as a sheet, a very elastic, malleable, and stress-sensitive fabric
on which marbles move that we can identify as masses, celestial bodies.
Einstein goes beyond and from the equations of general relativity comes to predict the
existence of ripples within this "sheet".
When a very heavy object undergoes very sharp and intense movements, modifies the local
curvature in space, producing waves similar to those left by a speedboat sailing at sea.
During particularly extreme events, as well as accelerated electric charges,
emit radiation under forms of electromagnetic waves,
enormous accelerated masses should emit energy in the form of gravitational waves, deformations
of space-time that spreads like waves. The space-time fabric thus ripples and sways
like the surface of a lake and the oscillation is transmitted and propagated at the speed of light
We’ ve had to wait a century, but nowadays gravitational waves have been finally detected.
Before now, we were only able to detect its presence indirectly. The first clue to their
existence was obtained by studying the behavior of a pulsar orbiting another neutron star,
a system discovered in 1974 by Joseph Taylor and Russell Hulse.
Gravitational waves were detected directly for the first time on 14 September 2015 and announced
to the world on 11 February 2016. The wave was produced by a catastrophic event: the collision
of two black holes, of mass equivalent to about 29 and 36 solar masses, which approached each
other ﬁno to penetrate and give rise to a new gigantic black hole of about 62 solar masses:
the 3 solar masses missing from the total sum was expelled in the form of energy carried by
gravitational waves: imagine 3 whole «Suns» vaporized in a few moments that have travelled
millions of years through interstellar space and have come down to us in the form of waves!
Gravitational waves have a great advantage: in their journey through space-time, they interact
very weakly with matter and arrive on Earth intact and genuine as they are created at the source.
They are not absorbed and thus give us access to phenomena that occurred when the universe
and the celestial bodies were still in formation, ﬁno to push us to very remote times, dating back
to the very first moments of the universe after the Big Bang. The price to pay to be able to
listen to these imperceptible "vagaries of the cosmos" is the absolute of ﬃcoltà in the detection
that must be frighteningly precise and is subject to a whole series of disturbances and noises that
overlap and alter the correct measurement of the signal. Any terrestrial vibration (an engine on
a distant highway, a train, ocean waves miles away), produces much more relevant effects...
To give an idea of what a gravitational wave produces, we can say that the oscillation
following its passage, would change the distance between two spacecraft that fly
to 5 million kilometers of the distance of about a picometer, 100 million times less than the width
of a human hair. Isolating the detectors from any external disturbance is a sﬁda to the limit of the
possible. The sﬁda can only be won by combining the results of several different detectors,
placed in places very far from each other. At least two detectors are needed,
thousands of miles apart. Only the signal detected by both of them deserves to be analyzed.
The world scientific community has already begun to build a "network" of gravitational wave
detectors which has been gradually improved over the years and will soon be expanded.
A sort of "global ear" constantly listening to the imperceptible signals that come from
deep space. This network today includes the 2 main detectors of the LIGO experiment,
which are located in the United States, the VIRGO detector located in
Italy, in the countryside around Pisa, GEO600, which operates in Germany, near
Hannover and KAGRA (Kamioka Gravitational Wave Detector)located in the Kamioka mine in Japan.
To improve the accuracy of the survey, the network will be further expanded in the coming years.
LIGO-India will be a third "twin" of interferometers LIGO,
will be built in India and will enter into operation in 2025. And other ground and satellite
detectors are currently under construction. In addition, the detectors, since 2015 to date
have already been further refined and enhanced in their sensitivity to detect waves.
In addition, on December 9, 2 specific satellites were launched in orbit to trace signals of cosmic
events that can produce gravitational waves, such as the fusion of two neutron stars.
These are Gecam (Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor),
two twin telescopes made by the National Space Science Center of the Chinese Academy of Sciences.
LIGO (Laser Interferometer Gravitational-Wave Observatory)
is the detector that on September 14, 2015, heard, the first gravitational wave in history.
LIGO is a project that involves scientists from Caltech and the Massachusetts Institute of
Technology (MIT) in Boston and only the phase of its construction (finished in 1999) It cost $365
million and is the largest and most ambitious project ever funded by the US National Science
Foundation. One of its creators is the scientist Kip Thorne who is famous among the general public
for having been the scientific advisor to director Christopher Nolan during the writing
of the script of the ﬁlm "Interstellar", 2014. Thorne was convinced that gravitational waves
existed to the point of betting with Stephen Hawking on their discovery. LIGO is located
in the United States and consists of two twin but separate interferometers,
managed as a single large observatory that monitors and processes the data of both.
One facility is located in Livingston, Louisiana, the other in Hanford, Washington, over 3,000 miles
away. Each interferometer is 4 kilometers long and consists of two arms arranged at right angles
(in the shape of "L"), inside which run laser rays riﬂessi hundreds of times by a system of mirrors.
The lasers travel back and forth inside high vacuum tubes (diameter 1.2 meters) that allow
measuring with very high precision the distance of the events between the two super-precision
mirrors on which these rays are located. The technique used is that of interferometry:
the semi-transparent mirror divides the laser beam into two parts,
then sent into the two arms of the interferometer ﬁno to meet two other mirrors that bounce the
laser. And these mirrors are the real sensors: passing through the detectors,
the gravitational waves disturb, even if by a very little, the journey of the laser beams.
LIGO was already able to detect a variation of
this distance ﬁno at the threshold of one thousandth of the size of a proton.
With the new updates, it even pushes ﬁno to a sensitivity ten times higher, coming to
perceive contractions or elongations of a tenth of a billionth of a billionth of a meter (10^-19 m)!
VIRGO
VIRGO is a giant laser interferometer located on Italian soil,
near Cascina, a town near Pisa, on the site of the European Gravitational Observatory (EGO). was
designed and built by a collaboration between the French Centre National de la Recherche Scientiﬁque
(CNRS) and the Italian INFN. The detector has 2 arms 3 km long each and is, therefore,
a little less sensitive than LIGO, however since its first activation, which occurred in 2007,
Virgo has been in time upgraded and updated. The latest update is an advanced optical system,
already previously installed and tested on the German-British gravitational wave detector
GEO600, which has greatly improved the sensitivity of Virgo making it very similar to that of Ligo.
As technology and upgrades progress, all gravitational detectors will be able to
pick up waves from ever-deeper portions of the universe, detecting gravitational wave
sources from increasingly distant and ancient areas of the universe.
What kind of event can produce a gravitational wave?
There are various possibilities: they are all extreme cosmic events,
rather rare and complex, involving different actors, and radiating the universe of
matter and energy. The most common is the collision of black holes or pulsars,
the rotation of a neutron star, and the explosion of a supernova.
Moreover, even immediately after the big bang, in the primordial moments of the universe,
a good number of gravitational waves could have been generated
and in the cosmos, there are many other types of sources yet to be discovered.
Each event produces characteristic gravitational waves, which have characteristic
"imprint" recognizable. Gravitational waves have a very wide spectrum of variability!
In this way the collaboration between detectors and telescopes,
scientists and observatories installed all over the world on the ground and in orbit
becomes central, to gather in a cross-analysis the "messages" that come to us from the cosmos.
So far "the global ear" has picked up signals in the cosmos in 3 distinct periods, during which the
detectors have already picked up 67 gravitational waves in total that are still being studied.
Of these, 56 were revealed in the last observation period, carried out in two tranches in 2019,
compared to only 8 surveys during the two previous observation periods,
to demonstrate that the updates on the detectors made over the years are paying off!
Today because of the Covid emergency the interferometers of the earth have been
turned off from 27 past March but the beginning of the fourth phase of surveys, that will see fully
active also the Japanese interferometer Kagra, is currently in the calendar for the half of 2022.
Gravitational waves will greatly expand our scientific horizon ﬁco: the network of terrestrial
and space gravitational detectors will open a new observation ﬁnestra on the universe. We can also
use these new objects to investigate precisely the first moments of life in the universe.
If in the future we will be able to systematically combine information from electromagnetic
phenomena, neutrinos, and gravitational waves (the so-called "multi-messenger") in an overview,
we will be able in the future to have a new and detailed view of the Universe
in a new Galilean revolution. When Galileo invented the telescope
he opened a "new sky", much wider than the Ptolemaic or Copernican conceptions.
What is happening now is very similar. We still need a little patience
but what is certain is that a new era for astrophysics is upon us!