Theory of relativity
1 676, Danish astronomer Orr christiansen Romer first discovered that light travels at a limited but very high speed. If you look at Jupiter's moons, you will notice that they disappear from sight from time to time because they walk behind this huge planet. Eclipses of these Jupiter moons should occur regularly, but Romer observed that the intervals of these eclipses were not equal. Satellites are in their orbits. Do they sometimes accelerate and sometimes slow down? He gave another explanation. If light travels at infinite speed, then we will see solar eclipses at regular intervals on the earth at the same time, just like the ticking of the cosmic clock. Since light travels any distance at the same time, this situation will not change whether Jupiter approaches or leaves the earth.
Now imagine that light travels at a finite speed. If so, we will see it sometime after each eclipse. This delay depends on the speed of light and the distance between Jupiter and the earth. If Jupiter does not change its distance from the earth, it will delay the eclipse by the same time every time. However, Jupiter is sometimes closer to the earth. In this case, the distance of the "signal" from each successive eclipse is getting shorter and shorter, so that if the distance is kept constant, it will arrive earlier than the "signal" from Jupiter. For similar reasons, when Jupiter retreated from the earth, we saw that the eclipse was delayed more than expected. The degree of this early or late arrival depends on the speed of light, which enables us to measure it. This is exactly what Romer did. He noticed that in a year, when the earth approached Jupiter's orbit, the solar eclipse of one of Jupiter's satellites appeared in advance, but was delayed when the earth left. He used this difference to calculate the speed of light. However, he can't accurately measure the change of the distance between the earth and Jupiter. Compared with the modern value of 186000 miles per second, his light speed value is 140000 miles per second. Nevertheless, Romer not only proved that light travels at a limited speed, but also measured that speed, and his achievements were remarkable. You should understand that this was 1 1 year before Newton published Mathematical Principles of Natural Philosophy.
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The speed of light and the moment of solar eclipse.
The observation time of Jupiter satellite eclipse depends on the actual time of eclipse and the time required for light to travel from Jupiter to the earth. In this way, when Jupiter moves towards the earth, the frequency of eclipses is higher, while when Jupiter is far away from the earth, the frequency of eclipses is lower. This figure exaggerates the effect for the sake of clarity.
1865, British physicist james clerk maxwell put forward a theory, which successfully unified some previous theories used to describe electricity and magnetism. Only then did we get the correct theory of light propagation. Although electricity and magnetism were known in ancient times, it was not until the 8th century A.D./KLOC-0 that British chemist Henry cavendish and French physicist Charles Augustine Coulomb established a quantitative law to limit the electricity between two charged objects. Decades later, in the early19th century, some physicists established similar laws of magnetism. Maxwell mathematically proved that these electric and magnetic forces are not caused by the direct interaction between particles; On the contrary, every charge and current will generate an electric field in the surrounding space, which will act on other charges and currents in the space. He found that every single field has electricity and magnetism, so electricity and magnetism are two inseparable aspects of the same force. He called this force electromagnetic force and the field carrying this force electromagnetic field.
Maxwell's equation predicts that there may be wavy disturbances in the electromagnetic field, which propagate at a fixed speed, just like ripples on the surface of a pond. When he calculated this speed, he found that it coincided with the speed of light! Today, we know that when the wavelength of Maxwell wave is between 40 parts per million and 80 parts per million, human eyes can regard it as light. Waves are a series of continuous peaks and valleys; Wavelength is the distance between peaks or valleys. Waves with shorter wavelengths than visible light are called ultraviolet rays, x-rays and gamma rays. Waves with longer wavelengths are called radio waves (1 m or longer), microwaves (about 1 cm) or infrared rays (shorter than one ten thousandth of a centimeter but longer than the wavelength of visible light).
Maxwell's theory means that radio waves or light waves travel at a fixed speed. This is difficult to reconcile with Newton's view that there is no static absolute standard. Because if there is no such standard, it is impossible to have a general view of the speed of objects. To understand why, imagine that you are playing table tennis on the train again. If you hit the ball in front of the train and your opponent measures that the speed of the ball is 10 mph, then you can expect an observer on the platform to find that the ball moves at the speed of10 mph-10 mph is its movement relative to the train, and 90 mph is the movement of the train relative to the platform. What is the speed of the ball, per hour 10 miles or per hour 100 miles? How do you define it-relative to the train or relative to the ground? Without absolute rest, you can't specify the absolute speed of the ball. It is also reasonable to say that any speed of the same ball depends on the frame of reference for measuring speed. According to Newton's theory, the above statement should also hold true for light. So, what does light wave propagation at a fixed speed mean to Maxwell's theory?
In order to coordinate Maxwell's theory with Newton's law, people put forward that there is a substance called ether, which is ubiquitous and even exists in an "empty" vacuum. The concept of ether has some additional attraction for scientists, who feel that just like water is to water waves or air is to sound waves, after all, some medium is needed to load the fluctuation of electromagnetic energy. According to this view, light waves propagate in the ether just like sound waves propagate in the air, so their "velocities" derived from Maxwell's equations must be measured relative to the ether. According to this view, different observers will see light coming at them at different speeds, but the speed of light relative to the ether will remain the same.
People can test this idea. Imagine emitting light from a light source. According to the ether theory, light travels through the ether at the speed of light. If you move towards it through the ether, the speed at which you approach the light will be the sum of the speed at which the light passes through the ether and the speed at which you pass through the ether. Light will approach you faster than assuming that you are still or moving in other directions. However, it is very difficult to measure the influence of this speed difference, because the speed of light is so great compared with the speed at which we move towards the light source.
Wavelength is the distance between consecutive peaks or valleys.
1887, Albert Michelson (who later became the first American to win the Nobel Prize in Physics) and Edward Morey made a very delicate and difficult experiment at the Case Institute of Applied Science in Cleveland (now case western reserve university). They realized that because the earth revolves around the sun at a speed of almost 20 miles per second, their laboratory itself must pass through the ether at a relatively high speed. Of course, no one knows the direction and speed of the ether relative to the sun, or whether it is moving. But repeating the experiment at different times of the year, because the earth's position in its orbit is different, they are expected to explain this unknown factor. In this way, Michelson and Molly set out to conduct an experiment to compare the speed of light in the direction of the earth's movement in the ether (when we are moving towards the light source) and the direction at right angles to the movement (when we are not moving towards the light source). They found that the speed in both directions was exactly the same, which surprised them!
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Different speeds of table tennis
According to the theory of relativity, although the speed of an object measured by an observer can be different, everyone's measurement is equally effective.
Between 1887 and 1905, scientists tried to save the ether theory several times. The most famous one was completed by the Dutch physicist Haendly Lorenz, who tried to explain the results of Michelson-Morey experiment according to the fact that objects and clocks contract and slow down respectively when they move in ether. However, in 1905, Albert Einstein, an employee of the Swiss Patent Office who was still unknown at that time, pointed out in a famous paper that as long as people are willing to abandon the concept of absolute time, the concept of ether is purely redundant (we will soon know why). A few weeks later, the famous French mathematician Henri Poincare put forward a similar view. Einstein's argument is closer to physics than Poincare's. Poincare regarded this problem as pure mathematics and refused to accept Einstein's theoretical explanation until his death.
As the name implies, the basic assumption of Einstein's theory of relativity is that the laws of science must be the same for all observers who move freely at any speed. This applies to Newton's laws of motion, but Einstein has now extended this idea to include Maxwell's theory. In other words, because Maxwell's theory points out that the speed of light has a given value, any free-moving observer, no matter how fast he leaves or approaches the light source, will certainly measure the same value. This simple idea certainly explains the meaning of the speed of light in Maxwell's equation-and does not require the use of ether or any other advanced reference system-and it also has some impressive and often counterintuitive inferences.
For example, asking all observers to agree that the speed of light propagation forces us to change the concept of time. Imagine a fast train. In the fourth chapter, we see that although someone playing table tennis on the train will say that the ball has only moved a few inches, people standing on the platform will think that the ball has moved about 40 meters. Similarly, if an observer on the train flashes, two observers cannot agree on the distance that light travels. Since speed is distance divided by time, if they don't agree with the distance of light, the only way to get them to agree with the speed of light is that they don't agree with the time it takes to travel. In other words, relativity ends the concept of absolute time! On the contrary, each observer must have his own time measurement, which is recorded by his own clock. The same clock carried by different observers may not be synchronized.
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coordinates in space
When we say that space has three dimensions, we mean that we need to use three numbers or coordinates to represent a point. If we add time to the description, then space becomes a four-dimensional space-time
There is no need to introduce the concept of ether into the theory of relativity. As the Michelson-Morey experiment showed, the existence of ether could not be detected at all. On the contrary, relativity forces us to fundamentally change our concepts of space and time. We must accept that time cannot be completely separated from space, independent of space, but combined with space to form an object called time and space. These ideas are not easy to master, and it took even physicists many years to generally accept the theory of relativity. Einstein was able to give birth to the theory of relativity, which showed his unparalleled imagination. He also drew many inferences from the theory of relativity, although it seems to lead to strange conclusions, which shows that he is confident in his logic.
It is common sense that we can use three numbers or coordinates to describe the position of a point in space. For example, we can say that a point in the room is 7 meters away from one wall, 3 meters away from the other wall and 5 meters higher than the floor. Or we can represent a point at a certain latitude, longitude and altitude. We are free to use any three appropriate coordinates, although they are only effective in a limited range. It is unrealistic to indicate the position of the moon according to how many miles northwest of Piccadilly Circus and how many feet above sea level. On the contrary, we can describe it according to the distance from the sun, the distance from the planet's orbital plane, and the angle between the straight line from the moon to the sun and the straight line from the sun to nearby stars such as proxima centauri. These coordinates are not very useful even when describing the position of the sun in our galaxy, or the position of our galaxy in our local galaxy group. In fact, we can use a set of overlapping coordinate fragments to describe the whole universe. In each segment, we can use three different sets of coordinates to represent the position of a point.
In the relativistic space-time, any event can be represented by four numbers or coordinates-that is, anything that happens at a specific time and at a specific point in space. Similarly, the choice of coordinates is arbitrary: we can use any three clearly defined spatial coordinates and any time measure. But in the theory of relativity, it is impossible to really distinguish space and time coordinates, just as it is impossible to really distinguish any two space coordinates. We can choose a new set of coordinates, for example, the first spatial coordinate is the combination of the old first and second spatial coordinates. So, in order to measure the position of a point on the ground, we can use the miles northeast and northwest of Piccadilly instead of the miles northwest and north of Piccadilly. Similarly, we can use the new time coordinate, which is the old time (in seconds) plus the distance north of Piccadilly Street (in seconds).
Another well-known relativistic inference is that mass and energy are equivalent. This is summarized in Einstein's famous equation E=mc2 (where e is energy, m is mass and c is the speed of light). People often use this equation to calculate, for example, how much energy will be generated when some substances are converted into pure electromagnetic radiation. (Because the speed of light is a big number, the answer is that there is a lot of energy-the material converted into energy in Hiroshima atomic bomb weighs less than 1 ounce. But this equation also tells us that if the energy of an object increases, its mass will also increase, that is, its resistance to acceleration or speed changes will also increase.
One form of energy is the energy of motion, which is called kinetic energy. Just as it takes energy to move your car, it takes energy to increase the speed of everything. The kinetic energy of a moving object is equal to the energy consumed to make it move. Therefore, the faster an object moves, the greater its kinetic energy. However, according to the equivalence of energy and mass, kinetic energy increases the mass of an object, so the faster the object moves, the harder it is to further improve its speed.
This effect is really meaningful only when the object moves at a speed close to the speed of light. For example, when the speed of light of an object is 10%, its mass is only 0.5% larger than normal, and when its speed of light is 90%, its mass will be more than twice the normal mass. When an object approaches the speed of light, its mass will rise faster and faster, which requires more and more energy to accelerate further. According to the theory of relativity, an object can never reach the speed of light, because then its mass will become infinite, and because of the equivalence of mass and energy, it needs infinite energy to achieve its goal. This is why any normal object is always restricted by relativity and moves at a slower speed than the speed of light. Only light, or other waves without intrinsic mass, can move at the speed of light.
Einstein 1905' s theory of relativity is called special relativity. This is because, although it successfully explains that the speed of light is the same for all observers and what happens when an object moves at a speed close to the speed of light, it is not in harmony with Newton's theory of gravity. Newton's theory says that at any given moment, objects attract each other, and their gravity depends on the distance between them at that moment. This means that if you move one of the objects, the force exerted on the other object will change immediately. For example, the sun suddenly disappears, and Maxwell's theory tells us that the earth will not turn black until 8 minutes later (because that's when light reaches us from the sun). But according to Newton's gravity theory, the earth will immediately realize that the attraction of the sun no longer exists and fly out of orbit. In this way, the gravitational effect of the disappearance of the sun reaches us at an infinite speed, not at or below the speed of light required by special relativity. Between 1908 and 19 14, Einstein made some unsuccessful attempts to find a theory of gravity compatible with special relativity. 19 15 years, he finally put forward a more revolutionary theory, which we now call general relativity.