The official day of discovery (detection) of gravitational waves is February 11, 2016. It was then, at a press conference held in Washington, the leaders of the LIGO collaboration announced that a team of researchers had managed to record this phenomenon for the first time in human history.
Prophecies of the great Einstein
Albert Einstein suggested that gravitational waves exist at the beginning of the last century (1916) within the framework of the General Theory of Relativity (GR) formulated by him. It remains only to be amazed 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 found confirmation in the next century (slowing down the flow of time, changing the direction of electromagnetic radiation in gravitational fields, etc.), until recently, it was not possible to practically detect the presence of this type of wave interaction of bodies.
Is gravity an illusion?
In general, in the light of the theory of relativity, gravity can hardly be called a force. disturbances or distortions of the space-time continuum. A stretched piece of cloth is a good example to illustrate this postulate. A depression is formed under the weight of a massive object placed on such a surface. Other objects moving near this anomaly will change their trajectory, as if "gravitating". And the greater the weight of the object (the greater the diameter and depth of curvature), the higher the "force of attraction". When it moves along the tissue, you can observe the emergence of diverging "ripples".
Something similar is happening in world space. Any rapidly moving massive matter is a source of fluctuations in the density of space and time. A gravitational wave with 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 spatio-temporal ripples. The gravitational wave affects the bodies and objects encountered, compressing and stretching them. The deformation values \u200b\u200bare very small - about 10 -21 of the original size. The whole difficulty of detecting this phenomenon was that researchers needed 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 is several kilowatts.
The speed of propagation of gravitational waves slightly depends on the properties of the conducting medium. The vibration amplitude gradually decreases with distance from the source, but never reaches zero. The frequency ranges from several tens to hundreds of hertz. The speed of gravitational waves in the interstellar medium approaches the speed of light.
Indirect evidence
For the first time, the American astronomer Joseph Taylor and his assistant Russell Hulse managed to obtain a theoretical confirmation of the existence of gravitational waves in 1974. Studying the vastness of the universe with the radio telescope of the Arecibo Observatory (Puerto Rico), the researchers discovered the PSR B1913 + 16 pulsar, which is a binary system of neutron stars revolving around a common center of mass with a constant angular velocity (a rather rare case). Each year, the orbital period, which was initially 3.75 hours, is reduced by 70 ms. This value is quite consistent with the conclusions from the equations of general relativity, which predict an increase in the speed of rotation of such systems due to the expenditure of energy to generate gravitational waves. Later, several binary pulsars and white dwarfs with similar behavior were discovered. Radio astronomers D. Taylor and R. Hals in 1993 was awarded Nobel Prize in physics for the discovery of new possibilities for studying gravitational fields.
Escaping gravity wave
The first application for the detection of gravitational 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 resonance detector was a well-vibration-insulated one-piece 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 to repeat the "success" of the American physicist with the help of similar equipment did not bring positive results. A few years later, Weber's work in this area was deemed untenable, but gave impetus to the development of a "gravitational boom", which attracted many specialists to this area of \u200b\u200bresearch. By the way, Joseph Weber himself until the end of his days was sure that he received gravitational waves.
Improvement of receiving equipment
In the 70s, scientist Bill Fairbank (USA) developed a gravitational-wave antenna, cooled using SQUIDs - ultrasensitive magnetometers. The technologies existing at that time did not allow the inventor to see his product, realized in "metal".
This is the principle behind the Auriga gravitational detector at the Legnard National Laboratory (Padua, Italy). The design is based on an aluminum-magnesium cylinder, 3 meters long and 0.6 m in diameter. The receiving device weighing 2.3 tons is suspended in an insulated vacuum chamber cooled to almost absolute zero. For fixing and detecting shocks, an auxiliary kilogram resonator and a computer-based measuring complex are used. The declared sensitivity of the equipment is 10 -20.
Interferometers
The operation of the 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 split into two streams. After multiple reflections and travels along the arms of the device, the streams are again brought together, and the final judgment is used to judge whether any disturbances (for example, a gravitational wave) affected the path of the rays. Similar equipment has been created in many countries:
- GEO 600 (Hannover, 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), leading the hunt for gravity waves since 2002.
The latter is worth considering in more detail.
LIGO Advanced
The project was initiated by scientists from the Massachusetts and California Institute of Technology. It includes two observatories, separated by 3 thousand km, in and Washington (the cities of Livingston and Hanford) with three identical interferometers. The length of the perpendicular vacuum tunnels is 4,000 meters. These are the largest such structures currently in operation. Until 2011, numerous attempts to detect gravitational waves did not bring any results. The significant modernization (Advanced LIGO) has 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 coveted value of 10 -21. The 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, improved 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 observed Universe - the merger of two large black holes with masses 29 and 36 times the mass of the Sun. In the course of the process, which took place more than 1.3 billion years ago, in a matter of fractions of a second, about three solar masses of matter were spent on 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, and alternative interpretations of the data obtained were carefully cut off. Finally, last year, the direct registration of the phenomenon predicted by Einstein was announced to the world community.
A fact that illustrates the titanic work of researchers: the amplitude of fluctuations in the size of the arms of the interferometers was 10 -19 m - this value is as much less than the diameter of an atom, as much as it itself is smaller than an orange.
Further prospects
The discovery made once again confirms that the General theory of relativity is not just a set of abstract formulas, but in principle a New Look on 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 disturbances of gravitational fields. The intensification of work in this direction can tell a lot about the main stages of the development of the Universe, about the processes, the observation of which in traditional ranges is difficult or impossible. Undoubtedly, black holes, whose gravitational waves will be recorded in the future, will tell a lot about their nature.
To study the relict 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 the assurances of experts, is possible no earlier than in 30-40 years.
The free surface of a liquid in equilibrium in the gravity field is flat. If, under the influence of some external influence, the surface of the liquid in some place is removed from its equilibrium position, then motion arises in the liquid. This movement will propagate along the entire surface of the liquid in the form of waves, called gravitational waves, since they are caused by the action of the gravity field. Gravitational waves occur mainly on the surface of the liquid, capturing its inner layers, the smaller the deeper these layers are.
We will consider here such gravitational waves in which the speed of the moving particles of the liquid is so small that in the Euler equation one can neglect the term in comparison with It is easy to find out what this condition means physically. During a time interval of the order of the period of oscillations made by the liquid particles in the wave, these particles travel a distance of the order of the amplitude a of the wave, therefore their velocity of motion is of the order waves). Therefore, the derivative of the velocity with respect to time is of the order and with respect to coordinates is of the order. Thus, the condition is equivalent to the requirement
that is, the amplitude of the oscillations in the wave must be small compared to the wavelength. In § 9 we saw that if the term in the equation of motion can be neglected, then the motion of the fluid is potential. Assuming the fluid is incompressible, we can therefore use equations (10.6) and (10.7). In equation (10.7) we can now neglect the term containing the square of the velocity; putting and entering the term in the gravity field, we get:
(12,2)
We choose the axis, as usual, vertically upward, and as the plane x, y we choose the equilibrium flat surface of the liquid.
We will denote - the coordinate of points of the liquid surface by; is a function of coordinates x, y and time t. In equilibrium, so there is a vertical displacement of the liquid surface during its oscillations.
Let constant pressure act on the surface of the liquid.Then we have on the surface according to (12.2)
The constant can be eliminated by redefining the potential (adding to it a coordinate-independent quantity.Then the condition on the liquid surface takes the form
The smallness of the amplitude of oscillations in the wave means that the displacement is small. Therefore, we can assume, in the same approximation, that the vertical component of the speed of movement of points on the surface coincides with the time derivative of the displacement But so we have:
Due to the smallness of oscillations, in this condition, we can take the values \u200b\u200bof the derivatives at instead of Thus, we finally obtain the following system of equations that determine the motion in a gravitational wave:
We will consider waves on the surface of a liquid, assuming this surface is unlimited. We will also assume that the wavelength is small compared to the depth of the liquid; then the liquid can be considered as infinitely deep. Therefore, we do not write boundary conditions at the lateral boundaries and at the bottom of the liquid.
Consider a gravitational wave propagating along the axis and uniform along the axis in such a wave, all quantities do not depend on the coordinate y. We will look for a solution that is a simple periodic function of time and coordinates x:
where (is the cyclic frequency (we will talk about it simply as a frequency), k is the wave vector of the wave, is the wavelength. Substituting this expression into the equation, we obtain the equation for the function
Its solution, which decays deep into the liquid (i.e., at):
We must still satisfy boundary condition (12.5) Substituting (12.5) into it, we find the relationship between the frequency b by the wave vector (or, as they say, the wave dispersion law):
The distribution of velocities in the fluid is obtained by differentiating the potential along the coordinates:
We see that the speed drops exponentially in the direction of the liquid. At each given point in space (i.e., for given x, z), the velocity vector rotates uniformly in the x-plane, remaining constant in magnitude.
Let us also define the trajectory of the liquid particles in the wave. Let us temporarily denote by x, z the coordinates of a moving fluid particle (and not the coordinates of a fixed point in space), but by - x values \u200b\u200bfor the equilibrium position of the particle. Then a on the right-hand side of (12.8) can be approximately written instead, taking advantage of the smallness of the oscillations. Integration over time then gives:
Thus, liquid particles describe circles around points with a radius exponentially decreasing in the direction into the depth of the liquid.
The velocity U of wave propagation is, as will be shown in § 67, Substituting here we find that the velocity of propagation of gravitational waves on an unbounded surface of an infinitely deep fluid is
It grows with increasing wavelength.
Long gravitational waves
Having considered gravitational waves, the length of which is small compared to the depth of the liquid, let us now dwell on the opposite limiting case of waves, the length of which is large compared to the depth of the liquid.
Such waves are called long waves.
Let us first consider the propagation of long waves in a channel. The length of the channel (directed along the x axis) will be considered unbounded. The channel section can have an arbitrary shape and can vary along its length. The cross-sectional area of \u200b\u200bthe fluid in the channel is denoted by The depth and width of the channel are assumed to be small compared to the wavelength.
We will consider here longitudinal long waves in which the liquid moves along the channel. In such waves, the velocity component along the channel length is large in comparison with the components
By simply denoting v and omitting the small terms, we can write the -component of the Euler equation in the form
and -component - in the form
(we omit the terms quadratic in the velocity, since the wave amplitude is still considered small). From the second equation we have, noting that on free surface ) should be
Substituting this expression into the first equation, we get:
The second equation for determining the two unknowns can be derived by a method similar to the derivation of the continuity equation. This equation is essentially the equation of continuity as applied to the case under consideration. Let us consider the volume of liquid enclosed between two planes of the channel cross-section located at a distance from each other. In a unit of time, a volume of liquid will enter through one plane, and a volume will exit through the other plane.Therefore, the volume of liquid between both planes will change by
A hundred years after the theoretical prediction made by Albert Einstein within the framework of general relativity, scientists were able to confirm the existence of gravitational waves. The era of a fundamentally new method of studying distant space - gravitational wave astronomy - begins.
Discoveries are different. There are random ones, they are often found in astronomy. There are some not entirely random ones, made as a result of a thorough "combing the area", such as, for example, the discovery of Uranus by William Herschel. There are serendipities - when they were looking for one thing and found another: for example, they discovered America. But planned discoveries occupy a special place in science. They are based on clear theoretical prediction. What is predicted is sought primarily in order to confirm the theory. Such discoveries include the detection of the Higgs boson at the Large Hadron Collider and the registration of gravitational waves using the laser-interferometric gravitational-wave observatory LIGO. But in order to register some phenomenon predicted by theory, you need to understand quite well what exactly and where to look, as well as what tools are needed for this.
Gravitational waves are traditionally called the prediction of the general theory of relativity (GR), and this is indeed so (although now such waves are present in all models that are alternative to GR or complement it). The appearance of waves is caused by the finiteness of the speed of propagation of the gravitational interaction (in general relativity, this speed is exactly equal to the speed of light). Such waves are perturbations of space-time propagating from a source. For the appearance of gravitational waves, it is necessary for the source to pulsate or accelerate, but in a certain way. Let's say movements with perfect spherical or cylindrical symmetry are not suitable. There are many such sources, but they often have a small mass, insufficient to generate a powerful signal. After all, gravity is the weakest of the four fundamental interactions, so it is very difficult to register a gravitational signal. In addition, for registration, it is necessary that the signal changes rapidly over time, that is, has a sufficiently high frequency. Otherwise, we will not be able to register it, since the changes will be too slow. This means that the objects must also be compact.
Initially, great enthusiasm was caused by supernova explosions that occur in galaxies like ours every few decades. This means that if you can achieve a sensitivity that allows you to see the signal from a distance of several million light years, you can count on several signals per year. But later it turned out that the initial estimates of the power of energy release in the form of gravitational waves during a supernova explosion were too optimistic, and such a weak signal could be recorded only if a supernova had erupted in our Galaxy.
Another option for massive, compact objects that move quickly are neutron stars or black holes. We can see either the process of their formation, or the process of interaction with each other. The last stages of the collapse of stellar cores, leading to the formation of compact objects, as well as the last stages of the merger of neutron stars and black holes, last on the order of a few milliseconds (which corresponds to a frequency of hundreds of hertz) - just what you need. At the same time, a lot of energy is released, including (and sometimes mainly) in the form of gravitational waves, since massive compact bodies perform certain fast movements. These are our ideal sources.
True, supernovae break out in the Galaxy every few decades, neutron stars merge once every couple of tens of thousands of years, and black holes merge with each other even less often. But the signal is much more powerful, and its characteristics can be accurately calculated. But now we need to learn to see a signal from a distance of several hundred million light years in order to cover several tens of thousands of galaxies and detect several signals per year.
Having decided on the sources, let's start designing the detector. To do this, you need to understand what the gravitational wave does. Without going into details, we can say that the passage of a gravitational wave causes a tidal force (normal lunar or solar tides are a separate phenomenon, and gravitational waves have nothing to do with it). So you can take, for example, a metal cylinder, equip with sensors and study its vibrations. It is not difficult, therefore, such installations began to be made half a century ago (they are also in Russia, now an improved detector, developed by the team of Valentin Rudenko from the SAI MSU, is being mounted in the Baksan underground laboratory). The problem is that such a device will see the signal without any gravitational waves. There are tons of noises that are difficult to deal with. It is possible (and it was done!) To install the detector underground, try to isolate it, cool it down to low temperatures, but still, in order to exceed the noise level, a very powerful gravitational-wave signal is needed. And strong signals are rare.
Therefore, a choice was made in favor of a different scheme, which was put forward in 1962 by Vladislav Pustovoit and Mikhail Hertsenstein. In an article published in ZhETF (Journal of Experimental and Theoretical Physics), they proposed using a Michelson interferometer to register gravitational waves. A laser beam runs between mirrors in two arms of the interferometer, and then the beams from different arms are added. By analyzing the result of the interference of the rays, the relative change in the length of the arms can be measured. These are very accurate measurements, so if you beat the noise, you can achieve fantastic sensitivity.
In the early 1990s, it was decided to build several detectors according to this scheme. The first to enter service were relatively small units, GEO600 in Europe and TAMA300 in Japan (the numbers correspond to the length of the arms in meters) for the technology run-in. But the main players were to be the LIGO installations in the USA and VIRGO in Europe. The size of these devices is already measured in kilometers, and the final planned sensitivity should have allowed seeing dozens, if not hundreds of events per year.
Why multiple appliances are needed? Primarily for cross-validation, as there is localized noise (eg seismic). The simultaneous registration of the signal in the northwestern United States and in Italy would be excellent evidence of its external origin. But there is a second reason: gravitational-wave detectors are very poor at determining the direction to the source. But if there are several spaced detectors, it will be possible to indicate the direction quite accurately.
Laser giants
In their original form, LIGO detectors were built in 2002, and VIRGO detectors in 2003. According to the plan, this was only the first stage. All installations were in operation for several years, and in 2010-2011 they were stopped for revision, in order to then reach the planned high sensitivity. The LIGO detectors were first operational in September 2015, VIRGO is due to join in the second half of 2016, and starting from this stage, the sensitivity allows us to hope to register at least a few events a year.
Since the start of LIGO, the expected rate of bursts was approximately one event per month. Astrophysicists have estimated in advance that the first expected events should be mergers of black holes. This is due to the fact that black holes are usually ten times heavier than neutron stars, the signal is more powerful, and it is "visible" from large distances, which more than compensates for the lower rate of events per galaxy. Fortunately, it didn't take long. On September 14, 201 5, both installations recorded an almost identical signal, which was named GW150914.
With pretty simple analysis data such as black hole masses, signal strength, and distance to the source can be obtained. The mass and size of black holes are related in a very simple and well-known way, and from the signal frequency one can immediately estimate the size of the energy release region. In this case, the size indicated that a black hole with a mass of more than 60 solar masses was formed from two holes of 25-30 and 35-40 solar masses. Knowing this data, you can get the full energy of the burst. Almost three masses of the Sun have passed into gravitational radiation. This corresponds to the luminosity of 1023 luminosities of the Sun - about the same as during this time (hundredths of a second) all stars in the visible part of the Universe emit. And from the known energy and the magnitude of the measured signal, the distance is obtained. The large mass of the merged bodies made it possible to register an event that occurred in a distant galaxy: the signal went to us for about 1.3 billion years.
A more detailed analysis makes it possible to clarify the mass ratio of black holes and understand how they rotated around their axis, as well as to determine some other parameters. In addition, the signal from two installations allows you to approximately determine the direction of the burst. Unfortunately, so far the accuracy is not very high here, but with the commissioning of the updated VIRGO it will increase. And in a few years, the Japanese KAGRA detector will start receiving signals. Then one of the LIGO detectors (initially there were three, one of the installations was dual) will be assembled in India, and it is expected that then many dozens of events will be recorded a year.
The era of new astronomy
The most important result of LIGO's work so far is the confirmation of the existence of gravitational waves. In addition, the very first burst made it possible to improve the constraints on the graviton mass (in general relativity it has zero mass), as well as to more strongly limit the difference between the propagation velocity of gravity and the velocity of light. But scientists hope that in 2016 they will be able to get a lot of new astrophysical data using LIGO and VIRGO.
First, data from gravitational-wave observatories are a new channel for studying black holes. If earlier it was possible only to observe the flows of matter in the vicinity of these objects, now you can directly “see” the process of merging and “calming down” of the resulting black hole, how its horizon vibrates, taking on its final shape (determined by rotation). Probably, up to the discovery of Hawking evaporation of black holes (so far this process remains a hypothesis), the study of mergers will provide the best direct information about them.
Secondly, observations of mergers of neutron stars will provide a lot of new, badly needed information about these objects. For the first time, we will be able to study neutron stars the way physicists study particles: observe their collisions to understand how they work inside. The mystery of the structure of the bowels of neutron stars worries both astrophysicists and physicists. Our understanding of nuclear physics and the behavior of matter at ultrahigh density is incomplete without addressing this issue. It is quite probable that gravitational-wave observations will play a key role here.
It is believed that mergers of neutron stars are responsible for short cosmological gamma-ray bursts. In rare cases, it will be possible to simultaneously observe an event both in the gamma range and on gravitational wave detectors (the rarity is due to the fact that, firstly, the gamma signal is concentrated in a very narrow beam, and it is not always directed at us, but secondly, we will not register gravitational waves from very distant events). Apparently, it will take several years of observation to be able to see this (although, as usual, you can get lucky, and it will happen today). Then, among other things, we can very accurately compare the speed of gravity with the speed of light.
Thus, laser interferometers will work together as a single gravitational-wave telescope, bringing new knowledge to both astrophysicists and physicists. Well, sooner or later the well-deserved Nobel Prize will be awarded for the discovery of the first bursts and their analysis.
, USA
© REUTERS, Handout Gravitational waves are finally open
Popular ScienceOscillations in space-time were discovered a century after they were predicted by Einstein. A new era in astronomy begins.
Scientists were able to detect fluctuations in space-time caused by the merging of black holes. This happened a hundred years after Albert Einstein predicted these "gravitational waves" in his general theory of relativity, and a hundred years after physicists started looking for them.
This landmark discovery was reported today by researchers at the LIGO Laser Interferometric Gravitational Wave Observatory. They confirmed the rumors that had surrounded the analysis of the first set of data they had collected for months. Astrophysicists say that the discovery of gravitational waves allows a new look at the universe and makes it possible to recognize distant events that cannot be seen with optical telescopes, but you can feel and even hear their faint tremors reaching us through space.
“We have detected gravitational waves. We did it!" Announced David Reitze, executive director of the 1,000-member research team, speaking today at a press conference in Washington at the National Science Foundation.
Gravitational waves are perhaps the most elusive phenomenon from Einstein's predictions; the scientist discussed this topic with his contemporaries for decades. According to his theory, space and time form stretching matter, which bends under the influence of heavy objects. Feeling gravity means getting into the curves of this matter. But can this space-time tremble like the skin of a drum? Einstein was confused, he didn't know what his equations meant. And he repeatedly changed his point of view. But even the staunchest supporters of his theory believed that gravitational waves were too weak to be observed anyway. They cascade outward after certain cataclysms, and as they move, alternately stretch and contract space-time. But by the time these waves reach the Earth, they stretch and compress every kilometer of space by a tiny fraction of the diameter of an atomic nucleus.
© REUTERS, Hangout LIGO Observatory Detector in Hanford, Washington
It took patience and caution to detect these waves. The LIGO observatory launched laser beams back and forth along the four-kilometer elbows of two detectors at right angles, one in Hanford, Washington and the other in Livingston, Louisiana. This was done in search of coinciding expansions and contractions of these systems during the passage of gravitational waves. Using state-of-the-art stabilizers, vacuum instruments and thousands of sensors, the scientists measured changes in the length of these systems, amounting to only one thousandth of the size of a proton. Such sensitivity of instruments was unthinkable a hundred years ago. It seemed incredible in 1968, when Rainer Weiss from Massachusetts institute of Technology conceived an experiment called LIGO.
“It's a great miracle that in the end they succeeded. They were able to detect these tiny vibrations! " - said the theoretical physicist from the University of Arkansas, Daniel Kennefick, who wrote in 2007 the book Traveling at the Speed \u200b\u200bof Thought: Einstein and the Quest for Gravitational Waves (Traveling with the speed of thought. Einstein and the search for gravitational waves).
This discovery marked the beginning of a new era in gravitational wave astronomy. It is hoped that we will have more accurate ideas about the formation, composition and galactic role of black holes - these superdense mass balls that distort space-time so dramatically that even light cannot escape from there. When black holes come close to each other and merge, they generate an impulse signal - space-time oscillations that increase in amplitude and tone, and then abruptly end. The signals that the observatory can record are in the audio range - however, they are too weak to be heard by the naked ear. You can recreate this sound by running your fingers over the piano keys. “Start at the lowest note and work up to the third octave,” Weiss said. "This is what we hear."
Physicists are already amazed at the number and strength of signals that have been recorded at the moment. This means that there are more black holes in the world than previously thought. “We're lucky, but I've always counted on such luck,” said Caltech astrophysicist Kip Thorne, who created LIGO with Weiss and Ronald Drever, who are also from Caltech. "This usually happens when a completely new window opens up in the universe."
Having eavesdropped on gravitational waves, we can form completely different ideas about space, and perhaps we will discover unimaginable cosmic phenomena.
“I can compare this to the moment when we first pointed a telescope up into the sky,” said theoretical astrophysicist Janna Levin of Barnard College, Columbia University. "People realized that there was something there, and you can see it, but they could not predict the incredible set of possibilities that exist in the universe." Likewise, Levin noted, the discovery of gravitational waves could show that the universe is "full of dark matter that we can't just detect with a telescope."
The story of the discovery of the first gravitational wave began on Monday morning in September, and it began with a clap. The signal was so clear and loud that Weiss thought: "No, this is nonsense, nothing will come of it."
Intensity of emotions
This first gravitational wave swept across the upgraded LIGO's detectors - first in Livingston and seven milliseconds later in Hanford - during a simulation run early in the morning of September 14, two days before the official start of data collection.
The detectors were "run-in" after a five-year upgrade that cost $ 200 million. They were equipped with new mirrored suspensions for noise reduction and an active feedback to suppress extraneous vibrations in real time. The modernization gave the improved observatory more high level sensitivity compared to the old LIGO, which found "absolute and pure zero" between 2002 and 2010, as Weiss put it.
When a powerful signal came in September, scientists in Europe, where it was morning at that moment, began hastily to sleep on their american colleagues e-mail messages. When the rest of the group woke up, the news spread very quickly. Almost everyone was skeptical about this, Weiss said, especially when they saw the signal. It was a real textbook classic, and so some people thought it was a fake.
Misconceptions in the search for gravitational waves have been repeated many times since the late 1960s, when Joseph Weber of the University of Maryland believed he had discovered resonant vibrations in an aluminum cylinder with sensors in response to waves. In 2014, an experiment called BICEP2 took place, according to the results of which it was announced that the original gravitational waves were detected - the space-time oscillations from the Big Bang, which have now stretched out and permanently frozen in the geometry of the universe. Scientists from the BICEP2 team announced their discovery with great fanfare, but then their results were independently verified, during which it turned out that they were wrong, and that this signal came from cosmic dust.
When Arizona State University cosmologist Lawrence Krauss heard about the LIGO team's discovery, he first thought it was a "blind stuff". During the operation of the old observatory, simulated signals were surreptitiously inserted into data streams to check the response, and most of the team did not know about it. When Krauss learned from a knowledgeable source that this time it was not "blind stuffing", he could hardly contain his joyful excitement.
On September 25, he tweeted to his 200,000 Twitter followers: “Rumors of a gravitational wave detected on the LIGO detector. Amazing if true. I'll give you the details, if it's not a linden tree. " This is followed by a January 11 entry: “Earlier rumors about LIGO have been confirmed by independent sources. Follow the news. Perhaps gravitational waves are discovered! "
The official position of scientists was as follows: do not dwell on the received signal until there is one hundred percent certainty. Thorne, bound hand and foot by this commitment to secrecy, did not even say anything to his wife. “I celebrated alone,” he said. To begin with, the scientists decided to go back to the very beginning and analyze everything to the smallest detail in order to find out how the signal propagated through the thousands of measurement channels of various detectors, and to understand if there was something strange at the moment the signal was detected. They found nothing out of the ordinary. They also eliminated the hackers who should have known best about the thousands of data streams during the experiment. “Even when the team throws in, they are not perfect enough and leave a lot of footprints in their wake,” Thorne said. "And there were no traces here."
In the weeks that followed, they heard another, weaker signal.
Scientists analyzed the first two signals, and they received more and more. In January, they presented their research papers in Physical Review Letters. This issue is on the Internet today. According to their estimates, the statistical significance of the first, most powerful signal exceeds the "5-sigma", which means that researchers are 99.9999% confident in its authenticity.
Listening to gravity
Einstein's equations of general relativity are so complex that it took most physicists 40 years to agree: yes, gravitational waves exist and can be detected - even theoretically.
At first, Einstein thought that objects could not release energy in the form of gravitational radiation, but then he changed his point of view. In his historical work, written in 1918, he showed what objects can do this: dumbbell-shaped systems that rotate simultaneously around two axes, for example, binaries and supernovae that explode like firecrackers. It is they who can generate waves in space-time.
© REUTERS, Handout Computer model illustrating the nature of gravitational waves in the solar system
But Einstein and his colleagues continued to hesitate. Some physicists have argued that even if waves exist, the world will vibrate with them, and it will be impossible to feel them. And only in 1957, Richard Feynman (Richard Feynman) closed this issue, demonstrating in the course thought experimentthat if gravitational waves exist, theoretically they can be detected. But no one knew how common these dumbbell systems were in outer space, or how strong or weak the resulting waves were. “Ultimately, the question was: will we ever be able to find them?” Kennefick said.
In 1968, Rainer Weiss was a young professor at the Massachusetts Institute of Technology and was assigned to teach a course in general relativity. As an experimenter, he knew little about it, but suddenly there was news of Weber's discovery of gravitational waves. Weber built three desk-sized resonance detectors out of aluminum and placed them in different American states. Now he said that all three detectors recorded "the sound of gravitational waves."
Weiss's students were asked to explain the nature of gravitational waves and express their opinion about the message sounded. Studying the details, he was amazed at the complexity of the mathematical calculations. “I couldn't figure out what the hell Weber was doing, how the sensors interact with the gravitational wave. I sat for a long time and asked myself: "What is the most primitive thing I can think of to detect gravitational waves?" And then an idea came to my mind, which I call the conceptual basis of LIGO. "
Imagine three objects in space-time, say, mirrors at the corners of a triangle. “Send a light signal from one to the other,” Weber said. "See how long it takes to move from one mass to another, and check if the time has changed." It turns out, the scientist noted, that this can be done quickly. “I entrusted this to my students as a scientific assignment. Literally the whole group was able to do these calculations. "
In subsequent years, when other researchers tried to replicate the results of Weber's experiment with a resonant detector, but constantly failed (it is not clear what he observed, but these were not gravitational waves), Weiss began to prepare a much more accurate and ambitious experiment: the gravitational wave interferometer. The laser beam reflects off three L-shaped mirrors to form two beams. The spacing of the peaks and troughs of light waves accurately indicates the length of the “G” knees that create the X and Y axes of spacetime. When the scale is stationary, the two light waves bounce off the corners and cancel each other out. The signal in the detector is zero. But if a gravitational wave passes through the Earth, it stretches the length of one arm of the letter "G" and compresses the length of the other (and vice versa in turn). The mismatch of the two light beams creates a signal in the detector, showing slight fluctuations in space-time.
At first, fellow physicists were skeptical, but soon the experiment found support in the person of Thorne, whose group of theorists from Caltech investigated black holes and other potential sources of gravitational waves, as well as the signals they generate. Thorne was inspired by Weber's experiment and similar efforts by Russian scientists. After speaking in 1975 at a conference with Weiss, “I began to believe that the detection of gravitational waves would be successful,” Thorne said. "And I wanted Caltech to be involved in this too." He arranged with the institute to hire Scottish experimenter Ronald Driever, who also announced that he would build a gravitational-wave interferometer. Over time, Thorne, Driver and Weiss began to work as one team, and each of them solved their share of countless tasks in preparation. practical experiment... The trio formed LIGO in 1984, and when prototypes were built and the ever-growing team began collaborating, they received $ 100 million in funding from the National Science Foundation in the early 1990s. Blueprints were drawn up for the construction of a pair of giant L-shaped detectors. A decade later, the detectors started working.
In Hanford and Livingston, in the center of each of the four-kilometer bends of the detectors there is a vacuum, thanks to which the laser, its beam and mirrors are maximally isolated from the constant vibrations of the planet. To insure even more, LIGO scientists monitor their detectors as they operate with thousands of instruments, measuring everything they can: seismic activity, atmosphere pressure, lightning, appearance cosmic rays, vibration of equipment, sounds in the area of \u200b\u200bthe laser beam, and so on. They then filter out this unwanted background noise from their data. Perhaps the main thing is that they have two detectors, and this allows you to compare the received data, checking them for the presence of coinciding signals.
Context
Gravitational Waves: Completed What Einstein Started in Bern
SwissInfo 13.02.2016How black holes die
Medium 19.10.2014Inside the vacuum created, even when the lasers and mirrors are completely isolated and stabilized, “strange things happen all the time,” says Marco Cavaglià, deputy spokesman for the LIGO project. Scientists must track these "goldfish", "ghosts", "incomprehensible sea monsters" and other extraneous vibrational phenomena, finding out their source in order to eliminate it. One difficult case occurred during the validation phase, said Jessica McIver, a research scientist with the LIGO team, who studies such extraneous signals and interference. A series of periodic single frequency noises often appeared in the data. When she and her colleagues converted the vibrations of the mirrors into audio files, “the phone was ringing clearly,” MacIver said. "It turned out that it was communications advertisers who were calling on the phone inside the laser room."
In the next two years, scientists will continue to improve the sensitivity of the detectors of the modernized Laser Interferometric Gravitational Wave Observatory LIGO. And in Italy, a third interferometer, called Advanced Virgo, will start working. One answer that the data obtained will help give is how black holes are formed. Are they the product of the collapse of the earliest massive stars, or are they the result of collisions within dense star clusters? “These are just two assumptions, I suppose there will be more when everyone calms down,” says Weiss. As LIGO begins to accumulate new statistics in the course of its upcoming work, scientists will begin to listen to stories about the origin of black holes that space will whisper to them.
In shape and size, the first, loudest pulsed signal originated 1.3 billion light-years from where, after an eternity of slow dance, under the influence of mutual gravitational attraction, two black holes, each about 30 times the mass of the sun, finally merged. Black holes circled faster and faster, like a whirlpool, gradually coming closer. Then there was a merger, and in the blink of an eye they released gravitational waves with an energy comparable to that of three Suns. This fusion has become the most powerful energetic phenomenon ever recorded.
“It's like we've never seen the ocean during a storm,” Thorne said. He has been waiting for this storm in space-time since the 1960s. The feeling Thorne experienced as the waves rolled in wasn't exactly excitement, he says. It was something else: a feeling of deepest satisfaction.
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The key difference is that if sound needs the environment it travels in, gravitational waves move the environment — in this case, spacetime itself. “They literally crush and stretch the fabric of spacetime,” says Chiara Mingarelli, a gravitational wave astrophysicist at Caltech. To our ears, the waves detected by LIGO will sound like a gurgle.
How exactly will this revolution take place? LIGO now has two detectors that act as "ears" for scientists, and there will be more detectors in the future. And if LIGO is the first to find it, it clearly won't be the only one. There are many types of gravitational waves. In fact, there is a whole spectrum of them, just as there are different types of light, with different wavelengths, in the electromagnetic spectrum. Therefore, other collaborations will enter the hunt for waves at a frequency that LIGO is not designed for.
Mingarelli works with the NanoGRAV (North American Nanohertz Gravitational Wave Observatory) Collaboration, part of a large international consortium that includes the European Pulsar Timing Array and Parkes Pulsar Timing Array in Australia. As the name suggests, NanoGRAV scientists hunt low-frequency gravitational waves in the 1 to 10 nanohertz mode; LIGO's sensitivity is in the kilohertz (audible) part of the spectrum, looking for very long waves. 
This collaboration draws on pulsar data collected by the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia. Pulsars are rapidly spinning neutron stars that form when stars larger than the Sun explode and collapse into themselves. They spin faster and faster as they contract, just as the weight at the end of a rope spins faster the shorter the rope gets.
They also emit powerful bursts of radiation as they rotate, like a beacon, which are detected as pulses of light on Earth. And this periodic rotation is extremely precise - almost as accurate as an atomic clock. It makes them ideal space-based gravitational wave detectors. The first indirect evidence came from studying pulsars in 1974, when Joseph Taylor, Jr. and Russell Hulse discovered that a pulsar orbiting a neutron star slowly contracts over time - an effect you would expect if it converted some of its mass into energy in the form of gravitational waves.
In the case of the NanoGRAV, the smoking gun will have a kind of flicker. The pulses should come at the same time, but if a gravitational wave hits them, they will come a little earlier or later, since spacetime will contract or stretch as the wave passes.
Pulsar time grid arrays are especially sensitive to gravitational waves produced by merging supermassive black holes a billion or ten billion times the mass of our Sun, such as those lurking at the center of the most massive galaxies. If two such galaxies merge, the holes in their centers will also merge and emit gravitational waves. “LIGO sees the very end of the merger when the pairs get very close,” says Mingarelli. "With the MPVR we could see them at the beginning of the spiral phase, when they just enter each other's orbit."
And there is still space mission LISA (Laser Interferometer Space Antenna). On Earth, LIGO is excellent at detecting gravitational waves equivalent to parts of the spectrum of audible sound - such as those produced by our merging black holes. But many interesting sources of these waves produce low frequencies. Therefore, physicists must travel to space to discover them. The main task of the current LISA Pathfinder () mission is to test the detector's operation. “With LIGO, you can stop the tool, open the vacuum and fix it,” says MIT's Scott Hughes. “But you can't open anything in space. We'll have to do it right away to make it work. "
The goal of LISA is simple: using laser interferometers, spacecraft will attempt to accurately measure the relative position of two 1.8-inch gold-platinum cubes in free fall. Housed in separate electrode boxes 15 inches apart, test objects will be protected from solar wind and other external forces, so it will be possible to detect tiny motion caused by gravitational waves (hopefully).
Finally, there are two experiments designed to search for prints left by primary gravitational waves in relict radiation (afterglow of the Big Bang): BICEP2 and the Planck satellite mission. BICEP2 announced the discovery of one in 2014, but it turned out that the signal was fake (space dust is to blame).
Both collaborations continue to hunt in the hopes of shedding light on the early history of our universe - and hopefully confirming key inflationary predictions. This theory predicted that shortly after its birth, the universe experienced a rapid growth, which could not help but leave powerful gravitational waves left as an imprint in the relict radiation in the form of special light waves (polarization).
Each of the four modes of gravitational waves will open four new windows to the universe for astronomers.
But we know what you're thinking: it's time to start the warp drive, dudes! Will the opening of LIGO help build the Death Star next week? Of course not. But the better we understand gravity, the wider we will understand how to build such things. After all, this is the work of scientists, this is how they earn their bread. By understanding how the universe works, we can rely more on our own capabilities.