Einstein's theory of relativity has always seemed abstract and incomprehensible to me. Let's try to describe Einstein's theory of relativity in simple words. Imagine being outside in heavy rain with the wind blowing at your back. If you start running fast, raindrops will not fall on your back. The drops will be slower or not reach your back at all, this is a scientifically proven fact, and you can check it yourself in a rainstorm. Now imagine if you turned around and ran against the wind with rain, the drops would hit your clothes and face harder than if you just stood.
Scientists previously thought that light acted like rain in windy weather. They thought that if the Earth moved around the Sun, and the Sun moved around the galaxy, then it would be possible to measure the speed of their movement in space. In their opinion, all they have to do is measure the speed of light and how it changes relative to two bodies.
Scientists did it and found something very strange. The speed of light was the same, no matter what, no matter how the bodies moved and no matter in which direction the measurements were taken.
It was very strange. If we take the situation with a rainstorm, then under normal circumstances the raindrops will affect you more or less depending on your movements. Agree, it would be very strange if a rainstorm blew at your back with equal 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 move, and no matter what 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 Lorentz, figured out that there was only one way to explain how all this 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 that you were moving slower. You will feel like everything else is happening faster., everything around you will move, like in a movie in fast forward.
So now let's imagine that you are again in a windy downpour. How is it possible that rain will affect you the same even if you are running? It turns out that if you were trying to run away from the rain, then your time would slow down and the rain would speed up. Raindrops would hit your back at the same speed. Scientists call this time dilation. No matter how fast you move, your time slows down, at least for the speed of light this expression is true.
Duality of dimensions
Another thing that Einstein and Lorentz figured out was 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 light always moving at the same speed.
Let's do a thought experiment
Imagine that you are standing in the center of your room and you have installed 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 travels, imagine that you turn on a lamp.
As soon as you turn on the lamp, the light will begin to spread out and illuminate. Since both walls are at the same distance, the light will reach both walls at the same time.
Now imagine that there is a large window in your room, 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 will seem 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 will be right. From your point of view, light hits both walls at the same time. From your friend's point of view, this is not so. There is nothing wrong.
This is why scientists say that “simultaneity is relative.” If you measure two things that are supposed to happen at the same time, then someone moving at a different speed or in a different direction will not be able to measure them in the same way as you.
This seems very strange to us, because the speed of light is instantaneous for us, and we move very slowly in comparison. Since the speed of light is so high, we do not notice the speed of light until we carry out 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 become shorter.
Again, let's imagine that the speed of light is very slow. Imagine that you are traveling on a train and you have installed a lamp in the middle of the carriage. Now imagine that you turn on a lamp, like in a 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 carriage by measuring how long it took the light to reach both sides.
Let's do the calculations:
Let's imagine that it takes 1 second to travel 10 meters and it takes 1 second for the light to spread from the lamp to the wall of the carriage. This means that the lamp is located 10 meters from both sides of the car. Since 10 + 10 = 20, this means the length of the car is 20 meters.
Now let's imagine that your friend is on the street, watching a train pass by. Remember that he sees things differently. The rear wall of the carriage moves towards the lamp, and the front wall moves away from it. This way, the light will not touch the front and back of the wall of the car at the same time. The light will reach the back first and then the front.
Thus, if you and your friend measure the speed of light propagation from the lamp to the walls, you will get different values, but from a scientific point of view, both calculations will be correct. Only for you, according to the measurements, the length of the carriage will be the same size, but for a friend, the length of the carriage will be less.
Remember, it's all about how and under what conditions you take measurements. If you were inside a rocket moving at the speed of light, you would not feel anything unusual, unlike the people on the ground measuring your movement. You wouldn't be able to realize that time was moving slower for you, or that the front and back of the ship had suddenly become closer to each other.
At the same time, if you were flying on a rocket, it would seem to you as if all the planets and stars were 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 their sizes should 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 rather than complex mathematics to come up with his most famous and elegant equation. Einstein is known for predicting strange but true phenomena, such as astronauts in space aging slower than people on Earth and the shapes of solid objects changing at high speeds.
But what's interesting is that if you pick up a copy of Einstein's original 1905 paper on relativity, it's fairly easy to decipher. 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 strong point. He loved to think visually, to perform experiments in his imagination and to think through them until the physical ideas and principles became crystal clear.
This is where Einstein's thought experiments began when he was just 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 ill-concealed disdain for his German roots and Germany's authoritarian teaching methods had already taken its toll, and he had been kicked out of high school, so he moved to Zurich in hopes of attending the Swiss Federal Institute of Technology (ETH).
But first, Einstein decided to spend a year of preparation at a school in the neighboring town of Aarau. At this point, he soon found himself wondering what it would be like to run next to a beam of light.
Einstein had already learned in physics class what a beam of light was: a set of oscillating electric and magnetic fields moving at 300,000 kilometers per second, the measured speed of light. If he ran nearby at the same speed, Einstein realized, he could see many oscillating electric and magnetic fields next to him, as if frozen in space.
But this was impossible. First, stationary fields would violate Maxwell's equations, the mathematical laws that underlie everything 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, no exceptions.
Worse, stationary fields did not fit with the principle of relativity, which 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 anything he could see when moving near a beam of light, including stationary fields, must be something mundane that physicists could create in the laboratory. But no one has ever observed this.
This problem would haunt Einstein for another 10 years, as he studied and worked at ETH and moved on to the Swiss capital of Bern, where he would become an examiner at the Swiss patent office. It is there that he will resolve the paradox once and for all.
1904: Measuring light from a moving train
It wasn't easy. Einstein tried every solution he could think of, but nothing worked. Almost in despair, he began to think about a simple, yet radical solution. Perhaps Maxwell's equations worked for everything, he thought, but the speed of light had always been constant.
In other words, when you see a beam of light fly by, it doesn't matter whether its source is moving towards you, away from you, away from you, or anywhere else, and it doesn't matter how fast its source is moving. The speed of light that 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 that is fired along a railway embankment while a train passes by in the same direction at, say, 3000 kilometers per second.
Someone standing near the embankment would have to measure the speed of the light beam and get the standard number of 300,000 kilometers per second. But someone on a train will see light moving at 297,000 kilometers per second. If the speed of light is not constant, Maxwell's equation inside the carriage should look different, Einstein concluded, and then the principle of relativity would be violated.
This apparent contradiction gave Einstein pause for almost a year. But then, one fine morning in May 1905, he was walking 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 quite 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 an observer is again standing next to the railway and a train is rushing past him. At the moment when the center point of the train passes the observer, lightning strikes 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 would be fair to say that lightning strikes simultaneously.
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 any direction. But because the train is moving, the light coming from the rear 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, we can conclude that the lightning strikes are not simultaneous - one occurs faster.
Einstein realized that it is precisely this simultaneity that is relative. And once you accept this, the strange effects we now associate with relativity are resolved using simple algebra.
Einstein feverishly wrote down his thoughts and submitted his work for publication. The title was “On the Electrodynamics of Moving Bodies,” and it reflected Einstein’s attempt to connect Maxwell’s equations with the principle of relativity. Besso received special thanks.
September 1905: mass and energy
This first work, however, was not the last. Einstein was obsessed with relativity until the summer of 1905, and in September he submitted a second paper for publication, this time in retrospect.
It was based on 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 remain in place, but since each pulse carries away a certain amount of energy, the energy contained in the object will decrease.
Now, Einstein wrote, 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 moves forward in the direction of travel will have higher energy than one that moves in the opposite direction.
Adding a little algebra, Einstein showed that for this to be consistent, the object must not only lose energy when sending out light pulses, but also mass. Or mass and energy should 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 constructed and would remain unshakable in the future - only the details remained to be clarified. But in the first decades of the twentieth century, physical views changed radically. This was the consequence of a “cascade” of scientific discoveries made during an extremely short historical period, covering the last years of the 19th century and the first decades of the 20th, many of which were completely inconsistent with the understanding 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 - SRT) refers to processes in the study of which gravitational fields can be neglected; general theory of relativity(hereinafter referred to as GTR) is a theory of gravitation that generalizes Newton’s.
Special, or special 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 the framework of SRT, it is an approximation for low speeds.
One of the reasons for Albert Einstein's success is that he valued experimental data over theoretical data. When a number of experiments revealed results that contradicted the generally accepted theory, many physicists decided that these experiments were wrong.
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 the mysterious ether - a medium in which, according to generally accepted assumptions, light waves should propagate, like acoustic waves, the propagation of which requires air, or another medium - solid, liquid or gaseous. Belief in the existence of the ether led to the belief that the speed of light should vary depending on the speed of the observer in relation to the ether. Albert Einstein abandoned the concept of the ether 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 reference frames—simply put, objects that move at a constant speed relative to each other. Einstein explained that when two objects are moving at constant speed, one should consider their motion relative to each other, rather than taking one of them as an absolute frame of reference. So if two astronauts are flying on two spacecraft and want to compare their observations, the only thing they need to know is the speed relative to each other.
The special theory of relativity considers only one special case (hence the name), when the motion is rectilinear and uniform.
Based on the impossibility of detecting absolute motion, Albert Einstein concluded that all inertial reference systems are equal. He formulated two most important postulates that formed the basis of a new theory of space and time, called the Special Theory of Relativity (STR):
1. Einstein's principle of relativity - this principle was a generalization of Galileo’s principle of relativity (states the same thing, 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 ones. It reads: all physical processes under the same conditions in inertial reference systems (IRS) proceed in the same way. This means that no physical experiments carried out inside a closed ISO can establish whether it is at rest or moving uniformly and rectilinearly. Thus, all IFRs are completely equal, and the physical laws are invariant with respect to the choice of IFRs (i.e., the equations expressing these laws have the same form in all inertial reference systems).
2. The principle of the constancy of the speed of light- the speed of light in a 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 is 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 about the relationship between mass and energy E=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 ceased to be 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. If the speed of light is reached, 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; Accelerating a body with infinite mass requires an infinite amount of energy, 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 independent validity and found themselves combined into a single law, which can be called the law of conservation of energy or mass." Thanks to the fundamental connection between these two concepts, matter can be turned into energy, and vice versa - energy into matter.
General theory of relativity- a 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 a material body accelerates or turns to the side, the laws of STR no longer apply. Then GTR comes into force, which explains the movements 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 space-time itself in which they are located. This deformation is related, in part, to the presence of mass-energy.
General relativity is currently the most successful theory of gravity, well supported by observations. GR generalized SR to accelerated ones, i.e. non-inertial systems. The basic principles of general relativity boil down to the following:
- limitation of the applicability of the principle of constancy of the speed of light to regions where gravitational forces can be neglected(where gravity is high, 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 gravity, it 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 directly calculate the effect of inertial forces on any physical system, and this gives us the opportunity to know the effect of the gravitational field, abstracting from its heterogeneity, which is often very insignificant.
A number of important conclusions were obtained from general relativity:
1. The properties of space-time depend on moving matter.
2. A ray of light, which has an inert and, therefore, 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 was little experimental evidence of general relativity. The agreement between theory and experiment is quite good, but the purity of experiments is violated by various complex side effects. However, the effects of spacetime curvature can be detected even in moderate gravitational fields. Very sensitive clocks, for example, can detect time dilation on the Earth's surface. To expand the experimental base of general relativity, new experiments were carried out in the second half of the 20th century: the equivalence of inertial and gravitational masses was tested (including by laser ranging of the Moon);
using radar, the movement of Mercury's perihelion was clarified; the gravitational deflection of radio waves by the Sun was measured, and radar was carried out on the planets of the Solar System; 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 assessed, 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 same 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 inert 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 thanks to the influence of bodies
With large masses, the paths of light rays are bent. 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 to small areas of space-time. The cardinal difference between the general theory of relativity and the fundamental physical theories that preceded it is 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 emerged.
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 of movement close to the speed of light, but without taking into account the gravitational field. When the speed of movement decreases, it reduces to classical mechanics, which, thus, turns out to be its special case.” 1
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.” 2 Such systems are called inertial, since the movement in them is subject to the law of inertia: “Every body maintains a state of rest or uniform rectilinear motion, unless it is forced to change it under the influence of moving forces.” 3
From the principle of relativity it follows that between rest and motion - if it is uniform and rectilinear - there is no fundamental difference. The only difference is the point of view.
Thus, the word “relatively” in the name of Galileo’s principle does not hide anything special. It has no other meaning than the one we put into motion, that motion or rest is always motion or rest relative to something that serves as a frame of reference for us. This, of course, does not mean that there is no difference between rest and uniform motion. But the concepts of rest and motion 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 frames of reference, 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 frame of reference, both inertial and non-inertial. Non-inertial reference systems are those moving with deceleration or acceleration.
In accordance with the special theory of relativity, which unites space and time into a single four-dimensional space-time continuum, the space-time properties of bodies depend on the speed of their movement. Spatial dimensions are reduced in the direction of motion as 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 reference frame, that is, moving parallel and at the same distance from the measured system, it is impossible to 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 reference systems proceed in the same way. But if the system is non-inertial, then relativistic effects can be noticed and changed. Thus, if an imaginary relativistic ship such as a photon rocket goes to distant stars, then after it returns to Earth, the time in the ship’s system will pass significantly less than on Earth, and this difference will be greater the further the flight is made, and the speed of the ship will be closer to 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 and 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 registered now in measurements of the path length of mesons that arise when particles of primary cosmic radiation collide with the nuclei of atoms on Earth. Mesons exist for 10 -6 - 10 -15 s (depending on the type of particles) and after their occurrence they decay at a short distance from the place of birth. All this can be recorded by measuring devices based on the traces of particle travel. 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 (thousands and tens of thousands of times), and the path length from birth to decay increases accordingly.
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 propagation of light (in vacuum) is the same in all inertial frames of reference.
But why is this speed so important that a judgment about it is equated in importance to the principle of relativity? The fact is that here we are faced with the second universal physical constant. The speed of light is the highest of all speeds in nature, the maximum speed of physical interactions. The movement of light is fundamentally different from the movement of all other bodies whose speed is less than the speed of light. The speed of these bodies always adds up to other speeds. In this sense, speeds are relative: their magnitude depends on the point of view. And the speed of light does not add up to other speeds, it is absolute, always the same, and when talking about it, we do not need to indicate a reference system.
The absoluteness of the speed of light does not contradict the principle of relativity and is completely 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 impart a speed to a body 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 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 maximum speed of information transfer. And the maximum speed of any physical interactions, and indeed all conceivable interactions in the world.
The speed of light is closely related to 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 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 provided a physical basis 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 is based on the principle of equivalence of inertial and gravitational masses, the quantitative equality of which was long ago established in classical physics. Kinematic effects arising under the influence of gravitational forces are equivalent to the effects arising under the influence of acceleration. So, if a rocket takes off with an acceleration of 2 g, then the rocket crew will feel as if they are in twice the gravity field of the Earth. 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 frame of reference, both inertial and non-inertial.
How can one imagine the curvature of space, which is described by the general theory of relativity? Let's imagine a very thin sheet of rubber, and we will assume that this is a model of space. Let's place large and small balls on this sheet - 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 slowdown of time in strong gravitational fields. Even the gravity of the Sun - a fairly small star by cosmic standards - affects the pace of time, 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 will take longer in this case than when there is nothing in the path of this signal. The slowdown near the Sun is about 0.0002 s.
One of the most fantastic predictions of the general theory of relativity is the complete stop of time in a very strong gravitational field. The stronger the gravity, the greater the time dilation. Time dilation manifests itself in the gravitational redshift of light: the stronger the gravity, the more the wavelength increases and its frequency decreases. Under certain conditions, the wavelength can rush to infinity, and its frequency - to zero.
With the light emitted by the Sun, this could happen if our star suddenly shrank 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 is truly infinite.
This will never actually happen with our Sun. But other stars, whose masses are three or more times the mass of the Sun, at the end of their lives most likely actually experience rapid catastrophic compression under the influence of their own gravity. This will lead them to a black hole state. A black hole is a physical body that creates such a strong gravitational force that the redshift of light emitted near it can become infinite.
Physicists and astronomers are absolutely sure that black holes exist in nature, although so far they have not been detected. The difficulties of astronomical searches are associated with the very nature of these unusual objects. After all, the infinite red shift, due to which the frequency of the received light goes to zero, 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 one hope to notice a black hole, for example, in a binary star system, where its partner would be an ordinary star. From observations of the motion of a visible star in the common gravitational field of such a pair, it would be possible to estimate the mass of the invisible star, and if this value exceeds the mass of the Sun by three or more times, it would be possible to claim that we have found a black hole.
There are now 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 until these estimates are refined, astronomers prefer to call these objects black hole candidates.
The gravitational dilation of time, the measure and evidence of which is the red shift, is very significant near a neutron star, and near a black hole, at 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, falling from a distance of 1 million km to the gravitational radius takes only about an hour. But according to the clock, which rests far from the black hole, the free fall of a 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 where time slows down near a black hole. Thus, matter influences the properties of space and time.
The ideas about space and time formulated in Einstein's theory of relativity are by far the most consistent. But they are macroscopic, since they rely on the experience of studying macroscopic objects, large distances and long periods of time. When constructing theories describing the phenomena of the microworld, this classical geometric picture, which assumes 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 microworld. 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, is based on 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 are coming to a strange conclusion: now it begins to seem to us that space plays a primary role, and matter must be obtained from space, so to speak, at the next stage. We have always considered matter to be primary and space to be secondary. Space, figuratively speaking, is now taking revenge and “eating” 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 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 fundamental property of spacetime. He presented a series of equations that described the effect of the curvature of spacetime on the energy and motion of the matter and radiation present in it.
A hundred years later, the general theory of relativity (GTR) became the basis for the construction of modern science, it withstood all the tests with which scientists attacked it.
But until recently it was impossible to conduct experiments under extreme conditions to test the theory's stability.
It's amazing how strong the theory of relativity has proven to be in 100 years. We still use 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
The general theory of 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 you put a heavy bowling ball 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 inside, not even light. Based on the theory, evidence was found for the generally accepted opinion today that the Universe is expanding and accelerating.
General 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 relies on the effect of subtle changes in radiation bent by the gravitational field of the star around which the planet orbits.
Testing Einstein's theory
General relativity works well for ordinary gravity, as shown by experiments carried out on Earth and observations of the planets of the solar system. But it has never been tested under conditions of extremely strong fields in spaces lying on the boundaries of physics.
The most promising way to test the theory under such conditions is by observing changes in spacetime called gravitational waves. They appear as a result of large events, the merger of two massive bodies, such as black holes, or especially dense objects - neutron stars.
A cosmic fireworks display of this magnitude would only reflect 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 located a meter apart on Earth by one thousandth 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 of detecting gravitational waves in the next two years.
Clifford Will
The Laser Interferometer 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 spacetime ripples pass through the detectors, they stretch and compress space, causing the detector to change dimensions. And LIGO can measure them.
LIGO began a series of launches in 2002, but failed to achieve results. Improvements were made in 2010, and the organization's successor, Advanced LIGO, should be operational 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 must continue to search for evidence of general relativity in order to be sure that it is correct.
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 connect together Einstein’s research, that is, the science of the macrocosm, and quantum mechanics, the reality of the smallest objects.
Advances in this area, quantum gravity, may require changes to general relativity. It is possible that quantum gravity experiments would require so much energy that they would be impossible to carry out. “But who knows,” says Will, “maybe there is an effect in the quantum universe that is insignificant, but searchable.”
