Ever since school days, we know that the speed of light, according to Einstein's laws, is an insurmountable maximum in the Universe. Light travels from the Sun to the Earth in 8 minutes, which is approximately 150,000,000 km. It takes only 6 hours to reach Neptune, but it takes decades for spacecraft to overcome such distances. But not everyone knows that the value of the speed can vary significantly depending on the medium in which the light passes.
Formula for the speed of light
Knowing the speed of light in vacuum (c ≈ 3 * 10 8 m / s), you can determine it in other media, based on their refractive index n. The very formula for the speed of light resembles the laws of mechanics from physics, or rather, the definition of distance using time and the speed of an object.
For example, let's take glass, which has a refractive index of 1.5. According to the formula for the speed of light v = c \ n, we get that the speed in this medium is approximately equal to 200,000 km / s. If we take a liquid, such as water, then the speed of propagation of photons (particles of light) in it is 226,000 km / s with a refractive index of 1.33.
The formula for the speed of light in air
Air is also a medium. Consequently, it has the so-called If in a vacuum photons do not encounter obstacles on their way, then in a medium they spend some time excitation of atomic particles. The denser the environment, the more time it takes for this very excitement. The refractive index (n) in air is 1.000292. And this does not deviate much from the limit of 299,792,458 m/s.
American scientists have managed to slow down the speed of light to almost zero. Greater than 1/299,792,458 sec. light speed can not overcome. The thing is that light is the same electromagnetic wave as x-rays, radio waves or heat. The only difference is the difference between wavelength and frequency.
An interesting fact is the absence of mass in a photon, and this indicates the absence of time for this particle. Simply put, for a photon that was born several million, or even billions of years ago, not a second of time has passed.
Light is one of the key concepts of optical physics. Light is electromagnetic radiation that is visible to the human eye.
For many decades, the best minds have struggled with the problem of determining how fast light travels and what it is, as well as all the calculations that go with it. In 1676 a revolution took place in the circle of physicists. A Danish astronomer named Ole Römer disproved the claim that light travels through the universe at an unlimited speed.
In 1676, Ole Roemer determined that the speed of light in a vacuum is 299792458 m/s.
For convenience, this figure has been rounded up. A nominal value of 300,000 m / s is still in use.
This rule, under normal conditions for us, applies to all objects without exception, including X-rays, light and gravitational waves of the spectrum that is tangible to our eyes.
Modern physicists studying optics have proven that the value of the speed of light has several characteristics:
- constancy;
- unreachable;
- limb.
The speed of light in different media
It should be remembered that a physical constant directly depends on its environment, especially on the refractive index. In this regard, the exact value can change, because it is due to frequencies.
The formula for calculating the speed of light is written as c = 3 * 10^8 m/s.
Light at all times occupied an important place in the survival of people and the creation of an advanced civilization that we see today. The speed of light throughout the history of human development has excited the minds of first philosophers and naturalists, and then scientists and physicists. This is the fundamental constant of the existence of our universe.
Many scientists at different times sought to find out what the propagation of light in various media is. Highest value for science was the calculation of the value that has the speed of light in a vacuum. This article will help you understand this issue and learn a lot of interesting things about how light behaves in a vacuum.
Light and the question of speed
Light in modern physics plays a key role, because, as it turned out, it is impossible to overcome the value of its speed at this stage in the development of our civilization. It took many years to measure what the speed of light is. Prior to this, scientists have done a lot of research, trying to answer the most important question "what is the speed of propagation of light in a vacuum?".
At this point in time, scientists have proven that the speed of light (CPC) has the following characteristics:
- she is constant;
- she is unchanging;
- she is unattainable;
- she is finite.
Note! The speed of light at the current moment in the development of science is an absolutely unattainable value. Physicists have only some assumptions about what happens to an object that hypothetically reaches the value of the speed of propagation of a light flux in a vacuum.
Light speed
Why is it so important how fast light travels in a vacuum? The answer is simple. After all, the vacuum is in space. Therefore, having learned what digital indicator has the speed of light in a vacuum, we will be able to understand with what maximum possible speed it is possible to move through the expanses solar system and beyond.
The elementary particles that carry light in our universe are photons. And the speed with which light moves in a vacuum is considered an absolute value.
Note! SRS refers to the speed at which electromagnetic waves move. Interestingly, light simultaneously represents elementary particles (photons) and a wave. This follows from the corpuscular-wave theory. According to it, in certain situations, light behaves like a particle, and in others, like a wave.
At this point in time, the propagation of light in space (vacuum) is considered to be a fundamental constant, which does not depend on the choice of the used inertial frame of reference. This value refers to physical fundamental constants. In this case, the value of CPC characterizes in general the basic properties of the space-time geometry.
Modern ideas characterize CPC as a constant, which is the maximum allowable value for the movement of particles, as well as the propagation of their interaction. In physics, this quantity is denoted by the Latin letter "c".
History of the study of the issue
In ancient times, surprisingly, even ancient thinkers wondered about the propagation of light in our universe. Then it was believed that this is an infinite value. The first estimate of the physical phenomenon of the speed of light was given by Olaf Remer only in 1676. According to his calculations, the propagation of light was approximately 220 thousand km / s.
Note! Olaf Remer gave an approximate value, but, as it turned out later, not very far from the real one.
The correct value for the speed at which light travels in a vacuum was only determined half a century after Olaf Roemer. This was done by the French physicist A.I.L. Fizeau by conducting a special experiment.
Fizeau experiment
He was able to measure it physical phenomenon by measuring the time it took the beam to travel through a defined and precisely measured area.
The experience looked like this:
- the source S emitted a luminous flux;
- it was reflected from the mirror (3);
- after that, the luminous flux was interrupted by means of a toothed disk (2);
- then it passed the base, the distance of which was 8 km;
- after that, the light flux was reflected by the mirror (1) and went on its way back to the disk.
During the experiment, the light flux fell into the gaps between the teeth of the disk, and it could be observed through the eyepiece (4). Fizeau determined the time of passage of the beam from the speed of rotation of the disk. As a result of this experiment, he obtained the value c = 313,300 km/s.
But this is not the end of the research that has been devoted to this issue. The ultimate formula for calculating a physical constant came about thanks to many scientists, including Albert Einstein.
Einstein and vacuum: final results of the calculation
Today, every person on Earth knows that the maximum permissible value for the movement of material objects, as well as any signals, is considered to be the speed of light in vacuum. The exact value of this indicator is almost 300 thousand km / s. To be precise, the speed of light in vacuum is 299,792,458 m/s.
The theory that it is impossible to exceed this value was put forward by the famous physicist of the past Albert Einstein in his special theory relativity or SRT.
Note! Einstein's theory of relativity is considered unshakable until there is real evidence that signal transmission is possible at speeds exceeding the CPC in a vacuum.
Einstein's theory of relativity
But today, some researchers have discovered phenomena that may serve as a prerequisite for the fact that Einstein's SRT can be changed. Under certain specially given conditions, it is possible to track the appearance of superluminal velocities. It is interesting that in this case the violation of the theory of relativity does not occur.
Why can't you move faster than light?
To date, there are some "pitfalls" in this issue. For example, why under normal conditions the CPC constant cannot be overcome? According to the accepted theory, in this situation the fundamental principle of the structure of our world, namely, the law of causality, will be violated. He argues that the effect, by definition, is not able to outstrip its cause. Figuratively speaking, it cannot be that at first the bear falls dead, and only then the shot of the hunter who shot him will be heard. But if the CPC is exceeded, then events should begin to occur in the reverse order. As a result, time will begin its reverse run.
So what is the speed of propagation of a light beam?
After numerous studies that were cited in order to determine the exact value of what the CPC is, specific numbers were obtained. Today c = 1,079,252,848.8 km/h or 299,792,458 m/s. and in Planck units, this parameter is defined as one. This means that the energy of light travels 1 Planck unit of length in 1 unit of Planck time.
Note! These figures are valid only for the conditions that exist in a vacuum.
Constant value formula
But in physics, for a simpler way of solving problems, a rounded value is used - 300,000,000 m / s.
This rule under normal conditions applies to all objects, as well as X-rays, gravitational and light waves of the spectrum visible to us. In addition, scientists have proven that particles with mass can approach the speed of a light beam. But they are unable to reach or exceed it.
Note! The maximum speed, close to the speed of light, was obtained in the study of cosmic rays accelerated in special accelerators.
It is worth noting that this physical constant depends on the medium in which it is measured, namely the refractive index. Therefore, its actual rate may vary depending on the frequencies.
How to calculate the value of a fundamental constant
To date, there are various methods for determining the SRS. It can be:
- astronomical methods;
- improved Fizeau method. Here, the gear wheel is replaced with a modern modulator.
Note! Scientists have proven that the CPC indicators in air and in vacuum are almost the same. And it is less than about 25% water.
The following formula is used to calculate the amount of propagation of a light beam.
Formula for calculating the speed of light
This formula is suitable for vacuum calculations.
Conclusion
Light in our world is very important and the moment when scientists can prove the possibility of the existence of superluminal speeds can completely change our familiar world. What this discovery will mean for people is even difficult to assess. But it will definitely be an incredible breakthrough!
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> speed of light
Find out which speed of light in vacuum is a fundamental constant in physics. Read what is the speed of light m / s, the law, the measurement formula.
The speed of light in a vacuum is one of the fundamental constants in physics.
Learning task
- Compare the speed of light with the refractive index of the medium.
Key Points
- The maximum possible indicator of light speed is light in vacuum (constant).
- C is the symbol for the speed of light in a vacuum. Reaches 299,792,458 m/s.
- When light hits a medium, its speed slows down due to refraction. Calculated by the formula v = c/n.
Terms
- Special speed of light: reconciliation of the principle of relativity and constancy of light speed.
- The refractive index is the ratio of the speed of light in air/vacuum to another medium.
speed of light
The speed of light acts as a point of comparison to define something as extremely fast. But what is it?
The light beam moves from the Earth to the Moon in the time interval required for the passage of a light pulse - 1.255 s at an average orbital distance
The answer is simple: we are talking about the speed of a photon and light particles. What is the speed of light? Light speed in vacuum reaches 299,792,458 m/s. This is a universal constant applicable in various fields of physics.
Take the equation E = mc 2 (E is energy and m is mass). It is the equivalent of mass-energy, using the speed of light to link space and time. Here one can find not only an explanation for energy, but also reveal obstacles to speed.
The speed of a wave of light in a vacuum is actively used for various purposes. For example, special relativity states that this is the natural speed limit. But we know that the speed depends on the medium and refraction:
v = c/n (v is the actual speed of light passing through the medium, c is the speed of light in vacuum, and n is the refractive index). The refractive index of air is 1.0003, and the speed of visible light is 90 km/s slower than c.
Lorentz coefficient
Rapidly moving objects show certain characteristics that conflict with the position of classical mechanics. For example, long contacts and time are expanding. These effects are usually minimal, but are more pronounced at such high speeds. The Lorentz coefficient (γ) is the factor where time expansion and length contraction occur:
γ \u003d (1 - v 2 / s 2) -1/2 γ \u003d (1 - v 2 / s 2) -1/2 γ \u003d (1 - v 2 / s 2) -1/2.
At low speeds, v 2 /c 2 approaches 0, and γ is approximately = 1. However, when the speed approaches c, γ increases towards infinity.
Doctor technical sciences A. GOLUBEV.
In the middle of last year, a sensational report appeared in the magazines. A group of American researchers have discovered that a very short laser pulse travels hundreds of times faster in a specially selected medium than in a vacuum. This phenomenon seemed absolutely incredible (the speed of light in a medium is always less than in a vacuum) and even gave rise to doubts about the validity of the special theory of 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 an amplifying medium, the initial region of a 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 the movement of material objects or the propagation of any signals is the speed of light in a 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 of 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 sinusoidal waves with different amplitudes, frequencies and initial phases - a group 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 - that reached him 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 the movement of material bodies and the transmission of signals. 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 the linear speed of the 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 a change in the concentration of photons during the propagation of the pulse - a change due to a change in the gain of the medium up to a 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 the analogy would be: a ball thrown at a wall would end up on the other side of the wall, or the undulating motion imparted to 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. Such a situation contradicts not only common sense, but, in essence, the theory of relativity as well.
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 into 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 devastating blow to fundamental principles. But not all physicists believe that the problem is settled. Professor A. Ranfagni, of the Italian research group 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. Works 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 an electromagnetic field) for 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 the Einstein 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. Such a reservation was made because recently new hypotheses have appeared in astrophysics and cosmology 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, most likely, will remain purely mathematical constructions of theoretical physicists living in our Universe and trying to find the worlds closed to us with the power of their thoughts ...
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