Einstein's theory of relativity has always been something abstract and incomprehensible to me. Let's try to describe Einstein's theory of relativity in simple words. Imagine you are outside in heavy rain and the wind is blowing on your back. If you start running fast, the rain drops will not fall on your back. Drops will be slower or not reach your back at all, this is a scientifically proven fact, and you yourself can check this in a downpour. Now imagine if you turned around and ran against the wind with rain, the drops would fall harder on your clothes and face than if you just stood.
Previously, scientists thought light acted like rain on windy days. They thought that if the Earth moves around the Sun, and the Sun moves around the galaxy, then it is possible to measure the speed of their movement in space. In their opinion, all that remains for them to do is to measure the speed of light and how it changes relative to two bodies.
Scientists have done this found something very strange. The speed of light was the same, no matter how the bodies moved and no matter in what direction to take measurements.
It was very strange. If we take a rainstorm situation, then under normal circumstances, raindrops will affect you more or less depending on your movements. Agree, it would be very strange if the downpour blew in your back with the same force, both when running and when stopping.
Scientists have discovered that light does not have the same properties as raindrops or anything else in the universe. No matter how fast you are moving, and no matter which direction you are heading, the speed of light will always be the same. This is very confusing and only Albert Einstein was able to shed light on this injustice.
Einstein and another scientist, Hendrik Lorenz, figured out that there is only one way to explain how it all could be. This is only possible if time slows down.
Imagine what would happen if time slowed down for you and you didn't know you were moving slower. You will feel like everything else is happening faster., everything around you will move like in a fast-forward movie.
So now let's pretend you're in a downpour again. How is it possible that the rain will affect you in the same way even if you are running? It turns out that if you tried to run away from the rain, then your time would slow down and the rain would speed up. Raindrops would fall on your back at the same speed. Scientists call this expansion of time. No matter how fast you move, your time slows down, at least for the speed of light, this expression is true.
Duality of measurements
Another thing that Einstein and Lorentz found out is that two people under different circumstances can get different calculated values, and the strangest thing is that they will both be right. This is another side effect of the fact that light always travels at the same speed.
Let's do a thought experiment
Imagine that you are standing in the center of your room and you have placed a lamp right in the middle of the room. Now imagine that the speed of light is very slow and you can see how it spreads, imagine that you have turned on the lamp.
As soon as you turn on the lamp, the light will begin to diverge and illuminate. Since both walls are at the same distance, the light will reach both walls at the same time.
Now imagine that your room has a large window and a friend of yours drives by. He will see something else. To him, it will look like your room is moving to the right, and when you turn on the lamp, he will see the left wall moving towards the light. and the right wall moves away from the light. He will see that the light first hit the left wall, and then the right. It seems to him that the light did not illuminate both walls at the same time.
According to Einstein's theory of relativity, both points of view would be right.. From your point of view, the light hits both walls at the same time. From your friend's point of view, this is not the case. There is nothing wrong.
That's why scientists say that "simultaneity is relative." If you are measuring two things that should happen at the same time, then someone who is moving at a different speed or in a different direction will not be able to measure them the same way as you.
This seems very strange to us, because the speed of light for us is instantaneous, and we move very slowly compared to it. Because the speed of light is so fast, we don't notice the speed of light unless we do special experiments.
The faster an object moves, the shorter and smaller it is
Another very strange side effect that the speed of light does not change. At the speed of light, moving things get shorter.
Again, let's imagine that the speed of light is very slow. Imagine that you are on a train and you have installed a lamp in the middle of the car. Now imagine that you have turned on the lamp, as in the room.
The light will spread and simultaneously reach the walls in front and behind the car. This way you can even measure the length of the wagon by measuring how long it took for the light to reach both sides.
Let's do the calculations:
Imagine that it takes 1 second to travel 10 meters and it takes 1 second for the light to travel from the lamp to the wall of the car. This means that the lamp is located at a distance of 10 meters from both sides of the car. Since 10 + 10 = 20, it means that the length of the car is 20 meters.
Now let's imagine that your friend is on the street, watching the train go by. Remember that he sees things differently. The rear wall of the car moves towards the lamp, while the front wall moves away from it. Thus, for him, the light will not touch the front and back of the wall of the car at the same time. First, the light will reach the back, and then to the front.
Thus, if you and your friend measure the speed of light from the lamp to the walls, you will get different values, while from the point of view of science, both calculations will be correct. Only for you, according to the measurements, the length of the wagon will be the same size, and for a friend, the length of the wagon will be less.
Remember, it's all about how and under what conditions you measure. If you were inside a flying rocket that moves at the speed of light, you would not feel anything unusual, unlike people on the ground measuring your movement. You wouldn't be able to tell that time was running slower for you, or that the front and back of the ship were suddenly closer together.
At the same time, if you were flying on a rocket, then it would seem to you as if all the planets and stars are flying past you at the speed of light. In this case, if you try to measure their time and size, then logically for them, time should slow down and size decrease, right?
All this was very strange and incomprehensible, but Einstein proposed a solution and combined all these phenomena into one theory of relativity.
The revolutionary physicist used his imagination, not complex mathematics, to come up with his most famous and elegant equation. Einstein is known for predicting strange but true phenomena, such as slower aging of astronauts in space compared to humans on Earth and changes in the shape of solid objects at high speeds.
But the interesting thing is that if you take a copy of Einstein's original 1905 paper on relativity, it's pretty easy to parse. The text is simple and clear, and the equations are mostly algebraic - any high school student can understand them.
This is because complex mathematics was never Einstein's forte. He liked to think figuratively, to conduct experiments in his imagination and comprehend them until the physical ideas and principles became crystal clear.
Here's how Einstein's thought experiments began when he was only 16 years old, and how they eventually led him to the most revolutionary equation in modern physics.
By this point in Einstein's life, his thinly concealed disdain for his German roots, the authoritarian teaching methods in Germany, had already played a role, and he was kicked out of high school, so he moved to Zurich in hopes of enrolling at the Swiss Federal Institute of Technology (ETH).
But first, Einstein decided to spend a year of training at a school in the nearby city of Aarau. At this point, he soon found himself wondering what it was like to run next to a beam of light.
Einstein had already learned in physics class what a ray of light is: lots of oscillating electric and magnetic fields moving at 300,000 kilometers per second, the measured speed of light. If he ran close at that speed, Einstein realized, he could see many oscillating electric and magnetic fields near him, as if frozen in space.
But it was impossible. First, stationary fields would violate Maxwell's equations, the mathematical laws that contained everything that physicists knew about electricity, magnetism, and light. These laws were (and still are) quite strict: any waves in these fields must travel at the speed of light and cannot stand still, bar none.
Worse, stationary fields did not fit in with the principle of relativity that had been known to physicists since the days of Galileo and Newton in the 17th century. Essentially, the principle of relativity says that the laws of physics cannot depend on how fast you are moving: you can only measure the speed of one object relative to another.
But when Einstein applied this principle to his thought experiment, a contradiction arose: relativity dictated that everything he could see moving near a beam of light, including stationary fields, must be something mundane that physicists could create in the lab. But no one has ever seen this.
This problem will worry Einstein for another 10 years, throughout his journey of studying and working at ETH and moving to the capital of Switzerland, Bern, where he will become an examiner at the Swiss patent office. It is there that he will resolve the paradox once and for all.
1904: light measurement from a moving train
It wasn't easy. Einstein tried every solution that came to his mind, but nothing worked. Almost despairing, he began to consider a simple yet radical solution. Maybe Maxwell's equations work for everything, he thought, but the speed of light has always been constant.
In other words, when you see a beam of light passing by, it doesn't matter if its source is moving towards you, away from you, to the side, or somewhere else, and it doesn't matter how fast its source is moving. The speed of light you measure will always be 300,000 kilometers per second. Among other things, this meant that Einstein would never see stationary oscillating fields, since he would never be able to catch a beam of light.
This was the only way Einstein saw to reconcile Maxwell's equations with the principle of relativity. At first glance, however, this solution had its own fatal flaw. He later explained it with another thought experiment: imagine a beam being fired along a railroad embankment while a train is passing by in the same direction at, say, 3,000 kilometers per second.
Someone standing near the embankment would have to measure the speed of the light beam and come up with a standard number of 300,000 kilometers per second. But someone on the train will see the light moving at 297,000 kilometers per second. If the speed of light is not constant, Maxwell's equation inside the car must look different, Einstein concluded, and then the principle of relativity will be violated.
This seeming contradiction kept Einstein thinking for almost a year. But then, one fine morning in May 1905, he went to work with his best friend Michel Besso, an engineer he had known since his student days in Zurich. The two men talked about Einstein's dilemma, as they always did. And suddenly Einstein saw the solution. He worked on it all night, and when they met the next morning, Einstein said to Besso, “Thank you. I completely solved the problem."
May 1905: lightning strikes a moving train
Einstein's revelation was that observers in relative motion perceive time differently: it is entirely possible for two events to occur simultaneously from the point of view of one observer, but at different times from the point of view of another. And both observers will be right.
Einstein later illustrated his point with another thought experiment. Imagine that the observer is again standing next to the railway and the train is speeding past him. At the moment when the central point of the train passes the observer, lightning strikes at each end of the train. Since lightning strikes at the same distance from the observer, their light enters his eyes at the same time. It is fair to say that the lightning strikes at the same time.
Meanwhile, another observer sits exactly in the center of the train. From his point of view, the light from two lightning strikes travels the same distance and the speed of light will be the same in either direction. But because the train is moving, the light coming from the back lightning has to travel a greater distance, so it arrives at the observer a few moments later than the light from the beginning. Since the light pulses arrive at different times, it can be concluded that the lightning strikes are not simultaneous - one occurs faster.
Einstein realized that it is precisely this simultaneity that is relative. And once you admit it, the strange effects that we now associate with relativity are resolved with simple algebra.
Einstein feverishly wrote down his thoughts and submitted his paper for publication. The title was On the Electrodynamics of Moving Bodies, and it reflected Einstein's attempt to link Maxwell's equations with the principle of relativity. Besso received a special thanks.
September 1905: mass and energy
This first work, however, did not become the last. Einstein was obsessed with relativity until the summer of 1905, and in September he submitted a second paper for publication, already after the fact, retroactively.
It was based on yet another thought experiment. Imagine an object at rest, he said. Now imagine that it simultaneously emits two identical pulses of light in opposite directions. The object will stay in place, but since each pulse carries a certain amount of energy, the energy contained in the object will decrease.
Now, wrote Einstein, what would this process look like to a moving observer? From his point of view, the object will simply continue to move in a straight line while the two pulses fly away. But even if the speed of the two pulses remains the same - the speed of light - their energies will be different. An impulse that is moving forward in the direction of travel will have a higher energy than one that is moving in the opposite direction.
Adding a bit of algebra, Einstein showed that for all of this to be consistent, the object must not only lose energy when sending light pulses, but also mass. Or mass and energy must be interchangeable. Einstein wrote down an equation that connects them. And it became the most famous equation in the history of science: E = mc 2 .
Even at the end of the 19th century, most scientists were inclined to the point of view that the physical picture of the world was basically built and would remain unshakable in the future - only the details had to be clarified. But in the first decades of the twentieth century, physical views changed radically. This was the result of a "cascade" of scientific discoveries made during an extremely short historical period, spanning the last years of the 19th century and the first decades of the 20th, many of which did not fit at all into the representation of ordinary human experience. A striking example is the theory of relativity created by Albert Einstein (1879-1955).
Theory of relativity- physical theory of space-time, that is, a theory that describes the universal space-time properties of physical processes. The term was introduced in 1906 by Max Planck to emphasize the role of the principle of relativity.
in special relativity (and, later, general relativity).
In a narrow sense, the theory of relativity includes special and general relativity. Special theory of relativity(hereinafter referred to as SRT) refers to processes in the study of which gravitational fields can be neglected; general theory of relativity(hereinafter referred to as GR) is a theory of gravitation that generalizes Newton's.
Special, or private theory of relativity
is a theory of the structure of space-time. It was first introduced in 1905 by Albert Einstein in his work "On the Electrodynamics of Moving Bodies". The theory describes movement, the laws of mechanics, as well as the space-time relationships that determine them, at any speed of movement,
including those close to the speed of light. Classical Newtonian mechanics
within SRT is an approximation for low velocities.
One of the reasons for Albert Einstein's success is that he put experimental data ahead of theoretical data. When a number of experiments showed results that contradicted the generally accepted theory, many physicists decided that these experiments were erroneous.
Albert Einstein was one of the first who decided to build a new theory based on new experimental data.
At the end of the 19th century, physicists were in search of a mysterious ether - a medium in which, according to generally accepted assumptions, light waves should have propagated, like acoustic waves, for the propagation of which air is needed, or another medium - solid, liquid or gaseous. Belief in the existence of the aether led to the belief that the speed of light must change with the speed of the observer with respect to the aether. Albert Einstein abandoned the concept of aether and assumed that all physical laws, including the speed of light, remain unchanged regardless of the speed of the observer - as experiments showed.
SRT explained how to interpret motions between different inertial frames of reference - simply put, objects that are moving at a constant speed relative to each other. Einstein explained that when two objects move at a constant speed, one should consider their motion relative to each other, instead of taking one of them as an absolute frame of reference. So if two astronauts are flying on two spaceships and want to compare their observations, the only thing they need to know is their speed relative to each other.
Special relativity considers only one special case (hence the name), when the motion is straight and uniform.
Based on the impossibility of detecting absolute motion, Albert Einstein concluded that all inertial frames of reference are equal. He formulated two important postulates that formed the basis of a new theory of space and time, called the Special Theory of Relativity (SRT):
1. Einstein's principle of relativity - this principle was a generalization of Galileo's principle of relativity (states the same, but not for all laws of nature, but only for the laws of classical mechanics, leaving open the question of the applicability of the principle of relativity to optics and electrodynamics) to any physical. It says: all physical processes under the same conditions in inertial reference systems (ISF) proceed in the same way. This means that no physical experiments carried out inside a closed IRF can determine whether it is at rest or moving uniformly and rectilinearly. Thus, all IFRs are completely equal, and physical laws are invariant with respect to the choice of IFR (ie, the equations expressing these laws have the same form in all inertial frames of reference).
2. The principle of constancy of the speed of light- the speed of light in vacuum is constant and does not depend on the movement of the source and receiver of light. It is the same in all directions and in all inertial frames of reference. The speed of light in a vacuum - the limiting speed in nature - this is one of the most important physical constants, the so-called world constants.
The most important consequence of SRT was the famous Einstein's formula on the relationship between mass and energy E \u003d mc 2 (where C is the speed of light), which showed the unity of space and time, expressed in a joint change in their characteristics depending on the concentration of masses and their movement, and confirmed by the data of modern physics. Time and space were no longer considered independently of each other, and the idea of a space-time four-dimensional continuum arose.
According to the theory of the great physicist, when the speed of a material body increases, approaching the speed of light, its mass also increases. Those. the faster an object moves, the heavier it becomes. In the case of reaching the speed of light, the mass of the body, as well as its energy, become infinite. The heavier the body, the more difficult it is to increase its speed; an infinite amount of energy is required to accelerate a body with infinite mass, so it is impossible for material objects to reach the speed of light.
In the theory of relativity, "two laws - the law of conservation of mass and conservation of energy - lost their validity independent of each other and turned out to be combined into a single law, which can be called the law of conservation of energy or mass." Due to the fundamental connection between these two concepts, matter can be turned into energy, and vice versa - energy into matter.
General theory of relativity- The theory of gravity published by Einstein in 1916, which he worked on for 10 years. It is a further development of the special theory of relativity. If the material body accelerates or turns to the side, the SRT laws no longer apply. Then GR comes into force, which explains the motions of material bodies in the general case.
The general theory of relativity postulates that gravitational effects are caused not by the force interaction of bodies and fields, but by the deformation of the very space-time in which they are located. This deformation is associated, in particular, with the presence of mass-energy.
General Relativity is currently the most successful theory of gravity, well supported by observations. General relativity has generalized SRT to accelerated ones, i.e. non-inertial systems. The basic principles of general relativity are as follows:
- limiting the applicability of the principle of constancy of the speed of light to areas where gravitational forces can be neglected(where gravity is strong, the speed of light slows down);
- extension of the principle of relativity to all moving systems(and not just inertial ones).
In general relativity, or the theory of gravitation, he also proceeds from the experimental fact of the equivalence of inertial and gravitational masses, or the equivalence of inertial and gravitational fields.
The principle of equivalence plays an important role in science. We can always calculate directly the action of the forces of inertia on any physical system, and this gives us the opportunity to know the action of the gravitational field, abstracting from its inhomogeneity, which is often very insignificant.
A number of important conclusions have been drawn from GR:
1. The properties of space-time depend on the moving matter.
2. A beam of light, which has an inert, and, consequently, gravitational mass, must be bent in the gravitational field.
3. The frequency of light under the influence of the gravitational field should shift towards lower values.
For a long time, there were few experimental confirmations of general relativity. The agreement between theory and experiment is quite good, but the purity of the experiments is violated by various complex side effects. However, the effect of space-time curvature can be detected even in moderate gravitational fields. Very sensitive clocks, for example, can detect time dilation on the Earth's surface. In order to expand the experimental base of general relativity, in the second half of the 20th century, new experiments were carried out: the equivalence of the inertial and gravitational masses was tested (including by laser ranging of the Moon);
with the help of radar, the movement of the perihelion of Mercury was clarified; the gravitational deflection of radio waves by the Sun was measured, the planets of the solar system were radar-located; the influence of the gravitational field of the Sun on radio communications with spacecraft that were sent to the distant planets of the solar system was evaluated, etc. All of them, one way or another, confirmed the predictions obtained on the basis of general relativity.
So, the special theory of relativity is based on the postulates of the constancy of the speed of light and the sameness of the laws of nature in all physical systems, and the main results to which it comes are as follows: the relativity of the properties of space-time; relativity of mass and energy; equivalence of heavy and inertial masses.
The most significant result of the general theory of relativity from a philosophical point of view is the establishment of the dependence of the space-time properties of the surrounding world on the location and movement of gravitating masses. It is due to the influence of bodies
with large masses there is a curvature of the paths of movement of light rays. Consequently, the gravitational field created by such bodies ultimately determines the space-time properties of the world.
The special theory of relativity abstracts from the action of gravitational fields and therefore its conclusions are applicable only for small areas of space-time. The fundamental difference between the general theory of relativity and the fundamental physical theories preceding it is in the rejection of a number of old concepts and the formulation of new ones. It is worth saying that the general theory of relativity has made a real revolution in cosmology. On its basis, various models of the Universe have appeared.
The special theory of relativity, created in 1905 by A. Einstein, was the result of a generalization and synthesis of the classical mechanics of Galileo - Newton and electrodynamics of Maxwell - Lorentz. “It describes the laws of all physical processes at speeds close to the speed of light, but without taking into account the gravitational field. With a decrease in the speed of motion, it reduces to classical mechanics, which, therefore, turns out to be its special case. one
The starting point of this theory was the principle of relativity. The classical principle of relativity was formulated by G. Galileo: “If the laws of mechanics are valid in one coordinate system, then they are valid in any other system moving rectilinearly and uniformly relative to the first one.” 2 Such systems are called inertial, since the movement in them obeys the law of inertia: “Every body retains a state of rest or uniform rectilinear motion, unless it is forced to change it under the influence of moving forces.” 3
It follows from the principle of relativity that there is no fundamental difference between rest and motion - if it is uniform and rectilinear. The difference is only in the point of view.
Thus, the word "relatively" in the name of Galileo's principle does not hide anything special in itself. It has no other meaning than that which we put into motion, that motion or rest is always motion or rest relative to something that serves us as a frame of reference. This, of course, does not mean that there is no difference between rest and uniform motion. But the concepts of rest and movement acquire meaning only when a reference point is indicated.
If the classical principle of relativity asserted the invariance of the laws of mechanics in all inertial reference frames, then in the special theory of relativity this principle was also extended to the laws of electrodynamics, and the general theory of relativity asserted the invariance of the laws of nature in any reference frames, both inertial and non-inertial. Non-inertial reference systems are called, moving with deceleration or acceleration.
In accordance with the special theory of relativity, which combines space and time into a single four-dimensional space-time continuum, the space-time properties of bodies depend on the speed of their movement. The spatial dimensions are reduced in the direction of motion when the speed of bodies approaches the speed of light in vacuum (300,000 km/s), time processes slow down in fast-moving systems, and body mass increases.
Being in a comoving frame of reference, that is, moving parallel and at the same distance from the measured frame, one cannot notice these effects, which are called relativistic, since all spatial scales and parts used in measurements will change in exactly the same way. According to the principle of relativity, all processes in inertial frames of reference proceed in the same way. But if the system is non-inertial, then relativistic effects can be noticed and changed. So, if an imaginary relativistic ship like a photon rocket goes to distant stars, then after its return to Earth, the time in the ship’s system will pass significantly less than on Earth, and this difference will be the greater, the farther the flight is made, and the speed of the ship will be closer to the speed of light. The difference can even be measured in hundreds and thousands of years, as a result of which the crew of the ship will immediately be transported to the near or distant future, bypassing the intermediate time, since the rocket, along with the crew, fell out of the course of development on Earth.
Similar processes of slowing down the passage of time depending on the speed of movement are actually recorded now in measurements of the path length of mesons arising from the collision of particles of primary cosmic radiation with the nuclei of atoms on Earth. Mesons exist for 10 -6 - 10 -15 s (depending on the type of particles) and after their appearance they decay at a small distance from the place of birth. All this can be registered by measuring devices on traces of particle runs. But if the meson moves at a speed close to the speed of light, then the time processes in it slow down, the decay period increases (by thousands and tens of thousands of times), and, accordingly, the path length from birth to decay increases.
So, the special theory of relativity is based on Galileo's extended principle of relativity. In addition, it uses another new position: the speed of light propagation (in a vacuum) is the same in all inertial frames of reference.
But why is this speed so important that the judgment about it is equated in value with the principle of relativity? The fact is that we are confronted here with the second universal physical constant. The speed of light is the largest of all speeds in nature, the limiting speed of physical interactions. The movement of light is fundamentally different from the movement of all other bodies, the speed of which is less than the speed of light. The speed of these bodies always adds up with other speeds. In this sense, the speeds are relative: their magnitude depends on the point of view. And the speed of light does not add up with other speeds, it is absolute, always the same, and, speaking of it, we do not need to specify the frame of reference.
The absoluteness of the speed of light does not contradict the principle of relativity and is fully compatible with it. The constancy of this speed is a law of nature, and therefore - precisely in accordance with the principle of relativity - it is valid in all inertial frames of reference.
The speed of light is the upper limit for the speed of movement of any bodies in nature, for the speed of propagation of any waves, any signals. It is maximum - this is an absolute speed record.
“For all physical processes, the speed of light has the property of infinite speed. In order to give a body a speed equal to the speed of light, an infinite amount of energy is required, and that is why it is physically impossible for any body to reach this speed. This result was confirmed by measurements that were carried out on electrons. The kinetic energy of a point mass grows faster than the square of its speed, and becomes infinite for a speed equal to the speed of light” 1 . Therefore, it is often said that the speed of light is the limiting speed of information transfer. And the ultimate speed of any physical interactions, and indeed of all conceivable interactions in the world.
Closely related to the speed of light is the solution to the problem of simultaneity, which also turns out to be relative, that is, depending on the point of view. In classical mechanics, which considered time to be absolute, simultaneity is also absolute.
In the general theory of relativity, new aspects of the dependence of space-time relations on material processes were revealed. This theory summed up the physical foundations for non-Euclidean geometries and connected the curvature of space and the deviation of its metric from the Euclidean one with the action of gravitational fields created by the masses of bodies. The general theory of relativity proceeds from the principle of equivalence of inertial and gravitational masses, the quantitative equality of which was established long ago in classical physics. The kinematic effects arising under the action of gravitational forces are equivalent to the effects arising under the action of acceleration. So, if a rocket takes off with an acceleration of 2g, then the rocket crew will feel as if they are in twice the Earth's gravity field. It was on the basis of the principle of equivalence of masses that the principle of relativity was generalized, which asserts in the general theory of relativity the invariance of the laws of nature in any frames of reference, both inertial and non-inertial.
How can one imagine the curvature of space that general relativity speaks of? Let's imagine a very thin sheet of rubber, and we will consider that this is a model of space. Let's place on this sheet large and small balls - models of stars. These balls will bend the rubber sheet the more, the greater the mass of the ball. This clearly demonstrates the dependence of the curvature of space on the mass of the body and also shows that the usual Euclidean geometry does not work in this case (the geometries of Lobachevsky and Riemann work).
The theory of relativity established not only the curvature of space under the influence of gravitational fields, but also the slowing down of time in strong gravitational fields. Even the gravity of the Sun - a rather small star by cosmic standards - affects the rate of time passing, slowing it down near itself. Therefore, if we send a radio signal to some point, the path to which passes near the Sun, the journey of the radio signal in this case will take longer than when there is nothing in the way of this signal. The deceleration near the Sun is about 0.0002 s.
One of the most fantastic predictions of general relativity is the complete stoppage of time in a very strong gravitational field. The slowing down of time is the greater, the stronger the gravity. Time dilation is manifested in the gravitational redshift of light: the stronger the gravitation, the more the wavelength increases and its frequency decreases. Under certain conditions, the wavelength can tend to infinity, and its frequency - to zero.
With the light emitted by the Sun, this could happen if our star were suddenly compressed and turned into a ball with a radius of 3 km or less (the radius of the Sun is 700,000 km). Because of this compression, the gravitational force on the surface, where the light comes from, increases so much that the gravitational redshift turns out to be truly infinite.
With our Sun, this will never actually happen. But other stars, whose masses are three or more times the mass of the Sun, at the end of their lives and really experience, most likely, a rapid catastrophic compression under the influence of their own gravity. This will lead them to the state of a black hole. A black hole is a physical body that creates such a strong gravity that the redshift for light emitted near it can go to infinity.
Physicists and astronomers are quite sure that black holes exist in nature, although so far they have not been detected. The difficulties of astronomical searches are connected with the very nature of these unusual objects. After all, the infinite redshift, due to which the frequency of the received light vanishes, makes them simply invisible. They do not shine, and therefore in the full sense of the word they are black. Only by a number of indirect signs can we hope to notice a black hole, for example, in a binary star system, where an ordinary star would be its partner. From observations of the movement of a visible star in the general gravitational field of such a pair, it would be possible to estimate the mass of an invisible star, and if this value exceeds the mass of the Sun by three or more times, it will be possible to assert that we have found a black hole.
Now there are several well-studied binary systems in which the mass of the invisible partner is estimated at 5 or even 8 solar masses. Most likely, these are black holes, but astronomers prefer to call these objects candidates for black holes until these estimates are refined.
Gravitational time dilation, which is measured and evidenced by redshift, is very significant near a neutron star, and near a black hole, near its gravitational radius, it is so great that time seems to freeze there.
For a body falling into the gravitational field of a black hole formed by a mass equal to 3 solar masses, the fall from a distance of 1 million km to the gravitational radius takes only about an hour. But according to the clock that rests far from the black hole, the free fall of the body in its field will stretch in time to infinity. The closer the falling body is to the gravitational radius, the slower this flight will appear to a distant observer. A body observed from afar will approach the gravitational radius indefinitely and never reach it. This is how time slows down near a black hole. Thus, matter influences the properties of space and time.
The concepts of space and time formulated in Einstein's theory of relativity are by far the most consistent. But they are macroscopic, as they are based on the experience of studying macroscopic objects, large distances and long time intervals. When constructing theories describing the phenomena of the microcosm, this classical geometric picture, assuming the continuity of space and time (space-time continuum), was transferred to a new area without any changes. There are no experimental data that contradict the application of the theory of relativity in the microcosm. But the very development of quantum theories may require a revision of ideas about physical space and time. The developed theory of superstrings, which represents elementary particles as harmonic vibrations of these strings and connects physics with geometry, proceeds from the multidimensionality of space. And this means that at a new stage in the development of science, at a new level of knowledge, we are returning to the predictions of A. Einstein in 1930: “We come to a strange conclusion: now it begins to seem to us that space plays the primary role, while matter must be obtained from space, so to speak, in the next step. We have always regarded matter as primary and space as secondary. Space, figuratively speaking, is now taking revenge and “eats” matter” 1 . Perhaps there is a quantum of space, a fundamental length L. By introducing this concept, we can avoid many of the difficulties of modern quantum theories. If its existence is confirmed, then L will become the third (besides Planck's constant and the speed of light in a vacuum) fundamental constant in physics. The existence of a quantum of space also implies the existence of a quantum of time (equal to L/c), which limits the accuracy of determining time intervals.
A hundred years ago, in 1915, a young Swiss scientist, who at that time had already made revolutionary discoveries in physics, proposed a fundamentally new understanding of gravity.
In 1915, Einstein published the general theory of relativity, which characterizes gravity as a basic property of spacetime. He presented a series of equations describing the effect of the curvature of space-time on the energy and motion of the matter and radiation present in it.
One hundred years later, the general theory of relativity (GR) became the basis for the construction of modern science, it has withstood all the tests with which scientists attacked it.
But until recently, it was not possible to conduct experiments under extreme conditions to test the stability of the theory.
It's amazing how strong the theory of relativity has proven to be over 100 years. We are still using what Einstein wrote!
Clifford Will, theoretical physicist, University of Florida
Scientists now have the technology to search for physics beyond general relativity.
A new look at gravity
General relativity describes gravity not as a force (as it appears in Newtonian physics), but as a curvature of space-time due to the mass of objects. The Earth revolves around the Sun, not because the star attracts it, but because the Sun deforms space-time. If a heavy bowling ball is placed on a stretched blanket, the blanket will change shape - gravity affects space in much the same way.
Einstein's theory predicted some crazy discoveries. For example, the possibility of the existence of black holes, which bend space-time to such an extent that nothing can escape from the inside, not even light. Based on the theory, evidence was found for the generally accepted opinion today that the universe is expanding and accelerating.
The general theory of relativity has been confirmed by numerous observations. Einstein himself used general relativity to calculate the orbit of Mercury, whose motion cannot be described by Newton's laws. Einstein predicted the existence of objects so massive that they bend light. This is a gravitational lensing phenomenon that astronomers often encounter. For example, the search for exoplanets is based on the effect of subtle changes in the radiation curved by the gravitational field of the star around which the planet revolves.
Testing Einstein's Theory
The general theory of relativity works well for ordinary gravity, as shown by experiments on Earth and observations of the planets of the solar system. But it has never been tested under conditions of extremely strong influence of fields in spaces lying on the boundaries of physics.
The most promising way to test the theory under such conditions is to observe changes in spacetime, which are called gravitational waves. They appear as a result of large events, during the merger of two massive bodies, such as black holes, or especially dense objects - neutron stars.
A cosmic firework of this magnitude would only have the smallest ripples in space-time. For example, if two black holes collided and merged somewhere in our Galaxy, gravitational waves could stretch and compress the distance between objects on Earth a meter apart by one thousandth of the diameter of an atomic nucleus.
Experiments have appeared that can record changes in space-time due to such events.
There is a good chance to fix gravitational waves in the next two years.
Clifford Will
The Laser Interferometric Gravitational Wave Observatory (LIGO), with observatories near Richland, Washington, and Livingston, Louisiana, uses a laser to detect minute distortions in dual L-shaped detectors. As space-time ripples pass through the detectors, they stretch and compress space, causing the detector to change dimensions. And LIGO can measure them.
LIGO started a series of launches in 2002 but didn't hit the mark. Improvements were made in 2010, and the organization's successor, the Advanced LIGO Observatory, should be up and running again this year. Many of the planned experiments are aimed at searching for gravitational waves.
Another way to test the theory of relativity is to look at the properties of gravitational waves. For example, they can be polarized, like light passing through polarized glasses. The theory of relativity predicts the features of such an effect, and any deviations from the calculations may become a reason to doubt the theory.
unified theory
Clifford Will believes that the discovery of gravitational waves will only strengthen Einstein's theory:
I think we need to keep looking for proof of general relativity to be sure it's right.
Why are these experiments needed at all?
One of the most important and elusive tasks of modern physics is the search for a theory that will link together Einstein's research, that is, the science of the macrocosm, and quantum mechanics, the reality of the smallest objects.
Advances in this direction, quantum gravity, may require changes to the general theory of relativity. It is possible that experiments in the field of quantum gravity will require so much energy that they will be impossible to carry out. “But who knows,” Will says, “maybe there is an effect in the quantum universe, insignificant, but searchable.”
