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 a team of researchers had succeeded in recording this phenomenon for the first time in the history of mankind.
Prophecies of the great Einstein
Even at the beginning of the last century (1916), Albert Einstein suggested that gravitational waves exist within 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 the many other predicted physical phenomena that were confirmed in the next century (slowing down the flow of time, changing the direction of electromagnetic radiation in gravitational fields, etc.), it was not possible to practically detect the presence of this type of wave interaction of bodies until recently.
Gravity is an illusion?
In general, in the lightThe theory of relativity can hardly call gravity a force. This is a consequence of perturbation or curvature of the space-time continuum. A good example illustrating this postulate is a stretched piece of cloth. Under the weight of a massive object placed on such a surface, a recess is formed. 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 the curvature), the higher the "force of attraction". When it moves through the fabric, you can observe the appearance of a divergent "ripple".
Something similar happens in world space. Any rapidly moving massive matter is a source of fluctuations in the density of space and time. A gravitational wave with a significant amplitude, formed by bodies with extremely large masses or when moving with huge accelerations.
Physical characteristics
The fluctuations of the space-time metric manifest themselves as changes in the gravitational field. This phenomenon is otherwise called space-time ripples. The gravitational wave acts on the encountered bodies and objects, compressing and stretching them. The deformation values are very small - about 10-21 from the original size. The whole difficulty of detecting this phenomenon was that the researchers had to learn how to measure and record such changes with the help of appropriate equipment. The power of gravitational radiation is also extremely small - for the entire solar system it isa few kilowatts.
The speed of propagation of gravitational waves slightly depends on the properties of the conducting medium. The oscillation amplitude gradually decreases with distance from the source, but never reaches zero. The frequency lies in the range from several tens to hundreds of hertz. The speed of gravitational waves in the interstellar medium is approaching the speed of light.
Circumstantial evidence
For the first time, the theoretical confirmation of the existence of gravity waves was obtained by the American astronomer Joseph Taylor and his assistant Russell Hulse in 1974. Studying the expanses of the Universe using 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 rotating around a common center of mass with a constant angular velocity (a rather rare case). Each year, the revolution period, which was originally 3.75 hours, is reduced by 70 ms. This value is quite consistent with the conclusions from the GR equations predicting an increase in the rotation speed of such systems due to the expenditure of energy for the generation of gravitational waves. Subsequently, several double pulsars and white dwarfs with similar behavior were discovered. Radio astronomers D. Taylor and R. Hulse were awarded the Nobel Prize in Physics in 1993 for discovering new possibilities for studying gravitational fields.
Escaped gravity wave
First statement aboutdetection of gravity waves came from the University of Maryland scientist Joseph Weber (USA) in 1969. For these purposes, he used two gravitational antennas of his own design, separated by a distance of two kilometers. The resonant detector was a well-vibrated one-piece two-meter aluminum cylinder equipped with sensitive piezoelectric sensors. The amplitude of the fluctuations allegedly recorded by Weber turned out to be more than a million times higher than the expected value. Attempts by other scientists using such equipment to repeat the "success" of the American physicist did not bring positive results. A few years later, Weber's work in this area was recognized as untenable, but gave impetus to the development of a "gravitational boom" that attracted many specialists to this area of research. By the way, Joseph Weber himself was sure until the end of his days that he received gravitational waves.
Improvement of receiving equipment
In the 70s, scientist Bill Fairbank (USA) developed the design of a gravitational wave antenna cooled by liquid helium using SQUIDs - supersensitive magnetometers. The technologies that existed at that time did not allow the inventor to see his product, realized in "metal".
The gravitational detector Auriga was made in this way at the National Legnard Laboratory (Padua, Italy). The design is based on an aluminum-magnesium cylinder, 3 meters long and 0.6 m in diameter. A receiving device weighing 2.3 tonssuspended in an isolated vacuum chamber cooled almost to absolute zero. An auxiliary kilogram resonator and a computer-based measuring complex are used for fixing and detecting shaking. Declared equipment sensitivity 10-20.
Interferometers
The operation of interference detectors of gravitational waves is based on the same principles as the Michelson interferometer. The laser beam emitted by the source is divided into two streams. After multiple reflections and travels along the shoulders of the device, the streams are again brought together, and the final interference image is used to judge whether any perturbations (for example, a gravitational wave) affected the course of the rays. Similar equipment has been created in many countries:
- GEO 600 (Hanover, Germany). The length of the vacuum tunnels is 600 meters.
- TAMA (Japan) 300m shoulders
- VIRGO (Pisa, Italy) is a joint Franco-Italian project launched in 2007 with 3km tunnels.
- LIGO (USA, Pacific Coast), hunting for gravity waves since 2002.
The last one is worth considering in more detail.
LIGO Advanced
The project was initiated by scientists from the Massachusetts Institute of Technology and the California Institute of Technology. Includes two observatories separated by 3 thousand km, in the states of Louisiana and Washington (the cities of Livingston and Hanford) with three identical interferometers. Length of perpendicular vacuumtunnels is 4 thousand meters. These are the largest such structures currently in operation. Until 2011, numerous attempts to detect gravity waves did not bring any results. The significant modernization carried out (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 such a coveted value of 10-21. The updated project started in September 2015, and the efforts of more than a thousand employees of the collaboration were rewarded with results.
Gravity waves detected
On September 14, 2015, advanced LIGO detectors with an interval of 7 ms recorded gravitational waves that reached our planet from the largest phenomenon that occurred on the outskirts of the observable 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, about three solar masses of matter were spent on the radiation of gravity waves in a matter of fractions of a second. The initial frequency of the gravitational waves was recorded as 35 Hz, and the maximum peak value reached 250 Hz.
The results obtained were repeatedly subjected to comprehensive verification and processing, and alternative interpretations of the data obtained were carefully cut off. Finally, on February 11 last year, the direct registration of the phenomenon predicted by Einstein was announced to the world community.
Fact illustrating the titanic work of researchers: the amplitude of fluctuations in the dimensions of the interferometer arms was 10-19m - this value is as much smaller than the diameter of an atom as it is smaller than an orange.
Further prospects
The discovery 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 in general.
In further research, scientists have high hopes for the ELSA project: the creation of a giant orbital interferometer with arms of about 5 million km, capable of detecting even minor perturbations of gravitational fields. The intensification of work in this direction can tell a lot about the main stages in the development of the Universe, about processes that are difficult or impossible to observe in traditional bands. There is no doubt that black holes, whose gravitational waves will be fixed in the future, will tell a lot about their nature.
To study the relic gravitational radiation, which can tell about the first moments of our world after the Big Bang, more sensitive space instruments will be required. Such a project exists (Big Bang Observer), but its implementation, according to experts, is possible not earlier than in 30-40 years.
