We were taught from school that it is impossible to exceed the speed of light, and therefore the movement of a person in outer space is a big insoluble problem (how to fly to the nearest solar system if light can overcome this distance only in a few thousand years?). Perhaps American scientists have found a way to fly at superspeeds, not only without cheating, but also following the fundamental laws of Albert Einstein. In any case, Harold White, the author of the project of the space deformation engine, says so.
We in the editorial office considered the news absolutely fantastic, so today, on the eve of Cosmonautics Day, we are publishing a report by Konstantin Kakaes for Popular Science magazine about a phenomenal NASA project, if successful, a person will be able to go beyond the solar system.
In September 2012, several hundred scientists, engineers and space enthusiasts came together for the group's second public meeting called 100 Year Starship. The group is led by former astronaut May Jemison and founded by DARPA. The goal of the conference is "to make possible human travel beyond the solar system to other stars within the next hundred years." Most of the conference participants admit that progress in manned space exploration is too small. Despite the billions of dollars spent in the last few quarters, the space agencies can do almost as much as they could in the 1960s. Actually, 100 Year Starship is convened to fix all this.
But more to the point. After a few days of the conference, its participants reached the most fantastic topics: organ regeneration, the problem of organized religion on board the ship, and so on. One of the more intriguing presentations at the 100 Year Starship meeting was called Warp Field Mechanics 102, and was delivered by NASA's Harold "Sonny" White. An agency veteran, White runs the Advanced Pulse Program at the Johnson Space Center (JSC). Together with five colleagues, he created the "Space Propulsion Systems Roadmap," which outlines NASA's goals for future space travel. The plan lists all kinds of propulsion projects, from advanced chemical rockets to far-reaching developments like antimatter or nuclear machines. But White's area of research is the most futuristic of all: it concerns the space warp engine.
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this is how Alcubierre's bubble is usually depicted |
According to the plan, such an engine will provide movement in space at a speed exceeding the speed of light. It is generally accepted that this is impossible, since it is a clear violation of Einstein's theory of relativity. But White argues otherwise. As confirmation of his words, he appeals to the so-called Alcubierre bubbles (equations derived from Einstein's theory, according to which a body in outer space is capable of reaching superluminal speeds, unlike a body under normal conditions). In the presentation, he told how he recently managed to achieve theoretical results that directly lead to the creation of a real space warp engine. |
It is clear that this all sounds absolutely fantastic: such developments are a real revolution that will untie the hands of all astrophysicists in the world. Instead of spending 75,000 years traveling to Alpha Centauri, the closest star system to our own, astronauts on a ship with such an engine could make the journey in a couple of weeks.
In light of the shutdown of the shuttle program and the growing role of private flights to low Earth orbit, NASA says it is refocusing on far-reaching, much bolder plans that go far beyond traveling to the moon. These goals can only be achieved through the development of new propulsion systems - the sooner the better. A few days after the conference, NASA chief Charles Bolden echoed White's words: "We want to travel faster than the speed of light and non-stop on Mars."
HOW DO WE KNOW ABOUT THIS ENGINE
The first popular use of the expression "space warp engine" dates back to 1966, when Jen Roddenberry released " star way". For the next 30 years, this engine existed only as part of this fantasy series. A physicist named Miguel Alcubierre watched an episode of the series just as he was working on his doctorate in general relativity and was wondering if it was possible to create a space warp drive in reality. In 1994, he published a paper setting out this position.
Alcubierre imagined a bubble in space. In the front of the bubble, time-space is shrinking, and in the back it is expanding (as it was with the Big Bang, according to physicists). The deformation will cause the ship to glide smoothly through outer space, as if it were surfing a wave, despite the surrounding noise. In principle, a deformed bubble can move arbitrarily fast; the limitations in the speed of light, according to Einstein's theory, apply only in the context of space-time, but not in such distortions of space-time. Inside the bubble, Alcubierre predicted, space-time would not change and space travelers would not be harmed.
Einstein's equations in general relativity are tricky to solve in one direction, figuring out how matter curves space, but it's doable. Using them, Alcubierre determined that the distribution of matter is a necessary condition for the creation of a deformed bubble. The only problem is that the solutions led to an indefinite form of matter called negative energy.
talking plain language, gravity is the force of attraction between two objects. Each object, regardless of its size, exerts some force of attraction on the surrounding matter. According to Einstein, this force is a curvature of space-time. Negative energy, however, is gravitationally negative, that is, repulsive. Instead of connecting time and space, negative energy repels and separates them. Roughly speaking, for this model to work, Alcubierra needs negative energy to expand the space-time behind the ship.
Despite the fact that no one has ever specifically measured negative energy, according to quantum mechanics, it exists, and scientists have learned how to create it in the laboratory. One way to recreate it is through the Kazimirov effect: two parallel conductive plates placed close to each other create some amount of negative energy. The weak point of the Alcubierre model is that its implementation requires a huge amount of negative energy, several orders of magnitude higher than scientists estimate it can be produced.
White says he has found a way around this limitation. In a computer simulation, White altered the geometry of the warp field so that, in theory, it could produce a deformed bubble using millions of times less negative energy than Alcubierra estimated required, and perhaps little enough for a spacecraft to carry its means of production. "The discoveries," says White, "change Alcubierre's method from impractical to quite plausible."
REPORT FROM WHITE'S LAB
The Johnson Space Center is located next to the Houston lagoons, from where the path to Galveston Bay opens. The center is a bit like a suburban college campus, only aimed at training astronauts. On the day of my visit, White meets me at Building 15, a multi-story maze of corridors, offices, and engine testing labs. White is wearing an Eagleworks polo shirt, as he calls his engine experiments, embroidered with an eagle hovering over a futuristic spaceship.
White began his career as an engineer doing research as part of a robotic group. Over time, he took command of the entire ISS robotic wing while completing his PhD in plasma physics. It wasn't until 2009 that he shifted his focus to the study of motion, and this topic captured him enough to become the main reason he went to work for NASA.
"He's quite an unusual person," says his boss, John Applewhite, who heads the propulsion systems division. - He is definitely a big dreamer, but at the same time a talented engineer. He knows how to turn his fantasies into a real engineering product.” Around the same time he joined NASA, White asked permission to open his own laboratory dedicated to advanced propulsion systems. He himself came up with the name Eagleworks and even asked NASA to create a logo for his specialization. Then this work began.
White leads me to his office, which he shares with a colleague who searches for water on the Moon, and then leads me down to Eagleworks. On the way, he tells me about his request to open a laboratory and calls it "a long and difficult process of finding an advanced movement to help man explore space."
White shows me the object and shows me its central function, something he calls a "Quantum Vacuum Plasma Thruster" (QVPT). This device looks like a huge red velvet donut with wires tightly braided around the core. This is one of two Eagleworks initiatives (the other is the warp engine). It's also a secret development. When I ask what it is, White replies that he can only say that this technology is even cooler than the warp engine). According to a 2011 NASA report written by White, the craft uses quantum fluctuations in empty space as its fuel source, meaning that a QVPT-powered spacecraft does not require fuel.
The engine uses quantum fluctuations in empty space as a fuel source,
which means spaceship
powered by QVPT, does not require fuel.
When the device works, White's system looks cinematically perfect: the color of the laser is red, and the two beams are crossed like sabers. Inside the ring are four ceramic capacitors made of barium titanate, which White charges up to 23,000 volts. White has spent the last two and a half years developing the experiment, and he says that capacitors show tremendous potential energy. However, when I ask how to create the negative energy needed for warped space-time, he evades the answer. He explains that he signed a non-disclosure agreement, and therefore cannot reveal details. I ask with whom he made these agreements. He says: “With people. They come and want to talk. I can't give you more details."
OPPOSITORS OF THE ENGINE IDEA
So far, the warped travel theory is pretty intuitive - warping time and space to create a moving bubble - and it has a few significant flaws. Even if White significantly reduces the amount of negative energy Alcubierra asks for, it will still require more than scientists can produce, says Lawrence Ford, a theoretical physicist at Tufts University who has written numerous papers on the topic of negative energy over the past 30 years. Ford and other physicists claim that there are fundamental physical limitations, and it's not so much engineering imperfections, but that such an amount of negative energy cannot exist in one place for a long time.
Another difficulty: to create a deformation ball that moves faster than light, scientists would need to generate negative energy around and above the spacecraft. White doesn't think this is a problem; he replies rather vaguely that the engine will most likely work due to some existing "apparatus that creates the necessary conditions." However, creating these conditions in front of the ship would mean providing a constant supply of negative energy traveling faster than the speed of light, again contradicting general relativity.
Finally, the space warp engine raises a conceptual question. In general relativity, FTL travel is equivalent to time travel. If such an engine is real, White creates a time machine.
These obstacles give rise to some serious doubts. “I don’t think the physics we know and its laws allow us to assume that he will achieve anything with his experiments,” says Ken Olum, a physicist at Tufts University, who also participated in the debate about exotic movement at the Starship 100th Anniversary meeting. ". Noah Graham, a physicist at Middlebury College who read two of White's papers at my request, emailed me: "I see no valuable scientific evidence other than references to his previous work."
Alcubierre, now a physicist at the National Autonomous University of Mexico, has his own doubts. “Even if I'm standing on a spaceship and I have negative energy available, there's no way I can put it where it's needed,” he tells me over the phone from his home in Mexico City. - No, the idea is magical, I like it, I wrote it myself. But it has a couple of serious flaws that I already see over the years, and I don’t know a single way to fix them. ”
THE FUTURE OF SUPERSPEEDS
To the left of the Johnson Science Center's main gate, a Saturn-B rocket lies on its side, its stages disengaged to reveal its contents. It's gigantic - the size of one of the many engines is the size of a small car, and the rocket itself is a couple of feet longer than a football field. This, of course, is quite eloquent evidence of the peculiarities of space navigation. Besides, she's 40 years old and the time she represents - when NASA was part of a huge national plan to send a man to the moon - is long gone. JSC today is just a place that was once great but has since left the space avant-garde.
A breakthrough in traffic could mean a new era for JSC and NASA, and to some extent part of that era is already beginning. The Dawn probe, launched in 2007, studies the ring of asteroids using ion thrusters. In 2010, the Japanese commissioned Icarus, the first interplanetary starship powered by a solar sail, another kind of experimental propulsion. And in 2016, the scientists plan to test VASMIR, a plasma-powered system made specifically for high propulsion at the ISS. But when these systems possibly get astronauts to Mars, they still won't be able to take them outside the solar system. To achieve this, White said, NASA will need to take on more risky projects.
The Warp Drive is perhaps the most far-fetched of NASA's motion design efforts. The scientific community says that White cannot create it. Experts say it works against the laws of nature and physics. Despite this, NASA is behind the project. “It's not being subsidized at the high government level it should be,” says Applewhite. - I think that the management has some special interest in him continuing his work; it's one of those theoretical concepts that, if successful, completely changes the game."
In January, White assembled his warp interferometer and moved on to his next target. Eagleworks has outgrown its own home. The new lab is larger and, as he enthusiastically states, "seismically isolated," meaning that it is protected from vibrations. But perhaps the best thing about the new lab (and most impressive) is that NASA gave White the same conditions that Neil Armstrong and Buzz Aldrin had on the Moon. Well, let's see.
The speed of light is one of the universal physical constants, it does not depend on the choice of inertial reference frame and describes the properties of space-time as a whole. The speed of light in vacuum is 299,792,458 meters per second, and this is the limiting speed of particles and propagation of interactions. This is what school books on physics teach us. You can also remember that the mass of a body is just not constant and tends to infinity as the speed approaches the speed of light. That is why photons move at the speed of light - particles without mass, and it is much more difficult for particles with mass.
However, the international team of scientists from the large-scale OPERA experiment, located near Rome, is ready to argue with the elementary truth.
He managed to detect neutrinos, which, as experiments have shown, move at a speed greater than the speed of light,
the press service of the European Organization for Nuclear Research (CERN).
The OPERA experiment (Oscillation Project with Emulsion-tRacking Apparatus) studies the most inert particles in the Universe - neutrinos. They are so inert that they can fly through the entire globe, stars and planets, and in order for them to hit an iron barrier, the size of this barrier must be from the Sun to Jupiter. Every second, about 10 14 neutrinos emitted by the Sun pass through the body of every person on Earth. The probability that at least one of them will hit the tissues of a person throughout his life tends to zero. For these reasons, it is extremely difficult to register and study neutrinos. The laboratories that do this are located deep under the mountains and even under the ice of Antarctica.
OPERA receives a beam of neutrinos from CERN, where the Large Hadron Collider is located. Its "little brother" - the superproton synchrotron (SPS) - directs the beam directly underground towards Rome. The resulting neutrino beam passes through the thickness of the earth's crust, thereby being cleansed of other particles that the substance of the crust retains, and goes straight to the laboratory in Gran Sasso, hidden under 1200 m of rock.
An underground path of 732 km is overcome by neutrinos in 2.5 milliseconds.
The detector of the OPERA project, consisting of about 150 thousand elements and weighing 1300 tons, "catches" neutrinos and studies them. In particular, the main goal is to study the so-called neutrino oscillations - transitions from one type of neutrino to another.
The stunning results of exceeding the speed of light are supported by serious statistics: the laboratory in Gran Sasso observed about 15 thousand neutrinos. Scientists have found that
neutrinos travel at 20 millionths faster than the speed of light - the "infallible" speed limit.
This result came as a surprise to them, its explanation has not yet been proposed. Naturally, to refute or confirm it, independent experiments carried out by other groups on other equipment are required - this principle of "double-blind control" is also implemented at the CERN Large Hadron Collider. The OPERA collaboration immediately published its results to enable colleagues around the world to verify them. A detailed description of the work is available on the preprint website Archive.Org.
The official presentation of the results will take place today at a seminar at CERN at 18.00 Moscow time, will be conducted online streaming.
“This data came as a complete surprise. After months of collecting, analyzing and cleaning data, as well as cross-checking, we did not find a possible source of system error in either the data processing algorithm or the detector. Therefore, we publish our results, continue our work, and also hope that independent measurements of other groups will help to understand the nature of this observation, ”said Antonio Ereditato, head of the OPERA experiment from the University of Bern, quoted by the CERN press service.
“When experimental scientists discover some implausible result and cannot find an artifact that would explain it, they turn to their colleagues from other groups to begin a wider study of the issue. This is a good scientific tradition, and the OPERA collaboration is now following it.
If the observations of the speed of light are confirmed, this may change our understanding of physics, but we must make sure that they do not have another, more banal explanation.
This is what independent experiments are for,” said CERN scientific director Sergio Bertolucci.
The measurements carried out in OPERA are extremely accurate. Thus, the distance from the point of neutrino launch to the point of their registration (more than 730 km) is known with an accuracy of 20 cm, and the time of flight is measured with an accuracy of 10 nanoseconds.
The OPERA experiment has been running since 2006. Approximately 200 physicists from 36 institutes and 13 countries, including Russia, take part in it.
But it turned out that it is possible; now it is believed that we will never be able to travel faster than light ... ". But in fact it is not true that someone once believed that it was impossible to travel faster than sound. Long before supersonic aircraft appeared, it was already known that bullets fly faster than sound. controlled supersonic flight, and that was the mistake. SS movement is a completely different matter. It was clear from the start that supersonic flight was hampered by technical problems that simply had to be solved. But it is completely unclear whether the problems that hinder the SS movement can ever be solved. The theory of relativity has a lot to say about this. If SS travel or even signal transmission is possible, then causality will be violated, and absolutely incredible conclusions will follow from this.
We will first discuss simple cases of CC motion. We mention them not because they are interesting, but because they resurface again and again in discussions of the STS movement and therefore have to be dealt with. Then we will discuss what we consider to be difficult cases of STS movement or communication and consider some of the arguments against them. Finally, we will consider the most serious assumptions about the real STS movement.
Simple SS move
1. The phenomenon of Cherenkov radiation
One way to move faster than light is to first slow down the light itself! :-) In a vacuum, light travels at a speed c, and this value is a world constant (see the question Is the speed of light constant), and in a denser medium like water or glass, it slows down to the speed c/n, where n is the refractive index of the medium (1.0003 for air; 1.4 for water). Therefore, particles can move faster in water or air than light travels there. As a result, Vavilov-Cherenkov radiation appears (see question ).
But when we talk about SS motion, we, of course, mean exceeding the speed of light in a vacuum c(299 792 458 m/s). Therefore, the Cherenkov phenomenon cannot be considered an example of SS motion.
2.Third party
If the rocket BUT flies away from me at a speed 0.6c west and the other B- from me with speed 0.6c east, then the total distance between BUT and B in my frame of reference increases with speed 1.2c. Thus, an apparent relative velocity greater than c can be observed "from a third party".
However, this speed is not what we usually understand by relative speed. Real rocket speed BUT regarding the rocket B- this is the rate of increase in the distance between the rockets, which is observed by the observer in the rocket B. Two velocities must be added according to the relativistic formula for adding velocities (see the question How to add velocities in particular relativity). In this case, the relative speed is approximately 0.88c, that is, is not superluminal.
3. Shadows and bunnies
Think about how fast the shadow can move? If you create a shadow on a distant wall from your finger from a nearby lamp, and then move your finger, then the shadow moves much faster than your finger. If the finger moves parallel to the wall, then the speed of the shadow will be D/d times the speed of the finger, where d is the distance from the finger to the lamp, and D- distance from the lamp to the wall. And you can get even more speed if the wall is located at an angle. If the wall is very far away, then the movement of the shadow will lag behind the movement of the finger, since the light will still have to fly from the finger to the wall, but still the speed of the shadow will be as many times greater. That is, the speed of the shadow is not limited by the speed of light.
In addition to shadows, bunnies can also move faster than light, for example, a speck from a laser beam directed at the moon. Knowing that the distance to the Moon is 385,000 km, try to calculate the speed of the bunny if you move the laser slightly. You can also think of a sea wave hitting the shore obliquely. With what speed can the point at which the wave breaks move?
Similar things can happen in nature. For example, a light beam from a pulsar can comb through a cloud of dust. A bright flash generates an expanding shell of light or other radiation. When it crosses the surface, it creates a ring of light that grows faster than the speed of light. In nature, this occurs when the electromagnetic pulse from lightning reaches upper layers atmosphere.
All these were examples of things moving faster than light, but which were not physical bodies. With the help of a shadow or a bunny, you cannot transmit a CC message, so communication faster than light is not possible. And again, this is apparently not what we want to understand by CC motion, although it becomes clear how difficult it is to determine what exactly we need (see the question FTL Shears).
4. Rigid bodies
If you take a long hard stick and push one end of it, does the other end move immediately or not? Is it possible to carry out the SS transmission of the message in this way?
Yes it was would could be done if such solid bodies existed. In reality, the influence of a blow to the end of a stick propagates along it at the speed of sound in a given substance, and the speed of sound depends on the elasticity and density of the material. Relativity imposes an absolute limit on the possible hardness of any bodies so that the speed of sound in them cannot exceed c.
The same thing happens if you are in the field of attraction, and first hold the string or pole vertically by the upper end, and then release it. The point that you let go will start moving immediately, and the lower end will not be able to start falling until the influence of letting go reaches it at the speed of sound.
It is difficult to formulate a general theory of elastic materials in terms of relativity, but the basic idea can also be shown using the example of Newtonian mechanics. The equation for the longitudinal motion of a perfectly elastic body can be obtained from Hooke's law. In variables mass per unit length p and Young's modulus Y, longitudinal displacement X satisfies the wave equation.
Plane wave solution moves at the speed of sound s, and s 2 = Y/p. This equation does not imply the possibility of a causal influence propagating faster s. Thus, relativity imposes a theoretical limit on the amount of elasticity: Y < pc2. Practically, there are no materials even close to it. By the way, even if the speed of sound in the material is close to c, matter in itself is not required to move with relativistic velocity. But how do we know that, in principle, there can be no substance that overcomes this limit? The answer is that all substances are made up of particles, the interaction between which obeys the standard model of elementary particles, and in this model no interaction can propagate faster than light (see below about quantum field theory).
5. Phase velocity
Look at this wave equation:
It has solutions like:
These solutions are sine waves moving at a speed
But this is faster than light, so we have the equation of the tachyon field in our hands? No, this is just the usual relativistic equation of a massive scalar particle!
The paradox will be resolved if we understand the difference between this speed, also called the phase speed vph from another speed, called the group speed v gr which is given by the formula,
If the wave solution has a frequency spread, then it will take the form of a wave packet , which moves with a group velocity not exceeding c. Only wave crests move with phase velocity. It is possible to transmit information using such a wave only with a group velocity, so the phase velocity gives us another example of superluminal speed, which cannot carry information.
7. Relativistic rocket
A controller on Earth watches a spacecraft leaving at a speed of 0.8 c. According to the theory of relativity, even after taking into account the Doppler shift of the signals from the ship, he will see that the time on the ship is slowed down and the clocks there go slower by a factor of 0.6. If he calculates the quotient of the distance traveled by the ship divided by the elapsed time measured by the ship's clock, he will get 4/3 c. This means that the ship's passengers travel through interstellar space at an effective speed greater than the speed of light they would have if measured. From the perspective of the ship's passengers, interstellar distances are subject to Lorentzian contraction by the same factor of 0.6, which means they too must admit that they cover known interstellar distances at a rate of 4/3 c.
This is a real phenomenon and in principle it can be used by space travelers to overcome huge distances during their lifetime. If they accelerate at a constant acceleration equal to the acceleration of free fall on Earth, then not only will they have perfect artificial gravity on the ship, but they will still have time to cross the Galaxy in just 12 of their years! (See the question What are the equations of a relativistic rocket?)
However, this is not a real SS movement. The effective speed is calculated from distance in one frame of reference and time in another. This is not real speed. Only the ship's passengers benefit from this speed. The dispatcher, for example, will not have time in his life to see how they fly a gigantic distance.
Difficult cases of SS movement
9. Paradox of Einstein, Podolsky, Rosen (EPR)
10. Virtual photons
11. Quantum tunneling
Real Candidates for the SS Travelers
This section contains speculative but serious assumptions about the possibility of FTL travel. These will not be the kind of things that are usually put in a FAQ, as they raise more questions than they answer. They are presented here mainly to show that serious research is being carried out in this direction. Only a brief introduction is given in each direction. More detailed information can be found on the Internet.
19. Tachyons
Tachyons are hypothetical particles that locally travel faster than light. To do this, they must have an imaginary mass, but their energy and momentum must be positive. It is sometimes thought that such CC particles should be impossible to detect, but in fact, there is no reason to believe so. Shadows and bunnies tell us that stealth does not follow from the CC of the movement.
Tachyons have never been observed and most physicists doubt their existence. It was once stated that experiments were carried out to measure the mass of neutrinos emitted during the decay of Tritium, and that these neutrinos were tachyon. This is highly doubtful, but still not excluded. There are problems with tachyon theories, because in terms of possible violations of causality, they destabilize the vacuum. It may be possible to get around these problems, but then it will be impossible to use tachyons in the SS message we need.
The truth is that most physicists consider tachyons to be a sign of an error in their field theories, and interest in them from the general public is fueled mainly by science fiction (see Tachyons article).
20. Wormholes
The most well-known supposed possibility of STS travel is the use of wormholes. Wormholes are tunnels in space-time that connect one place in the universe to another. They can move between these points faster than light would take its usual path. Wormholes are a phenomenon of the classical general relativity, but to create them, you need to change the topology of space-time. The possibility of this may be contained in the theory of quantum gravity.
Huge amounts of negative energy are needed to keep wormholes open. Misner and Thorn suggested that the large-scale Casimir effect can be used to generate negative energy and Visser proposed a solution using cosmic strings. All these ideas are highly speculative and may simply be unrealistic. An unusual substance with negative energy may not exist in the form necessary for the phenomenon.
Thorne discovered that if wormholes could be created, they could create closed time loops that would make time travel possible. It has also been suggested that the multivariate interpretation of quantum mechanics suggests that time travel will not cause any paradoxes, and that events will simply unfold differently when you get into the past. Hawking says that wormholes may simply be unstable and therefore unusable in practice. But the topic itself remains a fruitful area for thought experiments, allowing you to figure out what is possible and what is not possible based on both known and assumed laws of physics.
refs:
W. G. Morris and K. S. Thorne, American Journal of Physics 56
,
395-412 (1988)
W. G. Morris, K. S. Thorne, and U. Yurtsever, Phys. Rev. letters 61
, 1446-9 (1988)
Matt Visser, Physical Review D39, 3182-4 (1989)
see also "Black Holes and Time Warps" Kip Thorn, Norton & co. (1994)
For an explanation of the multiverse see, "The Fabric of Reality" David Deutsch, Penguin Press.
21. Deformer motors
[I have no idea how to translate this! The original warp drive. - approx. translator
translated by analogy with the article on Membrane]
The warp could be a mechanism for twisting space-time so that an object can travel faster than light. Miguel Alcabière became famous for having developed the geometry that describes such a deformer. Space-time distortion makes it possible for an object to travel faster than light while remaining on a time-like curve. The obstacles are the same as when creating wormholes. To create a deformer, you need a substance with a negative energy density u. Even if such a substance is possible, it is still not clear how it can be obtained and how to use it to make the deformer work.
ref M. Alcubierre, Classical and Quantum Gravity, 11
, L73-L77, (1994)
Conclusion
First, it was not easy to define in general what an SS travel and SS message means. Many things, like shadows, make CC move, but in such a way that it cannot be used, for example, to transmit information. But there are also serious possibilities of real SS movement, which are proposed in the scientific literature, but their implementation is still technically impossible. The Heisenberg uncertainty principle makes it impossible to use apparent CC motion in quantum mechanics. In general relativity there are potential means of SS propulsion, but it may not be possible to use them. It seems extremely unlikely that in the foreseeable future, or at all, technology will be able to create spacecraft with SS engines, but it is curious that theoretical physics, as we now know it, does not close the door to SS propulsion for good. SS movement in the style of science fiction novels is apparently completely impossible. For physicists, the question is interesting: "why, in fact, is this impossible, and what can be learned from this?"
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 special theory 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 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: it is unattainable to move not only at superluminal speed, but also at a speed equal speed light, - you can only approach it. It follows from the theory of relativity that with an increase in the speed of movement, three circumstances arise: the mass of a moving object increases, its size decreases in the direction of movement, and the passage of time on this object slows down (from the point of view of an external "resting" observer). At ordinary speeds, these changes are negligible, but as we approach the speed of light, they become more and more noticeable, and in the limit - at a speed equal to 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.
Soon after the discovery of lasers - in the early 60s - the problem arose of obtaining short (with a duration of the order of 1 ns = 10-9 s) high-power light pulses. To do this, a short laser pulse was passed through an optical quantum amplifier. The pulse was split by a beam-splitting mirror into two parts. One of them, more powerful, was sent to the amplifier, and the other propagated in the air and served as a reference pulse, with which it was possible to compare the pulse that passed through the amplifier. Both pulses were fed to photodetectors, and their output signals could be visually observed on the oscilloscope screen. It was expected that the light pulse passing through the amplifier would experience some delay in it compared to the reference pulse, that is, the speed of light propagation in the amplifier would be less than in air. What was the amazement of the researchers when they discovered that the pulse propagated through the amplifier at a speed not only greater than in air, but also several times greater than the speed of light in vacuum!
After recovering from the first shock, physicists began to look for the reason for such an unexpected result. No one had even the slightest doubt about the principles of the special theory of relativity, and this is precisely what helped to find the correct explanation: if the principles of SRT are preserved, then the answer should be sought in the properties of the amplifying medium.
Without going into details here, we only point out that a detailed analysis of the mechanism of action of the amplifying medium has completely clarified the situation. The point was a change in the concentration of photons during the propagation of the pulse - a change due to a change in the gain of the medium up to a negative value during the passage of the rear part of the pulse, when the medium is already absorbing energy, because its own reserve has already been used up due to its transfer to the light pulse. Absorption does not cause an increase, but a decrease in the impulse, and thus the impulse is strengthened in the front and weakened in the back of it. Let us imagine that we observe the pulse with the help of an instrument moving at the speed of light in the medium of an amplifier. If the medium were transparent, we would see an impulse frozen in immobility. In the medium in which the process mentioned above takes place, the strengthening of the leading edge and the weakening of the trailing edge of the pulse will appear to the observer in such a way that the medium, as it were, has moved the pulse forward. But since the device (observer) moves at the speed of light, and the impulse overtakes it, then the speed of the impulse exceeds the speed of light! It is this effect that was registered by the experimenters. And here there really is no contradiction with the theory of relativity: it's just that the amplification process is such that the concentration of photons that came out earlier turns out to be greater than those that came out later. It is not photons that move with superluminal speed, but the envelope of the pulse, in particular its maximum, which is observed on the oscilloscope.
Thus, while in ordinary media there is always a weakening of light and a decrease in its speed, determined by the refractive index, in active laser media, not only amplification of light is observed, but also propagation of a pulse with superluminal speed.
Some physicists have tried to experimentally prove the presence of superluminal motion in the tunnel effect, one of the most amazing phenomena in quantum mechanics. This effect consists in the fact that a microparticle (more precisely, a microobject that exhibits both the properties of a particle and the properties of a wave under different conditions) is able to penetrate the so-called potential barrier - a phenomenon that is completely impossible in classical mechanics(in which the analogy would be: a ball thrown at a wall would end up on the other side of the wall, or the undulating motion imparted to a rope tied to the wall would be transmitted to a rope tied to the wall on the other side). The essence of the tunnel effect in quantum mechanics is as follows. If a micro-object with a certain energy encounters on its way an area with a potential energy exceeding the energy of the micro-object, this area is a barrier for it, the height of which is determined by the energy difference. But the micro-object "leaks" through the barrier! This possibility is given to him by the well-known Heisenberg uncertainty relation, written for the energy and interaction time. If the interaction of the micro-object with the barrier occurs for a sufficiently certain time, then the energy of the micro-object, 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 micro-object. 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 impulse," says P. Milonni, a physicist at the Los Alamos National Laboratory in the United States.
Most physicists believe that new job does not deal a devastating blow to fundamental principles. But not all physicists believe that the problem is settled. Professor A. Ranfagni, of the Italian research 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.
Works recent years show that under certain conditions superluminal speed may 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 Einstein's formula for adding velocities, but in a fundamental way this is confirmed by the principle of causality. Einstein himself, considering the question of superluminal signal transmission, wrote that in this case "... we are forced to consider a signal transmission mechanism possible, when using which the achieved action precedes the cause. But, although this result from a purely logical point of view does not contain itself, in my opinion, no contradictions, it nevertheless contradicts the character of all our experience 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 spaceship, heading from the Earth towards a star, which 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 faster. 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 one 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!
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