The official day of discovery (detection) of gravitational waves is February 11, 2016. It was then, at a press conference in Washington, that the leaders of the LIGO collaboration announced that the team of researchers was able to fix this phenomenon for the first time in human history.
Prophecies of the Great Einstein
The fact that gravitational waves exist was already at the beginning of the last century (1916) suggested by Albert Einstein in the framework of the General Theory of Relativity (GR) formulated by him. One can only marvel at the brilliant abilities of the famous physicist, who, with a minimum of real data, was able to draw such far-reaching conclusions. Among many other predicted physical phenomena that have been confirmed in the next century (slowing down the flow of time, changing the direction of electromagnetic radiation in gravitational fields, etc.), it was practically impossible to detect the presence of this type of wave interaction of bodies until recently.
Is gravity an illusion?
In general, in the light of the Theory of Relativity, gravity can hardly be called force. This is a consequence of the perturbation or curvature of the space-time continuum. A good example illustrating this postulate is a stretched piece of fabric. Under the weight of a massive object placed on such a surface, a depression forms. Other objects moving near this anomaly will change the trajectory of their movement, as if "attracted". And the greater the weight of the object (the greater the diameter and depth of curvature), the higher the "gravity". When it moves along the fabric, one can observe the appearance of diverging "ripples".
Something similar is happening in world space. Any accelerated moving massive matter is a source of fluctuations in the density of space and time. A gravitational wave with a significant amplitude is formed by bodies with extremely large masses or when moving with huge accelerations.
physical characteristics
Oscillations of the space-time metric manifest themselves as changes in the gravitational field. This phenomenon is otherwise called spatiotemporal ripple. A gravitational wave acts on the bodies and objects encountered, compressing and stretching them. Deformation values are very insignificant - about 10 -21 from the original size. The whole difficulty in detecting this phenomenon was that the researchers needed to learn how to measure and record such changes using appropriate equipment. The power of gravitational radiation is also extremely small - for the entire solar system it is several kilowatts.
The propagation velocity of gravitational waves slightly depends on the properties of the conducting medium. The amplitude of the oscillations with distance from the source gradually decreases, but never reaches zero. The frequency lies in the range from several tens to hundreds of hertz. The speed of gravitational waves in an interstellar medium approaches the speed of light.
Indirect evidence
For the first time, the theoretical confirmation of the existence of gravitational waves was obtained by the American astronomer Joseph Taylor and his assistant Russell Hals in 1974. Studying the vastness of the universe with the radio telescope of the Arecibo Observatory (Puerto Rico), the researchers discovered the pulsar PSR B1913 + 16, which is a binary system of neutron stars orbiting a common center of mass with a constant angular velocity (a rather rare case). Every year, the circulation period, initially of 3.75 hours, is reduced by 70 ms. This value is consistent with the conclusions of the equations of general relativity predicting an increase in the speed of rotation of such systems due to the expenditure of energy on the generation of gravitational waves. Subsequently, several double pulsars and white dwarfs with similar behavior were discovered. In 1993, radio astronomers D. Taylor and R. Hals were awarded the Nobel Prize in Physics for discovering new possibilities for studying gravitational fields.
Escaping gravitational wave
The first application for the detection of gravitational waves came from University of Maryland Joseph Weber (USA) in 1969. For these purposes, he used two gravitational antennas of his own design, spaced two kilometers apart. The resonance detector was a well-insulated, one-meter, two-meter aluminum cylinder equipped with sensitive piezoelectric sensors. The amplitude of the oscillations allegedly recorded by Weber turned out to be more than a million times higher than the expected value. Attempts by other scientists using similar equipment to repeat the "success" of the American physicist did not bring positive results. After several years, Weber’s work in this area was declared insolvent, but gave impetus to the development of the “gravitational boom”, which attracted many specialists to this field of research. By the way, Joseph Weber himself until the end of his days was sure that he received gravitational waves.
Improving the receiving equipment
In the 70s, the scientist Bill Fairbank (USA) developed the design of a gravitational wave antenna cooled with liquid helium using squids - ultra-sensitive magnetometers. The technologies existing at that time did not allow the inventor to see his product implemented in "metal".
According to this principle, the Auriga gravitational detector was made at the National Legniar Laboratory (Padova, Italy). The design is based on an aluminum-magnesium cylinder with a length of 3 meters and a diameter of 0.6 m. The receiver, weighing 2.3 tons, is suspended in an insulated vacuum chamber cooled to almost absolute zero. An auxiliary kilogram resonator and a computer-based measuring complex are used to fix and detect tremors. The declared sensitivity of the equipment is 10 -20 .
Interferometers
The functioning of interference detectors of gravitational waves is based on the same principles by which the Michelson interferometer works. The laser beam emitted by the source is divided into two streams. After multiple reflections and travels on the shoulders of the device, the flows are again brought together, and the final interference image is used to judge whether any disturbances (for example, a gravitational wave) affected the course of the rays. Similar equipment was created in many countries:
- GEO 600 (Hanover, Germany). The length of the vacuum tunnels is 600 meters.
- TAMA (Japan) with shoulders of 300 m.
- VIRGO (Pisa, Italy) is a joint Franco-Italian project launched in 2007 with three-kilometer tunnels.
- LIGO (USA, Pacific Coast), a hunt for gravity waves since 2002.
The latter is worth considering in more detail.
LIGO Advanced
The project was created on the initiative of scientists at the Massachusetts and California Institute of Technology. It includes two observatories, spaced over 3 thousand km, in the states of Louisiana and Washington (the cities of Livingston and Hanford) with three identical interferometers. The length of the perpendicular vacuum tunnels is 4 thousand meters. These are the largest operating similar structures to date. Until 2011, numerous attempts to detect gravitational waves brought no results. A significant modernization (Advanced LIGO) increased the sensitivity of the equipment in the range of 300-500 Hz by more than five times, and in the low-frequency region (up to 60 Hz) by almost an order of magnitude, reaching the desired value of 10 -21 . An updated project was launched in September 2015, and the efforts of more than a thousand collaborators were rewarded with the results.
Gravitational waves detected
On September 14, 2015, advanced LIGO detectors with an interval of 7 ms detected gravitational waves reaching our planet from the largest phenomenon that occurred on the outskirts of the observed Universe - the merger of two large black holes with masses 29 and 36 times the mass of the Sun. During the process, which took place more than 1.3 billion years ago, in just a fraction of a second, about three solar masses of the substance were consumed by the radiation of gravitational waves. The recorded initial frequency of gravitational waves was 35 Hz, and the maximum peak value reached 250 Hz.
The results obtained were repeatedly subjected to comprehensive verification and processing; alternative interpretations of the obtained data were carefully cut off. Finally, on February 11 of last year, direct registration of the phenomenon predicted by Einstein was announced to the world community.
A fact illustrating the titanic work of researchers: the amplitude of fluctuations in the size of the arms of interferometers was 10 -19 m - this value is as many times smaller than the diameter of an atom, as much as it is smaller than an orange.
Further perspectives
The discovery made once again confirms that the General Theory of Relativity is not just a set of abstract formulas, but a fundamentally new look at the essence of gravitational waves and gravity as a whole.
In further studies, scientists have high hopes for the ELSA project: the creation of a giant orbital interferometer with shoulders of about 5 million km, capable of detecting even minor disturbances of gravitational fields. The intensification of work in this direction is able to tell a lot of new things about the main stages of the development of the Universe, about processes that are difficult or impossible to observe in traditional ranges. There is no doubt that black holes, whose gravitational waves will be recorded in the future, will tell a lot about their nature.
To study the relic gravitational radiation that can tell about the first moments of our world after the Big Bang, more sensitive space tools will be required. Such a project exists ( Big Bang Observer ), but its implementation, according to experts, is possible no earlier than in 30-40 years.