To achieve close to the speed of light, a multi-stage rocket would need to shed some of its mass as the speed increases, as the Super Haas rocket pictured here does.
Let's say you want to go on an interstellar journey and get to your destination as quickly as possible. You may not be able to make it until tomorrow, but if you had all the necessary tools and technology, and a little help from Einstein's relativity, would you be able to get there in a year? What about approaching the speed of light? That's what our reader is asking this week's question:
I was recently reading a book where the author tried to explain the twin paradox by imagining a spaceship flying at 1 g for 20 years and then returning back. Is it possible to maintain such an acceleration during such a time? If, for example, you start your journey on the first day of the new year and fly at an acceleration of 9.8 meters per second per second, then, according to the calculations, you can reach the speed of light before the end of the year. How can I accelerate further after that?
To travel to the stars, it is absolutely necessary to maintain such an acceleration.
This launch spaceship Columbia in 1992 shows that a rocket does not accelerate instantly - it takes a long time to accelerate
The most advanced rockets and jet propulsion systems created by mankind are not powerful enough for such a task, because they achieve not so much acceleration. They are impressive because they accelerate a huge mass for quite a long time. But the acceleration of rockets such as Saturn-5, Atlas, Falcon and Soyuz does not exceed the acceleration of any sports car: from 1 to 2 g, where g is 9.8 meters per second squared. What is the difference between a rocket and a sports car? The car will reach its limit in 9 seconds, at around 320 km / h. A rocket can accelerate this way much longer - not seconds or minutes, but a quarter of an hour.
NASA was the first to launch an Apollo 4 rocket from the Cape Kennedy Space Center. Although it accelerated as fast as a sports car, its key to success was sustaining that acceleration for a long time.
This is how we can overcome the Earth's gravitational pull and go into orbit, reach other worlds in our solar system, or even escape the sun's pull. But at some point, we will reach the limit - you can accelerate for a limited time due to restrictions on the amount of fuel carried. The rocket fuel we use is, unfortunately, extremely inefficient. You saw Einstein's famous equation, E = mc 2 , which describes mass as a form of energy, and that energy can be stored as matter. Our wonderful rocket fuel is horribly inefficient.
First test run of the SpaceX Raptor engine in early 2016
Using chemical reactions, the fuel converts no more than 0.001% of its mass into energy, severely limiting the maximum speed available to the spacecraft. And that is why it takes a rocket weighing 500 tons to launch 5 tons of payload into geostationary orbit. Nuclear rockets would be more efficient, converting about 0.5% of their mass into energy, but the ideal result would be matter and antimatter fuel achieving 100% efficiency in E = mc 2 conversion. If you had a rocket of a certain mass, no matter what, and only 5% of this mass was contained in antimatter (and another 5% in disposable matter), annihilation in time could be controlled. As a result, you would get a constant and steady acceleration of 1 g over a much longer period of time than any other fuel will give you.
An artist's idea of a reactive propulsion system using antimatter. Matter/antimatter annihilation produces the highest physical energy density of any known substance
If you need constant acceleration, then matter/antimatter annihilation, which is a few percent of the total mass, will allow you to accelerate at this rate for several months in a row. In this way, you can reach up to 40% of the speed of light if you spend the entire annual budget of the United States on the creation of antimatter, and you accelerate 100 kg of payload. If you need to accelerate even longer, you need to increase the amount of fuel you take with you. And the more you accelerate, the closer you get to the speed of light, the more relativistic effects you will notice.
How does your speed increase over time if you keep accelerating 1 g for several days, months, years or a decade
After ten days of flying at 1 g, you are already past Neptune, the last planet in the solar system. In a few months, you will begin to notice time slowing down and distances shrinking. In a year, you will already have reached 80% of the speed of light; in 2 years you will get close to 98% of the speed of light; After 5 years of flying with an acceleration of 1 g, you will be moving at a speed of 99.99% of the speed of light. And the longer you accelerate, the closer you get to the speed of light. But you will never reach it. Moreover, over time, it will require more and more energy.
On a logarithmic scale, you can see that the longer you accelerate, the closer you will get to the speed of light, but never reach it. Even after 10 years, you will get close to 99.9999999% of the speed of light, but you will not reach it
For the first ten minutes of acceleration, a certain amount of energy will be required, and by the end of this period you will be moving at a speed of 6 km / s. In another 10 minutes, you will double your speed to 12 km/s, but it will take three times as much energy. In another ten minutes you will be moving at a speed of 18 km/s, but it will require 5 times more energy than in the first ten minutes. This scheme will continue to work in the future. In a year, you will already be using 100,000 times more energy than at the beginning! In addition, the speed will increase less and less.
Lengths shorten and time stretches. The graph shows how a spacecraft moving at an acceleration of 1 g for a hundred years can travel to almost any point in the visible universe, and return from there, over the course of one human life. But by the time he returns, extra time will have passed on Earth.
If you want to accelerate a 100 kg ship for a year at 1 g, you need 1000 kg of matter and 1000 kg of antimatter. In a year, you will be moving at 80% of the speed of light, but you will never surpass it. Even if you had an infinite amount of energy. Constant acceleration requires a constant increase in thrust, and the faster you move, the more of your energy is wasted on relativistic effects. And until we figure out how to control the deformation of space, the speed of light will remain the ultimate limit of the universe. Everything that has mass cannot reach it, let alone surpass it. But if you start today, in a year you will be where no macroscopic object has ever gone before!
March 25th, 2017
FTL travel is one of the foundations of space science fiction. However, 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 denoted by the letter c and is almost 300 thousand kilometers per second; exact value c = 299 792 458 m/s.
The speed of light in vacuum is one of the fundamental physical constants. The impossibility of achieving speeds exceeding c follows from Einstein's special theory of relativity (SRT). 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 velocities greater than c. However, recent experimental studies have revealed some very interesting phenomena, 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 c, 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: movement is unattainable not only at superluminal speed, but also at a speed equal to the speed of 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 c - 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 an "all-penetrating" particle - a neutrino, which, like a photon, cannot move at a speed less than c.)
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 of movement of the phase of a sinusoidal wave - the so-called phase speed - can in a medium under certain conditions exceed the speed of light in a vacuum. 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 may exceed c or even lose its meaning, but then it does not apply to signal propagation. In SRT, it is established that it is impossible to transmit a signal at a speed greater than c.
Why is it so? Because the obstacle to the transmission of any signal with a speed greater than c is the same law of causality. 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 - a flash of light that reached him at a speed of a light flash, 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 c. 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 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 c.
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 to only one of these sixteen states, corresponding to almost absolute zero temperature on the Kelvin scale (-273.15 ° 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 (usual) 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 Vf > s ). This is the anomalous dispersion, in which the pattern of light propagation in a substance changes radically. The group velocity Vgr 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 Vgr > c 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 dispersive 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 than c, 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. Anyone who observed only the beginning and end of the experiment would only see a pulse of light that "jumped" forward in time, moving faster than c.
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. However, Wong admits that the effect he discovered makes it impossible to transmit information faster than c.
"The information here is already contained in the leading edge of the pulse," says P. Milonni, a physicist at the Los Alamos National Laboratory in the US.
Most physicists believe that the new work 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 normal air at a speed 25% faster than c.
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 an electromagnetic field) for the impossibility of transmitting signals at a speed greater than c. 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 to such an extent that the impossibility of the assumption V > c seems to be sufficiently proved. The principle of causality is the cornerstone that underlies the impossibility of superluminal signaling. And, apparently, all searches for superluminal signals, without exception, will stumble over this stone, no matter how much experimenters would like to detect such signals, because such is the nature of our world.
But still, let's imagine that the mathematics of relativity will still work at superluminal speeds. This means that theoretically we can still find out what would happen if the body happened to exceed the speed of light.
Imagine two spaceships heading from Earth towards a star that is 100 light-years away from our planet. The first ship leaves Earth at 50% the speed of light, so it will take 200 years to complete the journey. The second ship, equipped with a hypothetical warp drive, will depart at 200% the speed of light, but 100 years after the first. What will happen?
According to the theory of relativity, the correct answer largely depends on the perspective of the observer. From Earth, it will appear that the first ship has already traveled a considerable distance before being overtaken by the second ship, which is moving four times as fast. But from the point of view of the people on the first ship, everything is a little different.
Ship #2 is moving faster than light, which means it can outrun even the light it emits. This leads to a kind of "light wave" (analogous to sound, only light waves vibrate here instead of air vibrations), which gives rise to several interesting effects. Recall that the light from ship #2 moves slower than the ship itself. The result will be a visual doubling. In other words, at first the crew of ship #1 will see that the second ship appeared next to them as if from nowhere. Then, the light from the second ship will reach the first ship with a slight delay, and the result will be a visible copy that will move in the same direction with a slight lag.
Something similar can be seen in computer games when, as a result of a system failure, the engine loads the model and its algorithms at the end point of the movement faster than the motion animation itself ends, so that multiple takes occur. This is probably why our consciousness does not perceive that hypothetical aspect of the Universe in which bodies move at superluminal speed - perhaps this is for the best.
P.S. ... but in the last example, I didn’t understand something, why is the real position of the ship associated with the "light emitted by it"? Well, even though they will see him somehow in the wrong place, but in reality he will overtake the first ship!
sources
The solar system has not been of particular interest to science fiction writers for a long time. But, surprisingly, our “native” planets do not cause much inspiration for some scientists, although they have not yet been practically explored.
Having barely cut a window into space, humanity is torn into unknown distances, and not only in dreams, as before.
Sergei Korolev also promised to soon fly into space “on a trade union ticket”, but this phrase is already half a century old, and a space odyssey is still the lot of the elite - too expensive. However, two years ago, HACA launched a grandiose project 100 Year Starship, which involves the gradual and long-term creation of a scientific and technical foundation for space flights.
This unprecedented program should attract scientists, engineers and enthusiasts from all over the world. If everything is successful, in 100 years humanity will be able to build an interstellar ship, and we will move around the solar system like trams.
So what are the problems that need to be solved to make stellar flight a reality?
TIME AND SPEED ARE RELATIVE
Strange as it may seem, the astronomy of automatic vehicles seems to some scientists to be an almost solved problem. And this despite the fact that there is absolutely no point in launching automata to the stars with current snail speeds (about 17 km / s) and other primitive (for such unknown roads) equipment.
Now the American spacecraft Pioneer 10 and Voyager 1 have left the solar system, there is no longer any connection with them. Pioneer 10 is moving towards the star Aldebaran. If nothing happens to him, he will reach the vicinity of this star ... in 2 million years. In the same way crawl across the expanses of the Universe and other devices.
So, regardless of whether a ship is habitable or not, to fly to the stars, it needs a high speed close to the speed of light. However, this will help solve the problem of flying only to the nearest stars.
“Even if we managed to build a star ship that could fly at a speed close to the speed of light,” K. Feoktistov wrote, “the travel time only in our Galaxy will be calculated in millennia and tens of millennia, since its diameter is about 100,000 light years. But on Earth, much more will pass during this time.
According to the theory of relativity, the course of time in two systems moving relative to one another is different. Since at large distances the ship will have time to develop a speed very close to the speed of light, the difference in time on Earth and on the ship will be especially large.
It is assumed that the first goal of interstellar flights will be alpha Centauri (a system of three stars) - the closest to us. At the speed of light, you can fly there in 4.5 years, on Earth ten years will pass during this time. But the greater the distance, the greater the difference in time.
Remember the famous Andromeda Nebula by Ivan Efremov? There, flight is measured in years, and earthly ones. A beautiful story, to say the least. However, this coveted nebula (more precisely, the Andromeda galaxy) is located at a distance of 2.5 million light years from us.
According to some calculations, the astronauts' journey will take more than 60 years (according to starship hours), but an entire era will pass on Earth. How will the space "Neanderthals" be met by their distant descendants? And will the Earth be alive at all? That is, the return is basically meaningless. However, like the flight itself: we must remember that we see the Andromeda galaxy as it was 2.5 million years ago - so much of its light reaches us. What is the point of flying to an unknown target, which, perhaps, has not existed for a long time, in any case, in its former form and in the old place?
This means that even flights at the speed of light are justified only up to relatively close stars. However, vehicles flying at the speed of light, so far live only in a theory that resembles science fiction, however, scientific.
A SHIP THE SIZE OF A PLANET
Naturally, first of all, scientists came up with the idea to use the most efficient thermonuclear reaction in the ship's engine - as already partially mastered (for military purposes). However, for round trip travel at close to light speed, even with an ideal system design, a ratio of initial mass to final mass of at least 10 to the thirtieth power is required. That is, the spaceship will look like a huge train with fuel the size of a small planet. It is impossible to launch such a colossus into space from Earth. Yes, and collect in orbit - too, it is not for nothing that scientists do not discuss this option.
The idea of a photon engine using the principle of matter annihilation is very popular.
Annihilation is the transformation of a particle and an antiparticle during their collision into any other particles that are different from the original ones. The most studied is the annihilation of an electron and a positron, which generates photons, the energy of which will move the spaceship. Calculations by American physicists Ronan Keane and Wei-ming Zhang show that on the basis of modern technologies it is possible to create an annihilation engine capable of accelerating a spacecraft to 70% of the speed of light.
However, further problems begin. Unfortunately, using antimatter as a rocket fuel is very difficult. During annihilation, flashes of the most powerful gamma radiation occur, which are detrimental to astronauts. In addition, the contact of positron fuel with the ship is fraught with a fatal explosion. Finally, there are still no technologies to obtain enough antimatter and store it for a long time: for example, an antihydrogen atom "lives" now for less than 20 minutes, and the production of a milligram of positrons costs $25 million.
But, let's assume, over time, these problems can be resolved. However, a lot of fuel will still be needed, and the starting mass of a photon starship will be comparable to the mass of the Moon (according to Konstantin Feoktistov).
BROKEN THE SAIL!
The most popular and realistic starship today is considered to be a solar sailboat, the idea of which belongs to the Soviet scientist Friedrich Zander.
A solar (light, photon) sail is a device that uses the pressure of sunlight or a laser on a mirror surface to propel a spacecraft.
In 1985, the American physicist Robert Forward proposed the design of an interstellar probe accelerated by microwave energy. The project envisaged that the probe would reach the nearest stars in 21 years.
At the XXXVI International Astronomical Congress, a project was proposed for a laser starship, the movement of which is provided by the energy of optical lasers located in orbit around Mercury. According to calculations, the path of a starship of this design to the star Epsilon Eridani (10.8 light years) and back would take 51 years.
“It is unlikely that we will be able to make significant progress in understanding the world in which we live, based on data obtained from travels in our solar system. Naturally, thought turns to the stars. After all, earlier it was understood that flights around the Earth, flights to other planets of our solar system are not the ultimate goal. To pave the way to the stars seemed to be the main task.These words do not belong to a science fiction writer, but to the spacecraft designer and cosmonaut Konstantin Feoktistov. According to the scientist, nothing particularly new in the solar system will be found. And this despite the fact that man has so far only flown to the moon ...
However, outside the solar system, the pressure of sunlight will approach zero. Therefore, there is a project to accelerate a solar sailboat with laser systems from some asteroid.
All this is still theory, but the first steps are already being taken.
In 1993, a 20-meter-wide solar sail was deployed for the first time on the Russian ship Progress M-15 as part of the Znamya-2 project. When docking the Progress with the Mir station, its crew installed a reflector deployment unit on board the Progress. As a result, the reflector created a bright spot 5 km wide, which passed through Europe to Russia at a speed of 8 km/s. The patch of light had a luminosity roughly equivalent to that of the full moon.
So, the advantage of a solar sailboat is the lack of fuel on board, the disadvantages are the vulnerability of the sail design: in fact, it is a thin foil stretched over a frame. Where is the guarantee that the sail will not get holes from cosmic particles along the way?
The sail version may be suitable for launching robotic probes, stations and cargo ships, but is unsuitable for manned return flights. There are other starship designs, but they somehow resemble the above (with the same massive problems).
SURPRISES IN INTERSTELLAR SPACE
It seems that many surprises await travelers in the Universe. For example, just leaning out of the solar system, the American device Pioneer 10 began to experience a force of unknown origin, causing weak deceleration. Many suggestions have been made, up to yet unknown effects of inertia or even time. There is still no unambiguous explanation for this phenomenon, a variety of hypotheses are considered: from simple technical ones (for example, the reactive force from a gas leak in an apparatus) to the introduction of new physical laws.
Another spacecraft, Voyager 1, detected an area with a strong magnetic field at the edge of the solar system. In it, the pressure of charged particles from interstellar space causes the field created by the Sun to thicken. The device also registered:
- an increase in the number of high-energy electrons (about 100 times) that penetrate into the solar system from interstellar space;
- a sharp increase in the level of galactic cosmic rays - high-energy charged particles of interstellar origin.
The space between the stars is not empty. Everywhere there are remnants of gas, dust, particles. When trying to move at a speed close to the speed of light, each atom colliding with the ship will be like a particle of high-energy cosmic rays. The level of hard radiation during such a bombardment will increase unacceptably even during flights to the nearest stars.
And the mechanical impact of particles at such speeds will be likened to explosive bullets. According to some calculations, every centimeter of the starship's protective screen would be fired continuously at a rate of 12 shots per minute. It is clear that no screen can withstand such exposure for several years of flight. Or it will have to have an unacceptable thickness (tens and hundreds of meters) and weight (hundreds of thousands of tons).
Actually, then the starship will consist mainly of this screen and fuel, which will require several million tons. Due to these circumstances, flights at such speeds are impossible, all the more so because along the way you can run into not only dust, but also something larger, or get trapped in an unknown gravitational field. And then death is inevitable again. Thus, even if it is possible to accelerate the spacecraft to subluminal speed, it will not reach the final goal - there will be too many obstacles on its way. Therefore, interstellar flights can only be carried out at significantly lower speeds. But then the time factor makes these flights meaningless.
It turns out that it is impossible to solve the problem of transporting material bodies over galactic distances at speeds close to the speed of light. It makes no sense to break through space and time with the help of a mechanical structure.
MOLE HOLE
Science fiction, trying to overcome the inexorable time, invented how to "gnaw holes" in space (and time) and "fold" it. They came up with a variety of hyperspace jumps from one point of space to another, bypassing intermediate areas. Now scientists have joined science fiction writers.
Physicists began to look for extreme states of matter and exotic loopholes in the universe, where you can move at a superluminal speed contrary to Einstein's theory of relativity.
This is how the idea of the wormhole was born. This burrow links the two parts of the Universe like a carved tunnel connecting two cities separated by a high mountain. Unfortunately, wormholes are only possible in absolute vacuum. In our universe, these burrows are extremely unstable: they can simply collapse before a spaceship gets there.
However, to create stable wormholes, you can use the effect discovered by the Dutchman Hendrik Casimir. It consists in the mutual attraction of conducting uncharged bodies under the action of quantum oscillations in a vacuum. It turns out that the vacuum is not completely empty, there are fluctuations in the gravitational field in which particles and microscopic wormholes spontaneously appear and disappear.
It remains only to find one of the holes and stretch it, placing it between two superconducting balls. One mouth of the wormhole will remain on Earth, the other will be moved by the spacecraft at near-light speed to the star - the final object. That is, the spaceship will, as it were, punch through a tunnel. Once the starship reaches its destination, the wormhole will open up for real lightning-fast interstellar travel, the duration of which will be calculated in minutes.
WARP BUBBLE
Akin to the theory of wormholes bubble curvature. In 1994, the Mexican physicist Miguel Alcubierre performed calculations according to Einstein's equations and found the theoretical possibility of wave deformation of the spatial continuum. In this case, the space will shrink in front of the spacecraft and simultaneously expand behind it. The spaceship is, as it were, placed in a warp bubble capable of moving at an unlimited speed. The genius of the idea is that the spacecraft rests in a bubble of curvature, and the laws of the theory of relativity are not violated. At the same time, the bubble of curvature itself moves, locally distorting space-time.
Despite the impossibility of traveling faster than light, nothing prevents space from moving or propagating the warp of space-time faster than light, which is believed to have happened immediately after the Big Bang at the formation of the Universe.
All these ideas do not yet fit into the framework of modern science, but in 2012, NASA representatives announced the preparation of an experimental test of the theory of Dr. Alcubierre. Who knows, maybe Einstein's theory of relativity will someday become part of a new global theory. After all, the process of learning is endless. So, one day we will be able to break through the thorns to the stars.
Irina GROMOVA
The current speed record in space has been held for 46 years. When will he be beaten? We humans are obsessed with speed. So, only in the last few months it became known that students in Germany set a speed record for an electric car, and in the USA they plan to improve hypersonic aircraft in such a way that they reach speeds five times the speed of sound, i.e. over 6100 km / h. Such aircraft will not have a crew, but not because people cannot move at such a high speed. In fact, people have already moved at a speed that is several times higher than the speed of sound. However, is there a limit, having overcome which our rapidly rushing bodies will no longer be able to withstand overloads? The current speed record is equally held by three astronauts who participated in the Apollo 10 space mission ", - Tom Stafford, John Young and Eugene Cernan. In 1969, when the astronauts flew around the moon and returned back, the capsule in which they were, developed a speed that on Earth would be equal to 39.897 km / h. "I think that one hundred years ago, we could hardly have imagined that a person would be able to move in space at a speed of almost 40 thousand kilometers per hour, "says Jim Bray from the aerospace concern Lockheed Martin. Bray is the director of the habitable module project for the advanced Orion ), which is being developed by the US Space Agency NASA. As conceived by the developers, the Orion spacecraft is a multi-purpose and partially reusable - should bring astronauts into low Earth orbit. It may well be that with its help it will be possible to break the speed record set for a person 46 years ago. The new super-heavy rocket, which is part of the Space Launch System, should, according to the plan, make its first manned flight in 2021. This will be a flyby of an asteroid in a near-lunar orbit. Then, many-month-long expeditions to Mars should follow. Now, according to the designers, the usual maximum speed of the Orion should be about 32,000 km/h. However, the speed reached by Apollo 10 could be surpassed even if the basic configuration of the Orion was maintained. what we are planning now. But even the Orion will not represent the peak of human speed potential. "Basically, there is no other limit to the speed at which we can travel other than the speed of light," says Bray. The speed of light is one billion km/h. Is there any hope that we will be able to overcome the gap between 40 thousand km / h and these values? Surprisingly, speed as a vector quantity denoting the speed of movement and direction of movement is not a problem for people in the physical sense, as long as it is relatively constant and directed in one direction. side. Therefore, people - theoretically - can move in space only slightly slower than the "velocity limit of the universe", i.e. the speed of light. But even assuming that we overcome the significant technological hurdles associated with building high-speed spacecraft, our fragile, mostly water bodies will face new dangers associated with the effects of high speed. And so far only imaginary dangers can arise, if humans can travel faster than the speed of light by exploiting loopholes in modern physics or by discoveries that break the pattern. How to withstand overloads However, if we intend to move at a speed of over 40 thousand km / h, we will have to achieve it, and then slow down, slowly and with patience. Rapid acceleration and equally rapid deceleration are fraught with mortal danger to the human body. This is evidenced by the severity of bodily injuries resulting from car accidents, in which the speed drops from several tens of kilometers per hour to zero. What is the reason for this? In that property of the Universe, which is called inertia or the ability of a physical body with mass to resist a change in its state of rest or motion in the absence or compensation of external influences. This idea is formulated in Newton's first law, which says: "Every body continues to be held in its state rest or uniform and rectilinear motion, as long as it is not forced by applied forces to change this state. "The state of rest and movement at a constant speed is normal for the human body," explains Bray. "We should rather worry about the state of a person at the moment of acceleration “About a century ago, the development of durable aircraft that could maneuver at speed led pilots to talk about strange symptoms caused by changes in speed and direction of flight. These symptoms included temporary loss of vision and a feeling of either heaviness or weightlessness. The reason is g-forces, measured in units of G, which is the ratio of linear acceleration to the acceleration due to gravity on the surface of the Earth under the influence of attraction or gravity. These units reflect the effect of free fall acceleration on the mass of, for example, the human body. An overload of 1 G is equal to the weight of a body that is in the Earth's gravity field and is attracted to the center of the planet at a speed of 9.8 m / s (at sea level). which a person experiences vertically from head to toe or vice versa are truly bad news for pilots and passengers. slowing down, blood rushes from the toes to the head, there is a feeling of oversaturation, as in a handstand. "Red veil" (the feeling that a person experiences when blood rushes to the head) occurs when the blood-swollen, translucent lower eyelids rise and they close the pupils of the eyes. And, conversely, during acceleration or positive g-forces, the blood drains from the head to the legs, the eyes and brain begin to experience a lack of oxygen, since the blood accumulates in the lower extremities. there is a loss of color vision and rolls, as they say, a "gray veil", then a complete loss of vision or a "black veil" occurs, but the person remains conscious. Excessive overloads lead to a complete loss of consciousness. This condition is called congestion-induced syncope. Many pilots have died due to the fact that a "black veil" fell over their eyes - and they crashed. The average person can endure an overload of about five Gs before losing consciousness. Pilots dressed in special anti-g suits and trained in a special way to strain and relax the muscles of the torso so that the blood does not drain from the head, are able to fly an airplane at g-forces of about nine Gs. "For short periods of time, the human body can withstand much more G-forces than nine Gs," says Jeff Sventek, executive director of the Association Aerospace Medicine, located in Alexandria, Virginia. - But very few people can withstand high G-forces for a long period of time. "We humans are able to endure huge G-forces without serious injury, however, only for a few moments. put US Air Force Captain Eli Bieding Jr. on a viabase Holloman in New Mexico. In 1958, when braking on a special rocket-powered sled, after accelerating to 55 km / h in 0.1 seconds, he experienced an overload of 82.3 G. This result was recorded by an accelerometer attached to his chest. Beeding's eyes were also covered with a "black veil", but he escaped with only bruises during this outstanding demonstration of the endurance of the human body. True, after the arrival, he spent three days in the hospital. And now into spaceAstronauts, depending on the vehicle, also experienced quite high G-forces - from three to five Gs - during take-offs and when returning to the dense layers of the atmosphere, respectively. prone position in the direction of flight. Upon reaching a stable cruising speed of 26,000 km / h in orbit, astronauts experience speed no more than passengers of commercial flights. If overloads are not a problem for long expeditions on Orion spacecraft, then with small space rocks - micrometeorites - are more and more difficult. These particles the size of a grain of rice can develop impressive and at the same time destructive speeds of up to 300 thousand km / h. To ensure the integrity of the ship and the safety of its crew, the Orion is equipped with an external protective layer, the thickness of which varies from 18 to 30 cm. In addition, additional shielding shields are provided, and clever placement of equipment inside the ship is used. important for the entire spacecraft, we must accurately calculate the approach angles of micrometeorites,” says Jim Bray. Rest assured, micrometeorites are not the only obstacle to space missions, during which high human flight speeds in vacuum will play an increasingly important role. during the expedition to Mars, other practical problems will have to be solved, for example, to supply the crew with food and counteract the increased risk of cancer due to the effects of space radiation on the human body. Reducing travel time will reduce the severity of such problems, so the speed of movement will become more and more desirable oh. Next Generation SpaceflightThis need for speed will raise new obstacles in the way of space travelers. NASA's new spacecraft that threaten to break the Apollo 10 speed record will continue to rely on time-tested rocket propulsion chemistry systems used since the first spaceflights. But these systems have severe speed limits due to the release of small amounts of energy per unit of fuel. Therefore, in order to significantly increase the speed of flight for people going to Mars and beyond, as scientists recognize, completely new approaches are needed. “The systems that we have today are quite capable of getting us there,” says Bray, “but we all would like to witness a propulsion revolution." Eric Davis, a senior research physicist at the Institute for Advanced Study in Austin, Texas, and a member of NASA's Breakthrough Motion Physics Program, a six-year research project that ended in 2002, identified three of the most promising means, from the point of view of traditional physics, capable of helping humanity achieve speeds reasonably sufficient for interplanetary travel. In short, we are talking about the phenomena of energy release during the splitting of matter, thermonuclear fusion and annihilation of antimatter. The first method consists in the fission of atoms and is used in commercial nuclear reactors. The second, thermonuclear fusion, is creating heavier atoms from simpler atoms—the kind of reactions that power the sun. This is a technology that fascinates, but is not given to the hands; it is "always 50 years away" - and it always will be, as the industry's old motto goes. "These are very advanced technologies," says Davis, "but they are based on traditional physics and have been firmly established since the dawn of the Atomic Age." According to optimistic estimates, propulsion systems based on the concepts of atomic fission and thermonuclear fusion, in theory, are capable of accelerating a ship to 10% of the speed of light, i.e. up to a very worthy 100 million km / h. The most preferred, albeit elusive, source of energy for a fast spacecraft is antimatter, the twin and antipode of ordinary matter. When two types of matter come into contact, they destroy each other, resulting in the release of pure energy .Technologies that allow the production and storage of - so far extremely insignificant - quantities of antimatter already exist today. At the same time, the production of antimatter in useful quantities will require new special capacities of the next generation, and engineering will have to enter into a competitive race to create an appropriate spacecraft. But , says Davies, quite a few great ideas are already being worked out on the drawing boards. Spaceships powered by antimatter energy could accelerate for months and even years and reach greater percentages of the speed of light. At the same time, overloads on board will remain acceptable for the inhabitants of the ships. At the same time, such fantastic new speeds will also be fraught with other dangers for the human body. Energetic hailAt a speed of several hundred million kilometers per hour, any speck of dust in space, from pulverized hydrogen atoms to micrometeorites, inevitably becomes a high-energy bullet that can pierce a ship's hull." When you move at a very high speed, this means that the particles flying towards you are moving at the same speeds," says Arthur Edelstein. Together with his late father, William Edelstein, a professor of radiology at the Johns Hopkins University School of Medicine, he worked on a scientific work that examined the effects of exposure to cosmic hydrogen atoms ( on people and equipment) during ultra-fast space travel in space. Although its content does not exceed one atom per cubic centimeter, hydrogen scattered in space can acquire the properties of intense radiation bombardment. Hydrogen will begin to decompose into subatomic particles that will penetrate the ship and expose radiation to both crew and equipment. At a speed equal to 95% of the speed of light, exposure to such radiation would mean almost instantaneous death. , will boil immediately. "These are all extremely unpleasant problems," remarks Edelstein with grim humor. move at a speed less than half the speed of sound. Then the people on board have a chance to survive. Mark Millis, a translational physicist and former head of NASA's Breakthrough Motion Physics Program, warns that this potential speed limit for spaceflight remains a problem for the distant future. "Based on of physical knowledge accumulated to date, it can be said that it will be extremely difficult to develop a speed of more than 10% of the speed of light, "says Millis. - We are not in danger yet. A simple analogy: why worry that we can drown, if we still did not enter the water. Faster than light? If we assume that we, so to speak, have learned to swim, can we then master the gliding through space time - if we develop this analogy further - and fly at superluminal speed? The hypothesis of an innate ability to survive in a superluminal environment, although doubtful, is not without certain glimpses of educated enlightenment in pitch darkness. One of these intriguing modes of transportation is based on technologies similar to those used in the "warp drive" or "warp drive" from the Star Trek series. The principle of operation of this propulsion system, also known as The "Alcubierre engine"* (named after the Mexican theoretical physicist Miguel Alcubierre) is that it allows the ship to compress the normal space-time described by Albert Einstein in front of it and expand it behind it. Essentially, the ship moves in some volume of space-time, a kind of "curvature bubble" that moves faster than the speed of light. Thus, the ship remains stationary in normal spacetime in this "bubble" without being deformed and avoiding violations of the universal speed limit of light. like a surfer rushing on a board along the crest of a wave. "There is a certain catch here. To implement this idea, an exotic form of matter with negative mass is needed to compress and expand space-time. "Physics does not contain any contraindications regarding negative mass," Davis says, "but there are no examples of it, and we have never seen it in nature. ".There is another catch. In a paper published in 2012, researchers at the University of Sydney speculated that the "warp bubble" would accumulate high-energy cosmic particles as it inevitably began to interact with the contents of the universe. Some of the particles would penetrate the bubble itself and pump the ship with radiation. Stuck at sub-light speeds? Are we really doomed to get stuck at sub-light speeds because of our delicate biology?! It's not so much about setting a new world (galactic?) speed record for humans, but about the prospect of humanity turning into an interstellar society .At half the speed of light - which is the limit that Edelstein's research suggests our bodies can withstand - a round-trip journey to the nearest star would take more than 16 years. (The effects of time dilation, which would cause the crew of a starship to pass less time in their coordinate system than people left on Earth in their coordinate system would not be dramatic at half the speed of light.) Mark Millis is full of hope. Considering that humanity has developed anti-g suits and protection against micrometeorites, allowing people to safely travel in the great blue distance and the star-studded blackness of space, he is confident that we can find ways to survive, no matter how high-speed frontiers we reach in the future. "Te the very technologies that can help us achieve incredible new speeds of movement, Millis muses, will provide us with new, as yet unknown capabilities for protecting crews. And in 1995, Russian theoretical physicist Sergei Krasnikov proposed the concept of a device for space travel faster than the speed of sound. The idea was called "Krasnikov's pipes". This is an artificial curvature of space-time according to the principle of the so-called wormhole. Hypothetically, the ship will move in a straight line from the Earth to a given star through curved space-time, passing through other dimensions. According to Krasnikov's theory, the space traveler will return back at the same time that he set off.
Doctor of 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 pulse 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, recent experimental studies have revealed some very interesting phenomena, 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: movement is unattainable not only at superluminal speed, but also at a speed equal to the speed of 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 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. 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". With the help of 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 dispersive 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 pulse," says P. Milonni, a physicist at the Los Alamos National Laboratory in the US.
Most physicists believe that the new work 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 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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