Astronomers studying gravitational waves have stumbled upon a gold mine. Sensation: gravitational waves from neutron stars detected for the first time Gemini in the Universe

Gravitational waves, theoretically predicted by Einstein back in 1917, are still awaiting their discoverer.

Alexey Levin

At the end of 1969, University of Maryland physics professor Joseph Weber made a sensational statement. He announced that he had discovered gravitational waves coming to Earth from the depths of space. Until that time, no scientist had made such claims, and the very possibility of detecting such waves was considered far from obvious. However, Weber was known as an authority in his field, and therefore his colleagues took his message very seriously.

However, disappointment soon set in. The amplitudes of the waves allegedly recorded by Weber were millions of times higher than the theoretical value. Weber argued that these waves came from the center of our Galaxy, obscured by dust clouds, about which little was then known. Astrophysicists have suggested that there is a giant black hole hiding there, which annually devours thousands of stars and throws out part of the absorbed energy in the form of gravitational radiation, and astronomers began a futile search for more obvious traces of this cosmic cannibalism (it has now been proven that there really is a black hole there, but it leads behave quite decently). Physicists from the USA, USSR, France, Germany, England and Italy began experiments on detectors of the same type - and achieved nothing.

Scientists still don’t know what to attribute the strange readings from Weber’s instruments. However, his efforts were not in vain, although gravitational waves have still not been detected. Several installations to search for them have already been built or are being built, and in ten years such detectors will be launched into space. It is quite possible that in the not too distant future, gravitational radiation will become as observable a physical reality as electromagnetic oscillations. Unfortunately, Joseph Weber will no longer know this - he died in September 2000.

What are gravitational waves

It is often said that gravitational waves are disturbances of the gravitational field propagating in space. This definition is correct, but incomplete. According to the general theory of relativity, gravity arises due to the curvature of the space-time continuum. Gravity waves are fluctuations of the space-time metric, which manifest themselves as fluctuations in the gravitational field, so they are often figuratively called space-time ripples. Gravitational waves were theoretically predicted in 1917 by Albert Einstein. No one doubts their existence, but gravitational waves are still waiting for their discoverer.

The source of gravitational waves is any movement of material bodies that leads to a non-uniform change in the force of gravity in the surrounding space. A body moving at a constant speed does not radiate anything, since the nature of its gravitational field does not change. To emit gravitational waves, accelerations are necessary, but not just any acceleration. A cylinder that rotates around its axis of symmetry experiences acceleration, but its gravitational field remains uniform and gravitational waves do not arise. But if you spin this cylinder around a different axis, the field will begin to oscillate and gravitational waves will run from the cylinder in all directions.

This conclusion applies to any body (or system of bodies) that is asymmetrical about the axis of rotation (in such cases the body is said to have a quadrupole moment). A mass system whose quadrupole moment changes with time always emits gravitational waves.

Gravity beacons of space

Gravitational radiation from terrestrial sources is extremely weak. A steel column weighing 10,000 tons, suspended from the center in a horizontal plane and spun around a vertical axis up to 600 rpm, emits a power of approximately 10 -24 W. Therefore, the only hope of detecting gravitational waves is to find a cosmic source of gravitational radiation.

In this regard, close double stars are very promising. The reason is simple: the power of gravitational radiation of such a system grows in inverse proportion to the fifth power of its diameter. It is even better if the trajectories of the stars are very elongated, since this increases the rate of change of the quadrupole moment. It is quite good if the binary system consists of neutron stars or black holes. Such systems are similar to gravitational beacons in space - their radiation is periodic.

There are also “pulse” sources in space that generate short but extremely powerful gravitational bursts. This happens when a massive star collapses before a supernova explosion. However, the star's deformation must be asymmetric, otherwise the radiation will not occur. During collapse, gravitational waves can carry away up to 10% of the total energy of the star! The power of gravitational radiation in this case is about 10 50 W. Even more energy is released during the merger of neutron stars, here the peak power reaches 10 52 W. An excellent source of radiation is the collision of black holes: their masses can exceed the mass of neutron stars by billions of times.

Another source of gravitational waves is cosmological inflation. Immediately after the Big Bang, the Universe began to expand extremely quickly, and in less than 10 -34 seconds its diameter increased from 10 -33 cm to its macroscopic size. This process immeasurably strengthened the gravitational waves that existed before it began, and their descendants persist to this day.

Indirect confirmations

The first evidence of the existence of gravitational waves comes from the work of American radio astronomer Joseph Taylor and his student Russell Hulse. In 1974, they discovered a pair of neutron stars orbiting each other (a radio-emitting pulsar with a silent companion). The pulsar rotated around its axis with a stable angular velocity (which is not always the case) and therefore served as an extremely accurate clock. This feature made it possible to measure the masses of both stars and determine the nature of their orbital motion. It turned out that the orbital period of this binary system (about 3 hours 45 minutes) is reduced by 70 μs annually. This value agrees well with the solutions of the equations of the general theory of relativity, which describe the loss of energy of a stellar pair due to gravitational radiation (however, the collision of these stars will not happen soon, after 300 million years). In 1993, Taylor and Hulse were awarded the Nobel Prize for this discovery.

Gravitational wave antennas

How to detect gravitational waves experimentally? Weber used meter-long solid aluminum cylinders with piezoelectric sensors at the ends as detectors. They were isolated with maximum care from external mechanical influences in a vacuum chamber. Weber installed two of these cylinders in a bunker under the University of Maryland golf course, and one at Argonne National Laboratory.

The idea of ​​the experiment is simple. Space is compressed and stretched under the influence of gravitational waves. Thanks to this, the cylinder vibrates in the longitudinal direction, acting as a gravitational wave antenna, and piezoelectric crystals convert the vibrations into electrical signals. Any passage of cosmic gravitational waves almost simultaneously affects detectors separated by a thousand kilometers, which makes it possible to filter gravitational impulses from various types of noise.

Weber's sensors were able to detect displacements of the ends of the cylinder equal to only 10 -15 of its length - in this case 10 -13 cm. It was precisely such fluctuations that Weber was able to detect, which he first reported in 1959 in the pages of Physical Review Letters. All attempts to repeat these results have been futile. Weber's data also contradicts the theory, which practically does not allow us to expect relative displacements above 10 -18 (and values ​​​​less than 10 -20 are much more likely). It is possible that Weber made a mistake when statistically processing the results. The first attempt to experimentally detect gravitational radiation ended in failure.

Subsequently, gravitational wave antennas were significantly improved. In 1967, American physicist Bill Fairbank proposed cooling them in liquid helium. This not only made it possible to get rid of most of the thermal noise, but also opened up the possibility of using SQUIDs (superconducting quantum interferometers), the most accurate ultra-sensitive magnetometers. The implementation of this idea turned out to be fraught with many technical difficulties, and Fairbank himself did not live to see it. By the early 1980s, physicists from Stanford University had built an installation with a sensitivity of 10 -18, but no waves were detected. Now in a number of countries there are ultra-cryogenic vibration detectors of gravitational waves operating at temperatures only tenths and hundredths of a degree above absolute zero. This is, for example, the AURIGA installation in Padua. The antenna for it is a three-meter cylinder made of aluminum-magnesium alloy, the diameter of which is 60 cm and the weight is 2.3 tons. It is suspended in a vacuum chamber cooled to 0.1 K. Its shocks (with a frequency of about 1000 Hz) are transmitted to an auxiliary resonator weighing 1 kg, which vibrates with the same frequency, but with a much larger amplitude. These vibrations are recorded by measuring equipment and analyzed using a computer. The sensitivity of the AURIGA complex is about 10 -20 -10 -21.

Interferometers

Another method for detecting gravitational waves is based on the abandonment of massive resonators in favor of light rays. It was first proposed by Soviet physicists Mikhail Herzenstein and Vladislav Pustovoit in 1962, and two years later by Weber. In the early 1970s, Robert Forward, an employee of the research laboratory of the Hughes Aircraft Corporation (a former graduate student of Weber, and later a very famous science fiction writer), built the first such detector with quite decent sensitivity. At the same time, Massachusetts Institute of Technology (MIT) professor Rainer Weiss performed a very in-depth theoretical analysis of the possibilities of recording gravitational waves using optical methods.

These methods involve the use of analogues of the device with which 125 years ago physicist Albert Michelson proved that the speed of light is strictly the same in all directions. In this installation, a Michelson interferometer, a beam of light hits a translucent plate and is divided into two mutually perpendicular beams, which are reflected from mirrors located at the same distance from the plate. Then the beams merge again and fall on the screen, where an interference pattern appears (light and dark stripes and lines). If the speed of light depends on its direction, then when the entire installation is rotated, this picture should change; if not, it should remain the same as before.

The gravitational wave interference detector works in a similar way. A passing wave deforms space and changes the length of each arm of the interferometer (the path along which light travels from the splitter to the mirror), stretching one arm and compressing the other. The interference pattern changes, and this can be registered. But this is not easy: if the expected relative change in the length of the arms of the interferometer is 10 -20, then with a tabletop size of the device (like Michelson's) it results in oscillations with an amplitude of the order of 10 -18 cm. For comparison: waves of visible light of 10 trillion. times longer! You can increase the length of the shoulders to several kilometers, but problems will still remain. The laser light source must be both powerful and stable in frequency, the mirrors must be perfectly flat and perfectly reflective, the vacuum in the pipes through which the light travels must be as deep as possible, and the mechanical stabilization of the entire system must be truly perfect. In short, a gravitational wave interference detector is an expensive and bulky device.

Today, the largest installation of this kind is the American complex LIGO (Light Interferometer Gravitational Waves Observatory). It consists of two observatories, one of which is located on the Pacific coast of the United States, and the other near the Gulf of Mexico. Measurements are made using three interferometers (two in Washington state, one in Louisiana) with four-kilometer-long arms. The installation is equipped with mirror light accumulators, which increase its sensitivity. “Since November 2005, all three of our interferometers have been operating normally,” LIGO complex representative Peter Saulson, a professor of physics at Syracuse University, told Popular Mechanics. “We constantly exchange data with other observatories trying to detect gravitational waves with a frequency of tens and hundreds of hertz, which arose during the most powerful supernova explosions and mergers of neutron stars and black holes. Currently in operation is the German GEO 600 interferometer (arm length - 600 m), located 25 km from Hannover. The 300-meter Japanese TAMA instrument is currently being upgraded. The three-kilometer Virgo detector near Pisa will join the effort in early 2007, and at frequencies below 50 Hz it will be able to surpass LIGO. Installations with ultracryogenic resonators operate with increasing efficiency, although their sensitivity is still somewhat less than ours.”

1. In empty space they propagate at the speed of light. Moreover, this speed is almost always preserved when encountering material objects, so that gravitational waves do not undergo refraction. Extremely superdense matter can reduce the speed of gravitational waves, but in other cases this effect is negligible. The amplitudes of gravitational waves fade away with distance from the source, but do not fall to zero: once a gravitational wave arises, in a certain sense, it is doomed to eternal life. In particular, the Universe must be permeated with relic waves inherited from the inflationary phase. They encode information about the structure of the “embryo” Universe, which, however, still needs to be deciphered. 2. Gravity waves are transverse. Such a wave distorts the structure of space in a plane perpendicular to the vector of its propagation. A solid body caught in the region of the front of a gravitational wave will experience deformations in precisely this plane (which ones depend on the nature of the wave). 3. Gravitational waves carry away the energy that they take from the matter emitting them. Therefore, over time, the stars of the binary system come closer and the period of their revolution around the common center of mass decreases.

Prospects

What does the near future hold for gravitational wave detection methods? Professor Rainer Weiss told Popular Mechanics about this: “In a few years, more powerful lasers and more advanced detectors will be installed in the observatories of the LIGO complex, which will lead to a 15-fold increase in sensitivity. Now it is 10 -21 (at frequencies of about 100 Hz), and after modernization it will exceed 10 -22. The upgraded complex, Advanced LIGO, will increase the depth of penetration into space by 15 times. Moscow State University professor Vladimir Braginsky, one of the pioneers in the study of gravitational waves, is actively involved in this project.

The launch of the LISA (Laser Interferometer Space Antenna) space interferometer with an arm length of 5 million kilometers is planned for the middle of the next decade, this is a joint project of NASA and the European Space Agency. The sensitivity of this observatory will be hundreds of times higher than the capabilities of ground-based instruments. It is primarily designed to search for low-frequency (10 -4 -10 -1 Hz) gravitational waves, which cannot be detected on the Earth's surface due to atmospheric and seismic interference. Such waves are emitted by double star systems, quite typical inhabitants of the Cosmos. LISA will also be able to detect gravitational waves generated when ordinary stars are absorbed by black holes. But to detect relict gravitational waves that carry information about the state of matter in the first moments after the Big Bang, more advanced space instruments will most likely be required. Such an installation, Big Bang Observer, is now being discussed, but it is unlikely that it will be created and launched earlier than in 30-40 years.”

Physicists at the LIGO (Laser Interferometric Gravitational Observatory) first discovered gravitational waves - disturbances of space-time predicted a hundred years ago by the creator of the general theory of relativity, Albert Einstein. About the opening during a live broadcast organized by Lenta.ru and Moscow State University (MSU) named after M.V. Lomonosov, scientists from the Faculty of Physics, participants in the international LIGO collaboration. Lenta.ru talked to one of them, Russian physicist Sergei Vyatchanin.

What are gravitational waves?

According to Newton's law of universal gravitation, two bodies are attracted to each other with a force inversely proportional to the square of the distance between them. This theory describes, for example, the rotation of the Earth and the Moon in flat space and universal time. Einstein, having developed the special theory of relativity, realized that time and space are one substance, and proposed a general theory of relativity - a theory of gravity based on the fact that gravity manifests itself as the curvature of space-time that matter creates.

Doctor of Physical and Mathematical Sciences Sergei Vyatchanin has headed the Department of Oscillation Physics of the Physics Faculty of Moscow State University since 2012. His research interests focus on the study of quantum non-perturbative measurements, laser gravitational wave antennas, dissipation mechanisms, fundamental noise and nonlinear optical effects. The scientist collaborated with the California Institute of Technology in the USA and the Max Planck Society in Germany.

You can imagine an elastic circle. If you throw a light ball at it, it will roll in a straight line. If you put a heavy apple in the center of the circle, the trajectory will bend. From the equations of general relativity, Einstein immediately learned that gravitational waves are possible. But at that time (at the beginning of the twentieth century) the effect was considered extremely weak. You could say that gravitational waves are ripples in space-time. The bad thing is that this is an extremely weak interaction.

If we take similar (electromagnetic) waves, then there was the experiment of Hertz, who placed the emitter in one corner of the room and the receiver in the other. This doesn't work with gravitational waves. Too weak interaction. We can only rely on astrophysical catastrophes.

How does a gravity antenna work?

There is a Fabry-Perot interferometer, two masses separated by four kilometers. The distance between the masses is controlled. If the wave comes from above, the distance changes slightly.

Is gravitational disturbance essentially a distortion of the metric?

You can say that. Mathematics describes this as a slight curvature of space. Herzenstein and Pustovoit proposed using a laser to detect gravitational waves in 1962. It was such a Soviet article, a fantasy... Great, but still a flight of fancy. The Americans thought and decided in the 1990s (Kip Thorne, Ronald Drever and Rainer Weiss) to make a laser gravitational antenna. Moreover, two antennas are required, since if there are events, it is necessary to use a coincidence scheme. And then it all began. It's a long story. We have been cooperating with Caltech since 1992, and switched to a formal contractual basis in 1998.

Don't you think that the reality of gravitational waves was beyond doubt?

In general, the scientific community was confident that they existed, and it was a matter of time to discover them. Hulse and Taylor were awarded the Nobel Prize for the actual discovery of gravitational waves. What did they do? There are double stars - pulsars. Since they spin, they emit gravitational waves. We cannot observe them. But if they emit gravitational waves, they give off energy. This means that their rotation is slowing down, as if due to friction. The stars move closer to each other and a change in frequency can be seen. They looked - and saw (in 1974 - approx. "Tapes.ru"). This is indirect evidence of the existence of gravitational waves.

Now - direct?

Now - direct. A signal arrived and was registered on two detectors.

Is the reliability high?

It's enough to open.

What is the contribution of Russian scientists to this experiment?

Key. In initial LIGO (an early version of the antenna - approx. "Tapes.ru") ten-kilogram masses were used, and they hung on steel threads. Our scientist Braginsky already expressed the idea of ​​​​using quartz threads. A paper was published that proved that quartz filaments make much less noise. And now the masses (in advanced LIGO, a modern installation - approx. "Tapes.ru") hang on quartz threads.

The second contribution is experimental and related to charges. The masses, separated by four kilometers, need to be somehow adjusted using electrostatic activators. This system is better than the magnetic one that was used previously, but it senses the charge. In particular, every second a huge number of particles - muons - pass through a person’s palm, which can leave a charge. Now they are struggling with this problem. Our group (Valery Mitrofanov and Leonid Prokhorov) is participating in this experimentally and has become significantly more experienced.

In the early 2000s, there was an idea to use sapphire filaments in advanced LIGO, since formally sapphire has a higher quality factor. Why is it important? The higher the quality factor, the less noise. This is a general rule. Our group calculated the so-called thermoelastic noise and showed that it is still better to use quartz rather than sapphire.

And further. The sensitivity of the gravitational antenna is close to the quantum limit. There is the so-called standard quantum limit: if you measure a coordinate, then according to the Heisenberg uncertainty principle you immediately perturb it. If you continuously measure a coordinate, then you are perturbing it all the time. It is not good to measure the coordinate very accurately: there will be a large reverse fluctuation effect. This was shown in 1968 by Braginsky. Calculated for LIGO. It turned out that for initial LIGO the sensitivity is approximately ten times higher than the standard quantum limit.

The hope now is that advanced LIGO will reach the standard quantum limit. Maybe it will go down. This is actually a dream. Can you imagine this? You will have a quantum macroscopic device: two heavy masses at a distance of four kilometers.

Gravitational waves were detected on September 14, 2015 at 05:51 a.m. Eastern Daylight Time (13:51 Moscow time) at the twin detectors of the LIGO Laser Interferometer Gravitational-Wave Observatory located in Livingston (Louisiana) and Hanford (Washington State). ) in USA. The LIGO detectors detected relative fluctuations of ten to the minus 19 meters (this is approximately equal to the ratio of the diameter of an atom to the diameter of an apple) of pairs of test masses separated by four kilometers. The disturbances are generated by a pair of black holes (29 and 36 times heavier than the Sun) in the last fractions of a second before they merge into a more massive rotating gravitational object (62 times heavier than the Sun). In a fraction of a second, three solar masses turned into gravitational waves, the maximum radiation power of which was about 50 times greater than from the entire visible Universe. The merger of black holes occurred 1.3 billion years ago (this is how long it took for the gravitational disturbance to reach the Earth). Analyzing the moments of arrival of the signals (the Livingston detector recorded the event seven milliseconds earlier than the Hanford detector), scientists assumed that the signal source was located in the southern hemisphere. The scientists submitted their results for publication in the journal Physical Review Letters.

At first glance, this is not very compatible.

This is what is paradoxical. That is, it turns out to be fantastic. It seems to smack of charlatanism, but in reality it’s not, everything is honest. But for now these are dreams. The standard quantum limit has not been reached. There you still need to work and work. But it is already clear that it is close.

Is there any hope that this will happen?

Yes. The standard quantum limit needs to be overcome, and our group has been involved in developing methods for how to do this. These are the so-called quantum non-perturbing measurements, what specific measurement scheme is needed - this or that... After all, when you study theoretically, calculations cost nothing, and experimentation is expensive. LIGO achieved an accuracy of ten to minus 19 meters.

Let's remember a child's example. If we reduce the Earth to the size of an orange, and then reduce it by the same amount, we get the size of an atom. So, if we reduce the atom by the same amount, then we get ten meters to the minus 19 degree. This is crazy stuff. This is an achievement of civilization.

This is very important, yes. So what does the discovery of gravitational waves mean for science? It is believed that this could change the observational methods of astronomy.

What do we have? Astronomy in the usual range. Radio telescopes, infrared telescopes, X-ray observatories.

Is everything in the electromagnetic ranges?

Yes. In addition, there are neutrino observatories. There is registration of cosmic particles. This is another channel of information. If the gravitational antenna produces astrophysical information, then researchers will have at their disposal several observation channels at once, through which they can test the theory. Many cosmological theories have been proposed, competing with each other. It will be possible to weed out something. For example, when the Higgs boson was discovered at the Large Hadron Collider, several theories immediately fell away.

That is, this will contribute to the selection of working cosmological models. Another question. Is it possible to use a gravitational antenna to accurately measure the accelerated expansion of the Universe?

So far the sensitivity is very low.

What about in the future?

In the future, it can also be used to measure the relict gravitational background. But any experimenter will tell you: “Ay-yay!” That is, this is still a long way off. God grant that we register an astrophysical catastrophe.

Black hole collision...

Yes. After all, this is a disaster. God forbid you end up there. We wouldn't exist. And here is such a background... For now... “they feed the hopes of the young, they give joy to the elders.”

Could the discovery of gravitational waves provide further evidence of the existence of black holes? After all, there are still those who do not believe that they exist.

Yes. How do they work at LIGO? The signal is being recorded, to explain which scientists develop patterns and compare them with observational data. A collision of neutron stars, a neutron star falls into a black hole, a supernova explosion, a black hole merges with a black hole... We will change parameters, for example, the mass ratio, the initial moment... What should we see? Recording is in progress, and at the moment of the signal the performance of the templates is assessed. If the pattern designed for the collision of two black holes matched the signal, then that's proof. But not absolute.

Is there no better explanation? Is the discovery of gravitational waves most simply explained by the collision of black holes?

At the moment - yes. The scientific community now believes that it was a merger of black holes. But a collective community is the opinion of many, a consensus. Of course, if some new factors arise, it can be abandoned.

When will it be possible to detect gravitational waves from less massive objects? Doesn't this mean that new and more sensitive observatories need to be built?

There is a next generation program called LIGO. This is the second one. There will be a third. There are a lot of options there. You can increase the distance, increase the power, and the suspension. Now all this is being discussed. At the brainstorming level. If the observation of a gravitational signal is confirmed, it will be easier to obtain money to improve the observatory.

Is there a boom in the construction of gravitational observatories?

Don't know. It's expensive (LIGO cost about $370 million - approx. "Tapes.ru"). After all, the Americans offered Australia to build an antenna in the Southern Hemisphere and agreed to provide all the equipment for this. Australia refused. Too expensive toy. The maintenance of the observatory would take up the entire scientific budget of the country.

Is Russia financially involved in LIGO?

We cooperate with the Americans. What will happen next is unclear. So far we have good relations with scientists, but politicians rule everything... Therefore, we need to watch. They appreciate us. We deliver results that are truly up to par. But they are not the ones who decide whether to be friends with Russia or not.

Unfortunately yes.

This is life, let's wait.

The LIGO observatory is funded by the US National Science Foundation. Research at LIGO is carried out as part of a collaboration of the same name by more than a thousand scientists from the United States and 14 other countries, including Russia, represented by two groups from Moscow State University and the Institute of Applied Physics of the Russian Academy of Sciences (Nizhny Novgorod).

Are there any plans to build a gravitational observatory in Russia?

Not planned yet. In the 1980s, the Sternberg State Astronomical Institute of Moscow State University wanted to build the same gravitational antenna in the Baksan Gorge, only on a smaller scale. But perestroika came, and everything was covered with a copper basin for a long time. Now the traffic police of Moscow State University is trying to do something, but so far the antenna has not worked...

What else can you try to check using a gravitational antenna?

The validity of the theory of gravity. After all, most existing theories are based on Einstein's theory.

No one can refute it yet.

She occupies a leading position. Alternative theories are designed in such a way that they basically lead to the same experimental consequences as it does. And this is natural. Therefore, we need new facts that would sweep away incorrect theories.

Briefly, how would you formulate the meaning of the discovery?

In fact, gravitational astronomy began. And for the first time, the waves of space curvature were hooked. Not indirectly, but directly. A person admires himself: what a son of a bitch I am!

Anya Grushina

Temporal, or time, crystals are a new idea in physics that has been widely discussed in recent years. They are physical systems that repeat themselves “by themselves” over time. Despite the exotic nature of the concept, researchers are already considering possible areas of application of the idea and are looking for the most successful “recipes” for preparing “crystalline time.”

Frank Wilczek, 2004 Nobel laureate and author of the time crystal concept. Photo: Kenneth C. Zirkel/Wikimedia Commons/CC BY-SA 3.0.

The "recipe" for a temporal crystal from Christopher Monroe's experiment: laser radiation, shown by the orange and green arrows, flips the magnetic moments (spins); the laser light, shown by the red arrow, introduces disorder and causes interactions between the spins. As a result, the system of spins oscillates between two stable states that are resistant to changes in the pump frequency.

The beauty of the laws of nature goes hand in hand with symmetry. Strictly speaking, symmetry in physics implies that some property remains unchanged under a certain transformation: this could be a rotation or shift in space, a mirror reflection. Simply put, no matter how you twist an object or the Universe, the laws of physics do not change. Symmetry can be continuous or discrete. For example, a homogeneous ball can be rotated to any angle - nothing will change. But the cube “repeats itself” only when rotated at a certain angle. These are examples of continuous and discrete rotational symmetry.

Interesting physics begins where symmetry changes, or rather, breaks. Let's say a crystal is less symmetrical than a homogeneous liquid consisting of the same atoms, so it can be considered as a violation of spatial symmetry. The atoms in it are located in the nodes of the so-called crystal lattice with clearly defined distances and angles. In order to obtain the same crystal when moving in space, it must be moved by a clearly defined distance (the so-called lattice constant - the size of the elementary cell, the repetition of which can reproduce the entire crystal) or rotated by the appropriate angle. The specific characteristics of crystals directly depend on how exactly the symmetry was broken: the number of electrons on the outer shell of atoms, magnetic moments, temperature - all this affects the interactions between atoms and ultimately determines the properties of the material. Physicists have been studying crystals for a long time and have even learned to create similar systems using lasers or microwaves, where the role of lattice nodes can be played not only by atoms and electrons, but also by photons or quasiparticles, such as phonons. The symmetry of the medium is also disrupted by magnetization and the flow of electric current.

But a discrete violation of temporal, or temporal, symmetry (the continuous flow of time only forward) is still unexplored territory. Frank Wilczek, winner of the 2004 Nobel Prize for describing the interaction between quarks and gluons, began to think in 2012 about why time symmetry is never broken spontaneously (that is, due to random interactions between elements of the system) and whether it is possible to create the conditions , in which this would be possible. As a result, he came up with temporal crystals as a way to break temporal symmetry.

Temporal crystals are hypothetical structures that pulsate without expending energy, like a mechanical watch that does not require winding. The sequence is repeated in time, just as the atoms of a crystal are repeated in space. At first glance, the temporal crystal is more reminiscent of the World of the Great Crystal of science fiction writer Vladislav Krapivin than of strict physics, but such a structure may have good physical reasons for its existence.

One possible implementation of a temporal crystal is a ring of atoms that should rotate, regularly returning to its original state. Its properties would be eternally synchronized in time, similar to how the positions of atoms in a crystal are interconnected. By the definition of a temporal crystal, such a system must be in a state with the lowest energy so that movement does not require energy from the outside. In a sense, the temporal crystal would be a perpetual motion machine, except that it would not produce any useful work.

The scientific community for the most part considered the idea provocative. Nevertheless, Frank Wilczek stood his ground, confident that the problem was more subtle than it seemed at first glance, and that temporal crystals represented a new type of order. Moreover, perpetual motion has precedents in the quantum world: theoretically, superconductors conduct electric current forever (although the flow in this case is uniform and therefore does not change over time).

The temporal crystal paradox interested Haruki Watanabe, a graduate student at the University of California at Berkeley. When he presented his work on symmetry breaking in space, he was asked about the implications of Wilczek's idea of ​​a temporal crystal. Watanabe could not answer and decided to look into this issue by focusing on correlations between remote parts of the system in time and space. In 2015, together with physicist Masaki Oshikawa from the University of Tokyo, Watanabe proved a theorem according to which the creation of a temporal crystal in the lowest energy state is impossible. They also proved that temporal crystals are impossible for any equilibrium system that has reached a stable state at any energy value.

At this point, the physical community considered the question of the existence of temporal crystals closed. However, the evidence left a loophole. It did not exclude the possibility of the existence of temporal crystals in systems in which equilibrium had not yet been established. And theorists around the world began to think about how they could create alternative versions of temporal crystals to circumvent the theorem.

The breakthrough unexpectedly came from a field of physics in which researchers had not thought at all about the topic. Theorist Shivaji Sondhi and his colleagues from Princeton University studied the behavior of an isolated quantum system consisting of a “soup” of interacting particles that was regularly “kicked” energetically. If you believe the textbooks, then such a system should heat up and eventually become completely chaotic. But Szondi's group showed that when certain conditions are met, the particles cluster together and form a "pattern" that repeats over time.

This research caught the attention of Chetan Nayak, one of Wilczek's former students. Nayak and his colleagues suggested that the strange, out-of-equilibrium form of matter could be a type of temporal crystal, although not exactly the kind that Wilczek originally spoke of. The difference is that such a system is not in a state with the lowest energy and needs to be supplied with energy from the outside to maintain pulsations. But such a “soup” has its own rhythm, different from the pumping frequency, which actually means a violation of time symmetry.

Christopher Monroe from the University of Maryland at College Park, despite his skepticism, nevertheless tried to create a similar temporal crystal using cold atoms. The intricate "recipe" contains three main ingredients: the force that acts on the system, the interaction between atoms, and an element of random disorder. This combination limits the particles in the amount of energy they can absorb, allowing them to remain in an ordered state.

In the experiment, a chain of ten ytterbium ions was alternately illuminated by two lasers. The first laser flipped the magnetic moments of atoms, and the second forced them to interact with each other randomly. This led to oscillations in the projection of the magnetic moment of the system with a period twice as long as the period of laser spin pumping. Moreover, even if the first laser strayed from the desired radiation frequency, the oscillations in the system did not change. Just as ordinary crystals resist attempts to move atoms from their positions in the crystal lattice, so the temporal crystal has retained its periodicity in time.

A group of physicists from Harvard University led by Mikhail Lukin (who is also a co-founder of the Russian Quantum Center) took a different route and implemented a temporal crystal using diamond. For this purpose, a special sample was synthesized containing about a million disordered defects, each of which had its own magnetic moment. When such a crystal was exposed to pulses of microwave radiation to flip the spins, physicists recorded the system's response at a frequency that was only a fraction of the frequency of the exciting radiation.

Theoretical physicist Norman Yao, who took part in both experiments, emphasizes that systems in the lowest energy state, by definition, should not change over time. Otherwise, it would mean that they have extra energy that they can expend, and eventually the movement must stop. Yao compared the result of the experiments to a jump rope: the hand makes two turns, but the rope only makes one, and this is a weaker violation of symmetry than originally conceived by Wilczek, who believed that the rope could vibrate on its own.

The results of both experiments were published in the journal Nature and are certainly interesting, but the definition of a temporal crystal in both cases can be considered a bit far-fetched. Physicists agree that both systems spontaneously break time symmetry in some way and therefore satisfy the requirements of a temporal crystal from a mathematical point of view. But whether they can really be considered as such is a subject of scientific debate.

Whether Monroe and Lukin succeeded in getting temporal crystals or not, time will tell. In any case, these experiments are interesting because for the first time they demonstrated the simplest examples of new phases of matter in the relatively unexplored region of nonequilibrium states. This new state of matter consists of a group of quantum particles that continuously changes, never reaching a stable state. Stability is achieved through random interactions that would upset the balance in any other type of matter.

Moreover, these results may have practical implications. Temporal crystals can be useful as super-precise sensors. The behavior of the magnetic moments of defects in diamond is already used to record the slightest changes in temperature and magnetic fields. But this approach has its limitations: when too many defects “crowd” in a small volume, interactions between them destroy quantum states. In a temporal crystal, on the contrary, interactions stabilize the system, so millions of defects can be used together to amplify the signal. This will make it possible to study, in particular, living cells and materials of atomic thickness.

Another example of the use of such systems is quantum computing at fairly high temperatures. Quantum computers are a promising and long-awaited technology that is still far from practical implementation. The point is that the fragile quantum bits that do the calculations need to be insulated from the quantum-destroying effects of thermal motion and other environmental “side effects” while still being able to encode and read information from them. Physicists use very low temperatures to do this, just nanodegrees above absolute zero. A temporal crystal is essentially a quantum system that exists at significantly higher temperatures. In the case of the Lukin diamond, this is generally true at room temperature.

In an interview that can be read in “Science and Life” No. 12 for 2013, Mikhail Lukin spoke precisely about such unexpected practical “side effects” of what at first glance is a completely fundamental science. And perhaps it is the fantastic-sounding concept of a temporal crystal that will open the way to quantum computing without the need for complex and expensive cryogenics.

Space presented scientists with a gift for the centenary of Einstein's general theory of relativity - gravitational waves were detected

In mid-February this year, members of the international LIGO collaboration, uniting hundreds of scientists from seventeen countries, including Russia, announced the first direct detection of gravitational waves emitted by two merging black holes with a total mass of more than 60 suns 1.3 billion years ago. This is a scientific event, without exaggeration, on a cosmic scale, and it happened last September at the laser gravitational-wave observatory-interferometer LIGO (USA). For a detailed comment, we turned to the head of the laboratory of theoretical physics of the Institute of Electrophysics of the Ural Branch of the Russian Academy of Sciences, academician Mikhail Sadovsky.

- Dear Mikhail Vissarionovich, first of all, explain to an amateur what a gravitational wave is?

Imagine four balls hanging crosswise. If a gravitational disturbance occurs, two balls will deviate from each other by a certain distance, and the other two will simultaneously rush towards each other; in the next phase of the wave their movement will be opposite. As a result, under the influence of a gravitational wave, all four balls will begin to oscillate synchronously. But this is an imaginary experiment. In everyday life, no one feels or observes gravitational waves; they have no effect on anything, because gravitational interactions are very weak compared, for example, to electromagnetic ones. And although most theoretical physicists never doubted the existence of gravitational waves, the task of experimentally registering them under terrestrial conditions seemed very difficult. We could only hope for space - powerful gravitational disturbances occur there, and the waves caused by them can reach the Earth.

- So, the current discovery cannot be called unexpected?

The existence of gravitational waves was theoretically predicted by Albert Einstein exactly 100 years ago in his 1916 paper. This naturally followed from the general theory of relativity, or the modern theory of gravity. If electromagnetic waves exist, then there must also be gravitational disturbances, which propagate in the form of waves at the speed of light and locally change the geometry of space and time. The prediction of the existence of gravitational waves made it possible, for example, to explain the change in the rate of convergence of close systems of double stars.

For the first time, the American physicist Joseph Weber tried to solve the problem of direct registration of gravitational effects back in the 1960s. He developed the first detectors - two massive aluminum cylinders suspended at a great distance from each other. According to Weber, a large gravitational wave would cause them to oscillate in unison, and thus its passage could be recorded. In 1968, he announced the detection of gravitational waves with his detectors, but the results of his experiments were questioned by other researchers. Unfortunately, Joseph Weber did not live to see the current triumph of the movement he founded. However, the scientist’s contribution to gravitational-wave astronomy is recognized by the scientific community.

- Have our compatriots made attempts to register gravitational waves?

In the USSR and Russia, the pioneer of gravitational wave research was a corresponding member of the Russian Academy of Sciences Vladimir Braginsky. He was skeptical about Weber's experiments, believing that nothing could be registered with such detectors, but he continued to work in this direction.

The scheme implemented in the current experiment was also proposed by domestic scientists - Professor Mikhail Herzenstein and academician Vladislav Pustovoit in an article published in the Journal of Experimental and Theoretical Physics in 1962. This scheme is quite simple. It is built on a Michelson interferometer, the operating principle of which is as follows: a beam of light from a source is directed to a mirror located at some distance from it, reflected from the mirror and returned back, and a second light signal is sent in a perpendicular direction, it is also reflected from the mirror and returns. At the point where the light signals intersect on the detectors, you can see the interference pattern. If a gravitational wave passes, the mirrors begin to tremble synchronously, and the interference pattern changes. Due to the fact that optics is a very precise science, it becomes possible to detect even a very weak gravitational effect.

- Does the interferometer, where the sensational discovery was made, work on this principle?

Yes. The LIGO observatory consists of two installations: one is located in Hanford, Washington, the other in Livingston, Louisiana, at a distance of about 3 thousand kilometers. Each interferometer has two “arms” 4 km long, located perpendicular to each other. These are pipes inside which a laser beam is fired. If a gravitational wave arrives, then a characteristic interference pattern should synchronously appear in both interferometers on the detector at the point where the beams intersect.

The initiators of the LIGO project in the 1980s were professors at the California Institute of Technology Kip Thorne(by the way, one of the authors of the script for the space action movie “Interstellar”) and Ronald Driver, and also a professor at the Massachusetts Institute of Technology Rainer Weiss.
The list of participants in the international collaboration, numbering more than 200 people, includes our compatriots, including the already named corresponding member Vladimir Braginsky, professor Valery Mitrofanov(MSU), corresponding members Alexander Sergeev And Efim Khazanov(Institute of Applied Physics RAS, Nizhny Novgorod) and other researchers.

The work of Russian project participants was partially supported by grants from the Russian Foundation for Basic Research. Unfortunately, the ridiculous grant conditions adopted by the Russian Science Foundation completely exclude support for this kind of collective research. Thus, according to the rules of the foundation, work financed by the Russian Science Foundation cannot be supported by any other funds or grants. This requirement is as strict as it is unconstructive. After all, any major scientific project, especially an international one, receives support from dozens of different foundations, and the LIGO collaboration is an example of this.

Laser Interferometer Gravitational-Wave Observatory LIGO (Laser Interferometer Gravitational-Wave Observatory). The total cost of the project is about 620 million dollars

Meanwhile, the LIGO project is very expensive. The observatory cost $300 million to build, plus operating and modernization costs. LIGO was launched in 2002 and operated until 2010. However, at that time it was not possible to register gravitational waves; only various noises were recorded. The interferometer was then shut down for upgrades. A similar LIGO interferometer, Virgo, with three-kilometer arms, began operating in 2007 in Italy, near Pisa. It has been undergoing modernization since 2011, and should be launched again in the second half of this year. And the improved Advanced LIGO complex began operation in early autumn 2015.

- It turns out that the discovery occurred shortly after the launch?

Exactly. On September 14, the LIGO detector detected a signal that looked “suspicious” from the point of view of observing gravitational waves. The changes in the interference pattern were fully consistent with the calculations that the collaboration participants made in advance in the event of a gravitational disturbance. This was exactly what should have happened during the passage of a gravitational wave generated by the collision of two black holes - massive stars in the last stage of life, “weighing” 29 and 36 solar masses. As a result of the cosmic cataclysm, a black hole of 62 solar masses was formed, and the energy of three solar masses turned into gravitational radiation, which reached us after 1.3 billion light years. If the Virgo interferometer was already functioning at the time of fixation, it would be possible to determine where the gravitational wave came from. This time it was not possible to do this, but scientists hope that it will be possible in the future, when LIGO and Virgo work in parallel.

- And finally, a few words about the significance of the event...

The discovery of such “heavy” black holes is in itself a major discovery in astronomy. And direct registration of gravitational waves is essentially the birth of a new scientific direction, gravitational-wave astronomy. By studying gravitational effects, we may be able to peer into the earliest periods of the formation of the Universe. After all, from the earliest stages of the evolution of the “fireball” that arose as a result of the Big Bang, light signals do not pass through, but gravitational waves emitted at this stage of the expansion of the Universe can reach us. It is also remarkable that the general theory of relativity has now been almost completely experimentally tested at the classical (non-quantum) level and indeed describes gravity very accurately. So the discovery became a bright “gift” for the centennial anniversary of this theory.

Of course, it is difficult to talk about the practical meaning of registering gravitational waves, but it is possible that it will be revealed in the future. At the beginning of the 20th century, no one could have imagined that, for example, modern GPS navigators would correctly determine your location only taking into account the effects of general relativity. And gravitational-wave astronomy is apparently just around the corner.

First recorded gravitational wave signal

Gravitational wave from binary black hole mergers detected by LIGO detectors at Hanford and Livingston On the left is data from the detector in Hanford (H1), on the right is in Livingston (L1). Time is counted from September 14, 2015, 09:50:45 UTC. Top row: voltages h in the detectors. The GW150914 signal first arrived at L1 and after 6.9+0.5?0.4 ms at H1; for visual comparison, data from H1 are shown in the L1 plot in reversed and time-shifted form (to account for the relative orientation of the detectors). Second row: voltage h from the gravitational wave signal, passed through the same bandpass filter, 35 - 350 Hz. The solid line is the result of numerical relativity for a system with parameters compatible with those found based on the study of the GW150914 signal obtained by two independent codes with a resulting match of 99.9. The gray thick lines are the 90% confidence regions of the waveform reconstructed from the detector data by two different methods. The dark gray line models the expected signals from a black hole merger, the light gray line does not use astrophysical models, but represents the signal as a linear combination of sinusoidal-Gaussian wavelets. The reconstructions overlap by 94%. Third row: residual errors after extracting the filtered prediction of the numerical relativity signal from the filtered signal of the detectors. Bottom row: A representation of the voltage frequency map, showing the increase in the dominant frequency of the signal over time.

What did the LIGO detectors see?

We saw a signal that looked exactly as predicted for the merger of a pair of black holes. The relative stretching of the interferometer under the influence of a gravitational wave is shown. The vertical scale is 10–21, which means that the four-kilometer arm of the interferometer is stretched by 2.5 x 10–15 cm (they can measure stretches of up to 10–17 cm, no matter how fantastic this may seem). The figure shows the expansion and contraction of two detectors (shown in different colors) located at a distance of 3000 km. First there is noise, in which obvious waves begin to appear, which come more and more often, and then end abruptly. Each wave is half a revolution of the system of two black holes. They quickly converge, so the time between peaks decreases. The last wave is practically one black hole, albeit highly deformed.

How, looking at a picture, can you estimate the mass of merged black holes and the distance to them?
It is necessary to estimate the rotation period of the merging objects at the last moment. We look at the figure and see that the distance between the last peaks is about ten times less than between the risks, that is, about 5 milliseconds. This is half the rotation period of a still highly deformed black hole. At what linear speed does its surface rotate? Comparable to the speed of light, but less, about a third (the limiting Kerr hole) - regardless of size.

Then the semicircle of rotation will be approximately 500 km, divide by?, we get a radius of 170 km. The radius of a solar mass black hole is 3 km, which means the mass of the system is about 60 solar. In fact - 62. Amazing accuracy, especially considering that we estimated the time between peaks by eye.

Now let's try to estimate the distance. It's a little more complicated. The amplitude of a gravitational wave (relative deformation of space) is inversely proportional to the distance to the source. The deformation in the source is enormous, well, not unity, of course, but 0.1 is quite realistic (calculations give exactly this order of magnitude). We have 10–21 (see units on the vertical axis), which means we are about 1020 times farther from the source than its size - 170 km (see above). We get 1.7 x 107 cm x 1020 = 1.7 x 1027 cm = 0.6 gigaparsecs (actually 0.4 gigaparsecs). Again, a remarkable hit despite the fact that there is still uncertainty in the orientation of the equatorial plane of the system relative to the line of sight.