The speed of light - absolute value propagation speed electromagnetic waves in a vacuum. In physics, it is traditionally denoted by the Latin letter "c" (pronounced as [tse]). The speed of light in a vacuum is a fundamental constant independent of choice inertial system reference (ISO). It refers to the fundamental physical constants that characterize not just individual bodies, but the properties of space-time as a whole. According to modern concepts, the speed of light in vacuum is the limiting speed of particles and propagation of interactions. Also important is the fact that this value is absolute. This is one of the postulates of SRT.
In a vacuum (emptiness)
In 1977, it was possible to calculate the approximate speed of light, equal to 299,792,458 ± 1.2 m / s, calculated on the basis of a 1960 reference meter. On the this moment consider that the speed of light in vacuum is a fundamental physical constant, by definition exactly equal to 299,792,458 m/s, or approximately 1,079,252,848.8 km/h. The exact value is due to the fact that since 1983 the standard of the meter has been the distance traveled by light in a vacuum in a time interval equal to 1/299,792,458 seconds. The speed of light is denoted by the letter c.
Michelson's fundamental experience for SRT showed that the speed of light in vacuum does not depend on the speed of the light source, nor on the speed of the observer. In nature, the speed of light propagates:
actual visible light
other types of electromagnetic radiation (radio waves, x-rays, etc.)
From special theory Relativity implies that the acceleration of particles having a rest mass to the speed of light is impossible, since this event would violate the fundamental principle of causality. That is, the excess of the speed of light by the signal, or the movement of mass at such a speed, is excluded. However, the theory does not exclude the motion of particles in space-time with superluminal speed. Hypothetical particles moving at superluminal speeds are called tachyons. Mathematically, tachyons easily fit into the Lorentz transformation - these are particles with an imaginary mass. The higher the speed of these particles, the less energy they carry, and vice versa, the closer their speed is to the speed of light, the greater their energy - just like the energy of ordinary particles, the energy of tachyons tends to infinity when approaching the speed of light. This is the most obvious consequence of the Lorentz transformation, which does not allow the particle to accelerate to the speed of light - it is simply impossible to give the particle an infinite amount of energy. It should be understood that, firstly, tachyons are a class of particles, and not just one kind of particles, and, secondly, no physical interaction can propagate faster speed Sveta. From this it follows that tachyons do not violate the principle of causality - they do not interact with ordinary particles in any way, and the difference between their velocities also cannot be equal to the speed of light.
Ordinary particles moving slower than light are called tardyons. Tardions cannot reach the speed of light, but can only approach it as close as they like, since in this case their energy becomes infinitely large. All tardions have a rest mass, unlike massless photons and gravitons, which always move at the speed of light.
In Planck units, the speed of light in vacuum is 1, that is, light travels 1 Planck unit of length per unit of Planck time.
In a transparent environment
The speed of light in a transparent medium is the speed at which light travels in a medium other than vacuum. In a medium with dispersion, phase and group velocity are distinguished.
The phase velocity relates the frequency and wavelength of monochromatic light in a medium (λ=c/ν). This speed is usually (but not necessarily) less than c. The ratio of the phase speed of light in vacuum to the speed of light in a medium is called the refractive index of the medium. The group speed of light in an equilibrium medium is always less than c. However, in nonequilibrium media it can exceed c. In this case, however, the leading edge of the pulse still moves at a speed not exceeding the speed of light in vacuum.
Armand Hippolyte Louis Fizeau proved by experience that the movement of a medium relative to a light beam can also affect the speed of light propagation in this medium.
Denial of the postulate about the maximum speed of light
AT last years there are often reports that in the so-called quantum teleportation interaction propagates faster than the speed of light. For example, August 15, 2008 research group Dr. Nicolas Gisin of the University of Geneva, examining bound photon states separated by 18 km in space, allegedly showed that "the interaction between particles is carried out at a speed of about one hundred thousand times the speed of light." The so-called Hartmann paradox was also discussed earlier - superluminal speed with tunnel effect.
Scientific analysis of the significance of these and similar results shows that they cannot in principle be used for superluminal transmission of any signal or movement of matter.
History of measurements of the speed of light
Ancient scientists, with rare exceptions, considered the speed of light to be infinite. In modern times, this issue became the subject of discussion. Galileo and Hooke assumed that it was finite, although very large, while Kepler, Descartes and Fermat still defended the infinity of the speed of light.
The first estimate of the speed of light was given by Olaf Römer (1676). He noticed that when the Earth and Jupiter are in different sides from the Sun, the eclipses of Jupiter's moon Io are delayed by 22 minutes compared to the calculations. From this he obtained a value for the speed of light of about 220,000 km/sec - inaccurate, but close to the true value. Half a century later, the discovery of aberration made it possible to confirm the finiteness of the speed of light and to refine its estimate.
In the 19th century, several scientific experiments took place that led to the discovery of a number of new phenomena. Among these phenomena is the discovery by Hans Oersted of the generation of magnetic induction electric shock. 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, as a result of which new theory electromagnetism, formulated by Wilhelm Weber, was based on long-range action. Later, Maxwell defined the concept of electric and magnetic fields, which are able to generate each other, which is an electromagnetic wave. Subsequently, Maxwell used in his equations the so-called "electromagnetic constant" - with.
By that time, scientists had already come close to the fact that light has an electromagnetic nature. The physical meaning of the electromagnetic constant is the propagation speed 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.
Prior to this discovery, humanity shared light, electricity and magnetism. Maxwell's generalization made it possible to take a fresh look at the nature of light, as a fragment of electric and magnetic fields propagating independently in space.
The figure below shows a diagram of the propagation of an electromagnetic wave, which is also light. Here H is the intensity vector magnetic field, E is the intensity vector electric field. Both vectors are perpendicular to each other, as well as to the direction of wave propagation.
Michelson's experiment - the absoluteness of the speed of light
The physics of that time was largely built taking into account 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 propagation velocity should have depended on the velocity 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-built theory was in a shaky state.
Experiments have shown that the speed of light does not really depend on the speed of the source, which means that a theory is required that can explain such a strange fact. The best theory at that time was the theory of "ether" - a certain medium in which light propagates, just as sound propagates in air. Then the speed of light would be determined not by the speed of the source, but by the features of the medium itself - the ether.
Many experiments have been undertaken to discover the ether, the most famous of which is the experience of the American physicist Albert Michelson. In short, we know that the Earth is moving 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 highest degree selfishness, but simply cannot be caused by anything. If the Earth moves through some medium in which light propagates, then it is logical to assume that there is an addition of velocities. That is, the propagation of light should depend on the direction of motion of the Earth, which flies through the ether. As a result of his experiments, Michelson did not find any difference between the speed of propagation of light in both directions from the Earth.
The Dutch physicist Hendrik Lorentz tried to solve this problem. According to his assumption, the "ethereal wind" influenced the 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 apparatus 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 moment of the death of the theory of the ether, nevertheless, scientists felt that this theory"pulled by the ears." So the ether had to have a number of "fabulous" properties, including weightlessness and the absence of resistance to moving bodies.
The end of the history of the ether came in 1905, along with the publication of the article "On the Electrodynamics of Moving Bodies" by then little known Albert Einstein.
Albert Einstein's special theory of relativity
Twenty-six-year-old Albert Einstein expressed a completely new, different view of 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 reference frame the observer is in, the speed of light relative to him is always one 300,000 km / s. From this followed the impossibility of applying the addition of velocities 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 the change in the parameters of bodies moving at speeds close to the speed of light. So, for example, the length of such bodies will be reduced, 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 includes also the Lorentz factor ( E= ymc2), which in general case equates to unity, in the case when the speed of the body v equals zero. As the speed of the body approaches v to the speed of light c Lorentz factor y rushes to infinity. It follows from this that in order to accelerate the body to the speed of light, an infinite amount of energy is required, and therefore it is impossible to go over this speed limit.
In favor of this statement, there is also such an argument as "the relativity of simultaneity".
Paradox of relativity of simultaneity SRT
In short, the phenomenon of 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.
This also implies such a paradox that event B, which is a consequence of event A, can occur simultaneously with it. In addition, one can choose frames of reference in such a way that event B occurs earlier than the event A that caused it. Such a phenomenon violates the principle of causality, which is quite firmly established in science and has never been questioned. However, this hypothetical situation is observed only when the distance between events A and B is greater than the time interval between them, multiplied by the "electromagnetic constant" - with. So the constant c, which is equal to the speed of light, is maximum speed transfer of information. Otherwise, the principle of causality would be violated.
How is the speed of light measured?
Observations by Olaf Römer
Scientists of antiquity for the most part believed that light moves at an infinite speed, and the first estimate of the speed of light was obtained as early as 1676. Danish astronomer Olaf Römer 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 satellite Io was 22 minutes late compared to the calculated time. The only solution that Olaf Römer found is that the speed of light is the limit. 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. Roemer calculated that the speed of light was 220,000 km/s.
James Bradley's observations
In 1727, the English astronomer James Bradley discovered the phenomenon of light aberration. The essence of this phenomenon is that when the Earth moves around the Sun, as well as during the Earth's own rotation, a shift of stars in the night sky is observed. Since the observer on Earth and the Earth itself are constantly changing their direction of motion 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 causes the stars in the sky to describe an ellipse during the year. This experiment allowed James Bradley to estimate the speed of light - 308,000 km / s.
The Louis Fizeau Experience
In 1849, the French physicist Louis Fizeau put laboratory experience by measuring the speed of light. The physicist set up a mirror in Paris at a distance of 8,633 meters from the source, but according to Römer's calculations, light will travel this distance in a hundred-thousandths of a second. Such clock accuracy was then unattainable. Then Fizeau used a gear wheel, which 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 the light beam from the source to the mirror passed between the teeth, and hit the tooth on the way back, the physicist doubled the speed of the wheel. With the increase in the speed of rotation of the wheel, the light practically ceased to disappear, until the rotation speed reached 12.67 revolutions per second. At that 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 315,000 km/sec.
A year later, another French physicist Léon 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, the Fizeau method was improved so much that a similar experiment set up in 1950 by E. Bergstrand gave a speed value of 299,793.1 km / s. Given 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 precision 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 unrealizable, not because of the technical imperfection of the instruments, but because of the error of the meter standard itself. For this reason, in 1983, the 17th General Conference on Weights and Measures defined the meter as the distance traveled by light in a vacuum in a time equal to 1/299,792,458 of a second.
Summing up
So, from all of the above, it follows that the speed of light in vacuum is a fundamental physical constant that appears in many fundamental theories. This rate is absolute, that is, it does not depend on the choice of reference system, and is also equal to the limiting rate of information transfer. Not only electromagnetic waves (light) move with this speed, but also all massless particles. Including, presumably, graviton - a particle of gravitational waves. In addition, due to relativistic effects, the proper time for light is literally worth it.
Such properties of light, in particular 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 based precisely on this nature of light.
Although in everyday life it is rare for anyone to directly calculate what the speed of light is, interest in this issue manifests itself in childhood. Surprisingly, every day we all face the sign of the speed constant of propagation of electromagnetic waves. The speed of light is a fundamental quantity, due to which the entire Universe exists exactly in the form in which we know it.
Surely, everyone, watching in childhood a flash of lightning and the subsequent clap of thunder, tried to understand what caused the delay between the first and second phenomena. Simple mental reasoning quickly led to a logical conclusion: the speed of light and sound is different. This is the first acquaintance with two important physical quantities. Subsequently, someone received necessary knowledge and could easily explain what was going on. What is the cause of the strange behavior of thunder? The answer is that the speed of light, which is about 300,000 km/s, is almost a million times the speed of propagation in air (330 m/s). Therefore, a person first sees from lightning and only after a while hears the roar of thunder. For example, if there is 1 km from the epicenter to the observer, then the light will overcome this distance in 3 microseconds, but the sound will need as much as 3 s. Knowing the speed of light and the time delay between flash and thunder, the distance can be calculated.
Attempts to measure it have been made for a long time. Now it is quite funny to read about the ongoing experiments, however, in those distant times, before the advent of precision instruments, everything was more than serious. When trying to find out what is the speed of light, one interesting experience. At one end of the fast-moving train car was a man with an accurate chronometer, and at the opposite side, his assistant in command opened the lamp damper. According to the idea, the chronometer was supposed to determine the speed of propagation of photons of light. Moreover, by changing the positions of the lamp and the chronometer (with the same direction of train movement), it would be possible to find out whether the speed of light is constant, or it can be increased / decreased (depending on the direction of the beam, theoretically, the speed of the train could affect the speed measured in the experiment ). Of course, the experiment was not successful, since the speed of light and registration with a chronometer are incomparable.
For the first time, the most accurate measurement was made in 1676 thanks to observations by Olaf Roemer noticed that the actual appearance of Io and the calculated data differed by 22 minutes. As the planets approached, the delay decreased. Knowing the distance, it was possible to calculate the speed of light. It amounted to about 215 thousand km/s. Then, in 1926, D. Bradley, studying the change in the apparent positions of stars (aberration), drew attention to the pattern. The location of the star changed depending on the time of year. Consequently, the position of the planet relative to the Sun had an influence. You can give an analogy - raindrops. Without wind, they fly vertically down, but it is worth running - and their apparent trajectory changes. Knowing the speed of rotation of the planet around the Sun, it was possible to calculate the speed of light. It amounted to 301 thousand km / s.
In 1849 A. Fizeau held next experience: between the light source and the mirror, remote at 8 km, there was a rotating one. The speed of its rotation was increased until the flux of reflected light in the next gap turned into a constant (non-flickering) one. Calculations gave 315 thousand km / s. Three years later, L. Foucault with a rotating mirror and received 298 thousand km / s.
Subsequent experiments became more and more accurate, taking into account refraction in air, etc. At present, the data obtained using a cesium clock and a laser beam are considered relevant. According to them, it is equal to 299 thousand km / s.
The speed of light is the distance that light travels per unit time. This value depends on the medium in which the light propagates.
In vacuum, the speed of light is 299,792,458 m/s. This is the highest speed that can be reached. When solving problems that do not require special accuracy, this value is taken equal to 300,000,000 m/s. It is assumed that all types of electromagnetic radiation propagate at the speed of light in a vacuum: radio waves, infrared radiation, visible light, ultraviolet radiation, x-rays, gamma radiation. Designate it with a letter with .
How is the speed of light determined?
In ancient times, scientists believed that the speed of light was infinite. Later, discussions on this issue began in the scientific community. Kepler, Descartes and Fermat agreed with the opinion of ancient scientists. And Galileo and Hooke believed that, although the speed of light is very high, it still has a finite value.
Galileo Galilei
One of the first to measure the speed of light was the Italian scientist Galileo Galilei. During the experiment, he and his assistant were on different hills. Galileo opened the damper on his lantern. At that moment, when the assistant saw this light, he had to do the same with his lantern. The time during which the light traveled from Galileo to the assistant and back turned out to be so short that Galileo realized that the speed of light is very high, and at such short distance it is impossible to measure it, since light propagates almost instantly. And the time recorded by him shows only the speed of a person's reaction.
The speed of light was first determined in 1676 by the Danish astronomer Olaf Römer using astronomical distances. Observing with a telescope the eclipse of Jupiter's moon Io, he found that as the Earth moves away from Jupiter, each subsequent eclipse comes later than it was calculated. The maximum delay, when the Earth moves to the other side of the Sun and moves away from Jupiter at a distance equal to the diameter of the Earth's orbit, is 22 hours. Although at that time the exact diameter of the Earth was not known, the scientist divided its approximate value by 22 hours and came up with a value of about 220,000 km / s.
Olaf Römer
The result obtained by Römer caused distrust among scientists. But in 1849 the French physicist Armand Hippolyte Louis Fizeau measured the speed of light using the rotating shutter method. In his experiment, light from a source passed between the teeth of a rotating wheel and was directed to a mirror. Reflected from him, he returned back. Wheel speed increased. When it reached a certain value, the beam reflected from the mirror was delayed by the moved tooth, and the observer at that moment did not see anything.
Fizeau's experience
Fizeau calculated the speed of light as follows. Light goes the way L from the wheel to the mirror in a time equal to t1 = 2L/s . The time it takes the wheel to make a ½ slot turn is t 2 \u003d T / 2N , where T - wheel rotation period, N - the number of teeth. Rotation frequency v = 1/T . The moment when the observer does not see the light comes at t1 = t2 . From here we get the formula for determining the speed of light:
c = 4LNv
After calculating this formula, Fizeau determined that with = 313,000,000 m/s. This result was much more accurate.
Armand Hippolyte Louis Fizeau
In 1838, the French physicist and astronomer Dominique François Jean Arago proposed using the method of rotating mirrors to calculate the speed of light. This idea was put into practice by the French physicist, mechanic and astronomer Jean Bernard Léon Foucault, who in 1862 obtained the value of the speed of light (298,000,000 ± 500,000) m/s.
Dominique Francois Jean Arago
In 1891, the result of the American astronomer Simon Newcomb turned out to be an order of magnitude more accurate than Foucault's result. As a result of his calculations with = (99 810 000±50 000) m/s.
The studies of the American physicist Albert Abraham Michelson, who used an installation with a rotating octahedral mirror, made it possible to more accurately determine the speed of light. In 1926, the scientist measured the time during which light traveled the distance between the tops of two mountains, equal to 35.4 km, and received with = (299 796 000±4 000) m/s.
The most accurate measurement was made in 1975. In the same year, the General Conference on Weights and Measures recommended that the speed of light be considered equal to 299,792,458 ± 1.2 m/s.
What determines the speed of light
The speed of light in vacuum does not depend on the frame of reference or on the position of the observer. She stays constant value, equal to 299 792 458 ± 1.2 m/s. But in various transparent media this speed will be lower than its speed in vacuum. Any transparent medium has an optical density. And the higher it is, the slower the light propagates in it. So, for example, the speed of light in air is higher than its speed in water, and in pure optical glass it is less than in water.
If light passes from a less dense medium to a more dense one, its speed decreases. And if the transition occurs from a denser medium to a less dense one, then the speed, on the contrary, increases. This explains why the light beam is deflected at the boundary of the transition of two media.
The speed of light in a vacuum- the absolute value of the propagation velocity of electromagnetic waves in vacuum. In physics, it is denoted by the Latin letter c.
The speed of light in a vacuum is a fundamental constant, independent of the choice of inertial frame of reference.
By definition, it is exactly 299 792 458 m / s (approximate value of 300 thousand km / s).
According to the special theory of relativity, is the maximum speed for the propagation of any physical interactions that transmit energy and information.
How is the speed of light determined?
The speed of light was first determined in 1676 O. K. Römer by changing the time intervals between eclipses of Jupiter's satellites.
In 1728 it was installed by J. Bradley, based on his observations of the aberration of stellar light.
In 1849 A. I. L. Fizeau he was the first to measure the speed of light by the time it takes light to travel a precisely known distance (base); since the refractive index of air differs very little from 1, ground-based measurements give a value very close to s.
In Fizeau's experiment, a beam of light from a source S, reflected by a semitransparent mirror N, was periodically interrupted by a rotating toothed disk W, passed the base MN (about 8 km) and, reflected from the mirror M, returned to the disk. When the light hit the tooth, the light did not reach the observer, and the light that fell into the gap between the teeth could be observed through the eyepiece E. The time of passage of the light through the base was determined from the known disk rotation speeds. Fizeau obtained the value c = 313,300 km/s.
In 1862 J. B. L. Foucault realized the idea of D. Arago expressed in 1838, using a rapidly rotating (512 rpm) mirror instead of a toothed disk. Reflecting from the mirror, the beam of light was directed to the base and, upon returning, fell again on the same mirror, which had time to turn through a certain small angle. With a base of only 20 m, Foucault found that the speed of light is 29800080 ± 500 km/s. The schemes and main ideas of the experiments of Fizeau and Foucault were repeatedly used in subsequent works on the determination of p.
