Relativity is a theory about the motion of matter and the relationship between time and space. It is one of the theoretical foundations of modern physics. Relativity was established and developed by Einstein and others at the beginning of this century on the basis of summarizing experimental facts (such as Michelson-Morey experiment). Before that, when people explained the light propagation according to the classic concept of time and space (mainly represented by galilean transformation), it led to a series of sharp contradictions. The new space-time phenomenon and the motion law of high-speed objects in physics are established, which is of great significance to the future development of physics. Relativity can be divided into two parts: special relativity and general relativity. 1938+the basic principles of the special theory of relativity established in 0905;
In any inertial reference system, the laws of nature are the same, which is the so-called relativity principle.
In any inertial system, the speed of light in vacuum is the same, that is, the principle of constant speed of light.
It is concluded that when the quantity of time and space is transformed from one inertial system to another, it should satisfy Lorentz transformation, not galilean transformation, and thus many important conclusions are derived, such as:
① The sequence of two events or whether they are "simultaneous" is different in different frames of reference (but the law of causality still holds).
② When measuring the length of an object, the length of the moving object in its moving direction will be shorter than that at rest. Similarly, when measuring the time course, we will see that the moving clock is slower than the stationary clock.
(3) The mass of an object increases with the increase of speed, and the relation is static mass, which is called static mass.
Nothing can travel faster than the speed of light.
⑤ The mass and energy of an object satisfy the mass-energy relationship.
The above conclusions are consistent with the current experimental facts, but the effect is only significant when moving at high speed. In general, the relativistic effect is very small, so classical mechanics can be considered as an approximation of low-speed relativistic mechanics. 19 16 years, the general theory of relativity was established, and its basic principles are as follows:
The principle of general relativity, that is, the laws of nature can be expressed in the same mathematical form in any reference system.
(2) Equivalence principle, that is, the gravitational force in a small volume range and the inertial force in the acceleration system are equivalent to each other.
According to the above principle, gravity is caused by the existence and certain distribution of matter, which makes the quality of space-time uneven (so-called space-time bending); The theory of gravitational field holds. Special relativity is a special case of general relativity in the case of weak gravitational field. Some important conclusions can be drawn from general relativity, such as the precession law of Mercury's perihelion; Light bends in the gravitational field; In the strong gravitational field, the clock slows down (or the spectral line in the gravitational field moves to the red end). These conclusions are basically consistent with the later observation results. In recent years, the conclusion of general relativity has been confirmed with higher accuracy by measuring the time delay of radar wave propagating back and forth in the solar gravitational field. Relativity is of great historical significance, but there are still many problems to be studied.
Space bending
Experiments to verify general relativity
1, Einstein pointed out three verification experiments.
1905, when Einstein published the famous historical document "On Electrodynamics of Moving Objects" and established the special theory of relativity, his theory was not accepted by people, and many people (including some prestigious scientists) doubted or even opposed it, because the brand-new concept of special relativity was quite different from the classical concept of physics, which was incredible, because of the lack of experimental verification, and because Einstein was only in his twenties.
Einstein was not deterred by this, and he continued to consider extending the theory of relativity to non-inertial systems. From 1907 to 19 16, Einstein published many articles continuously, constantly improving the theory of general relativity and introducing Riemann bending space. In "Fundamentals of General Relativity" published by 19 16, Einstein pointed out that Newton's theory of gravity can be regarded as a first-order approximation of relativistic gravity theory. Einstein also pointed out that the general theory of relativity can be verified by measuring the precession of perihelion in planetary orbit, the bending of light in gravitational field and the gravitational red shift of galaxy lines.
2. precession of perihelion in planetary orbit
According to Newton's law of motion and inverse square law's law of universal gravitation, the orbits of planets in the solar system should be a strict ellipse and a closed curve, and the sun is located at a focal point of the ellipse. But from 1859, astronomers found that the orbit of the planet is not a strictly closed ellipse. Every time a planet rotates around the sun, the long axis of its elliptical orbit rotates slightly, which is usually called the near (or far) day of the planet. As shown in the figure 1, especially for Mercury, which is closest to the sun, its precession observation value is once every 100 years. It is generally believed that Mercury is attracted not only by the sun, but also by other planets in the solar system. Moreover, people observe from the non-ideal inertial system in which the earth is also rotating and revolving, so there is a slow precession. Calculated by Newton's gravity theory, considering the above effects, the annual difference is still smaller than the actual observation value. Although the numerical value is small, it is beyond the allowable error range of observation accuracy. Moreover, other planets in the solar system have similar perigee redundant precession, and the value is very small.
In order to explain this difference, the astronomer Levi, who successfully predicted the existence of Neptune, predicted that there was an undiscovered asteroid near the sun, that is, an "underwater planet" in the orbit of Mercury. The gravitational pull of this underwater planet on Mercury leads to the appearance of the redundant age difference. However, the predicted sky area has been carefully searched for many years. This imaginary underwater planet has never been discovered. Underwater planet has become an unsolved problem in Newton's theory of gravity for many years. According to the general theory of relativity, the greater the mass of a celestial body, the more curved the space-time around it. The planet moves along the short-distance line in curved space-time Mercury is the closest planet to the sun. The gravitational field here is much stronger than other planets in the solar system, and the space-time is also very curved. In addition, the orbital eccentricity of Mercury is large. Therefore, the excess precession of Mercury's perihelion is larger than that of other planets. The excess precession of Mercury perihelion calculated by Einstein in 19 15 according to general relativity is quite consistent with the actual observation. Therefore, the precession of Mercury's perihelion is regarded as the first major experimental verification in the early days of the establishment of general relativity. The precession of the perihelion of the Earth, Venus and other planets measured later is also quite consistent with the calculation of general relativity.
3. Deflection of light in gravitational field
According to the general theory of relativity, light will bend and deflect in the gravitational field. But because this deflection is very small, it is not easy to observe on earth. Einstein pointed out in 19 1 1 that if we take advantage of the special opportunity of the total solar eclipse, we can measure the positions of the planets that seem to be located near the sun when measuring the total solar eclipse, and then compare them with the usual positions of these planets. This deviation should be observed. 19 16, and he calculated that the deflection angle of light passing near the sun is. 19 19 When a total solar eclipse occurred in the southern hemisphere, Britain sent two expeditionary observation teams led by astronomer A.S. Eddington to conduct synchronous measurements in West Africa and Brazil respectively. The deviation of measurement is obtained.
Similar observations during the future total solar eclipse also support the conclusion of general relativity. After all, there are fewer opportunities for total solar eclipse, and scientists hope that such experiments can be carried out at other times. Radio astronomy, developed after the 1960s, enables people to measure radio sources covered by the sun with radio telescopes at ordinary times, and the resolution is greatly improved. 19438+0975 The observed deflection angle of radio waves passing near the surface of the sun is the same as that predicted by general relativity.
4. Gravitational redshift of spectral lines
According to the general theory of relativity, when light propagates in the gravitational field, its frequency will change. When light propagates from a place with strong gravitational field (such as near the sun) to a place with weak gravitational field (such as near the earth), its frequency will decrease slightly and its wavelength will increase slightly, that is, gravitational redshift will occur. When light propagates in the opposite direction, its frequency will increase and its wavelength will shorten, that is, gravitational blue shift will occur. Einstein in 1965438+.
Comparison of the sizes of various stars
White dwarfs have large mass and small radius, and the gravitational red shift effect of the light emitted is obvious. 1925, astronomer w s ADAMS observed a white dwarf Sirius a, and the measured gravitational redshift was basically consistent with the theory of general relativity. The uncertainty between the gravitational redshift of solar spectral lines measured in 1960s and 1970s and the theoretical value is less than 5% ~ 7%.
The light propagating between two points with a height difference of tens of meters near the ground should also produce gravitational redshift, but the change of gravitational redshift is smaller, only one order of magnitude. The discovery of Mossbauer effect in 1958, which is difficult to observe by general experimental means, provides the possibility for accurately completing the gravitational redshift experiment on the ground. Pound (R.V.Pound) and Rybka (C. Rebka) shoot the rays emitted by cobalt 57 from the top of the tower with a height of 22.6m to the receiver on the ground, and measure the frequency at the bottom of the tower by using Mossbauer effect.
5. The fourth verification experiment-radar echo delay
In addition to the three verification experiments discussed above, I.Shapiro proposed in 1964 to test the general theory of relativity with the radar echo delay experiment. According to the general theory of relativity, the existence and motion of matter cause the bending of surrounding space-time, and the bending of light near a massive object can be regarded as a kind of refraction, which is equivalent to the slowdown of light speed. When radar waves reach the surface of the planet and reflect back to the earth, the time required for a round trip can be measured. The delay time of radar echo can be obtained by comparing the round-trip time of radar wave propagating near the sun and the round-trip time far from the sun.
Shapiro's team has carried out radar echo delay experiments on Mercury, Venus and Mars successively, and the uncertainty between the later experimental data and the theoretical value of general relativity is about 65438 0%. In the early 1980s, the uncertainty of the experimental value of radar echo delay was reduced to 0.65,438+0% by using the Viking probe that landed on the surface of Mars, which strongly supported the general theory of relativity.
6. It is indirectly proved that the imported technology has force wave-pulse binary star observation.
According to the general theory of relativity, matter accelerates the generation of gravitational waves in an asymmetric way. Einstein proved that gravitational waves travel at the same speed as electromagnetic waves. There is no gravitational wave in Newton's theory of gravity. If gravitational waves can be observed, it will be a great victory of general relativity. However, because the gravitational effect is many orders of magnitude weaker than the electromagnetic effect, it is impossible to artificially generate detectable gravitational waves on the earth with existing materials and experimental means. People have to pin their hopes on the detection of gravitational waves produced by astrophysical processes with great quality.
1967, astronomers S.J. Bell and A.Hewish discovered pulsars with radio telescopes. It turned out that pulsars are neutron stars. The pulse signal received by the radio telescope is the electromagnetic wave emitted by the magnetic pole when the neutron star rotates. In 1974, R.A.Hulse and J.H. Taylor Sr1913+16). According to the general theory of relativity, pulsed binary stars radiate gravitational waves when they rotate. Pulsed binary stars (PSR1913+16) radiate gravitational waves with a power of W, but the binary stars are too far away from the earth, and the gravitational wave energy flow density reaches the ground. At present, there is no way to detect such a weak gravitational wave. However, according to the general theory of relativity, gravitational radiation damping is due to the inevitable energy loss when pulsed binary stars radiate gravitational waves, that is, the energy of binary star system will decrease and the period will slow down. After nearly 20 years' observation, it is found that the motion period of pulse binary stars is decreasing steadily, and the rate of change of the period slowing is quite consistent with the theoretical value of general relativity. Therefore, the observation of pulse binary stars is considered as an indirect proof of the existence of gravitational waves. Hall and Taylor won a prize of 6500 pounds for discovering pulse binary stars.
The direct detection of gravitational waves is one of the main topics in experimental physics, which will further test the general theory of relativity. Western developed countries have invested a lot of manpower and material resources in research, but so far they have not got satisfactory data.
The accelerated expansion of the universe
The vastness and magic of the universe are amazing and fascinating. The basic component of blossoming galaxies is baryon-type matter that shines like a star, but there are also quite a few baryon-type matters in the universe, such as nebulae, planets, dwarfs and black holes. Baryon-type substances refer to ordinary substances or their different transformations that people often encounter on earth or in laboratories.
Collapse of supernovae
Surprisingly, all baryon matter accounts for only about 4% of the universe, of which 26% is called dark matter and 70% is called dark energy. These "dark matter" and "dark energy" are brand-new forms, and people don't know much about them. The universe is not only special in composition, but also not as static as it seems at first glance. The universe is actually expanding dynamically.
Through a high-precision telescope, we can observe that the spectral lines of distant stars move to the red end. The spectral redshift indicates that the star is regressing, and the farther away from the star, the greater the spectral redshift. If the universe expands at the same speed, the distance is proportional to the redshift, which is the famous Hubble law. 1998, scientific research has made great progress. Scientists have found that the universe is not what people have always imagined. Because only matter exists, it should slow down and expand. On the contrary, the expansion of the universe is accelerating. This indicates that the main component of the universe is not matter, but a brand-new form: "dark energy". In 2000, there was an important evidence of the accelerated expansion of the universe, that is, the accurate measurement of the anisotropy of the cosmic microwave background.
In the early 1960s, scientists discovered that there was 3K-degree microwave background radiation in the universe. In fact, it is the residual temperature of the universe after the full expansion of high-temperature thermal explosion, and it is quite uniform and isotropic. However, the 3K microwave background radiation has one-tenth of the uneven fluctuation, which contains extremely rich valuable information of the early universe. It is from the analysis of this information that people know that the universe is flat. Moreover, it has the proportion of the above components. Different methods of astronomical observation, such as X-ray distribution of galaxy clusters, gravitational lens, inference of cosmic age and large-scale structural evolution, give more evidence. The universe contains less than 40% of "matter", including baryon type and dark type. The cross-validation of various achievements basically confirms our overall view of the universe, which is a very remarkable achievement.
However, the challenges are also severe. The nature of dark matter is unknown. There is no electromagnetic and strong interaction between it and our ordinary "baryon matter", so it is difficult to detect. We only rely on its large-scale gravitational effect to infer its existence. Quite a few scientists hope that "dark matter" is the so-called "neutral particle" in the supersymmetric extended version of the particle standard model hypothesis. However, its existence is far from being confirmed by experiments. As for dark energy, it is even more mysterious. Particle physics knows that vacuum energy, as one of the possible forms of dark energy, is called cosmological constant, but it is always dozens of orders of magnitude higher than the dark energy needed in cosmology. This shows that our basic physics has major defects and is facing a crisis.
In addition, the existence of a kind of "vacuum energy" similar to the "cosmological constant" will dominate the future of the universe, and all the galaxies we can see today will fly out of our sight at an accelerated speed and never come back. Therefore, we must combine the latest achievements of particle physics and cosmology to study the severe challenges brought by the universe to basic theory and show a more reassuring future prospect of the universe.
strange
Looking back on the scientific development process from Aristotle to Newton to Einstein, we can realize that any physical theory has its own success and failure, or its own effective and ineffective scope. Solving old problems and raising new ones are often two sides of the same theory. Newton solved many problems that Aristotle did not solve, but he also left his own difficulties. Einstein solved many difficulties in Newton's theory, but it also brought new problems.
One of the biggest problems of Einstein's general relativity is singularity. There are singularities in the solution of black holes and cosmology. The final result of gravitational collapse is singularity. The starting point of the big bang is also a singularity.
Singularity has a series of strange properties, such as infinite material density, infinite pressure, infinite curved space-time and so on. In addition, at the singularity, all forms of causality disappear, which makes it impossible for us to talk about the past or predict the future. For a time, physicists thought that singularities might only be brought about by mathematical forms, but they could actually be avoided. Without a perfectly symmetrical geometric structure, there may be no singularity. But since 1970s, Hawking and Panrossi have proved that singularity is a universal and inevitable thing in general relativity. When the general theory of relativity is applied to the universe, singularities will inevitably appear, just as Newtonian mechanics inevitably encounters some infinity in cosmology.
Some unreasonable infinity in Newton's system shows that Newton's theory is no longer applicable under certain conditions, and the inevitability of singularity in general relativity may also be a manifestation of the limitations of general relativity. Einstein himself saw the importance of these singularities in this way. He said: "People can't assume that these equations are still valid for high field density and material density, nor can they conclude that the beginning of expansion must mean a mathematical singularity. In short, we must understand that these equations cannot be extended to such areas. " Therefore, we must seek theories that can be extended to such fields.
After Einstein, people developed Einstein's theory mainly from two aspects, one is to combine general relativity with quantum theory, the other is to unify general relativity with other basic interactions.