The speed of light is the most unusual measurement known to date. The first person who tried to explain the phenomenon of light propagation was Albert Einstein. It was he who deduced the well-known formula E = mc² , where E is the total energy of the body, m is the mass, and c is the speed of light in vacuum.
The formula was first published in Annalen der Physik in 1905. Around the same time, Einstein put forward a theory about what would happen to a body moving at absolute speed. Based on the fact that the speed of light is a constant value, he came to the conclusion that space and time must change.
Thus, at the speed of light, an object will shrink indefinitely, its mass will increase indefinitely, and time will practically stop.
In 1977, it was possible to calculate the speed of light, a figure of 299,792,458 ± 1.2 meters per second was named. For more rough calculations, a value of 300,000 km/s is always taken. It is from this value that all other cosmic measurements are repelled. This is how the concept " light year"and" parsec "(3.26 light years).
Neither to move at the speed of light, nor, moreover, to overcome it is impossible. At least at this stage of human development. On the other hand, science fiction writers have been trying to solve this problem in the pages of their novels for about 100 years. Perhaps one day fantasy will become a reality, because back in the 19th century, Jules Verne predicted the appearance of a helicopter, an airplane and an electric chair, and then it was pure fantasy!
Doctor technical sciences A. GOLUBEV.
In the middle of last year, a sensational report appeared in the magazines. A group of American researchers found that a very short laser pulse travels hundreds of times faster in a specially selected medium than in a vacuum. This phenomenon seemed completely unbelievable (the speed of light in a medium is always less than in a vacuum) and even gave rise to doubts about the validity special theory relativity. Meanwhile, a superluminal physical object - a laser pulse in an amplifying medium - was first discovered not in 2000, but 35 years earlier, in 1965, and the possibility of superluminal motion was widely discussed until the early 70s. Today, the discussion around this strange phenomenon has flared up with renewed vigor.
Examples of "superluminal" motion.
In the early 1960s, high-power short light pulses began to be obtained by passing a laser flash through a quantum amplifier (a medium with an inverse population).
In the amplifying medium, the initial region of the light pulse causes stimulated emission of atoms in the amplifier medium, and its final region causes energy absorption by them. As a result, it will appear to the observer that the momentum is moving faster than light.
Lijun Wong experiment.
A beam of light passing through a prism of a transparent material (such as glass) is refracted, that is, it experiences dispersion.
A light pulse is a set of oscillations of different frequencies.
Probably everyone - even people far from physics - knows that the maximum possible speed of movement of material objects or the propagation of any signals is the speed of light in vacuum. It is marked with the letter with and is almost 300 thousand kilometers per second; exact value with= 299 792 458 m/s. The speed of light in vacuum is one of the fundamental physical constants. The impossibility of achieving speeds exceeding with, follows from the special theory of relativity (SRT) of Einstein. If it were possible to prove that the transmission of signals with superluminal speed is possible, the theory of relativity would fall. So far, this has not happened, despite numerous attempts to refute the ban on the existence of speeds greater than with. However, in experimental studies Recently, some very interesting phenomena have been discovered, indicating that under specially created conditions it is possible to observe superluminal velocities without violating the principles of the theory of relativity.
To begin with, let us recall the main aspects related to the problem of the speed of light. First of all: why is it impossible (under normal conditions) to exceed the light limit? Because then the fundamental law of our world is violated - the law of causality, according to which the effect cannot outstrip the cause. No one has ever observed that, for example, a bear first fell dead, and then a hunter shot. At speeds exceeding with, the sequence of events becomes reversed, the time tape rewinds. This can be easily seen from the following simple reasoning.
Let's assume that we are on a certain cosmic miracle ship moving faster than light. Then we would gradually catch up with the light emitted by the source at earlier and earlier points in time. First, we would catch up with photons emitted, say, yesterday, then - emitted the day before yesterday, then - a week, a month, a year ago, and so on. If the light source were a mirror reflecting life, then we would first see the events of yesterday, then the day before yesterday, and so on. We could see, say, an old man who gradually turns into a middle-aged man, then into a young man, into a youth, into a child ... That is, time would turn back, we would move from the present to the past. Cause and effect would then be reversed.
Although this argument completely ignores the technical details of the process of observing light, from a fundamental point of view, it clearly demonstrates that the movement at a superluminal speed leads to a situation that is impossible in our world. However, nature has set even more stringent conditions: it is unattainable to move not only at superluminal speed, but also at a speed equal speed light, - you can only approach it. It follows from the theory of relativity that with an increase in the speed of movement, three circumstances arise: the mass of a moving object increases, its size decreases in the direction of movement, and the passage of time on this object slows down (from the point of view of an external "resting" observer). At ordinary speeds, these changes are negligible, but as we approach the speed of light, they become more and more noticeable, and in the limit - at a speed equal to with, - the mass becomes infinitely large, the object completely loses its size in the direction of motion and time stops on it. Therefore, no material body can reach the speed of light. Only light itself has such a speed! (And also the "all-penetrating" particle - the neutrino, which, like the photon, cannot move at a speed less than with.)
Now about the signal transmission speed. Here it is appropriate to use the representation of light in the form electromagnetic waves. What is a signal? This is some information to be transmitted. An ideal electromagnetic wave is an infinite sinusoid of strictly one frequency, and it cannot carry any information, because each period of such a sinusoid exactly repeats the previous one. The speed at which the phase of the sine wave moves - the so-called phase speed - can exceed the speed of light in a vacuum under certain conditions. There are no restrictions here, since the phase speed is not the speed of the signal - it does not exist yet. To create a signal, you need to make some kind of "mark" on the wave. Such a mark can be, for example, a change in any of the wave parameters - amplitude, frequency or initial phase. But as soon as the mark is made, the wave loses its sinusoidality. It becomes modulated, consisting of a set of simple sine waves with different amplitudes, frequencies and initial phases- groups of waves. The speed of movement of the mark in the modulated wave is the speed of the signal. When propagating in a medium, this velocity usually coincides with the group velocity characterizing the propagation of the above group of waves as a whole (see "Science and Life" No. 2, 2000). Under normal conditions, the group velocity, and hence the speed of the signal, is less than the speed of light in vacuum. It is no coincidence that the expression "under normal conditions" is used here, because in some cases the group velocity can also exceed with or even lose meaning, but then it does not apply to signal propagation. It is established in the SRT that it is impossible to transmit a signal at a speed greater than with.
Why is it so? Because the obstacle to the transmission of any signal at a speed greater than with the same law of causality applies. Let's imagine such a situation. At some point A, a light flash (event 1) turns on a device that sends a certain radio signal, and at a remote point B, under the action of this radio signal, an explosion occurs (event 2). It is clear that event 1 (flash) is the cause, and event 2 (explosion) is the effect that occurs later than the cause. But if the radio signal propagated at a superluminal speed, an observer near point B would first see an explosion, and only then - reaching it with a speed with flash of light, the cause of the explosion. In other words, for this observer, event 2 would have happened before event 1, that is, the effect would have preceded the cause.
It is appropriate to emphasize that the "superluminal prohibition" of the theory of relativity is imposed only on motion material bodies and signal transmission. In many situations it is possible to move at any speed, but it will be the movement of non-material objects and signals. For example, imagine two rather long rulers lying in the same plane, one of which is located horizontally, and the other intersects it at a small angle. If the first line is moved down (in the direction indicated by the arrow) at high speed, the intersection point of the lines can be made to run arbitrarily fast, but this point is not a material body. Another example: if you take a flashlight (or, say, a laser that gives a narrow beam) and quickly describe an arc in the air, then line speed light spot will increase with distance and at a sufficiently large distance will exceed with. The spot of light will move between points A and B at superluminal speed, but this will not be a signal transmission from A to B, since such a spot of light does not carry any information about point A.
It would seem that the question of superluminal speeds has been resolved. But in the 60s of the twentieth century, theoretical physicists put forward the hypothesis of the existence of superluminal particles, called tachyons. These are very strange particles: they are theoretically possible, but in order to avoid contradictions with the theory of relativity, they had to be assigned an imaginary rest mass. Physically imaginary mass does not exist, it is a purely mathematical abstraction. However, this did not cause much concern, since tachyons cannot be at rest - they exist (if they exist!) only at speeds exceeding the speed of light in vacuum, and in this case the mass of the tachyon turns out to be real. There is some analogy with photons here: a photon has zero rest mass, but that simply means that the photon cannot be at rest - light cannot be stopped.
The most difficult thing was, as expected, to reconcile the tachyon hypothesis with the law of causality. Attempts made in this direction, although they were quite ingenious, did not lead to obvious success. No one has been able to experimentally register tachyons either. As a result, interest in tachyons as superluminal elementary particles gradually faded away.
However, in the 60s, a phenomenon was experimentally discovered, which at first led physicists into confusion. This is described in detail in the article by A. N. Oraevsky "Superluminal waves in amplifying media" (UFN No. 12, 1998). Here we briefly summarize the essence of the matter, referring the reader interested in the details to the said article.
Shortly after the discovery of lasers, in the early 1960s, the problem arose of obtaining short (with a duration of the order of 1 ns = 10 -9 s) high-power light pulses. To do this, a short laser pulse was passed through an optical quantum amplifier. The pulse was split by a beam-splitting mirror into two parts. One of them, more powerful, was sent to the amplifier, and the other propagated in the air and served as a reference pulse, with which it was possible to compare the pulse that passed through the amplifier. Both pulses were fed to photodetectors, and their output signals could be visually observed on the oscilloscope screen. It was expected that the light pulse passing through the amplifier would experience some delay in it compared to the reference pulse, that is, the speed of light propagation in the amplifier would be less than in air. What was the amazement of the researchers when they discovered that the pulse propagated through the amplifier at a speed not only greater than in air, but also several times greater than the speed of light in vacuum!
After recovering from the first shock, physicists began to look for the reason for such an unexpected result. No one had even the slightest doubt about the principles of the special theory of relativity, and this is precisely what helped to find the correct explanation: if the principles of SRT are preserved, then the answer should be sought in the properties of the amplifying medium.
Without going into details here, we only point out that a detailed analysis of the mechanism of action of the amplifying medium has completely clarified the situation. The point was in the change in the concentration of photons during the propagation of the pulse - a change due to a change in the amplification factor of the medium up to negative value during the passage of the rear part of the pulse, when the medium is already absorbing energy, because its own reserve has already been used up due to its transfer to the light pulse. Absorption does not cause an increase, but a decrease in the impulse, and thus the impulse is strengthened in the front and weakened in the back of it. Let us imagine that we observe the pulse with the help of an instrument moving at the speed of light in the medium of an amplifier. If the medium were transparent, we would see an impulse frozen in immobility. In the medium in which the process mentioned above takes place, the strengthening of the leading edge and the weakening of the trailing edge of the pulse will appear to the observer in such a way that the medium, as it were, has moved the pulse forward. But since the device (observer) moves at the speed of light, and the impulse overtakes it, then the speed of the impulse exceeds the speed of light! It is this effect that was registered by the experimenters. And here there really is no contradiction with the theory of relativity: it's just that the amplification process is such that the concentration of photons that came out earlier turns out to be greater than those that came out later. It is not photons that move with superluminal speed, but the envelope of the pulse, in particular its maximum, which is observed on the oscilloscope.
Thus, while in ordinary media there is always a weakening of light and a decrease in its speed, determined by the refractive index, in active laser media, not only amplification of light is observed, but also propagation of a pulse with superluminal speed.
Some physicists have tried to experimentally prove the presence of superluminal motion in the tunnel effect - one of the most amazing phenomena in quantum mechanics. This effect consists in the fact that a microparticle (more precisely, a microobject that exhibits both the properties of a particle and the properties of a wave under different conditions) is able to penetrate the so-called potential barrier - a phenomenon that is completely impossible in classical mechanics (in which such a situation would be analogous : a ball thrown at a wall would end up on the other side of the wall, or the undulating motion given by a rope tied to the wall would be transmitted to a rope tied to the wall on the other side). The essence of the tunnel effect in quantum mechanics is as follows. If a micro-object with a certain energy encounters on its way an area with a potential energy exceeding the energy of the micro-object, this area is a barrier for it, the height of which is determined by the energy difference. But the micro-object "leaks" through the barrier! This possibility is given to him by the well-known Heisenberg uncertainty relation, written for the energy and interaction time. If the interaction of the microobject with the barrier occurs for a sufficiently definite time, then the energy of the microobject, on the contrary, will be characterized by uncertainty, and if this uncertainty is of the order of the barrier height, then the latter ceases to be an insurmountable obstacle for the microobject. It is the rate of penetration through the potential barrier that has become the subject of research by a number of physicists who believe that it can exceed with.
In June 1998, an international symposium on the problems of superluminal motions was held in Cologne, where the results obtained in four laboratories - in Berkeley, Vienna, Cologne and Florence were discussed.
And finally, in 2000, two new experiments were reported in which the effects of superluminal propagation appeared. One of them was carried out by Lijun Wong and co-workers at a research institute in Princeton (USA). His result is that a light pulse entering a chamber filled with cesium vapor increases its speed by a factor of 300. It turned out that the main part of the pulse leaves the far wall of the chamber even before the pulse enters the chamber through the front wall. This situation is contrary not only common sense, but, in essence, also the theory of relativity.
L. Wong's report provoked intense discussion among physicists, most of whom are not inclined to see in the results obtained a violation of the principles of relativity. The challenge, they believe, is to correctly explain this experiment.
In the experiment of L. Wong, the light pulse entering the chamber with cesium vapor had a duration of about 3 μs. Cesium atoms can be in sixteen possible quantum mechanical states, called "ground state hyperfine magnetic sublevels". Using optical laser pumping, almost all atoms were brought to only one of these sixteen states, corresponding to almost absolute zero temperature on the Kelvin scale (-273.15 o C). The length of the cesium chamber was 6 centimeters. In a vacuum, light travels 6 centimeters in 0.2 ns. As the measurements showed, the light pulse passed through the chamber with cesium in a time 62 ns shorter than in vacuum. In other words, the transit time of a pulse through a cesium medium has a "minus" sign! Indeed, if we subtract 62 ns from 0.2 ns, we get a "negative" time. This "negative delay" in the medium - an incomprehensible time jump - is equal to the time during which the pulse would make 310 passes through the chamber in vacuum. The consequence of this "time reversal" was that the impulse leaving the chamber managed to move away from it by 19 meters before the incoming impulse reached the near wall of the chamber. How can such an incredible situation be explained (unless, of course, there is no doubt about the purity of the experiment)?
Judging by the discussion that has unfolded, an exact explanation has not yet been found, but there is no doubt that the unusual dispersion properties of the medium play a role here: cesium vapor, consisting of atoms excited by laser light, is a medium with anomalous dispersion. Let us briefly recall what it is.
The dispersion of a substance is the dependence of the phase (ordinary) refractive index n on the wavelength of light l. With normal dispersion, the refractive index increases with decreasing wavelength, and this is the case in glass, water, air, and all other substances transparent to light. In substances that strongly absorb light, the course of the refractive index reverses with a change in wavelength and becomes much steeper: with a decrease in l (increase in frequency w), the refractive index sharply decreases and in a certain range of wavelengths becomes less than unity (phase velocity V f > with). This is the anomalous dispersion, in which the pattern of light propagation in a substance changes radically. group speed V cp becomes greater than the phase velocity of the waves and can exceed the speed of light in vacuum (and also become negative). L. Wong points to this circumstance as the reason underlying the possibility of explaining the results of his experiment. However, it should be noted that the condition V gr > with is purely formal, since the concept of group velocity was introduced for the case of small (normal) dispersion, for transparent media, when a group of waves almost does not change its shape during propagation. In regions of anomalous dispersion, however, the light pulse is rapidly deformed and the concept of group velocity loses its meaning; in this case, the concepts of signal velocity and energy propagation velocity are introduced, which in transparent media coincide with the group velocity, while in media with absorption they remain less than the speed of light in vacuum. But here's what's interesting about Wong's experiment: a light pulse, passing through a medium with anomalous dispersion, does not deform - it retains its shape exactly! And this corresponds to the assumption that the impulse propagates with the group velocity. But if so, then it turns out that there is no absorption in the medium, although the anomalous dispersion of the medium is due precisely to absorption! Wong himself, recognizing that much remains unclear, believes that what is happening in his experimental setup can be clearly explained as a first approximation as follows.
A light pulse consists of many components with different wavelengths (frequencies). The figure shows three of these components (waves 1-3). At some point, all three waves are in phase (their maxima coincide); here they, adding up, reinforce each other and form an impulse. As the waves propagate further in space, they are out of phase and thus "extinguish" each other.
In the region of anomalous dispersion (inside the cesium cell), the wave that was shorter (wave 1) becomes longer. Conversely, the wave that was the longest of the three (wave 3) becomes the shortest.
Consequently, the phases of the waves also change accordingly. When the waves have passed through the cesium cell, their wavefronts are restored. Having undergone an unusual phase modulation in a substance with anomalous dispersion, the three considered waves again find themselves in phase at some point. Here they add up again and form a pulse of exactly the same shape as that entering the cesium medium.
Typically in air, and indeed in any normally dispersed transparent medium, a light pulse cannot accurately maintain its shape when propagating over a remote distance, that is, all of its components cannot be in phase at any remote point along the propagation path. And under normal conditions, a light pulse at such a remote point appears after some time. However, due to the anomalous properties of the medium used in the experiment, the pulse at the remote point turned out to be phased in the same way as when entering this medium. Thus, the light pulse behaves as if it had a negative time delay on its way to a remote point, that is, it would have arrived at it not later, but earlier than it passed the medium!
Most physicists are inclined to associate this result with the appearance of a low-intensity precursor in the dispersive medium of the chamber. The fact is that in the spectral decomposition of the pulse, the spectrum contains components of arbitrarily high frequencies with negligible amplitude, the so-called precursor, which goes ahead of the "main part" of the pulse. The nature of the establishment and the form of the precursor depend on the dispersion law in the medium. With this in mind, the sequence of events in Wong's experiment is proposed to be interpreted as follows. The incoming wave, "stretching" the harbinger in front of itself, approaches the camera. Before the peak of the incoming wave hits the near wall of the chamber, the precursor initiates the appearance of a pulse in the chamber, which reaches the far wall and is reflected from it, forming a "reverse wave". This wave, propagating 300 times faster with, reaches the near wall and meets the incoming wave. The peaks of one wave meet the troughs of another so that they cancel each other out and nothing remains. It turns out that the incoming wave "returns the debt" to the cesium atoms, which "borrowed" energy to it at the other end of the chamber. Someone who watched only the beginning and end of the experiment would see only a pulse of light that "jumped" forward in time, moving faster with.
L. Wong believes that his experiment is not consistent with the theory of relativity. The statement about the unattainability of superluminal speed, he believes, is applicable only to objects with a rest mass. Light can be represented either in the form of waves, to which the concept of mass is generally inapplicable, or in the form of photons with a rest mass, as is known, equal to zero. Therefore, the speed of light in a vacuum, according to Wong, is not the limit. Nevertheless, Wong admits that the effect he discovered does not make it possible to transmit information at a speed greater than with.
"The information here is already contained in the leading edge of the impulse," says P. Milonni, a physicist at the Los Alamos National Laboratory in the United States.
Most physicists believe that new job does not deal a crushing blow to fundamental principles. But not all physicists believe that the problem is settled. Professor A. Ranfagni, of the Italian research team that carried out another interesting experiment in 2000, says the question is still open. This experiment, carried out by Daniel Mugnai, Anedio Ranfagni and Rocco Ruggeri, found that centimeter-wave radio waves propagate in ordinary air at a speed exceeding with by 25%.
Summarizing, we can say the following. The works of recent years show that under certain conditions, superluminal speed can indeed take place. But what exactly is moving at superluminal speed? The theory of relativity, as already mentioned, forbids such a speed for material bodies and for signals carrying information. Nevertheless, some researchers are very persistent in their attempts to demonstrate the overcoming of the light barrier specifically for signals. The reason for this lies in the fact that in the special theory of relativity there is no rigorous mathematical justification (based, say, on Maxwell's equations for electromagnetic field) the impossibility of transmitting signals at a speed greater than with. Such an impossibility in SRT is established, one might say, purely arithmetically, based on Einstein's formula for adding velocities, but in a fundamental way this is confirmed by the principle of causality. Einstein himself, considering the question of superluminal signal transmission, wrote that in this case "... we are forced to consider a signal transmission mechanism possible, when using which the achieved action precedes the cause. But, although this result from a purely logical point of view does not contain itself, in my opinion, no contradictions, it nevertheless contradicts the character of all our experience so much that the impossibility of supposing V > c appears to be sufficiently proven." The principle of causality is the cornerstone that underlies the impossibility of superluminal signal transmission. And this stone, apparently, will stumble all searches for superluminal signals, without exception, no matter how much experimenters would like to detect such signals because that is the nature of our world.
In conclusion, it should be emphasized that all of the above applies specifically to our world, to our Universe. This stipulation was made because recent times in astrophysics and cosmology, new hypotheses appear that allow the existence of many universes hidden from us, connected by topological tunnels - jumpers. This point of view is shared, for example, by the well-known astrophysicist N. S. Kardashev. For an outside observer, the entrances to these tunnels are marked by anomalous gravitational fields, similar to black holes. Movements in such tunnels, as suggested by the authors of the hypotheses, will make it possible to circumvent the limitation of the speed of movement imposed in ordinary space by the speed of light, and, consequently, to realize the idea of creating a time machine... things. And although so far such hypotheses are too reminiscent of plots from science fiction, one should hardly categorically reject the fundamental possibility of a multi-element model of the structure of the material world. Another thing is that all these other universes are likely to remain pure mathematical constructions theoretical physicists living in our Universe and trying to find the worlds closed to us by the power of their thoughts...
See in a room on the same topic
Man has always been interested in the nature of light, as evidenced by myths, legends, philosophical disputes and scientific observations that have come down to us. Light has always been an occasion for discussions of ancient philosophers, and attempts to study it were made even at the time of the emergence of Euclidean geometry - 300 years BC. Even then it was known about the rectilinearity of the propagation of light, the equality of the angles of incidence and reflection, the phenomenon of light refraction, the causes of the rainbow were discussed. Aristotle believed that the speed of light is infinitely great, and therefore, logically reasoning, light is not subject to discussion. A typical case when the problem is ahead of the era of understanding the answer in its depth.
Some 900 years ago, Avicenna suggested that no matter how large the speed of light is, it still has a finite value. This opinion was not only he, but no one was able to prove it experimentally. The ingenious Galileo Galilei proposed an experiment of mechanistic understanding of the problem: two people, standing at a distance of several kilometers from each other, give signals by opening the shutter of the lantern. As soon as the second participant sees the light from the first lamp, he opens his shutter and the first participant fixes the time of receiving the response light signal. Then the distance increases and everything repeats. It was expected to fix the increase in delay and on this basis to perform the calculation of the speed of light. The experiment ended in nothing, because "everything was not sudden, but extremely fast."
The first to measure the speed of light in a vacuum in 1676 was the astronomer Ole Remer - he took advantage of the discovery of Galileo: he discovered four in 1609 in which the time difference between two satellite eclipses was 1320 seconds for half a year. Using the astronomical information of his time, Roemer obtained the value of the speed of light equal to 222,000 km per second. It turned out to be amazing that the method of measurement itself is incredibly accurate - using the now known data on the diameter of Jupiter and the delay time of the obscuration of the satellite gives the speed of light in vacuum, at the level contemporary meanings obtained by other methods.
At first, there was only one claim to Roemer's experiments - it was necessary to carry out measurements by earthly means. Almost 200 years have passed, and Louis Fizeau built an ingenious installation in which a beam of light reflected from a mirror at a distance of more than 8 km and came back. The subtlety was that it passed along the road back and forth through the cavities of the cogwheel, and if the speed of rotation of the wheel is increased, then the moment will come when the light will no longer be visible. The rest is a matter of technique. The measurement result is 312,000 km per second. We now see that Fizeau was even closer to the truth.
The next step in measuring the speed of light was made by Foucault, who replaced the gear wheel. This made it possible to reduce the dimensions of the installation and increase the measurement accuracy to 288,000 km per second. No less important was Foucault's experiment, in which he determined the speed of light in a medium. To do this, a pipe with water was placed between the mirrors of the installation. In this experiment, a decrease in the speed of light during its propagation in a medium was established, depending on the refractive index.
In the second half of the 19th century came the time of Michelson, who devoted 40 years of his life to measurements in the field of light. The culmination of his work was the installation on which he measured the speed of light in vacuum using an evacuated metal tube more than one and a half kilometers long. Michelson's other fundamental achievement was the proof that for any wavelength the speed of light in vacuum is the same and, as a modern standard, is 299792458+/- 1.2 m/s. Such measurements were carried out on the basis of updated values of the reference meter, the definition of which has been approved since 1983 as an international standard.
Wise Aristotle was wrong, but it took almost 2000 years to prove it.
Really, how? How to measure the highest speed in universe in our humble Earth conditions? We no longer need to puzzle over this - after all, for several centuries so many people have worked on this issue, developing methods for measuring the speed of light. Let's start the story in order.
speed of light is the propagation velocity of electromagnetic waves in vacuum. It is denoted by the Latin letter c. The speed of light is approximately 300,000,000 m/s.
At first, no one thought at all about the question of measuring the speed of light. There is light - that's great. Then, in the era of antiquity, the opinion that the speed of light was infinite, that is, instantaneous, dominated among scientific philosophers. Then it was Middle Ages with the Inquisition, when the main question of thinking and progressive people was the question "How not to get into the fire?" And only in the era Renaissance and Enlightenment the opinions of scientists have bred and, of course, divided.
So, Descartes, Kepler and Farm were of the same opinion as the scientists of antiquity. But he believed that the speed of light is finite, although very high. Actually, he made the first measurement of the speed of light. More precisely, he made the first attempt to measure it.
Galileo's experience
Experience Galileo Galilei was brilliant in its simplicity. The scientist conducted an experiment to measure the speed of light, armed with simple improvised means. At a great and well-known distance from each other, on different hills, Galileo and his assistant stood with lit lanterns. One of them opened the shutter on the lantern, and the second had to do the same when he saw the light of the first lantern. Knowing the distance and time (the delay before the assistant opens the lantern), Galileo expected to calculate the speed of light. Unfortunately, in order for this experiment to succeed, Galileo and his assistant had to select hills that are several million kilometers apart. I would like to remind you that you can by filling out an application on the site.
Roemer and Bradley experiments
The first successful and surprisingly accurate experiment in determining the speed of light was the experience of the Danish astronomer Olaf Römer. Roemer applied the astronomical method of measuring the speed of light. In 1676, he observed Jupiter's moon Io through a telescope and found that the time of the satellite's eclipse changes as the Earth moves away from Jupiter. Max Time the delay was 22 minutes. Assuming that the Earth is moving away from Jupiter at a distance of the diameter of the Earth's orbit, Roemer divided the approximate value of the diameter by the delay time, and received a value of 214,000 kilometers per second. Of course, such a calculation was very rough, the distances between the planets were known only approximately, but the result turned out to be relatively close to the truth.
The Bradley Experience. In 1728 James Bradley estimated the speed of light by observing the aberration of stars. aberration is a change in the apparent position of a star caused by the movement of the earth in its orbit. Knowing the speed of the Earth and measuring the angle of aberration, Bradley got a value of 301,000 kilometers per second.
Fizeau's experience
To the result of the experiment of Römer and Bradley, the then academia reacted with disbelief. However, Bradley's result was the most accurate for more than a hundred years, right up to 1849. That year the French scientist Armand Fizeau measured the speed of light using the rotating shutter method, without observing celestial bodies but here on earth. In fact, this was the first laboratory method after Galileo to measure the speed of light. Below is a diagram of its laboratory setup.
The light, reflected from the mirror, passed through the teeth of the wheel and was reflected from another mirror, 8.6 kilometers away. The speed of the wheel was increased until the light was visible in the next gap. Fizeau's calculations gave a result of 313,000 kilometers per second. A year later, a similar experiment with a rotating mirror was carried out by Léon Foucault, who obtained the result of 298,000 kilometers per second.
With the advent of masers and lasers, people have new opportunities and ways to measure the speed of light, and the development of the theory also made it possible to calculate the speed of light indirectly, without making direct measurements.
The most accurate value for the speed of light
Mankind has accumulated vast experience in measuring the speed of light. To date, the most accurate value of the speed of light is considered to be the value 299 792 458 meters per second received in 1983. It is interesting that further, more accurate measurement of the speed of light turned out to be impossible due to errors in the measurement meters. Now the value of the meter is tied to the speed of light and equals the distance that light travels in 1/299,792,458 seconds.
Finally, as always, we suggest watching an informative video. Friends, even if you are faced with such a task as independently measuring the speed of light with improvised means, you can safely turn to our authors for help. you can fill out an application on the website of the Correspondence. We wish you a pleasant and easy study!
The speed of light - absolute value velocity of propagation of electromagnetic waves in 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.)
It follows from the special theory of relativity that the acceleration of particles having 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 than the speed of light. It follows from this 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's paradox - superluminal speed in the tunnel effect - was also discussed earlier.
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.
