The 19th century saw several scientific experiments that led to the discovery of a number of new phenomena. Among these phenomena is Hans Oersted's discovery of the generation of magnetic induction by electric current. Later, Michael Faraday discovered the opposite effect, which was called electromagnetic induction.
James Maxwell's equations - the electromagnetic nature of light
As a result of these discoveries, the so-called “interaction at a distance” was noted, resulting in the new theory of electromagnetism formulated by Wilhelm Weber, which was based on long-range action. Later, Maxwell defined the concept of electric and magnetic fields, which are capable of generating each other, which is an electromagnetic wave. Subsequently, Maxwell used the so-called “electromagnetic constant” in his equations - With.
By that time, scientists had already come close to the fact that light is electromagnetic in nature. The physical meaning of the electromagnetic constant is the speed of propagation of electromagnetic excitations. To the surprise of James Maxwell himself, the measured value of this constant in experiments with unit charges and currents turned out to be equal to the speed of light in vacuum.
Before this discovery, humanity separated light, electricity and magnetism. Maxwell's generalization allowed us to take a new look at the nature of light, as a certain fragment of electric and magnetic fields that propagates independently in space.
The figure below shows a diagram of the propagation of an electromagnetic wave, which is also light. Here H is the magnetic field strength vector, E is the electric field strength vector. Both vectors are perpendicular to each other, as well as to the direction of wave propagation.
Michelson experiment - the absoluteness of the speed of light
The physics of that time was largely built on Galileo's principle of relativity, according to which the laws of mechanics look the same in any chosen inertial frame of reference. At the same time, according to the addition of velocities, the speed of propagation should depend on the speed of the source. However, in this case, the electromagnetic wave would behave differently depending on the choice of reference frame, which violates Galileo's principle of relativity. Thus, Maxwell's seemingly well-formed theory was in a shaky state.
Experiments have shown that the speed of light really does not depend on the speed of the source, which means a theory is required that can explain such a strange fact. The best theory at that time turned out to be the theory of “ether” - a certain medium in which light propagates, just as sound propagates in the air. Then the speed of light would be determined not by the speed of movement of the source, but by the characteristics of the medium itself - the ether.
Many experiments have been undertaken to discover the ether, the most famous of which is the experiment of the American physicist Albert Michelson. In short, it is known that the Earth moves in outer space. Then it is logical to assume that it also moves through the ether, since the complete attachment of the ether to the Earth is not only the highest degree of egoism, but simply cannot be caused by anything. If the Earth moves through a certain medium in which light propagates, then it is logical to assume that the addition of velocities takes place here. That is, the propagation of light must depend on the direction of motion of the Earth, which flies through the ether. As a result of his experiments, Michelson did not discover any difference between the speed of light propagation in both directions from the Earth.
The Dutch physicist Hendrik Lorentz tried to solve this problem. According to his assumption, the “ethereal wind” influenced bodies in such a way that they reduced their size in the direction of their movement. Based on this assumption, both the Earth and Michelson's device experienced this Lorentz contraction, as a result of which Albert Michelson obtained the same speed for the propagation of light in both directions. And although Lorentz was somewhat successful in delaying the death of the ether theory, scientists still felt that this theory was “far-fetched.” Thus, the ether was supposed to have a number of “fairy-tale” properties, including weightlessness and the absence of resistance to moving bodies.
The end of the history of the ether came in 1905 with the publication of the article “On the Electrodynamics of Moving Bodies” by the then little-known Albert Einstein.
Albert Einstein's special theory of relativity
Twenty-six-year-old Albert Einstein expressed a completely new, different view on the nature of space and time, which went against the ideas of the time, and in particular grossly violated Galileo’s principle of relativity. According to Einstein, Michelson's experiment did not give positive results for the reason that space and time have such properties that the speed of light is an absolute value. That is, no matter what frame of reference the observer is in, the speed of light relative to him is always the same, 300,000 km/sec. From this it followed the impossibility of applying the addition of speeds in relation to light - no matter how fast the light source moves, the speed of light will not change (add or subtract).
Einstein used the Lorentz contraction to describe changes in the parameters of bodies moving at speeds close to the speed of light. So, for example, the length of such bodies will decrease, and their own time will slow down. The coefficient of such changes is called the Lorentz factor. Einstein's famous formula E=mc 2 actually also includes the Lorentz factor ( E= ymc 2), which in general is equal to unity in the case when the body speed v equal to zero. As the body speed approaches v to the speed of light c Lorentz factor y rushes towards infinity. It follows from this that in order to accelerate a body to the speed of light, an infinite amount of energy will be required, and therefore it is impossible to cross this speed limit.
There is also an argument in favor of this statement called “the relativity of simultaneity.”
Paradox of the relativity of simultaneity of SRT
In short, the phenomenon of the relativity of simultaneity is that clocks that are located at different points in space can only run “at the same time” if they are in the same inertial frame of reference. That is, the time on the clock depends on the choice of reference system.
From this follows the paradox that event B, which is a consequence of event A, can occur simultaneously with it. In addition, it is possible to choose reference systems in such a way that event B will occur earlier than the event A that caused it. Such a phenomenon violates the principle of causality, which is quite firmly entrenched in science and has never been questioned. However, this hypothetical situation is observed only in the case when the distance between events A and B is greater than the time interval between them multiplied by the “electromagnetic constant” - With. Thus, the constant c, which is equal to the speed of light, is the maximum speed of information transmission. Otherwise, the principle of causality would be violated.
How is the speed of light measured?
Observations by Olaf Roemer
Scientists of antiquity for the most part believed that light moves at infinite speed, and the first estimate of the speed of light was obtained already in 1676. Danish astronomer Olaf Roemer observed Jupiter and its moons. At the moment when the Earth and Jupiter were on opposite sides of the Sun, the eclipse of Jupiter's moon Io was delayed by 22 minutes compared to the calculated time. The only solution that Olaf Roemer found is that the speed of light is limiting. For this reason, information about the observed event is delayed by 22 minutes, since it takes some time to travel the distance from the Io satellite to the astronomer’s telescope. According to Roemer's calculations, the speed of light was 220,000 km/s.
Observations by James Bradley
In 1727, the English astronomer James Bradley discovered the phenomenon of light aberration. The essence of this phenomenon is that as the Earth moves around the Sun, as well as during the Earth’s own rotation, a displacement of stars in the night sky is observed. Since the earthling observer and the Earth itself are constantly changing their direction of movement relative to the observed star, the light emitted by the star travels different distances and falls at different angles to the observer over time. The limited speed of light leads to the fact that the stars in the sky describe an ellipse throughout the year. This experiment allowed James Bradley to estimate the speed of light - 308,000 km/s.
The Louis Fizeau Experience
In 1849, French physicist Louis Fizeau conducted a laboratory experiment to measure the speed of light. The physicist installed a mirror in Paris at a distance of 8,633 meters from the source, but according to Roemer's calculations, the light will travel this distance in hundred thousandths of a second. Such watch accuracy was unattainable then. Fizeau then used a gear wheel that rotated on the way from the source to the mirror and from the mirror to the observer, the teeth of which periodically blocked the light. In the case when a light beam from the source to the mirror passed between the teeth, and on the way back hit a tooth, the physicist doubled the speed of rotation of the wheel. As the rotation speed of the wheel increased, the light almost stopped disappearing until the rotation speed reached 12.67 revolutions per second. At this moment the light disappeared again.
Such an observation meant that the light constantly “bumped” into the teeth and did not have time to “slip” between them. Knowing the speed of rotation of the wheel, the number of teeth and twice the distance from the source to the mirror, Fizeau calculated the speed of light, which turned out to be equal to 315,000 km/sec.
A year later, another French physicist Leon Foucault conducted a similar experiment in which he used a rotating mirror instead of a gear wheel. The value he obtained for the speed of light in air was 298,000 km/s.
A century later, Fizeau's method was improved so much that a similar experiment carried out in 1950 by E. Bergstrand gave a speed value of 299,793.1 km/s. This number differs by only 1 km/s from the current value of the speed of light.
Further measurements
With the advent of lasers and increasing accuracy of measuring instruments, it was possible to reduce the measurement error down to 1 m/s. So in 1972, American scientists used a laser for their experiments. By measuring the frequency and wavelength of the laser beam, they were able to obtain a value of 299,792,458 m/s. It is noteworthy that a further increase in the accuracy of measuring the speed of light in a vacuum was impossible, not due to the technical imperfections of the instruments, but due to the error of the meter standard itself. For this reason, in 1983, the XVII General Conference on Weights and Measures defined the meter as the distance that light travels in a vacuum in a time equal to 1/299,792,458 seconds.
Let's sum it up
So, from all of the above it follows that the speed of light in a vacuum is a fundamental physical constant that appears in many fundamental theories. This speed is absolute, that is, it does not depend on the choice of reference system, and is also equal to the maximum speed of information transmission. Not only electromagnetic waves (light), but also all massless particles move at this speed. Including, presumably, the graviton, a particle of gravitational waves. Among other things, due to relativistic effects, light’s own time literally stands still.
Such properties of light, especially the inapplicability of the principle of addition of velocities to it, do not fit into the head. However, many experiments confirm the properties listed above, and a number of fundamental theories are built precisely on this nature of light.
The speed of light in different media varies significantly. The difficulty is that the human eye does not see it in the entire spectral range. The nature of the origin of light rays has interested scientists since ancient times. The first attempts to calculate the speed of light were made as early as 300 BC. At that time, scientists determined that the wave propagated in a straight line.
Quick response
They managed to describe with mathematical formulas the properties of light and the trajectory of its movement. became known 2 thousand years after the first research.
What is luminous flux?
A light beam is an electromagnetic wave combined with photons. Photons are understood as the simplest elements, which are also called quanta of electromagnetic radiation. The luminous flux in all spectra is invisible. It does not move in space in the traditional sense of the word. To describe the state of an electromagnetic wave with quantum particles, the concept of the refractive index of an optical medium is introduced.
The light flux is transferred in space in the form of a beam with a small cross section. The method of movement in space is derived by geometric methods. This is a rectilinear beam, which, at the border with various media, begins to refract, forming a curvilinear trajectory. Scientists have proven that the maximum speed is created in a vacuum; in other environments, the speed of movement can vary significantly. Scientists have developed a system in which a light beam and a derived value are the basis for the derivation and reading of certain SI units.
Some historical facts
About 900 years ago, Avicena suggested that, regardless of the nominal value, the speed of light has a finite value. Galileo Galilei tried to experimentally calculate the speed of light. Using two flashlights, the experimenters tried to measure the time during which a light beam from one object would be visible to another. But such an experiment turned out to be unsuccessful. The speed was so high that they were unable to detect the delay time.
Galileo Galilei noticed that Jupiter had an interval between eclipses of its four satellites of 1320 seconds. Based on these discoveries, in 1676, Danish astronomer Ole Roemer calculated the speed of propagation of a light beam as 222 thousand km/sec. At that time, this measurement was the most accurate, but it could not be verified by earthly standards.
After 200 years, Louise Fizeau was able to calculate the speed of a light beam experimentally. He created a special installation with a mirror and a gear mechanism that rotated at high speed. The light flux was reflected from the mirror and returned back after 8 km. As the wheel speed increased, a moment arose when the gear mechanism blocked the beam. Thus, the speed of the beam was set at 312 thousand kilometers per second.
Foucault improved this equipment, reducing the parameters by replacing the gear mechanism with a flat mirror. His measurement accuracy turned out to be closest to the modern standard and amounted to 288 thousand meters per second. Foucault made attempts to calculate the speed of light in a foreign medium, using water as a basis. The physicist was able to conclude that this value is not constant and depends on the characteristics of refraction in a given medium.
A vacuum is a space free of matter. The speed of light in vacuum in the C system is designated by the Latin letter C. It is unattainable. No item can be overclocked to such a value. Physicists can only imagine what might happen to objects if they accelerate to such an extent. The speed of propagation of a light beam has constant characteristics, it is:
- constant and final;
- unattainable and unchangeable.
Knowing this constant allows us to calculate the maximum speed at which objects can move in space. The amount of propagation of a light beam is recognized as a fundamental constant. It is used to characterize space-time. This is the maximum permissible value for moving particles. What is the speed of light in a vacuum? The current value was obtained through laboratory measurements and mathematical calculations. She equal to 299.792.458 meters per second with an accuracy of ± 1.2 m/s. In many disciplines, including school ones, approximate calculations are used to solve problems. An indicator equal to 3,108 m/s is taken.
Light waves in the human visible spectrum and X-ray waves can be accelerated to readings approaching the speed of light. They cannot equal this constant, nor exceed its value. The constant was derived based on tracking the behavior of cosmic rays at the moment of their acceleration in special accelerators. It depends on the inertial medium in which the beam propagates. In water, the transmission of light is 25% lower, and in air it will depend on temperature and pressure at the time of calculations.
All calculations were carried out using the theory of relativity and the law of causality derived by Einstein. The physicist believes that if objects reach a speed of 1,079,252,848.8 kilometers/hour and exceed it, then irreversible changes will occur in the structure of our world and the system will break down. Time will begin to count down, disrupting the order of events.
The definition of meter is derived from the speed of a light ray. It is understood as the area that a light beam manages to travel through in 1/299792458 of a second. This concept should not be confused with the standard. The meter standard is a special cadmium-based technical device with shading that allows you to physically see a given distance.
To determine speed (distance traveled/time taken) we must choose distance and time standards. Different standards may give different speed measurements.
Is the speed of light constant?
[In fact, the fine structure constant depends on the energy scale, but here we are referring to its low-energy limit.]
Special theory of relativity
The definition of the meter in the SI system is also based on the assumption of the correctness of the theory of relativity. The speed of light is constant in accordance with the basic postulate of the theory of relativity. This postulate contains two ideas:
- The speed of light does not depend on the movement of the observer.
- The speed of light does not depend on coordinates in time and space.
The idea that the speed of light is independent of the speed of the observer is counterintuitive. Some people can't even agree that this idea is logical. In 1905, Einstein showed that this idea was logically correct if one abandoned the assumption of the absolute nature of space and time.
In 1879, it was believed that light must travel through some medium in space, just as sound travels through air and other substances. Michelson and Morley conducted an experiment to detect the ether by observing changes in the speed of light when the direction of the Earth's motion relative to the Sun changes throughout the year. To their surprise, no change in the speed of light was detected.
Doctor of Technical Sciences A. GOLUBEV
The concept of wave propagation speed is simple only in the absence of dispersion.
Lin Westergaard Heu near the installation where a unique experiment was carried out.
Last spring, scientific and popular science magazines around the world reported sensational news. American physicists conducted a unique experiment: they managed to reduce the speed of light to 17 meters per second.
Everyone knows that light travels at enormous speed - almost 300 thousand kilometers per second. The exact value of its value in vacuum = 299792458 m/s is a fundamental physical constant. According to the theory of relativity, this is the maximum possible signal transmission speed.
In any transparent medium, light travels more slowly. Its speed v depends on the refractive index of the medium n: v = c/n. The refractive index of air is 1.0003, of water - 1.33, of various types of glass - from 1.5 to 1.8. Diamond has one of the highest refractive index values - 2.42. Thus, the speed of light in ordinary substances will decrease by no more than 2.5 times.
In early 1999, a group of physicists from the Rowland Institute for Scientific Research at Harvard University (Massachusetts, USA) and Stanford University (California) studied the macroscopic quantum effect - the so-called self-induced transparency, passing laser pulses through a medium that is normally opaque. This medium was sodium atoms in a special state called the Bose-Einstein condensate. When irradiated with a laser pulse, it acquires optical properties that reduce the group velocity of the pulse by 20 million times compared to the speed in vacuum. Experimenters managed to increase the speed of light to 17 m/s!
Before describing the essence of this unique experiment, let us recall the meaning of some physical concepts.
Group speed. When light propagates through a medium, two velocities are distinguished: phase and group. Phase velocity v f characterizes the movement of the phase of an ideal monochromatic wave - an infinite sine wave of strictly one frequency and determines the direction of light propagation. The phase velocity in the medium corresponds to the phase refractive index - the same one whose values are measured for various substances. The phase refractive index, and therefore the phase velocity, depends on the wavelength. This dependence is called dispersion; it leads, in particular, to the decomposition of white light passing through a prism into a spectrum.
But a real light wave consists of a set of waves of different frequencies, grouped in a certain spectral interval. Such a set is called a group of waves, a wave packet or a light pulse. These waves propagate through the medium at different phase velocities due to dispersion. In this case, the impulse is stretched and its shape changes. Therefore, to describe the movement of an impulse, a group of waves as a whole, the concept of group velocity is introduced. It makes sense only in the case of a narrow spectrum and in a medium with weak dispersion, when the difference in the phase velocities of the individual components is small. To better understand the situation, we can give a clear analogy.
Let's imagine that seven athletes lined up on the starting line, dressed in different colored jerseys according to the colors of the spectrum: red, orange, yellow, etc. At the signal of the starting pistol, they simultaneously start running, but the “red” athlete runs faster than the “orange” one. , "orange" is faster than "yellow", etc., so that they stretch into a chain, the length of which continuously increases. Now imagine that we are looking at them from above from such a height that we cannot distinguish individual runners, but just see a motley spot. Is it possible to talk about the speed of movement of this spot as a whole? It is possible, but only if it is not very blurry, when the difference in the speeds of different colored runners is small. Otherwise, the spot may stretch over the entire length of the route, and the question of its speed will lose meaning. This corresponds to strong dispersion - a large spread of speeds. If runners are dressed in jerseys of almost the same color, differing only in shades (say, from dark red to light red), this becomes consistent with the case of a narrow spectrum. Then the speeds of the runners will not differ much; the group will remain quite compact when moving and can be characterized by a very definite value of speed, which is called group speed.
Bose-Einstein statistics. This is one of the types of so-called quantum statistics - a theory that describes the state of systems containing a very large number of particles that obey the laws of quantum mechanics.
All particles - both those contained in an atom and free ones - are divided into two classes. For one of them, the Pauli exclusion principle is valid, according to which there cannot be more than one particle at each energy level. Particles of this class are called fermions (these are electrons, protons and neutrons; the same class includes particles consisting of an odd number of fermions), and the law of their distribution is called Fermi-Dirac statistics. Particles of another class are called bosons and do not obey the Pauli principle: an unlimited number of bosons can accumulate at one energy level. In this case we talk about Bose-Einstein statistics. Bosons include photons, some short-lived elementary particles (for example, pi-mesons), as well as atoms consisting of an even number of fermions. At very low temperatures, bosons congregate at their lowest—basic—energy level; then they say that Bose-Einstein condensation occurs. The condensate atoms lose their individual properties, and several millions of them begin to behave as one, their wave functions merge, and their behavior is described by a single equation. This makes it possible to say that the atoms of the condensate have become coherent, like photons in laser radiation. Researchers from the American National Institute of Standards and Technology used this property of the Bose-Einstein condensate to create an “atomic laser” (see Science and Life No. 10, 1997).
Self-induced transparency. This is one of the effects of nonlinear optics - the optics of powerful light fields. It consists in the fact that a very short and powerful light pulse passes without attenuation through a medium that absorbs continuous radiation or long pulses: an opaque medium becomes transparent to it. Self-induced transparency is observed in rarefied gases with a pulse duration of the order of 10 -7 - 10 -8 s and in condensed media - less than 10 -11 s. In this case, a delay of the pulse occurs - its group velocity decreases greatly. This effect was first demonstrated by McCall and Khan in 1967 on ruby at a temperature of 4 K. In 1970, delays corresponding to pulse velocities three orders of magnitude (1000 times) less than the speed of light in vacuum were obtained in rubidium vapor.
Let us now turn to the unique experiment of 1999. It was carried out by Len Westergaard Howe, Zachary Dutton, Cyrus Berusi (Rowland Institute) and Steve Harris (Stanford University). They cooled a dense, magnetically held cloud of sodium atoms until they returned to the ground state, the lowest energy level. In this case, only those atoms were isolated whose magnetic dipole moment was directed opposite to the direction of the magnetic field. The researchers then cooled the cloud to less than 435 nK (nanokelvins, or 0.000000435 K, almost absolute zero).
After this, the condensate was illuminated with a “coupling beam” of linearly polarized laser light with a frequency corresponding to its weak excitation energy. The atoms moved to a higher energy level and stopped absorbing light. As a result, the condensate became transparent to the following laser radiation. And here very strange and unusual effects appeared. The measurements showed that, under certain conditions, a pulse passing through a Bose-Einstein condensate experiences a delay corresponding to the slowing of light by more than seven orders of magnitude - a factor of 20 million. The speed of the light pulse slowed down to 17 m/s, and its length decreased several times - to 43 micrometers.
The researchers believe that by avoiding laser heating of the condensate, they will be able to slow down the light even further - perhaps to a speed of several centimeters per second.
A system with such unusual characteristics will make it possible to study the quantum optical properties of matter, as well as create various devices for quantum computers of the future, for example, single-photon switches.
epigraph
The teacher asks: Children, what is the fastest thing in the world?
Tanechka says: The fastest word. I just said, you won’t come back.
Vanechka says: No, light is the fastest.
As soon as I pressed the switch, the room immediately became light.
And Vovochka objects: The fastest thing in the world is diarrhea.
I was once so impatient that I didn’t say a word
I didn’t have time to say anything or turn on the light.
Have you ever wondered why the speed of light is maximum, finite and constant in our Universe? This is a very interesting question, and right away, as a spoiler, I’ll give away the terrible secret of the answer to it - no one knows exactly why. The speed of light is taken, i.e. mentally accepted for a constant, and on this postulate, as well as on the idea that all inertial frames of reference are equal, Albert Einstein built his special theory of relativity, which has been pissing scientists off for a hundred years, allowing Einstein to stick his tongue out at the world with impunity and grin in his grave over the dimensions the pig that he planted on all of humanity.
But why, in fact, is it so constant, so maximum and so final, there is no answer, this is just an axiom, i.e. a statement taken on faith, confirmed by observations and common sense, but not logically or mathematically deducible from anywhere. And it is quite likely that it is not so true, but no one has yet been able to refute it with any experience.
I have my own thoughts on this matter, more on them later, but for now, let’s keep it simple, on your fingers™ I’ll try to answer at least one part - what does the speed of light mean “constant”.
No, I won’t bore you with thought experiments about what would happen if you turn on the headlights in a rocket flying at the speed of light, etc., that’s a little off topic now.
If you look in a reference book or Wikipedia, the speed of light in a vacuum is defined as a fundamental physical constant that exactly equal to 299,792,458 m/s. Well, that is, roughly speaking, it will be about 300,000 km/s, but if exactly right- 299,792,458 meters per second.
It would seem, where does such accuracy come from? Any mathematical or physical constant, whatever, even Pi, even the base of the natural logarithm e, even the gravitational constant G, or Planck’s constant h, always contain some numbers after the decimal point. In Pi, about 5 trillion of these decimal places are currently known (although only the first 39 digits have any physical meaning), the gravitational constant is today defined as G ~ 6.67384(80)x10 -11, and the constant Plank h~ 6.62606957(29)x10 -34 .
The speed of light in vacuum is smooth 299,792,458 m/s, not a centimeter more, not a nanosecond less. Want to know where this accuracy comes from?
It all started as usual with the ancient Greeks. Science, as such, in the modern sense of the word, did not exist among them. The philosophers of ancient Greece were called philosophers because they first invented some crap in their heads, and then, using logical conclusions (and sometimes real physical experiments), they tried to prove or disprove it. However, the use of real-life physical measurements and phenomena was considered by them to be “second-class” evidence, which cannot be compared with first-class logical conclusions obtained directly from the head.
The first person to think about the existence of light's own speed is considered to be the philosopher Empidocles, who stated that light is movement, and movement must have speed. He was objected to by Aristotle, who argued that light is simply the presence of something in nature, and that’s all. And nothing is moving anywhere. But that's something else! Euclid and Ptolemy generally believed that light is emitted from our eyes, and then falls on objects, and therefore we see them. In short, the ancient Greeks were as stupid as they could until they were conquered by the same ancient Romans.
In the Middle Ages, most scientists continued to believe that the speed of propagation of light was infinite, among them were, say, Descartes, Kepler and Fermat.
But some, like Galileo, believed that light had speed and therefore could be measured. The experiment of Galileo, who lit a lamp and gave light to an assistant located several kilometers from Galileo, is widely known. Having seen the light, the assistant lit his lamp, and Galileo tried to measure the delay between these moments. Naturally, he didn’t succeed, and in the end he was forced to write in his writings that if light has a speed, then it is extremely high and cannot be measured by human effort, and therefore can be considered infinite.
The first documented measurement of the speed of light is attributed to the Danish astronomer Olaf Roemer in 1676. By this year, astronomers, armed with the telescopes of that same Galileo, were actively observing the satellites of Jupiter and even calculated their rotation periods. Scientists have determined that the closest moon to Jupiter, Io, has a rotation period of approximately 42 hours. However, Roemer noticed that sometimes Io appears from behind Jupiter 11 minutes earlier than expected, and sometimes 11 minutes later. As it turned out, Io appears earlier in those periods when the Earth, rotating around the Sun, approaches Jupiter at a minimum distance, and lags behind by 11 minutes when the Earth is in the opposite place of the orbit, and therefore is further from Jupiter.
Stupidly dividing the diameter of the earth's orbit (and it was already more or less known in those days) by 22 minutes, Roemer received the speed of light 220,000 km/s, missing the true value by about a third.
In 1729, the English astronomer James Bradley, observing parallax(by a slight deviation in location) the star Etamin (Gamma Draconis) discovered the effect aberrations of light, i.e. a change in the position of the stars closest to us in the sky due to the movement of the Earth around the Sun.
From the effect of light aberration, discovered by Bradley, it can also be concluded that light has a finite speed of propagation, which Bradley seized upon, calculating it to be approximately 301,000 km/s, which is already within an accuracy of 1% of the value known today.
This was followed by all the clarifying measurements by other scientists, but since it was believed that light is a wave, and a wave cannot propagate on its own, something needs to be “excited,” the idea of the existence of a “luminiferous ether” arose, the discovery of which the American failed miserably physicist Albert Michelson. He did not discover any luminiferous ether, but in 1879 he clarified the speed of light to 299,910±50 km/s.
Around the same time, Maxwell published his theory of electromagnetism, which means that the speed of light became possible not only to directly measure, but also to derive from the values of electrical and magnetic permeability, which was done by clarifying the value of the speed of light to 299,788 km/s in 1907.
Finally, Einstein declared that the speed of light in a vacuum is a constant and does not depend on anything at all. On the contrary, everything else - adding velocities and finding the correct reference systems, the effects of time dilation and changes in distances when moving at high speeds and many other relativistic effects depend on the speed of light (because it is included in all formulas as a constant). In short, everything in the world is relative, and the speed of light is the quantity relative to which all other things in our world are relative. Here, perhaps, we should give the palm to Lorentz, but let’s not be mercantile, Einstein is Einstein.
The exact determination of the value of this constant continued throughout the 20th century, with each decade scientists found more and more numbers after decimal point at the speed of light, until vague suspicions began to arise in their heads.
Determining more and more accurately how many meters light travels in a vacuum per second, scientists began to wonder what it is we are measuring in meters? After all, in the end, a meter is just the length of some platinum-iridium stick that someone forgot in some museum near Paris!
And at first the idea of introducing a standard meter seemed great. In order not to suffer with yards, feet and other oblique fathoms, the French in 1791 decided to take as a standard measure of length one ten-millionth of the distance from the North Pole to the equator along the meridian passing through Paris. They measured this distance with the accuracy available at that time, cast a stick from a platinum-iridium (more precisely, first brass, then platinum, and then platinum-iridium) alloy and put it in this very Parisian Chamber of Weights and Measures as a sample. The further we go, the more it turns out that the earth's surface is changing, the continents are deforming, the meridians are shifting, and by one ten-millionth part they have forgotten, and began to count as a meter the length of the stick that lies in the crystal coffin of the Parisian "mausoleum."
Such idolatry does not suit a real scientist, this is not Red Square (!), and in 1960 it was decided to simplify the concept of the meter to a completely obvious definition - the meter is exactly equal to 1,650,763.73 wavelengths emitted by the transition of electrons between the energy levels 2p10 and 5d5 of the unexcited isotope of the element Krypton-86 in a vacuum. Well, how much more clear?
This went on for 23 years, while the speed of light in a vacuum was measured with increasing accuracy, until in 1983, finally, even the most stubborn retrogrades realized that the speed of light is the most accurate and ideal constant, and not some kind of isotope of krypton. And it was decided to turn everything upside down (more precisely, if you think about it, it was decided to turn everything back upside down), now the speed of light With is a true constant, and a meter is the distance that light travels in a vacuum in (1/299,792,458) seconds.
The real value of the speed of light continues to be clarified today, but what is interesting is that with each new experiment, scientists do not clarify the speed of light, but the true length of the meter. And the more accurately the speed of light is found in the coming decades, the more accurate the meter we will eventually get.
And not vice versa.
Well, now let's get back to our sheep. Why is the speed of light in the vacuum of our Universe maximum, finite and constant? This is how I understand it.
Everyone knows that the speed of sound in metal, and in almost any solid body, is much higher than the speed of sound in air. This is very easy to check; just put your ear to the rail, and you will be able to hear the sounds of an approaching train much earlier than through the air. Why is that? It is obvious that the sound is essentially the same, and the speed of its propagation depends on the medium, on the configuration of the molecules from which this medium consists, on its density, on the parameters of its crystal lattice - in short, on the current state of the medium through which the sound transmitted.
And although the idea of luminiferous ether has long been abandoned, the vacuum through which electromagnetic waves propagate is not absolutely absolute nothing, no matter how empty it may seem to us.
I understand that the analogy is somewhat far-fetched, but that’s true on your fingers™ same! Precisely as an accessible analogy, and in no way as a direct transition from one set of physical laws to others, I only ask you to imagine that the speed of propagation of electromagnetic (and in general, any, including gluon and gravitational) vibrations, just as the speed of sound in steel is “sewn into” the rail. From here we dance.
UPD: By the way, I invite “readers with an asterisk” to imagine whether the speed of light remains constant in a “difficult vacuum.” For example, it is believed that at energies of the order of temperature 10–30 K, the vacuum stops simply boiling with virtual particles, and begins to “boil away,” i.e. the fabric of space falls to pieces, Planck quantities blur and lose their physical meaning, etc. Would the speed of light in such a vacuum still be equal to c, or will this mark the beginning of a new theory of “relativistic vacuum” with corrections like Lorentz coefficients at extreme speeds? I don't know, I don't know, time will tell...
